Field Behavior of Steel Threaded Micropiles under Axial Loads in Cohesive Soils

  • Author / Creator
    Guo, Zhengyang
  • Steel threaded micropiles are a new deep foundation system recently introduced to the piling industry of North America. This pile consists of a steel threaded tubular shaft that is tapered at the lower segment. Steel threaded micropiles are quick to install, easy to dismantle, and have been increasingly used to support new and existing structures. Therefore, it is important to study the performance of steel threaded micropiles installed in different soil types subject to various loading conditions. Although conventional micropiles have been investigated in a limited number of studies, there is not any study of the axial performance of steel threaded micropiles. Field tests of steel threaded full-scale micropiles with the diameter varying from 76 mm to 114 mm and length varying from 1.6 m to 3 m were carried out to investigate the axial bearing capacities, the load-transfer mechanism, the torque mechanism, and the axial cyclic response of the piles. Two cohesive soil sites were selected for the field tests: University of Alberta South Campus Site and Sherwood Park Site. Comprehensive site investigations including cone penetration tests (CPT), Shelby tube soil sampling, and laboratory tests were carried out for both sites. At the South Campus site, forty load tests were performed on piles subject to static axial compressive and tensile loads. Four test piles were instrumented with several strain gauge stations. Results showed the piles behave as frictional piles and reached the limit state before the displacement exceeded 10% of the pile diameter. The adhesion coefficient of the top smooth shaft at limit state was less than 0.1. The failure mode along the cylindrical threaded shaft was confirmed to be 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 based on preceding load-transfer mechanism and the results agreed reasonably well with the measured capacities. A theoretical torque model was proposed to estimate the end installation torques using the CPT-based remolded soil strength; the results matched the measured end torques very well. At the Sherwood Park site, twenty-five axial monotonic load tests and three axial cyclic load tests were performed. Three tests were instrumented with five strain gauge stations to investigate the unit shaft resistance development during the monotonic and cyclic loading. Results showed that the piles behave as frictional piles and skin friction is mobilized when the displacement reaches 5% to 10% of the pile diameter during monotonic load tests. With the similar installation torques, the compressive capacity of all test piles is greater than tensile capacity by an average of 27%. When subject to vertical earthquake loading, the steel threaded micropiles underwent acceptably small cumulative displacements of less than 2 mm. The magnitude of the cumulative displacement decreased with the pile length and pile diameter. Cyclic loading redistributed the load transfer along different segments of the pile and the negative skin friction was developed along the smooth pile shaft when the pile underwent decreasing axial loading. No degradation of pile axial stiffness was observed for the three piles tested under the range of applied cyclic loading.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.