Chapter 2 sub-surface investigation

CONTENTS
 Foundation
 Functionsof foundation
 Sub-surfaceinvestigation
 Purposeof siteexploration
 Depth of exploration
 Choice of method of site
exploration
 Methodsof siteexploration
 Soil samplesand samplers
 Bearing capacity of soil
 Methods of determining
bearing capacity of soils
 Methods of improving bearing
capacity of soils 2

FOUNDATION
• “The foundation of a building is that part of walls, piers and
columns in direct contact with the ground and transmitting
loads to the ground.”
• Every building needsafoundation of somekind.
• Because of the variety of soil, rock, and water conditions that
are encountered below the surface of the ground and the
unique demands that buildings make upon their foundations,
foundation design is a highly specialized field combining
aspectsof geotechnical and civil engineering.
3

FUNCTIONS OF FOUNDATION
• To distribute the load of the structure over a large bearing area
so as to bring the intensity of load within the safe bearing
capacity of soil.
• To load the bearing surface at a uniform rate to avoid
differential settlement.
• To prevent thelateral movement of supporting material.
• To attain alevel and firm bed for building operations.
• To increasethestability of thestructureasawhole. 4

Ground Level
Super-structure
foundation
Distribute building load to the ground
 The size and depth of a
foundation is determined by
the structure and size of a
building. It supports and the
nature and bearing
capacity of the ground
supporting it.
5

SUB-SURFACE INVESTIGATION
 The Surface And Subsurface Investigation Or Site
Exploration Is Carried Out To Collect The Information
About Physical Properties And Characteristics Of The
Subsoil Material As Well As The Details Of Other
Geological FeaturesOf TheSiteArea.

PURPOSEOFSITEEXPLORATION
The site exploration or investigation is carried out to
collect the complete details of the site for the following
purposes:
i.To fix the depth of foundation up to which it must be taken
insidetheground.
ii.To fix thevalueof safebearing capacity of thesoil.
iii.To predict the likely settlement of the selected foundation
and to maketheallowancefor thesamein thedesign . 7

i. To know theunderground water level.
ii. To select an economical and safetypeof foundation.
iii.To evaluate the earth pressure against the walls
,basements , abutments , etc. and to make the provision
against difficultiesduring construction.
8

DEPTH OF EXPLORATION
• The depth of exploration or a trial pit or bore hole will
depend upon thecharacteristics of thesoil aswell asthetype
of structure,itsshape, sizeand loading condition.
• As a thumb rule , the depth should be one and half times the
probablewidth of thefooting or 1.5 m whichever ismore.
• In case of weak soils , however , the pits or the bore holes
should be taken to a depth at which the loads can be carried
by thesoil without undesirablesettlement.
• The number and spacing of the trail pits or bore holes to be
adopted for asitewill depend upon theareaof plot aswell as
typeof structureto bebuilt.
9

CHOICEOFMETHODOFSITE
EXPLORATION
The choice of particular exploration method depends on the
following factors:
Natureof ground
Topography
Cost of exploration
10

NATUREOFGROUND
• In clayey soils, borings are suitable for deep exploration and
pitsfor shallow exploration.
• In sandy soils, boring is easy but special equipment should
be used for taking representative sample below the water
table.
• In rocks, borings are suitable in hard rocks and pits in soft
rocks. 11

TOPOGRAPHY
• In hilly country, the choice between vertical openings and
horizontal opening may depend upon geological structure, since
steeply inclined strata are most effectively explored by heading
and horizontal strataby trial pitsand borings.
• Swamps and areas overlain by water are best explored by
boringswhich may haveto beput down from afloating craft.
12

COST OF EXPLORATION
• For deep exploration, boringsarepreferred to deep shafts.
• In case of shallow exploration in soil, the choice between pits
and borings will depend upon the nature of the ground and
theinformation required.
• Particularly for shallow exploration in rock, the use of core
drilling can be justified, if number of bore holes required to
be drilled are large. Alternatively, the trial pits prove to be
moreeconomical. 13

