2. OBJECTIVE Nature of soil deposit Depth and thickness of
various soil strata Location of ground water table Collection of
soil and rock sample and determination of engineering properties
In-situ properties by field test
3. WHAT IS THE NEED ? To determine the type of foundation
required for the proposed project at the site, i.e. shallow
foundation or deep foundation. To make recommendations regarding
the safe bearing capacity or pile load capacity Ultimately, it is
the subsoil that provides the ultimate support for the
structures
4. HOW ? The three important aspect are Planning Execution
Report writing Planning To minimize cost of explorations and yet
give reliable data. Decide on quantity and quality depending on
type, size and importance of project and whether investigation is
preliminary or detailed
5. HOW (Contd..) Execution: Collection of disturbed and/or
undisturbed samples of subsurface strata from field. Conducting
in-situ tests of subsurface material and obtaining properties
directly or indirectly. Study of ground water conditions and
collection of sample for chemical analysis. Laboratory testing on
samples
6. HOW (Contd..) Report writing: Description of site conditions
topographic features, hydraulic conditions, existing structures,
etc. supplemented by plans/drawings. Description of nature, type
and importance of proposed construction Description of field and
lab tests carried out. Analysis and discussion of data collected
Preparation of charts, tables, graphs, etc. Calculations performed
Recommendations
7. SITE INVESTIGATION A complete site investigation will
consist of : Preliminary work Collecting general information and
already existing data such as study of geologic , seismic maps,
etc. at or near site. Study site history if previously used as
quarry, agricultural land, industrial unit, etc. Site
Reconnaissance: Actual site inspection. To judge general
suitability various tests and Decide exploration techniques
8. METHODS OF EXPLORATION Direct methods Trial pits or Trenches
Semi-direct methods Borings Indirect methods Penetration tests
Geophysical methods
9. DIRECT AND SEMI DIRECT METHODS Test pits Adopted for minor
structures, Small buildings Suitable upto small depths (< 2m)
Geotechnical characteristics of disturbed samples Boring Used for
exploration at greater depths where direct methods fail Provide
both disturbed as well as undisturbed samples depending upon the
method of boring
10. BORING (Contd..) The different types of boring methods are
: Auger boring Wash boring Rotary drilling Percussion drilling
11. AUGER BORING This method is fast and economical Suitable
for soft to stiff cohesive soils Soil sample collected is disturbed
Not suitable for very hard or cemented soils, very soft soils, as
then the flow into the hole can occur
12. AUGER BORING (Contd..) Used for advancing borehole into the
ground Suitable when the borehole is kept dry and unsupported Hand
operated (3-5 m) or power driven Post hole and helical auger
Rotating and pressing mechanism Testing of samples collected in the
Auger
13. AUGER BORING (Contd..)
14. POWER DRIVEN AUGER BORING
15. WASH BORING Suitability Exploration below GWT For all soils
except gravel and boulders Technique Making of hole for short depth
using an auger Installation of casing pipe (manually/power)
