TABLE OF CONTENTS
 Objectives
 Stages in Sub-Surface Exploration
 Extent of Exploration
 Methods of Exploration
 Samplers
 Geophysical Methods

OBJECTIVES
 To classify the soil
 To check the properties of soil
 To select type and depth of foundation
 To check the water level beneath the ground
 To check the site preferred for “Borrow Pits”
 To identify the stratification of soil below ground
surface
 To investigate safety of existing structures

STAGES IN SUB-SURFACE
EXPLORATION
o Reconnaissance
o To know general topography and existence of drains
o To observe settlement cracks in existing structure
o To observe evidence of landslides
o Location of high flood marks
o To know about drainage pattern and type of vegetation
o Preliminary Exploration
• To determine the depth,thickness,extent and composition of each
soil stratum at the site.
• The depth of the bed rock and the ground water is also
determined.

STAGES IN SUB-SURFACE
EXPLORATION
o Detailed exploration
• To determine the engineering properties of the soils in different
strata.
• It includes an extensive boring programme, sampling and
testing of sample in a laboratory.
• For complex project involving heavy structures such as bridge,
dams, multi-storey builiding , it is essential to have detailed
exploration.
• For small projects ,detailed investigation may not be require.

EXTENT OF EXPLORATION
The Depth of soil exploration is carried out upto
“Significant Depth” i.e. where stresses due to the
loading are reduced to 20%. Depth of soil exploration
depends on:
 Type of structure
 Depth and type of foundation
 Height of embankment
 Scour Depth in case of flowing water
 Capillary Rise

EXTENT OF EXPLORATION
Minimum 5 boreholes are required for an area of about
0.4 hectares that can be reduced to 1 in case of
compact sites. The spacing of boreholes is kept as
following:
 10 to 30 m depending upon subsurface condition
 150 to 300 m incase of highways
 40 to 80 m in case of concrete dams

METHODS OF EXPLORATION
 Direct Methods
 Trenches and Pits
 Drifts and Shafts
 Boring
 Auger Boring
 Wash Boring
 Rotary Drilling
 Percussion Drilling
 Core Boring
 Indirect Methods
 Seismic Method
 Resistivity Method

Rotary Drilling by MAIT Machine

Trenches and pits are excavated at the site to inspect the
strata. The size of the pit should be sufficient to provide
necessary working space IS 4453-1967 recommend a
clear working sapce of 1.2 *1.2 m at the bottom of pit.
Trenches and pits are normally preferred in case depth of
exploration does not exceed 6m. Shallow pits upto a
depth of 3m.
Drifts and Shafts. Drifts are horizontal tunnels made in
the hill side to determine the nature and structure of the
geological formation. IS:4453-1980 recommends that a
drift Should have the minimum clear dimensions of 1.5
m width and 2.0 m height in hard rock.

In soft rock an arch roof is more advantages tha a flat roof.
Shafts are large size vertical holes made in the geological
formation. These may be rectangular or circular in section.
The min width of a rectangular shaft is 2.4 m and circular
shaft min dia is 2.4 m. Shaft are also used to extend the
exploration below the river bed. In other hand Boreholes are
meant for exploring the soil where significant depth is more
than 6m. In this method the sample of soil which is collected
with the help of samplers, is tested in laboratory

 Auger Boring preferred for maximum depth of 12 m.
 Auger boring is not used when there are large cobbles,
boulders or other obstructions are encountered.
 In wash boring hole is advanced by chopping bit and
water jet.
 Wash Boring is not preferred in hard soils, rocks or soils
containing boulders.
 Rotary Drilling make use of cutting bit by rotation
 Rotary Drilling is not preferred for soils containing large
percentage of gravels as they start rotating along with bit.

 In Percussion method a heavy chisel is alternately
lifted and droped in a vertical hole.
 The material gets pulverised.
 Percussion Drilling is preferred where soils consist of
rocks or boulders.
 Percussion Drilling is also used for constructing
Tube Wells.
 Core Drilling is preferred to collect the core by
rotating the hollow drilling rod.

