Finished dissertation UP691938

School of Earth and
Environmental
Sciences
Slope stability analysis and rockfall hazard
zonation of Royat, France
Name: Katie Acton
Course: BSc (Hons) Geological Hazards
Student No: 691938
Year: 2015/16

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Disclaimer
The author is required to make the following disclaimer to the reader:

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School of Earth and Environmental Sciences
Slope stability analysis and rockfall hazard
zonation of Royat, France
Katie Acton

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Abstract
Geological hazards affect the lives of people all around the world, with slope failures being one of
the main hazards. Field evidence with rockfall modelling and stereographic projects resulted in the
production of a hazard zonation map. This report with maps shows the areas of highest rockfall
hazard in eight selected locations around Royat Park, France. Trachybasaltic aa lava flow runs
through the valley of Royat and are responsible for the slope stability problems within the town.
There are three surface morphologies discussed within this investigation and they are: rubbly lava,
massive lava and columnar jointing.
Rockfall hazard zoning is considered a quantitative method and does not take into account of the
frequency. The hazard zones corresponding to different kinetic energy levels of the fallen block.
This paper explores the effect discontinuities have on slope geometry, through observing
stereographic projections which highlights Royat can be affected by wedge failure, planar failure
and toppling failure which are common slope failure types. Rockfall modelling results show a clear
pattern, the parameter of Royat Park is considered more hazardous than the centre of the Park.
This approach focused on a small area within the town, and does not take into account of
vulnerability or risk.

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Acknowledgments
The author would like to thank the following people for the help and guidance they have provided
in the production of this dissertation:
 Dr Dave Giles – Many thanks for the support provided given when planning France trip,
as well as the guidance given when carrying out the rockfall modelling
 Dr Benjamin van Wyk de Vries – Support in France, and supplying lidar data that was used
within this report
 Dr Philip Benson- Project Supervision
 Dr Malcom Whitworth – Project support
 Julian Edwards – Providing support with dissertation writing techniques and planning
 Emma Hazell- provided company while carrying out fieldwork in Royat, France
 Emeline Wavelet and Chloé Sevilla – Helping organise accommodation in France, as well
as showing us around the town of Clermont Ferrand
 Robert Holness- Support in France, and putting up with my moaning
 Ryan Rheeston- Spent the time to read the dissertation providing ideas for improvements
Thank you to my family, friends and partner for their endless support and encouragement
throughout the stressful time of writing this dissertation.

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Table of Contents
DISCLAIMER 1
ABSTRACT 3
ACKNOWLEDGMENTS 4
1.0 INTRODUCTION 12
1.1 RATIONALE 12
1.2 BACKGROUND 12
1.3 RESEARCH AIMS AND OBJECTIVES 13
2.0 LITERATURE REVIEW 14
2.1 SLOPE FAILURE MECHANISMS 15
2.1.1 CIRCULAR FAILURE (ROTATIONAL) 15
2.1.2 PLANAR FAILURE 15
2.1.3 WEDGE FAILURE 16
2.1.4 TOPPLING FAILURE 16
2.2 STABILITY ANALYSIS AND ROCKFALL MODELLING 18
2.3 HAZARDS 18
3.0 STUDY AREA 19
3.1 GEOGRAPHICAL LOCATION 19
3.2 GEOLOGICAL HISTORY 21
3.3 GEOLOGICAL SETTING 24
3.4 GEOLOGY 27
3.4.1 GRANITE 28
3.4.2 ALLUVIUM SUPERFICIAL DEPOSIT 28
3.4.3 TRACHYBASALT 30
4.0 METHODOLOGY: 36
4.1 DATA COLLECTION: 36
4.1.1 WALKOVER SURVEY 37
4.1.2 GEOLOGICAL MAPPING: 37
4.1.3 DISCONTINUITY SURVEY 37
4.2 ANALYSIS TECHNIQUES 38

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4.2.1 STEREOGRAPHIC PROJECTION AND KINEMATIC ANALYSIS 38
4.2.2 ROCKFALL MODELLING 38
5.0 RESULTS 43
5.1 GEOMORPHOLOGICAL MODEL 43
5.2 KINEMATIC ANALYSIS RESULTS: 45
5.2.1 KINEMATIC SLOPE STABILITY ANALYSIS OF DIRECT TOPPLING 45
5.2.2 KINEMATIC SLOPE STABILITY ANALYSIS OF PLANAR SLIDING 50
5.2.3 KINEMATIC SLOPE STABILITY ANALYSIS OF WEDGE SLIDING 52
5.2.4 STEREOGRAPHIC PROJECTIONS HAZARD RATING 54
5.3 ROCKFALL MODELLING: 56
5.4 ROCKFALL HAZARD ZONATION 64
6.0 DISCUSSION 66
7.0 CONCLUSION 68
7.1 LIMITATIONS: 68
7.2 FUTURE WORK 69
8.0 REFERENCE 70
9.0 APPENDICES 76
List of Tables:
Table 1: Hazard rating score for location 1 to 8, based on stereographic projections...................54
List of Figures:
Figure 1. Shows the main types of slope failures and there stratigraphic projections which gives
rise to those particular structural conditions. A) Circular failure B) Planar failure C) Wedge failure
D) Toppling failure Source: Hoek and Bray, (1981)....................................................................17
Figure 2. Geographical location of study area at 1:50 000m scale. Royat is highlighted by the red
circular point, near the centre of France. (Map produced using ArcGIS® software) ...................19

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Figure 3. Geographical location of study area at 1:400m scale. Arrow points to the location of
Royat Park. (Map produced using ArcGIS® software) ...............................................................20
Figure 4. Distribution of the main volcanic provinces of the French Massif Central. Royat is
situated within the Chaîne des Puys province. Source: (Jannot et al, 2005)................................21
Figure 5 Simplified geological history of the Auvergne region. (Source: Van Wyk de Vries, 2010)
.......................................................................................................................................................23
Figure 6a. Schematic map of showing the distribution trachytic volcanoes (Black filled dots)
Figure 6b. Lidar image of the volcanoes. (Source: Miallier et al, 2004) .....................................24
Figure 7. Simplified Geological map that highlights key features of the Chaine des Puy, basaltic
lava that runs through the town of Royat. The sample numbers are not relevant to this study.
(Source: Hamelin, 2008)................................................................................................................25
Figure 8 Map showing the Tiretaine lava flows and surrounding lavas. Royat is indicated by
‘outcrop location’. F1, F2, F3 and F4 displays the different Royat Flows. (Source: Loock, 2008)
.......................................................................................................................................................26
Figure 9. Geology map of Royat, France, 1:10 000m scale. Geology map shows there are 3
lithological units within the study area, with Granite dominating the west site of Royat. The
trachybasalt runs through the valley..............................................................................................27
Figure 10. Vertical section created to show the major flow structures within an aā flow...........31
Figure 11. Map showing the locations of the 8 localities that will be the focus of this investigation.
Data collected from these 8 localities will produce: stereographic projections, rockfall modelling
and hazard zonation map. The locations throughout the test, will refers to those within this figure.
.......................................................................................................................................................40
Figure 12. 2D shaded relief surface map of Royat Park with a 0.5m resolution. Map was created
using surfer® 13 software. Surface geometry from this image is used within the rockfall modelling.
.......................................................................................................................................................41
Figure 13. 3D surface map, produced using Surfer to display the digital terrain of the rockfall
study area.......................................................................................................................................41

