1. i
Case Histories: Ground movements arising
from the construction of new shafts in clay.
Sunil Vaghela
Supervised by Dr S. Divall
City University London
Submitted to the School of Engineering and
Mathematical Sciences
In partial fulfilment of the requirements for the
Degree of Bachelor of Engineering in Civil
Engineering
June 2015

2. ii
ABSTRACT
Due to the demand of increased infrastructure, there are now many construction projects
taking place. To aid the construction of tunnels and other underground structures, shafts
are built as a form of access for people and equipment. However, when a shaft is built,
there are certain ground movements that arise as a result of excavating into the ground.
During the excavation process, the surrounding soil displaces to form a horizontal wall
deflection and vertical surface settlement. In greenfield situations, ground movements
induced by shafts may not be a cause for concern, however, it may be in urban areas where
there is a high volume of infrastructure. In congested cities, induced ground movements
have the capability of damaging adjacent buildings and underground structures. For
example, a settlement in the ground due to the construction of a shaft could potentially
cause a railway track to shift by a few millimeters. This shows that it is very important that
ground movements are accurately predicted to mitigate the likelihood of potential damage
to surrounding infrastructure. This study investigates a range of case studies that document
ground surface settlements induced by circular shafts in clay. Once the case studies were
reviewed and analysed, a new and improved prediction method for surface settlements
could be established whilst relying on previous empirical methods (New and Bowers, 1994).
Furthermore, subsurface soil displacements around circular shafts are discussed with a
review of how building damage due to shafts can be assessed.

3. CONTENTS
ABSTRACT……………………………………………………………………………………………..
LIST OF TABLES.................................................................................................
LIST OF FIGURES…………………………………………………………………………………………..
LIST OF SYMBOLS………………………………………………………………………………………….
1. INTRODUCTION ............................................................................................................
1.1 The Problem.....................................................................................................................
1.2 Aims ..................................................................................................................................
1.3 Scope.................................................................................................................................
2. BACKGROUND...............................................................................................................
2.1 Sources of ground movement.........................................................................................
2.2 Type of soil .......................................................................................................................
2.3 The different forms of excavation..................................................................................
3. LITERATURE REVIEW..................................................................................................
3.1 Preface..............................................................................................................................
3.2 Ground movements.........................................................................................................
3.3 Square excavations..........................................................................................................
3.4 Peck’s prediction method ...............................................................................................
3.5 Clough and O’Rourke’s prediction method ...................................................................
3.6 Circular excavations ........................................................................................................
3.7 New and Bowers (1994) settlement prediction ...........................................................
3.8 Crossrail project settlement prediction.........................................................................
4. CASE STUDIES ...............................................................................................................
4.1 Terwillegar Shaft (Canada).............................................................................................

4. 4.2 Circular shaft constructed for the Heathrow Express trial tunnel..............................
4.3 The construction of a deep cylindrical shaft in Japan…..…………………………………………
4.4 A deep circular shaft constructed for Crossrail ............................................................
5. ANALYSIS .......................................................................................................................
5.1 Normalisation of data…………………………………………………………………………………………………
5.2 Interpretation of case studies for circular excavations……………………………………………
5.3 Curve fitting .....................................................................................................................
6. DISCUSSION……………………………………………………………………………………………………………
6.1 Variation of empirical constant α…………………………………………………………………………….
6.2 Subsurface settlement profiles around circular excavations……………………………………….
6.3 Effects of ground movements on buildings...................................................................
7. CONCLUSION……………………………………………………………………………………………………………
7.1 Summary of work undertaken……………………………………………………………………………….
7.2 Conclusions………………………………………………………………………………………………………………..
7.3 Limitations………………………………………………………………………………………………………………….
7.4 Recommendations for future work.......................................................................................
REFERENCES…………………………………………………………………………………………..
TABLES…………………………………………………………………………………………………..
FIGURES………………………………………………………………………………………………….

5. LIST OF TABLES
Table 5.0.1 Overview of selected case studies for square excavations in clay.
Table 5.0.2 Overview of case studies for circular excavations in clay.
Table 6.1 Example of calculations for (Wong and Kaiser, 1988 – Axes OA) that were
carried out for curve fitting.
LIST OF FIGURES
Figure 2.1 The three specific ground movements that occur once excavated into the
ground (Burland and Wroth, 1974).
Figure 2.2.1 Pre-cast concrete segmental underpinning components: (a) surface collar; (b)
underpinning; (c) finishing works completed; (d) alternative structural floor
arrangement (Allenby and Kilburn, 2015).
Figure 2.2.2 Pre-cast concrete segmental open caisson components: (a) surface collar; (b)
caisson-sinking; (c) concrete tremie plug placed; (d) finishing works completed
(Allenby and Kilburn, 2015).
Figure 3.3.1 Settlement predictions developed by Peck (1969) normalised with maximum
excavation depth.
Figure 3.3.2 Settlement prediction developed by Clough and O’Rourke (1990) normalised
with maximum excavation depth and maximum soil settlement.
Figure 3.4.1 New and Bowers (1994) prediction method using the two monitoring lines, S
and T.
Figure 4.2 Overview of instrumentation for measuring movements around the circular
shaft for the Heathrow Express trial tunnel (New and Bowers, 1994).
Figure 5.0.1 Normalised data showing trend of surface settlements for square excavations
in clay.
Figure 5.0.2 Normalised data showing trend of surface settlements for circular excavations
in clay.
Figure 5.2 Observed settlement data for three case studies depending on the relevant
maximum excavation depth (Wong and Kaiser, 1988; New and Bowers, 1994;
Muramatsu and Abe, 1996).

