6 site investigation

NATURE OF SITE
INVESTIGATION
At any site, the ground conditions need to be assessed to enable safe and
cost-effective design, construction and operation of civil engineering projects.
This will generally include sub-surface ground investigation (GI), which
needs to be focused on the particular project needs and unknowns.
The requirements for GI will be very different for a tunnel compared to the
design of foundations for a high-rise building or for stability assessment of a
cut slope.
There needs to be a preliminary review of the nature of the project, the
constraints for construction and the uncertainties about the engineering
geological conditions at the site.
The British Code of Practice for Site Investigation, BS 5930 (BSI, 1999),
sets out the objectives broadly as follows:
R. R. GADGIL, DEPT. OF EARTH SCIENCE, GOA UNIVERSITY 2

NATURE OF SITE
INVESTIGATION
1. Suitability: to assess the general suitability of a site and its environs for the
proposed works.
2. Design: to enable an adequate and economic design, including for temporary
works.
3. Construction: To plan the best method of construction and, for some projects, to
identify sources of suitable materials such as concrete aggregate and fill and to
locate sites for disposal of waste.
4. Effect of changes: to consider ground and environmental changes on the works
(e.g. intense rainfall and earthquakes) and to assess the impact of the works on
adjacent properties and on the environment.
5. Choice of site: where appropriate, to identify alternative sites or to allow
optimal planning of the works.

OVERALL SCOPE OF SITE
INVESTIGATION
1) Hazards and constraints during construction and in the
longer term:
2) Previous site use – obstructions, contamination
3) Any history of mining or other underlying or adjacent projects
(e.g. tunnels or pipelines)
4) Sensitive receivers – such as neighbours that might be affected
by noise, dust, vibration and changes in water levels
5) Regulatory restrictions
6) Natural hazards, including flooding, wind, earthquakes,
subsidence and landslides

INVESTIGATION
2) Assess and record site characteristics:
 Access constraints for investigation and construction
 Need for traffic control, access for plant and waste disposal
 Access to services
 Site condition survey (partly as a record for any future dispute)
3) Geological profile at site:
 Distribution and nature of soil and rock underlying the site, to
an adequate degree, to allow safe and cost-effective design
 Usually this will require a sub-surface ground investigation

INVESTIGATION
4) Physical properties of soil and rock units and design parameters:
 Key parameters:
– mass strength (to avoid failure)
– deformability (to ensure movements are tolerable)
– permeability (flow to and from site, response to rainfall and
loading/unloading)
 Other factors:
– chemical stability (e.g. reactivity in concrete, potential for dissolution)
– potential for piping and collapse
– abrasivity (sometimes a major consideration for construction)

INVESTIGATION
5) Changes with time:
 install instruments to check physical nature of the
site – e.g. groundwater response to rainfall
– install instruments to monitor settlement and
effect on adjacent structures during construction
– consider the potential for deterioration and need
for maintenance

STAGES IN SITE
INVESTIGATION
Stage 1: Desk study at project conception stage:
– Identification of key geological and environmental hazards at optional sites based
on broad desk study and possibly site visits.
– Consider site constraints, engineering considerations and economic factors.
Stage 2: Detailed desk study and reconnaissance survey
– Collect and review all documents relevant to the preferred site, including
topographic and geological maps, aerial and terrestrial photographs and any previous
investigation reports. Review site history including previous building works and
mining. Look for hazards such as landslides.
– Site mapping, possibly with advance contract allowing safe access, vegetation
clearance and trial pits or trenches.

STAGES IN SITE
INVESTIGATION
Stage 3: Preliminary ground investigation linked to basic engineering
design
– Consider use of geophysical techniques to investigate large areas and volumes.
– Preliminary boreholes designed to prove geological model (rather than design parameters).
– Instrumentation as appropriate (e.g. to establish groundwater conditions and seismicity).
Stage 4: Detailed ground investigation
– Further investigation to prepare detailed ground model and allow detailed design.
– In situ and laboratory testing to establish parameters.
– Detailed instrumentation and monitoring.

