Question

You are designing the foundation for a natural gas import terminal. The subsurface is predominantly alluvial clays in the upper 6 – 15 m below the ground surface (BGS), which has resulted in large post construction settlements on nearby structures. To prevent large settlements and differential settlements, the new gas tanks will be supported by pile foundations. You have determined that pre-stressed concrete (PSC) piles are the most economical solution. The pile layout will be a square with center-to-center spacing of 1.8 m throughout the center area of the tank and with two piles along the outer ring. Approximately 2,000 piles are needed for each new tank. Each pile must be designed for an allowable compression capacity of 890 kN and a factor of safety of 2.0 after discounting the downdrag loads from the upper clay layer. Neglecting any group effects, lateral loads, settlement, or downdrag, calculate the required length and width of a PSC pile to support the allowable load. A detailed subsurface investigation was conducted, data are provided below. The ground water table level was around 1.6 m. The surficial layer in the upper 1.5 m BGS was hydraulic fill placed more than 30 years ago. It was a mixture of clay, silt, and sand with varying proportions. Please refer to Figure 1 for the specifications of the soil properties.