August 17, 2017

1.     This blog describes how to setup a remoulded sand sample for a triaxial test in a standard triaxial cell using VJ Tech’s updated sample former with attachment for a suction device.

2.     In addition to a load frame and cell, the equipment in Figure 1 is also required.

Equipment Required for Sand Sample Preparation

 3.    Ensure the Back Pressure line and base pedestal are flushed with de-aired water and are free from leaks. Clean the base of the cell.

4.     Place the first porous disc onto the base pedestal of the cell (Figure 2 ).

Porous Disc Placed onto Base Pedestal

5.     Next place the membrane over the base pedestal (Figure 3 ).

Membrane placed over based pedestal

  6. UsUsing the ‘O’ Ring tool place two ‘O’ Rings over the membrane to secure it on the base pedestal (Figure 4 ).

Securing membrane onto base pedestal

7.     Take the silicone grease and sample former. Remove the clamp (see Figure 5 ) from the former so the two halves can be separated.

Sample former and grease

8.     Along the inside edges of the sample former (shown in Figure 6 ) smear a small amount of silicone grease (other hydrocarbon based greases can damage the membranes).

Greased inside edges of sample former

9.     Place the two halves of the former over the base pedestal taking care not to trap the membrane between them Figure 7 (L).

10.  Then place the former clamp over the former and once in place tighten using the locking screw Figure 7 (R).

Sample former fitted onto base pedestal

11. Fold membrane over sample and add suction device (Fig 8).

Suction device fitted to former


12.  Apply suction using the suction device so that the membrane is stretched tight against the inside of the former (Figure 9 ).

Membrane stretched against former

13.  The sand can now be poured into the former to create the required density of sample. The sand can be tamped down using the sample tamper and by gently tapping the side of the former (Figure 10 ).

Tamper inside former

14. Once the sand has filled the former and has been levelled place the final porous disc on top of the sample (Figure 11 ).

Top porous disc in place

15.  Place an ‘O’ Ring over the top cap and place the top cap on the top porous disc (Figure 12 ).

Top cap on top of sample

16.  Fold up the excess membrane over the top cap. Using the ‘O’ Ring tool place the ‘O’ Ring over the membrane and top cap (Figure 13 ).

O ring in place over top cap

17. Remove the suction device from the former. A small vacuum will now need to be applied to the sample. The easiest way to do this is to attach the suction device (used for stretching the membrane) to the valve on the base of the triaxial cell that is on the opposite side to the pore pressure valve connection (Figure 14 ). Then create suction using the suction device, this may need to be repeated a few times to remove a large amounts of air from large samples. Once the air is removed and the suction holds shut the valve that the suction device is connected to. Using this device typically applies a small negative pressure of approximately -15 to -20 kPa. By applying a small negative pressure to the sample the sample shouldn’t collapse once the former is removed. Other methods can be used but are not detailed in this document.

Applying suction to the sample

18.  Carefully undo the sample former clamp and remove it. Then remove the two halves of the sample former again taking care not to damage the sample or membrane. Carefully fold down the excess membrane. Clean any excess sand way from the base of the triaxial cell (Figure 15 ).

Prepared sand sample

The triaxial cell can then be assembled (take care not to damage the sample when lifting on the top half of the cell) and then filled with water for testing.

Tips

  • Before attempting saturation the following can be attempted to reduce the time and pressures required when saturating a sand sample:
  • Flush the sample with de-aired water. This should be done from base to top. To do this, increase the cell pressure to approximately 30 kPa. Remove the suction device to release the vacuum on the sample. Connect a line to the same valve as used by the suction device from the de-aired water tank. Open the Back Pressure valve on the cell and allow water to flow up the sample. Do this until no air is seen coming out of the Back Pressure line. After this flush, set the sample up for testing. Continue with the normal saturation procedures.
  • After flushing the sample (as described above) close the valve to the de- aired water supply and the Back Pressure valve. Increase the Cell Pressure to a pressure greater than 300 kPa. Leave the sample like this for 12 hours (overnight). After this, flush the sample again with clean de-aired water. Then continue with the normal saturation process.
  • The sample can be flushed with CO2. CO2 will dissolve more easily into water than air so reducing the saturation time.

For more information or to speak with one of our experts about the preparation for a triaxial cell sand sample, please contact us.