METHODS OF SITE EXPLORATION
1) Trial pits
2) Probing
3) Subsurface soundings
4) Geo physical methods
 Seismic refraction method
 Electrical resistivity method
 Magnetic method
 Gravitational method
5) Boring
Auger boring
Auger and shell boring
Wash boring
Percussion boring
Rotary boring
14

TRAIL PITS
• Theexcavation of trial pitsisasimpleand reliablemethod.
• Thedepth islimited to 4-5m only.
• Thein-situ conditionsareexamined visually
• It iseasy to obtain disturbed and undisturbed samples
• Block samples can be cut by hand tools and tube samples
can betaken from thebottom of thepit.
• Walls of the test pit indicate four layers (1) Clayey silt (2)
Sandy silt (3) Clean sand (4) Sandy gravel 15

PROBING
• This method is suitable for soft soils such as clay , gravel and
sand. In this method , a steel bar of 25 to 40mm diameter having
a pointed end is driven in the ground until a hard strata is met .
The bar is allowed to fall vertically under its own weight or it is
driven by drop hammer .
• The bar is drawn out at some interval , so that the rough idea of
the nature of strata is obtained from the soil sticking to the sides
of the bar. The experienced persons can be engaged in the
execution of the driving operation . Fig. shows various types of
17

SUB-SURFACE SOUNDINGS
• This method consists of measuring the variation in the resistance
offered by the soil with the depth by means of a tool known as
penetrometer.
• The penetrometer may consist of a 50 mm diameter mild steel
cone fitted loosely to a steel rod or it may be a tool known as
standard split spoon sampler.
• The resistance offered by the soil to penetration is co-related
with the engineering properties of soil , such as – density ,
consistency , permeability and bearing capacity.
19

GEOPHYSICAL METHODS
Geo-physical methods are used when soil exploration is to be
carried out over large area and where speed is of prime
importance. These soil exploration methods are based on
principle that physical properties like electrical conductivity,
elasticity or seismicity, magnetic susceptibility, density etc. vary
for different types of soils. There are two soil exploration
methods of geo physical survey, namely, (1 ) Seismic refraction
method (it) Electrical resistivity method,
20

SEISMIC REFRACTION METHOD
• This soil exploration method is based on the principle that
sound waves travel faster in rock than in soil. This is on
account of the fact that velocity of sound waves is
different in different media.
• In this method shock waves are created into the soil at
ground level or at a certain depth below it, either by
striking a plate on the soil with the hammer or by
exploding small charge in the soil. The shock waves so
produced travel down in the sub-soil strata and get
21

• The refracted or radiated shock waves are picked up by the
vibration detector (also known as geophone) where the time of
travel of the shock waves gets recorded. Knowing the time of
travel of the primary and refracted waves at various geophones,
tune and distance graphs are drawn based on which it is possible
to evaluatethedepth of variousstratain thesub-soil.
• Different materials such as clay, gravel, silt rock, hard rock etc.
have characteristics seismic velocities and hence it is possible to
establish their identity in the sub-soil based on time distance
graph. 22

ELECTRICAL PROFILING METHOD
• The resistivity of a material depends upon the type of material,
its water content, density, concentration of dissolved ions, etc.
Rocks and dry soils have a grater resistivity then saturated clays.
Thisfact isused in thismethod.
24

• This soil exploration method is based on the principle that each
soil has different electrical resistivity, depending upon the type of
soil, its water content, compaction and composition. Thus
saturated soil has lower electrical resistivity as compared to loose
dry gravel or solid rock. In this method 4 electrodes are driven in
the ground at equal distance apart and in a straight line. The
distance between two electrodes being the depth of exploration or
depth up to which the ground resistance is to be measured. A
current is passed between the two outer electrodes and the
potential drop between the inner electrodes is measured by use of
25

• Theelectrical resistivity Pisgiven by
P = 2πD.V/I
Where,
P=mean resistivity(ohm/m)
D=distancebetween electrodes(m)
V=potential drop between two inner electrodes(volts)
I=current applied between two outer electrodes(Amperes)
26

BORING
 The various boring methods adopted for soil exploration are as
follows:
 Auger boring
 Auger and shell boring
 Wash boring
 Percussion boring
 Rotary boring 27

