The specimens required for testing in this type of deformation cell are a right cylinder with a diameter/height ratio o f 0.5. All the Barbados samples were approximately 38 mm in diameter and about 76 mm in length (BS 1377, 1990). However, for materials with extremely low permeability such as the Lichi Mélange scaly clay from Taiwan, this size was too great to allow full consolidation within the time constraints of the project. A smaller sample size was therefore used. Details o f this reduction in size are described later in this chapter.
The 38/76 mm right cylindrical specimens o f Barbados samples were produced using a standard soil lathe (Figure 5.2.4). A small block with the long axis parallel to the vertical direction recorded in the field was cut carefully from the undisturbed, preserved sample (Head 1982). To prevent specimen drying out during the preparation, which causes surface cracks to appear, the samples were repeatedly sprayed with distilled water. On removal of the specimen from the soil lathe, it was cut to the required length and the ends made plane and normal to the specimen axis within 0.5 degree (BS 1377, 1990). Due to the pervasive scaly fabric and the inhomogeniety o f the material sample specimens broke into pieces easily and therefore extra care had to be paid during sample preparation.
The Lichi Mélange scaly clay contains numerous clasts o f varying size and hardness and it was not possible to produce specimens using the soil lathe without damaging the sample and its fabric. An alternative low-vibration coring technique was therefore employed instead o f the soil lathe. The Lichi Mélange scaly clay was very stiff when in its natural state but became muddy very quickly when in contact with water. Coring had, therefore, to be slow enough to minimise disturbance to the natural fabric, but fast enough to prevent the sample softening and turning into its remoulded state.
As mentioned previously, a smaller specimen size was needed for the Lichi Mélange scaly clay. In theory, the time required for completion o f consolidation o f a sample has a root square function relationship to the distance that the pore fluid travels. As a result, reduction in size, both length and diameter, by a factor o f two will shorten the time required for consolidation by a factor o f four. The sample size was reduced by a factor of 1.5 which gives a specimen approximately 51 mm in length and 25.4 mm in diameter. This follows the work o f Petley (1994) who determined that a reduction in the size of the test specimen by a factor o f 1.5 does not have a measurable effect on the experimental results. Samples smaller than this are affected by inhomogeneity o f the material
properties o f the test material which introduces variability in the test results (Petley, 1994).
Once a cylindrical specimen was produced, it was enclosed in a thin latex rubber membrane sealed to both the top cap and bottom platen of the cell using rubber o-ring seals (Figure 5.2.5, Plate 5.2.2). Sometimes two membranes were used because the sand and coarser particles tended to puncture the membrane under pressure. The membrane acts as a seal to separate the specimen from the confining fluid. Between the specimen and both the top cap and bottom platen filters were placed (Figure 5.2.5) to prevent detached particles invading the drainage system. Drainage connections to the top and base o f the sample were made through the top cap and pedestal (Figure 5.2.5). Top and bottom pore fluid pressures can therefore be measured independently, and fluids drained from the specimen through these lines.
The Barbados samples received no further treatment before they went into the cell. However, for the Taiwan material, reduction in specimen size did not adequately shorten the consolidation time. To further accelerate consolidation side drainage was employed. A filter paper, trimmed to the right size and shape, was placed around the sample (Head, 1986; Yassir, 1989) allowing pore fluid to drain from the sample, not only vertically but also horizontally. Although side drainage speeds up the consolidation remarkably, there was a disadvantage. In such experiments pore fluid pressure measured at the undrained end o f the sample did not represent the pore fluid pressure in the interior of the specimen. This is because the filter paper increases the fluid communication between top and bottom o f the specimen, over the surface of the sample cylinder, and the pressure at the surface is reduced compared with that in the interior o f the sample. The extent of this depends on how impermeable the test material is. The consolidation path for a low permeability material using side drainage will thus have a form shown in Figure 5.2.6. An apparent rapid increase in mean effective stress without appreciable volume strain is recorded during the first part o f the consolidation. This is due to the reduction in measured pore fluid pressures at the boundary o f the specimen. Most o f the volume strain occurs subsequently when top and bottom pore fluid pressures have equalised. Therefore, real pore fluid pressure and effective stress can only be determined by closing the drainage line for a period to allow the measured pore fluid pressures to equalise with those in the interior o f the sample. The consolidation paths o f experiments TWL-400 (Figure 5.4.38) and TWS-400 (Figure 5.4.44) show the typical response to closing the drainage line during consolidation. The upper curves in Figures 5.4.38 & 44 are the recorded experimental behaviour using the re-equilibrated pore fluid pressures, and the lower curves, the true consolidation paths.
‘A’
§
Figure 5 .2 .4 M anual soil lathe and trimming saw used to produce cylindrical specim en for triaxial testing
Filter paper Membranes Top cap O' nngs Porous disk Sample Filter paper Porous disk O' rings Pedestal
Figure 5 .2 .5 Sam ple in the membrane
' ' i & i c
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a #
1.35-1
Experimental consolidation curve obtained using side drainage — • Reconstructed "true" consolidation curve
1 .3 0 - d> s 1.20 - 1.15 100 200 300 400 500 600 0
Mean Effective Stress (kPa)
Figure 5.2.6 Typical consolidation curve for using side drainage and reconstructed consolidation path.