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Figure 14. Map produced to show the normal and tangential coefficient of restitution which is
imported into the RocFall 5.0 Software to produce slope material...............................................42
Figure 15. Geological ground model showing the 3 lava flow morphological features of
trachybasaltic lava within Royatonic Park ....................................................................................44
Figure 16. Location one- Direct toppling analysis using pole vectors and intersections ............46
Figure 17. Location two- Direct toppling analysis using pole vectors and intersections.............47
Figure 18. Location three- Direct toppling analysis using pole vectors and intersections...........47
Figure 19. Location four - Direct toppling analysis using pole vectors and intersections ...........48
Figure 20. Location five - Direct toppling analysis using pole vectors and intersections............48
Figure 21. Location six- Direct toppling analysis using pole vectors and intersections ..............49
Figure 22. Location seven- Direct toppling analysis using pole vectors and intersections..........49
Figure 23. Location eight- Direct toppling analysis using pole vectors and intersection ............50
Figure 24. Location one- planar sliding kinematic analysis, pole vector mode ...........................51
Figure 25. Location two: - planar sliding kinematic analysis, pole vector mode.........................51
Figure 26. Location one- wedge sliding kinematic analysis, intersection points and contours ...52
Figure 27. Location two- Wedge sliding kinematic analysis, intersection points and contours. .53
Figure 28. Hazard rating map of Royat. This map is used to illustrate the different hazards zones
within Royat, based on stereographic projections. The different colours of each zone indicate the
hazard rating. The run out distance is not displayed in this map...................................................55
Figure 29. Location 1- Rockfall modelling results. (Top) displays the kinetic energy of the fallen
block as it moves down the slope. (Bottom) Displays the rockfall model and trajectory of the fallen
block. Used Rocscience RocFall 5.0 software. .............................................................................56

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Figure 37. Hazard zonation map of Royat Park, 1:20m Scale. The highest hazard zone is located
around the parameter of Royat Park. While the lowest hazard zone is within the centre of the Park
.......................................................................................................................................................65
Figure 38. Slope aspect map of Royat displaying the slope directions. Figure created using
ArcGIS...........................................................................................................................................76
Figure 39. Hillshade map, with contours, of Royat. Created using ArcGIS. ...............................76
Figure 40. Stratigraphic Column showing a simplified geological time scale.............................77

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Figure 41. Regions to be considered for direct toppling. Zone 1 and are the primary and secondary
critical zone. Zone 3 is the critical intersection zone. (Source: Direct Toppling, 2014)...............78
List of Plates:
Plate 1. Large granite outcrop displaying evidence of faulting. (Location: 45°46’10.46”N,
3°02’57.93”E)................................................................................................................................28
Plate 2. Alluvium deposits at location 45°45’57.40”N, 3°03’13.85”E. Plate 2a. Shows bedding
and fractures within the outcrop 2b. Displays a close up of the alluvium deposit, showing it is
poorly sorted. 2c. Displays laminations within the outcrop, which shows different depositional
environments. ................................................................................................................................29
Plate 3. Trachybasalt outcrop at location 45°45’55.74”N, 3°02’58.98”E. 3a. Shows homes have
been constructed around lava flows. b) Displays vesicles within the trachybasalt outcrop (pencil is
20 cm long)....................................................................................................................................30
Plate 4. Lava Tube found at the side of a road, location 45°45’57.57”N, 3°02’53.78”E. The plate
highlights there are instabilities issues within the rock mass, as there are unsupported blocks that
contain fractures. ...........................................................................................................................31
Plate 5. Aā lava flow showing massive and rubbly zone at location 45°45’56.56”N, 3°03’18.13”E.
The rubbly zone is contains loose, fragmented clasts, that are unsupported and have the potential
to fall. The massive zone contains smooth lava, which has randomly spaced discontinuities......32
Plate 6a. Two types of flow morphology, rubbly and massive within Royat Park. 6b. Close up of
the rubbly lava zone. Both plates are located at 45°45’56.56”N, 3°03’18.13”E. .........................32
Plate 7. Grotto de Laveuses, displaying a basaltic lava cave. 7a. Rockfall is being mitigates with
rock bolts, warning signs present to warn the public against the hazard. 7b. Left side of the interior
of the washers cave. 7c. Right side of the interior of the wash .....................................................33
Plate 8 Columnar jointing present near a car park within Royat (location 45°45’54.47”N,
3°03’04.47” N). .............................................................................................................................34
Plate 9. Columnar jointing present within the Tiretaine Valley (Location 45°45’54.21” N,
3°03’02.55 E). Image shows slope stability mitigation methods such as rockfall mesh, rock bolts
and anchors....................................................................................................................................35

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Plate 10. Columnar jointing present within the Tiretaine Valley (Location 45°45’55.3” N,
3°03’00.02 E). Columnar jointing is altered, the colour is lighter than other localities as a results
of chemical alterations from water. ...............................................................................................35
Plate 11. Location 5, Le Grotte Siméoni. Shows discontinuities such as fractures within the rock
mass. Also the cave has been blocked off to stop access due to health and safety regulations. ...79
Plate 12. Massive lava within Royat Park. Image shows buildings have been constructed above
the lava. Rockfall mitigation has been implemented within this area. A fence has been placed to
stop people getting close to the rock slope, to reduce the risk of being affected by any rock falls.
.......................................................................................................................................................80
Plate 13. Evidence of Planar or wedge sliding occuring within the lava......................................80

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1.0 Introduction
1.1 Rationale
This report presents new findings relating the slope stability hazards within Royat, Central France
to key structural data collected in the town. Slope failures are a natural occurrence in mountainous
terrains (Wyllie, 2015), where steep slopes are exposed to weathering (Turner & Schuster 2012).
Failures involving small volumes of material have the potential to cause harm and can pose as a
significant hazard, therefore, a better understanding of rock slope stability on a local scale is very
important. Royat is a town that is located on basaltic lava (Plenier, 2007). Using standard mapping
techniques followed up with slope stability models, the potential for rock slope failure types is
examined.
1.2 Background
Royat is an ancient spa town which draws an influx of tourism through its natural springs
(Prendergast et al, 2011). Royat Park is the focus of this investigation and is situated west of
Royatonic Spa. Although literature regarding rockfall within Royat does not exist, field evidence
alone suggests there are rock slope stability problems within the town. Field observations such as
rock bolts and anchors have been recorded. Furthermore, shotcrete has also been used to cover
rubbly, loose fragmented lava flow debris. Therefore, it’s assumed that mitigation techniques have
been implemented in order to reduce the rockfall potential, to lessen the hazard. In order for
mitigation methods to be successful, it is essential to understand the hazards.
Rockfalls are common worldwide and are referred to as a type of fast mass movement that is
usually triggered by either natural or human induced causes (Palma el al, 2012). Rock slopes tend
to produce slope failures that often generate movements that propagate far from the source (Hantz
et al, 2003). There are four main slope failure mechanisms associated with rock masses and they
are circular, planar, wedge and topple (Bye & Bell, 2001). Rockfalls are also considered a type of
slope failure (Turner & Schuster, 2012).
Geology has a significant impact on stability analysis of any given slopes, particularly regarding
the potential risks of dipping slopes (Lee & Wang, 2011). Slope stability analysis is conducted by
applying discontinuity data collected from field investigation coupled with stereographic
projections in order to evaluate both the mode failure and the stability of the slope. By segmenting
spatial geometries of a given slope coupled with slope material parameters, rockfall modelling can
take place.

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1.3 Research aims and objectives
Rockfall hazards within the town of Royat are assessed, focusing on the park within the centre of
town. The principal aim is to produce a new hazard map that is created using field observations
combined with rockfall modelling in order to illustrate the different hazard zones.
To achieve these aims, this rockfall hazard and risk study has the following objectives:
 Review existing literature on the geological setting of the area while also including both
geological and topographic maps
 Present new data from a field study of the area focusing on rockfall potential areas,
collecting discontinuity data sets
 Create a hazard rating system to summarise the slope stability hazards
 Complete a series of rockfall models in order to record both run out distance using RocFall
Software
 Prepare a rockfall hazard map

