Direct shear test.pdf

1

Direct Shear Test for soils(under consolidated drained conditions)

ASTM D 3080

CE-325 Foundation Engineering I

NUST Institute of Civil Engineering (NICE)

2Objective

To determine the consolidated-drained shearing strength of the sandy to silty soils using the direct shear apparatus. The test is performed by deforming a specimen at a controlled strain rate on a single predefined shear plane.

Direct shear test(ASTM D 3080)

Need and scopeIn many engineering problems such as design of foundation, 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. Direct shear test is used to predict these parameters quickly.


3

The strength of a material is the greatest stress it can sustain.The safety of any geotechnical structure is dependent on the strength of the soil.If the soil fails, the structure founded on it can collapse.

Shear strengthDirect shear test(ASTM D 3080)


4

Failure due to inadequate strength at shear interface

Shear failure in soilsDirect shear test(ASTM D 3080)


5Shear failure in soils (contd.)Direct shear test(ASTM D 3080)


6Bearing capacity failureDirect shear test(ASTM D 3080)


7

West side of foundation sank 24-ft

Transcosna Grain Elevator Canada (Oct. 18, 1913)

Bearing capacity failure (contd.)Direct shear test(ASTM D 3080)


8Significance of shear strengthDirect shear test(ASTM D 3080)

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


9

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

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

Shear strength in soils depends primarily on interactions between particles

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

Shear strength in soilsDirect shear test(ASTM D 3080)


10Shear strength in soils (contd.)Direct shear test(ASTM D 3080)

Soil derives its shear strength from two sources:Cohesion between particles (stress independent component)

Cementation between sand grains

Electrostatic attraction between clay particles

Frictional resistance between particles (stress dependent component)


11CohesionDirect shear test(ASTM D 3080)

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


12Internal frictionDirect shear test(ASTM D 3080)

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


13Mohr-Coulomb failure criteriaDirect shear test(ASTM D 3080)

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.

The relationship between normal stress and shear is given as

φσ ′′+′= tancs

S = shear strengthc’ = cohesionφ’ = angle of internal friction

Shea

r Stre

ngth

, S

Normal Stress, σn = σ′ = γ h

c′

φ′


14Determination of shear strength parametersDirect shear test(ASTM D 3080)

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

Direct Shear Test

Triaxial Shear Test

Soil

Normal stress σn

Shear stress σ3

σ3

σ1

Triaxial test Direct shear test


15

Direct shear test is Quick and Inexpensive.

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

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

ASTM D 3080.

Direct shear testDirect shear test(ASTM D 3080)


16

The test equipment consists of a metal box in which the soil specimen is placed

The box is split horizontally into two halves

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

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

Soil

Normal stress σn

Shear stress σ3

Direct shear test (contd.)Direct shear test(ASTM D 3080)


17Direct shear test (contd.)Direct shear test(ASTM D 3080)


18Direct shear test (contd.)Direct shear test(ASTM D 3080)


19Direct shear test dataDirect shear test(ASTM D 3080)

Shea

r str

ess

Residual Strength

Peak Strength


20Direct shear test dataDirect shear test(ASTM D 3080)

ΔH


21Apparatus Direct shear test(ASTM D 3080)

Direct shear box

Direct shear apparatus

Porous stones

Axial-loading device, & axial load-measuring device

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

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

Displacement indicators

Equipment for remolding or compacting specimens


22Test specimenDirect shear test(ASTM D 3080)

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

Take extreme care in preparing undisturbed specimen.

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.

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

Min diameter to thickness ratio: 2:1


23Specimen preparationDirect shear test(ASTM D 3080)

Undisturbed specimen – prepare undisturbed specimens from large undisturbed samples.

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.


24Specimen preparation (contd.)Direct shear test(ASTM D 3080)

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

Allow specimen to stand prior to compaction in accordance with the following guide:

ClassificationASTM D 2487

Minimum standing time

SW, SP No requirementSM 3 h

SC, ML, CL 18 hMH, CH 36 h


25ProcedureDirect shear test(ASTM D 3080)

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 (see ASTM D 2435).


26Procedure (contd.)Direct shear test(ASTM D 3080)

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.

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.



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.



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 = 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)f

fr

f

td

d

tt

=

= 5050

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 df = 0.2 in.



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.


30Data collectedDirect shear test(ASTM D 3080)

Specimen dataDiameter or side of specimen (in.)Initial height of specimen (in.)Mass of specimen at beginning of test (g)Initial water content data:

Mass of wet soil sample plus can at the beginning of test (g)Mass of oven-dried sample plus can (g)Mass of can (g)

Final water content data:Mass of wet soil sample plus can at the end of test (g)Mass of oven-dried sample plus can (g)Mass of can (g)

Shear stress dataNormal load on test specimen (lb)Rate of shear (rate of horizontal displacement) (in./min)Vertical dial readings, horizontal displacement dial readings, and proving ring

dial readings (in.)

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Direct shear test.pdf