6. Figure 6.1.1 New prediction curve for surface settlements compared with observed data
from case study conducted by Wong and Kaiser (1988).
Figure 6.1.2 New prediction curve for surface settlements compared with observed data
from case study conducted by Muramatsu and Abe (1996).
Figure 6.2 Form of surface and subsurface settlement profiles above a tunnel (Mair et al.,
1993).
Figure 6.3.1 Deformation of a building above a tunnel (Mair et al., 1996).
Figure 6.3.2 Categories of building damage relating to horizontal strain and deflection ratio
for L/H=1, hogging mode (Burland, 1995; Mair et al., 1996).
LIST OF SYMBOLS
Sv(d) Vertical settlement at corresponding distance away from the retaining wall
Svmax Maximum vertical settlement
α Empirical constant
d Radial distance or distance away from the retaining wall
H Maximum excavation depth
D Diameter of a shaft
Et Extent of a settlement trough
δ Difference in x-values at corresponding y-value
∑ The sum of
θ Rotation
ω Tilt
∆ Relative deflection
∆/L Deflection ratio
β Angular distortion

7. 1 Introduction
This research will focus on the analysis of ground movements generated by the construction
of circular shafts. Specifically, the soil settlement profiles that develop from a square and
circular excavation, investigating whether or not they share similar ground movements.
1.1 The Problem
A plane strain shape of a retaining wall consists of dimensions with horizontal displacement
and vertical displacement whereas a circular shape of a retaining wall consists of dimensions
such as the circumference and radial distance. Since there is a difference in these
parameters and the shape, the two types of excavation may not share similar ground
movement profiles.
1.2 Aims
i. To provide an accurate description on the sources of ground movement due to
excavating into the ground, discussing two types of shaft construction techniques.
ii. To gather and review a large number of circular excavation case studies, deducing
key parameters.
iii. To extract surface settlement data from specific case studies of circular excavations
in clay.
iv. To discuss the difference in ground movements induced by square and circular
shafts.
v. Further analysis by comparing the case studies for circular excavations, hence
forming key relationships

8. vi. To produce a useful database to predict surface settlements for circular excavations
in clay to aid future design.
vii. To generate a reasonable theory that predicts subsurface settlements surrounding
circular excavations.
viii. To gather information on how building damage can be assessed due to the effect of
ground movements.
2 Background
In urban areas, there are huge amounts of infrastructure. Due to cities which are
overpopulated with the population continuously increasing, there is a need for an efficient
transport system besides roads and bridges, therefore, engineers are forced to consider
constructing structures under the ground such as tunnels. For example, the London
underground tubes have been built as an alternative for public transport. Besides carrying
people, tunnels are also made to transport sewerage and water. In order to maintain these
underground structures and provide access, shafts are constructed. A shaft is an
underground vertical or inclined passageway that provides access to an underground
structure and can also be used for supplying equipment, personnel and support systems to a
horizontal tunnel. However, there are a variety of important points to take into
consideration before excavating into the ground. When a shaft is constructed, there is a
substantial amount of soil being removed from the excavation site which induces ground
movements. When dealing with urban cities, these particular ground movements can have
an effect on existing building infrastructure and existing underground structures. Induced
ground movements are capable of distorting and damaging overlying structures in severe
cases. In which case, repair of damages could be costly and time consuming. In addition,

9. individuals are protective over their own assets in the surrounding area. Therefore, if there
is any excavation work taking place, extra precaution is taken to ensure ground movements
are to a minimum. Hence, it is extremely important that ground movements are accurately
predicted by engineers before a shaft is constructed to prevent potential hazards from the
early stages of a project.
2.1 Sources of ground movement
The construction of shafts will inevitably be accompanied by movement around them. There
have been many studies that describe structural distortion of soil around shallow
excavations. Burland and Wroth (1974) suggested reliable and widely accepted definitions
that anticipate the sources of ground movement based on the displacements of a number of
distinct points on the foundations of a building. These definitions are illustrated in Figure
2.1. It is proposed that there are three specific movements that occur once excavated into
the ground:
a) Settlement s, relative settlement δs, rotation θ and angular strain α;
b) Relative deflection ∆ and deflection ratio ∆/L;
c) Tilt ω and relative rotation (angular distortion) β
However, it should be noted that these definitions only apply to in-plane deformations as
opposed to circular.
2.2 Shaft construction methods
Shafts can be constructed using a variety of methods, depending on the type of soil being
excavated in to and its relative ground conditions such as the influence of groundwater. Two
techniques of shaft construction which shall be discussed in further detail are ‘underpinning
in free air’ and ‘dry and wet open caisson-sinking’. Allenby & Kilburn (2014) reviewed these

10. two techniques including two other traditional, shaft-sinking techniques which are
‘pneumatic caisson-sinking’ and ‘underpinning in compressed air’.
2.2.1 Underpinning in free air
Underpinning is a process of shaft construction whereby the ground is first excavated
followed by the installation of a pre-cast segmental ring which is complete with watertight
gaskets. Once the ring is placed into position, it is then fully grouted with cement grout
which is a substance that cements soil grains together reducing its ability to move. The
process of underpinning is outlined in Figure 2.2.1.
Firstly, as shown in Figure 2.2.1(a), a surface collar is excavated which is normally two rings
deep and erected to plane. The annulus formed is then backfilled with concrete to ‘lock’ the
collar into the ground. Following this, as shown in the diagram, a guard rail is then erected
allowing the shaft centre-line to be established whilst the depth of the shaft can be
measured by suspending a tape inside the shaft, referencing it to a surface bench mark.
Thereafter, the process of underpinning begins whereby the shaft floor is carefully
excavated and the side walls are cut off appropriately to allow the first underpinned ring to
drop into position. This procedure is repeated until full excavation depth is achieved, taking
into account that every ring is grouted once placed into position (Figure 2.2.1(b)). The
construction of the shaft floor then commences using reinforced concrete, as shown in
Figure 2.2.1(c).
2.2.2 Dry and wet open caisson-sinking
Allenby et al. (2009) defined open caisson-sinking as ‘a technique which permits a shaft
structure to be progressively sunk, either under its own weight or with the aid of caisson

11. jacks in a controlled manner from the surface to a predetermined depth’. This is illustrated
in Figure 2.2.2.
The construction of caissons can be accomplished by either using manhole rings fitted with
watertight gaskets, a pre-cast concrete segmental shaft or as a monolith in reinforced
concrete. It typically begins with a concrete surface collar constructed which helps provide a
foundation and reaction for the caisson jacks and outlines the position and shape of the
caisson.
Figure 2.2.2(a) illustrates the initial position of the steel cutting shoe, concrete cutter and
choker ring with at least the first caisson ring inside the collar. The collar is then filled with a
lubricant such as bentonite to stabilise the overcutting, reduce groundwater inflow and
minimize skin friction on the caisson walls.
The next stage, Figure 2.2.2(b), shows the caisson jacks fitted into place. Sinking begins
whereby a known jacking load is applied to the top caisson ring whilst the caisson floor is
excavated in a controlled sequence that pushes the caisson ring, one by one, into the
ground. After each stage of excavation, the caisson jack is withdrawn allowing the next ring
to be positioned with lubricant added. This sequence is repeated until the full depth is
required.
Figure 2.2.2(b) also shows a steel cutting edge at the bottom of the caisson which must be
buried to create a ‘cut-off’ that supports the ground before sinking. In ‘dry open caisson-
sinking’, the ‘cut-off’ depth is fulfilled when there is no flow of material into the caisson
since it is above the groundwater table. However, in ‘wet open caisson-sinking’, below the
groundwater table, there must be a balance between the hydrostatic pressure externally
and the water level inside the caisson in order to prevent ground loss. In a dry open caisson,
grouting is achieved by a method of working up the caisson injecting grout and displacing

