Post on 02-Jun-2018
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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
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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 !
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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
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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
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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
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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
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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
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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
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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.
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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
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4. Hollow Stem Auger
Auger Boring
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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.
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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.
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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.
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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
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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.
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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)
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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)
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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
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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 %
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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
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Cyclic Triaxial Test (High strain amplitude test)
Dynamicproperties ofsoil using CyclicTriaxial system:
1. ShearModulus (G)
2. Dampingratio (D)
Cyclic Triaxial Test
DDampingEModulusYoungDynamic
dStressDynamic aStrainAxial
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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
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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
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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
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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.
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Thank You