STAGES IN SITE
INVESTIGATION
Stage 5: Construction
– Review of ground models during construction (including logging of
excavations).
– Testing to confirm design parameters.
– Instrumentation to monitor behaviour and check performance against
predictions.
– Revision to design as necessary.
Stage 6: Maintenance
– Ongoing review – e.g. of settlement, slope distortion, groundwater changes and
other environmental impacts, possibly linked to a risk management system.

EXTENT OF GROUND
INVESTIGATION
Important questions are, how much ground investigation is required and
how should it be done?
There are no hard and fast rules, even though some authors try to provide
guidance on the basis of site area or volume for particular types of operation.
In reality, it depends upon the complexity of the geology at the site, how
much is already known about the area, the nature of the project and cost.
For sites with simple geology, the plan might be for boreholes at 10m to
30m spacing, for discrete structures like a building.
For a linear structure like a road or railway project, the spacing might be
anywhere between 30 and 300m spacing, depending on perceived variability
(Clayton et al., 1995).

For example,
Figure shows the
route of a planned
tunnel in Hong
Kong, with
potential hazards
identified, together
with a rationale for
their mitigation
and additional GI.

Where
steeply
dipping
geological
structures such
as faults are
anticipated,
inclined
boreholes may
be required.

EXTENT OF GROUND
INVESTIGATIONAs a general rule, at any site, at least one borehole should be put down to prove
ground conditions to a depth far greater than the depth of ground to be stressed
significantly by the works. Generally, for foundations, at least one borehole should be
taken to at least 1.5 times the breadth (B) of the foundation

EXTENT OF GROUND
INVESTIGATION
Poulos (2005) discusses the consequences of
‘geological imperfections’ on pile design and
performance.
Boreholes are often terminated once
rock has been proved to at least 5m, but
this may be inadequate to prove bedrock
in weathered terrain (Hencher &
McNicholl, 1995).
Whether or not one has reached in situ
bedrock might be established by geological
interpretation of consistent rock fabric or
structure across a site, but elsewhere it may be
more difficult, in which case it is best to take
one or more boreholes even deeper if
important to the design. R. R. GADGIL, DEPT. OF EARTH SCIENCE, GOA UNIVERSITY 15

PROCEDURES FOR SITE
INVESTIGATION
Desk Study: Sources of information
For any site, it is important to conduct a thorough document search.
This should include topographic and geological maps. Hazard maps are
sometimes available. These include broad seismic zoning maps for countries
linked to seismic design codes. In some countries, there are also local seismic
micro-zoning maps showing locations of active faults and hazards such as
liquefaction susceptibility.
Air Photograph interpretation
Air photographs can be extremely useful for examining sites. Pairs of
overlapping photographs can be examined in 3D using stereographic viewers,
and skilled operators can provide many insights into the geology and
geomorphological conditions. R. R. GADGIL, DEPT. OF EARTH SCIENCE, GOA UNIVERSITY 16

PLANNING A GROUND
INVESTIGATION
The three verbal equations of Knill (1976) are.
1.The first part is to consider geological factors: material and
mass strengths and other properties.
2.The second is to assess the influence of environmental factors
such as in situ stress, water and earthquakes.
3.The final consideration is how these factors affect, and are
affected by the construction works.
A very similar process has been proposed for addressing risk by
Pöschl & Kleberger (2004), particularly for tunnels.

PLANNING A GROUND
INVESTIGATION

MATERIAL SCALE
The material scale is that of the intact soil and rock making up the site.
It is also the scale of laboratory testing, which is usually the source of engineering parameters
for design.
Hazards might include adverse chemical attack on foundations or ground anchors,
liquefaction during an earthquakeor swelling or low shear strength due to the
presence of smectite clays.
Inherent site hazards associated with geology include harmful minerals such as asbestos.
Gas hazards are especially important considerations for tunnelling and mining but are also an
issue for completed structures, as illustrated by the Abbeystead disaster of 1984 when methane
that migrated from coal-bearing strata accumulated and exploded killing 16 people (Health and
Safety Executive, 1985). These are all material-scale factors linked to the geological nature of
the rocks at a site.