12. lubricant until the annulus is fully grouted. On the other hand, in a wet open caisson, it is
normal to place a concrete tremie plug in the shaft (Figure 2.2.2(c)), dewater the caisson
and then grout the lubrication annulus. Once this is done, the caisson floor can be
constructed (Allenby & Kilburn, 2014).
When a shaft is being constructed, adjacent soil grains are prevented from entering into it.
As a result, this process creates an undercutting effect which leads to the ground collapsing,
and in turn, forms a maximum vertical settlement (Svmax) and maximum horizontal wall
deflection (Shmax). For this study, wall deflection shall be ignored with focus only on ground
surface settlements.
2.3 Type of Soil
Different shaft sinking techniques are carried out depending on the nature of its ground
conditions. Typical ground conditions for underpinning are dry, stiff, homogeneous, un-
weathered clays. It may also be suitable for sinking through chalk and a variety of rock
types. Underpinning can still take place in less ideal conditions, however, the presence of
certain geological features such as fissures and faults must be investigated to check whether
it is suitable. If not, then an alternative technique should be considered such as caisson-
sinking. Open caisson-sinking is ideally suited in ground conditions such as water-bearing
silts, sands, gravels, alluvium and glacial deposits. In addition, highly compressible ground
such as peats, water-bearing chalk and a variety of clays ranging from thinly bedded,
laminated, highly variable clay strata to high plasticity clays such as marine clay can be
suitable for this particular sinking technique. On the other hand, open caisson-sinking is
generally not suitable if construction takes place through a thick bed of very stiff or hard
clay.

13. For the purpose of this study, the type of soil which will be specifically focussed on is clay, as
opposed to sand or gravel.
2.4 The different forms of excavation
Deep excavations into the ground may consist of different shapes depending on what would
be more feasible on a particular project. They can either be constructed forming a circular
shape, square shape or elliptical shape. However, primarily, this study will focus only on
square and circular shafts. In some cases, circular shafts can be more suitable than square
shafts as they are more structurally stable since the shaft support is placed in ring
compression due to earth loads. The benefit of a circular shaft is that the need for internal
support is disregarded and reinforcement in the structure can be reduced. For this reason,
circular excavations are beginning to become more and more popular in the construction
industry.
3 Literature Review
3.1 Preface
As mentioned previously, circular excavations are a commonly used method in the industry
because of different reasons. When surpassing a depth of 5m, square excavations usually
require propping whereas circular excavations make use of the hoop forces in the retaining
structure and are structurally stable, therefore, they do not require propping. For this
reason, circular shafts may be more straightforward. As it has been stated by Powderham
(2000), the maximisation of space for construction operations, the reduction of bulk
excavation and the simplicity of the construction sequence make the circular excavation a
‘dramatically simple solution’. The wall length can furthermore be reduced in comparison to

14. a rectangular excavation with the same floor area and therefore provides an economic
solution (Gaba et al., 2003). Due to vertical and horizontal arching effects, earth loading on
the walls is smaller than under plane strain conditions (Wong and Kaiser, 1988)
3.2 Ground Movements
This section highlights the main components of ground movement due to square and
circular excavations. In order to understand the fundamental, current knowledge of such
movements, a literature review of the published material was carried out.
There are a number of methods used for predicting ground movement due to excavations:
 Empirical approaches are widely used to predict ground movements during the initial
stage of a design process by using data from numerous case studies. The reason for
this is so that designers have a preliminary idea on the type of construction method
and wall type that will be utilised in a project. This is referring to Eurocode 7 of the
(British Standards Institution, 2004a, p.110) which states that ‘a cautious estimate of
the distortion and displacement of retaining walls, and the effects on supported
structures and services shall always be made on the basis of comparable experience’.
Peck (1969) and Clough and O’Rourke (1990) have developed methods for non-
circular excavations.
 Physical modelling is a method that has been used to provide an additional insight.
This can be practiced in the form of centrifuge models whereby interactions
between the soil and structure can be understood. Phillips (1986) performed an
experimental study comprised of a series of centrifuge tests to investigate the
ground deformation of an excavation in overconsolidated clay.

15.  Finally, numerical modelling is an alternative approach which can be used in the case
that insufficient data is gathered. This can be a very complex method. Finite Element
analysis and Mobilised Strength Design (MSD) are common techniques that can
accommodate various elements of shaft construction and differing conditions.
Furthermore, there are a selection of key parameters that affect the magnitude and
distribution of soil settlement profiles:
 The type of shaft, therefore, whether it is of a circular shape or square shape.
 The diameter of the shaft in the case of circular excavations.
 The excavation geometry, therefore, the width and length of a square shaft.
 The excavation depth, as a large depth will induce a large maximum settlement
because of the amount of volume loss in the soil.
Besides this, there are also a few minor parameters that will not be applied in this study,
such as the groundwater conditions, the method of shaft construction, the type of retaining
wall and the existence of adjacent buildings.
3.3 Square excavations
(Long, 2001) complied a database of 300 case histories of non-circular excavations for wall
and ground movements. It is mentioned that there are two alternative approaches available
when analysing the data. The data could either be accessed on a local basis, using a small
number of case histories from that particular location which would prove to be a more
reliable method. Alternatively, data could be accessed from worldwide locations which
could be a weaker approach because of the dissimilar ground conditions in certain areas.

