3.0 soil exploration

SOIL EXPLORATION
Prof. E.Saibaba Reddy
B.Tech, M.E.(Hons) Roorkee, Ph.D (Nottingham, UK)
Post Doc,(Halifax Canada), Post Doc (Birmingham UK)
&
Eadala Rakesh Reddy
B.Tech (JNTUH), M.Tech (VSSUT-Gold Medal),
(Ph.D)- Andhra University-DST-Inspire Fellow
Chief Consultant –EE Engineering Construction Services

COMMON STAGES IN INVESTIGATION
 Desk Study
 Site Reconnaissance
 Field Investigation- a) Preliminary b) Detailed
 Laboratory Testing
 Report Writing
 Follow up Investigations during design & construction’
 Appraisal of performance

SOIL INVESTIGATION
 Determination of surface and subsurface soil conditions and features
in an area of proposed construction that may influence the design
and construction and address expected post construction problems.
 SCOPE OF INVESTIGATION:
 Simple visual examination of soil at the surface or from shallow test
pits.
 Detailed study of soil and groundwater to a reasonable depth
(influence zone) by sampling from bore holes, shafts and audits and
in-situ and laboratory tests.

PURPOSE OF EXPLORATION
 To determine the general suitability of the site.
 To find the nature of each stratum and engineering properties of the soil and rock, which
may affect the design and mode of construction of the proposed structure and foundation.
 To find out the sources of construction material (backfilling, sub grade for roads).
 To ensure the safety of surrounding existing structures.
 To locate the ground water level and possible corrosive effect of soil and water on
foundation material.
 To predict the settlements.
 Selection of suitable construction technique.
 Selection of type and depth of foundation.

PLANNING OF EXPLORATION
 Exploration program involves location and depth of borings, test pits or other
methods to be used, and methods of sampling and tests to be carried out to
determine the stratification and engineering properties of the soils underlying
the site. The principal properties of interest will be the shear strength,
deformation and hydraulic characteristics of soil.
 The planning of a subsurface exploration program includes the following
steps:
 Gather all available information
 Reconnaissance
 Preliminary Exploration
 Detailed Exploration

GATHER ALL AVAILABLE INFORMATION
 Assemble all information on dimensions, column spacing, type and
use of structure, basement requirements, and any special
architectural considerations of the proposed building.
 For bridges the Geotechnical Engineer should have access to type
and span lengths as well as pier loadings. This information will
indicate any settlement limitations, and can be used to estimate
foundation loads.

RECONNAISSANCE
 This step includes visual inspection carried out at site without drilling
bore holes to reveal surface and subsurface information. This
includes:
 Collection of information about adjacent sites and structures.
 Ground water levels that can be determined by checking nearby
levels.
 Type of vegetation.
 The general topography of the site, the possible exsistence of
drainage trenches.
 Soil stratification from deep cuts.

PRELIMINARY EXPLORATION
 In this step few borings are made to establish in a general manner:
 To know the stratification, types of soil to be expected, and possibly
the location of the groundwater table.
 If the initial borings indicate the upper soil is loose or highly
compressible, one or more borings should be taken to rock, or hard
strata.
 A feasibility exploration program should include collection of enough
site data and sample recovery to approximately determine the
properties of soil, foundation design and identify the construction
procedures.
 To find the thickness and composition of each soil layer.

DETAILED EXPLORATION
 Here we make a detailed planning for soil exploration in the form of trial pits
or borings, their spacing and depth. Accordingly, the soil exploration is carried
out.
 The details of the soils encountered, the type of field tests adopted and the
type of sampling done, presence of water table if met with are recorded in the
form of bore logs.
 The soil samples are properly labeled and sent to laboratory for evaluation of
their physical and engineering properties.
 The report is prepared with clear description of the soils at the site, methods
of exploration, soil profile, test methods and results and the location of the
ground water. This should include information and/or explanations of any
unusual soil water bearing stratum and soil and ground water condition that
may be troublesome during construction.

