1

Geotechnical Testing Methods I

Ajanta SachanAssistant ProfessorCivil EngineeringIIT Gandhinagar

Geotechnical Engg Structures… Buried right Under your Feet…!!

Hiding World of Geotechnical Engg…!!

Foundations

Tunneling

Shoring

Soil Exploration

2

You pay for soil investigation whether you carry out or not. Infact you eventually pay more without a soil investigation.

Leaning Tower of Pisa

… Our Blunders become Monuments !

3

Unfortunately, soils are made by nature and not by man, and the products of nature are always complex…

As soon as we pass from steel and concrete to earth, the omnipotence of theory ceases to exist. Natural soil is never uniform…

Soil Investigation is unique for each soil site!

Terzaghi says:(Father of Soil Mechanics) Karl Terzaghi (1883-1963)

Typical Geotechnical Project

construction site

Geo-Laboratory~ for testing

Design Office~ for design & analysis

soil properties

4

Purpose of Geotechnical Testing?

ground

Can the soils Support the structure?

What is the impact of Excavation or Filling?

Are the earth and rock Slopes stable?

What type of Foundation is best suited for the structure?

How will the site respond to an Earthquake?

Is the site Contaminated?

Determine potential problems and Avoid surprises!!

S : Solid Soil particle

W: Liquid Water (electrolytes)

A: Air Air

v

s

Ve

VVoid ratio,

Three Phases in Soils

5

Grain Size Distribution

In Coarse grained soils …... By Sieve analysis (Dry/Wet)

Sieve Analysis Hydrometer Analysis

soil/water suspension

hydrometer

stack of sieves

sieve shaker

In Fine grained soils …... By Hydrometer analysis

Soil Groups

Fine grain soils

Coarse grain soils

0.002 200(300)

63(80)

2.36 (4.75; IS code)

0.075

Grain size (mm)

BoulderClay Silt Sand Gravel Cobble

Granular soils or Cohesion less soils

Cohesive soils

6

Rounded Subrounded

Subangular Angular

Sand and Gravelparticle size > 75 mm

Clay particle size < 2 mm

Soil particle shapes & sizes

Silt particle size = 75-2 mm

Typical Geotechnical Testing Plan

Borings: No. of bore holes, spacing

Ground Water Monitoring: measure the ground water level

Soil sampling: sampler (split spoon sampler, Shelby tube), Specimen (undisturbed, disturbed)

Laboratory Test: Index properties, Consolidation, Shear strength properties, Relative density, Permeability etc.

Field Test: In-situ dry density, Shear Strength, Plate Load Test

7

Borings: Number & Spacing

No hard and fast rule

Some guidelines – IS:1892-1979

For small projects

On plane site – 4 or 5 borings sufficient

On uneven site – add 1 or 2 more borings

For large projects: 50-100 m spacing in grid pattern

It is important to conduct borings as close as possible to column locations and strip footing locations.

Depth of Exploration

8

Other Exploration Techniques

Test Pits: Unlike boring, soil can be visually observed from the sides of the test pit. Pit is made by excavating ground (typical size =1.2mx1.2m) considering sufficient working space.

Trenches: Trenches are long shallow pits. They are more suitable for exploration on slopes than pits.

Suggestions:

Test pits suggested if required exploration depth = 2-4m

Trenches suggested for slopes (small)

Boring suggested for exploration depth > 4m

Indentifying the Weak Plane: Boring

Estimated Slip Surface

9

Boring Techniques

Auger Boring

Wash Boring

Rotary Boring

Percussion Boring

Use depends on

Nature of soil

Water table Depth

Sample Disturbance

Accuracy of soil exploration

Auger Boring for soils which can stay open without casing or drilling mud. It is not possible for sands below water table. Good for Highways, railways projects where small depth of soil exploration is needed.

18

Auger Boring

Push and rotate the auger until annular space of auger fills up

Withdraw the auger and clean it Repeat the process

Pro

ced

ure

1. Hand Auger – for shallow depth (3 - 5 m)

2. Power Driven Auger – for larger depth

3. Sand Bailer – Heavy duty pipe with cutting

edge– Lifted and then left to fall freely

under self weight. Additional weight (sinker) may be added for ease of sinking

10

19

4. Hollow Stem Auger

Auger Boring

20

Wash Boring

A casing pipe of 2-3 m length is driven into the soil by a heavy drop hammer.