16. 3.3.1 Peck’s prediction method
Peck (1969) developed a method of measuring ground surface settlements for square
excavations in a graphical form using a large number of case histories prior to 1969.
Depending on a variety of factors such as the type of soil, the settlement curve was
categorised into three zones, I, II and III (Figure 3.3.1). The excavations from the case
histories are supported by sheet piles or soldier piles with lagging. For very soft to soft clay,
it is suggested that the maximum soil settlement experienced is about 1% of the maximum
excavation depth. However, it should also be noted that when using diaphragm walls, which
is a new construction method to the industry, the maximum settlements are generally
smaller than defined.
3.3.2 Clough and O’Rourke’s prediction method
The study conducted by Clough and O’Rourke (1990) uses a new approach to predictions as
to Peck (1969) and has proved to be the most widely used in practice (Long, 2001). It is
proposed that the settlement profiles for excavations in stiff clay are triangular with
maximum settlement occurring nearer to the wall. In the case of soft to medium clays, it is
proposed that the settlement profile will form a trapezoidal shape whereby the maximum
settlement occurs at a point distance away from the wall. This is shown in Figure 3.3.2.
3.4 Circular excavations
For circular excavations, it is noted that there are very limited case studies worldwide
compared to square excavations. In addition to this, the number of circular excavations in
clay made the research even more difficult. Therefore, designers can hardly find field
studies that have similar matches in ground conditions, excavation depth and shaft
diameter. In terms of circular excavations, estimation of surface settlement only relies on

17. one study conducted by the Transport Research Laboratory on the Heathrow Express Trial
Tunnel (New and Bowers, 1994). In relation to the small number of case studies that were
found, it is highly unlikely to fulfil the requirements imposed by codes and standards.
Eurocode 7 (British Standards Institution, 2004a) and CIRIA C580 (Gaba et al., 2003) clearly
recommend that case studies which can be compared should be used when predicting
ground movements.
3.4.1 New and Bowers (1994) settlement prediction
(New and Bowers, 1994) conducted a well-documented case study on a circular shaft that
has been frequently used to predict ground movements. The case study dealt with a deep
circular shaft for the Heathrow Express trial tunnel that was sunk in London Clay. The
settlement predictions were represented on a graph using two lines (S and T line). These
two lines were monitored from five and six movement stations that were radiated from the
shaft wall. The vertical settlement Sᵥ(d) is plotted against distance d behind the shaft wall
whereby both axes are normalised with the excavation depth H (Figure 3.4.1). A parabolic
curve was then presented using the data to obtain an empirical formula for the surface
settlement Sᵥ(d) behind the shaft wall,
𝑆v(𝑑) =
∝ (𝐻 − 𝑑)2
𝐻
(3.8)
Where α, taken as 0.0006, is a constant depending on the method of construction and soil
conditions. Since this equation is for a circular shaft, a certain parameter which is not
applied is the diameter of the shaft which could serve to be an important factor when
predicting settlements for circular excavations. Consequently, the diameter of the shaft and
uncertainty of α are the disadvantages of this particular equation.

18. 3.4.2 Settlement predictions for shafts in the Crossrail project
The Crossrail project is the construction of a major railway link underneath the city of
London which consisted of several box and circular excavations. Geotechnical Consulting
Group (2007) conducted a survey on the ground movements, specifically surface
settlements, in relation to the circular excavations. The settlement guidelines provided
mainly rely on the settlement predictions conducted by New and Bowers (1994), however,
Geotechnical Consulting Group (2007) add a factor of the diameter D to the equation. This
method is slightly more conservative as New and Bowers (1994) due to high uncertainty.
Therefore, the maximum vertical settlement Sv,max (which is equal to Sv at d=0) is established
as:
𝑆v,max =∝ 𝐻.
𝐷
10𝑚
(3.91)
An upper bound is set for D>25m to prevent settlements of high value for large diameter
shafts.
𝑆 𝑣,𝑚𝑎𝑥 = 15 × 10−4
. 𝐻 (3.92)
It is assumed that the extent of the settlement trough Et is equal to 1H for D>10m and 2H
for D>25m. It is linearly interpolated for diameters between 10m and 25m. The formula for
the settlement distribution is given below:
𝑆 𝑉(𝑑) = 𝑆 𝑣,𝑚𝑎𝑥. exp(0.5) . exp⁡(−
(𝑑 + 2𝐸𝑡 3⁄ )2
8 𝐸𝑡
2
9⁄
) (3.93)
In relation to horizontal soil movement, there are rarely any case studies of circular shafts
that can be used to derive a reliable formula. For this case, Geotechnical Consulting Group
(2007) made an assumption that the same Gaussian distribution formed as that of the
settlement, which is the same method used for rectangular excavations. Therefore, the

19. maximum horizontal soil movement Sh,max is assumed to be equal to the maximum vertical
surface settlement Sv,max, i.e. Sh,max/Sv,max=1.
4 Case Studies
4.1 Terwillegar Shaft (Canada)
The Terwillegar shaft in Edmonton, Canada is a circular shaft with an excavation depth and
wall depth of 20m. The diameter of the shaft was changed from 3.2 to 2.4m and was sunk
into the ground using conventional shaft sinking methods. At each stage of excavation, a
temporary lining of corrugated and flanged steel plates were installed within the shaft to
form a support ring. In terms of the soil conditions, the ground below consisted of brown,
silty clay between loose, moist sand of medium grain size which was followed by glacial till
consisting of a combination of sand, silt and clay. The bedrock consisted mainly of
interbedded clay shales.
The field instrumentation were laid out consisting of 2 inclinometers, 2 multipoint
extensometers and 18 surface settlement points and were positioned along radial axes (A, B
and C). The vertical soil displacements were measured using the surface settlement points
whereby all readings were referred to a fixed reference point, a bench mark at about 60 m
from the shaft.
It was observed that the maximum settlement of about 7.8mm was experienced along the
axes OC with axes OA experiencing the least amount of settlement of about 5.2mm. The
large surface settlement along axes OC was most likely due to loss of ground in a sand lense,
which controls the behaviour of the shaft locally (Wong and Kaiser, 1988). It was also noted
that the maximum amount of displacement appeared nearer to the shaft with the
settlement decreasing gradually away from the shaft.