DEPTH OF EXPLORATION
 Generally soil exploration should be advanced to a depth up to which
the increase in pressure due to structural loading will have no
damaging effect (such as settlement & shear failure) on the structure.
In other words, the depth at which soil does not contribute settlement
of foundation. This depth is termed as significant depth.
Significant
Depth

FACTORS AFFECTING SIGNIFICANT DEPTH
 Type of Structure
 Weight of Structure
 Dimension of Structure
 Disposition of the loaded area
 Soil profile and layer properties

THUMB RULES TO DECIDE THE DEPTH OF
SOIL EXPLORATION

THUMB RULES TO DECIDE THE DEPTH OF
SOIL EXPLORATION
B – Width of Foundation
L – Length of Foundation

SOIL INVESTIGATION - TRIAL PITS
 They are excavated at site for inspection of strata so as to provide
necessary working space. According to IS 4453:1967, a clear
working space at the bottom of the pit should be 1.2 m x 1.2 m.
 Shallow pits (up to 3m) do not require lateral support. For depth
greater than 3m and GWT arises then lateral support in the form of
sheeting and bracing is required.
Trial Pit Excavated

TRIAL PIT WITH BRACING & SHEETS

TRENCHES
 They can be defined as long shallow pits. They are continuous
over a considerable length and provide exposure along a line.
On slopes trenches are more suitable than pits.

TRENCH DUG PROVIDED WITH ANCHORS

DRIFTS AND SHAFTS
 Drifts: They are the horizontal tunnels made in the hill sides to
determine the nature and structure of the geological strata.
According to IS 4453-1980 a drift should be 1.5 m wide and 2
m height in hard rock. In soft arched roof can be provided.
 Shafts: Large size vertical holes made in geological formation
are called shafts. Shafts are done usually for depth greater
than 4 m.
 For Circular Diameter = 2.4 m (min)
 For Rectangular Width = 2.4 m

DRIFT (HORIZONTAL EXPLORATION)

DISADVANTAGES OF WASH BORING
 Highly disturbed samples
 Finer particles (clay, loam etc.) wont settle, and heavier particles not
brought up
 Exact strata identification not possible due to mixing of soil particles
 Slow in c. grained and stiff soils
 Not effective in hard soils, rocks , boulders
 Can be used in most type of soil but the progress is slow in coarse gravel strata
 Some indications about strata from slurry colour and drill penetration resistance
 It is only used for advancing the borehole to enable tube samples to be taken or
field test to be carried at the hole bottom.

FEATURES MEASURING DEGREE OF
DISTURBANCE

DESIGN FEATURES AFFECTING DEGREE OF
DISTURBANCE

OPEN TUBE SAMPLER/SHELBY TUBES
(IS:2132-1986)

SPLIT SPOON SAMPLER (IS 9640-1980)

HANDLING, PRESERVATION AND
TRANSPORTATION OF SAMPLES

STANDARD PENETRATION TEST
(IS 2131-1986)

SPT CORRELATIONS FOR COHESIONLESS
SOILS

POINT CONE RESISTANCE VS. SPT
CORRELATION

PLATE LOAD TEST AND STANDARD
REFERANCE (IS:1888-1982)
i) Plate Load Test is a field test for determining the
ultimate bearing capacity of soil and the likely
settlement under a given load.
ii) Circular or square bearing plates of mild steel not less
than 25mm in thickness and varying in size from
300 - 750mm.
iii) The subgrade modulus is defined as the load intensity ‘p’
applied on the standard plate per unit deflection i.e.
k=p/d, value of d=1.25mm. The test load is gradually
increased till the plate starts to sink at a rapid rate.
iv) 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 safebearing capacity of soil.

APPLICATIONS OF ELECTRICAL PROFILING

APPLICATION OF RESISTIVITY SOUNDINGS

APPLICATION OF ELECTRICAL RESISTIVITY
STUDIES

ELECTRICAL RESISTIVITY INSTRUMENTS

ELECTRICAL RESISTIVITY MEASUREMENTS

ELECTRICAL RESISTIVITY METHOD- PROS
AND CONS

SCHEMATIC REPRESENTATION OF
REFRACTION METHOD

ROLE OF GEOPHYSICAL METHODS IN
SOLVING GEOTECHNICAL PROBLEMS

CONTENTS OF A SUBSOIL INVESTIGATION
REPORT

SUBSOIL EXPLORATION REPORT –
GRAPHICAL REPRESENTATIONS

3.0 soil exploration

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