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Figure 8.1 shows the general arrangement for a horizontal flow test in the Pitsea compression cell. The top platen seals were inflated during the test to prevent leakage of leachate through the gap between the top platen and cylinder (section 3.3).

Horizontal flow was induced across the sample between the two sets of diametrically opposite ports in the cylinder wall (section 3.4): inflow being through the set

connected to the leachate supply tanks and outflow through the opposite set. All eleven sets of ports could be used when the sample was lightly compressed, but in later compression stages the sample height was reduced below the level of the upper sets and so these could not be used.

Each inlet port was connected to individual header tanks (section 3.5 and Figure 3.5) via flexible hose connections. These header tanks were connected to a common supply tank to maintain the same level of leachate in each tank and hence the same pressure head at each inlet port. Outlet pressure heads were governed by the elevation of the outlet inverted u-bends positioned at a common height below the elevation of the header tank water level in order to induce leachate flow across the sample. A more flexible arrangement was adopted for the later sample DN1 tests as used for the vertical hydraulic conductivity tests (section 7.2 , Figures 7.2 and 7.3). This allowed the outlet u-bend elevations to be controlled by switching valves to either 4.00, 5.00, 7.00 or 9.00 m above ground level (a.g.l.). This, in combination with two possible inlet pressure heads using either high or low level header tanks at elevations of 9.37 or 5.31 m a.g.l. respectively, extended the possible range of flow rates that could be used and permitted tests to be carried out at different pore water pressures (section 2.4.7). In all cases the outlet elevation had to be below that of the inlet (to induce horizontal flow) and also above the top of the sample (to maintain the sample in saturated conditions).

Supply Tank Header tanks Cylinder Gravel layer Isolated ports Seal Bottom platen Inlet Ports Inlet Valves Gravel layer Flow outlets (isolated) Outlets Piezometer tubes inserted in sample at various positions New Seals Top platen Top platen outlets (isolated) T o m ea su rin g cy lin d er Waste Sample P ie zo m et ric h ea d

Figure 8.1 Arrangement of confined horizontal hydraulic conductivity test using three inlet and three outlet ports

Inlet flow rates through each horizontal flow port were measured by briefly shutting off the leachate supply to the relevant header tank, and timing the discharge of a measured volume of leachate from the tank. The leachate supply to the tank was then re-established. Outlet flow rates were measured by timing the discharge from each outlet pipe into a measuring cylinder. Inlet and outlet flow rates were compared to ensure steady state conditions had been achieved. At high applied stresses, inlet flow rates became too low to measure using the above method and only stabilised outflow rates were measured.

Pressure heads within the waste were measured using standpipes connected to open- ended piezometer tubes inserted into the sample through ports in the cylinder wall. These were positioned throughout the depth of the sample. In most tests three sets of piezometer tubes were used: one set with the end of the piezometer inserted near the centre of the sample (1 m from the cylinder wall), one set positioned in the vicinity of the inlets, and the other set near to the outlets (typically 30 cm to 50 cm from the cylinder wall). Later tests included piezometer tubes with ends positioned only a few centimetres from the inlets and outlets in an attempt to obtain in greater detail the pattern of head changes in these areas. Other piezometers were incorporated into the inlet pipework at the entry to the inlet ports to enable any head loss between the header tanks and inlet ports to be measured.

Several different tests were carried out at each applied stress using a variety of input and output port configurations. The configurations could be changed not only by varying the head of the inlet and/or outlet ports as described above, but also by changing the number of inlet and outlet ports. A few tests were run with outflow also allowed via the top and bottom gravel layers (which were maintained at the same head as the outlet ports). These are designated as ‘unconfined’ conditions. Normal tests with horizontal flow between the two sets of ports are referred to as ‘confined’.

Following the observations that vertical hydraulic conductivity was significantly affected by gas accumulation and pore water pressure (section 7.4), horizontal flow tests on sample DN1 were carried out in both ‘purged’ and gas accumulated conditions (this was not done on the aged sample AG2 which showed less signs of gas activity). Nominally purged conditions were attained by draining the sample and then inducing an upward

flow of leachate at a high flow rate. Gas accumulated conditions were usually attained by maintaining static saturated conditions for several days, with gas allowed to freely vent from the sample. Measurements of the volume of leachate displaced by gas accumulation and / or changes in sample weight according to load cell measurements were used to determine when gas accumulation had attained a threshold. Flow was then gradually established by opening the control valves in stages. This minimised gas displacement. The final stabilised flow rate was measured (i.e. when it was established that no further gas accumulation occurring). The process was carried out in both low and high pore water pressures by using the different header tank and outlet elevations described above. The above method, combined with the similar vertical hydraulic conductivity procedure, allowed separate kh : kv assessments to be made by comparing vertical and horizontal flow results according to gas accumulation and pore water pressure conditions.