20. 4.2 A circular shaft constructed for Heathrow Express trial tunnel
For the Heathrow Express trial tunnel, a circular shaft with an excavation depth of 26m and
a diameter of 11m was constructed to provide access to the underground tunnel. The
ground conditions appeared to be a small amount of made ground and Taplow Terrace
gravel, mainly consisting of London Clay. The instrumentation of movement stations were
installed prior to construction in which the layout was carefully focussed to provide cost
effective monitoring of ground movements. The instrumentation were positioned along five
lines (S, T, A, B and C). Specifically for this study, lines A, B and C are ignored as they lie
perpendicular to the tunnel, therefore, ground movements in relation to these lines are not
solely affected by the shaft (Figure 4.2).
The S line data was rejected as a settlement prediction due to heavy plant movement,
however, the T line data provided reliable readings with a maximum settlement (Sᵥmax) of
approximately 15mm (Figure 3.4.1). In addition, the ground movements induced by the
shaft are generally small of less than 14mm, besides the location affected by heavy plant
movement. Likewise, at a distance of 26m away from the shaft (same as excavation depth),
the ground movements became insignificant (New and Bowers, 1994).
4.3 The construction of a deep cylindrical shaft in Japan
The shaft described in this case study is a deep cylindrical shaft, constructed together with a
regulating reservoir of underground river type, and has a diameter of 28.2m with a
maximum excavation depth of 60.3m. A diaphragm wall was chosen as the wall type, having
a depth of 98m and a wall thickness of 1.2m. The installation of the wall formed a cylinder
with 28 elements in total. Figure 4.31 shows a longitudinal section of the shaft. The
diaphragm wall was constructed by inverted lining method, illustrated by the steps shown in

21. Figure 4.31 (circled numbers). The excavation was incorporated in 6 steps, starting from the
first excavation level and continuing until the final floor bedding level was reached. From
the fourth excavation level, chemical grout was injected to improve the bottom of the shaft
and to prevent swelling from occurring. In terms of the geological profile, a depth of 8m
from the surface consisted of Musashino and Tachikawa loam layers. Directly below this
layer was the presence of the Tokyo layer group overlaying the Joso layer group.
Ground deformations induced from the construction of the cylindrical shaft were measured
using monitoring equipment as shown in Figure 4.32. Measurements of ground settlement
and horizontal displacement of the wall at the surface were carried out using the measuring
lines A to E on Figure 4.32. For measuring displacement, distances were surveyed at points
established on measuring lines ① to ③, as shown on Figure 4.32. The observed data
showed that the ground experienced a 4-7mm settlement at distances ranging from near
the wall to an extended distance of 10m away from the wall. At a distance away from the
shaft of 50m or more, no settlement occurred. Ground settlement commenced as soon as
the wall construction started, with deformations increasing as excavation proceeded. It was
noted that majority of the settlement could be observed when excavation reached the
middle of the desired design depth. Although after this stage, there was only a slight
increase in settlement (Muramatsu and Abe, 1996).
4.4 A deep circular shaft constructed for Crossrail
From within the basement of a new development, Moorhouse, near Moorgate in the City of
London, a 37.5m deep circular shaft with a diameter of 8.2m was constructed for the
Crossrail project. The construction of the shaft began after completion of the new
development which meant that careful and detailed planning was essential. The presence of

22. foundations within the basement placed tight constraints on acceptable ground movements
induced by the construction of the shaft as these soil displacements could have a negative
impact on the Moorhouse development. The site stratigraphy consisted of a 2.5-3.5m mix
between made ground and Terrace Gravel over about 30m of London Clay. Directly below
lay about 18m of Lambeth Group, where two water-bearing layers were present (McNamara
et al, 2008).
In relation to measuring ground movements, an extensive monitoring system was devised
and implemented. The main objective of this system was to not only measure the
magnitude of ground movements during the construction process, but to enable the
designer to make a decision on whether measures should be taken to prevent excessive
movement due to the presence of adjacent foundations. Monitoring included the use of
inclinometers, extensometers and piezometers with inclinometers associated with
measuring ground movement. Figure 8 shows the layout of the monitoring system whereby
IG1 and IG2 represent the inclinometers which measure soil displacement (McNamara et al,
2008).
Measured data showed that the surrounding ground experienced a maximum vertical
settlement of 26mm along the circumference of the shaft. Therefore, maximum settlement
occurred at a radial distance of zero due to shaft construction. Additionally, the measured
data also indicates that the amount of settlement gradually decreases as the radial distance
away from the shaft increases, giving a minimum settlement of 0.4mm at a radial distance
of 56m (Geotechnical Consulting Group, 2007).

23. 5 Analysis
This chapter documents a series of case studies and publications on excavations for circular
shafts. In order to gain a valuable understanding of surface settlements induced by circular
shafts, square excavations were first analysed. The settlement data for various square
excavations were extracted from case studies and plotted with prediction curves developed
by previous studies i.e. Clough and O’Rourke, 1990 and Peck, 1969 whereby a general trend
can be seen. This is represented on Figure 5.0.1 with information on a selected number of
case studies on Table 5.0.1. Referring to Figure 5.0.1, it can be seen that the case studies
more or less fit within the predicted settlement. This consequently shows that settlement
data for square excavations are well-documented. It can also be conformed from Table 5.0.1
and Figure 5.0.1 that the maximum amount of settlement normally occurs at a certain
distance away from the shaft wall.
Once ground movements surrounding square shafts were well understood, analysis could
then be carried out for circular shafts. Settlement data for circular excavations was
therefore extracted from case studies and plotted with prediction curves developed by New
and Bowers (1994) and Crossrail/GCG (2007). These are represented on Figure 5.0.2 and
Table 5.0.2. It can be seen that settlements induced by circular shafts follow a similar
pattern. However, as opposed to square shafts, the maximum surface settlements for
circular shafts occur at a negligible distance away from the wall, therefore, along the
circumference of the shaft. It should be noted that the curves shown on Figures 5.0.1 and
5.0.2 are normalised data from various case studies.

