Geotechnical Testing Methods I_AS

8/10/2019 Geotechnical Testing Methods I_AS

1/26

1

Geotechnical Testing Methods I

Ajanta SachanAssistant ProfessorCivil EngineeringIIT Gandhinagar

Geotechnical Engg StructuresBuried right Under your Feet!!

Hiding World of Geotechnical Engg!!

Foundations

Tunneling

Shoring

Soil Exploration


2/26

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 !


3/26

3

Unfortunately, soils are made by nature and not byman, 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 & analysissoil properties


4/26

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

andAvoid surprises!!

S : Solid Soil particle

W: Liquid Water (electrolytes)

A: Air Air

v

s

Ve

VVoid ratio,

Three Phases in Soils


5/26

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 orCohesion less soils

Cohesivesoils


6/26

6

Rounded Subrounded

Subangular Angular

Sand and Gravelparticle size > 75 mm

Clay particle size < 2 mm

Soil particle shapes & sizesSilt 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 strengthproperties, Relative density, Permeability etc.

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


7/26

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 columnlocations and strip footing locations.

Depth ofExploration


8/26

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 forexploration 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


9/26

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 drillingmud. It is not possible for sands below water table.Good for Highways, railways projects where small depth of soilexploration is needed.

18

Auger Boring

Push and rotate the auger untilannular space of auger fills up

Withdraw the auger and clean it Repeat the process

P

rocedure

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 freelyunder self weight. Additionalweight (sinker) may be added forease of sinking


10/26

10

19

4. Hollow Stem Auger

Auger Boring

20

Wash Boring

A casing pipe of 2-3 mlength is driven into thesoil by a heavy drophammer.

The soil inside thecasing is removed bymeans of a chopping bitattached to a drill rodwhich forces water athigh pressure.

Soil mixed with watermoves up in annulargap between drill rodand casing.

Samples are obtained atcertain depth byremoving drill rod andpushing a sampler

instead.


11/26

11

Rotary Boring

Boring is done by rapidly rotatingdrilling bits attached to bottom ofdrilling rod.

Soil/rock cuttings removed bycirculating drilling fluid

Samples are taken a certain depths byremoving drill rod and placingsampler.

Mud Rotary Drilling: Hollow drillingrods are used to flow mud slurry(Bentonite) to check caving in of thematerial (soil) at bottom.

Core Drilling: Core barrels withdiamond bit are used.

Design similar to wash boring

Useful when soil is resistant to auguring or washboring

Percussion Boring

Dry boring or water circulated to removeloose soil

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

Percussion drilling rods may be replaced bycables.


12/26

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 inPiezometer is recommended)

Piezometers may be installed in bore hole for seasonalvariations in High permeability soils. Chemical analysis ofground water may be performed if its constituents can be

damaging to foundation.


13/26


14/26

14

Shelby Tube(Thin-wall) Sampler

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

Sampler pushed into the groundhydraulically

Sample extruded from tube andUndisturbed soil sample is obtained

Sealing ofSampling Tube

After removing the samplerfrom ground, it is sealed on

both sides using melted waxto preserve moisture


15/26

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 withindustrial 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 testUndisturbed soil samples are used to perform these tests.


16/26

16

Consolidation Test: Oedometer Test

Input: Vertical Load, VerticalDisplacement

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

Direct Shear Test(Recommended for Cohesionless soils)

Input: Vertical Load, VerticalDisplacement, Lateral LoadLateral Displacement

Output: shear strength, friction angle (f)


17/26

17

Unconfined Compression Test (UC test)

(Recommended for Cohesive soils)

Input: Vertical Load, VerticalDisplacement

Output: Shear Strength underUndrained Conditions (Su)

Triaxial Test:

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

Loading conditions :Static loading(compression is common)


18/26

18

Triaxial Testing Setup

Soil specimen

Triaxial setup

Control

Panel

Input: Vertical Load, VerticalDisplacement, Porepressure, Cell pressure

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

Triaxial & Cyclic Triaxial


19/26

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 %


20/26

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


21/26

21

Cyclic Triaxial Test (High strain amplitude test)

Dynamicproperties ofsoil using CyclicTriaxial system:

1. ShearModulus (G)

2. Dampingratio (D)

Cyclic Triaxial Test

DDampingEModulusYoungDynamic

dStressDynamic aStrainAxial


22/26

22

Cyclic Simple Shear Test (High strain amplitude test)Digitally controlled Electro-mechanical actuators are usedto apply the stress or straincontrolled loading

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

Cyclic Simple Shear Test

DDampingGusShearModul

gnShearStraisShearStres


23/26

23

Resonant Column Test

(Low strain amplitude test)

The basic principle of the resonant columndevice is to excite one end of a confinedcylindrical soil specimen in a fundamentalmode of vibration by means of torsional orlongitudinal excitation.

Once the fundamental mode of resonancefrequency is established, measurementsare made of the resonance frequency andamplitude of vibration from which wave

propagation velocities and strainamplitudes are calculated using the theoryof elasticity.

The Resonant Column Test provideslaboratory 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 ispicked 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 theresonant frequency it ispossible to back-calculatethe velocity (vs or vl) of the

wave propagation andthereby G or E

After measuring theresonant condition, the drivesystem is cut of and thespecimen is brought to astate of free vibration.Damping is determined byobserving the decay pattern


24/26

24

tiCt e)(

Acc.

ff

Resonant freq.f1

+

Sample Geometry

+

End restraint

+

Wave equation (torsion)

( 2

1220 2

Ts

F

fHvG

Resonant Column Test:

Determination of Shear Modulus of soil (G)

Resonant Column Test:Damping properties of soil (D

D = 1/21


25/26

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 involtage.

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

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

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


26/26

Thank You

Date post:	02-Jun-2018
Category:	Documents
Upload:	sd
View:	236 times
Download:	1 times

Geotechnical Testing Methods I_AS

Documents