Self-weight consolidation plays a major role in the creation of land using mud, the ripening of mud layers and, also, in the storage of contaminated mud and slurries in ponds. There is a significant compaction due to self-weight consolidation at lower stresses (0.1 to 10 kPa) which gives the need for special laboratory tests and accurate modeling. The self-weight consolidation theory according to Gibson was extended to other time dependent processes: Gas production has been included in this theory and in a numerical code (Wichman 1999a and 1999b) and this was validated in the laboratory and the field (Wichman et al. 2000). The userfriendly numerical tool FSCongas has been developed, including several gas production scenarios with gas dissolution, and a choice of boundary conditions. To this tool also a simple creep model has been added (Greeuw et al. 1999).
In the hydraulic consolidation tests GASCON and HYDCON the loading condition in the Slufter disposal site – with up to 20 metres of mud at maximum effective stress of 10 kPa – has been successfully simulated (Wichman et al. 2000). In these tests the mud sample was left to consolidate and stiffen for some days, and next it was loaded gently by applying a hydraulic gradient across the mud sample. In the GASCON-tests, after the initial self- weight consolidation was finished, the hydraulic gradient was increased a several times. Settlement and pore pressures at short distances across the sample were monitored. The GASCON-cell allows for the monitoring of gas production and gas accumulation throughout the consolidation process, whereas the HYDCON-cell is suitable for saturated samples, only. In this previous research the experimental conditions were chosen carefully, resembling the field situation, as effects of thixotropy and creep were expected.
Recently, effects of thixotropy and creep were studied more closely by means of the Suction Induced Consolidation test (SIC). Some results are discussed in this article. Initial sample conditions, creep and thixotropy may significantly influence consolidation performance and containment structure stability in the short and long term. Permeability of the sample affects the consolidation speed. The response of the sample permeability to a varying imposed sucking flow rate was studied and compared to the field situation.
Two processes were found to have a major effect at low stress levels, i.e. thixotropic stiffening and creep. Mitchell (Mitchell 2005) says: ‘Thixotropy is defined as an isothermal, reversible, time-dependent process occurring under conditions of constant composition and volume, whereby a material stiffens while at rest and softens or liquefies upon remolding’. This effect is largest in a soil structure with high water content and active clay content, as the soil particles have more freedom to rearrange, react and form bonds. Sills (Sills 1995) has shown that creep can be a mayor effect at low effective stress levels, i.e. the volume effect induced by creep is one order of magnitude larger than at higher stress levels (say 50 kPa). Creep can be considered as a time dependent adjustment of soil structure at constant effective stress, which gives a reduction in pore volume and an increase in strength. In addition, in permeability testing care should be taken not to disturb the sensitive soil structure. Permeability values might depend on the imposed sucking flow speed, which is discussed in this article.
The experimental set-up of the SIC is shown in Figure 1. A sample of freshly mixed mud is placed in a ring in a triaxial cell, which is partly filled with water. The sample can be drained at both sides or at the upper side only. The latter was the case in the consolidation tests presented here. The sample is loaded by pressing the sample against a fixed stamp, by using the load control at the bottom of the triaxial cell. At the bottom of the sample a piston pump is attached that can be used to extract a chosen discharge from the sample. In this way the sample can be consolidated by sucking water from it, but this was not done so in our research. By sucking a small amount of pore water, a permeability test can be done. The load, displacement, differential pore water pressure across the sample and cell pressure (set to 3 Bar) are measured.