ch-1 foundation.pdf

CHAPTER ONE
SITE EXPLORATION
Out line
1.0 Introduction
1.1 Purpose of Site Exploration
1.2 Plan and Method of Site Exploration
1.3 Evaluation of Field Tests Data
1.4 Geotechnical Report
3/1/2021 Foundation Engineering by Estifanos B. 1

1.0 Introduction
• The stability of the foundation of a building, a bridge, an
embankment or any other structure built on soil depends on the
strength and compressibility characteristics of the subsoil.
• Site investigation or soil explorations are done for obtaining
the information about subsurface conditions at the site of
proposed construction.
Soil exploration consists of
• Determining the profile of the natural soil deposits at the site,
• Taking the soil samples ,determining the engineering properties of
soils.
• It also includes in-situ testing of soils.

Conti…
Site investigation, in general deals with determining
the suitability of the site for the proposed
construction.
Sub-surface investigations may be subdivided into
three classes:
a) Foundation investigations to investigate sites for
new structures.
b) Stability or failure investigations to investigate
causes of distress or failure of existing structures.
c) Earthwork investigations to evaluate suitability of
natural materials for construction purposes.

Conti…
• Soil exploration is a must in the present age for
the design of foundations of any project.
• Cost of soil exploration reaches between 0.05%
to 0.2% of the total cost of the entire project.
• In some unusual condition it reach up to 1% of
the total cost of entire project.

Goals of Soil Investigation are
1. Estimating Geometry of soil Strata and ground
water
2. Field Investigation
Identifying materials and layering, retrieving
samples and engineering properties through insitu
tests
3. Laboratory Testing
Determining index and engineering properties
from samples

1.1 Purpose of Site Exploration
The site investigation provides first hand information for;
 Selection of foundation type.
 Design of foundations.
 Contractors to quote realistic and competitive tenders.
 Planning construction techniques.
 Selection of appropriate construction equipment (especially for
excavation and foundations).
 Feasibility studies of the site.
 Estimating development cost for the site.
 Study of environmental impacts of the proposed construction.

Conti..
 Selection of borrow areas for embankments.
 Need for any suitable soil improvements.
 Requirement of any surface or subsurface drainage.
 Selection of the most suitable and economical route for highways with respect
to soil conditions.
 Selecting areas (better soil) for engineering structures where there is
flexibility in relocating the structure thus realizing considerable savings in
foundation costs.
 The design of extension works to existing structures.
 The investigation of the cases where failure has occurred, to know the causes
and design of remedial works.

1.2 Plan and Method of Site Exploration
1.2.1 Plan of Site Exploration
The principle of soil exploration remains the same for all the projects but the
program and methodology may vary from project to project.
The planning of a program for soil exploration depends upon
i. The nature of sub-soil
ii. The type of structure
iii. The importance of structure

The actual planning of subsurface exploration program includes some or all of
the following:
1. Disk Study
Assembly of Available Information on type and use of the structure, and also of
the general topographic and geological character of the site.
2. Reconnaissance Survey
This involves inspection of behavior of adjacent structures, rock outcrops, cuts,
etc.
3. Preliminary Ground Investigation
This is usually in the form of a few borings or a test pit to establish the types of
materials, Stratification of the soil, and possibly the location of the ground water
level.
4. Detailed Ground Investigation
For complex projects or where the soil is of poor quality and/or erratic, a more
detailed investigation may be undertaken this may involve sinking several
boreholes, taking soil samples for laboratory investigations, conducting sounding
and other field tests.

1.2.2 Methods Of Exploration
Methods of determining the stratification and engineering
characteristics of sub-surface are
 Test pits
 Boring and sampling
 Field tests
 Geophysical methods
 Laboratory tests

Test pits
• The simplest and cheapest method of shallow soil exploration is
to sink test pit to depths of 3 to 4 m.
• Test pits enable the in-situ soil conditions to be examined visually.
• It is relatively easy to obtain disturbed or undisturbed soil
samples .

