CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
PURPOSE:
The parameters of the shear strength relationship provide a means of evaluating the load carrying
capacity of soils, stability of slopes, and pile capacity. The direct shear test is one of the methods
for obtaining the frictional resistance component (generally termed the angle of internal friction), the
cohesive component, c, and the shear stress-strain characteristics. Properly interpreted, these
values may be used to determine the ultimate shear resistance of a soil.
It can be pretty tough to watch a hillslope evolve. We just don't have the time in our short lives. So
geomorphologists frequently turn to experiments to get insight into how real-world systems work.
This lab will simulate how a hillslope evolves through time by looking at slope failures in different
materials.
ASTM REF:
D 3080
EQUIPMENT:
Controlled strain direct shear testing machine, shear box, tamper, caliper,
funnels, ring mold, steel plates, slope failure simulation box, ruler, scale.
PROCEDURES:
Angle of Repose
1.
Place a 12 in x 12 in glass plate on a level surface with the roughened side up.
2.
Cover the small opening of the funnel with your fingers and fill the funnel with dry sand.
Position the small opening of the funnel over the center of the glass plate and allow the
sand to flow from the funnel into a conical mound. During flow, keep the opening of the
funnel approximately ½ inch above the surface of the conical mound.
3.
Using the caliper, determine the diameter of the conical mound at three separate locations.
Use these measures to determine the average radius of the conical mound. Position the
height indicator rod so that it just touches the top of the sand mound. Pour off the sand and
reposition the glass plate under the indicator rod. Use the caliper to determine the height of
the conical mound.
Direct Shear Tests
1.
Remove the shear box and mold from shear testing machine. Record the internal diameter
of the mold and the mass of the brass loading cylinder. Reassemble the mold and place the
brass set rods in place so that both halves of the mold are prevented from sliding.
2.
Fill the small aluminum tin with sand and strike flush. Loosely fill the mold by passing the
sand through a small funnel. Compact the sand using the tamper. Place the brass loading
cylinder onto the compacted sand.
3.
Carefully position the shear box onto the direct shear testing machine. Adjust the counter
balance so that the cross-arm applies minimal additional normal load to the specimen.
Increase the normal load on the specimen to the desired value by adding weights to the
suspended platform. Position the horizontal and vertical deformation indicators and zero
appropriately.
CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
4.
Shear the specimen using a controlled strain rate of approximately 0.04 in/min (Setting 12).
Obtain readings of proving ring deformations and vertical displacements versus shear box
deformation until a total horizontal displacement of 0.20 inches has been attained.
Determine the peak shear load value from the test results.
5.
Remove the suspended weights and reverse the direction of movement of the shear device.
Manually slide the top of the shear box towards the retreating load piston until it returns to
its pre-sheared position. Continue reversing the load piston until a small gap between the
piston and shear box is visible. Remove the shear box from the direct shear machine taking
care not to damage the horizontal and vertical deformation indicators.
6.
Repeat steps 2 - 5 for three additional trials, increasing the normal load by approximately
100% for each subsequent trial.
Slope Stability
1.
Position the slope failure simulation box on the counter and clamp in place such that the
sliding wall is free to move vertically without restriction.
2.
Raise the sliding wall to its highest position and backfill the shear box with a selected
slope material. Densify the slope material by rodding and slowly lower the sliding wall
until the elevation is equal to the backfill materials. Record the height of the wall.
3.
Carefully lower the sliding wall in increments of 1 in. Take care not to create any
vibrations while lowering the wall. After each wall movement, capture all slope materials
and determine the mass of the materials and the profile of the slope, noting any scarps
or steep toes.
4.
Continue lowering the wall until it is completely removed from the simulation box. Record
the shape and surface texture of the slope materials.
5.
Repeat steps 1 through 4 for each slope material.
CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
CALCULATIONS
1.
Use the results of the angle of repose tests to determine the slope angle of the conical sand
mound (angle of repose). Use this value as an estimate of the angle of internal friction for
sand in a loose condition, L. (DR ~ 0%)
2.
Using the shear mold measurements, determine the area of the shear plane in square
inches. Combining all loads on the shear plane, determine the effective normal stress on
the shear plane for each test trial in units of psi.
3.
Using the peak load data from each test, determine the maximum shear stress in units of
psi. Construct a plot of maximum shear stress vs normal stress and determine the angle of
internal friction for the sand in a dense condition, D. (DR ~ 100%)
4.
Using the data from the direct shear tests at maximum normal load, construct a plot of shear
stress vs horizontal displacement and a separate plot of vertical displacement vs horizontal
displacement.
4.
Comment of the results obtained and the plots generated from the test data, indicating how
your values/plots compare to typical results provided in lecture.
5.
Prepare a plot of the failed mass of slope material vs wall drop for each slope material.
Comment on the observed trends.
6.
Prepare a plot of slope profiles versus wall position for each slope material. Comment on
the trends observed for each slope material.
CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
DATA SHEET 1
Angle of Repose Tests
Measurement
1
2
3
Diameter of Mound, in
Mound Height, in
Angle of Repose
Direct Shear Tests
Internal Mold Diameter, in _____________________________
Shear Test Number
1
2
3
4
Weight of Sand Above Shear
Plane, lb
0.2
0.2
0.2
0.2
Weight of Suspended Load, lb
3.1
6.0
9.1
21.3
Weight of Brass Loading
Cylinder, lb
Total Normal Load, lb
Normal Stress, psi
Maximum Shear Stress, psi
CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
DATA SHEET 2
Suspended Load, lb ___3.1________
Horizontal
Displacement
in
Vertical
Displacement
in
Proving Ring
Deformation, R
(0.0001”)
* Proving Ring Load (lb) = 0.235 R
Shear
Load
Lb*
Comments
CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
DATA SHEET 2
Suspended Load, lb ___6.0________
Horizontal
Displacement
in
Vertical
Displacement
in
Proving Ring
Deformation, R
(0.0001”)
* Proving Ring Load (lb) = 0.235 R
Shear
Load
Lb*
Comments
CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
DATA SHEET 2
Suspended Load, lb ___9.1________
Horizontal
Displacement
in
Vertical
Displacement
in
Proving Ring
Deformation, R
(0.0001”)
* Proving Ring Load (lb) = 0.235 R
Shear
Load
Lb*
Comments
CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
DATA SHEET 2
Suspended Load, lb ___21.3________
Horizontal
Displacement
in
Vertical
Displacement
in
Proving Ring
Deformation, R
(0.0001”)
* Proving Ring Load (lb) = 0.235 R
Shear
Load
Lb*
Comments
CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
Slope Material ________________ Shape _____________
Wall
Position, in
Weight of Failed
Materials, g
Slope
Profile
Texture ________________
Comments
CEEN 162 - Geotechnical Engineering
Laboratory Session 7 - Direct Shear Testing & Slope Stability
Slope Material ________________ Shape _____________
Wall
Position, in
Weight of Failed
Materials, g
Slope
Profile
Texture ________________
Comments