soil dynamic properties
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Contents:
INTRODUCTION
IMPORTANCE & USES OF DYNAMIC PROPERTIES
LAB TESTS FOR DYNAMIC PROPERTIES
FIELD TESTS FOR DYNAMIC PROPERTIES
REFERENCES
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INTRODUCTION
SOIL DYNAMICS: Soil dynamics is the branch of soil mechanics that deals
with the behavior of soil under dynamic load, including
the analysis and the stability of earth-supported and
earth retaining structures.
Soil Dynamic Properties: Properties (Behavior) of Soil under dynamic loading.
Most commonly, these properties include Shear wave
velocities Vs, Shear Modulus G, Damping D and Poisons
Ratio ๐.
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IMPORTANCE OF DYNAMIC SOIL PROPERTIES
โข Earthquake, ground vibration, and wave propagation
through soils
โข Dynamic stress, deformation, and strength properties of soils
โข Dynamic earth pressure problem
โข Dynamic bearing capacity problems and design of shallow foundations
โข Problems related to soil liquefaction
โข Design of foundations for machinery and vibrating equipment
โข Design of embedded foundations and piles under
dynamic loads
โข Stability of embankments under earthquake loading
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1. Shear Wave Velocity, Vs
โข For Soil Characterization
โข As Particle motion is perpendicular to the wave motion, so it can be used for determining shear properties of soil
skeleton.
2. Shear Modulus, G
โข Used for defining stiffness matrices in FEM of Earth
structures and foundation soils
3. Damping Ratio, D
โข Ability of System to absorb dynamic energy
โข Controls the duration and mode of Vibration
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i. Laboratory tests and results
ii. Field tests and measurements
iii. Empirical correlations for the shear modulus
and damping ratio
Shear Modulus and Damping Ratio are
obtained from field and lab test and are the
most important parameters for design work.
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1) For Saturated Clays: Unconsolidated - Undrained Trixial Tests
Cu = Undrained Shear Strength
๐๐ข(๐ ๐ก๐๐ก๐๐ )= โ๐๐๐๐ฅ
2 ; โ๐๐๐๐ฅ = ๐1๐ โ ๐3
๐๐ข(๐ ๐ก๐๐ก๐๐ )= ๐๐ข(๐๐ฆ๐๐๐๐๐)
For Strain of 0.5%
or less
For Strain of 42%
to 50%
For most of the practical cases
๐ช๐(๐ ๐๐๐๐๐๐)
๐ช๐(๐๐๐๐๐๐) โ 1.5 (Carroll 1963)
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Fig. Typical Triaxial Test Apparatus
bad
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SHEAR STRENGHT UNDER RAPID LOADING (Contdโฆ)
2) For Sands:
โข Vacuum Triaxial tests on dry sands
โข Compressive Strength, โ๐๐๐๐ฅ = ๐ 1๐ โ ๐ 3
Whereas, ๐ 1๐ = Effective Major Principal stress at failure
๐ 3 = Effective Minor Principal stress
Drained soil friction angle, ๐ = ๐ ๐๐โ1๐ 1๐โ๐ 3
๐ 1๐+๐ 3
Similarly, for Dynamic Friction Angle,
๐๐ ๐๐๐๐๐๐ = ๐โ ๐๐
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โข Transient Load is defined as Short timed, sudden
change of Load.
โข Typical example is Blast Loading.
