Ground heave around driven piles in clay
Large diameter piles have increasingly been used, especially for offshore foundations. Most of the piles that have been used offshore have been open-ended. There is evidence from previous work that closed-ended piles have merits over open-ended piles; they have a greater ultimate axial capacity and are easier to drive than open-ended piles of the same outer diameter. However, because a greater volume of soil is displaced, closed-ended piles give rise to much greater ground heave. This additional heave may cause unacceptable distortion to the structure as each leg is fixed to the sea-bed in turn. For this reason, this research was commissioned. Laboratory studies of ground heave around driven open- and closed-ended piles (16mm o.d.) were carried out. Two different types of specimen reconstituted from Speswhite Kaolin were prepared; semi-cylindrical and full cylindrical specimens each 450mm in diameter and about 450mm in height. The semi-cylindrical specimens were used to obtain qualitative data about the movements of soil around the driven pile. The full cylindrical specimens were used to take accurate measurements of the ground heave during pile driving and to study the influence of the ground heave on a small surcharged collar through which a pile was driven. A further series of tests investigated the effect of ground heave when a group of piles was driven through a larger surcharged collar which was used to represent the mudmat of an offshore structure. The lateral movement of soil at around the mid-depth of a driven pile agreed with the cavity expansion model (plane strain analysis) assuming undrained conditions and no vertical movement. Closed-ended piles were found to generate about four times the magnitude of ground heave compared to open-ended piles with an area ratio of 34%. Ground heave appeared to be unaffected by overconsolidation ratio and rate of pile driving (22 to 150 blows/min.). However, maximum vertical consolidation pressure seemed to have a small effect. A remarkably good agreement was obtained between the field data and the results of this investigation when both were made dimensionless. The surcharged collar and model mudmat generally settled during pile driving. However, when very low surcharge pressures were used, the collar and mudmat were lifted up. For pile groups, the surcharged mudmat tilted towards the pile being driven. The magnitude of this tilt for the mudmat was greater when closed-ended piles were used.