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1. AIM:
Determination of Shearing Strength of Soils by Direct Shear Test.
2. Apparatus Used:
(i) Shear box, container for shear box, grid plates (Two pairs: one perforated and one without perforation) porous stones, base plate, loading pad (IS 11229)
(ii) Loading frame for applying shear force at a constant rate of shearing displacement
(iii) Loading yoke for applying normal loads.
(iv) Weights when applied on the hanger induce normal load intensities of 0.05, 0.10, 0.50 and 1.00 kg/cm^2.
(v) Proving Ring
(vi) Dial gauges (2 nos.)
(vii) Stop watch, balance, spatula, straight edge.
3. Procedure:
(i) Assemble the two halves of the shear box using the connecting pins.
(ii) Place the shear box inside the container.
(iii) Place the base plate inside the shear box.
(iv) Over the base plate, keep one grid plate without perforations such that the grids ar perpendicular to the direction of shear.
(v) Carefully place the soil sample inside the shear box so that it rests on the grid plate.
(vi) Over the top of the sample another grid plate without perforations such that the grids are perpendicular to the direction of shear.
(vii) Slightly press this grid plate evenly so that the grids are buried in the sample.
(viii) Place the loading pad on the top grid plate.
(ix) Bring the proving ring assembly in contact with U-arm provided for the top half of the shear box.
(x) Place a steel ball on the spherical groove provided on the loading pad.
(xi) Seat the loading yoke on this ball.
(xii) Remove the pins connecting the upper and lower halves of the shear box.
(xiii) Fix two dial gauges, one on the loading yoke to measure vertical displacement and the other on the bracket provided on the shear box container to measure shearing displacement.
(xiv) Apply the required normal load on the hanger of the loading level.
(xv) Apply the shearing force to cause a shearing displacement of 1.25 mm/minute . immediately after applying the normal load.
(xvi) Note the readings on the two displacement dial gauges and the proving ring dial gauge at regular interval of time.
(xvii) Continue applying the shear force, till the specimen fails which is indicated by a kick-back of the pointer in the proving ring dial gauge.
(xviii) If such a failure does not occur, continue till the specimen undergoes a shearing displacement.
(xix) Conduct at least three tests on separate specimens having same density and water content but applying different normal loads.
4. Observation Table:
Time Shear dial reading Shear displacement Normal dial reading Normal displacement Proving ring
(minute) (mm) ∊ (mm)
------ ---------- -------- ----------- ------------ ----------
----- ---------- -------- ----------- ------------ ----------
----- ---------- -------- ----------- ------------ ----------
----- ---------- -------- ----------- ------------ ----------
----- ---------- -------- ----------- ------------ ----------
----- ---------- -------- ----------- ------------ ----------
Shear Force (kg) = -------------------
Shear Stress = Shear Force / Area
5. Plotting the Results:
(i) Draw a graph by plotting the normal stresses as the abscissa and shear stresses as the ordinates corresponding to failure states.
(ii) Join the points corresponding to the state of failure by a line. This line gives the Mohr envelope for the soil.
(iii) Measure the angle which the line (Mohr's envelope) makes the horizontal axis. This gives the angle of shearing resistance (ฮฆ)
(iv) Measure the intercept which the line (envelope) makes with the shear stress axis. This gives the cohesion value c in kg/cm^2.
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