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SITE INVESTIGATION
SITE INVESTIGATION
Site Investigation is the gathering of the
information about the proposed
location of a project, e.g. highway or
buildings.
The Purpose of Site Investigation
1. The site investigation is aimed at providing
sufficient reliable subsurface information for
most economical, satisfactorily safe
foundation for the proposed structure.
2. The site investigation should reveal sufficient
subsurface information for the design and
construction of a stable foundation safe from
both collapse and detrimental movements.
The Scope of Site Investigation
Topography
Soil profile
Ground-water condition
The Stages of Site Investigation
In general, a site investigation program should
comprise four stages, i.e. :
Desk study and site reconnaissance,
Preliminary ground investigation,
Detailed ground investigation,
Monitoring
Desk study and site reconnaissance
The desk study is the first stage of the site
investigation process which involves researching the
site to gain as much information as possible, both
geological and historical.
A good starting point is to use Ordinance survey
maps which allow the selection of the site by
obtaining accurate grid reference through the maps.
In addition to present maps, old maps are used to
gain historical information such as former uses of
the site; concealed mine workings; in filled ponds;
old pits; disused quarries; changes in potential
landslide areas, etc.
The source of information that
useful in desk study:
1. Geological map
Geological maps are probably most important source of
information as these give and excellent indication of the sort of
ground conditions like to be encountered.
2. Aerial photography
Aerial photography is another extremely useful source of
information on topography and ground conditions.
3. Records of previous investigation
Records of previous investigation reports also helpful in a
desk study. The many sources of site investigation data
include previous company and Public Works Departement.
The reconnaissance phase of a site
investigation
This site investigation is done through a site
visit or walk-over survey.
Important evidences to look for are site lay out,
surface condition, climate and hazards water
levels, etc.
Generally the desk study and reconnaissance
is aimed at the feasibility study of the being
planned.
If the desk study shows that the site is feasible
for the structure, then preliminary investigation
should follows.
Preliminary Investigation
Preliminary Investigation is aimed at predicting the
geological structures, soil profiles and the position of
ground water table by geophysical method or by
making a few boreholes.
The investigation should give information on the
existence on ground structures that may need closer
examination: for example,
1. The extent of disturbed strata,
2. The location and extend of natural cavities and mine workings.
3. Fractures and river crossings or alluvial areas that may have
buried soft material or pet, their liability to cause subsidence,
surface movements or instability
4. Information on suitability of soil for fills work, ground water
condition and the possibility of flooding should be provided at
this stage.
Detailed Investigation
At this stage, the extent of the test,
number and depth of boreholes,
selection of appropriate equipment for
field testing and the choice of
laboratory testing are made.
Soil exploration consists of three steps:
1. Boring and in-situ testing,
2. Sampling,
3. Laboratory testing.
Monitoring
Monitoring during construction and maintenance
period is required whether the expectations of the
proceeding investigation have been realize.
No one can ensure that the soil parameters used for
design is the most representative of the soil conditions
at the site unless the response is observed.
Field observation can help for early diagnosis and
redemption of any problem that might be encountered
during construction.
Among the measurement made during the monitoring
stage are the settlement, displacement, deformation,
inclination, and pore water pressure.
Steps of Soil Exploration
A. BORING
Soil borings are the most common method of
subsurface exploration in the field. A bore hole is used
to determine the nature of the ground in a qualitative
manner and then recover disturbed and undisturbed
samples for quantitative examination.
Some types of borings are hand/mechanical auger
borings, wash borings, percussion drilling, rotary
drilling, and core borings. An auger is a screw-like tool
used to bore a hole.
Some augers are operated by hand: others are power
operated
Hand/Mechanical Auger
Hand augers may be used for boring to a
depth of about 6 m.
Power augers may be used for boring to a
depth of about 10 to 30 m.
As the hole is bored a short distance, the
auger may be lifted to removed soil. The
removed soil can be used for field
classification and laboratory testing, but it must
not be considered as an undisturbed soil
sample.
Power auger set with a drill rig can be used to
obtain samples from deeper strata. Some rigs
can be used to drill a hole to 100 m depth.