24. 5.1 Normalisation of data
The majority of case studies represented their settlement data on a graph that contained
the relative vertical settlement on the y-axis with the x-axis containing the distance away
from the retaining wall. Alternatively, various other case studies represented their
settlement data on a graph with the relative vertical settlement versus the distance away
from the retaining wall, but with both axes normalised with the maximum excavation depth
of that specific project.
For this particular study, the observed settlements of each case study were extracted and
plotted onto Figure 5.0.1 and Figure 2.0.2. For the x-axis, the distance away from the wall
was normalised with the maximum excavation depth and for the y-axis, the relative vertical
settlement was normalised with the maximum vertical settlement. By normalising the data,
the observed settlement profiles from various case studies can be more easily comparable
as it does not depend on the maximum excavation depth and maximum vertical settlement.
This method therefore allowed for the key parameters to be further evaluated, such as the
maximum excavation and further interpretation of case studies for circular excavations.
5.2 Interpretation of case studies for circular excavations
In relation to the observed soil displacement for the case studies plotted on Figure 5.2, it
can be seen that shafts consisting of larger excavation depths usually experience the most
amount of surface settlement. However, in the case of the study conducted by Muramatsu
and Abe (1996), this may not be completely true. Despite its large dimensions of 60m depth
and 28m diameter, which is larger than the shaft constructed for Crossrail, it experienced a
maximum settlement of about 4mm only. This is well below what was expected compared
to the Crossrail project which experienced a maximum settlement of 26mm. It is suggested

25. that the reason for this is the construction method used which was the installation of a
diaphragm wall compared to the process of shaft sinking used for the alternative projects. It
has been proven that all approaches predict an overestimate of the settlements for
Muramatsu and Abe (1996). This therefore indicates that diaphragm wall shafts are likely to
induce smaller ground displacements than shaft sinkings.
It can be observed on Figure 5.2 that the settlement curve representing Wong and Kaiser
(1988) lies between the measurements from Muramatsu and Abe (1996) and New and
Bowers (1994). Wong and Kaiser (1988) and New and Bowers (1994) both utilise shaft
sinking as their method of construction and both consist of similar excavation depths.
However, it is reasonable to assume that Wong and Kaiser (1988) have smaller surface
settlements than New and Bowers (1994) because of its smaller shaft diameter (Schwamb,
2014).
5.3 Curve fitting
Once all case studies for circular excavations were gathered and plotted onto Figure 5.0.2,
curve fitting was carried out. This was achieved by using Microsoft excel. It involves
modifying the existing empirical method for New and Bowers (1994), by varying the
empirical constant α, so that a new prediction curve is developed which has a reasonable fit
to the points of the case studies conducted by Wong and Kaiser (1988) and Muramatsu and
Abe (1996). Consequently, the same equation for New and Bowers (1994) was used, but
with an alteration to the empirical constant α. Following this, an improved method can then
be developed relying on the prediction methods conducted by New and Bowers (1994).

26. 6 Discussion
This chapter focuses on the results found in the previous chapter. Included are details of
how the prediction curve from New and Bowers (1994) was modified to provide a more
accurate prediction of settlements for circular excavations in clay. In addition, reasonable
assumptions of subsurface settlement around circular shafts are made with an appropriate
description of how building damage can be assessed due to excavations.
6.1 Variation of empirical constant α
As mentioned previously in terms of curve fitting, the prediction line for New and Bowers
(1994) was adjusted to match the relevant case studies in the following way. Firstly, all key
parameters that have an effect on the magnitude and distribution of induced ground
surface settlements were extracted. In the case of circular excavations, these parameters
are the diameter of the shaft D, the maximum depth of excavation H, the radial distance
away from the retaining wall d and an empirical constant α. An initial value was calculated
using the equation developed by New and Bowers (1994) for a range of radial distances
away from the wall. These values would act as the new prediction line for this study. After
this, the observed settlement data for each case study was inserted into a table, taking
Table 6.1 as an example for the case study conducted by Wong and Kaiser (1988). Referring
to Table 6.1, the ‘VLOOKUP’ function was firstly used for each corresponding x-value along
the points of the case study. After this stage, the difference in x-values (δ) was calculated
followed by the differences squared (δ²). The sum of the differences squared (∑δ²) was then
found out. Once this was done, the correlation factor could be obtained which is the sum of
the square root of the differences squared (∑(√δ)²), as shown in Table 6.1. The objective of
this process was to keep the correlation factor to a minimum because the lower the

27. correlation factor, the better fit that the curve will have with the relevant case studies. The
solve function on Microsoft excel was used to match the new prediction line with each
individual case study, depending on the parametric factor empirical constant α. Therefore,
when the value of the empirical constant α is varied, the prediction line will shift. Once the
solver function was used for each individual case study, a relationship between the
empirical constant and shape of prediction curve could be derived. Through many attempts
of trial and error, a finalised value of the empirical constant could be then be obtained
which would provide a line of best fit that matches all relevant case studies of circular
excavations in clay. It should be noted that the empirical constant is a value that depends on
the method of construction, soil condition, position of groundwater table, etc. Figure 6.1.1
and Figure 6.1.2 illustrates the proposed prediction curve, for this study, that predicts
surface settlements compared to the case studies conducted by Wong and Kaiser (1988)
and Muramatsu and Abe (1996). The x-axis consists of the radial distance away from the
wall and the y-axis measures surface settlement. Hence, for this study, the proposed
equation that best predicts surface settlement profiles induced by circular shafts is the case
study conducted by New and Bowers (1994), which is equation 3.8.
However, based on the curve fitting carried out in this study, the proposed value of the
empirical constant α is 0.266473. As opposed to the value provided by New and Bowers
(1994), which is 0.0006, this new α value is based on the case studies plotted on Figures
6.1.1 and 6.1.2 which provides a prediction of surface settlements for circular excavations
in clay. Consequently, designers can use the above equation including the new α value to
make a reasonably accurate prediction of surface settlements for a proposed project. This
can be carried out by simply inputting the relative parameters of the excavation depth and
radial distance away from the wall for a particular project. Although this may be an

28. improved method, it does not consider the diameter of the shaft which still plays an
important factor when measuring the magnitude and distortion of ground movements.
6.2 Subsurface settlement profiles around circular excavations
Mair et al. (1993) analysed data from several tunnelling projects of surface and subsurface
settlements above the position of a tunnel. Based on surface settlements above tunnels
forming the shape of a Gaussian distribution, it was assumed that subsurface settlements
would be characterised by the same shape. It was found that field measurements of
subsurface settlements were in fact narrower with a larger maximum settlement compared
to the surface settlement. This is shown in Figure 6.2.1 In terms of subsurface settlements
for circular excavations, it was difficult researching on this topic as there were no known
publications found that reported these types of observations. However, it is still assumed
that there is some sort of subsurface soil movement surrounding circular shafts. Based on
the case study conducted by Mair et al. (1993), it can be anticipated that subsurface
settlements are characterised by a similar shape to that of surface settlements for circular
shafts. In addition, it can also be presumed that subsurface movement would take place at a
certain distance below the maximum depth of excavation, forming a similar curve to that of
surface settlements but with a narrower width. Therefore, subsurface settlements would
become negligible at a smaller radial distance away from the wall compared to surface
settlements. This theory is illustrated on Figure 6.2.2.
6.3 Effects of ground movements on buildings
Burland and Wroth (1974) and Burland et al. (1977) addressed the issue of damage to
buildings caused by settlement which is briefly described in Chapter 2. Following this,
Boscardin and Cording (1989) investigated several case studies of excavations whilst