Boring and sampling
Boring
This is the most widely used method.
It provides samples from shallow to deeper depths for visual
inspection as well as laboratory tests.
The most commonly used methods of boring are:
Auger boring
Wash boring
Percussion drilling
Rotary drilling

Auger boring
It is operated by hand or by power
Two types of hand augers, the posthole type and helical type are
shown. The depth of holes by augers is limited to a maximum of
10m. The size of hole may very from 10cm to 15cms.
Mechanically operated augers are used for greater depths and they
can be used in gravely soils also. Extension rods of 1.0m lengths can
Power operated augers (helical) can be used to great depths, even
to 30m be added as work progresses.

Conti…

Wash boring
• Power operated.
• Hole is advanced by chopping, twisting action of
a light chopping bit and jetting action of drilling
fluid.
• This method best suits in sandy and clayey soils
and not in very hard soil strata (i.e. boulders)
and rocks.
• Depth of boring could be up to 60m or more.

conti

Percussion drilling
• It consist of breaking up of the formation by
repeated blows from a bit or chisel. Water
should be added to the hole at the time of
boring and the debris baled out at intervals.
• Hole is advanced by repeated blows of a heavy
chisel into the bottom of the hole.
• Because of the deep disturbance of the soil this
method of boring is not favored.
• Maximum depth of boring is 60m.

Conti..

Rotary drilling
• Power operated.
• Hole is advanced by a rapidly rotating bit.
• This is the most rapid method for penetrating highly resistant
materials (e.g. bed rock).

Soil Sampling
Objective: to obtain soils of satisfactory size with minimum
disturbance for observations and laboratory tests.
Laboratory test results are mainly dependent on the quality of soil
samples.
There are two main types of soil samples which can be recovered from
bore holes or trial pits.
1. Disturbed and
2. Undisturbed samples
Sampling Apparatus:
 Split-Spoon Sampling
 Sampling with a Thin-Walled Tube
 Sampling with a Piston Sampler

Disturbed Samples
• These are samples where the structure of the
natural soil has been disturbed to a considerable
degree by the action of the boring tolls or
excavation equipment.
• However, these samples represent the
composition and the mineral content of the soil.
• Disturbed samples are satisfactory for
performing classification tests such as, sieve
analysis, Atterberg limits etc.

Undisturbed Samples
 These are samples, which represent as closely as is
practicable, the true in-situ structure and water content
of the soil.
 Undisturbed samples are required for determining
reliable information on the shearing resistance and
stress-deformation characteristics of a deposit.
 It is practically impossible to obtain totally undisturbed
samples. This is due to that:
 The process of boring, driving the coring tool, raising and
withdrawing the coring tool and extruding the sample from the
coring tool, all conspire to cause some disturbance.
 In addition, samples taken from holes may tend to swell as a
result of stress relief.

Requirement of sampler for undistured Sampling
Although sample disturbance depends on factors such as rate of
penetration, whether the cutting force is obtained by pushing or
driving, and presence of gravel, it also depends on the ratio of the
volume of soil displaced to the volume of collected sample,
expressed as an area ratio Ar.
Well-designed sample tubes should have an area ratio of less than
about 10 percent.

Split-Spoon Sampling
• to obtain soil samples that are generally
disturbed, but still representative.
• samplers having Di and Do up to 63.5 mm and
76.2 mm,respectively, are also available.
• Used in SPT-test

Thin-Walled Tube
• Thin-walled tubes are sometimes referred to as
Shelby tubes.
• They are made of seamless steel
• They used to obtain undisturbed clayey soils.
• The most common thin-walled tube samplers have outside
diameters of 50.8 mm and 76.2 mm.
• The bottom end of the tube is sharpened.
• The tubes can be attached to drill rods.

Piston Sampler
• When undisturbed soil samples are very soft or larger
than76.2 mm in diameter, they tend to fall out of the
sampler. Piston samplers are articularly useful under such
conditions
the presence of the piston prevents distortion in the sample
by
 not letting the soil squeeze into the sampling tube very
fast and
 not admitting excess soil.
Consequently, samples obtained in this manner are less
disturbed than those obtained by Shelby tubes.

Conti..
.