Wash Boring
Wash borings consists of simultaneous
drilling and jetting action. A hole is bored
through a casing by using a drilling bit.
Jetting action is accomplished by
pumping water downward through the
drilling bit to soften the soil. Samples
taken using the wash boring methods
are disturbed sample.
Percussion Drilling
Percussion Drilling is the process of making
boreholes by striking the soil then removing it.
The tools are repeatedly dropped down the
borehole while suspended by wire from the
power winch.
Water is circulated to bring the soil cuttings to
the ground surface.
A casing and a pump are required to circulate
the water.
Rotary Drilling
Rotary Drilling uses rotation of the drill bit with
the simultaneous application of pressure to
advance the hole.
This method is the most rapid method of
advancing a hole in soil and rock.
Drilling mud may be needed to prevent soil
cave-in.
Sample obtained from drilling by this method is
relatively less disturbed as compared to
samples obtained by the preceding methods.
17
Boring tools
Auger boring Power drills
18
SOIL BORING
19
Boring Logs
Boring Logs
20
B. SAMPLING
Sampling refers to the taking of soil sample
from bored hole.
There are two types of samples:
1. Disturbed samples
This sample are usually needed for index properties
of soil.
2. Undisturbed samples
This sample are usually needed for determining the
engineering properties such as shear strength and
consolidation characteristic of the soil.
The sampling procedures varies according to the type of
strata in which the investigation takes place. Undisturbed
samples are normally needed for clays at every 1.5 m
depth or change of stratum.
If undisturbed sample cannot be retrieved at a specific
depth, then bulk samples should be taken.
Undisturbed sample are not practically for sand and
gravel due to the lack of cohesion.
Bulk samples to be taken every 1 m or every change of
stratum while alternate disturbed and undisturbed
samples should be taken for silt layer at 0.75 m intervals.
Undisturbed sample may be possible for soft rock such
as chalks and marls.
A sampling program should be consistent with
the required accuracy of design and the scale of
the structures.
Disturbed sample can be obtained from auger
boring, core boring, split spoon sampler in
standard penetration test (pit and trench, and
some types of sampler such as thick walled
sampler, displacement sampler, and
Beggemann sampler.
Undisturbed sample are generally required
during a detail subsurface exploration to provide
specimens for laboratory testing.
If a test pit is available in clay soil, an undisturbed
sample may be obtained by simply carving a sample
very carefully out of the side of the test pit. Such a
sample should then be coated with paraffin wax and
placed in an airtight container.
A more common method of obtaining an undisturbed
sample is to push a thin tube into the soil, thereby
trapping the undisturbed sample inside the tube and then
to remove the tube and the intact sample.
The most popular tube is the open drive sampler while
the recommended sampler for the soft soil is the piston
sampler.
Several types of piston samplers are available, for
instance the fixed piston sample, free piston sampler,
and restraint sampler.
The term undisturbed is considered relative because the
process of extracting the sample from a depth in soil,
transporting the samples to laboratory and preparing the
specimen for testing my introduce disturbance that can
cause the result of laboratory testing will not be
representative of in-situ condition.
To ensure the quality of the sample, some step should
be taken after obtaining the undisturbed sample
appropriate tube.
Immediately after the tube containing the sample is
brought to the ground surface, the ends of the tube
should be sealed with paraffin wax.
After sealing the tube, the following data should be
attached to the sampling tube:
1. Project name,
2. Name of drilling operator,
3. Date of the sampling,
4. Borehole number and sample number,
5. Depth of sample.
Care should be taken during shipment and
stored of the sealed tube for testing in the
laboratory because these processes may result
in serious sample disturbance.
On arrival at the laboratory, it is important to
check the conditions of the samples and
compare them with the states recorded in the
field.
The samples should be stored in a room where
the temperature and humidity are kept constant
and similar to the in situ-conditions.
Visual inspection of undisturbed samples
should be made to ensure that there is:
1. no visible distortion of strata in the sample,
2. no opening or softening of the material,
3. specific recovery ratio (SRR) should not be
less than 95%,
4. area ratio (Ar) should be less than 15 %.