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2.0 Literature Review
Rockfalls are considered a rapid type of mass movement (Palma et al, 2012), and are defined by
as the detachment of a rock mass from a steep slope or cliff, in which minimal or no shear
displacement has occurred Li et al., (2009). Once the rock mass has detached from the source the
material it will either free fall, bounce, roll, topple or slide down the slope face (Li et al, 2009;
Vick, 2015). It is believed that rockfalls are initiated by both internal and external influences
(Turner & Schuster, 2012). External influences or “triggers” are defined as those forces acting
externally for changing the rock mass stability (Wyllie, 2012). These forces may, in turn, be
influenced by freeze-thaw, earthquakes, snow melt, and weathering which are just a few examples
of external influences. Internal forces include friction and cohesion.
Discontinuities are defined as the planes of weakness within a rock mass and can control the size
of the ensuing rock blocks (Hustrulid et al, 2000). Joints, bedding planes, foliation, and faults are
geological structures that are also considered to be discontinuities (e.g. Wyllie, Mah and Hoek,
2004), which form weak zones, and control the stability of the rock mass. As mentioned by Hungr
and Evans (1988) rockfalls are hazardous as they have the potential to travel long distances
downslope as a result of increased kinetic energy. Hence this material has the potential to cause
catastrophic consequences.
Turner and Schuster (2012) discuss the different type of rock slope failures, and promotes the idea
that columnar jointed basalt is susceptible to toppling, secondary toppling and ravelling failure
modes. Ritchie, (1963) and Dorren, (2003) both describe the characteristics of motion of a ‘falling
rock’ indicating there are different types of ‘falling’ motions. Nevertheless, it is clear the trajectory
of the fallen rock mass is influenced by many factors, such as slope characteristics (orientation,
slope angle and height) (Hoek and Bray, 1981) as well as characteristics of the block such as
strength shape and size (Ritchie, 1963; Azzoni et al., 1995; Fityus, 2013). This information can be
used to derive a list of variables that need to be measured and recorded during field work. It is
essential to first understand the slope stability parameters, especially those relating to rock
strength. The parameters of interest are based on Kliche’s (1999) work and are defined:
Internal angle of friction - Also referred to as the friction angle, is defined as the angle in which
a body resting on an inclined surface, overcomes the frictional resistance, and begins to slide
Generally materials with high friction angles tend to have low cohesion.
Cohesion – Cohesion is the intermolecular force that causes mineral grains to attract. Cohesion is
a property of both soil and rock and is a component of shear strength that is independent of normal
the normal effective stress in mass movement.

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Daylight envelope - Daylight envelopes are key aspects of stereographic projections that are
primarily used in slope analysis. It is widely viewed that poles plotted within the daylight envelope
have the potential to slide. Hence, planes that daylight from a given slope are represented by the
daylight envelope.
2.1 Slope failure mechanisms
Both Bell (2004) and Kliche (1999) have stated there are four primary slope failure types within
rock masses and they are: circular, planer, wedge and topple which are illustrated in figure 1
alongside their stratigraphic projections. Rockfall as mentioned previously is slope failure
mechanism.
2.1.1 Circular failure (Rotational)
The failure surface of a rotational failure forms either a circular or non-circular curve. In general,
homogenous soil conditions are associated with circular slips, with non-homogenous conditions
linked to non-circular slips (Craig, 2013): a typical circular failure displayed in figure 1a. Circular
failures are greatly influenced by the mechanical properties of the particles within the rock slope
or soil. When the individual particles are very small in comparison to the overall outcrop, a circular
failure can occur. Highly weathered rocks tend to behave like a soil when closely spaced, or when
randomly orientated. This is typical of rapidly cooled basalt, as discussed by Wyllie, Mah and
Hoek (2004).
2.1.2 Planar Failure
Planar failure describes the process where sliding occurs along a plane of weakness as a result of
changes in shear strength and/or ground water conditions (Hoek & Bray, 1981). Figure 1b shows
a typical plane failure within a rock mass. Wyllie, Mah and Hoek (2004) hold the view that plane
failure is a special case due to the geometric conditions needed for plane failure to occur. The
favourable conditions of planar failures have been listed by Prudencio and Jan (2007), Wyllie
(2012), and Wyllie et al., (2004) and include:
 The discontinuity dip direction must strike within ±20
o
of the dip direction of the slope
face.
 The dip of the plane must be less than the dip of the slope face, ψ p < ψ f, hence the sliding
plane is ‘day lighting’ in the slope face
 The angle of friction of the surface must be smaller than the dip of the sliding plane ψ p >
Φ
The following authors go in the greater detail of planar failure: Hoek and Bray (1981) and
González de Vallejo and Ferrer, (2011)

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2.1.3 Wedge failure
According to Wyllie et al. (2004) wedge failures can occur across a diverse range of geometric
and geological conditions, making them more frequent than planar sliding. A wedge failure is the
result of two discontinuities intersecting while sliding occurs along the two planes simultaneously.
In this paper only the standard case of wedge failure will be discussed, hence only taking into
account of the top slope, slope face, and two discontinuities. A typical wedge failure is illustrated
in figure 1c). Kinematic conditions needed for wedge failure to occur according to Park and West
(2001) and Norrish and Wyllie (1996):
 The dip direction of the slope face must be similar to the strike of the line of the intersection
 The dip direction of the intersection must be less than the dip of the slope face
 The angle of friction needs to be smaller than the dip direction of the line of intersection
Stereographic analysis is a simple tool used to analyse kinematic stability and is a generally
accepted method used by both Wyllie (2012) and Park and West (2001). In the literature by Ahmad
et al, (2013), kinematic analysis is used to indicate the possibility of a wedge failure to occur.
Wedge failure analysis has been discussed extensively in geotechnical literature and draws upon
the work of Hoek and Bray (1981) and Londe (1965).
2.1.4 Toppling Failure
Toppling failure is discussed in great detail by Goodman and Bray (1976) who employ
mathematical solutions for describing this process. They also differentiate between the different
types of failure. For the purpose of this investigation, the precise kinematic conditions needed for
toppling failure to occur will be used. According to Bell (2013) toppling is considered a special
type of rockfall that involves large amounts of material.
This form of slope failure happens when the weight vector of the rock mass that is resting on an
inclined plane, falls outside the base of the rock mass (Kliche, 1999). This failure type may occur
in undercutting beds (Hoek and Bray, 1981). The basic conditions needed for toppling and sliding
to occur are:
 The angle of dip must equal the friction angle
 The rock mass rests on an inclined surface
 The joint sets are perpendicular to the inclined surface

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Figure 1. Shows the main types of slope failures and there stratigraphic projections which gives rise to those
particular structural conditions. A) Circular failure B) Planar failure C) Wedge failure D) Toppling failure
Source: Hoek and Bray, (1981)
A
B
C
D

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2.2 Stability analysis and rockfall modelling
A detailed discussion of numerical modelling of rock slope stability has been carried out by
Coggan et al. (1998) who divide the topic into continuum and discontinuum modelling analysis
methods. Continuum modelling involves both finite element and finite difference models whereas
discontinuum implies distinct element and DDA methods. As suggested by Turner and Schuster
(2012), there are a variety of commercial products available that support rock slope stability
analysis through evaluating discontinuity data bases in the form of stereonets. Dips (RocScience)
is the software used to display discontinuity data. Rockpack III (Rocksware 2011), is an alternative
software program to Dips. The development of these commercial products has enabled widespread
use of stereonets in conjunction with probabilistic algorithms in order to assess how discontinuities
affect the slope stability and factor of safety.
2.3 Hazards
Around the world rockfall is considered a major hazard and hence it is addressed by engineers as
reported by Alcántara-Ayala and Goudie (2010), geomorphological hazards referrers to: floods,
landslides, soil erosion etc. A significant volume of literature discusses large scale rock fall events,
and the severe hazards posed by these. For example, Azzoni et al., (1995) discuss the detachment
of a large section of rockmass where nearby valleys have steep sides, typical in alpine settings.
Paronuzzi (2009) outlines the effect of a detachment of a single block and how the potential hazard
is enhanced due to the ensuing ‘rolling’ motion. The development of rockfall hazard rating system
(RHRS) combining all these effects is essential to identify areas that are at risk. The development
of this system began in Oregon (USA), in 1984 (Turner & Schuster, 2012). This system is
qualitative and is subjected to probability, hence the system needs to be developed and related to
specific sites in the Royat area and, in order to quantify the data, and produce a spatial map.