Field tests
The most commonly used field tests are:
1)Penetration or sounding tests
2)Vane shear test
3)Plate loading test
4)Pile loading test
5) Dilatometer Test (DMT)
6) Pressuremeter Test (PMT)

1)Penetration or sounding tests
 They are conducted mainly to get information on the relative density of
soils with little or no cohesion.
 The tests are based on the fact that the relative density of a soil stratum
is directly proportional to the resistance of the soil against the
penetration of the drive point.
 From this, correlations between values of penetration resistance versus
angle of internal friction (Φ), bearing pressure, density and modulus of
compressibility have been developed.
 Penetration tests are classified as: Static and dynamic penetration tests.

Penetration Tests
Static Cone Penetration Test (Dutch Cone Penetrometer Test)
This method is widely used in Europe.
 The test consists of a cone (apex angle 600, overall diameter 35.7mm, end area 10cm2, rods
(⅝” ), casing pipe ( ¾”).
 The rod is pushed hydraulically into the ground at a rate of 10mm/sec.
 The pressure exerted on the rod is measured with a proving ring, manometer or a strain
gauge.
 The cone is 1st pushed into the ground. The force required to push the cone 20cm into
the soil is recorded.
 The casing pipe is then advanced to join the cone. The force required to push the pipe
is also recorded.
 The readings thus taken are plotted against depth.

Conti..
• .

Dynamic Penetration Tests
1. Standard Penetration Test (SPT):
 The method has been standardized as ASTM D 1586.
 This is the most common of the field tests and measures the resistance
of the soil to dynamic penetration by a 50mm diameter split spoon
sampler which is driven into the soil at the bottom of a borehole
(sometimes cased).
 The sampler is attached to drill rods and the dynamic driving force is a
63.5kg mass falling through a height of 76cm onto the top of the rods.
 The sampler is initially driven 15cm below the bottom of the borehole.
It is then further driven 30cm. The number of blows required to drive
the last 30cm is termed as the standard penetration value denoted by N.

Conti..

2. Dynamic Cone Penetration (DCP) Test
• This is another useful test, which is normally used to
determine the relative resistance offered by the different
soil layers.
• The cone is fixed to the bottom of a rod and driven into the
ground in the same way as a SPT is performed.
• The number of blows required to penetrate 30cm depth is
called as Nc value.
• In the case of dynamic cone penetration test no borehole is
used.

Conti..
 Experiments carried out indicate that beyond about 6m
depth, frictional resistance on the rod increases which gives
erroneous results for Nc value. The maximum depth
suggested for this test is about 6 m.
 If the test has to be conducted beyond 6 m depth, one has to
use drilling mud (bentonite slurry) under pressure forced
through the pipe and the cone.
 The mud solution coming out of the cone rises above along
the drill rod eliminating thereby the frictional resistance
offered by the soil for penetration. The former method is
called as dry method and the latter wet method.

Conti..

Sounding Test
I. Swedish Weight Sounding Test
 This method of testing is widely used in
Scandinavia.
 The depth of penetration is measured for each
loading after which the number of half-turns is
counted by 100kg load; the penetration depth is
then measured after 25 half-turns.
 If the penetration after 25 half-turns is less than
5cm the rod is unloaded and driven down by a 5
to 6kg hammer.

Conti..
Swedish weight sounding equipment, penetration diagram

Vane shear test
It is used to determine the undrained shear
strength of soft clays soils.
The shear vane device consists of four thin metal
blades (vanes) welded orthogonally (900) to a
rod .
Vane head (torsion head), complete with pointer,
stop pin, circumferential graduated scale,
calibrated torsion spring.
The vane is pushed, usually from the bottom of a
borehole, to the desired depth.

Conti..

Conti..
In most cases a hole is drilled to the desired
depth, where the vane shear test is planned to
be performed & the vane is carefully pushed into
the soil.
A torque is applied at a rate of 60 per minute by
a torque head device located above the soil
surface and attached to the shear vane rod.
The maximum torque is then measured from
which the shearing strength is determined.

Conti..
From the measured maximum torque one may
estimate the shearing resistance of the tested
clay from the following formula:
Where : T = Torque
D = Diameter of Vane
H = Height




















6
2
2
12
3
2
2 D
H
D
T
D
H
D
T




Plate loading test
In this test a gradually increasing static load is applied to
the soil through a steel plate, and readings of the
settlement and applied load are recorded, from which a
relationship between bearing pressure and settlement for
the soil can be obtained. The test procedure:
1. Pit for the test must be at least 5 times the size of the
plate.
2. The plate should be properly placed in the soil. In the
case of cohesionless soil (to prevent early displacement
of soil under the edges of the plate), the plate must be
positioned in cast in-situ concrete.
3. Loading platform should be properly erected.

