The Behaviour of Model Screw Piles in Cohesive Soils

Modelling helical screw piles in soft clay and design implications

“…Additional helical plates cause the failure mode to extend between the outer edges of the helical plates. This is consistent with the proposition of Rao et al (1991Rao et al ( , 1993) that a cylindrical failure mechanism occurs and is clearly apparent for all the multi-helix configurations tested here. This is true regardless of whether the loading applied to the pile was compressive or tensile, or whether s/D was 1 .…”

“…Based upon these observations Rao et al (1991) proposed simple design equations for the ultimate capacity of helical screw piles loaded in either compression or tension, assuming a cylindrical failure surface between the uppermost and lowermost helical plate. Predictions of the capacities in the model tests made using these design equations were reasonable for s/D over the range of 1 .…”

“…Rao et al (1991) investigated the impact of the number of helical plates on ultimate capacity in clay using model helical screw piles of 75 mm diameter in a bed of compacted clay. The number of helical plates over a fixed length of helical screw pile significantly affected the capacity and stiffness of the response of the piles, leading to the conclusion that maximum capacity was attained when s/D was between 1 .…”

“…The S f factor was determined empirically using the experimental results reported in Rao et al (1991) and compared favourably to limited field data (three tests) from full-scale pullout tests reported by Mooney et al (1985). It should be noted that, as the clay used by Rao et al (1991) was remoulded, it probably had a very low sensitivity, both in absolute terms and relative to that of natural clay deposits.…”

Foundations

“…They presented the penetration mechanism of the screw pile and the evaluation of end bearing capacity based on theˆeld installation and loading tests. The beneˆt of using helical plates for screw piles/anchors for uplift resistance was examined earlier by Rao et al (1991) and others. Karkee (1999) summarized the developments related to the installation of low noise and vibration methods of pile installation, pointing out the drastic change in the use of impact driven or vibration driven pile installation method from 75z in early 1970s to 13z in 1980s, to other methods such as the pre-boring method, the inner excavation method and the screw-in method.…”