Introduction to Site Investigation Methods

Site investigation or sub-soil exploration is the process of
obtaining information about subsurface conditions at a
site proposed for construction.
 The process involves determining the profile of the
natural soil deposits at the site, taking soil samples, and
determining the engineering properties of soils using
laboratory tests as well as in-situ testing methods.

According to US 5930: 1981, site investigation aims to
determine all the information relevant to site usage,
including meteorological, hydrological and environmental
information.
Ground investigation aims only to determine the ground
and groundwater conditions at and around the site; this is
normally achieved by boring and drilling exploratory
holes, and carrying out soil and rock testing.
In common engineering parlance, however, the terms site
investigation and ground investigation are used
interchangeably.

Soil is a very complex engineering material; its properties
vary not only in vertical and horizontal directions but also
at the same location with different environmental
conditions (change in water content, temperature,
presence of chemicals etc.).
Therefore, for designing any geotechnical structure no
readymade solutions are available and each time engineer
have to start from scratch.

There are frequent cases of foundation failures every year
due to improper or no geotechnical investigation. For
better appreciation of the importance of geotechnical
investigation few cases have been discussed

Leaning tower of Pisa is one of the blunders which became
monument.
The construction of tower started in the year 1173 and was
completed during 1370.
After construction it was observed that tower is having a
tilt of 5.50 towards south direction.
The structure was resting on soft saturated clay which
started consolidating over the period of time and thus the
tower started tilting.
To keep the structure stable a lot of ground improvement
work has been carried out.

In 2008, while carrying out construction of railway
embankment between Eranakulam and Mulanthuruthi
section just adjacent to the existing track, newly
constructed 6m high embankment failed suddenly. The
second attempt of reconstruction also ended with failure.
It was noticed that heaving occurred at 15m to 20m away
from toe.

After this failure when detail subsoil investigation has
been carried out, the borehole data and SPT test revealed
the presence of soft clay with SPT valve less than 5 at a
depth of 4.5m. The estimated pressure due to the
embankment was about 100 KN/m2 which was close the
bearing capacity and resulted into failure.
Comments: The construction Engineers involved in
doubling project assumed that the adjacent track is
running fine so no need of subsoil investigations.

Safety is of paramount and there could be no question on
it, therefore IS code as well as other codes recommends
appropriate factor of safety.
The second perspective of any construction project is to
reduce the cost.
The role of Civil Engineer is to ensure safety as well as
optimizing the cost. The following case study will give
better understanding of the importance of economy.

One office cum residential complex at Jodhpur was
planned for construction.
The geotechnical investigation carried out by a less
experienced agency, they have taken 8 boreholes and, in
all boreholes, rocky strata have been found except at
borehole 5 where rocky strata was encountered at 1.5 m
depth, where nothing was planned to be constructed.
Based on this the agency recommended foundation depth
of 1.5 m and entire area were blasted. Blasting amounted
to 25000 m3 boulder and debris which was a herculean
task to dispose off.

Based on this the agency recommended foundation depth
of 1.5 m and entire area were blasted. Blasting amounted
to 25000 m3 boulder and debris which was a herculean
task to dispose off.
Comments: In this case only 10% area was required to be
blasted where open foundation is to be constructed. The
investigating agency recommended 1.5m foundation depth
which could be reduced to 0.5m to 1m.

Selection of the type and the depth of foundation suitable
for a given structure.
Evaluation of the load-bearing capacity of the foundation.
Estimation of the probable settlement of a structure.
Determination of potential foundation problems (for
example, expansive soil, collapsible soil, sanitary landfill,
and so on).

Establishment of ground water table.
Prediction of lateral earth pressure for structures like
retaining walls, sheet pile bulkheads, and braced cuts.
Establishment of construction methods for changing
subsoil conditions.

1. Site selection
The construction of certain major projects, such as earth
dams, is dependent on the availability of a suitable site.
Clearly, if the plan is to build on the cheapest, most
readily available land, geotechnical problems due to the
high permeability of the sub-soil, or to slope instability
may make the final cost of the construction prohibitive.
Since the safety of lives and property are at stake, it is
important to consider the geotechnical merits or demerits
of various sites before the site is chosen for a project of
such magnitude.

