The Impact of Long Opening Times on Shaft Friction in Cohesive Soil

In recent years deep foundation have become deeper and pile diameters and panel thickness have increased.  The pile and panel excavation times have increased from less than a day to more than 5 days when allowance is made for weekend work.  In addition, rig break downs and construction problems can extend this opening time. In cohesive soils the clay next to the pile shaft is known to soften and swell with extended opening times.  With bentonite support fluids the water flow rate into the clay is so slow that a filter cake does not have time to develop. The results of pile load tests with extended opening times of up to 3 days are summarised to assess the reduction of shaft adhesion factor (alpha) with time. The focus of the paper is the numerical modelling of the pile / panel excavation process with the swelling of the adjacent soil prior to concreting.  In addition, the horizontal concrete fluid pressures which develop during tremie concreting are considered in the model.  Once the concrete sets it forms a rigid boundary at the soil interface.  The adjacent soil experiences dissipation of excess pore water pressures and increases in effectives stresses.  The final effective stresses and water contents in the adjacent soil are considered and related to the pile shaft friction. In addition, the changes to the undrained strength at the soil boundary are assessed by extracting the earth and water pressures on a soil element in the model mesh.  This mesh element is then subjected to an undrained triaxial stress path to failure.  The drop in the undrained strength next to the pile is noted.  The results are compared with the adhesion factors from the pile load tests.  This numerical modelling enables the drop in shaft friction to be extended well beyond 3 days.