SNPIT & RC,

UMRAKH

Guided By:- Proff. Krunal A. Shah

Subject:- Foundation Engineering

Topic:- Sub Soil Exploration

Nagma Modi

Mrunali Mehta

Prakruti Pathak

Hirvi Vimawala

Sagar Padhiyar

130490106065

130490106064

130490106094

130490106121

140493106015

Prepared by…

SUBSOIL

EXPLORATION

UNIT 1

Sub soil Exploration???

The process of collection soil data for the assessment soil properties at a site through series of laboratory and field investigation is collectively called Sub-soil Exploration

Enables the engineers to draw soil profile indicating the sequence of soil strata and the properties of soil involved.

Main Objectives

Determination of

Nature of Soil deposit

Depth and Thickness of soil strata

Horizontal extent of soil deposit

Depth of GWT and its fluctuations

Engineering properties of soil

Insitu soil properties

Collection of soil and rock sample

Introduction

WHAT?

Attempt at understanding the subsurface conditions such as:

Soil and rock profile

Geological features of the region

Position and variation of ground water table

Physical properties of soil and rock

Contamination, if any

General data of adjacent structures, hydrological data, topography, soil maps, seismicity, etc.

Engineering properties of soil

Introduction

WHY?

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.

Failures

Leaning Tower of Pisaand Sinkholes

Introduction

HOW?

The three important aspect are planning, execution and 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.

Introduction

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.

Geophysical exploration, if necessary.

Laboratory testing on samples

Introduction

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

Introduction

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

Decide exploration techniques

Introduction

Exploration

Preliminary Investigations: Exploratory borings or shallow test pits, representative sampling, geophysical investigations, etc

Detailed Investigations: Deep boreholes, extensive sampling, in-situ testing, lab testing, etc.

Depth and spacing: In general, depth of investigation should be such that any/all strata that are likely to experience settlement or failure due to loading. Spacing depends upon degree of variation of surface topography and subsurface strata in horizontal direction. Refer to AlamSingh.

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Boring Logs

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Methods of soil Exploration

Exploration methods

Direct Methods Semi Direct In Direct Methods

Test pits, Trial pits, Trenches

Borings•Auger•Auger and shell•Wash Boring•Percussion drilling•Rotary Drilling

Sounding or penetration

Tests and Geophysical

methods

Test pits

Depth upto 3m

Uneconomical at greater depths.

Supports are required at greater depths. Especially in case of weak strata

Problems with GWT and the same should be lowered

Open type Exploration

Soils are investigated in natural condition

Soil samples are collected for determining strength and Engineering properties

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1

2

3

4

Walls of the test pit indicate four layers (1) Clayey silt (2) Sandy silt (3) Clean sand (4) Sandy gravel

Stratigraphy and Finds

Layer Soil Soil Colour Finds ChronologyL1 Sandy soil Gray 7.5YR 5/1 Modern Rubbish (filled soil) 1980s

L2 Sandy soil Pinkish white 7.5YR 8/2 Modern rubbish (filled soil) 1980s

L3 Sandy soil Reddish yellow 7.5YR 7/6 Modern rubbish (filled soil) 1980s

L4 Sandy soil Gray 7.5YR 6/1 Modern rubbish (filled soil) 1980s

L5 Loamy soil Reddish yellow 5YR 6/6 Nil (original decomposed soil)

L6 Loamy soil Reddish yellow 5YR 6/8 Nil (original decomposed soil)

L7 Loamy soil, with

some

decomposed

bed rock texture

Light red 2.5YR 6/8 Nil (original decomposed soil)

Test Pit Wall Photograph

Western Wall Section

Test Pit Wall Drawing

Western Wall Section Drawing

Excavated test pit

Boring

Drilling a hole into the soil strata upto specified depth is known as boring

1. Auger boring

2. Auger and shell boring

3. Wash boring

4. Percussion drilling

5. Rotary drilling

Auger Boring

Drilling is made using a device called Soil Auger

Power driven (upto 3 to 5m) and Hand operated (Greater than 5m)

Advancement is made by drilling the auger by simultaneous rotating and pressing it into the soil

Dry and unsupported bore holes

When the auger gets filled with soil same, it is taken out and the soil sample collected

Soil augers

Auger and Shell Boring

Casing is provided in case of weak strata

First the casing is driven and then the auger

Boring rig is used for power driving (hand rig for depth upto 25 m)

Soft rocks are broken using chisel bits

Sand pumps are used in the case of sandy soils.

Disadvantage:

Whenever the casing is to be extended, the auger has to be withdrawn which hinders the quick progress of the work.

Wash Boring

Below GWT. May not be used for soils mixed with gravel and boulders

Initially, the hole is advanced for a short depth by using an auger.

