Estimation of Characteristics Strength and Design Optimization of Cast-in-Place Piles

Abstract

The evaluation of soil characteristic strength values is fundamental to the Limit States Design method. The Limit States Design approach and design verification process were adopted to support a potash mine project in Saskatchewan. In this project, over two thousand cast-in-place piles were installed as part of the mine facility foundations. Firstly, geotechnical information database was established from historical data spanned over 6 years. The data included results from borehole drilling, Cone Penetration Testing (CPT), field and laboratory test results, and historic bi-directional static load tests using Osterberg cell (O-cell) method completed during the initial stages of the design. In the final stage of the design, relevant information from the database was extracted and processed to develop the soil characteristic profiles. Finally, an additional 12 bi-directional static load tests were performed in 2021 for design verification purposes. This paper summarizes the process of estimating the axial resistance of piles using static analysis and the soil characteristic values defined in Limit States Design approach and compares the ultimate (unfactored) pile axial resistance with those obtained from the 2021 O-cell test program. In this case study, it was found that the pile load test results support the use of higher adhesion factors for cohesive soils than those adopted in the static analysis. This leads to an increase of the soil design resistance parameters and an optimized pile design. This paper demonstrates the benefit of a site specific geotechnical investigation and data analysis program for design optimization, it also highlights the role of pile verification testing in achieving cost-effective and safe design.