Field behaviour of screw micropiles subjected to axial loading in cohesive soils

Abstract

Field tests of full-scale screw micropiles with a diameter varying from 76 to 114 mm and a length varying from 1.6 to 3 m were undertaken to investigate the axial pile capacities, load-transfer mechanism, and end installation torques of the piles in cohesive soils. Forty tests were performed on piles subjected to axial compressive and tensile loads. Six tests were instrumented with strain gauges on the pile shaft. Results showed the piles reached the limit state before the displacement exceeded 10% of the shaft diameter. The majority of axial load was transferred to the threaded segment. The adhesion coefficient of the top smooth shaft at the limit state was less than 0.1. The failure mode along the cylindrical threaded shaft was cylindrical shearing along the edge of the threads; the threads increased the axial capacities of the segment. Axial capacities of the threaded tapered segment were 43% on average greater than that of a cylindrical segment with the equivalent volume. Compressive capacities of all test piles were estimated and the results agreed reasonably well with the measured capacities. A theoretical torque model was proposed to estimate the end installation torques based on the cone penetration test results; the theoretical results matched the measured end torques very well.Key words: field test, screw micropile, tapered, axial behaviour, installation torque.

Field loading tests of screw micropiles under axial cyclic and monotonic loads

“…Because screw micropiles were installed under torque, the end installation torque over the last cycle of rotation of the pile drive was recorded as T end and listed in Table 2. The magnitude of T end can serve as an indicator of the in situ soil strength along the pile shaft [12]. The value of T end ranged from about 0.6 kN-m for P3 to 3.9 kN-m for P1.…”

“…Understanding the installation torque of a screw pile may be useful for designing screw piles. Guo and Deng [12] presented an end torque model that correlates T end to the soil residual strength obtained from CPT f s and the pile surface area. The torque model assumes that the soil-pile interface strength can be represented by the measured CPT f s .…”

“…The estimated a values of three segments are 0.075, 0.85, and 0.79, respectively, from the top to the bottom segments bounded by SG-1 and SG-4. Guo and Deng [12] carried out an extensive field test program for screw micropiles under axial compressive loads at another cohesive soil site; they observed that a was 0.0926 on average for the smooth segment and an a of 1.0 for the cylindrical threaded segment. Given the heterogeneity of in situ soil properties, the estimated a in present study is considered consistent with those in Guo and Deng [12].…”

“…Guo and Deng [12] carried out an extensive field test program for screw micropiles under axial compressive loads at another cohesive soil site; they observed that a was 0.0926 on average for the smooth segment and an a of 1.0 for the cylindrical threaded segment. Given the heterogeneity of in situ soil properties, the estimated a in present study is considered consistent with those in Guo and Deng [12]. Note that the theoretical capacity model was developed by Guo and Deng [12] based on a large number of field tests of the piles.…”

“…Despite the possible advantages in increased capacity, installation, and cost efficiency, published research on screw micropiles has been very limited. Guo and Deng [12] carried out field investigation of the performance of several screw micropiles under monotonic axial compression and tension in a cohesive soil; the authors discussed the load-transfer mechanisms along the pile shaft, developed a model to estimate the final installation torque, and noticed the increased unit shaft resistance at the tapered segment.…”