A series of pile tests provided a level of understanding that informed foundation design for all of the T5 buildings. Four straight shafted test piles were constructed by specialist subcontractor Bachy Soletanche on the T5 site in 2002. In order to determine the extent of skin friction and therefore pile shaft capacity, we installed large polypropylene bags filled with water into the pile bores at the base of the pile reinforcement before concrete was poured.

The bags were connected to the surface by hoses, and by opening valves on the hoses once concrete had cured it was possible to isolate the pile bases completely from the bottom of the pile bores – the water bags had created a 600mm deep void beneath each pile. This enabled the piles to be jacked down in up to four 150mm increments without the effects of end bearing influencing the test results.

Mott MacDonald carried out testing of the piles at three month intervals, enabling a clear picture to be built up of the piles’ initial capacity compared with their residual capacity – that is, their strength before and after being moved. This simulated the effect of the swelling ground moving relative to the piles. Testing started with a single pile. After three months we moved the first pile a second time and another pile for the first time. Another three months on and we included a third pile in the tests. And we tested the last pile for the first time at the end of a year. Staggering the tests illustrated what changes were likely to occur in the interaction between soil and structure over time.

We measured end bearing capacity on two further piles using the innovative Osterberg Cell technique. Before concrete was poured we had placed a steel plate and high capacity jack in the toe of each pile. Our team activated jacks to push down against the bearing plate and up against the pile. We gained an accurate value for end bearing strength by measuring force exerted by the jack and displacement of clay beneath the pile.