Direct shear test is performed to determine the consolidated-drained shear strength of a sandy to silty soil. Shear strength of the soil may be expressed by the equation:
𝛕 = c + 𝛔 ′ 𝐭𝐚𝐧 𝝓
Where:
c = cohesion (zero for sandy soil)
𝛔 ′ = effective normal stress
𝝓 = angle of friction of soil
The angle of friction, is a function of the relative density, compaction of sand, grain size, shape and distribution in a given soil mass. For a given sand, an increase in the void ratio (i.e., a decrease in the relative density of compaction) will result in a decrease of the magnitude of 𝝓. However, for a given void ratio, an increase in the angularity of the soil particles will give a higher value of the soil friction angle.
Direct Shear Test Apparatus
- Direct shear test machine (strain controlled)
- Load and deformation dial gauges
- Balance sensitive to 0.1 g
- Large porcelain evaporating dish
- Tamper (for compacting sand in the direct shear box)
- Spoon
Direct Shear Test Machine
It consists primarily of a direct shear box, which is split into two halves (top and bottom) and which holds the soil specimen; a proving ring to measure the horizontal load applied to a specimen; two dial gauges (one horizontal and one vertical) to measure the deformation of the soil during the test; and a yoke by which a vertical load can be applied to the soil specimen. A horizontal load to the top half of the shear box is applied by a motor and gear arrangement. In a strain-controlled unit, the rate of movement on the top half of the shear box can be controlled.
Shear Box
The shear box is split into two halves-top and bottom. The top and bottom halves of the shear box can be held together by two vertical pins. There is a loading head which can be slipped from the top of the shear box to rest on the soil specimen inside the box. There are also three vertical screws and two horizontal screws on the top half of the shear box.
Procedure
- Weigh some dry sand in a large porcelain dish, W1.
- Measure the internal dimensions of the shear box.
- Carefully assemble the shear box. Then place a porous stone and a filter paper in the shear box.
- Fill the shear box with sand in small layers. A tamper may be used to compact the sand layers. The top of the compacted specimen should be about 1/4 in. (6.4 mm) below the top of the shear box.
- Weigh the porcelain dish again and compute the mass of soil used, W2.
- Level the surface of the sand specimen.
- Place a filter paper, a porous stone, and loading head on top of the sand.
- Put the shear box assembly in place in the direct shear machine.
- Apply the desired normal load, N, on the specimen. This can be done by hanging dead weights to the vertical load yoke. The top crossbars will rest on the loading head of the specimen which, in turn, rests on the soil specimen.
- Attach the horizontal and vertical dial gauges (0.001 in./small div) to the shear box to measure the displacement during the test.
- Apply horizontal load, S, to the top half of the shear box. The rate of shear displacement should be between 0.1 to 0.02 in./min (2.54 to 0.51 mm/min). For every tenth small division displacement in the horizontal dial gauge, record the readings of the vertical dial gauge and the proving ring gauge (which measures horizontal load, S). Continue this until after
- the proving ring dial gauge reading reaches a maximum and then falls, or
- the proving ring dial gauge reading reaches a maximum and then remains constant.
Observations and Calculations
| Item | Quantity |
| Specimen length, L (in.) | — |
| Specimen width, B (in.) | — |
| Specimen height, H (in.) | — |
| Mass of porcelain dish + dry sand (before use), W1 (g) | — |
| Mass of porcelain dish + dry sand (after use), W2 (g) | — |
| Dry unit weight of specimen γd (lb/ft3) = [(W1-W2)/LBH] x 3.808 | — |
| Specific gravity of soil solids, Gs | — |
| Void ratio, e = (Gs x γw/γd)-1 | — |
| Normal Stress 𝛔 ′ (lb/in2) | Horizontal Disp. (in.) | Vertical Disp. (in.) | No. of Div. in proving ring dial gauge | Proving ring calibration factor (lb/div) | Shear force, S (lb) | Shear Stress (𝛕) (lb/in2) |
| — | ||||||
| — | ||||||
| — |
Repeat the test for different values of normal stress and plot a graph between horizontal displacement and shear stress as given below:
Plot a graph between normal stress and shear stress using the peak values of shear stress from the above graph.
From the graph above shear strength of the soil can be calculated using the formula:
𝛕 = c + 𝛔 ′ 𝐭𝐚𝐧 𝝓
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Advantages and Disadvantages
Advantages of direct shear test are:
- Simple, quick, and economical (typically around $200 to $250/ envelope)
- Efficient for testing sands and sandy silts
- Can be used to determine interface friction between two different materials (i.e. geomembrane on clay)
- Thin samples can be tested rapidly due to short drainage path
Disadvantages:
- Stresses are non-uniform and straining is difficult to define
- Soil is required to shear on a horizontal plane, rather than a plane of potential weakness.
- Difficult to control drainage conditions.
- The test only provides “drained” strengths (i.e. effective strength conditions).
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