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3.0 Study area
3.1 Geographical Location
The town of Royat is situated in Central France (Figure 2), located in the department of Puy De
Dôme, within the region of Auvergne-Rhône-Alpes (conseil-general.com, 2015). The study area
(Figure 3) is located approximately 2.0km South-West from the nearest town of Chamalières and
3.2km from Clermont-Ferrand.
Royat is considered a desired tourist location due to its history, as it was the location of the first
Roman spa which was found in 20 BC (Prendergast et al, 2011). Thermal spas are often found in
regions of young volcanic activity Watson (1997), making Royat a site of volcanic interest within
the department of the Puy De Dôme.
Figure 2. Geographical location of study area at 1:50 000m scale. Royat is highlighted by the red circular
point, near the centre of France. (Map produced using ArcGIS® software)

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Figure 3. Geographical location of study area at 1:400m scale. Arrow points to the location of Royat Park.
(Map produced using ArcGIS® software)

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3.2 Geological History
Jannot et al (2005) states that the largest magmatic province within the west-European Rift System
is the French Massif Central area which is shown in figure 4. The Tertiary and Quaternary
volcanoes of the Massif Central have been extensively studied, but nevertheless the source of its
volcanism is still being questioned. Froidevaux et al. (1974), and Granet et al. (1995) suggest that
the activity originated as a result of magma upwelling from the mantle, acting as a hotspot. While
Dèzes et al (2004) has argued that the formation of the Alpine lithospheric root resulted in the
presence of an asthenospheric flow.
The Chaîn des Puys is both the most northern and most recent volcanic unit of the Massif Central
in France as highlighted by Wallecan (2010). The Massif Central formed approximately 400
million years ago, initiating at the start of the Hercynian orogeny. Europe experienced widespread
deformation, thrusting and metamorphism of the basement rock which is discussed in greater detail
by Jung (1946).
Figure 4. Distribution of the main volcanic provinces of the French Massif Central. Royat is situated within
the Chaîne des Puys province. Source: (Jannot et al, 2005)

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The geological history of Massif Central is complex and has undergone significant tectonic
processes. The Hercynian orogeny was the result of a period of uplift throughout the Devonian
and the Carboniferous period which then resulted in the formation of the basement rocks (Boivan,
2004). The basement rocks comprised of both granites and metamorphic rocks.
Throughout the Oligocene extensional stresses along with crustal thinning in the lithosphere gave
rise to both the European Cenzozoic Rift System and the grabens of the Limagne (Wallecan, 2010)
which were filled with sediments rich in marl and limestone. These were then uplifted over 20Ma
(Nehlig et al., 2001). The Late Miocene- Pleistocene experienced the main phase of volcanisms
within the Auvergne region, which resulted in major volcanic edifices (Wallecan, 2010) that
became the volcanoes of central and Monts-Dore (Nehlig et al., 2001; Wallecan, 2010). It is during
the Pleistocene and Holocene (between 90,000 and 6,000 years ago) that the Chaîne des Puys
formed which then triggered the formations of fissure-related Strombolian scoria cones (Boivin et
al., 2004; Wallecan, 2010). A simplified geological timescale is shown in figure 5 and is described
in more deal by Benjamin Van Wyk de Vries (2010). A stratigraphic column is found within the
appendix.

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Figure 5 Simplified geological history of the Auvergne region. (Source: Van Wyk de Vries, 2010)

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3.3 Geological Setting
According to Jannot et al (2005) the Chaîne des Puys contains over 90 volcanos that are relatively
young in age, 95-7 Ka (Boivin et al., 2004), mainly scoria cones (Plenier et al, 2007). Figure 6
shows the extent of the distribution of volcanoes in relation to the Limagne Fault, with black dots
representing trachytic volcanoes. Vernet et al, (1998) has concluded that the basaltic phreatic
activity along the western fault scarp of the Limagne developed between 160 and 70ka, and then
the Chaîne des Puys began.
The volcanic products of the Chaîne des Puys range from basalt to Trachyte in composition (Jannot
et al, 2005; Plenier et al, 2007). 90% of the lava flows of Chaîne des Puys are basaltic lava flows
(Scarth, 2001. pp. 221) as shown in Figure 7. Basaltic lava runs through the study area of Royat,
mimicking the flow path of the Tiretaine River (Figure 8).
A B
Figure 6a. Schematic map of showing the distribution trachytic volcanoes (Black filled dots) Figure 6b.
Lidar image of the volcanoes. (Source: Miallier et al, 2004)

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N
Royat Basaltic Lava
Flow
Figure 7. Simplified Geological map that highlights key features of the Chaine des Puy, basaltic lava that runs
through the town of Royat. The sample numbers are not relevant to this study. (Source: Hamelin, 2008)

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Figure 8 is produced by Loock et at (2008) and describes the various different basaltic flows that
flow for Royat. As mentioned previously, it is difficult to delineate between the different ages of
basaltic lava, hence during fieldwork basaltic lava delineation has been based on morphological
features. The different morphological features of basaltic lava flows have been discussed by
numerous authors (e.g. Kilburn (2004), Boivin et al., (2004) & Fityus et al., (2013), with Kilburn
(2004) differentiating the different sections of basaltic lava flow.
Figure 8 Map showing the Tiretaine lava flows and surrounding lavas. Royat is indicated by ‘outcrop location’.
F1, F2, F3 and F4 displays the different Royat Flows. (Source: Loock, 2008)

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3.4 Geology
Royat is situated on the outskirts of Clermont Ferrand, and the very edge of the Limagne Basin,
with the majority of the town being built on top of lava. Though carrying out fieldwork a geological
map was produced (figure 9) which shows there are three lithological units that are present within
the study area, which include: granite, trachybasalt and alluvium; trachybasalt being the focus of
this rockfall investigation.
Figure 9. Geology map of Royat, France, 1:10 000m scale. Geology map shows there are 3 lithological units
within the study area, with Granite dominating the west site of Royat. The trachybasalt runs through the valley.

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3.4.1 Granite
The granite is situated in the west and is the most abundant lithology within the area as shown in
figure 9. The granite is the oldest lithology within the study area, underlying the lava flows and
alluvium. The granite formed at depth from magma, and is referred to as plutonic rock (Petford et
al, 2000). The composition of the rock provides evidence of its origin. It is comprised of: orthoclase
feldspar, plagioclase feldspar quartz and biotite which acidic in nature. These minerals are known
as felsic minerals and contain large amounts of silica.
It can be assumed that the granite has experienced a series of uplift, resulting in increased stress
and faulting. Plate 1 demonstrated faulting that has occurred within the granite, sowing clear shear
plane.
Plate 1. Large granite outcrop displaying evidence of faulting. (Location: 45°46’10.46”N, 3°02’57.93”E)
3.4.2 Alluvium superficial deposit
The alluvium deposits are referred to as loose unconsolidated sediments that have been both eroded
and reshaped by water, and then deposited in non-marine environments. Alluvium deposits are
made up of fine grained clay, sand and gravel particles that that have been cemented into a
lithological unit. Plate 2 shows the alluvium deposits which consist of nearly horizontal strata,
consisting of a mixture of sand, sandstone clay and mudstone. The Alluvium deposit is poorly
sorted, thinly-bedded and contains medium grained clasts that are sub angular to sub rounded
within a fine grained cement (plate 2c). The alluvium deposit is older than the lava deposits and
has experienced a period of uplift.

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Plate 2. Alluvium deposits at location 45°45’57.40”N, 3°03’13.85”E. Plate 2a. Shows bedding and fractures
within the outcrop 2b. Displays a close up of the alluvium deposit, showing it is poorly sorted. 2c. Displays
laminations within the outcrop, which shows different depositional environments.

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3.4.3 Trachybasalt
Royat lava flows are located west of Clermont Ferrand and belong to the Quaternary Chaîne des
Puys. Research done by Sébastien Loock (2008) suggests there are 4 superimposed lava flows
(figure 8) that date back 45,000 years, with flow 1 and flow 2 being the focus. The flows are
controlled by the paleotopography of Royat, following the path of the Tiretaine River. Loock
(2008) also discusses that the Royat lavas originated from Petit Puy De Dôme which is a basaltic
strombolian cinder cone that produced aā lava flows. These exposures are found throughout Royat,
with plate 3 displaying a typical outcrop. Trachybasalt is a fine grained effusive igneous rock that
is intermediate in composition, between trachyte and basalt (Bowes, 1989).
Plate 3. Trachybasalt outcrop at location 45°45’55.74”N, 3°02’58.98”E. 3a. Shows homes have been constructed
around lava flows. b) Displays vesicles within the trachybasalt outcrop (pencil is 20 cm long).

