dynamic soil properties important

8/14/2019 Dynamic Soil Properties important

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Dynamic Soil Properties

Dynamic, cyclic, static differenceLow strain, high strain properties

2. Stress Conditions by Mohr Circle

Sign convention Fig. 6.1

Mohr Circle to explain the normal & shear stresses on a plane inclined at

angle, .

Eqs. 6.1, Fig. 6.2

Pole any line drawn thru the pole will intersect the Mohrs circle at a

point that describes the and on a plane parallel to that line

*Principal Plane, Principal Stresses

Major 1, minor = 3, intermediate 2

(usually 2= 3, assume)

Increase of shear stress

Mohrs circle grows (Fig. 6.4)

Rotation of principal stress axes.


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*Stress Paths

- Plotting the variation of the position of a single point on Mohr circle- p = (1+ 3)/2, q = (1- 3)/2

- effective stress paths

P` = 1' '

3 1 3 1 3( ) ( )

2 2 2

u uu p u

+ + += = =

q` = 1' '

3 1 3 1 3( ) ( )2 2 2

u u q

= = =

-Consolidation (K0-cond.) (AB)

v , h = const. (foundation

loading)

Axial comp.(AC)

v= const. h , (active cond.)

Lateral ext.(LE)

v , h= const. (excavation)

Axial ext.(AE)

v= const. h, (passive cond.)

lateral comp.(LC)

- Soil Element subjected to vertically propagating S-wave

change of Mohr circle

variation of Pole point rotation of principal axesvariation in stress paths

not considered in the lab. And field tests

p

3 1

ACLE

A

B

AELC

q


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Ch. 6 Behaviors of SoilsUnder Static and Dynamic L

Deformational Characteristics of Soils

or )

G1 or E2)G2 or E2)

0.1 0.2

Stress-Strain RelationshipStress-Strain Relationship

G (or E)

0.001 0.

Modulus-Strain RelatiModulus-Strain Relatio

(ora), %


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Deformational Characteristics of S

Intermediate sSmall strains (


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Dynamic Soil Properties

Dynamic Modulus : Youngs Modulus(E), Shear MConstraint Modulus(M)

Poissons Ratio() Damping Ratio(D) and attenuation coefficient(

Liquefaction Resistance and Shear Strength with

-> Apply to Dynamic AnalysisSoil-Structure Interaction Analysis

In-situ Seismic Test Crosshole, Downhole, SASW, HWAW, Refract Vs -> Gmax

Laboratory Test Cyclic TX, RC/TS, Simple shear test, Shaking G-log , D-log Liquefaction Potential


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Static, Cyclic, Dynamic Soil Propert

Modulus by Testing Condition

Strain

Stress Load

Strain

Stress Loa yclic Modulus- cyclic loading(low frequency)

- Cyclic TX, Torsion shear

Static Modulus- monotonic loading- Triaxial Tests etc..

Dynamic Modulus- cyclic loading

(high frequency)- Resonant Column etc..

Loa Inertia plays role

In the past, Gstatic GdynamicIn the past, Gstatic Gdynamic


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Measurement of Dynamic Soil Properties

*Measurement of low-strain properties wave propagation*Measurement of large-strain properties stress-strain

*Selection

Replicate the initial stress conditions

Replicate the anticipated cyclic loading conditions.

*Dynamic Soil Properties and Stress-strain Ralationhand out

*Source of uncertainty

Inherent variability

Inherent anisotropy(soil structure)

Inherent anisotropy(by stress conds)

Disturbance

Limits of testing equipments

Testing and interpretation errors

1. Field Tests

Advantage : not requiring sampling

Response of relatively large Volume of soil

Including similar soil deformation to real problems

Disdvantage : can not represent other conditions

Not control drainage

Properties obtained indirectly

(theoretically, correalations)

Surface Test. Vs. Borehole Test


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1.1. Low-strain Tests

Strains below 0.001%Based on elastic wave propagation(body wave, R wave)

P-wave : fastest

S-wave : using polarity characteristics

Averaging technique

Effects of Ground water

(a)Seismic Reflection Test

Determine V and H of surficial layers

H, Vp1

Vp2

2H/vp1

1/vp

S x R

t

x

Direct arrival

td= x/vp1

Reflected arrival

tr=2 2

1

4

p

H x

V

+ H = 2 2 2

1

1

2 r pt V x

Or use intersection

Inclined layer boundary

Difficulties of determining arrival time tr


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Seismic Refraction Test

using the arrival times of the first waves (P or S)

typical test setup

Horizontal Layering

direct arrival 1/ vxt nd = Snells Law

At the boundary reflect, refract Huygens Principal

- 36 -

critical angle of incident( )Ci

refracted wave parallel to the boundary

produce head wave

near source receiver dist. direct arrival first far dist. After critical distance head wave first

Cx


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- 37 -- 37 -


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- 38 -

1/1 v

2/1 v

12

12

2 vvvvxH c

+

=

1

2

v

iHCosC

dist.

Cx

For multi layers Fig. 6.14.

Basic Assumptions : Velocity increases w depth

2

1

21

90

v

viSin

v

Sin

v

iSinC

C==

o

* Inclined or Irregular Layering p 106~111 (Richart et. al)

* Example


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- 39 -


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- 40 -- 40 -

dynamic soil properties important

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