Construction of the model 50 

5.3.1 Construction of the model piles

Before construction of the model piles could take place, the exact distance of the top of the sample from the top of the tub was measured. The model was designed for a sample depth of 300mm, but as it can be difficult to measure the exact depth of the clay slurry when it is put into the tub before consolidation, this depth is not always achieved. The actual depth achieved varied between 295mm and 305mm so, once the exact depth was known, the guide for the pile cutting apparatus was adjusted to ensure model piles of the correct length were constructed.

The plate of the guide was bolted to the top of the tub to keep it in place during construction of the model piles. Fittings were attached to the top and bottom of the guide, guiding the pile cutting tool to cut half the length of the pile. The tool was rotated to ensure the clay that was being cut was sheared off at the bottom. The clay that was extracted was then cleaned from the tool before the fitting at the top of the guide was removed to enable the full length of the pile to be cut.

Once both the straight boreholes were cut, the fittings at the bottom of the guide were also removed and a further attachment to guide the rib-cutting tool was connected to the top of the guide. The rib-cutting tool was passed through the guide and into the borehole, and screwed into the top of the guide attachment to lock it into place. The handle at the top of the rib-cutting tool was then pushed down, forcing the teeth of the tool into the side of the borehole, and rotated anticlockwise to cut the rib profile. When the rib profile was cut the handle was

pulled up again so that the teeth retract, and the tool was unlocked and taken out. Once the profiles of both piles were complete, the plate of the guide was removed from the tub, and the boreholes were ready forming of the piles.

The resin was prepared before the tub was taken out of the consolidation press. 50g of filler and of each part of the resin were weighed out in plastic cups and left ready for mixing. After the cutting of the piles was complete and the guide removed from the tub, the resin could be mixed for the forming of the piles. Part A was first mixed with the filler, then Part B mixed in. The 2 parts start reacting straight away so the resin was mixed thoroughly and quickly in order to pour as soon as possible. The resin was poured directly into the borehole, slowly and steadily so as to avoid air bubbles forming while the resin set.

After the resin was poured into both boreholes, the pile head attachments were placed into the centre of each pile, the spacers connected to the aluminium plates to hold the attachments in place until the resin was set. The resin took approximately 5 minutes to set completely. In the meantime the model was prepared for testing.

5.3.2 Preparation of the model for testing

The LVDT apparatus was bolted to the tub (Figure 5.4) and the LVDTs adjusted so that there was plenty of space for the pins to move up and down. The tub was then weighed, as was the loading apparatus before being put on top of the tub over the LVDT apparatus. Care was taken to ensure all cables coming out of the apparatus from instrumentation were kept safe from getting in the way of the loading pins or getting stuck between the apparatus and the side of the tub, also with enough cable kept loose to enable movement of the instrumentation in the case of the load cells. The counterweight at the opposite end of the swing arm to the swing was then adjusted so that it would be balanced with the weight of the swing when the centrifuge was spun up.

The tub was then mounted onto the centrifuge swing and bolted onto the swing through its base plate. A camera and light were bolted onto the tub with the loading apparatus. The instrumentation was plugged into the corresponding channels on the junction box, and all loose cables tied down, including the stoneless PPT which was put into the standpipe. The plastic tube for flow into the standpipe and the inflow plastic tubes to the reservoirs of the loading apparatus were also plugged into the corresponding connections and tied down. The standpipe tube was connected to the valve at the bottom of the tub that is next to it and the valve on the opposite side was opened. The valve for controlling water flow into the standpipe, which is in the centrifuge control room, was then opened until water flowed cleanly out of the valve on the opposite side of the tub to the standpipe. This cleaned out the base of the tub. Lastly, 850ml of silicone oil was poured onto the surface of the sample via a funnel through a tiny gap in the apparatus attached to the top of the tub. This was to prevent drying out of the clay surface as the surface of the water table curves with the curvature of the centrifuge’s rotation, leaving up to 15ml depth of clay unsaturated in the centre of the sample. The standpipe valve was now opened again and left open, and the centrifuge was ready to be turned on.