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3.4.3.1 Massive and rubbly morphology
Within Aā flows the crust is being disrupted constantly (Brown et al, 2011) forming clinker while
allowing the heat to be lost in the core. Clinker is initially spinous, however during the later
advancement stages, they become rounded and rubbly (Loock, 2008), which is visible in several
localities around Royat. The clinker zone of a lava flow is highly permeable, and hence if water is
injected into the upper vesicular zone, it can result in the formation of lava tubes as shown in plate
4 .
Figure 10 demonstrates a vertical cross section that displays the major flow structures within an
aā flow based on field evidence. The core of the aā flow is known to have a massive morphology
that is both denser and stronger then the upper and lower rubbly crust (Hulme, 1974). The rubbly
base contains large amounts of vesicles that are deformed. One important observation to make is
the massive lava is smoother and less weathered that then rubbly lava. Whereas the rubbly basalt
is more flakey and weathered.
Plate 4 Lava Tube found at the side of a road, location 45°45’57.57”N, 3°02’53.78”E. The plate highlights there are
instabilities issues within the rock mass, as there are unsupported blocks that contain fractures.
Figure 10. Vertical section created
to show the major flow structures
within an aā flow.
The clinker represents a sharp lava
flow texture. The core is a massive
smooth texture. While the rubbly
lava, is sub- angular, and
fragmented.

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Plate 5. Aā lava flow showing massive and rubbly zone at location 45°45’56.56”N, 3°03’18.13”E. The rubbly
zone is contains loose, fragmented clasts, that are unsupported and have the potential to fall. The massive zone
contains smooth lava, which has randomly spaced discontinuities.
Plate 6a. Two types of flow morphology, rubbly and massive within Royat Park. 6b. Close up of the rubbly lava
zone. Both plates are located at 45°45’56.56”N, 3°03’18.13”E.
Scale:1.0m

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Plate 7. Grotto de Laveuses, displaying a basaltic lava cave. 7a. Rockfall is being mitigates with rock bolts,
warning signs present to warn the public against the hazard. 7b. Left side of the interior of the washers cave.
7c. Right side of the interior of the wash
According to Vernet (2003) the basaltic cave (Plate 7) formed as a result of vaporisation of water
occurring beneath the hot lava flow, which then resulted in huge bubbles of steam remaining
trapped, leaving behind basaltic caves. Plate 7a shows the massive core of the lava has been pushed
up as a result of the steam, and is flanked by scoria. Rock bolts have been used to stabilise the rock
mass as well as warning signs have been put in place to discourage the public from going within
the cave. The interior of the cave is shown in plate 7b and plate 7c, displaying that the roof of the

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cave is unsupported and has undergone chemical alterations. It is essential to note that there is a
spring that flows through the lava flow, which can cause water to go within the joints of the lava
flow.
3.4.3.2 Massive and rubbly morphology
Columnar jointing is another lava morphology present within Royat, and is formed as a result of
contraction and rapid cooling. Generally, the columns are symmetrical hexagonal with equal sides
and equal angles, however perfection is not always the case (Spry 1962). In this instance the
columns present in Royat are pentagonal columns (5-sided) with fluctuating angles (Plate 8). The
slower the cooling process, the wider the columns (Hetényi et al, 2012).
Plate 8 Columnar jointing present near a car park within Royat (location 45°45’54.47”N, 3°03’04.47” N).
Recent publications suggest that columnar jointing formed as a result of contractional cooling
(Guy and Le Coze 1990; Gilman 2009; Guy 2010), which is the widely accepted view. Columnar
jointing forms. The Joints form as a result of increased stress within the lava as it cools, forming
cracks as the lava continues to grow (Hetényi et al, 2012). Typically, the growth is perpendicular
to the surface of the flow. Both late 9 and 10 display columnar jointing, as well as demonstrating
the hazards that are present. As you can see from the image, there are unsupported rocks, which
have the potential to fall. Mitigation methods have been implemented at the site, as there are rock
bolts, shotcrete, and rockfall mesh, which act to stabilise to the slope.

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Plate 9. Columnar jointing present within the Tiretaine Valley (Location 45°45’54.21” N, 3°03’02.55 E). Image
shows slope stability mitigation methods such as rockfall mesh, rock bolts and anchors.
Plate 10. Columnar jointing present within the Tiretaine Valley (Location 45°45’55.3” N, 3°03’00.02 E).
Columnar jointing is altered, the colour is lighter than other localities as a results of chemical alterations
from water.

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4.0 Methodology:
Rock slope stability is influenced significantly by the structural geology hence geological
investigations are necessary. As stated by Fossen (2010) structural geology is referred to as the
study of deformation structures in the lithosphere in order to understand how structures form by
looking at its geometry. In this instance the investigation focuses on the naturally occurring breaks
within the rock unit which are known as discontinuities. Wyllie, Mah and Hoek (2004) highlights
that rock slope stability investigations should address the structural geology of the site which
involved two steps: mapping the geological outcrops to determine the engineering properties of
the rock slope and carrying out discontinuity surveys. Through using stereographic projection
method as a kinematic analysis the mode of slope failures has carried out, as suggested by Ahmad
et al, (2013). The data was then used to work out the factor of safety for each site and potential
hazards, vulnerability and risk where produced. The methodology for each of these processes are
described in more detail later in this chapter.
4.1 Data collection:
Literature on Royat was very limited due to its location as a result the fieldwork was a very
important element of the project. The investigation enabled the development of a more detailed
geological map of the valley of Royat, which differentiated between the different types of basaltic
lava flow types. The area of focus was the geology park situated next to Royatonic Spa, which
demonstrates clear basaltic lava flow exposures. The data collection enabled the production of:
 Field observations
 Geology map
 Lava morphological map
 Discontinuity survey
Analysis of the data collected in the filed allowed the production of the following features:
 Stratigraphic projections
 Kinematic analysis
 Rockfall Modelling
 Hazard Map

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4.1.1 Walkover survey
The walk-over survey was taken place in order to collect information on the site that could not be
obtained from the study of literature alone. Also referred to as the site reconnaissance (Bell, 2004)
the walk over survey was undertaken to confirm information that was collected during the desk
study. Through doing the walkover survey in conjunction with the desk study the accessibility of
site was reviewed. Once accessibility to the site was deemed suitable the site reconnaissance was
completed on the afternoon June 25th
2015. Walking trails were the main routes taken when doing
the initial primary data collection.
4.1.2 Geological Mapping:
The purpose of geological mapping is to distinguish the different geological units as well as
defining sets of discontinuities, or single features like faults which will all influence the stability
of the slope. The method for geological mapping has be adapted from Coe (2010). Bell (2004)
suggests that before a discontinuity survey commences, it’s essential to map the geology of the
area to determine both the rock types’ present and major structures.
The geological mapping scale was carried out on a 1:10000m scale as suggested by Coe (2010) in
order to produce a map similar to the map by Boivin and Benson (2009) (appendix 1). The study
area was small hence, the geology could be described in more detail. It can be noted that authors
such as Loock, et al (2008) have stated that the there are several lava flows that run through the
valley of Royat, referred to as the Tiretaine flows demonstrating different ages of flows. When
field observations were carried out it was hard to delineate between the different ages of flows.
Hence, the lava flows where categorized into different types of basaltic flows: rubbly, massive and
columnar (Bonney, 1876; Lyle, 2000; Waters, 1960)
The different types of flows can affect the overall stability of a rock slope or rock face, which is
why it’s important to know both the geological and engineering properties of the rock type.
4.1.3 Discontinuity survey
The first step of a discontinuity survey is to analyse the orientations and identify discontinuity sets,
or any potential unstable blocks. These should have been identified during the geological mapping.
Both Wyllie, Mah and Hoek (2004) and Turner and Schuster (2013) recommends using the terms
dip and dip direction for orientation that will be collected. The following definitions are taken
directly from Wyllie, Mah and Hoek (2004, pp 26):
1) Dip is the maximum inclination of a discontinuity to the horizontal (angle ).
2) Dip direction or dip azimuth is the direction of the horizontal trace of the line of dip
measured clockwise from north (angle).