Auger Boring
This type of augers are commonly used for clayey or sandy
soils or soft soils . It can be operated manually or mechanically.
The hand operated augers are used up to 6 m depth and the
mechanically operated augers areused for greater depth up to 15
m .
The auger is vertically driven into the ground by rotating its
handleand ispressed down during theprocessof rotation .
At every 30 cm depth of penetration , the auger is drawn out
and thesamplesof soilsarecollected separately for testing .
28

Auger and Shell Boring
• In thismethod ,different typesof toolsareused for boring.
• In case of soft clay , cylindrical auger having 75 to 200 mm
diameter hollow tubewith acutting edgeat itsbottom isused .
• In case of hard clay , the shells with cutting edge or teeth at its
lower end are used ; while in case of sandy soil or sand , the
pumpsareused for boring .
• The hand operated rigs are used 25 m depth up to 200 mm
diameter and themechanical rigsareused for 50m depth.
• Thesamplesof soil arecollected at regular interval for testing .
31

Wash Boring
–A casing is driven with a drop hammer. A hollow drill rod with
chopping bit isinserted insidethecasing.
–Soil is loosened and removed from the borehole using water or
adrilling mud jetted under pressure.
–The water is jetted in the hole through the bottom of a wash
pipe and leaves the hole along with the loose soil, from the
annual spacebetween theholeand wash pipe.
–Thewater reaches theground level wherethesoil in suspension
isallowed to settleand mud isre-circulated.
33

Percussion Boring
• This method is used for rocks and soils having boulders ;
which arebroken up by repeated blowsfrom abit or chisel.
• Thus , the pulverised material is converted into slurry by
pouring water in the bore and this slurry is bailed out at
regular interval and dried for testing.
35

Rotary Boring
• Primarily intended for investigation in rock, but also used in
soils.
• The drilling tool, (cutting bit or a coring bit) is attached to the
lower end of hollow drilling rods. The coring bit is fixed to the
lower end of acore.
• There are two forms of rotary drilling, open-hole drilling and
coredrilling.
• Open- hole drilling, which is generally used in soils and weak
rock, just for advancing thehole.
37

• The drilling rods can then be removed to allow tube samples
to betaken.
• In core drilling, which is used in rocks and hard clays, the
diamond or tungsten carbide bit cuts an annular hole in the
material and an intact core enters the barrel, to be removed as
a sample. Typical core diameters are 41, 54 and 76mm, but
can rangeup to 165 mm.
38

SOIL SAMPLES AND SAMPLERS
1)Soil samples:
• Disturbed samples
• Undisturbed samples
2)Soil samplers
• Open drivesampler
• Stationary piston sampler
• Rotary sampler 43

Soil Samples
1)Disturbed samples:
A disturbed sample is a sample in which the natural structure of soil
getspartly or fully modified and destroyed . However , by taking proper
precautions, thenatural water content in soil samplecan bepreserved .
This type of soil sample should maintain the original proportion of the
variousparticles.
2)Undisturbed samples:
An undisturbed sample is a sample which the natural structure and
propertiesremain preserved .
The sample disturbance depends on the design of samplers and the
method of sampling .
The sampling tube when forced into the ground should cause as little
remoulding and disturbanceaspossible. 44

Typesof Samplers
Thesamplersareclassified as–
i.Thick wall samplers
ii.Thin wall samplers
Depending upon thearearatio. Thethick wall samplersarethose
having theareagreater than 10% .
Depending on themodeon operation , thesamplersareclassified
as–
i.Open drivesampler
ii.Stationary piston sampler
iii.Rotary sampler 45

Open Drive Sampler
• The open drive sampler is a tube open at its lower end .the
sampler head is provided with vents (valves) to permit water
and air to escapeduring driving .
• The check valve helps to retain sample , when the sampler is
lifted up .
• The tube may be seamless or it may split in two parts , which
may beknown assplit spoon sampler .
46