2. Foundation and earthworks design.
Generally, factors such as the availability of land at the
right price, in a good location from the point of view of the
eventual user, and with the planning consent for its
proposed use are of over-riding importance.
For medium-sized engineering works, such as motorways
and multistorey structures, the geotechnical problems
must be solved once the site is available, in order to allow
a safe and economical design to be prepared.

3. Temporary works design
The actual process of construction may often impose
greater stress on the ground than the final structure.
While excavating for foundations, steep side slopes may be
used, and the in-flow of groundwater may cause severe
problems and even collapse.
These temporary difficulties, which may in extreme
circumstances prevent the completion of a construction
project, will not usually affect the design of the finished
works. They must, however, be the object of serious
investigation.

4. The effects of the proposed project on its environment.
The construction of an excavation may cause structural
distress to neighbouring structures for a variety of reasons
such as loss of ground, and lowering of the groundwater
table.
This will result in prompt legal action.

5. Investigation of existing construction.
The observation and recording of the conditions leading to
failure of soils or structures are of primary importance to
the advance of soil mechanics, but the investigation of
existing works can also be particularly valuable for
obtaining data for use in proposed works on similar soil
conditions.
The rate of settlement, the necessity for special types of
structural solution, and the bulk strength of the sub-soil
may all be obtained with more certainty from back-
analysis of the records of existing works than from small
scale laboratory tests.

6. The design of remedial works.
If structures are seen to have failed, or to be about to fail,
then remedial measures must be designed.
Site investigation methods must be used to obtain
parameters for design.

7. Safety checks.
Major civil engineering works, such as earth dams, have
been constructed over a sufficiently long period for the
precise construction method and the present stability of
early examples to be in doubt.
 Site investigations are used to provide data to allow their
continued use.

Apart from above requirement there are engineering/
technical requirements also. The technical objective of the
investigations can be defined as to:
Determine the sequence, thickness and lateral extent of
the soil strata and, where appropriate, the level of bedrock
Obtain representative samples of the soils (and necessary
for use in laboratory tests to determine relevant soil
parameters
Identify the groundwater conditions.

Geotechnical investigations are not only carried out for new
construction project as mentioned above but it can also be useful
for:
To know the source of material to be used in construction
Hydrogeology, hydrology and ground water condition.
Selection of dumping ground or waste disposal site
Determining the degree of contamination of soil and ground water
determination
Determining the safety of existing structure when new structure is
constructed
Determining remedial measure for slope stabilisation and
underpinning
Forensic investigations in case of failure

 There are many reasons why completing a site investigation is
needed for any development project that involves new construction.
 Along with saving money and reducing potential damages,
performing a site investigation helps to identify safety requirements
for the project and can assist in determining which materials should
be used during the construction process.

 In order to make sure that a site is suitable for a construction project, the site may
need to be treated. However, soil treatment may be unnecessary if the site already
accommodates the type of structure that is intended to be developed. A site
investigation will give the information that is required to ascertain if soil
treatment is needed.
 Treating the soil now will also save the money by reducing the risk of
complications once the construction process begins.
 For instance, weak foundations require continual maintenance. If the soil needs to
be treated beforehand, the maintenance costs can be avoided by ensuring a strong
foundation for the property.

 Likely the most beneficial aspect of completing a site investigation is that it will
assist in improving the safety of the project in question.
 Without having information on the characteristics of the soil, a new building could
be developed on soil that doesn’t properly support it.
 Constructing a new building on certain soil types can pose a threat to nearby
buildings as well as any on-site workers.
 One of the OSHA safety guidelines for construction projects involves making sure
that hazards are properly communicated throughout the construction process. If
site investigation is not conducted, workers may be unaware of the various
hazards that they could be exposed to.

 Performing a site investigation before construction begins will also help you
reduce the possibility of damages. If a building is constructed on problematic soil,
the foundation for the building as well as the building itself could be damaged in
the future.
 To make sure that the foundation is designed properly, it’s recommended to bring
in a geotechnical engineer to look at the site before the design of the foundation is
made. These professionals can identify potential risks of continuing development
as well as the solutions to these risks.
 The engineer may suggest to choose a different site altogether or that you use a
more suitable material that will add support to the foundation.