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It can be noted that dip and dip directions can be used to plot stereonets and analysis of
discontinuity orientations. During data collection the dip is written as 2-digits and the dip direction
is written as 3-digits which minimize the confusion of which data set is what. As well as orientation
data the following information is also recorded during a discontinuity survey: persistence, spacing,
roughness, aperture, strength, filling and seepage.
4.2 Analysis Techniques
4.2.1 Stereographic Projection and Kinematic analysis
Orientation data collected during fieldwork can be visualized in a stereographic projection. The
aim of stereographic projection is to allow three-dimension orientation data to be analysed in two-
dimensions. Orientation data collected will be used in the program Dips 7.0, part of the RocSceince
programs, and will allow structural data to be displayed and analysed. Both the dip and dip
direction at different site locations will displayed in different stereographic projections. Hoek and
Bray (1981) demonstrates in figure 1 how stereonets can be used to plot great circles which are
poles. Through plotting the poles of orientation data potential stability problems should be
recognizable. Once the type of block failure has been identified, kinematic analysis is then
conducted.
Kinematic analysis involves examining the block failure by recognizing the direction in which the
rock mass will slide, in order to assess stability conditions. RockFall 5.0 is the chosen numerical
simulator software that has been used, as suggested by Ahmad et al (2015). The software is also
part of the RocScience package and allows the maximum kinetic energies to be presented.
4.2.2 Rockfall modelling
Rockfall modelling is used to determine the run out zones of fallen material in order to assess the
hazards of rockfalls. As suggested by Turner and Schuster, 2012) the rockfall modelling program
used during this investigation was RocFall. RocFall program from RocScience, Inc. was developed
by Stevens (1998), with the most recent version RocFall 4.0 being offered commercially, allowing
the user to undertake rockfall simulations.
Modelling has been carried out at 8 locations within Royat, and are shown in figure 11. In order
to define the slopes of each profile, topographic sections were constructed using slope geometry
collected from the 2d shaded relief map, figure 12, and graphically represented in Rocfall 5.0
Software. A 3D surface map of the study area is shown within the figure 13 which uses colours to
emphasize the terrain.
Elevation profiles are imported into the software package to determine the geometry of the slope,
and through carrying out a literature review on the coefficient of restitution and friction angles of

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basalt, the slope material is defined. The slope material was defined as ‘bedrock outcrop’, ‘talus
slope’ and ‘asphalt’ for all the slopes. A surface map was produced based on field evidence,
displaying simply the surface types present within Royat presented in figure 14. The normal
coefficient of restitution (Rn) and tangential coefficient of restitution (Tn) where provided by
Rocsceince Coefficient of restitution table (2016) the values for which are shown within the table
in figure 14. A random approach when carrying out the rockfall modelling, hence Monte Carlo
sampling was used as the simulation setting design. ‘Rigid body’ was used, as suggested by
RocScience, (2015), which takes into account of block shapes. Once data is inputted into the
programme, rockfall modelling can takes place, with results being displayed in both graphs and
models, which will then be used during hazard analysis.

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Figure 11. Map showing the locations of the 8 localities that will be the focus of this investigation. Data collected
from these 8 localities will produce: stereographic projections, rockfall modelling and hazard zonation map.
The locations throughout the test, will refers to those within this figure.

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Figure 12. 2D shaded relief surface map of Royat Park with a 0.5m resolution. Map was created using surfer®
13 software. Surface geometry from this image is used within the rockfall modelling.
Figure 13. 3D surface map, produced using Surfer to display the digital terrain of the rockfall study area.

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Figure 14. Map produced to show the normal and tangential coefficient of restitution which is imported into
the RocFall 5.0 Software to produce slope material.

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5.0 Results
5.1 Geomorphological Model
A geomorphological model was created for the town of Royat, displaying the location of the 3
different morphologies of the trachybasalt (figure 15). The morphologies of the different lavas can
affect the overall stability of the rock mass, making it essential to examine the hazards.
Massive lava is the dominant morphology whereas the rubbly lava is the least abundant. The
massive lava is both smoother and stronger in strength in comparison to the rubbly lava with
outcrops being more visible around the town. This is a disadvantage as town is built on the massive
lava flow, which can experience rockfall in large blocks from the slopes causing servere harm.
The rubbly lava flows dominate within the Royatonic Park; it can be assumed that this area is not
built upon, due to poor foundations it would provide. It can be noted that the river contains large
angular rocks, indicating that rock falls have occurred in the past. Furthermore, there is a river that
runs through Royatonic Park, as well as springs that flow through some areas of the slopes,
increasing the hazard within the area. This is due to there being a combination of both rockfall
hazard and flooding hazard. In addition to this water can also increase pore pressures within the
rock, making it more susceptible to rockfall. The columnar jointing morphology outcrops close to
the Grotto de Laveuses. It is important to note that if rockfall was to occur it would produce large
blocks due to discontinuities being widely spaced.
Through field evidence alone it’s anticipated that the rubbly lava is more susceptible to rockfall in
comparison to the massive lava, due to it being more fragmented. Plate 5 and 6 demonstrates
clearly the different flow structures within a Royat flow, with clear evidence of rockfall within the
aā flow, which can pose as a hazard. Both the columnar lava and massive lava are more intact and
it can be assumed that if a rockfall is to occur, the material would fall as blocks rather than rubbly
fragmented pieces. The trajectory of rockfall material will be discussed later in the text.

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Figure 15. Geological ground model showing the 3 lava flow morphological features of trachybasaltic lava
within Royatonic Park

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5.2 Kinematic analysis results:
Through research of the structural geology of Royat Park, stereographic projections were applied
with kinematic analysis as method of analysing structural data in two dimensions, as recommend
by Wyllie, Mah and Hoek (2004). The following sliding kinematic analysis were taken place:
planar sliding, wedge sliding, direct topping.
5.2.1 Kinematic slope stability analysis of direct Toppling
Like planar and wedge sliding, direct toppling kinematic analysis was carried out within Dips 7.0
which uses concepts described by Hudson and Harrison (2000). The fundamental components of
direct toppling kinematic analysis according to Direct Toppling (2016) are:
- Two joint sets intersect in such a manor resulting in the intersection line dipping into the
slope resulting in discrete toppling blocks
- Release planes or sliding planes which allows blocks to topple due to the existence of a
third joint set
Direct toppling occurred in every location. The results are described below:
Location 1:
The stereographic projection for location one (figure 16) shows there are no pole vectors or critical
intersections with the primary and secondary critical zone (the highlighted red zone), which
implies there is no risks of toppling blocks. There is however a critical intersection within the
oblique toppling section (the highlighted yellow zone). This implies that toppling outside of the
lateral limit is more likely to occur, which can still pose a hazard. It can be noted the planes do not
fall with the critical base planes.
Location 2:
Figure 17 displays the direct toppling kinematic analysis failure for location which states that there
is a 0% chance of direct toppling occurring. There are two poles that are plotted in critical zone
which signify release planes which are also considered sliding planes, and are situated outside the
friction cone. There is only a 1.52% likelihood of oblique toppling to occur, which indicates
toppling happens near vertical intersections, outside the lateral limit.
Location 3:
Direct toppling and oblique toppling are not likely to occur at location 3, as shown in figure 18.
74% of poles are plotted within the critical base plane zones. Poles that are plotted within the
critical intersection zone and oblique topping and basic plane zone suggest that planes have the
potential to act as release surfaces for toppling surfaces.