EQUATION 1: GEOLOGY

MASS SCALE
One of the main geological hazards to engineering projects at the mass
scale is faults.
Faults can be associated with zones of fractured and weathered material, high permeability and
earthquakes.
Alternatively, faults can be tight, cemented and actually act as barriers to flow, as natural dams
rather than zones of high permeability.
Faults should always be looked for and their influence considered.
There are many cases of unwary constructers building on or across faults, with severe
consequences, sometimes leading to delays to projects or a need for redesign.
Consequence is sometimes difficult to predict but should be considered and investigated.
Other examples of mass factors that would significantly affect projects include boulders in
otherwise weak soil, which might preclude the use of driven piles or would comprise a hazard on a
steep slope.

MASS SCALE
Mass-scale factors include the distribution of different materials in different
weathering zones or structural regimes, as successive strata or as intrusions.

EQUATION 3: CONSTRUCTION
RELATED FACTORS

GEOPHYSICAL METHODS
Self study

SUBSURFACE EXPLORATION
The scale of the mapping will depend on the engineering
requirement, the complexity of the geology, and the staff and time
available.
Rock and soil types should be mapped according to their
lithology and, if possible, presumed physical behaviour, that is, in
terms of their engineering classification, rather than age.
Geomorphological conditions, hydrogeological conditions,
landslips, subsidences, borehole and field-test information all can
be recorded on geotechnical maps.
Particular attention should be given to the nature of the
superficial deposits and, where present, made-over ground.

There are no given rules regarding the location of boreholes or drillholes, or the depth
to which they should be sunk.
This depends upon the geological conditions and the type of project concerned.
The information provided by the desk study, the preliminary reconnaissance and
from any trial trenches should provide a basis for the initial planning and layout of
the borehole or drillhole programme.
Holes should be located so as to detect the geological sequence and structure.
Obviously, the more complex this is, the greater the number of holes needed.
In some instances, it may be as well to start with a widely spaced network of holes.
As information is obtained, further holes can be put down if and where necessary.

The results from a borehole or drillhole should be documented on a log.
Apart from the basic information such as number, location, elevation, date, client,
contractor and engineer responsible, the fundamental requirement of a borehole log is to
show how the sequence of strata changes with depth.
Individual soil or rock types are presented in symbolic form on a log.
The material recovered must be described adequately, and in the case of rocks
frequently include an assessment of the degree of weathering, fracture index and relative
strength.
The type of boring or drilling equipment should be recorded, the rate of progress made
being a significant factor.
The water level in the hole and any water loss, when it is used as a flush during rotary
drilling, should be noted, as these reflect the mass permeability of the ground.

The simplest method whereby data relating to
subsurface conditions in soils can be obtained is by hand
augering.
Soil samples that are obtained by augering are badly
disturbed and invariably some amount of mixing of soil
types occurs.
Critical changes in ground conditions therefore are
unlikely to be located accurately. Even in very soft soils it
may be very difficult to penetrate more than 7 m with
hand augers.

Basic soil sampling kit animation
Hand augers for heterogeneous soils
Horizontal Directional Drilling _ Boring (HDD)- How the Drill Bit is Steered -
YouTube
Rotary drilling, percussion drilling, cable tool drilling, coredrilling, diamond bits,
casing will not be dealt in here.

CORE LOGGING
Poor or inappropriate core logging can lead to major civil engineering
claims.
Accordingly, accurate core logging is one of the most important activities
for the engineering geologist.
Cores taken as part of a site investigation for an engineering project should
be logged by an engineering geologist rather than a geologist for the style,
content and emphasis of the descriptions will depend on the recognition of
the importance of the geological factors revealed in the cores to the particular
engineering project.
If the logging of a specific geology feature, such as structural geology,
fossil content, etc., is required, a geologist spe cialised in the required feature
should rather do this additionally.