Stationary Piston Sampler
• It consistsof asamplecylinder and thepiston system .
• During lowering of thesampler through hole, thelower end of
the sampler is kept closed with the piston. When the desired
sampling elevation is reached , the piston rod is clamped ,
thereby keeping the piston stationary and the sampler tube in
advanced down into the soil . The sampler is then lifted up
with piston rod clamped in position .
• This type of sampler is more suitable for sampling soft soils,
saturated sands. 47

Rotary Sampler
• The rotary sampler are the core barrel type having an outer
tube provided with a cutting teeth and a removable thin
wall liner inside.
• It is used for firm to hard cohesivesoils and cemented soils
.
48

BEARING CAPACITY OFSOIL
The maximum load per unit area which the soil or rock can carry
without yielding or displacement is termed as bearing capacity of
soil.
Therearefour typesof bearing capacity.
They areasfollows;-
1.Ultimatebearing capacity of soil. 4. Net pressureintensity.
2.Allowablebearing pressure.
3.Safebearing capacity.
49

ULTIMATE BEARING
CAPACITY OFSOIL
 It is defined as intensity of loading , at the base of
foundation , at which soil supportsfailsin shear.
50

SAFE BEARING
CAPACITY
 It is defined as maximum intensity of loading that the soil
will safely carry without risk of shear failure.
 It can be obtained by dividing the ultimate bearing
capacity by acertain factor of safety.
51

NET PRESSURE INTENSITY
 It is defined as loading acting on bottom of foundation
trench in excess of weight of soil removed from the
trench.
52

ALLOWABLE BEARING
PRESSURE
 It is defined as maximum allowable net loading intensity
which can beapplied to soil taking into account theultimate
bearing capacity, the amount and kind of settlement
expected and the ability of given structure to withdraw the
settlement.
53

THEBEARING CAPACITY OF
SOILCAN BEDETERMINEDBY
THEFOLLOWING METHODS
i. Plate load test
ii. Penetration test
iii.Safe bearing capacity values given in
codes
55

PLATE LOAD TEST
• Plate Load Test is a field test for determining the ultimate
bearing capacity of soil and the likely settlement under a
given load.
• The Plate Load Test basically consists of loading a steel plate
placed at the foundation level and recording the settlements
corresponding to each load increment. The test load is
gradually increased till the plate starts to sink at a rapid rate.
The total value of load on the plate in such a stage divided by
the area of the steel plate gives the value of the ultimate
bearing capacity of soil.
• The ultimate bearing capacity of soil is divided by suitable
factor of safety (which varies from 2 to 3) to arrive at the value
of safe bearing capacity of soil. 56

For better understanding, this Plate Load Test can be sub-
divided into thefollowing heads:-
1.Test set-up
2. Testing procedure
3. Interpretation of results
4. Limitationsof thetest
57

1. Test Setup:
• A test pit is dug at site up to the depth at which
the foundation is proposed to be laid. The width of the pit
should beat least 5 timesthewidth of thetest plate.
• At the centre of the pit a small square depression or hole is
made whose size is equal to the size of the test plate and bottom
level of which correspondsto thelevel of actual foundation. The
depth of the hole should be such that the ratio of depth to width
of the loaded area is approximately the same as the ratio of the
actual depth to width of the foundation.58

59
• The mild steel plate (also known as bearing plate) used in the
test should not be less than 25 mm in thickness and its size may
vary from 300 to 7 50 mm. The plate could be square or circular in
shape. Circular plate is adopted in case of circular footing and
square plate is used in all other types of footings. The plate is
machined on sideand edges.
2. Testing Procedure:
Theload isapplied to thetest platethrough acentrally placed
column. Thetest load istransmitted to thecolumn by oneof the
following methods.
(i)By gravity loading or reaction loading method
(ii) By loading trussmethod.

