Investigation into the factors affecting the shaft resistance of driven piles in sands

The paper presents the results of field tests performed to study the effects of the installation technique, degree of plugging, cyclic loading and ageing on the shaft resistance developed on openended piles in sand. Two instrumented model piles were jacked and driven into an artificially created loose sand deposit in Blessington, Ireland. The results from these tests indicated that the equalized radial effective stresses which are suggested to control the shaft capacity vary strongly with the degree of plugging and number of load cycles experienced during installation. A comparison of jacked and driven installations suggest similar radial stresses were developed provided the jacked pile had experienced a sufficient number load cycles. The degree of plugging experienced during installation controlled the radial stresses near the bottom of the pile, with closed-ended or plugged piles developing high stresses near the pile base and exhibiting friction fatigue up the shaft, compared with open-ended coring piles which exhibited relatively low stresses along the length of the pile shaft. A comparison with full scale 340 mm diameter pipe piles driven into the dense sand in Blessington noted comparable radial stresses when the pile was fully coring but exhibited a larger increase in radial stress near the pile toe as the pile became plugged. Further tests on the effects of ageing show a pile shaft capacity increase of 260% over 220 days after driving. Further research is underway in Blessington to investigate the mechanisms controlling this ageing behaviour.