โข tL = Loading Time
tD = Decay time
Transient Loading
๐๐ (๐๐๐๐๐๐๐๐๐)
๐๐ (๐๐๐๐๐๐)= ๐. ๐ ๐๐ ๐. ๐
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UNCONFINED COMPRESSIVE STRENGHT OF A CLAY FOR VARYING
TIME OF LOADING
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โข For determining Shear Modulus and Damping Ratio
of Soil
โข Sample Size: h = 20-30mm, d = 60-80mm
โข Sample under Vertical Effective Stress, ๐ ๐ฃ and Cyclic
Shear Stress, ๐
Damping Ratio = Decay of Oscillations after disturbance
= C/Cc Where,
C = Damping Coefficient
Cc = Critical Damping Coefficient
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Cyclic Simple Shear Test (Contdโฆ)
G = ๐จ๐๐๐๐๐๐๐๐ ๐๐ ๐ช๐๐. ๐บ๐.๐บ๐๐๐๐๐ ๐จ๐๐๐๐๐๐๐๐ ๐๐ ๐ช๐๐. ๐บ๐. ๐บ๐๐๐๐๐
;
Go Back
D = ๐
๐๐ (
๐จ๐๐๐ ๐๐ ๐ฏ๐๐.๐ณ๐๐๐
๐จ๐๐๐ ๐๐ โ๐ถ๐จ๐ฉ & ๐ถ๐จโฒ๐ฉโฒ)
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Cyclic Simple Shear Test (Contdโฆ)
and
and
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โข Used to determine Modulus of
Elasticity โEโ & Damping Ratio โDโ
โข Confining Pressure ๐3 and Axial
Cyclic Stress โ๐๐ is applied
Strain-Controlled Test (Servo-System is used)
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โข To calculate Shear Modulus, G
๐ = ๐๐๐๐ ๐๐โฒ๐ ๐ ๐๐ก๐๐
โข Damping Ratio, D
Cyclic Triaxial Test (Contdโฆ)
๐ฎ =๐ฌ
๐(๐ + ๐)
๐ซ =๐
๐๐ ร
๐จ๐๐๐ ๐๐ ๐ฏ๐๐๐๐๐๐๐๐๐ ๐ณ๐๐๐
๐จ๐๐๐ ๐๐ โ๐ถ๐จ๐ฉ & โ๐ถ๐จโฒ๐ฉโฒ (Same as that of Cyclic Simple Shear Test)
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โข Resonant Column Test
โข Cyclic Torsional Simple Shear Test
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Summary
And
or
And
Or
For
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Pre
pa
ratio
n
Trench is excavated at the depth of 0.75m (depth of foundation). Steel plate 25mm thick and 762mm diameter
Loa
din
g
Loading is done at the rate of 25% increment until negligible settlement rate (0.25mm/hour) U
nlo
ad
ing
Plate is unloaded in short time but gradually. Rebound of soil is noted and plate is again loaded to the next increment.
For Calculating Spring Constant K & Shear Modulus G
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โข ๐พ๐๐๐๐ก๐ =๐๐ด
๐๐
๐พ๐= (๐๐
๐๐) ร ๐พ๐๐๐๐ก๐
๐พ๐= (๐๐+๐๐
2ร๐๐) ร ๐พ๐๐๐๐ก๐
Wf = Foundation width
Wp = Plate Width
Cyclic Plate Load Test (Contdโฆ)
For Cohesive Soils
For Cohesion less Soils
๐ฎ =(๐ โ ๐)๐ช๐๐. ๐๐
ร ๐จ
๐ = ๐๐๐๐ ๐๐๐ ๐ ๐๐ก๐๐, ๐ถ๐ง = ๐๐๐. ๐๐ ๐๐ข๐๐๐๐๐๐ ๐ ๐๐๐๐ก๐๐๐, ๐ด = ๐ด๐๐๐ ๐๐ ๐๐๐๐ก๐
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Used for:
โข Wave Propagation
Velocities
โข Thickness of soil layers
Suitable for General
Site Investigation for
soil dynamics
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โข Best method for low strain shear wave velocity
โข At least Two bore holes required
Seismic Energy (S-waves)
generated from bottom of bore
hole
Time to travel to another hole is
calculated
Velocity is calculated from time and used to determine Shear
Modulus G.
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Cross-hole Technique (Contd.)
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โข Similar to Cross-Hole
Technique
โข Require only one
borehole
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โข Recently Developed Technique
โข Combination of Down-hole technique with standard CPT
โข Less expensive than other seismic techniques
Shear Wave Velocity โVsโ is calculated by dividing the difference in travel paths b/w two depth by time diff. of two signals recorded.
Shear force is induced at surface with drop hammer. Rest of the process is like Down hole technique
Penetration is paused temporarily
Seismic Receiver is attached to the cone
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REFERENCES
Determination of dynamic soil properties using geophysical methods by Ronaldo Luna and Houda Jadi
Soil Dynamics, Braja M. Das
In situ determination of dynamic soil properties, Jaehun Ahn
Seismic Cone Analysis using digital signal processing, R G Campanella & W. P. Stewert
Unified Facilities Criteria (ufc), Soil Mechanics: Department of Defense, USA
Soil Dynamic, Shamsher Prakash
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