(i) Gravity loading orreactionloadingmethod:
In case of gravity loading method, a loading platform is
constructed over thecolumn placed on thetest plateand test load
is applied by placing dead weight in the form of sand bags, pig
iron, concrete blocks, lead bars etc. on the platform. Many a
times a hydraulic jack is placed between the loading platform
and the column top for applying the load to the test plate – the
reaction of the hydraulic jack being borne by the loaded
platform. Thisform of loading istermed asreaction loading.
60

(ii)ReactionTruss Method:
• In case of reaction truss method, instead of constructing a loading platform,
a steel truss of suitable size is provided to bear the reaction of the hydraulic
jack.
• When the load is applied to the test plate, it starts sinking slowly. The
settlement of theplateisrecorded to an accuracy of 0.02 mm with thehelp of
sensitive dial gauges. At least two dial gauges are used to account for
differential settlement.
• The dial gauges are placed at diametrically opposite ends of the plate and
one dial gauge is mounted on independently supported references beam or
datum rod. As the plate sinks, the ram of the dial gauge moves down and the
settlement isrecorded. 62

63
• The magnitude of load is indicated on the load-gauge of the
hydraulic jack. The load is applied in regular increment of about
2KN or 1/5th of the expected ultimate bearing capacity,
whichever isless.
•Settlement should be observed for each increment of load after
an interval of 1,4,10,20,40 and 60 minutes and thereafter at
hourly intervals until the rate of settlement becomes less than
0.02 mm per hour. The maximum load to be applied for the test
should be about 15 times the expected ultimate bearing capacity
of thesoil.

3. Interpretationof Results:
Theload intensity and settlement observations of the plateload test
areplotted in theform of load settlement curves.
65

Curve I is typical for loose to medium non cohesive soils. It can
be seen that initially this curve is a straight line, but as the load
increasesit flattensout. Thereisno clear point of shear failure.
Curve II is typical for cohesive soils. This may not be quite
straight in the initial stages and leans towards settlement axis as
thesettlement increases.
Curve III istypical for partially cohesivesoils.
Curve IV istypical for purely densenon-cohesivesoil.
The safe bearing capacity is obtained by dividing the ultimate
bearing capacity by a factor of safety varying from 2 to 3. The
value of safe bearing capacity thus arrived at, is considered to be
based on creation of shear failure. 66

67
• Safe bearing capacity (SBC) based on permissible settlement. The
value of ultimate bearing capacity and hence the SBC in this case,
can be obtained from the load settlement curves by reading the
value of load intensity corresponding to the desired settlement of
test plate.
• The value of permissible settlement (Sf) for different types of
footings (isolated or raft) for different types structures are specified
in the I.S. code. The corresponding settlement of test plate (Sp) can
becalculated from thefollowing formula,
Sf = Sp {[B(Bp + 0.3) ]/[ Bp (B+ 0.3) ]}^2
Where,
B=width of footing in mm.
Bp = width of test platein mm.
Sp= settlement of test platein mm.
Sf = settlement of footing in mm.

4. Limitations of plateloadtest:
The plate load test, though very useful in obtaining necessary
information about soil for design of foundation has following
limitations,
(1 ) The test results reflect only the character of the soil located
within a depth of less than twice the width of bearing plate.
Normally the foundations are larger than the test plates, the
settlement and shear resistance of soil against shear failure will
depend on the properties of much thicker stratum. Thus the results
of test could be misleading if the character of the soil changes at
shallow depths.
68

69
(ii) The Plate Load Test being of short duration, does not give
theultimatesettlementsparticularly in caseof cohesivesoils.
(iii) For clayey soils the bearing capacity (from shear
consideration) for a large foundation, is almost same as that for
the smaller test plate. But in dense sandy soils the bearing
Capacity increases with the size of the foundation and hence the
test with smaller size test plate tends to give conservative values
in densesandy soils.

PENETRATION TEST
Thetest consistsof themeasurement of resistanceoffered by
thesoil to thepenetration.
Thus , based on the values of resistance the bearing capacity
of soil can bedetermined by using standard graphs.
In this method , a hole of 55 to 150 mm in diameter is made
in the ground with the help of suitable equipment for
conducting thetest.
70

• The test are made and the sample are taken out at every
changein stratum.
• In case of very fine or silty saturated sand, due to apparent in
resistance, theN-valuesobtained by thistest aremore.
73
• Terzaghi and peck have recommended the use of a
equivalent penetration resistance (Ns) in place of actually
observed valueof N ,whereN isgreater than 15.
Nsisgiven by theformula:-
Ns = 15 + ½ (N-15)