 The site investigation will help to identify which materials should be
used throughout the construction process.
 When there is better idea about the characteristics of the soil, it will
be able to make more informed decisions about the materials that
will work best for the project.
 It must be noted that certain materials don’t pair well with specific
soils, which may increase the development of cracks along the walls
once construction has finished.

 In the field of geotechnical engineering, a well-planned and executed geotechnical
campaign plays a crucial role in the success of any construction project.
 This thorough investigation of the site provides the crucial information about the
geological and geotechnical characteristics of the area, allowing us to make
informed decisions, minimize risks, and optimize the design and construction of
structures.

 A well-developed geotechnical campaign provides us with a detailed
understanding of the geological and geotechnical characteristics of
the site.
 Through sampling techniques, laboratory testing, and in-situ
assessments, we can determine soil bearing capacity, shear strength,
compressibility, and other key geotechnical parameters.
 This enables us to accurately evaluate the suitability of the ground
to support loads and design appropriate foundations and structures,
minimizing the risk of differential settlements and structural
failures.

 A comprehensive geotechnical campaign helps us identify and assess
potential geotechnical risks on the site.
 This includes identifying problematic soil conditions such as
expansive soils, soft clays, or areas with erosion potential.
 By detecting these risks early on, we can implement appropriate
mitigation measures such as soil stabilization, slope reinforcement,
or proper drainage to ensure long-term stability and safety of the
structures.

 The information gathered during the geotechnical campaign allows
us to optimize the design and construction of the structures.
 By understanding the geotechnical properties of the site, we can
select the most suitable construction techniques, appropriate
materials, and proper foundation methods.
 This not only ensures construction efficiency but also reduces costs
and potential project delays.

 Conducting a thorough geotechnical campaign in the initial stages of the project
can result in significant long-term cost savings.
 By understanding the site conditions and making informed decisions, design
changes, rework, and additional costs are minimized.
 Additionally, by anticipating and addressing geotechnical risks early on, costly
problems are avoided, and the service life and performance of the structures are
improved.

The process for a site investigation involves four different stages,
which extend to:
 Site Reconnaissance
 Preliminary site exploration
 Detailed site exploration
 Sub-soil investigation and exploration report

 Field investigations are little costly therefore at initial stage it is not
recommended to carry out detailed soil investigation like sampling,
field and laboratory testing.
 Site reconnaissance would help in deciding future program of field
investigations i.e. preliminary or/and detailed investigations. The
main constituents at this stage are:
 Desk Study of database information
 Field observations

 Desk study of database information is collection of as much as existing
information about the site as possible and analysing them. Some of the
constituents of desk study are:
i. Maps
a. Topographical
b. Air Photos:
Natural or man-made structures in the region, changes in site over time,
identifying major borrow areas for filling
c. Geological Maps (Bedrock and surface geology)
d. Coastal charts
e. Soil Survey Maps (Pedology)

ii. Water well logs
iii. Previous reports
a) Internal Studies
b) Local authority records, local library, old reports of the site and previous
consultant’s report
c) Local practices, foundation type for similar
d) Representative samples, nearby boreholes

 Inspection of the site is must at reconnaissance stage as it gives better
appreciation of site and overcomes some of the obvious mistakes.
 While performing site inspection some of the important points which are to be
observed are:
i. Slope failures
ii. Evidence of seepage
iii. Check outcrops or open cuts
iv. Check surface material
v. Problem with existing structures
vi. Shrinkage cracks
vii. Environmental contamination especially in industrial areas.
viii. Interviewing residents

 Interviewing residents is very important part as it gives idea about previous use
of site, even on untouched lands where changes in topography, removal of
overburden by excavation, landslide, erosion, deposition, diversions of streams,
rivers and drainage conditions could affect the performance of structure planned.
In city or developed areas, records of underground mining, mineral extraction,
quarrying operations, waste tipping underground tunneling and demolished
properties will give indications on probable issues.
 Special attention is to be given for presence of utility locations such as
underground (power, telephone, optic fibre, water, sewer etc.) and above ground
(power, telephone, cable, etc.)
 Geophysical methods may be used at reconnaissance stage as it is simple and
quick means for knowing stratification. It helps in deciding the course of detailed
investigation. The most commonly used methods for reconnaissance survey are
electrical resistivity method and seismic method.