29. achieving an important development. By using the analysis put forward by Burland et al.,
Boscardin and Cording (1989) showed that the categories of damage are linked to the
amount of tensile strain in the building. Moreover, Burland (1995) and Mair et al. (1996)
monitored the damage of buildings in terms of a deflection ratio ∆/L (∆ being the deflection
and L being the length of fragments of the building in hogging or sagging mode). Based on
this approach, Mair et al. (1996) described an appropriate method for assessing the risk of
building damage due to bored tunnelling. This is shown in Figure 6.3.1. It can be seen that
when there is a settlement in the ground, a hogging and sagging zone develops, which
increases the deflection ratio and therefore allows horizontal strains to be transferred from
the ground into the existing infrastructure (Mair et al., 1997). Although these factors may be
reduced depending on the inherent stiffness of the building, which was well illustrated by
Breth and Chambosse (1974), it can still contribute to a significant amount of damage.
Figure 6.3.2 represents a reasonably understandable graph that indicates the category of
damage that a building falls under, depending on the magnitude of the horizontal strain and
deflection ratio. This provides a well-defined method for assessing building damage for
tunnelling.
In terms of circular excavations, the settlement profile it forms is of a slightly different shape
to that of tunnelling. However, the ground settlement may still deform a building and
develop a hogging or sagging zone whereby horizontal strains can be transferred to existing
infrastructure. Therefore, by using the prediction curve for circular excavations developed
from this study (Figures 6.1.1 and 6.1.2), reasonable predictions can be made for the
horizontal strain and deflection ratio. This in turn will help identify which category of
damage the existing building falls under. Overall, the method established by Mair et al.

30. (1996) for tunnelling can be utilised as a technique for assessing building damage for circular
excavations.
7 Conclusion
This chapter will provide a summary of the research and work carried out over the duration
of this study including important conclusions that can be drawn out from the analysis and
discussion. Also included are limitations of the research and a brief overview of
recommendations for future works.
7.1 Summary of work undertaken
As a solid foundation to understanding the vertical ground movements induced by
excavations, a literature review was primarily conducted concerning the settlements
associated with square excavations and the current methods for prediction (Peck, 1969;
Clough and O’Rourke, 1990; Hsieh and Ou, 1998). Once this was understood, a literature
review for circular excavations was carried out to observe the shape of their surface
settlement profiles as compared to those of square excavations including current methods
for prediction (Crossrail/GCG, 2007; New and Bowers, 1994). The observed settlement data
showed that ground movements imposed by these two types of excavations are not similar.
The settlement data for square excavations were neglected due to the fact that their
current prediction methods are reasonably accurate and case studies for this type of
excavation are well-documented. On the other hand, settlement data for circular
excavations were very limited with not many well-documented case studies. Therefore, the
aim was to develop an alternative prediction method for surface settlements induced by
circular excavations. The key parameters were extracted and determined graphically by
adjusting curves to fit the settlement data.

31. 7.2 Conclusions
The conclusions that can be drawn from this research are as follows:
1. Square and circular excavations do not share similar surface settlement profiles.
2. Circular shafts experience a maximum settlement close to the wall, as opposed to
square shafts which experience a maximum settlement as some distance away from
the wall.
3. Circular shafts are more structurally stable than square shafts and is also a simpler
construction process.
4. Diaphragm wall shafts utilised in circular excavations are likely to induce much
smaller displacements than anticipated from prediction methods.
5. There is a significant amount of ground movement when shotcrete is chosen as
method of construction, as opposed to sheet piling.
6. Sheet piling tends to be a slower construction process than shotcrete.
7.3 Limitations
The results obtained from this research are limited to a certain extent. This study involves
gathering case studies for surface settlements induced by circular excavations in clay. Firstly,
the literature contains a scarce number of well-documented case studies on circular
excavations. Secondly, the information contained within these limited case studies is mostly
observed horizontal wall deflections with only a few which have included vertical surface
settlements. Lastly, the amount of case studies documenting circular excavations in clay was
very limited taking into account that the majority of soil was clay with small amounts of
sand and gravel. Therefore, only four case studies consisted of observed surface settlements
for circular excavations in clay.

32. 7.4 Recommendations for future work
From the research carried out, it is still difficult to deduce a relationship for surface
settlements induced by circular shafts. An important factor which has not been included in
the research is the diameter of a shaft. The prediction curve developed from this study is
only based on the excavation depth and empirical constant. Therefore, for future studies,
the diameter should be included into the research so that a relationship can be established
on how the size of the diameter of a shaft can affect the magnitude of ground surface
settlements. In addition, this particular study is only focussed on movements in clay. To aid
a better understanding of ground movements due to shafts, situations where sand or gravel
are involved should be studied, as opposed to clay.

33. References
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Proceedings of the Institution of Civil Engineers – Geotechnical Engineering 162 (1): 59-70
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techniques. Proceedings of the Institution of Civil Engineers – Geotechnical Engineering 168:
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Boscardin, M.D. and Cording, E.J. (1989). Building response to excavation induced
settlement. ASCE, Journal of Geotechnical Engineering, Vol. 115, No. 1, pp. 1-21.
Breth, H. and Chambosse, G. (1974). Settlement behaviour of buildings above subway
tunnels in Frankfurt Clay. Proc. Conf. Settlement of Structures, Cambridge, Pentech Press
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Part 1: General rules. BSI, London, p. 110.
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Ithaca, New York, pp 439-470.
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excavation in clay. J. Geotech. Engeng Div. Am. Soc. Civ. Engrs 115, No. 8, 1045- 1064
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walls - guidance for economic design. CIRIA, London.