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Location 4 and 6
Results for location four (figure 19) and location 6 (figure 21) display exactly the same results;
100% of poles are plotted within the critical base planes zones, which is the highest percentage.
The poles are plotted within zone 2 and 3 are not sliding planes, however they can act as release
planes. Direct toppling and oblique toppling are not likely to occur according to the results. It can
be noted that friction cone appears larger at location 6 in comparison to location 4.
Location 5
Similar to location 3, direct toppling and oblique toppling are not likely to occur at location 5 as
shown in figure20. However, 71% of poles fall within the critical base planes which can be
considered potential base planes, even though the planes may dip into the slope.
Location 7 and 8
Direct toppling kinematic analysis result for location 7 and 8 are shown in figure 22 and figure 23.
Both locations are likely to not experience direct toppling or oblique toppling according to the
direct topping results. At location 7 89% of poles are considered potential base planes as they are
located within the critical base plane (zone 2 and 3) which is a higher percentage than location 8.
Results show that no sliding occurs at either location 7 or 8.
Figure 16. Location one- Direct toppling analysis using pole vectors and intersections

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Figure 17. Location two- Direct toppling analysis using pole vectors and intersections
Figure 18. Location three- Direct toppling analysis using pole vectors and intersections

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Figure 19. Location four - Direct toppling analysis using pole vectors and intersections
Figure 20. Location five - Direct toppling analysis using pole vectors and intersections

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Figure 21. Location six- Direct toppling analysis using pole vectors and intersections
Figure 22. Location seven- Direct toppling analysis using pole vectors and intersections

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Figure 23. Location eight- Direct toppling analysis using pole vectors and intersection
5.2.2 Kinematic slope stability analysis of planar sliding
The planar sliding kinematic analysis failure mode was a test carried out in Dips 7.0 Rocsceince
software. As mentioned by (Planar Sliding, 2016) the critical pole vector zone for planar sliding
is defined by the region that is situated:
 Inside the daylight envelope and
 Outside the pole friction cone
 Inside the lateral limit
Hence, the poles that are situated within the critical pole vector zone are at risk of sliding. Location
one (figure 24) planar kinematic analysis shows that there are no poles that are critical of planar
sliding indicating that the slope is not at risk of planar sliding. However, the density concentration
is slightly within the critical pole vector zone, indicating there is a slight chance of planar failure.
One major drawback to this is it is hard to judge the planar sliding with only four poles plotted.
There is some evidence within figure 25 that may indicate location 2 may also be at risk of planar
sliding, as there are 2 poles that are located within the critical zone of planar sliding. Table 1
shows that location 2 has a higher hazard in comparison to location 1.

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Figure 24. Location one- planar sliding kinematic analysis, pole vector mode
Figure 25. Location two: - planar sliding kinematic analysis, pole vector mode

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5.2.3 Kinematic slope stability analysis of wedge sliding
Wedge sliding kinematic analysis failure mode was carried out using Dips 7.0 Rocscience
software, and is a test used for sliding of wedges that are formed by the intersection of two planes
(Turner and Schuster, 2013). Pole vectors are not used in this analysis. This form of kinematic
analysis uses dips vectors of individual planes. According to (Wedge Sliding, 2016) intersections
must be within the critical zone. Wedge failure is present at location one and location two.
At location one (figure 26) 67% of the intersections fall within the critical zone, which plays a
vital role in bringing that hazard score higher. Hence, intersections present within the critical zone
increases the risk of wedge sliding to occur. Location two (figure 27) has 44% of intersections
present within the critical zone. The findings indicate that there is lower hazard posed by wedge
sliding compared with location one. There is no hazard posed by wedge sliding at location three
to eight. Results are shown within the appendix.
Figure 26. Location one- wedge sliding kinematic analysis, intersection points and contours

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Figure 27. Location two- Wedge sliding kinematic analysis, intersection points and contours.

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5.2.4 Stereographic projections hazard rating
Table 1 has been used to display a hazard score for each location based on the potential of planar
sliding, wedge sliding and direct toppling failure. The score is calculate using:
Hazard score =
𝐓𝐡𝐞 𝐧𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐜𝐫𝐢𝐭𝐢𝐜𝐚𝐥 𝐩𝐨𝐥𝐞𝐬
𝐭𝐨𝐭𝐚𝐥 𝐧𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐩𝐨𝐥𝐞𝐬
The higher the frequency of critical poles, the higher the hazard score, the lower the frequency
score the smaller the hazard score.
Table 1: Hazard rating score for location 1 to 8, based on stereographic projections.
Location:
Lava
morphology
type
Hazard score
Planar
sliding
(Hp)
Wedge
Sliding
(Hs)
Direct
Toppling
(Ht)
Total hazard
score (out of
3)
One Massive 0 0.67 0.17 0.84
Two Massive 0.16 0.44 0.18 0.78
Three Columnar 0 0 0.74 0.74
Four Columnar 0 0 1.00 1.00
Five Massive 0 0 0.71 0.71
Six Columnar 0 0 1.00 1.00
Seven Massive 0 0 0.89 0.89
Eight N/A * 0 0 0.60 0.60
* N/A = not applicable due to location seven being the alluvium deposit.
Stereographic projections not mentioned in the report are found within the appendix.
The hazard rating is out of 3:
0- No hazard, 0.01 to 0.99 -low hazard, 1.00 to 1.99-medium hazard, 2.00 to 3.0-high hazard
H= Hp + Hw+ Ht
* H = Hazard, Hp = planar sliding hazard, Hw, wedge sliding hazard, Ht = direct toppling hazard.

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The results from the hazard scores are presented in table 1. The lowest hazard is location 8, which
is the alluvium deposits. Whereas the highest hazard is within the columnar jointing at location
four and six which have identical hazard scores. Results show that planar and wedge sliding is not
as probable as direct toppling. Location 2 however has the potential to produce planar, wedge and
direct toppling. The results are also shown in a simplified hazard map (figure 28).
Figure 28. Hazard rating map of Royat. This map is used to illustrate the different hazards zones within
Royat, based on stereographic projections. The different colours of each zone indicate the hazard rating. The
run out distance is not displayed in this map.

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5.3 Rockfall modelling:
Rockfall modelling is undertaken using the RocSceience software program, in order to determine
the areas that are considered hazardous. Figure 30 to figure 37 displays the results of the rockfall
modelling carried out at 8 different locations and starting positions.
Figure 29. Location 1- Rockfall modelling results. (Top) displays the kinetic energy of the fallen block as it
moves down the slope. (Bottom) Displays the rockfall model and trajectory of the fallen block. Used Rocscience
RocFall 5.0 software.

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36.5m 44.95m

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The result from the rockfall modelling shows that the closer to the rockfall source the higher the
hazard. The further away from the source the lower the hazard. It is clear that the slope appears
smooth and so the materials roll down the slope rather than bouncing which wasn’t expected. The
kinetic energy distribution fluctuates across each location. Generally, the further away from the
source, the lower the kinetic energy peaks, providing that slopes gradient deceases at the same
rate, as displayed in figure 33 (location 5) and figure 29 (location 1).
The hazard zones are based on fluctuates within the kinetic energy as it assumed that the higher
the kinetic energy the higher the impact, hence the higher the hazard. However even this approach
does not take into account of independent variables such as slope material type. Figure 36 (location
8) shows that the kinetic energy reached its peak in what is considered the low hazard zone.
The maximum kinetic energy is displayed within figure 30 (location 2) which is the location of
the massive lava (as shown in the geomorphology map). Whereas the lowest is location 8. Figure
30 has a steeper slope gradient whilst figure 36 has the lowest, which is the alluvium slope.
5.4 Rockfall Hazard Zonation
Rockfall modelling together with hazard zonation system discussed is a crude tool which is
regarded as a semi-qualitative approach. The rockfall trajectory was characterized quantitatively
by examining the runout distance, while the hazard zones were created based on divergences in
the kinetic energy.
The lowest hazard zone is the no hazard zone (figure 37) here the kinetic energy of the moving
material is zero, hence the material is no moving. The width of this zone varies from 20m in the
west, and 125m in the east. Due to there being no energy to move the material into this zone, there
is no hazard recorded. The hazard map highlights that the centre of Royat Park is not considered
hazardous, whereas the high hazard zone are close to the slope. Run out distance increases as you
move further away from the slope, thus the kinetic energy produce during rockfalls also decreases.