Conti..
4. Loading of the soil is conducted in steps (loading increment is
kept constant).
5. Once the test is completed, the plate is unloaded in the same
incremental steps (to draw the expansion curve).
Bearing capacity of non-cohesive soil is determined from
settlement consideration.
If the maximum permissible settlement, S, of a footing of width Bf is
given, the settlement, Sp, of a plate of width Bp under the same
intensity of loading is given by:

Conti..
• Using the value Sp, computed from the above equation, the
loading intensity under the footing could be read from the load
settlement curve.
• The settlement of footing in clay is normally determined from
principles of consolidation. However from plate load test, the
approximate settlement of footing of width B can be determined
using the following expression.

Limitation of Plate loading test
 Plate loading test is of short duration. Hence
consolidation settlement does not fully occur during the
test.
 For settlement consideration, its use is restricted to sandy
soils, and to partially saturated or rather unsaturated
clayey soils.
 Plate loading test can give very misleading information of
the soil is not homogeneous within the effective depth
(depth of stress influence) of the prototype foundation.
 Plate loading test should not be recommended in soils
which are not homogeneous at least to depth of 1½ to 2
times the width of the prototype foundation

Conti..

Pile loading test
This is the most reliable means for determining
the load carrying capacity of a pile.
The load arrangement and testing procedure are
more or less similar to the plate-loading test.
From the results of this test, the allowable
bearing capacity and load- settlement
relationship of a group of friction piles can be
estimated.

Conti..

GEOPHYSICAL METHODS
• These comprise the seismic and resistivity methods.
• These methods are usually limited to establishing
location of bedrock underlying softer material (by
seismic method) or locating gravel or sand deposits
(by resistivity method).
• The seismic method is based on the fact that sound
waves travel faster through rocks than through soils
• The resistivity method makes use of the fact some
soils (e.g. soft clays) have low electrical resistivity
than others (e.g. sand or gravel)
• These methods are normally employed as
preliminary or supplementary to other methods of
exploration

Ground Water Measurement
Ground water affects many elements of foundation
design and construction.
Because of this its location should be determined in
each job with reasonable accuracy.
The depth of water table is measured by lowering a
chalk-coated steel tape in the borehole.
The depth can also be measured by lowering the leads
of an electrical circuit. As soon as the open ends of the
leads touch the water in the borehole, the circuit is
completed. It is indicated by glow of the indicator
lamp

laboratory tests
The common laboratory tests that concern the
foundation engineers are
Grain size analysis
Atterberg limits
Natural moisture content
Unit weight
Unconfined compression test
Direct shear test
Triaxial compression test
Consolidation test
Compaction test

1.2.3 Soil Sampling and depth of boring
• For buildings minimum of three borings, where
the surface is level and the first two borings
indicate regular stratification, may be adequate.
• Five borings are generally preferable (at building
corners and center), especially if the site is not
level.
• Borings should extend below the depth where
the stress increase from the foundation load is
significant. This value is often taken as 10
percent (or less) of the contact stress qo. (
Bowles)

Conti..
Summarizing, there are no binding rules on either
the number or the depth of exploratory soil
borings. Each site must be carefully considered
with engineering judgment in combination with
site discovery to finalize the program and to
provide an adequate margin of safety. ( Bowles)
Budhu Muni”s Recommendation was
• a minimum of three boreholes should be drilled
for a building area of about 250 m2 and about fi
ve for a building area of about 1000 m2.

