Researcher: Adam Pellew

Supervisor: Dr. R. J. Jardine

Sponsor: EPSRC

Duration of Project: October 1997 to October 2000

Recent work on piles at IC, using the instrumented IC steel pile, has culminated in a design method for driven piles which follows an effective stress approach and uses the Mohr-Coulomb failure criterion at the pile/soil interface¹. The method has already been successfully applied in practice. This three year research programme aims to enlarge the scope of the earlier research under the following headings.
 

The first of these consists of examining the mechanisms that govern the behaviour of bored pile shaft resistance in stiff clay. The inherent difficulties in measuring u, F _r and J _rz at a concrete / soil interface have lead to simulating a bored pile. The shaft is augered in the usual way, then filled with a cementicious grout and the instrumented IC steel pile is lowered into the hole. The grout is therefore displaced up and out of the shaft so leaving a thin 5 mm annulus around the IC steel pile. Since shearing takes place at the soil / grout interface and the stress measurements are made at the grout / steel interface the success of the simulation depends on the grout being strong enough to withstand shearing while being not so stiff as to reduce excessively the total stress measurement sensitivity.

The simulation should reveal certain unknown general features of bored pile behaviour, such as the magnitude and time of effective stress equalisation and the nature of the stress path at the interface, for instance whether restrained dilation occurs during loading. It will also be possible to vary parameters associated with the shaft such as the augering process and the delay before pile installation.

The first of a series of field bored pile simulations has very recently been completed. The photograph shows the extracted pile with a thin coating of London clay around the grout and a section of the grout cut away from the IC steel pile
 

Pile ageing refers to the post-equalisation changes in pile behaviour that may be observed with time. The new driven pile design method acknowledges the strong influence of ageing on the capacities of driven piles in all soil types.

Two jacked and one driven pile have been tested recently at the Building Research Establishment’s Canons Park test site in London Clay. Some 17 years after installation, the results show marked increases in peak and post-peak capacity from those observed at just a few weeks following installation³. Previous tests with the instrumented IC pile at the site show that excess pore pressures due to installation were likely to have been almost fully dissipated after one week and that therefore these changes are due to some other process⁴

A trial pit was dug adjacent to two of the tested piles in order to take soil samples at different depths and radial distances away from the piles. These samples are now being used to measure soil properties; to examine soil fabric; and to evaluate any biological and chemical activity.

The photograph shows the trial pit at an early stage. The two piles may be seen in the immediate foreground and at the far side of the pit. The piles were cased some 2 m through the alluvial clayey gravel which overlies the London Clay.