Resultados de la observación: corporeidad

A potentially simpler alternative laboratory design to that of the compression cell would be a rectangular permeameter. Figure 8.16 shows the basis of a rectangular design used for measuring horizontal but not vertical hydraulic conductivity of wastes (TU

Braunschweig, Germany - unpublished). The waste sample is contained within a rectangular box and compressive stress is applied by a piston acting on the top plate. Horizontal flow is induced through the sample by the head difference between the inlet and outlet compartments. Flow is not strictly through a uniform cross sectional area of the sample, but providing the difference between inlet and outlet head is small, this and the small vertical flow component is fairly insignificant. Darcy’s law can therefore be applied directly to calculate horizontal hydraulic conductivity. Due to the low

hydraulic gradient, tests are limited to samples of medium to high hydraulic

conductivity. The low pore water pressure test conditions will be representative of very shallow leachate depths only.

Figure 8.16 Side view of horizontal flow permeameter (TU Braunschweig – unpublished)

The basic design could be modified to measure vertical flow as well as horizontal flow (Figure 8.17). This would require inlet holes on the base and top platen. During the vertical flow test, flow through the vertical screens (for horizontal flow) would have to be prevented to avoid the risk of short circuiting. It may be possible to have

interchangeable screens and solid panels to achieve the required configurations. Alternatively it may be possible to insert flexible but impermeable packing behind the screens as required. Baffles as described in section 8.7.1 could be fitted to the top and bottom plates to prevent short circuiting across the top and bottom of the sample during horizontal flow tests.

A sealing arrangement such as that shown in Figure 8.17 would allow hydraulic conductivity tests to be conducted at higher pore water pressure representative of deeper saturated zones. Essentially the design then becomes a square version of the Pitsea compression cell, but with a different sealing arrangement. It has the advantage of full inlet and outlet areas for flow in both planes (providing short circuiting can be satisfactorily prevented in tests in each plane) with a uniform cross sectional area of

High permeability screens

Dimensions (approx) in mm

FLOW _{Inlet water}

(constant head) Outlet water (constant head) Sample Head difference STRESS (1000 kPa max) 100 100 600 800

the sample for both vertical and horizontal flow. There is neither any enforced vertical component to horizontal flow (as is present with the compression cell inlet and outlet port arrangement) nor any horizontal element to the vertical flow. Darcy’s law could be applied directly to flow in both directions and so numerical modelling would not be required as it is for flow across a cylinder. Venting would be required at the top of the sample to release excess gas.

However sample packing in a square/rectangular receptacle is more problematic than in a round one, and the design may be more prone to frictional losses during

compression. Access to the sample would be more restrictive than the compression cell arrangement, requiring removal of both the top cover and the top platen screen.

Horizontal flow inlet/outlet Horizontal flow inlet / outlet Sample STRESS

High permeability screens for horizontal flow test. Need to be sealed during vertical flow test (top portion of screen also needs sealing as the sample is compressed)

Platen perforated for vertical flow test. This screen and the base screen need to be sealed during horizontal flow test

Base – perforated for vertical flow test

O-ring seal

Vertical flow inlet / outlet

Vertical flow inlet / outlet

Figure 8.17 Outline of suggested design of rectangular section permeameter for measurement of vertical and horizontal hydraulic conductivity in wastes

8.8 Summary

Horizontal hydraulic conductivity tests were carried out on samples AG2 and DN1 in the Pitsea compression cell. A horizontal flow of leachate was induced across the samples and flow rates were measured. A variety of test configurations were used using different numbers of ports. Both ‘confined’ (outflow via horizontal flow ports only) and ‘unconfined’ (outflow through horizontal flow ports and top and bottom of the sample) tests were carried out. Tests on sample DN1 were run in both gas purged and gas accumulated conditions, and at different pore water pressures and flow rates by altering inlet and outlet head configurations.

Groundwater Vistas and the USGS groundwater flow model, MODFLOW was used to assess horizontal hydraulic conductivity of each sample at each compression stage using horizontal flow rates obtained in tests in conjunction with previously determined vertical hydraulic conductivity values. The results showed that horizontal hydraulic conductivity was greater than vertical hydraulic conductivity with kh : kv ratios of both samples being between 5 and 10. This is the first time that such anisotropic flow has been systematically demonstrated for landfill wastes. Further findings of the test results are that kh : kv ratios tend to increase with stress but are unaffected by gas accumulation and possibly different pore water pressures.

The findings are highly significant for the modelling of leachate management and contaminant movement within landfill wastes for which isotropic conditions or an arbitrary anisotropic value previously have had to be assumed.