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					Pavements & Structures                                                   TEST METHOD   WA 144.1


                                   DIRECT SHEAR TEST


SCOPE

This method describes the procedure for the determination of the shearing resistance of a soil
using the direct shear test apparatus.


APPARATUS

1.        Direct shear machine (Figures 1 and 2) consisting of the following units:

1.1       A base frame, motorised gear box, loading screw and ball tracks.

1.2       A shear box assembly with a shear box size of 60mm square.

1.3       A calibrated proving ring.

1.4       A loading assembly, 16mm ball bearing and masses for the required normal loads.

2.        A balance of at least 5Kg capacity readable to 0.01 grams or better with a limit of
          performance of less than or equal to 0.06 grams.

3.        A thermostatically controlled oven with good air ventilation and capable of
          maintaining a temperature between 105°C and 110°C.

4.        150mm Vernier callipers.

5.        Suitable equipment as required for preparing the test sample from undisturbed samples
          in the laboratory.

6.        Stopwatch or other suitable timer.


PROCEDURE

1.        Preparation of Test Sample

1.1       Undisturbed Sample

1.1.1 Obtain a test sample from a selected position within a core or other sample.

1.1.2 Cut the test sample to a size slightly larger than the shear box from an undisturbed
      sample using a thin walled tube or other suitable sample preparation equipment.

1.1.3 Determine the mass to at least the nearest 1g, of the shear box, bottom and top grids
      and porous stones (if applicable).


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1.1.4 Position the bottom plate, porous plate if required, and grid into the shear box. Ensure
      that the grid which may be perforated or solid (Note a) is positioned so that the grid
      serrations are at right angles to the direction of shear.

1.1.5 Trim the test sample so that it will make good contact with the sides of the shear box.

1.1.6 Determine the height of the test sample using the Vernier callipers and insert it into
      the shear box.

1.1.7 Place the second grid horizontally on top of the test sample so that the grid serrations
      are at right angles to the direction of shear.

1.1.8 Determine the mass of the assembled shear box and sample.

1.1.9 Determine the moisture content, in accordance with Test Method WA 110.1 or 110.2,
      of a sample taken adjacent to the position from which the test sample was removed.

1.2       Disturbed Sample

1.2.1 Obtain a test sample taken in accordance with Test Method WA 100.1 or from a
      selected position within a core or other sample.

1.2.2 Position the bottom plate, porous plate if required, and grid into the shear box. Ensure
      that the grid which may be perforated or solid (Note a) is positioned so that the grid
      serrations are at right angles to the direction of shear.

1.2.3 Compact the test sample into the shear box to the required conditions of moisture and
      density.

1.2.4 Place the second grid horizontally on top of the test sample so that the grid serrations
      are at right angles to the direction of shear.

2.        Test Preparation

2.1       Place the carriage on the ball tracks and couple to the loading jack.

2.2       Position the assembled shear box into the carriage, as shown in Figure 1.

2.3       Ensure the jack is in the non-extended position by using the handwheel and then
          engage the friction clutch of the drive belt.

2.4       Position the pressure plate, with the 16mm diameter ball in place, on top of the upper
          grid or porous stone as required.

2.5       Fill the carriage with water unless otherwise specified.

2.6       Position a suitable proving ring between the carriage loading screw and the proving
          ring tail screw.

2.7       Zero the proving ring dial gauge.
3.        Unconsolidated Undrained Test (Note B)
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3.1       Assemble the loading cap and hanger and mount a vertical dial gauge on top of the
          load hanger to enable changes in specimen thickness to be monitored.

3.2       Place the normal load on the load hanger.

3.3       Remove the locating screws and replace in the tapped parting screw holes. Screw
          these down until in contact with the bottom half of the box. Slightly part the box by
          turning the screws another half turn then remove the screws. Zero the vertical dial
          gauge.

3.4       Select the shear rate of 40mm/hour and operate the forward control of the machine to
          set the upper half of the shear box into motion.