The SSR and A
r
can be defined as follows:
SSR = length of undisturbed sample recovered from the tube
Length of the tube
(2.1)
Where, Di = inside diameter and
Do = outside diameter
%100
2
1
2
1
2
x
D
DD
A
o
r
−
=
(2.2)
IN SITU TESTING
In some cases the data obtained from sampling and
laboratory testing is less reliable than those from in-
situ testing. Moreover, sampling can be more
expensive than in-situ testing or sounding.
Therefore, the program of sampling may be planned in
combination with in-situ testing.
Common types of field testing include the standard
penetration test (SPT), cone penetration test (CPT),
vane shear test (VST), pressure meter test (PMT), and
dilatometer test (DMT).
STANDARD PENETRATION TEST
(SPT)
The standard penetration test (SPT) is a dynamic test
and is a measure of the density of the soil. The SPT is
carried out in a borehole by lowering the split spoon
sampler of about 650 mm length, 50 mm external
diameter, and 35 mm internal diameter (Figure 2.5),
and driving it using repeated blows by a freely dropped
hammer at falling height of 765 mm.
There are two types of hammer : automatic trip
hammers and slip-rope-hammers but the standard
weight of the hammer is 63.5 kg (Figure 2.8).
The test procedure is standardized in ASTM D 1586.
The blow count is made in three steps of 150
mm. The strength of the soil is measured by
the number of blow count of the last 300 mm
penetration denoted as N blows/300 mm.
The blow count (N) may be corrected by field
conditions such as,
a) energy used for driving the rod into the soil (E
m
),
b) Variations in the test apparatus (C
s
and C
R
),
c) Size of drilling hole (C
B
)
The values of E
m
, C
s
, C
R
, and C
B
depend on the
SPT equipment.
Many of the correlations developed based
on hammer that have an efficiency of
60%, the results of other hammer should
be corrected to this efficiency factor.
Thus :
N
CCCE
NRSBm
6.0
60
=
(2.3)
The SPT data may also be influenced by
overburden pressure, thus the N value
should be corrected to a standard
effective overburden pressure (σ’
o
).
For a standard energy and effective
overburden pressure of 100 kPa, the
corrected N value (Terzaghi et al, 1996,
and Liao and Whitman, 1986) is:
5.0
'
100
'
==
o
N
NNCN
σ
(2.2)
The SPT test should be halted when soil shows some
refusal i.e. when more than 50 blows are required to
penetrate any 150 mm increment or 100 blows are
obtained for 30 mm penetration or if 10 successive blow
produce no advance in the penetration.
The N values can be correlated with the relative density
of the soil, and internal friction angle of cohesionless soil
(Table 2.1).
Even though not reliable for cohesive soil, relationship
between the N value and the consistency and the
undrained shear strength of cohesive soil was also
developed (Table 2.2)
Table 2.1
SPT
N
(blows/300
mm)
Relative
Density
(%)
Internal
friction angle
State of
packing
4
4 – 10
10 – 30
30 – 50
> 50
20
20 – 40
40 – 60
60 – 80
> 80
30
30 – 35
35 – 40
40 – 45
45
Very
loose
Loose
Compact
Dense
Very
dense
Table 2.2
SPT
N
(blows/300 mm)
Undrained shear
strength
Cu
(kPa)
Consistency
2
2 – 4
4 – 8
8 – 15
15 – 30
> 30
10
10 – 25
25 – 50
50 – 100
100 – 200
> 200
Very soft
Soft
Medium
Stiff
Very stiff
Hard
CONE PENETRATION TEST (CPT)
The CPT is used widely in Europe and other parts of the world
because of its versatility. The procedure has been standardized
in ASTM D3441.
Basic parts of this equipment include a cone to measure the tip
resistance and skin friction of soil, some rods, and measuring
devices.
Two type of cone currently available are mechanical cone and
electric cone. Both have two parts , a 35.7 mm diameter cone
shaped tip with a 60o apex angle and 35.7 mm diameter and
133.7 mm long cylindrical sleeve.
