Saturation Procedure for HAE Ceramic Discs in SWCC Systems
Adequate saturation of the High Air Entry (HAE) ceramic disc is important in order to perform a Soil Water Characteristic Curve (SWCC) test as it provides higher accuracy to the determination of the water volume changes. Saturation of the HAE disc allows a direct contact between the water in the soil pores and the water underneath the disc. In this way, water can move freely, under differential pressure, through the disc. In contrast, air cannot pass the HAE disc if it has a pressure lower than the Air Entry Value of the disc.
This document describes the procedure to saturate the HAE ceramic disc on a Multi-Purpose Consolidation Cell (MPCC) and it should be used in conjunction with the “SWCC system guide” and the glossary of terms.
It is assumed that all the hydraulic connections have been made (see the “SWCC system guide”) and the distribution panel has been flushed and is free from air. The water that will be used for the saturation of the HAE disc should be properly de-aired.
For this procedure, the following components will be used:
1. The Multi-Purpose Consolidation Cell (MPCC) with the appropriate kit for performing the SWCC tests (VJT0651-H_UNSAT70) fitted (Figure 1);
2. A hydraulic Automatic Pressure Controller (APCH) which is normally used for controlling the pore water pressure during the SWCC test (Fig 2);
3. A pneumatic Automatic Pressure Controller (APCP) which is normally used for controlling the pore air pressure during the SWCC test (Figure 3);
4. A water container where the pipe from the flush valve rests (Figure 4);
5. Tools to remove and re-fit the HAE ceramic disc (Allen key or screwdriver);
1. Remove the HAE disc from the base pedestal by undoing the screws that are holding it in place (Figure 5).
2. Inspect the HAE disc for damages or cracks – Figure 6 shows an example of a damaged disc.
3. Connect the pipe from the APCH to the PwP valve of the cell.
4. Apply a small pressure on the APCH and open the Pore Water Pressure Valve to flush water out from the spiral groove on the pedestal (Figure 7). Once no more air is extracted, close the Pore Water Pressure valve on the MPCC and stop the APCH. Leave some water on top of the the pedestal.
5. Attach the HAE disc onto the pedestal. Cross-tighten the bolts to fix it in place (Figure 8). Sweep any excess water that has escaped on the top.
6. Make sure that the pipe from the flush valve is resting inside the tray and is submerged under water (Figure 9).
7. Open the Pore Water Pressure valve and the Flush valve on the MPCC. Apply a small pressure on the APCH. Let water flush through the spiral groove and out from the Flush valve, removing all the air from the line. Once no more air bubbles are coming out of the flush pipe, close the Flush valve.
8. Apply a small pressure (less than 50 kPa) to the APCH. Do not exceed 50 kPa of pressure; otherwise there is a risk of the HAE disc detaching from the base.
9. Allow water to pass through the HAE disc. Observe any signs of water leak from the pedestal. If there is one, stop the procedure and inspect the HAE disc for damage. Under normal circumstances, water should only come out from the light-colored surface of the disc (Figure 10). Leave until a water bubble has been formed which covers the entire HAE disc surface. This may take between a few hours and a few days, depending on the rating of the HAE disc (higher AEV require longer times).
Signs of imperfections to the HAE disc may include water coming from the top of the disc through the glue, the screws or from the circumference of the metallic supporting disc (Figure 11).
10. Place the cutting ring and the support ring on the MPCC. Top up the de-aired water on the top of the HAE disc to a height of about 10-20 mm (Figure 12). Stop the APCH controlling the water pressure.
11. Assemble the MPCC. Apply air pressure to the MPC, using the APCP, to about 10% less than the rating of the HAE disc. For example, for a 5 Bar HAE disc apply 450 kPa air pressure. Leave the cell pressurised for at least 24 hours (higher AEV require longer times). The increased air pressure will initiate the dissolving of trapped air bubbles that existed within the HAE, into the de-aired water. The APCH must be in an idle state (i.e. it should not control the pressure) and the water pressure is expected to rise up to the same value as the air pressure.
12. While the MPCC is pressurised, apply a pressure of 20 kPa on the APCH for 10 seconds and then open the Flush valve on the MPCC. Observe any air bubbles coming out from the flush valve, close it once they are removed completely. Stop the Pore Water Pressure APCH.
13. Repeat step (12) a few more times within the 24 hours period, allowing the water pressure to build up to a value close to the air pressure before it is repeated.
14. Drop the water pressure to the APCH to 0 kPa. Stop the controller.
15. Reduce the air pressure to the APCP to 0 kPa. Stop the controller.
16. To prevent cavitation of he disc, leave a thin film of water on the surface, until the soil specimen is installed.
It is very important to reduce the water pressure first, when the saturation procedure ends, before dropping the air pressure.
Tip: For better saturation results leave the MPCC pressurised for longer periods.