3.5       Record proving ring and vertical dial gauge readings every 15 seconds for the first two
          minutes and then every 30 seconds.

3.6       Calculate the shear stress as the test progresses.

3.7       Continue the test until the maximum shear stress becomes evident or until the
          maximum shear displacement of 15mm is reached.

4.        Consolidated Undrained Test (Note C)

4.1       Assemble the loading cap and hanger and mount a vertical dial gauge on top of the
          load hanger to enable changes in specimen thickness to be monitored.

4.2       Place the normal load on the load hanger.

4.3       Record vertical dial gauge readings every 30 seconds. Start time (to) is the moment
          that the normal load is placed onto the load hanger.

4.4       Plot dial gauge readings versus time (log scale) until primary consolidation has taken
          place (Note d).

4.5       Remove the locating screws and replace in the tapped parting screw holes. Screw
          these down until in contact with the bottom half of the box. Slightly part the box by
          turning the screws another half turn then remove the screws. Zero the dial gauge.

4.6       Select the shear rate of 40mm/hour and operate the forward control to set the upper
          half of the shear box into motion.

4.7       Record proving ring and vertical dial gauge readings every 15 seconds for the first two
          minutes and then every 30 seconds.

4.8       Calculate the shear stress as the test progresses.

4.9       Continue the test until the maximum shear stress becomes evident or until the
          maximum shear displacement of 15mm is reached.
5.        Drained Test (Note e)


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5.1       Assemble the loading cap and hanger and mount a vertical dial gauge on top of the
          load hanger to enable changes in the specimen thickness to be monitored.

5.2       Place the normal load on the load hanger.

5.3       Record the vertical dial gauge readings every 30 seconds. Start time (to) is the
          moment that the normal load is placed onto the load hanger.

5.4       Plot consolidation versus time (log scale) until primary consolidation has taken place
          (Note d).

5.5       Remove the locating screws and replace in the tapped parting screw holes. Screw
          these down until in contact with the bottom half of the box. Slightly part the box by
          turning the screws another half turn then remove the screws. Zero the dial gauge.

5.6       Select a shear rate which is suitably slow and operate the forward control to set the
          upper half of the shear box into motion.

5.7       Record proving ring and vertical dial gauge readings every 15 seconds for the first two
          minutes and then at suitable intervals of time to allow the shear stress to be monitored.

5.8       Calculate the shear stress as the test progresses.

5.9       Continue the test until the maximum shear stress becomes evident or until the
          maximum shear displacement of 15mm is reached.

6.        Residual Shear Strength (Note F)

6.1       Perform the test as per Procedures either 3, 4 or 5 to the maximum travel of the box.

6.2       Restore the upper half of the shear box to its original position on the lower half of the
          box using the handwheel.

6.3       Repeat Procedures 6.1 and 6.2 until the shear stress becomes constant.


CALCULATIONS (Note g)

1.        Calculate horizontal displacement at each reading by use of the formula:

                    hd = as - ap

          where hd = horizontal displacement in mm

                    as = apparent shift which is calculated by multiplying the machine drive rate
                         in mm/min by the time in minutes from the start of the test

                 ap = actual proving ring displacement in mm
2.        Calculate shear displacement by use of the formula:



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                             hd
                    γ    =      x 100
                             60

          where γ        = shear displacement in percent

3.        Calculate the area of the failure surface by use of the formula:
                                            -6
                    Af = 60 (60 - hd) x 10

          where Af = the area of the failure surface in m²

4.        Calculate the shear stress by use of the formula:

                             Proving Ring Reading x Ring Factor (N)
                    τ    =               Af x 1000

          where τ        = shear stress in kPa

5.        Plot shear stress ( τ ) versus shear displacement ( γ ) and determine the shear stress
          (Note h).

6.        Calculate the normal stress by use of the formula:

                    σ    = Normal load (kg) x 9.81
                                 Af x 1000

          where σ        = normal stress in kPa

7.        Determine the soil strength parameters cohesion (C) and angle of internal friction (Ø)
          if required by plotting peak shear stress versus normal stress (Figure 3) (Note h).


