Compression results and discussion

Tables 5.1 and 5.2 respectively show the amount of compression of samples AG2 and DN1 at each compression stage. In Table 5.1 a separate entry is made for additional settlement of sample AG2 during “wet” testing mentioned above.

Table 5.1 Sample AG2 compression

Applied stress (kPa) 0 40 87 165 322 603

After completion of compression stage Sample height (mm) 2329 2037 1818 1654 1491 1377 % of original sample height 100% 87.5% 78.1% 71.0% 64.0% 59.1% after completion of wet testing Sample height (mm) - 1945 1778 1623 1480 1372 % of original sample height 83.5% 76.3% 69.7% 63.5% 58.9%

Table 5.2 Sample DN1 compression

Applied stress (kPa) 0 40 87 134 228 334 603

Sample height (mm) 2239 1663 1437 1313 1120 1029 933 % of original sample

height

100% 74.3% 64.2% 58.6% 50.0% 46.0% 41.7%

The results show that sample AG2 was less compressible than DN1 (AG2 compressed to 59 % of the original sample height at an applied stress of 603 kPa whereas DN1 compressed to just under 42 % of original height at 603 kPa). The comparatively low

compressibility of AG2 was probably due to it having previously undergone secondary settlement during degradation.

In Tables 5.1 and 5.2, applied stresses are shown. Additional stress may arise from the self-weight of the sample. This would be negligible at the top of the waste (the top gravel layer exerts a stress of approximately 1 kPa on the sample) but could increase with sample depth to typically between 10 and 20 kPa at the base of the sample (Table 5.3 and 5.4). This could result in an increase of sample density and therefore a

decrease in drainable porosity and hydraulic conductivity throughout sample - particularly at low applied stress as the stress exerted by the weight of the sample is significant in comparison with applied stress.

However the weight induced stress may be partly compensated by, or exceeded by, stress transmission losses arising from friction between the sample and the cylinder sidewall. The problem of transmission losses is more likely to increase with sample depth and so is a particular problem when testing deep samples. For this reason sample height (length) to diameter ratios of 0.25 or less are recommended for

permeameters with loading pistons for testing soil samples (Daniel, 1994). The height to diameter ratio of uncompressed samples in the Pitsea compression cell exceeds 1.0 and is therefore much higher than that recommended for soil permeameters.

Consequently stress transmission losses could potentially be significant. However these are difficult to assess with certainty. Beaven (2000) stated that the magnitude of stress loss is dependent on the sample depth, the friction angle (δ) between the sample and cylinder wall and the internal friction angle (φ’). The sidewall friction angle for loose household waste against a smooth steel surface was estimated by Beaven (2000) to be about 25o. Estimates for the internal friction angle of wastes vary between 20o and 40o (Jessberger and Kockel, 1991). Lower values may be expected in decomposed wastes or wastes with high water contents. Higher internal friction angles are likely to occur in waste subjected to high strains. The range of possible values is limited by the sidewall friction angle being less than the internal friction angle of the waste and from this and the above estimated sidewall friction angle and range of internal friction angles, the maximum theoretical stress transmission losses at the base of samples in the

stress loss at the base of the sample of samples AG2 and DN1, could exceed 50 %. Stress transmission losses of similar magnitude (up to 60 %) have been recorded at the base of compressible tyre shred samples in smaller scale (300 mm diameter)

permeameters (Benson et al., 2002, Warith et al., 2004) of similar height : diameter ratios to the Pitsea compression cell. Stress losses of this magnitude would be likely to have a significantly effect on density and hydraulic properties throughout the depth of the sample.

However it is possible that actual stress transmission losses may be significantly less than the theoretical maximum. Several methods were adopted in order to evaluate stress transmissions losses for the two samples tested. These were:

• the use of pressure cells installed in the waste sample (section 5.2) to directly measure transmitted stress – these failed to give reliable data

• the installation of a magnetic extensometer (section 5.2) in sample DN1 to directly measure differential settlement throughout the depth of the sample

• the use of drainable porosity data to detect changes in porosity throughout sample depth

• the examination of hydraulic conductivity data throughout sample depth The drainable porosity data for both samples (the plots are shown in Appendix D, section D4) were fairly consistent throughout sample depth at all compression stages. Within the data variations present in the plots, it was possible to conclude that stress transmission losses were significantly less than the theoretical maximum (in excess of 50% - above). The minimum stresses at the base of the samples according to the drainable porosity data are shown in Figure 5.3 and 5.4. It is probable that stress transmission losses are much less and possibly negligible. This is largely supported by the magnetic extensometer data (Appendix D, section D3) which mainly indicates uniform compression and the hydraulic conductivity data (section 7.4) which although is not consistent sample depth, does not indicate overall that hydraulic conductivity is

lower at the top of the samples (as would be expected if samples were preferentially compressed). However this cannot be stated categorically as all methods exhibit some inconsistencies. Consequently minimum and maximum error bars are shown in the presentation of hydraulic conductivity measurements in Figures 7.12.

Table 5.3 Range of possible stresses transmitted to the base of sample AG2

Applied stress (kPa) 0 40 87 165 322 603

Sample height (mm) drained

2329 2037 1818 1654 1491 1377

Stress due to weight of sample (assuming no frictional losses) 20.8 21.8 21.6 20.8 19.6 19.2 Maximum stress at base of sample (applied + sample weight stress) kPa

21 62 109 186 342 622

Minimum stress at base of sample (from drainable porosity data) kPa

Table 5.4. Range of possible stresses transmitted to the base of sample DN1

Applied stress (kPa) 0 40 87 134 228 334 603

Sample height (mm) drained

2239 1663 1437 1313 1120 1029 933

Stress due to weight of sample (assuming no frictional losses) 8.8 14.2 13.5 13.1 11.4 11.2 10.2 Maximum stress at base of sample (applied + sample weight stress) kPa

8.8 54 100.5 147 239 345 613

Minimum stress at base of sample (from drainable porosity data) kPa

- 22 66 107 177 270 490

It should be noted that the sample compression is essentially primary; the duration of each compression stage (about one week) is insufficient to take into account of

‘secondary compression’ arising from waste degradation. Prolonged measurements of waste settlement have shown (e.g. Sarsby, 2000, Watts et al., 2001, 2002, 2006) that secondary compression, although of a much smaller magnitude than primary

compression, will continue on a timescale lasting several months and possibly years (and therefore is impractical to replicate in theses tests). This may be of little consequence for the aged AG2 waste sample as it would already have undergone secondary settlement, but in the field situation fresh waste would be expected to undergo further settlement.