New framework improves bored pile foundations in weathered rock
Researchers have developed a new design framework for bored piles in weathered sedimentary rock, accounting for weathering effects to improve accuracy and reduce overdesign.
Large-diameter bored piles are critical for major infrastructure projects like bridges and high-rise buildings. However, designing these piles in weak, weathered sedimentary rocks such as siltstone and sandstone presents a challenge, as the rock's load-bearing capacity is significantly influenced by its in-situ condition, including weathering and fracturing.
Current design methods often rely on the uniaxial compressive strength of intact rock, which may not accurately represent the weaker rock mass surrounding the pile socket. This can lead engineers to adopt overly conservative designs, resulting in increased pile size, material usage, and construction costs.
A research team from Shibaura Institute of Technology in Japan has developed empirical design correlations specifically for bored piles installed in weathered siltstone and sandstone. Their study, published in Results in Engineering, analyzed data from 20 instrumented static axial load tests on large-diameter piles. The researchers measured unit shaft resistance and layer displacement along the pile depth, using hyperbolic fitting to estimate shaft resistance for layers with limited displacement.
A key innovation was the explicit inclusion of rock weathering. The team adjusted intact rock strength with a weathering-based reduction factor to derive an equivalent in-situ rock strength. This allowed for a comparison between conventional estimates and weathering-adjusted estimates, revealing that siltstone mobilized adhesion factors roughly twice those of sandstone under similar conditions. The weathering-adjusted correlations also reduced prediction bias and variability compared to methods relying solely on intact rock strength.
The proposed framework allows for preliminary design using intact rock strength, followed by a detailed design phase that incorporates the degree of weathering to calculate adjusted rock strength and apply specific adhesion-factor correlations. This field-calibrated approach can lead to more reliable shaft-resistance estimates, potentially reducing unnecessary overdesign while maintaining safety and serviceability for infrastructure built on weak sedimentary rock.
This development introduces a more accurate, field-calibrated method for designing deep foundations in challenging geological conditions. By accounting for rock weathering, it moves beyond simplified models, enabling more efficient use of materials and potentially lower construction costs for critical infrastructure. This aligns with the broader additive manufacturing push towards material efficiency and optimized structural performance.
Edited by the news editor with AI from the original report — please refer to the original source.