Direct shear test.pdf

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Direct Shear Test for soils (under consolidated drained conditions) ASTM D 3080

CE-325 Foundation Engineering I

NUS NUS T Instit Institut ute e of Civ Civil il Eng E ngine ineer ering ing (NICE (NIC E )

Objective

Direct shear test (ASTM D 3080)

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To determin ine e the consolidated-drained shearing strength of the sandy to silty soils using the direct shear apparatus.

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The The test is perfor formed by defor formin ing g a specimen at a controlled strain rate on a single predefined shear plane.

Need and sco s cope pe z

I n many eng engiinee neering proble problems such such as as de desi sign gn of founda oundati tion, on, retaining walls, slab bridges, pipes, sheet piling, the value of the angle of internal friction and cohesion of the soil involved are required for the design.

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Direct sh she ear test test is is use used to pre predi dict ct these these par parameter ters qui quickl ckly.

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Objective


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To determin ine e the consolidated-drained shearing strength of the sandy to silty soils using the direct shear apparatus.

z

The The test is perfor formed by defor formin ing g a specimen at a controlled strain rate on a single predefined shear plane.

Need and sco s cope pe z

I n many eng engiinee neering proble problems such such as as de desi sign gn of founda oundati tion, on, retaining walls, slab bridges, pipes, sheet piling, the value of the angle of internal friction and cohesion of the soil involved are required for the design.

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Direct sh she ear test test is is use used to pre predi dict ct these these par parameter ters qui quickl ckly.

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Shea hearr st strengt rength h


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The The strength of a material ial is the greatest stress it can can susta sustaiin.

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The The safet fety of any ge geote otechni chnica call structure is depe depend nde ent on the the stre strength ngth of the soil soil.

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I f the the soil oil fails, the the struct tructu ure foun ounded on it can can collapse.

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NUST Institute of Civil Engineering (NICE)

Shear failure in soils

Failure due to inadequate strength at shear interface

Direct shear test (ASTM D 3080) 4


Shear failure in soils (contd.)



Bearing capacity failure



Bearing capacity failure (contd.) TranscosnaGrain Elevator Canada (Oct. 18, 1913)

West side of foundation sank 24-ft



Significance of shear strength z


Engineers must understand the nature of shearing resistance in order to analyze soil stability problems such as; {

Bearing capacity

{

Slope stability

{

Lateral earth pressure on earth-retaining structures

{

Pavement


Shear strength in soils


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The shear strength of a soil is its resistance to shearing stresses.

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It is a measure of the soil resistance to deformation by continuous displacement of its individual soil particles

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Shear strength in soils depends primarily on interactions between particles

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Shear failure occurs when the stresses between the particles are such that they slide or roll past each other

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Shear strength in soils (contd.) z


Soil derives its shear strength from two sources: {

{

Cohesion between particles (stress independent component) Ê

Cementation between sand grains

Ê

Electrostatic attraction between clay particles

Frictional resistancebetween particles (stress dependent component)

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Cohesion z


Cohesion (C), is a measure of the forces that cement particles of soils. {

Dry sand with no cementation

{

Dry sand with some cementation

{

Soft clay

{

Stiff clay

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Internal friction z


Internal Friction angle (φ ), is the measure of the shear strength of soils due to friction.

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Mohr-Coulomb failure criteria


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This theory states that a material fails because of a critical combination of normal stress and shear stress, and not from their either maximum normal or shear stress alone.

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The relationship between normal stress and shear is given as

s = c′ + σ ′ tanφ ′ S , h t g n e r t S r a e h S

φ ′

c′

S = shear strength c’ = cohesion φ = angle of internal friction ’

Normal Stress, σn =σ′ =γ h



Determination of shear strength parameters z

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The shear strength parameters of a soil are determined in the lab primarily with two types of tests; {

Direct Shear Test

{

Triaxial Shear Test Normal stress σn

Shear stress σ3

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Soil

3


Direct shear test


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Direct shear test is Quick and Inexpensive.

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Shortcoming is that it fails the soil on a designated plane which may not be the weakest one.

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Used to determine the shear strength of both cohesive as well as non-cohesive soils.

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ASTM D 3080.

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Direct shear test (contd.) z

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The test equipment consists of a metal box in which the soil specimen is placed The box is split horizontally into two halves

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Vertical force (normal stress) is applied through a metal platen

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Shear force is applied by moving one half of the box relative to the other to cause failure in the soil specimen

Normal stress σn

Shear stress σ3

Soil


Direct shear test (contd.)



Direct shear test (contd.)