Piezocone is equiped with a pore pressure transducer to
measure pore pressure.
In recent year, the CPT or CPTU is supplemented by additional
sensors, such as seismic cone, lateral stress sensing, and
electrical resistivity for estimating in situ porosity or density.
Cone penetration test carried out by mechanically or
hydraulically pushing a cone into the ground at a
constant speed (20mm/sec) while measuring the tip
resistance and friction.
The cone penetration test measures the tip resistance
(designated as q
c
in kgf/cm
2
) and the friction resistance (f
s
in kgf/cm).
Friction ratio (Fr) represents the ratio between the friction
resistance and the cone resistance in percentage which
is very useful in the estimation of soil type.
For piezocone, pore pressure (u
b
in kgf/cm2) is
measured along depth of penetration.
Cone Penetration Test (CPT)
Procedures
The parameters obtained from cone
penetration test can be correlated with relative
density, soil classification, and unconfined
compression strength, sensitivity of clay,
degree of over-consolidation, pile design
parameter, bearing capacity and settlement.
Figure 2.12 shows a commonly used
correlation between cone resistance, friction
ratio, and the soil classification developed by
Robertson and Campanella in 1983.
The cone penetration resistanace can be related
to the undrained shear strength (c
u
) of cohesive
soil by the following equation:
In which σ’
o
is the overburden pressure and N
k
is
the cone factor which ranges from15 to 20
depending on the type cone used.
k
c
u
N
q
c
0
'
σ
−
=
(2.5)
Another correlation based on CPT data
is equal to 2.5 – 3.5 q
c
.
Other correlations relate the results of
cone penetration test with the N value
from Standard penetration test.
VANE SHEAR TEST (VST)
Vane shear test is commonly used to
measure the shear strength and
sensitivity of clay.
The equipment consists of four-bladed
rectangular vane, rotating rod, and
measuring device.
Vane Shear Test (VST) Procedures
The test is carried out in a borehole or directly
pushing the vane into the ground.
The vane rod is then rotated at a rate of
60/min, while the torque is read at interval of
30 seconds.
After maximum torque is achieved, the vane is
rotated at a higher rate to obtain the remolded
strength of the soils
Measure parameters include the peak torque
(T
peak
), and residual torque (T
res
).
The theoretical formula for relating the results of
vane shear test to the shear strength
parameters of the soil is :
Where: cu is the undrained shear strength of
soil, T is the maximum torque, d is the diameter
of the vane, and h is the height of the vane.
+
=
62
32
dhd
T
c
u
π
(2.6)
The test result may be affected by several factors i.e.
the disturbance due to vane insertion, blade thickness,
rate of rotation, time lapse between insertion of the
vane and the beginning of the test, and possible
friction of the rod and surrounding soils.
Type of soil and strength anisotropy may also affect
the results.
Skempton recommended multiplying the vane
diameter by 1.05 for interpretation of strength.
Bjerrum suggested a correction factor for the shear
strength of highly plastic clay obtained from vane
shear test (Figure 2.14)
Cohesive soils often lose some of their shear
strength if disturbed and most of the soil
samples obtained in the field are subject to
disturbance.
A parameter known as sensitivity indicates the
amount of strength lost by soil as a result of
thorough disturbance.
Vane shear test is usually performed to predict
the sensitivity of a cohesive soil by repeating
the test at the same point after remolding the
sample by completely rotating the blade.
The first maximum torque represents the
peak strength, while the second maximum
torque represent the residual strength of
the soil.
Sensitivity of the soil can be calculated
from (Equation 2.7).
res
peak
T
T
S=(2.7)
The Range Of The Sensitivity Of
Clays
The sensitivity of most clays ranges
between 2 and about 4.
For sensitive clays, the sensitivity ranges
from 4 to 8.
For extra sensitive clays, the sensitivity
ranges from 8 to 16.
Quick clays, the sensitivity greater than
16.
Pressuremeter Test (PT)
Pressuremeter test is carried out to estimate
the soil type, and to measure the undrained
shear strength (c
u
), modulus of horizontal sub-
grade reaction (E
m
), and insitu horizontal stress
in the ground (σ
ho
).
