El funcionamiento del SRPA en la Escuela de Trabajo El Redentor

5.1 Introduction

As discussed in section 2.4.4, stress is the main factor controlling the hydraulic conductivity of household wastes. The overburden stress acting on landfill waste at depth is replicated in the Pitsea compression cell by applying a compressive load, referred to as applied stress, to the samples. This is performed in several stages. At each compression stage the hydrogeological properties of the sample are determined (chapters 6 – 8) in order to evaluate changes in these properties throughout the depth of a landfill.

In this chapter the sample loading (section 5.2) and compression (section 5.3)

methodologies are described, and the settlement results for the two samples tested are given for each compression stage. The terms water content (section 5.4) and density are defined (section 5.5.1) and the results presented. The compression and density values for the two samples tested are compared with those for other wastes (section 5.5.2). Potential errors such as the effects of sidewall friction are discussed.

5.2 Sample loading

5.2.1 Methodology

All tests were carried out in the Pitsea compression cell described in chapter 3. Prior to loading the compression cell sides and base were cleaned. Grease was liberally applied to the inside walls of the cylinder to prevent rusting and possibly reduce sidewall friction during sample compression. The bottom platen of the compression cell was bolted in position and the O-ring type seal inflated to create a watertight joint between the platen and cylinder wall.

A layer of gravel (particle size 10 to 20 mm) was installed at the bottom of the cylinder and raked level (Figure 5.1). The purpose of the gravel was to evenly distribute

inflowing leachate across the sample (introduced through twelve holes in the bottom platen) during the following drainable porosity and hydraulic conductivity tests (chapters 6 and 7). The gravel was usually temporarily flooded before the waste sample was loaded to measure the drainable porosity of the layer - flow meter counters and / or the load cells were used to measure the amount of water admitted. The surface of the water also provided a useful guide for levelling the gravel layer. The thickness of the gravel layer was less than the 150 mm height of the dividing ring (shown on Figure 5.1 and 5.5) on the bottom platen to allow the ring to penetrate into the base of the waste sample. The same arrangement was used for the top gravel layer and top platen dividing ring. In vertical flow tests this permitted leachate flow rates through the inner core of the waste to be measured independently to that through the outer region. Comparison of these flow rates was used to assess if peripheral flow was occurring between the periphery of the waste and cylinder wall (section 7.3).

Figure 5.1 Gravel layer installed at the base of the compression cell ready for waste sample to be loaded. The tube on the left hand side of the photo is the extensometer tube for mounting magnets to assess differential settlement

Waste samples were loaded into the cylinder using a lorry-mounted hydraulic grab (Figure 5.2). The cylinder was tilted approximately 30o from the vertical position to provide sufficient clearance for the grab during loading and yet prevent the bottom gravel layer shifting. After each loading (of approximately 30 to 50 cm depth of waste) the cylinder was returned to the vertical position and the waste raked level. During loading, records were made of the sample depth and weight indicated by the load cells (the load cells had a resolution of 5 kg) under the compression cell

framework.

Total earth pressure cells were installed in the sample (Figure 5.3). These were vibrating wire type cells manufactured by Soil Instruments and were calibrated by the

manufacturer before installation. The purpose of these pressure cells was to measure the transmitted vertical stress at various depths in the sample as some reduction in stress (and compression) with sample depth was expected during compression due to friction between the sample and cylinder wall. In sample AG2 pressure cells were positioned at the top, mid-height and base of the sample. This was revised for sample DN1 to two pressure cells only, installed at the base of the sample. The pressure cells were packed in sand (if within the gravel layer) or vermiculite (if within the waste sample) to avoid direct contact with waste or gravel which may have affected readings.

Figure 5.3. Pressure cell positioned on top of bottom gravel layer

For sample DN1, a magnetic extensometer tube (Soil Instruments) was mounted vertically throughout the depth of the sample (Figure 5.1) The vertical positions of sliding ring-magnets spaced on this tube (Figure 5.5) were located with an

extensometer inserted in the tube, allowing settlement to be monitored throughout the sample depth (in addition to total settlement measured by the staff on the top platen – section 5.3.1).

The top gravel layer (6 to 7 cm thick) was installed on top of the sample and raked level (Figure 5.4). The sample was allowed to settle overnight and the settled sample depth recorded prior to testing. A diagrammatic view of a waste sample installed in the compression cell is shown in Figure 5.5.

Figure 5.4. Top gravel layer installed prior to lowering the top platen

5.2.2

Discussion

In section 2.4.10 it was observed that soil test results can be erroneous if the structure of the sample is not preserved during sampling. The situation may not be so critical for testing fresh wastes as they are artificially laid rather than occurring from natural processes. The method used of releasing large grab loads of waste into the

compression cell cylinder is considered to reasonably replicate the process of waste being deposited off the back of a lorry. Raking the waste level at regular intervals during loading should have minimised ‘edge effects’ near the cylinder wall. However it is difficult to prove or disprove whether a true landfilled waste structure (which may vary from site to site) has been achieved.

The structure of the aged waste is probably more difficult to reproduce as during degradation it would have undergone a degree of natural settlement. The resultant structure would have been totally destroyed during excavation. There is uncertainty whether recompressing a degraded waste (as performed for these tests) would have given a reasonable replication of the original structure. An alternative method of

sampling is suggested in section 8.7.2, but there was insufficient time and funding for this to be used for this research.

Figure 5.5. Schematic cross-section of sample and gravel layer arrangement in the compression cell

SAMPLE