Installation of tripod, pulley and winch Hollow drill bit screwed
to a hollow drill rod connected to a rope passing over a pulley The
hollow drill bit assembly placed in the hole Water is forced
through the swivel and the drilling rod using a pump the rods are
rotated and also moved up and down either manually or with a
mechanized rig The loosened soil comes out of the borehole in the
form of slurry through the annular space between the rod and side
of hole The suspension is led to a settling tank where the soil
particle settle Water collected in the sump is re-circulated
again
16. WASH BORING (Contd..)
17. WASH BORING (Contd..) Limitation Very disturbed sample
Cannot be used for evaluation of engineering properties Sample can
be extracted by replacing the drilling bit with a sampler
18. PERCUSSION DRILLING Suitable for hard soil and soft rock
where auger boring and wash boring can not be employed Hole is made
using auger Installation of casing pipe Heavy drill bit called
Churn bit is attached to drill rods Bore hole is extended by
repeated blows of the drill bit using winch system Water is forced
through the drill rod assembly for breaking stiff soil or rock
Slurry removed by bailers Method cannot be used in loose sand and
is slow in plasticity Formation gets badly disturbed by impact
19. PERCUSSION DRILLING (Contd..)
20. ROTARY DRILLING Suitable for rock strata and can also be
used for sands and silt Method is fast in rock formation Drill bit
fixed to drill rod is rotated by power The soil collected in the
drill bit can be removed and boring is continued Water/Bentonite
slurry can be forced under pressure through the drill rod Rock
cores may be obtained by using suitable diamond drill bit
21. PLANNING AN EXPLORATION PROGRAMME Include: Site plan of the
area A layout plan of proposed structure with column location and
expected loads Location of bore holes and field tests SPT, Vane
shear test Planning of other field tests SCPT, DCPT, Plate Load
Test Bore log data and different laboratory tests for evaluation of
strength and compressibility characteristics of different soil
Grain size, Specific gravity, Plasticity, Triaxial shear test,
consolidation test
22. PLANNING AN EXPLORATION PROGRAMME (CONTD.) Spacing of
boring: depends on type, size, weight of proposed structure Extent
of variation in soil conditionSl. No Name of Project Spacing , (m)