The soil inside the casing is removed by means of a chopping bit attached to a drill rod which forces water at high pressure.

Soil mixed with water moves up in annular gap between drill rod and casing.

Samples are obtained at certain depth by removing drill rod and pushing a sampler instead.

11

Rotary Boring

Boring is done by rapidly rotating drilling bits attached to bottom of drilling rod.

Soil/rock cuttings removed by circulating drilling fluid

Samples are taken a certain depths by removing drill rod and placing sampler.

Mud Rotary Drilling: Hollow drilling rods are used to flow mud slurry (Bentonite) to check caving in of the material (soil) at bottom.

Core Drilling: Core barrels with diamond bit are used.

Design similar to wash boring

Useful when soil is resistant to auguring or wash boring

Percussion Boring

Dry boring or water circulated to remove loose soil

Heavy drilling bit or chisel is dropped while inside the casing to chop the hard soil.

Percussion drilling rods may be replaced by cables.

12

Bore Hole Stabilization

Drilling Mud Use of Casing

Ground Water Observation

High Permeability Soils

Bore hole/Observation wells (Observation time = 24 to 48 Hrs)

Low Permeability Soils

Casagrande Piezometer (when water level in bore hole does not get stabilize in Piezometer is recommended)

Piezometers may be installed in bore hole for seasonal variations in High permeability soils. Chemical analysis of ground water may be performed if its constituents can be damaging to foundation.

13

Soil Sampling

Disturbed Samples: Natural soil structure is modified or destroyed during sampling Representative Samples:

Natural water content and mineral constituents of particular soil layer are preserved

Good for soil identification and water content

Non-representative Samples: Water content altered and soil layers mixed up

Of no use.

Undisturbed Samples: Soil structure and the other mineral properties are preserved to an extent. Some disturbance is always there, e.g. due to stress release.

However it should be minimized in order to have suitable sample for our analysis.

Standard Split Spoon Samplers

Thick wall (0.25in) cylinder

Sampling tube (dia 51 mm) is split along the length

Representative Disturbed soil samples

14

Shelby Tube (Thin-wall) Sampler

Thin wall (1/16in = 0.0625 in) sampling tube

Sampler pushed into the ground hydraulically

Sample extruded from tube and “Undisturbed” soil sample is obtained

Sealing of Sampling Tube

After removing the sampler from ground, it is sealed on both sides using melted wax to preserve moisture

15

Laboratory Test: Index Properties

Index Properties of soil:

Basic soil properties such as (a) Specific gravity (Gs)(b) Grain size distribution (dry/wet Sieve test, Hydrometer test), (c) Liquid Limit (LL), Plastic limit (PL) (d) OMC, Maximum Dry density(Compaction/Proctor test)(e) Permeability (Constant head/Falling head)(f) Relative Density (Minimum & Maximum density for cohesionless soils)

More tests for Problem soils: (a) Shrinkage Limit, Free swell, Swell pressure for Expansive soils(b) Pinhole test, Crumb test for Dispersive soils(c) Chemical Test (PH, Sulphite, Chloride, Iron etc) for soils (may affected with industrial waste or some other waste)(d) Furnace test for Organic Soils (peats etc)

“Representative Disturbed “soil samples are used to perform these tests.

Laboratory Test: Engineering Properties

Engineering Properties of soil:

Consolidation Properties (Oedometer setup)(i) Must to perform for Clayey soils; (ii) Soil parameters obtained: Cc,Cv,Cr, OCR, k

Shear Strength Properties(i) Direct Shear test (for cohesionless soil)(ii) Unconfined Compression test (for cohesive soil)

(iii) Triaxial test (for all soil types; cohesive, cohesionless)

Dynamic Properties(i) Cyclic Triaxial test(ii) Cyclic Simple Shear test(iii) Resonant Column test(iv) Bender Element test

“Undisturbed” soil samples are used to perform these tests.

