site investigation ,soil mechanics ,foundation engg

1. INVESTIGATION AT
SITE

2. SITE INVESTIGATION
Site Investigation is the gathering
of the information about the
proposed location of a project,
e.g. highway or buildings.

3. The Purpose of Site
Investigation1. 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.

4. The Scope of Site
Investigation
Topography
Soil profile
Ground-water
condition

5. 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

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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

13. 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.

14. 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.

15. 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.

16. 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
17
Auger boring Power drills

18. SOIL BORING
18

19. 19
Boring LogsBoring Logs

20. 20

21. 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.

22.  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.

23.  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.

24.  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.

25.  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.

26.  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.

27.  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.

28.  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 %.

29.  The SSR and Ar 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
%1002
1
2
1
2
x
D
DD
A o
r
−
=
(2.2)

30. 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).

31. 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.

32.  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 (Em),
b) Variations in the test apparatus (Cs and CR),
c) Size of drilling hole (CB)
 The values of Em, Cs, CR, and CB depend on the
SPT equipment.

33.  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
N RSBm
6.0
60 = (2.3)

34.  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)

35.  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)

36. 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

37. 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

38. 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.

39.  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 qc in kgf/cm2
) and the friction resistance
(fs 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 (ub in kgf/cm2) is
measured along depth of penetration.
Cone Penetration Test (CPT)
Procedures

40.  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.

41.  The cone penetration resistanace can be
related to the undrained shear strength (cu) of
cohesive soil by the following equation:
In which σ’o is the overburden pressure and Nk 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)

42.  Another correlation based on CPT data is equal to 2.5 –
3.5 qc.
 Other correlations relate the results of cone penetration
test with the N value from Standard penetration test.

43. 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.

44. 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 (Tpeak), and residual torque (Tres).

45.  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
cu
π
(2.6)

46.  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)

47.  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.

48.  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)

49. 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.

50. Pressuremeter Test (PT)
 Pressuremeter test is carried out to
estimate the soil type, and to measure
the undrained shear strength (cu),
modulus of horizontal sub-grade
reaction (Em), and insitu horizontal
stress in the ground (σho).
 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.

51.  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.

52. 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

53.  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.

54. 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.

55.  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.

56.  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.

57. 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.

58. 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.

59. 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.

60. 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.