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



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 𝝁.



IMPORTANCE OF DYNAMIC SOIL PROPERTIES

• Earthquake, ground vibration, and wave propagation

through soils

• Dynamic stress, deformation, and strength properties of soils

• Dynamic earth pressure problem

• Dynamic bearing capacity problems and design of shallow foundations

• Problems related to soil liquefaction

• Design of foundations for machinery and vibrating equipment

• Design of embedded foundations and piles under

dynamic loads

• Stability of embankments under earthquake loading



1. Shear Wave Velocity, Vs

• For Soil Characterization

• 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

• Used for defining stiffness matrices in FEM of Earth

structures and foundation soils

3. Damping Ratio, D

• Ability of System to absorb dynamic energy

• Controls the duration and mode of Vibration



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.





1) For Saturated Clays: Unconsolidated - Undrained Trixial Tests

Cu = Undrained Shear Strength

𝑐𝑢(𝑠𝑡𝑎𝑡𝑖𝑠)= ∆𝜎𝑚𝑎𝑥

2 ; ∆𝜎𝑚𝑎𝑥 = 𝜎1𝑓 − 𝜎3

𝑐𝑢(𝑠𝑡𝑎𝑡𝑖𝑠)= 𝑐𝑢(𝑑𝑦𝑛𝑎𝑚𝑖𝑐)

For Strain of 0.5%

or less

For Strain of 42%

to 50%

For most of the practical cases

𝑪𝒖(𝒅𝒚𝒏𝒂𝒎𝒊𝒄)

𝑪𝒖(𝒔𝒕𝒂𝒕𝒊𝒄) ≈ 1.5 (Carroll 1963)



Fig. Typical Triaxial Test Apparatus

bad



SHEAR STRENGHT UNDER RAPID LOADING (Contd…)

2) For Sands:

• Vacuum Triaxial tests on dry sands

• Compressive Strength, ∆𝜎𝑚𝑎𝑥 = 𝜎 1𝑓 − 𝜎 3

Whereas, 𝜎 1𝑓 = Effective Major Principal stress at failure

𝜎 3 = Effective Minor Principal stress

Drained soil friction angle, 𝜑 = 𝑠𝑖𝑛−1𝜎 1𝑓−𝜎 3

𝜎 1𝑓+𝜎 3

Similarly, for Dynamic Friction Angle,

𝝋𝒅𝒚𝒏𝒂𝒎𝒊𝒄 = 𝝋− 𝟐𝒐



• Transient Load is defined as Short timed, sudden

change of Load.

• Typical example is Blast Loading.

• tL = Loading Time

tD = Decay time

Transient Loading

𝒒𝒖 (𝒕𝒓𝒂𝒏𝒔𝒊𝒆𝒏𝒕)

𝒒𝒖 (𝒔𝒕𝒂𝒕𝒊𝒄)= 𝟏. 𝟓 𝒕𝒐 𝟐. 𝟎



UNCONFINED COMPRESSIVE STRENGHT OF A CLAY FOR VARYING

TIME OF LOADING



• For determining Shear Modulus and Damping Ratio

of Soil

• Sample Size: h = 20-30mm, d = 60-80mm

• Sample under Vertical Effective Stress, 𝜎 𝑣 and Cyclic

Shear Stress, 𝜏

Damping Ratio = Decay of Oscillations after disturbance

= C/Cc Where,

C = Damping Coefficient

Cc = Critical Damping Coefficient



Cyclic Simple Shear Test (Contd…)

G = 𝑨𝒎𝒑𝒍𝒊𝒕𝒖𝒓𝒆 𝒐𝒇 𝑪𝒚𝒄. 𝑺𝒉.𝑺𝒕𝒓𝒆𝒔𝒔 𝑨𝒎𝒑𝒍𝒊𝒕𝒖𝒓𝒆 𝒐𝒇 𝑪𝒚𝒄. 𝑺𝒉. 𝑺𝒕𝒓𝒂𝒊𝒏

;

Go Back

D = 𝟏

𝟐𝝅 (

𝑨𝒓𝒆𝒂 𝒐𝒇 𝑯𝒚𝒔.𝑳𝒐𝒐𝒑

𝑨𝒓𝒆𝒂 𝒐𝒇 ∆𝑶𝑨𝑩 & 𝑶𝑨′𝑩′)



Cyclic Simple Shear Test (Contd…)

and

and



• Used to determine Modulus of

Elasticity ‘E’ & Damping Ratio ‘D’

• Confining Pressure 𝜎3 and Axial

Cyclic Stress ∆𝜎𝑑 is applied

Strain-Controlled Test (Servo-System is used)



• To calculate Shear Modulus, G

𝜇 = 𝑃𝑜𝑖𝑠𝑜𝑛′𝑠 𝑅𝑎𝑡𝑖𝑜

• Damping Ratio, D

Cyclic Triaxial Test (Contd…)

𝑮 =𝑬

𝟐(𝟏 + 𝝁)

𝑫 =𝟏

𝟐𝝅×

𝑨𝒓𝒆𝒂 𝒐𝒇 𝑯𝒚𝒔𝒕𝒆𝒓𝒊𝒔𝒊𝒔 𝑳𝒐𝒐𝒑

𝑨𝒓𝒆𝒂 𝒐𝒇 ∆𝑶𝑨𝑩 & ∆𝑶𝑨′𝑩′ (Same as that of Cyclic Simple Shear Test)



• Resonant Column Test

• Cyclic Torsional Simple Shear Test



Summary

And

or

And

Or

For





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



• 𝐾𝑝𝑙𝑎𝑡𝑒 =𝑞𝐴

𝑆𝑒

𝐾𝑓= (𝑊𝑓

𝑊𝑝) × 𝐾𝑝𝑙𝑎𝑡𝑒

𝐾𝑓= (𝑊𝑓+𝑊𝑝

2×𝑊𝑝) × 𝐾𝑝𝑙𝑎𝑡𝑒

Wf = Foundation width

Wp = Plate Width

Cyclic Plate Load Test (Contd…)

For Cohesive Soils

For Cohesion less Soils

𝑮 =(𝟏 − 𝝁)𝑪𝒛𝟐. 𝟐𝟔

× 𝑨

𝜇 = 𝑃𝑜𝑖𝑠𝑜𝑛𝑠 𝑅𝑎𝑡𝑖𝑜, 𝐶𝑧 = 𝑀𝑜𝑑. 𝑜𝑓 𝑆𝑢𝑏𝑔𝑟𝑎𝑑𝑒 𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛, 𝐴 = 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑃𝑙𝑎𝑡𝑒



Used for:

• Wave Propagation

Velocities

• Thickness of soil layers

Suitable for General

Site Investigation for

soil dynamics



• Best method for low strain shear wave velocity

• 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.



Cross-hole Technique (Contd.)



• Similar to Cross-Hole

Technique

• Require only one

borehole



• Recently Developed Technique

• Combination of Down-hole technique with standard CPT

• Less expensive than other seismic techniques

Shear Wave Velocity ‘Vs’ 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



Seismic CPT (Contd.)



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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Soil Dynamic Properties