 The next phase of the site investigation involves general site
exploration, which is preliminary and not quite as detailed as the
next stage.
 During this phase of the process, site exploration occurs for light
structures, airfields, highways, and small projects.
 The primary objective of this process is to create an estimate of the
current sub-soil conditions without spending a significant sum of
money

 During the general site exploration, a soil sample will be collected
and tested in a laboratory. This testing will identify the density and
moisture content of the soil. A compressive strength test will also
take place.
 Some of the on-site tests that take place during a general
exploration include sounding tests, penetration tests, and
geophysical tests.
 Performing these tests will allow you to obtain the information
that’s needed to properly identify the strength characteristics and
density of any soils on the site.

The information that gathered during a general exploration of the
site includes:
 An estimate of the values for the compressive strength of the soil
 The groundwater table position
 The composition of soil
 The extent and depth of the layers of soil
 The overall depth of the hard stratum from the ground level
 Various engineering properties of the soil, which occurs by obtaining
a disturbed soil sample

 Detailed exploration goes further than general site exploration and
should be used when constructing major engineering projects, more
complex projects, and heavy structures.
 The heavier structures that detailed site exploration applies to
include high rise buildings, bridges, and dams.
 Unlike a general site exploration, the detailed site exploration
process requires a substantial amount of capital.
 If the budget is relatively limited, it’s recommended to stick to a
general site exploration, which should be adequate if the project is
small.

 During a detailed site exploration, many different on-site tests will
occur to make sure that a large construction project can take place
without issue. The main tests that are performed during a detailed
site exploration include:
 Vane shear test
 Plate load test
 The laboratory tests that are necessary during a detailed site
exploration extend to:
 Compressive strength test to identify exact soil properties
 Permeability tests

 The fourth and final stage of a site investigation involves the creation of a sub-soil
investigation and creation report, which is very detailed and provides construction
crews and developers with everything they should know about the construction
site.
 This report can only be generated once the general or detailed site exploration
process has been performed.

Sections Included in the Report:
 An introduction
 The scope of the site investigation
 A detailed description that highlights the type of structure that’s being built as
well as the main purpose of the site investigation
 Details from the site reconnaissance that was performed
 The site exploration details, which include any sampling details as well as the
location, depth, and number of boreholes on the site
 The techniques that were performed during on-site exploration as well as the
results that were obtained from these tests

Sections Included in the Report:
 Details and results of the laboratory tests that were performed
 Details of the level and position of the groundwater table
 Recommendations on improvements that can be made to the site if necessary
 Structural details and types of foundations that are recommended for the
structure in question
 A brief conclusion

 In practice, the way in which sites are investigated can vary very widely, and the
costs and time necessary will also be significantly different.
 The keys in selecting the most effective method of dealing with the inevitable
uncertainties which must arise are geotechnical knowledge and experience.
 Possible approaches which have been successfully used include the following.

 The minimum requirement for a satisfactory investigation is that a
desk study and walk-over survey are carried out by a competent
geotechnical specialist, who has been carefully briefed by the lead
technical construction professional (architect, engineer or quantity
surveyor) as to the forms and locations of construction anticipated at
the site.
 This approach will be satisfactory where routine construction is
being carried out in well-known and relatively uniform ground
conditions.

For most projects a more elaborate approach is needed, and will
generally follow this course:
 Site Reconnaissance
 Preliminary site exploration
 Detailed site exploration
 Sub-soil investigation and exploration report

 In some projects, it may be possible to carry out redesign during
construction, in order to reduce costs.
 Given the natural variability of the ground, geotechnical engineers
routinely use ‘moderately conservative’ soil parameters in design
calculations, and do not normally attempt genuine predictions of
values such as settlement, ground movements adjacent to
excavations, etc.

 This is a carefully considered approach to geotechnical design,
developed by Peck (1969).
 Peck argued that the methods available for coping with the
inevitable uncertainties which arise as a result of the natural
variability of soil and rock conditions broadly form three groups.
 Method 1: Carry out limited investigation, and adopt an excessive
factor of safety during design.
 Method 2: Carry out limited investigation, and make design
assumptions in accordance with general average experience.
 Method 3: Carry out very detailed investigation.

Introduction to Site Investigation Methods

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