The equipment consists of a probe, measuring
unit, and cable (Figure 2.15).
The test is performed in a borehole by pushing
the probe into the ground and loading it
horizontally until it reaches the limit pressure
or capacity of the device.
Normally the pressure increments are between
5 and 14 kPa.
There are three types of pressure-meter i.e.
borehole pressure-meter, self-boring pressure-
meter, and push-in pressure-meter.
The type of soil, the rate of expansion,
membrane stiffness and system compliance,
and size of drilling hole may affect the results
of the pressure-meter test.
Pressure may also be corrected for the
resistance of the probe with the pressure
volumeter, and hydrostatic effects.
Dilatometer Test
The test is similar to the pressure-meter test,
but the measurement is made through a blade
with a stainless-steel membrane mounted on
one side of the blade.
The test is carried out by pushing or
hammering a dilatometer blade into the soil at
rate between 10 – 30 mm/seconds, while
measuring penetration resistance and then
using gas pressure to expand the membrane
approximately 1.1 mm into the soil
Various parameters can be measured by ,
dilatometer; among these is dilatometer
modulus as an estimate of elastic Young’s
modulus (ED).
Calibration of membrane should be made at
ground surface before and after dilatometer
test for the gauge pressure necessary to suck
membrane against its support, and the
pressure necessary to moved it outward to the
1.10 mm position.
The result may be affected by disturbance due
to blade insertion, blade thickness, membrane
stiffness and thickness, and the soil type.
Observation of Ground Water
Information on the groundwater level and
any artesian pressure in particular strata is
very important and should be determined
carefully during site investigation.
Several problems related to the presence of
ground water table:
1. Shear strength of a soil may be reduced below
water table.
2. Foundation may be uplifted by the water.
3. Possibility of dewatering if the structure should be
constructed in dry conditions, etc.
The location of ground water table is usually
determined by measuring the depth of water surface in
a borehole after a suitable time lapse because water
table in boreholes may take some time to stabilize
depending on the permeability of the soil.
Common practices is to measure the depth of ground
water table after drilling and covering the hole with a
small piece of plywood.
In soil with high permeability such as sand and gravel,
24 hours is adequate for the water level to stabilize.
In soil with low permeability such as silts and clay, it
may take several days for the water level to stabilize.
In this case, measurement should be made at a
regular interval of time until it stabilizes.
For a regular condition, measurement
can be made using a tell tale, but if it is
desirable to obtain the water pressure in
a particular strata, then a piezometer
should be utilized.
Ground water sample may be taken for
chemical analysis because some
chemical may attack structural material
such as concrete and steel.
Laboratory Testing
In site investigation program, the
determination of soil properties is generally
made in soil mechanics laboratory. To get a
good quality of testing results, the samples
retrieved from the ground should be tested as
soon as after arrival at laboratory.
Standard laboratory testing may be grouped
based on its purpose as shown in Figure 2.18.
Laboratory Testing for Undisturbed
Samples
Undisturbed samples are needed for more
sophisticated laboratory test such as;
1. Shear strength, include the unconfined
compression test, direct shear or shear box test
and Triaxial test under unconsolidated undrained
(UU), consolidated undrained (CU), and
consolidated drained conditions (CD).
2. Consolidation test.
The consolidation test is usually performed on
standard oedometer cell.
Laboratory Testing for Disturbed
Samples
Disturbed samples are normally used for
determining index properties of the soil such
as;
1. The unit weight,
2. Specific gravity.
The samples also used for classification test
such as;
1. Sieve and hydrometer analysis to obtained the
particle size distribution,
2. Atterberg limit tests to find the consistency of
cohesive soil.
Soil Exploration Report
Soil exploration report should be presented upon the
completion of a soil exploration program.
The report should include the scope of investigation,
description of the proposed structure, and general site
conditions.
The report should present the general description of
soil strata, position of ground water table and other
information pertinent to the site.
The detail of field exploration should include the
number of borings, lay-out and depth of boring, type
of boring and other specifications of field test
conducted during the exploration.