This week, we are rolling back the clock to a job we accepted in the of summer 2012. Our client wanted us to undertake Parallel seismic testing at Wembley Central railway station. The purpose of the work was to provide information on the length and integrity of 2 piles on the disused track of platform 7 to assess the suitability for a proposed hotel and shopping complex.

Parallel seismic testing is a non-destructive method used to determine the depth of below ground entities such as foundations or piles. Firstly, trial pits were dug to locate piles suitable for testing. The positions of these trial pits were determined from historical information indicating the position and dimensions of piles. Of the four pits dug, two were considered suitable locations for the boreholes required for the seismic testing.

Construction records indicated 900mm diameter piles were installed to a length of 18.20m and under reamed at the base to a diameter of 2.40m. For the purpose of testing, if an under reamed section was not located during the borehole installation works, the bore hole was to continue to a depth of 30m. A bore hole was completed by means of shell and auger cable drilling equipment to a depth of 30m at a distance no greater than 400mm from the pile location. A 55mm diameter watertight pipe was then installed the full length of the borehole and grouted into place with a sand and bentonite grout mixture.

With the borehole completed, the parallel seismic testing could begin. The Method of parallel seismic testing involves striking the top of the pile with a hammer thus inducing a vibration wave and measuring the time taken for the wave to arrive at a hydrophone which is lowered at 500mm increments down the length of the borehole. Sound travels faster through concrete than clay. Therefore, it is possible to accurately calculate where the pile ends

The recorded hydrophone readings showed a gradient for both piles indicating vibration speed of 4.00m/ms to 4.20m/ms, which is comparable with the sonic velocities within cured concrete. A second distinct gradient showed readings of 1.6m/ms to 1.7m/ms, which correlates with the P wave velocities within clays and saturated soils typical of the London area.

The data collected matched that of the historical records. It also showed that the piles were in good condition with no sign of voids. These results satisfied our client and the works for the shopping centre and hotel commenced in November 2012 and were completed in summer 2015.

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