Then a casing pipe is pushed in and driven with a drop weight. The driving may be with the aid of power.

A hollow drill bit is screwed to a hollow drill rod connected to a rope passing over a pulley and supported by a tripod.

Water jet under pressure is forced through the rod and the bit into the hole.

This loosens the soil at the lower end and forces the soil-water suspension upwards along the annular surface between the rod and the side of the hole

This suspension is collected in a settling tank.

Soil particles are allowed to settle down and water is allowed to overflow into a sump which is then recirculated

Very disturbed sample is obtained. Hence cannot be used for determining engineering properties.

whenever a soil sample is required, the chopping bit is to be replaced by a sampler.

The change of the rate of progress and change of colour of wash water indicate changes in soil strata.

Typical set up for Wash boring

Percussion Drilling

A heavy drill bit called ‘churn bit’ is suspended from a drill rod or a cable and is driven by repeated blows.

Water is added to facilitate the breaking of stiff soil or rock.

The slurry of the pulverised material is bailed out at intervals.

Disadvantages

Cannot be used in loose sand and is slow in plastic clay.

The formation gets badly disturbed by impact.

Rotary Drilling

Suitable for rock formations.

A drill bit, fixed to the lower end of a drill rod, is rotated

by power while being kept in firm contact with the hole.

Drilling fluid or bentonite slurry is used under pressure which brings up the cuttings to the surface.

Even rock cores may be obtained by using suitable diamond drill bits.

Disadvantage

Not used in porous deposits as the consumption of drilling fluid would be high.

Indirect methods

Sounding or penetration Tests and

Geophysical methods

SPT “IS: 2131-1986—Standard Penetration Test”.

Generally used for cohesionless soils

To determine relative density , angle of shearing resistance, UCC

A bore hole is made using drilling tools and a hammer of weight 63.5 falling from the height of 750 mm at the rate of 30 blows/minute

After reaching the specified depth, the drilling tool is replaced by a split spoon sampler to collect soil sample.

First 150 mm penetration is taken as seating

drive and the no. of blows required for that

penetration is discarded

No of blows required for next 300mm

penetration after seating drive is taken as

standard penetration number (N)

No of blows greater than 50 are taken as

refusal and the test is discontinued

Corrections are applied to the observed N

value

Correction to N value

Dilatancy Correction

Overburden correction

Of these, overburden correction is applied first and to that corrected value, dilatancy Correction is applied

Dilatancy Correction

Due to the presence of fine sand and silt below the water table, negative pore pressure develops which increases, the observed N value. Hence correction is applied. (If N’<15 or N=15 , N’ = N)

Over burden correction

Soils having the same relative density will show higher N value at greater depth due to presence of over burden.

Cohesionless soils are greatly affected by confining pressure. Hence N value is corrected .σ <=280 kN/m2

SPT correlations for cohesionless soil

SPT correlations for Clays

This method is also used to skin friction values which is used to determine the length of the piles

The cone is pushed only by thrust and not by driving

In order to find out the cone resistance , the cone alone is pushed

Later the cone and sleeve is pushed together to find out the combined frictional and point resistance of the cone.

Hydraulic gauges are used for measuring pressure developed

Frictional resistance = Combined resistance –Cone resistance

Modified Cone penetrometer is known as Refined Dutch Cone

Cone penetration resistance is denoted as qc in kN/m2

Unlike SPT, this method is also suitable for clayey deposits

Unsuitable for gravels and dense sand. For such soil dynamic Cone penetration is used

Point cone resistance Vs SPT Correlation

Type of Soil Qc (kN/m2) VsSPT ‘N’

Gravel 800 to 1000

Sands 500 to 600

Silty sands 300 to 400

Silts and clayey silts 200

Cone and Friction assembly for SCPT

Cone used for SCPT

Typical Test Set up for SCPT

• The stratification of soils and rocks can be determined by geophysical methods of exploration which messures changes in certain physical characteristics of these materials,for example density,magnetism,electrical resistivity,etc.

The following two geophysical methods are commonly used:

(i)Seismic refraction method

(ii)Electrical resistivity method

(i)Seismic Refraction Method:

The seismic refraction method is based on the principal that seismic waves have different velocities in different types of soil.

The seismic refraction method is more suited to shallow exploration for civil engineering purposes.

Geophysical methods:

The electrical resistivity d is given by

ℓ=2*3.14*D*V/I

ℓ=mean resistivity (ohm/m)

D=distance between electrodes (m)

V =potential drop between two inner electrodes (volts)

I=current applied between two outer electrodes (Amperes)

Limitations:

(i)The methods are capable of detecting only the strata having different electrical resistivity.

(ii)The services of an expert in the field are needed.