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Figure 37. Hazard zonation map of Royat Park, 1:20m Scale. The highest hazard zone is located around the
parameter of Royat Park. While the lowest hazard zone is within the centre of the Park

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6.0 Discussion
This investigation concentrates on common rock slope stability problems that are found all around
the world, with the investigation focussing on the town of Royat in particular. This study draws
upon slope failures such as toppling, wedge and planar failures highlighting that toppling is
regarded as the highest hazard within Royat Park. As well emphasizing that highest rockfall hazard
is located near the slope.
Recalling upon an earlier statement, Turner and Schuster (2012) suggests that columnar jointed
basalt is susceptible to toppling failures. Through fieldwork, and stereographic kinematic analysis
the result collected also support this idea. However, the lithology is trachybasalt and not basalt. As
mentioned previous location 4 (plate 10) and location 6 (plate 9) produced the largest hazard score
within table 1. Looking on the hazard zonation map it is clear that 4 and 6 are within close
proximity of each other, therefore it’s presumed they were formed at the same times. This is further
supported by the geomorphological map which shows that the columnar jointing formed only in
the west side of Royat Park. Field observations also recorded that the both flow directions were in
a south-east direction, which is perpendicular to the growth (Hetényi et al, 2012).
Hoek and Bray (1981) state that slope failures are more likely to occur in fractured rock masses
depending on the structural control of discontinuities. Stereographic projections show that wedge
and planar failure is only likely to occur at location 1 (plate 5) and location 2. These locations also
correspond to the massive surface morphology of the aa lava flow, which are both similar in
structural properties, and are in close proximately of each other. Planar sliding occurs when the
dip of the discontinuity is dipping in the direction of the slope face and intersect which can initiate
sliding (Okubo, 2004). While wedge failure can be triggered by the intersection of two
discontinuities with the slope. It is known that if failure was to occur in these locations, the failure
block would undergo minimal rotation in comparison to toppling blocks. Using information shown
in the rockall hazard zonation map, it can be assumed the high hazard zone would be smaller for
a planar sliding or wedge sliding, in comparison to the rockfall displacement shown in figure 37.
The rockfall models where produce using RocSceince Software Rocfall 5.0. Although the models
produced used actual slope geometry imported from LIDAR data, the slopes appeared shallower
within the models then what was witnessed within the field. There could me numerous reasons
behind this, yet the main reason could be that the Lidar image (figure 12) used within this
investigation was not a good enough resolution. The LIDAR image was 0.5m resolution, which
produced slopes that were not as detailed as was required. The models produced falls that were far
less simple then was expected and only shown one generic rockfall path for each location, even
when using 50 samples was suggested by Ahmad et al (2015).

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The main weakness with this investigation was that the hazard map was created using peaks within
the kinetic energy graphs and run out distance as the parameters. These are not a realistic parameter
to use on their own. The basic assumption was the closer to the source the higher the kinetic energy,
hence the higher the hazard. One major drawback to this approach is it suggest there is a negative
correlation (inverse relationship) between these two parameters as run out distance increases,
kinetic energy decreases. This does not take into account of variables that may increase kinetic
energy as the material moves down a slope. Kinetic energy can fluctuate based on numerous
variables as described in more detail by Jaboyedoff et al, (2005). Location 8 (figure 36) is a typical
example where kinetic energy is not a good parameter to use when defining the hazard zones. The
maximum kinetic energy is within what is considered the ‘low hazard’ zone. The increased kinetic
energy can be due to slope material change, as well as change in slope gradient. Location 6 (figure
34) and location 4 (figure 32) also show several increases in kinetic energy within ‘low hazard
zone’. The strategies used for defining the hazard zones were not correct and will come with major
criticism.
Nevertheless, the hazard map produce, shows a generic rockfall hazard map, that follows similar
trends that are shown within the works of Guzzetti et al (2003) and Jaboyedoff et al, (2005).
Rockfalls and slope failures are a major hazard around the work, especially on major roads within
mountainous terrain. Badger and Lowell (1992) have stated that in the last 30 years nearly half a
dozen fatalities have occurred due to rock slope problems with 45% occurring as a result of
rockfalls in Washington, United Staates. Furthermore, Hungr and Evans (1989) recall that in an
87-year period, 13 deaths were also recorded as a result of rock related falls, in Canada. Therefore,
these particular events illustrate the importance of hazard mapping. Within Royat there are main
roads that run throughout the town, in particular Avenue de la Vallée and Avenue de Puy de Dôme
that connect the town of Royat to Orcines. Looking at other historical events around the world, if
a rockfall event was to occur it can cause significant problems. The implications of this study could
allow hazard maps to be used commercially to mitigate against or reduce the hazard from rockfalls
in zones considered to be vulnerable or at risk, such as busy roads within high hazard zones. It is
highly important to mention that that vulnerability and risk have not been discussed within this
report, which brings on the next section.

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7.0 Conclusion
Mountainous areas with steep sided slopes are frequently subjected to slope failures. This project
examines the potential hazards wedge slides, planar slides, direct toppling and rockfall have within
the town of Royat. The primary aim of this investigation was to create a rockfall hazard map
through using a combination of fieldwork mapping, stereographic projections and rockfall
modelling to enable the assessment of the various hazards zones within Royat Park with the report
drawing to the following conclusions:
 The field mapping showed there to be 3 lithological units within Royat, with trachybasalt
being the main source of slope stability problems
 Geomorphological mapping shows there to be 3 different lava morphology types, with
rubbly lava being the more susceptible to rockfall
 Location 4 and 6 has the highest hazard based on planar, wedge and direct toppling
kinematic analysis of stereographic projections
 Hazard is considered greatest near the rock face, on the outskirts of Royat park and reduces
the further away from the slopes
7.1 Limitations:
There are limitations with this research project that need to be acknowledged by those relying the
both the results and conclusion. As describe by Arosio et al (2009) it neither practical nor possible
to detect all potential rockfall hazards within a given area. However, the hazard map produced
during this investigation allows a generic overview of the different hazards zone throughout Royat
Park. However, there are recommendations that could be undertaken in order to improve this
report. In total there were 8 locations that were studied during this investigation, with only one
rockfall model taking place per locality. In order to improve the reliability of the results more than
one rockfall model should have took place. Rockfall are influenced by numerous of factors which
are listed within the literature review and described in more detail by Hoek and Braw (1981). Yet
during the investigation kinetic energy and run out distances are the only parameters investigated
when plotting the hazard map. Furthermore, stereographic kinematic analysis used only
investigated orientation data. Hence the results are not realistic and do not take into account of
external or internal forces that may affect slope stability within Royat. The limitations can be
simplified into the following points:
 Stereonets give an indication of the stability conditions, it does not however take into
account of external forces such as water pressure or reinforcement comprising tensioned
rock bolts as mentioned by Wyllie, Mah and Hoek (2004).

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 Rockfall hazards are triggered by numerous of factors hence they can either be both
dependant and independent of previous event and so consequently rockfall frequency is
unpredictable and there is no way of determining a precise rockfall period, as probability
always changes
 This report does not suggest mitigation techniques nor does it give an indication of how
the hazards can affect society in relation to vulnerability or risk
7.2 Future Work
Many studies take into account of vulnerability when looking at hazard in order to understand the
potential risk within an area, such as Guzzetti et al (2003). Risk analysis of rockfalls on major
roads have not been covered in great detail, hence geotechnical literature on this subject is very
limited. The following authors discuss probability of slope failure occurring resulting in either
damage, injury or death; Fell and Hartford (1997), and Lee (2004). However, all these authors
focus on landslides and not on rockfall hazards. Therefore, future work can look at the risks applied
to rockfalls on busy roads within Royat, extending the study to busier towns such as Clermont
Ferrand.

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9.0 Appendices
Figure 38. Slope aspect map of Royat displaying the slope directions. Figure created using ArcGIS.
Figure 39. Hillshade map, with contours, of Royat. Created using ArcGIS.

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Figure 40. Stratigraphic Column showing a
simplified geological time scale.
(Source: International Commission on
stratigraphy, UISG)

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Figure 41. Regions to be considered for direct toppling. Zone 1 and are the primary and secondary critical
zone. Zone 3 is the critical intersection zone. (Source: Direct Toppling, 2014)

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Plate 11. Location 5, Le Grotte Siméoni. Shows discontinuities such as fractures within the rock mass. Also the
cave has been blocked off to stop access due to health and safety regulations.

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Plate 12. Massive lava within Royat Park. Image shows buildings have been constructed above the lava.
Rockfall mitigation has been implemented within this area. A fence has been placed to stop people getting close
to the rock slope, to reduce the risk of being affected by any rock falls.
Plate 13. Evidence of Planar or wedge sliding occurring within the lava.

Finished dissertation UP691938

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