Direct shear test data


Peak Strength

s s e r t s r a e h S

Residual Strength


Direct shear test data


H


Apparatus


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Direct shear box

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Direct shear apparatus

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Porous stones

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Axial-loading device, & axial load-measuring device

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Shear-loading device, & and shear load-measuring device

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Tools for preparing specimen: cutting ring, wire saw, knife

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Displacement indicators

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Equipment for remolding or compacting specimens

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Test specimen


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Take sample sufficient to prepare three specimen. Prepare the specimen in controlled temperature and humidity environment to minimize moisture gain or loss.

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Take extreme care in preparing undisturbed specimen.

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Min specimen diameter for circular specimen (and width for square specimen): 2.0 in. (50 mm) but not less 10 times the maximum particle diameter.

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Min initial specimen thickness: 0.5 in. (12 mm) but not less than 6 times the maximum particle diameter.

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Min diameter to thickness ratio: 2:1


Specimen preparation


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Undisturbed specimen – prepare undisturbed specimens from large undisturbed samples.

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Compacted specimen {

Method 1: Compact the soil using Standard Proctor Test (ASTM D 698) or Modified Proctor Test (ASTM D 1557). Trim and prepare the specimen from compacted soil.

{

Method 2: Place a moist porous stone in the bottom of shear box. Place soil in layers in shear box and compact each layer by either kneading or tamping. The area of temper shall have an area equal to or less than area of mold. The top of each layer shall be scarified prior to addition of soil for next layer. Continue placing the compacting soil until the entire specimen is compacted.


Specimen preparation (contd.)


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Mix soil with sufficient water to produce desired water content before preparing the test specimen (by compaction method).

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Allow specimen to stand prior to compaction in accordance with the following guide: Classification ASTM D 2487

Minimum standing time

SW, SP

No requirement

SM

3h

SC, ML, CL

18 h

MH, CH

36 h


Procedure 1.


Measure diameter (or side), height, and mass of specimen. Assemble the apparatus. Note: For undisturbed samples from below the water table, the

porous stones are dampened.

2.

For consolidated test, consolidate the test specimen under the appropriate normal force. a)

After applying the initial appropriate normal force, fill the water reservoir to a point above the top of specimen. Maintain this water lever during the consolidation and subsequent shear phase.

b)

Allow the specimen to drain and consolidate under the desired normal force or increments thereof prior to shearing.

c)

During consolidation process, record normal displacement readings before each increment of normal force is applied (seeASTM D 2435).


Procedure (contd.)


d)

For each load increment, verify completion of primary consolidation before proceeding. Plot the normal displacement readings against elapsed time and use procedure as described for Consolidation test (ASTM D 2435).

e)

Allow each increment of normal force to remain until primary consolidation is complete.

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f) The final increment should produce the specified normal stress. Note: Application of force in one increment may be appropriate for

relatively firm soils. For soft soils, however, several increments may be necessary to prevent damage of the specimen. 3.

Separate the upper and lower halves of the shear box frames by a gap of approx. 0.025 in. (0.64 mm) to start shearing test.


Procedure (contd.)


4.

Position the shear-deformation (horizontal displacement) indicator and set both the vertical and horizontal displacement indicators to zero.

5.

Fill the shear box with water for saturated tests.

6.

Apply the shearing force and shear the specimen.

7.

After reaching failure, stop the test apparatus. This displacement may range from 10 – 20% of specimen’s original diameter or length.

8.

For all tests (except consolidated drained conditions), the rate of shear (i.e. the rate of horizontal displacement) should be 0.05 in./min.


Procedure (contd.) 9.


Obtain data readings of time, vertical and horizontal displacement, and shear force at desired interval of displacement. The displacement interval should be equal to 2% of the specimen diameter (or width).

10. For consolidated drained (CD) test, shear the specimen slowly to ensure complete dissipation of excess pore pressure. Use the following guidelines: tf = 50t50 dr =

d f tf

tf = Time of failure (min) t50 = Time required for 50% consolidation under normal

force (min) dr = displacement rate (in./min) df = estimated horizontal displacement at failure (in)

Note: The magnitude of df depends on many factors including type and

stress history of soil. As a guide use df = 0.5 in for normally or lightly over-consolidated fine grained soil; otherwise use d 0.2 i


Procedure (contd.)


11. At the completion of test, remove the normal force from specimen. For cohesive test specimens, separate the shear box halves with a sliding motion along the failure plane. Photograph, sketch or describe in writing the failure surface. This procedure is not applicable to cohesionless specimens. 12. Remove the specimen from the shear box and determine its water content. 13. Repeat entire procedure for two or more specimen at different normal loads.

Direct shear test.pdf

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