DESIGN AND INSTALLATION OF DEEP FOUNDATIONS IN SEDIMENTARY ROCK WITH SELECTED CASE STUDIES

This paper will explore some of the major considerations in the design and installation of Deep Foundations on sites underlain by sedimentary rock.

Specific rock characteristics may preclude the use of certain types of deep foundations because of the inability to meet standard installation tolerances like plumbness and plan location. Engineers utilizing other types of deep foundations must give careful consideration to the certain recommended capacities, such as uplift, in situations where the length of foundation is highly variable across a project site. Measured rock characteristics such as a low Rock Quality Designation (RQD) will greatly reduce the calculated design capacities when following the standard practice of treating the rock as a soil, however, this approach may be too conservative in some sedimentary rocks.

H-Piles driven to bedrock were used at one site to support a new chimney at a power plant. Geotechnical soil boring logs drilled in the immediate vicinity of the proposed chimney revealed approximately 50 feet of overburden soils underlain by weathered shale of variable thickness. A weathered limestone was encountered beneath the shale. The RQD in this rock was 0% and Unconfined Compression tests could not be performed because of inadequate rock sample sizes. The two major challenges were designing a pile with a high enough capacity to withstand the heavy chimney loads and selecting a pile length to accommodate the subsurface conditions. Rather than treating the rock as a soil, the structural capacity of the H-Pile was used with a large factor of safety. Static Compression Tests in accordance with ASTM D1143 were performed to verify the compression capacity of the driven H-Piles, with the test results yielding capacities that far exceed the calculated capacity of the pile if considering the rock as a soil.