In the world of geotechnical engineering, understanding the behavior of saturated soils under undrained conditions is crucial, particularly for the energy sector where foundations and infrastructure often rest on such soils. A recent discussion published in the journal *Yantu gongcheng xuebao* (translated to *Rock and Soil Mechanics*) challenges a commonly held belief about the inclination angle of undrained shear slip surfaces in saturated soils, potentially reshaping how engineers approach stability analyses in critical energy projects.
The discussion, led by LI Guangxin and YU Yuzhen from Tsinghua University’s State Key Laboratory of Hydroscience and Engineering and Department of Hydraulic Engineering, takes aim at a previous study that suggested the inclination angle of the slip surface under undrained conditions is a straightforward 45°. According to LI and YU, this oversimplification doesn’t hold up under closer scrutiny.
“According to the principle of effective stress, the strength and failure of soil are determined by the effective stress acting on the soil skeleton,” LI explains. “The Mohr-Coulomb strength theory tells us that when the major principal stress directions are respectively vertical and horizontal, this inclination angle should be 45°±φ′/2, not just 45°.”
This distinction is more than academic. In the energy sector, where massive structures like offshore wind farms, oil rigs, and pipelines are built on saturated soils, understanding the precise angle of potential slip surfaces is vital for ensuring stability and preventing catastrophic failures. The difference between 45° and 45°±φ′/2 might seem subtle, but it can translate to significant adjustments in design and construction practices.
LI and YU’s discussion highlights that a multitude of experimental results and engineering practices support the 45°±φ′/2 angle, making it the more reliable benchmark. They also emphasize that the analysis object in such studies can be either the saturated soil mass or the soil skeleton, each with its own mechanical properties and implications.
“In the former, the main mechanical properties of the material are φ = 0° and dεv = 0, at which point the Mohr-Coulomb strength theory will degenerate into the Tresca strength criterion,” YU notes. “In the latter, the main mechanical properties are the angle of internal friction φ′, compressibility and dilatancy (contraction), and at this time, dεv=0 is only the boundary condition of the analysis.”
To bolster their argument, the researchers conducted a triaxial undrained test on saturated sand, demonstrating that the inclination angle of the slip surface was indeed closer to 45°+φ′/2. This finding has significant commercial impacts for the energy sector, where accurate stability analyses can mean the difference between a successful project and a costly disaster.
As the energy sector continues to expand into more challenging environments, the insights from this discussion could shape future developments in geotechnical engineering. By adopting a more nuanced understanding of slip surface angles, engineers can design safer, more stable structures that withstand the test of time and nature.
For professionals in the energy sector, staying abreast of such research is not just about keeping up with academic trends—it’s about ensuring the safety and success of critical infrastructure. As LI and YU’s work shows, even small refinements in our understanding of soil mechanics can have far-reaching implications for the industry.