Resultados de la conversación informal

Although the modified compression cell fulfilled several important test criteria, some shortcomings of the design were apparent during testing. These, and possible remedies to them, are discussed in this section.

8.7.1 Suggested improvements to the compression cell design

Perhaps the most serious criticism of the compression cell design was the use of relatively small horizontal flow inlet and outlet ports (the size of the ports being limited in order not to weaken the cylinder – section 3.4). As detailed in Appendix J, flow appeared to have been affected by variations in waste permeability in the vicinity of the ports. Although the effect on flow is assumed to be averaged by the use of several ports in each test it may have been beneficial to conduct two sets of tests for each horizontal flow configuration, reversing the flow in the second test. This would have allowed the flow characteristics of each port to have been investigated but would have required more complex pipework and extended test times.

If further horizontal flow tests were to be carried out in the Pitsea compression cell, it would be worth considering abandoning the horizontal flow ports and replacing them with a pair of larger orifices set diametrically opposite each other in the cylinder wall

(Figure 8.12). Flow through the larger horizontal area would be less susceptible to localised variations in waste hydraulic conductivity. During compression, the orifices in the cylinder would have to be blanked off with a solid curved panel to prevent waste being squeezed out. This could remain in place during vertical hydraulic conductivity tests but removed for the horizontal flow tests. A mesh panel may have to be fitted during the horizontal flow tests to prevent the waste collapsing or being washed out in this area. Suitable strengthening of the cylinder would be required.

A further modification would be to fit gas venting pipes through the top platen to prevent gas build up in the upper regions of the sample during horizontal flow tests. This could potentially reduce flow or divert it through the lower regions of the sample.

The results of some tests had to be excluded from the final kh : kv assessments as exceptionally high flows were evident through the lower ports (Appendix J). It is assumed that leachate flow was short-circuiting from the bottom inlet port, across the bottom gravel layer to the bottom outlet port. This highlights a fundamental problem in the design of bi-planar flow test equipment – how to prevent the distribution layer necessary for flow in one of the planes affecting flow in tests conducted in the other plane. In the compression cell design the use of small ports for the horizontal flow would have been unlikely to affect vertical flow, but it appears that the gravel layers for the vertical flow may have allowed horizontal flow to short circuit in some tests. The above proposed orifice would not be positioned as low as the previous lower ports and this may be sufficient to prevent short circuiting. The path length between inlets and gravel layers could be increased further by confining the top and bottom gravel layer to the area within the dividing ring (Figure 8.12). Consequently gravel could not be used as the distribution medium as its low compressibility would prevent

compression of the outer ring of waste. Tyre shreds would probably be suitable, being highly permeable and exhibiting similar compression under load as wastes (Benson et al., 2002, Hudson et al., 2003, 2004). A disadvantage with this arrangement is that installation of samples would be more complicated. Furthermore a component of horizontal flow would be introduced in the vertical hydraulic conductivity test and so numerical analyses, rather than straightforward application of equation 2.1, would be required to determine vertical hydraulic conductivity.

Inlet or outlet for horizontal

flow _{Inlet or outlet}

for horizontal flow

Inlet or outlet for up or down vertical flow (isolated for horizontal flow test) Inlet or outlet for up or down vertical flow (isolated for horizontal flow test)

Gas vents in top platen – top of pipe

raised above inlet elevation

Reduced area high permeability layers to prevent short-circuiting Mesh screen for

horizontal flow (replaced by solid

panel in vertical flow test)

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Figure 8.12 Suggested modifications to the Pitsea compression cell for improved horizontal and vertical hydraulic conductivity tests

An alternative method of preventing horizontal flow across the bottom high

permeability layers would be to add vertical baffle plates to the bottom platen (Figure 8.13). In effect the top platen already has baffle plates across its diameter (Figure 8.14) and this may be why short-circuiting was not evident across the top gravel layer. A suggested pattern for the bottom platen is shown in Figure 8.13 showing two baffle plates across the existing dividing ring. These would protrude into the samples as shown in Figure 8.15, directing any flow across the bottom gravel layer upwards and back into the waste sample.

Figure 8.13 Sketch of suggested baffle plate arrangement on bottom platen to prevent short-circuiting via high permeability layer (view from above)

Figure 8.14 Compression cell bottom platen (in fully extended eject position) showing top surface normally covered by the bottom gravel layer. The dividing ring protrudes through the gravel layer into the waste sample

Baffle plates

Existing dividing ring Direction

Figure 8.15. Cross section of lower portion of suggested modified arrangement showing baffle plates

8.7.2 Alternative methods of obtaining waste samples

In section 2.4.10 it was questioned whether the structure of waste samples were realistically replicated by the loading process described in section 5.2. Of particular concern was the preservation of the structure of the aged waste sample AG2 (section 5.2.2).

An alternative approach may be the use of a large-scale sampling tube as used by Rosqvist (1999). A 1.93 m diameter x 2 m high steel tube was alternately excavated and driven into landfill waste. Top and bottom plates were then welded in place and the assembly lifted out. When installed in the laboratory the cylinder then served as the test column (no compression applied).

In effect this method is essentially a large scale version of a Shelby tube and piston sampler (section 2.4.10). The same advantages and disadvantages are apparent: the

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