REPORTING

Report the following values and general information if appropriate.

1.        Results

1.1       Normal stress and shear stress to the nearest 5kPa.

1.2       Cohesion to the nearest 5kPa.

1.3       The angle of internal friction to the nearest 1°.

2.        General Information

2.1       Sample reference numbers, date sampled, location, borehole number and depth.

2.2       Field description and Unified Soil Classification of the sample.

2.3       Type of test performed (eg drained or undrained).
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2.4       Specimen dimensions.

2.5       The rate of shear and percent displacement.

2.6       The initial wet and dry density for each stage to the nearest 0.001 t/m3.

2.7       The final wet and dry density to the nearest 0.001 t/m3.

2.8       The final moisture content to the nearest 0.1%.


NOTES

(a)       Non-perforated grids and non-porous plates are used during undrained tests. Perforated grids and
          porous plates are used during drained tests.

(b)       The unconsolidated undrained shear test (or quick test) may be used to obtain shear strength parameters
          of clays and sand clay mixes under total stress conditions.

          Effective stresses cannot be measured since pore pressures generated during shearing cannot be
          measured.

          To minimise drainage, the normal loads should be kept low and shearing should be commenced as soon
          as possible after the application of the normal load.

          When failure is reached there will often be a peak value followed by a lower constant value. The test
          should continue until the shear force is constant and both the peak and lower value should be reported.

(c)       As is the case with the unconsolidated undrained test only shear strength parameters at total stress
          conditions can be measured.

          Complete prevention of drainage is not possible as porous plates are used to allow consolidation.
          Drainage can be minimised by rapid shearing.

(d)       Three hours is considered to be a conservative time for primary consolidation to take place in a 25mm
          specimen for most clays, but this should be monitored for each specimen.

(e)       The drained shear test may be used to obtain approximate values of peak and residual shear strength
          parameters under effective stress conditions, since pore water pressures are allowed to dissipate.

          When testing clays, slow rates of shear are required to allow dissipation of pore water pressure. Clayey
          soils are generally sheared over a period of one working day (approximately 1 mm/hour shear rate), but
          some very plastic clays may require two days to test.

          When testing sands a fast shearing rate may be used. Sands may be tested saturated or unsaturated
          depending on modelling conditions.

(f)       Generally a plot of shear strength versus displacement will show the initial part of the curve reaching a
          peak, then a reduction in shear strength which eventually reaches a constant value. This is the residual
          shear strength.

          Very large strains (possible in excess of 1000%) may be required to reach the residual condition. This
          may be achieved by restoring the upper half of the shear box to its original position on the lower box
          using the manual handwheel when maximum travel has been reached and then continuing shearing.

(g)       In the case of residual shear strength tests, separate values of horizontal displacement, shear
          displacement, area of failure and shear stress are calculated for each 15mm shearing stage.

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(h)       In the case of residual shear strength tests, plot shear stress versus cumulative shear displacement.
          Cumulative shear displacement is determined by adding the shear displacement from each subsequent
          15mm shearing stage.

(i)       Soil strength parameters C and Ø require repeated tests using various normal loads or multi stage
          testing to be carried out.

          Cohesion is the value of the shear stress at the intercept of the line of best fit for peak shear stresses and
          the shear stress axis (Figure 3).

          The angle of shearing resistance is the angle produced by the line of best fit for peak shear stresses and
          the shear stress axis (Figure 3).




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                                    FIGURE 1




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                                                                      SHEAR BOX ASSEMBLY
                                                                      SHEAR BOX ASSEMBLY
                                                                           FIGURE 2




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                               NORMAL STRESS σ
                                  FIGURE 1




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