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assessment of risk of building damage due to bored tunnelling. Proc. Int. Symposium on
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36. Reference Location Excavation depth (m) Construction method
Maximum surface
settlement (mm)
Distance away
from wall (m)
Ng, C.W.W (1998) Cambridge, UK 10
Diaphragm wall, Top-
down construction
9.8 8.5
Peck (1969) Oakland, USA 19 Sheet piles, Multiprop 12.7
Liu, G.B. and Wang, Z.W. (2005) Shangai, China 15.5 Diaphragm wall 13.2 6.2
Wong and
Kaiser (1988)
Terwillegar
shaft
Edmonton,
Canda
7m sand/clay layers
14m till
clay shale
20 20 3.2 0.16 shaft sinking 7.8
New and
Bowers (1994)
Circular shaft
(Heathrow
Express tunnel)
London, UK
0.5m made ground
3m Taplow Terrace
gravel London Clay
26 26 11 0.42 shaft sinking 15
McNamara et al.
(2008)
Circular shaft
(Crossrail)
London, UK
26m London Clay
14m Lambeth Group
14m Thanet Sand
Chalk
40 0.35 40 8.2 0.2
segmental
lining &
secondary
lining
26
Muramatsu and
Abe (1996)
Cylindrical shaft
(Japan)
Musashino,
Japan
8m loam layers
15 Musashino gravel
Tokyo layer group
Joso layer group
98 1.2 60.3 28.2 0.47 diaphragm 7
Reference Ratio D/H [-]
Construction
method
Maximum surface
settlement (mm)
Excavation
depth (m)
Diameter
(m)
Title Location Ground conditions
Wall
depth (m)
Wall
thickness (m)
TABLES
Table 5.0.1 Overview of selected case studies for square excavations in clay.
Table 5.0.2 Overview of case studies for circular excavations in clay.

37. Table 6.1 Example of calculations for (Wong and Kaiser, 1988 – Axes OA) that were carried
out for curve fitting.
Correlation factor

38. FIGURES
Figure 2.1 The three specific ground movements that occur once excavated into the ground
(Burland and Wroth, 1974).

39. Figure 2.2.1 Pre-cast concrete segmental underpinning components: (a) surface collar; (b)
underpinning; (c) finishing works completed; (d) alternative structural floor arrangement
(Allenby and Kilburn, 2015).

40. Figure 2.2.2 Pre-cast concrete segmental open caisson components: (a) surface collar; (b)
caisson-sinking; (c) concrete tremie plug placed; (d) finishing works completed (Allenby and
Kilburn, 2015)

41. Figure 3.3.1 Settlement predictions developed by Peck (1969) normalised with maximum
excavation depth.
Figure 3.3.2 Settlement prediction developed by Clough and O’Rourke (1990) normalised
with maximum excavation depth and maximum soil settlement.

42. 0
0.01
0.02
0.03
0.04
0.05
0.06
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Sᵥ(d)/H(%)
d/H [-]
T line
S line
Figure 3.4.1 New and Bowers (1994) prediction method using the two monitoring lines, S and T.

43. Figure 4.2 Overview of instrumentation for measuring movements around the circular shaft
for the Heathrow Express trial tunnel (New and Bowers, 1994).

44. Figure 5.0.1 Normalised data showing trend of surface settlements for square excavations in clay.
-120
-100
-80
-60
-40
-20
0
0 1 2 3 4 5 6
Settlement/Maximumsettlement(Sv/Svmax)(%)
Distance/Maximum excavation depth (d/H)
Peck (1969)
Clough & O'Rourke (1990)
Hsieh & Ou (1998)
Finno, R.J.,Atmatzidis, D.K
and Perkins, S.B. (1989)
Ng, C.W.W. (1998)
Lui, G.B. And Wang, Z.W.
(2005)

45. -120
-100
-80
-60
-40
-20
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Relativedisplacement/Maximumdisplacement(Sv/Svmax)(%)
Radial distance/Maximum excavation depth (d/H)
New and Bowers (1994)
Crossrail/GCG (2007)
Wong and Kaiser - Axes OA (1988)
Wong and Kaiser - Axes OB (1988)
Wong and Kaiser - Axes OC (1988)
Muramatsu and Abe - No.1 (1996)
Muramatsu and Abe - No.2 (1996)
Muramatsu and Abe - No.3 (1996)
Figure 5.0.2 Normalised data showing trend of surface settlements for circular excavations in clay.

46. Figure 5.2 Observed settlement data for three case studies depending on the relevant maximum excavation depth (Wong and Kaiser, 1988;
New and Bowers, 1994; Muramatsu and Abe, 1996)
-0.07
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Settlement/Maximumexcavationdepth(Sv/H)(%)
Radial distance/Maximum excavation depth (d/H)
New and Bowers (1994)
Wong and Kaiser (1988) - Axes OA
Wong and Kaiser (1988) - Axes OB
Wong and Kaiser (1988) - Axes OC
Muramatsu and Abe (1996) - No.1
Muramatsu and Abe (1996) - No.2
Muramatsu and Abe (1996) - No.3

48. -9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 1 2 3 4 5 6 7 8 9 10
Settlement(mm)
Radial distance away from the wall (m)
Wong and Kaiser - Axes OA (1988)
New prediction curve
Wong and Kaiser - Axes OB (1988)
Wong and Kaiser - Axes OC (1988)
Figure 6.1.1 New prediction curve for surface settlements compared with observed data from case study conducted by Wong and Kaiser
(1988).

49. -8
-7
-6
-5
-4
-3
-2
-1
0
0 5 10 15 20 25 30 35 40 45 50
Settlement(mm)
Radial distance away from the wall (m)
Muramatsu and Abe - No.1 (1996)
Muramatsu and Abe - No.2 (1996)
Muramatsu and Abe - No.3 (1996)
New prediction curve
Figure 6.1.2 New prediction curve for surface settlements compared with observed data from case study conducted by Muramatsu and Abe
(1996).

50. Figure 6.2.1 Form of surface and subsurface settlement profiles above a tunnel (Mair et al.,
1993).
Figure 6.2.2 Subsurface settlement prediction surrounding circular excavations (based on
Mair et al.,1993).
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
0 5 10 15
Settlement(mm)
Radial distance (m)
Subsurface settlement
prediction
Surface settlement prediction

51. Figure 6.3.1 Deformation of a building above a tunnel (Mair et al., 1996)
Figure 6.3.2 Categories of building damage relating to horizontal strain and deflection ratio
for L/H=1, hogging mode (Burland, 1995; Mair et al., 1996)