Apparatus

The triaxial testing system employed for this study is schematically shown in Figure 3-7. A Perspex cylinder with top and bottom plate was used as the cell, so that a cylindrical sample (50mm in diameter and 100mm in height) and local (on specimen) transducers can be placed inside the cell. The cell was filled with de- aired water. Confining stress to the specimen was applied by water pressure. A Digital Pressure Volume Controller (DPVC) was used to apply/control cell pressure. The top and bottom platens of the specimen were connected to a second DPVC to control pore water pressure/volume inside the sample.

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A force actuator controlled by a Geotechnical Digital System (GDS) digital controller was used to apply load to the bottom of the specimen. Therefore, the bottom platen was the moving platen and “free” movement was facilitated by a linear roller bearing. A submersible load cell located just above the top platen, i.e. between the top platen and the shaft, was used to record the axial force.

3.3.1.1 Axial force

The applied load on the sample was recorded by a submersible load cell (2kN capacity) positioned above the top platen of the specimen inside the Perspex cylinder so that the recorded data was unaffected by any inevitable frictional forces. The load cell is manufactured by GDS with an accuracy of ±0.1% of total capacity i.e. ±2N. Thus for a sample with diameter of 50mm the accuracy is within 1kPa.

3.3.1.2 Cell Pressure

The cell pressure was applied by water pressure using de-aired water by a GDS Digital Pressure Volume Controller (DPVC). A DPVC is a microprocessor controlled linear actuator for precise regulation and measurement of liquid pressure and liquid volume change. It can be directly connected to a computer for computer control via a General Purpose Interface Bus (GPIB).

The principle of DPVC in soil testing is illustrated in Figure 3-8 and detailed description is given in the GDS Handbook (2000). The DPVC consists of a pressure cylinder which is filled with de-aired water. De-aired water is pressurized by a piston moving inside the cylinder and the applied pressure is displayed via a Liquid Crystal Display (LCD). The piston is activated by a ball screw turned in a captive ball nut by a stepping motor and gearbox that move rectilinearly on a ball slide. The pressure inside the cylinder is detected by means of an integral pressure transducer. Control algorithms are built into the programmable memory to cause the motor to seek to a target pressure or step to a target volume change. The latter is achieved by counting the steps of the stepping motor.

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The DPVC used for cell pressure had a maximum operational loading capacity of 2000kPa and a volume change capacity is 200 cc. The resolution of DPVC was 1kPa for pressure and 1 cu mm for volume change.

Figure 3-8 Schematic diagram of digital volume change controller (GDS Handbook, 2000)

3.3.1.3 Pore Water Pressure and Volume Change

The pore water pressure and the volume change of the soil specimen were monitored and controlled by another 200cc volume change capacity DPVC connected to both top and bottom drainage lines. The role of this DPVC depended on the testing conditions. Thus, in drained testing the DPVC was used to measure the volume change and controlled (kept constant) the pore water pressure. In undrained testing, it controlled (kept constant) the volume change and measured the pore water pressure. It can switch from one mode to another via computer control and therefore enables the shearing condition to switch smoothly from a drained condition to undrained condition during testing.

A pressure transducer was connected to the top platen of the specimen to have an alternative data source to cross check data from DPVC and to check pore water pressure uniformity (equilibrium) inside the sample.

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To properly understand the behavior of the soil during creep and its effect on undrained shear, a continuous and fast logging system is needed. External measurements are believed to be inadequate due to factors such as low resolution, seating errors, bedding errors (Baldi et al, 1988). Thus in addition to external measurements, on-specimen local LVDTs are required for accurate creep measurement and stiffness calculation particularly at small-strain values which are focus of this study.

Thus axial deformation of the sample was monitored by one external LVDT and two local/on-specimen LVDTs. The linear range of the local LVDT is ±5mm. LVDT are placed diametrically opposed and mounted on the specimen through the axial pads. Local axial strains are calculated as the average from both local LVDTs. The axial pads (see Figure 3-9) that held the LVDT and reaction plate were redesigned and made so they were thicker in order to accommodate the use of enlarged platens. Based on some trials, the gauge length rod was also redesigned and remade so it has a niche thus making it easier to screw it to the axial pad

Initially the axial pads were pinned to the specimen but then this was discarded due to a leaking problem. Thus the axial pads were glued using instant contact adhesive i.e. loctite glue.

3.3.1.5 Radial deformation

Although for undrained shear, radial measurement is theoretically not significant (due to zero volumetric strain), direct measurement of radial change is important for measuring deformation during creep. Therefore radial deformation of the sample was monitored by local radial LVDT through an in-house made radial calliper, adapted from Bishop (1962). The radial LVDT has linear range ± 5mm thus the maximum radial strain is about 10% (for 50mm sample diameter). The calliper was mounted on the test specimen by means of two diametrically opposed

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pads bonded to the membrane by loctite glue. The LVDT was positioned across the opening of the calliper where it measured the opening and closing of the jaws. The measurement position of the radial LVDT was the same distance from the centre of the positioning pads as they were from the hinge. Therefore the radial LVDT measurement was twice the equivalent change in specimen diameter.

(a) (b)

Figure 3-9 ‘In house’ axial LVDT pads (a) and gauge length (b)

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