BASIC REQUIREMENTS OF GOOD
CORE LOGGING
The contractor and consultant must ensure that the cores
are placed in good, well-labelled core boxes and are
stored so that their condition as recovered from the bore-
hole is maintained.
They should not be allowed to dry out, be eroded by
rain, frozen, etc.
A well-lit large table in a clean, dry, covered store
should be provided as a logging area.
Help should be available to move core boxes.

THE PROCESS OF CORE
LOGGINGIdeally, all the cores should be logged in one exercise and the logger must
be fully aware of and understand the nature of the engineering work so that
the log gives the information necessary for the design of that work.
Cores may be logged as they come out of the borehole for their description
may influence the further development of the investigation but this
provisional log should not be the final log.
 For the final log the logger must look at all the soils and rocks and decide
provisionally what they should be called and in what detail they should be
described.
A published system of description should be used and its selection justified.
The way in which the cores are described will depend upon the geology,
the engineering work, and the nature of the problem that the geology poses
for that work.
Photos of the core boxes against a scale should be made for later reference.R. R. GADGIL, DEPT. OF EARTH SCIENCE, GOA UNIVERSITY 34

THE PROCESS OF CORE
LOGGINGThe cores should be sorted out so that they fit
together as well as possible to allow Total Core
Recovery (TCR) and, if required, Solid Core
Recovery (SCR) and Rock Quality
Designation (RQD) to be measured.
TCR is the length of core recovered divided
by the length of hole drilled (the core run)
expressed as a percentage.
SCR is the total length of solid core cylinders as a
percentage of the core run.
Care has to be taken in defining what is meant by
‘solid’ (Norbury et al. 1986).

THE PROCESS OF CORE
LOGGINGRQD is the total length of core sticks longer than 4 inches (now commonly converted to
100 mm) expressed as a percentage of the core run.
According to Deere (1968) “ ‘The Rock Quality Designation’ (RQD) is based on a
modified core recovery procedure which, in turn, is based indirectly on the number of
fractures and the amount of softening or alteration in the rock mass as observed in the
rock cores in a borehole. ”
Fractures induced by handling or the drilling process should not be counted (the pieces
broken by such fractures should be fitted together and their total length measured) and the
pieces counted should be ‘hard and sound’.
If all the sticks of core measured are less than 100 mm long, RQD is zero and if there
are no breaks in the core RQD is 100%.
This raises the possibility that if all sticks were 99 mm long, RQD would be 0% and if
all were 101 mm long RQD would be 100%.
Clearly measuring mechanical discontinuity spacing against a fixed baseline has some
disadvantages. R. R. GADGIL, DEPT. OF EARTH SCIENCE, GOA UNIVERSITY 36

THE PROCESS OF CORE
LOGGINGSome rock materials are composed of thinly interlaminated sandy and clayey bands.
They may come out of the core barrel as solid sticks of rock but on drying out separate
into thin discs, splitting along the clayey layers.
RQD measured on the dried out cores would be very much lower than on those just
recovered from the borehole.
The logger is then faced with the difficult choice between including or ignoring the
drying-out breaks.
Inclusion gives a quality assessment of the rock material; exclusion gives an RQD
related only to genetically, tectonically, or geomorphologically induced discontinuities.
Such discing may also be a consequence of stress relief.
Whether or not included in RQD measurements such dicing would merit description and
discussion in the report.

RECORDING
The logging process falls mostly into two parts; first describing the materials and second,
describing the discontinuities.
It is often a good idea to do the discontinuity measurements (stick length, joint and
bedding dip etc.) first.
It is better to measure the depth and dip of every mechanical discontinuity so that
thereafter any calculations (RQD, frequency etc.) can be made.
If using a rock mass classification system somewhere in the project remember to gain the
data to fill in the ratings.
This may involve noting joint roughness, infilling etc., and also means that the rock mass
classification system must be chosen before beginning logging.