SAFEBEARING CAPACITY
VALUES BASEDON I.S. CODES
• The safe bearing capacity of soil should be determined on the
basisof soil testsdata.
• Peat may occur in avery soft spongy condition.
74

I.S. CODES
• IS: 456 – codeof practicefor plain and reinforced concrete.
 IS: 383 – specificationsfor fine& coarseaggregatefrom
natural sourcesfor concrete.
 IS: 2386 – methodsof testsfor aggregatefor concrete.
 IS: 2430 – methodsof sampling.
 IS: 4082 – specificationsfor storageof materials.
 IS: 2116 – permissibleclay, silt & finedust contentsin sand.
 IS: 3495 – methodsof testing of bricks.
 IS: 3025 – testson water samples.
75

IMPROVING SOIL BEARING
CAPACITY
• Sometimes, the safe bearing pressure of soil is so low that the
dimensions of the footings work out to be very large and
uneconomical. In such acase, it becomesessential to improvethe
safe bearing pressure, which can be done by the following
methods:
I.Increasing depth of foundation
II.Compacting the soil
III.Draining the soil
IV.Confining the soil
V.Grouting
VI.Chemical treatment
76

Increasing depth of foundation
• At deeper depths, the over burden pressure on soil is higher;
hence the soil is more compacted at deeper depth. As a
result it showshigher bearing capacity.
• Thisisapplicableonly for cohesionless soils.
• This method is not applicable if the subsoil material grows
wetteras depth increase.
• This method has a limited use because with increase in
depth, the weight and cost of foundation also77

Compacting the soil
• If we compact soil using appropriate method, then there will be
increase in its density and shear strength. As a result the
bearing capacity of soil also increases. Few methods of
compacting soil of them arementioned below:
• Compaction by surcharge load: The ground can be surcharged
with heavy loads from aggregates or other materialsto speed up
thesettlement and improvebearing capacity of soil 78

• Using sand piles: This method is very useful in sandy soil
or soft soils. Hollow pipes are driven in the ground, at close
interval. This results in the compaction of soil enclosed
between theadjacent pipes.
• These pipes are then gradually removed, filling and ramming
sand in the hole, resulting in the formation of sand piles.
79

• Vibration: Heavy vibratory rollers and compactors may
compact a layer of granular soils to a depth of 1 to 3 m. Using
an appropriate roller as per the soil type to move at a
specified speed.
80

• Vibro-flotation: It is a combination of vibration and jetting.
A heavy cylinder, known as vibro flot is inserted in the
ground (soil) while the cylinder vibrates, a water jet on the tip
of the vibro flot supplies a large amount of water under
pressure. As the vibro flot sinks, clean sand is added into a
crater that developson thesurface.
81

Draining the soil
• It is a well known fact that presence of water decreases
the bearing powerof soil, specially when it issaturated.
• Reason for it is low shearing strength of soil in presence
of excess water
• Drainageof thesoil resultsin:
• decrease in the voids ratio
• improvement of bearing capacity of soil
82

Confining the soil
• Sometimes the safe bearing pressure of the soil is low because
of settlements resulting due to the lateral movement of loose
granular soil.
• Such a tendency of lateral movement can be checked by
confining the soil, outside the perimeter of foundation area, by
driving sheet piles, thus forming an enclosure and confining
thesoil.
83

Grouting
• This method is useful in loose gravels and fissured rocky
strata.
• Bores holes in sufficient numbers are driven in the ground
and cement grout is forced through theseunder pressure.
• The cracks, voids and fissures of the strata are thus filled
with the grout, resulting in the increase in the bearing
value.
84

Chemical treatment
• This method of improving bearing capacity of soil is costly
and applied in exceptional cases.
• In this method, chemical solutions, like silicates of soda
and calcium chloride is injected with pressure into the
soil.
• These chemical along with the soil particles form a gel like
structureand develop acompact mass. Thisiscalled chemical
stabilization of soil and used to give additional strength to
soft soilsat deeper depths.
85

Chapter 2 sub-surface investigation

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