General guidance on the depth of boreholes
• In compressible soils such as clays, the borings should penetrate
to at least between 1 and 3 times the width of the proposed
foundation or until the stress increment due to the heaviest
foundation load is less than 10%, whichever is greater.
• In very stiff clays and dense, coarse-grained soils, borings should
penetrate 5 m to 6 m to prove that the thickness of the stratum is
adequate.
• Borings should penetrate at least 3 m into rock.
• Borings must penetrate below any fills or very soft deposits
below the proposed structure.
• The minimum depth of boreholes should be 6 m unless bedrock
or very dense material is encountered

Table: Guidelines for the Minimum Number or Frequency and
Depths of Boreholes for Common Geo structures ( Budhu muni)

Table: Requirements for Trial pit and boring layout. ( EBCS-7 , 1995Pg.22)

According to EBCS-7,1995
 The exploration is normally carried out to a depth which
includes all strata likely to be significantly affected by the
structural load.
 If rock is found , a penetration of at least 3.0m in more than one
borehole maybe requires to estabilish whether bedrock or
boulder is encountered.
 for structure on footing foundation, depth of exploration =
3(least width of footing ) and >=1.5m.
 For MAT Foundation, depth of exploration=1.5(width of mat) and
>=6.0m.
 For pile foundation, depth of exploration should be atleast up to
30 below end of the pile.

According to CES-159,2015
Handling, transport and storing of samples shall be
carried out in accordance with ISO22475-1.
The sampling categories, and the number of
samples to be taken shall be based on:
The aim of the ground investigation;
The geology of the site;
The complexity of the geotechnical structure.

Conti..
To establish geotechnical design requirements,
three Geotechnical Categories, 1, 2 and 3, may
be introduced. (CES-158,2015)
A preliminary classification of a structure
according to Geotechnical Category should
normally be performed prior to the geotechnical
investigations.
Geotechnical Category 1: only include small and
relatively simple structures

Conti..
Geotechnical Category 2: include conventional
types of structure and foundation with no
exceptional risk or difficult ground or loading
conditions.
• Designs for structures in Geotechnical Category 2 should
normally include quantitative geotechnical data and analysis to
ensure that the fundamental requirements are satisfied.
• Routine procedures for field and laboratory testing and for design
and execution may be used for Geotechnical Category 2 designs

Conti..
The following are examples of conventional structures or parts of structures
complying with Geotechnical Category 2:
 spread foundations;
 raft foundations;
 pile foundations;
 walls and other structures retaining or supporting soil or water;
 excavations;
 bridge piers and abutments;
 embankments and earthworks;
 ground anchors and other tie-back systems;
 tunnels in hard, non-fractured rock and not subjected to special water tightness or other
requirements.

Cont…
 Geotechnical Category 3 : include structures or parts of
structures, which fall outside the limits of Geotechnical
Categories 1 and 2.
Geotechnical Category 3 should normally include alternative
provisions and rules to those in this standard(CES).
Geotechnical Category 3 includes:
 very large or unusual structures;
 Structures involving abnormal risks, or unusual or exceptionally
difficult ground or loading conditions;
 Structures in highly seismic areas;
 Structures in areas of probable site instability or persistent
ground movements that require separate
 investigation or special measures.

1.3 Evaluation of Field Tests Data
• The main goal of conducting Field tests is to
know the shear strength of the soil. Which can
be predicted determined from the test directly
or after determing shear parameters from the
field test out puts.

Conti..
• The correlation between density of frictional soils and
consistency of cohesive soils and ht/m (half-turns per meter)
for Swedish Weight Sounding Test are as given below.

Conti..
• Correlation between Cone (Point) Resistance and Relative
Density of Frictional Soils for CPT
According to Meyerhof:
N = ¼ (Ckd)
where: N = Standard penetration number
Ckd = Static Cone resistance (kg/cm2)
For sand, modulus of compressibility (ES) can be estimated
from cone resistance from the following relationship.

Correction of Standard Penetration Test
several factors contribute to the variation of the standard
penetration number N at a given depth for similar soil profiles.
Among these factors are the SPT hammer efficiency, borehole
diameter, sampling method, and rod length (Skempton, 1986; Seed,
et al., 1985).
• The SPT hammer energy efficiency can be expressed as
• In the field, the magnitude of Er can vary from 30 to 90%. The
standard practice now in the U.S. is to express the N-value to an
average energy ratio of 60% (N60).

.

