FIELD INVESTIGATION AND NUMERICAL MODELLING OF MICRO SCREW PILES UNDER AXIAL AND LATERAL LOADINGS
Creators: Khidri, Mujtaba
Publisher: name: University of Alberta Library
Year: 2022

<p>A micro screw pile is a multi-sectional pile that consists of a smooth segment at the top, a threaded segment in the middle and a tapered segment at the bottom. Due to limited information on the performance, design and behaviour of micro screw piles, further research is required to study the axial, axial cyclic and lateral behaviors and capacities of the micro screw piles in cohesive and cohesionless soils. Therefore, six types of micro screw piles were tested at three sites with various soil compositions.</p>

<p>An axial load field test program was performed on full-scale micro screw piles installed in a cohesionless soil site (Sandpit) using the torque method. Selected piles were instrumented with axial strain gauges (SGs). A geotechnical site investigation was carried out involving cone penetration and standard penetration tests. A total of 41 piles, including eight instrumented piles, were tested. The ultimate capacities and the distributions of unit shaft resistances were determined. The shaft resistance was then compared with the tip resistance from cone penetrometer tests (CPTs). The coefficient of lateral earth pressure and combined shaft resistance factor was determined over the individual pile segment, and then an effective stress method based on the combined shaft resistance factor was used to estimate the capacity of 41 test piles. A theoretical torque model was adopted using the CPT sleeve friction. The model was verified by comparing the estimated torque to the measured torque of the test piles. In the end, empirical torque factors were developed.</p>

<p>An axial cyclic load field test program was carried out at Sandpit to examine the axial cyclic response of the micro screw piles. Six one-way compressive and load-controlled axial cyclic tests were performed. Three piles were instrumented with axial SGs to measure the distribution of the unit shaft resistance during the cyclic test. The pile-head cumulative displacement, stiffness and equivalent damping ratios were determined from the load-displacement curves. The effect of<br>
the initial factor of safety on cyclic behavior was examined. The re-distribution of the unit shaft resistances of the individual pile segments was obtained. The equivalent damping ratio and stiffness of the individual pile segments were obtained from the unit shaft resistance hysteresis.</p>

<p>A lateral load field test program was carried out that included six piles at a cohesive soil site in Sherwood Park, 22 at a cohesive soil site on South Campus and 18 at Sandpit. Initially, the lateral capacity, pile shaft response and failure mode of the pile were investigated. Afterward, the effectiveness of Broms’s method in estimating the piles’ capacities was assessed once the pile failure mode was determined. The estimated capacities of the piles using only shaft resistance and neglecting the effect of the thread are comparable to the measured capacities of the piles.</p>

<p>Numerical models based on the Beam-on-Nonlinear-Winkler-Foundation (BNWF) method were developed on the OpenSEES platform to predict the lateral responses of the micro screw piles at these three sites. Different components of the soil-pile interaction responses, including the lateral shaft resistance, the vertical shaft resistance, the bearing resistance of the threads and the lateral thread resistance, were represented using materials with uniaxial load-deformation responses, such as p-y, t-z, q-z and t-z curves, respectively. The failure mode was investigated by examining the distribution of the pile deformation, the bending moment and the shear stress of the pile as well as the lateral normal forces of the soil on the pile shaft. The contributions of different components of soil-pile reactions were assessed. The effect of the thread on the lateral capacity of the pile was evaluated. A sensitivity analysis was conducted to examine the effect of different soil properties and specific pile geometrical features on the lateral response of the pile.</p>