SAMPLERS
Types of Samples:
 Disturbed Soil Sample.
1. Change in stress condition.
2. Change in water content and void ratio.
3. Disturbance of the soil structure.
4. Chemical changes.
5. Mixing and segregation of soil constituents.
 Undisturbed Soil Sample
1. No change in stress condition.
2. No change in water content and void ratio.
3. No disturbance of the soil structure.
4. No change in constituents and Chemical properties.

SAMPLERS
Requirement of Good Sampler:
Considering different types of soil samplers available
in the market to collect soil samples while boring, a
good sampler should have following properties:
 The inside clearance should be 0.5% to 3%
 Sands, silts, soft clay should have 0.5%
 Stiff and hard clays below water table should have 1.5%
 Stiff expansive clays should have 3%
 The Area Ratio should be less than 10%
 The Outside Clearance should be less than 2%

SAMPLERS
Requirement of Good Sampler:
 Maximum Inside diameter should be 75mm
 Cutting edge angle should be less than 20o
 The sampler tube should be uniform and smooth
 Recovery Ratio should be 96% - 98%
 Inside wall should be properly oiled to reduce wall
friction

SAMPLERS
Types:
 Split Spoon Sampler
 Stationery Piston Sampler
 Rotary sampler

SAMPLERS
 Split Spoon Sampler
• It consists of tool-steel driving shoe at the bottom, a steel tube
(that is split longitudinally in to halves) in the middle, and a
coupling at the top.
• The steel tube in the middle has inside and out side diameters of
34.9mm and 50.8mm, respectively.
• When the bore hole is advanced to a desired depth, the drilling
tools are removed.
• The split-spoon sampler is attached to the drilling rod and then
lowered to the bottom of the bore hole. The sampler is driven into
the soil at the bottom of the bore hole by means of hammer blows.

SAMPLERS
• The hammer blows occur at the top of the drilling rod. The
hammer weights 623N. For each blow, the hammer drops a
distance of 0.762m.
• The number of blows required for driving the sampler through
three 152.4mm interval is recorded.
• The sum of the number of blows required for driving the last two
152.4mm intervals is referred to as the standard penetration
number; N. it is also commonly called the blow count.
• After driving is completed, the sampler is with drawn and the shoe
and coupling are removed.
• The soil sample collected inside the split tube is then removed and
transferred to the laboratory in small glass jars. Determination of
the standard penetration number and collection of split-spoon
samples are usually done at 1.5m.

 Stationery Piston Sampler
 A piston sampler consists of a thin walled tube with a piston
inside.
 The piston keeps the lower end of the sampling tube closed
when the sampler is lowered to the bottom of the hole.
 After the sampler has been lowered to desire depth , the piston
is prevented from moving downwards by a suitable arrangement
, which differs in different types of piston sample.
 The thin tube sampler is pushed past the piston to obtain the
sample.
 The piston remains in close contact with the top of the sample.
SAMPLERS

SAMPLERS
• The presence of the piston prevents rapid squeezing of the soft soil
into the tube and reduce the disturbance of the sample.
• A vacuum is created on the top of the sample, which helps in
retaining the sample.
• During the withdrawal of the sampler, the piston provides
protection against the water pressure otherwise would have
occurred on the top of sample.
• Piston sampler are used for getting undisturbed soil sample from
soft and sensitive clays.

SAMPLERS
 Rotary sampler
 A Rotary Sampler is a double –walled tube sampler with an inner
removable liner.
 The outer tube or the rotating barrel is provided with a cutting bit.
 The bit cuts an annular ring when the barrel is rotated.
 The inner tube which is stationary, slides over the cylindrical
sample cut by the outer rotating barrel. The sample is collected in
the inner liner.
 Rotary samples are useful for sampling in firm to hard cohesive
soils and particularly in rocks

GEOPHYSICAL METHODS
 Resistivity method
 Limitations
 Valid only for strata having different electricalresistivity
 Results are influenced by surface irregularities, wetness of strata
 Expertise isrequired
 Electrical resistivity changes gradually rather abruptly as assumed
 Seismic Method
 Limitations
 Cannot be used if harder surface overlies soft layer
 Cannot be used for areas covered by concrete or asphaltic pavement
 Cannot be used when surface is frozen
 Requires costlyequipment
 Expertise isrequired

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