16

Consolidation Test: Oedometer Test

Input: Vertical Load, Vertical Displacement

Output: Consolidation parameters (Cv, Cc & Cs); void ratio Vs overburden pressure curve

Direct Shear Test (Recommended for Cohesionless soils)

Input: Vertical Load, Vertical Displacement, Lateral Load Lateral Displacement

Output: shear strength, friction angle (f)

17

Unconfined Compression Test (UC test) (Recommended for Cohesive soils)

Input: Vertical Load, Vertical Displacement

Output: Shear Strength under Undrained Conditions (Su)

Triaxial Test:

Measures shear strength parameters of soil (shear strength properties: cohesion, friction angle)

Loading conditions : Static loading (compression is common)

18

Triaxial Testing Setup

Soil specimen

Triaxial setup

Control Panel

Input: Vertical Load, Vertical Displacement, Pore pressure, Cell pressure

Output: Shear Strength properties of soil under UU, CU, CD Conditions: friction angle (f), cohesion (c)

Triaxial & Cyclic Triaxial

19

Soil Properties

Monotonic Loading (Shear strength properties of soil)

Angle of Internal Friction (f)

Cohesion (c)

Dynamic Loading (Dynamic properties of soil)

Shear Modulus (G)

Damping Ratio (D)

Dynamic properties of Soil

Shear Modulus, G = .VS2

Shear wave velocity = VS (m/sec)

Mass density = (g/g) (Kg/m3)

Unit weight of soil = g (KN/m3)

Acceleration of gravity = g (m/sec2)

Damping, D = decay in energy

Shear Modulus (G) is measured in KN/m2 & Damping (D) in %

20

Dynamic properties of soil

Low Strain Amplitude test

For strains (10-6% to 10-4%)

Frequency range: 10 Hz to 200Hz

Vibratory loading (Rotating Machinery etc)

High Strain Amplitude test

For strains (10-4% to 10-2%)

Frequency range: 0.1 Hz to 2 Hz (in general)

Blast loading, Earthquake

Dynamic properties (Lab test)

High Strain Amplitude test

Cyclic Triaxial Test

Cyclic Direct Simple Shear Test

Low Strain Amplitude test

Resonant Column Test

Bender Element Test

21

Cyclic Triaxial Test (High strain amplitude test)

Dynamic properties of soil using Cyclic Triaxial system:

1. Shear Modulus (G)

2. Damping ratio (D)

Cyclic Triaxial Test

DDamping EModulus Young Dynamic

dStress Dynamic aStrain Axial

22

Cyclic Simple Shear Test (High strain amplitude test)

Digitally controlled Electro-mechanical actuators are used to apply the stress or strain controlled loading

Output: Shear modulus (G), Damping (D)

Cyclic Simple Shear Test

DDamping GusShearModul

gnShearStraisShearStres

23

Resonant Column Test (Low strain amplitude test)

The basic principle of the resonant column device is to excite one end of a confined cylindrical soil specimen in a fundamental mode of vibration by means of torsional or longitudinal excitation.

Once the fundamental mode of resonance frequency is established, measurements are made of the resonance frequency and amplitude of vibration from which wave propagation velocities and strain amplitudes are calculated using the theory of elasticity.

The Resonant Column Test provides laboratory values of Shear modulus (G) and Damping ratio (D).

Resonant Column Test (Low strain amplitude test)

(a) Specimen is excited at the bottom and the response is picked up at the top (velocity or acceleration)(b) Driving force is applied on the top. The response pickup is also placed on the top

With known value of the resonant frequency it is possible to back-calculate the velocity (vs or vl) of the wave propagation and thereby G or E

After measuring the resonant condition, the drive system is cut of and the specimen is brought to a state of free vibration. Damping is determined by observing the decay pattern

24

tiCt e)(

Acc.

ff

Resonant freq. f1

+

Sample Geometry

+

End restraint

+

Wave equation (torsion)

( 2

1220 2

T

sF

fHvG

Resonant Column Test:Determination of Shear Modulus of soil (G)

Resonant Column Test:Damping properties of soil (D

D = 1/2·D1

25

Bender Element Test (Low strain amplitude test)

Bender Elements (made by Piezoelectric material)

Bender Element Test (Low strain amplitude test)

Piezo-ceramic elements distort or bend when subjected to a change in voltage.

Two Piezoelectric bender elements are placed opposite one another and inserted a small distance into a soil sample. One bender element work as source and other as receiver.

The voltage in one element is varied creating shear waves through the sample, which are received by the opposite element. The input voltage, (created using a function generator) and the received signal are recorded continuously using an oscilloscope, allowing the travel time of the shear waves to be measured from which the dynamic elastic shear modulus (G) can be determined.

Bender elements provide a reliable, cost effective alternative to undertaking locally instrumented stress path triaxial tests and can be readily performed on unconfined samples in the laboratory.

26

Thank You