Correlations for N60 in Cohesive Soil
• the consistency of clay soils can be estimated
from the standard penetration number, N60.
consistency index (CI)

Conti..
Hara, et al. (1971) also suggested the following correlation between
the undrained shear strength of clay (cu) and N60
• On the basis of the regression analysis of 110 data points, Mayne
and Kemper (1988) obtained the Correlation b/n the over
consolidation ratio,(OCR) and Standatd Penetration Test( SPT)

Conti…
• Using the field test results of Mayne and Kemper (1988) and
others (112 data points), Kulhawy and Mayne (1990) suggested
the approximate correlation.
• Kulhawy and Mayne (1990) have also provided an approximate
correlation for the pre-consolidation pressure (𝝈′𝒄)of clay as

Correction for N60 in Granular Soil
• In granular soils, the value of N60 is affected by
the effective overburden pressure, (𝝈′
𝒐). For that
reason, the value of N60 obtained from field
exploration under different effective overburden
pressures should be changed to correspond to a
standard value of (𝝈′
𝒐).

Conti..

Correlation between N60 and Relative Density of Granular Soil.
Kulhawy and Mayne (1990) modified an empirical relationship for
relative density that was given by Marcuson and Bieganousky
(1977), which can be expressed as.

Correlation between Angle of Friction and Standard
Penetration Number
• Peck, Hanson, and Thornburn (1974) give a correlation between
N60 and ꬾ’ in a graphical form, which can be approximated as
(Wolff, 1989).

Conti..
• Correlation between Number of blows (N), Angle of Internal Friction and
Relative Density of Frictional Soils(Terzaghi and Peck).
• Correlation between Number of blows (N), Unconfined Compressive
Strength and Consistency of Cohesive Soils. (Terzaghi and Peck).

Conti..
The relationship between  and Dr may be
expressed approximately by the following equation
(Meyerhof):
For granular soil, containing more than 5% fine
sand and silt.
0=25+0.15Dr
For granular soil, containing less than 5% fine
sand and silt. In the equations Dr is expressed in
percent.
0=30+0.15Dr

Correlation between Modulus of Elasticity and Standard
Penetration Number

Conti..
To judge the consistency of soil from Nc values of
DCP test , the general practice is to convert Nc to N
values of SPT:
Nc = N/C
Where:
 N = blow count for SPT
 Nc = blow count for dynamic cone penetration test.
 C = Constant, lies b/n 0.8 & 1.2 when bentonite is used.
 Nc = 1.5N for depths up to 3m
 Nc = 1.75N for depths between 3m and 6m
 Nc Values need to be corrected for overburden pressure
in cohesionless soils like SPT

Data Presentation
The results of borings, samplings, penetration
tests and laboratory tests of a site are usually
plotted graphically on a sheet of drawing paper.
The graphical presentation should include.
1.A plot plan, showing the location of all
boreholes, test pits, etc and their identification
number.
2.A separate plot, showing the soil profile as
established from the drillings or test pits records.

Conti..
3. Soil profiles along given lines in the ground
surface, showing the boundaries between
identifiable soil layers, variation of thickness of
firm bottom layer, thickness of soft clay layers etc.
4. The penetration number, the unconfined
compression strength, Atterberg limits, natural
moisture content, and other appropriate
laboratory data may be shown on each boring on
the soil profile.
5. The location of ground water table should also be
shown on the soil profile.

Preparation of Boring Logs
The detailed information gathered from each borehole is presented in a
graphical form called the boring log
1. Name and address of the drilling company
2. Driller’s name
3. Job description and number
4. Number, type, and location of boring
5. Date of boring
6. Subsurface stratification, which can be obtained by visual observation of
the soil brought out by auger, split-spoon sampler, and thin-walled Shelby
tube sampler
7. Elevation of water table and date observed, use of casing and mud
losses, etc
8. Standard penetration resistance and the depth of SPT
9. Number, type, and depth of soil sample collected
10. In case of rock coring, type of core barrel used and, for each run, the
actual length of coring, length of core recovery, and RQD

Conti…
.

1.4 Geotechnical Report
Most reports have the following contents:
 Introduction: - Purpose of investigation, type of investigation
carried out.
 General description of the site: - general configuration and
surface features of the site.
 General geology of the area.
 Description of soil conditions found in bore holes (and test pits)
 Laboratory test results.
 Discussion of results of investigation in relation to foundation
design and constructions.
 Conclusion: recommendations on the type and depth of
foundations, allowable bearing pressure and methods of
construction.

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