High Capacity Micropiles in Intermediate Geomaterials: Partial versus Full Length Casing

Micropiles were used to support air quality control structures and equipment at a Southeast U.S. electric power plant in areas where construction equipment access for other types of deep foundations was very limited. The plant is located within the Piedmont physiographic province. The subsurface profile consists of upper medium dense silty and clayey sands, underlain by partially weathered rock (PWR), of varying thickness. The PWR is underlain by Gneiss bedrock with a relatively reduced degree of weathering, grading to a relatively unweathered rock. Due to the varying thickness of the PWR, the first Phase of the project included installation of two verification test piles to evaluate the contribution of PWR to pile capacity. Both test piles were installed with a partial length casing to stabilize the hole within the top portion of the sandy soils.The results of these tests indicated that PWR with sufficient thickness could be used as a load bearing stratum for micropile design; and that the surface casing could be used only as a means to stabilize the top of the drill hole. By using the PWR layer as a load bearing stratum and installing only partial length casing, the project team was able to considerably reduce the Phase 1 installation costs. However, due to variable subsurface conditions encountered during production, it was difficult to ascertain the depth to which the casing should be extended to stabilize the hole. Hence, a decision was made to install full-length casing for a Phase 2 test pile and subsequent production piles. The results of the Phase 2 verification test indicated side shear values within the Gneiss layer comparable to the Phase 1 test findings. The casing was found to contribute primarily as a borehole stabilizing mechanism and to haved no significant effect on the load distribution within the bond zone.