RECORDING
Features in the cores resulting from drilling, have to be identified.
Thus, ends of core runs tend to be more broken while core springs may produce scratch
marks simulating bedding.
These should be noted on the record. Because the core spring is situated above the cutting
face of the drilling bit extraction of the core may leave a core stub at the bottom of the
borehole.
If this is recovered in the next run then the core recovery for that run could exceed 100%.
If a stub is left at the bottom of the hole and the next core run completely fills the core
barrel then cores may be crushed.
If samples are extracted then something should be put in their place (ideally a block of
wood cut to length and labelled) and noted.
Core boxes should have a logging record pinned to the inside of the lid on which t he
logger notes who and when the cores were logged and also records any samples taken.R. R. GADGIL, DEPT. OF EARTH SCIENCE, GOA UNIVERSITY 39

RECORDING
In the initial stages of logging, errors of the driller in placing the cores in the box may be
encountered.
It is useful to be familiar with top and bottom criteria to identify cores that may have
been put in upside down.
Before logging rock it is useful to wash the cores and look at them wet, for wet cores
show more than dry ones.
In soils a little drying often helps reveal delicate sedimentary structures such as
lamination.
Cores may have to be broken to determine the rock type. If this is done note the locations
of breaks made.
If possible, at the end of the logging of a number of boreholes, all logs should be
examined again to check consistency of description.

SAMPLING IN SOILS
As far as soils are concerned, samples may be divided into two types,
disturbed and undisturbed.
In a disturbed sample, the material must be representative of the mass from
which it has been taken. Any type of soil sample should contain the same
ingredients as the mass from which it is extracted and in proper proportion,
including the moisture content.
Disturbed samples can be obtained by hand, by auger or from the shell of a
boring rig. Samples of fine soils should be approximately 0.5 kg in weight.
The samples are sealed in jars.
A larger sample is necessary if the particle size distribution of coarse soil is
required, and this may be retained in a tough plastic sack. Care must be
exercised when obtaining such samples to avoid loss of fines.

SAMPLING IN SOILS
An undisturbed sample can be regarded as one that is removed from its
natural condition without disturbing its structure, density, porosity,
moisture content and stress condition.
These samples are taken primarily for all shear and consolidation tests,
which generally are essential for the design and analysis of foundations.
Although it must be admitted that no sample is ever totally undisturbed,
every attempt must be made to preserve the original condition of such
samples.
Unfortunately, mechanical disturbances produced when a sampler is
driven into the ground distort the soil structure.
Furthermore, a change of stress condition occurs when a borehole is
excavated. R. R. GADGIL, DEPT. OF EARTH SCIENCE, GOA UNIVERSITY 42

SAMPLING IN SOILS

IN-SITU TESTING OF SOIL
There are two categories of penetrometer tests, the dynamic and the static. Both
methods measure the resistance to penetration of a conical point offered by the soil at
any particular depth.
Penetration of the cone creates a complex shear failure and thus provides an indirect
measure of the in situ shear strength of the soil.

The most widely used dynamic method is the standard
penetration test.
The SPT is probably the most commonly used in situ test,
whereby the number of blows to hammer a sample tube into
the ground is recorded.
The split spoon sampler used for the SPT is a steel tube with
a tapered cutting shoe.
It is lowered down the borehole, attached to connecting
rods, and then driven into the ground by a standard weight,
which drops a standard height, as illustrated in Figure.
The number of blows for each penetration of 75mm is
recorded; blows for the first 150mm are recorded but
essentially ignored (considered disturbed); the blows for the
final 300mm are added together as the N-value.
Standard Penetration Test (SPT)
Demonstration – YouTube
Standard-Penetration-Test, Demo –
YouTube
Geoprobe® 7822DT Geotechnical
Auto Drop Hammer Collecting Split
Spoon Samples - YouTube

The static cone penetrometer is a conical tool (like an SPT)
that is pushed rather than driven into the ground, usually from
a heavy lorry.
The end force on the cone tip, and drag on the sides of the
tool, are measured independently and can be interpreted in
terms of strength and deformability.
Clay, being cohesive, grips the side proportionally more
than sand or gravel, so the ratio between end resistance and
the side friction can be used to interpret the type of soil as
well as strength.
Cone Penetrometer Testing –
YouTube
Cone penetration test movie -
YouTube

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