1 Highway 300-600 2 Earth dam 30-60 3 Borrow pit 30-120 4
Multistory building 15-30 5 Single story building 30-90
23. PLANNING AN EXPLORATION PROGRAMME (CONTD.) Depth of boring:
Type of structure Should penetrate all strata that could
consolidate For bridge and tall building the boring should extend
to rock 1.5 times the width of footing below foundation level For
embankment and dam 0.5 2 times the height Single storey 3.5 m,
Double storey 6.5 m
24. SOIL SAMPLING In general soil samples are categorized as
shown in figure
25. SOIL SAMPLING (Contd..) Disturbed sample : In such sample
natural soil structure is modified or destroyed If water content
and mineral content are also modified then it is a
non-representative or remolded sample If water content and mineral
content are not modified then it is representative sample
Undisturbed sample : Natural soil structure, water content and
mineral content are preserved
26. SOIL SAMPLING (Contd..) For the purpose of atterbergs
limit, specific gravity, grain size analysis either representative
or undisturbed sample should be used Undisturbed sample are desired
for coefficient of permeability, consolidation parameter and shear
strength parameter,
27. FIELD METHODS TO DETERMINE BEARING CAPACITY OF SOIL Plate
load test Standard penetration test (SPT) Static cone penetration
test (SCPT) Dynamic cone penetration test (DCPT) Field vane shear
test
28. PLATE LOAD TEST Significance Determination of allowable
bearing capacity of sub soil Suitable for gravel/boulder strata
when SPT and DCPT does not give dependable results Also used to
determine the modulus of subgrade reaction (K) useful for design of
pavements Apparatus Bearing plates Square MS plates( 30, 45, 60 ,75
and 100 cm) Circular plates (30 cm , 75 cm dia.) Dial gauge (0.01
mm) Magnetic base Hydraulic jack stop watch Spirit level
29. PLATE LOAD TEST (Contd..) Test location Conducted at
proposed foundation level If GWT is above test level it is lowered
down to test level Selection of test plate Side of test plate
atleast equal to 4 times the maximum size of particle at the test
level Circular plates are used for circular footings and road
pavement General size of plates are 30 cm, 45cm, 60cm, 75 cm or 100
cm Test pit Excavated upto the level of foundation for proposed
structure
30. PLATE LOAD TEST (Contd..)
31. PLATE LOAD TEST (Contd..) Procedure of test Application of
seating load of 0.07 kg/cm2 Load removed after 5-10 min, Dial gauge
is set to zero The load is then applied in cumulative in equal
increment of 0.5 to 1 kg/cm2 For each increment, settlement are
noted down at different time interval of 1, 2.25, 4, 6.25, 9, 16,
25 min, 1hr, 2hr, till the rate of settlement is not more than 0.02
mm/min, not less than 1hr for sandy gravelly soils In clays, the
settlement measures are taken for 24 hours for each load increment
Application of next higher load and the process is repeated Test is
continued till a total settlement of 25 mm under normal condition
or 50 mm (under special condition like dense gravel, gravel-sand
mixture) is reached or till failure occurs whichever is earlier
Calculation of final settlement corresponding to each loading
intensity (Average of settlement of all three or four dial gauge
readings) When settlement does not reach 25 mm continue the test
till
32. STANDARD PENETRATION TEST (SPT) Significance Determination
of in-situ parameters of soil Determination of bearing capacity
Apparatus Augers, Split spoon sampler, Drive weight assembly,
A-rods, Tripod(fitted with winch and pulley), Boring guide
Procedure Bore hole with hand auger up to about 1.5 m depth sampler
is seated through 150 mm by giving blows of 63.5 kg hammer falling
freely through 750 mm height Number of blows for 150 mm penetration
is counted Sampler further driven by 300 mm and number of blows are
recorded for each 150 mm penetration The number of blows for first
150 mm penetration are discarded The total blows for the second and
third 150 mm penetration are recorded as Penetration resistance
(N-value)
33. STANDARD PENETRATION TEST (Contd) Collection of soil sample
Sample is taken out of borehole and is opened If sandy material,
the sample collected in polythene bag, in case of clay it should be
steel tube The tube is sealed with wax to avoid evaporation of soil
mixture Corrections of N Correction of overburden pressure
Correction factor Nc= 0.77 log 20/po Correction for dilatency
(N>15) Ncor = 15 + (Nc-15)
34. UTILITY OF NC VALUES Estimation of geotechnical properties
Estimation of allowable bearing capacity by settlement
criteriaCohesive soil Cohesionless soil
35. STATIC CONE PENETRATION TEST Significance Most useful where
soil properties gets disturbed by boring/blows Useful on very soft
and loose soils where transportation of heavy equipments required
for SPT and DCPT test may not be possible Useful for determination
of bearing capacity at different depths below foundation level Skin
friction values required to be used for determining the length of
piles
36. STATIC CONE PENETRATION TEST (Contd) Apparatus Steel cone
Friction jacket Sounding rods Sounding tubes/mantle tubes Driving
and measuring instrument Capacity of equipment available 3 ton 10
ton 20 ton
37. STATIC CONE PENETRATION TEST (Contd) Recording of cone and
friction jacket resistance Determination of cone penetration
resistance Determination of ultimate bearing capacity
38. FIELD VANE SHEAR TEST Significance Determination of in-situ
shear strength of saturated clay of very soft to medium consistency
Difficulty in sampling and underestimate of shear strength of such
soil in laboratory Undrained strength both in undisturbed and
remoulded samples are obtained for estimating the sensitivity of
the soil
39. FIELD VANE SHEAR TEST (Contd..) Vane Four mutually
perpendicular blades, L =2D Dia. 37.5 mm, 50 mm ,75 mm or 100 mm
Dia of central rod to which the blades are welded >12.5 mm
Torque applicator Attachment to secure the string of rods
connecting the vane Speed control (0.1o/sec) Rod system The rods
connect the vane to the torque applicator Rods are 1m and can
resist the maximum torque
40. FIELD VANE SHEAR TEST (Contd..) Procedure Bore hole is made
by suitable method up to the required depth Casing may be driven
upto the full depth of the hole Vane is then connected to 1m
extension rods as required The assembly is then lowered into the
borehole The vane is then pushed inside bottom surface to a depth 5
times the dia of the hole The assembly is then connected to torque
applicator through a connector Vane is rotated at the rate of
0.1o/s after a minimum period of five minutes The vane is rotated
till shear failure or when the torque indicator dial gauge moves
back Note the maximum divisions of the torque application Using the
calibration chart provided by the manufacturer convert the reading
into Torque cm-kg
41. FIELD VANE SHEAR TEST (Contd..) Sensitivity After the
determination of maximum torque, the vane is rotated rapidly
through a minimum of ten revolutions Remoulded strength is
determined within one minute after completion of revolutions
Sensitivity = Undisturbed strength/ Disturbed strength
42. References Geotechnical investigation for road work by Shri
Sudhir Mathur, CRRI, New Delhi www. nptel.ac.in
www.cdeep.iitb.ac.in
