Determination of Basal Heave Stability for Strutted Excavations Supported with Jet Grout Base Slab-Paper

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The paper evaluates basal heave stability in deep excavations in soft clay supported by jet grout pile (JGP) base slabs. Using 55 plane-strain finite element analyses and the shear strength reduction method, the author calculates factors of safety and compares them with two limit-equilibrium approaches: the slip-circle method and a modified Terzaghi method. A reliability analysis is also proposed to account for uncertainty in soil and JGP properties.<br /><br />The analyses show that the finite element factor of safety decreases as excavation width increases. JGPs improve stability, particularly in narrower excavations, and the improvement depends strongly on adhesion between the JGP slab and the retaining wall. For example, a 4 m slab increased the factor of safety by 31% for a 20 m-wide excavation, compared with 16% for a 30 m-wide excavation. The modified Terzaghi method generally agrees reasonably with the finite element results, while slip-circle predictions show greater scatter.<br /><br />Reliability results demonstrate that the same mean factor of safety can correspond to substantially different failure probabilities, depending on variability in soil strength and JGP–wall adhesion. Greater uncertainty in either property increases the probability of basal heave failure. The proposed reliability approach provides a systematic way to quantify these risks and support design decisions, rather than relying on a factor of safety alone.

Keywords

basal heave stability

deep excavation

soft clay

jet grout pile

finite element analysis

shear strength reduction

factor of safety

reliability analysis

limit equilibrium

terzaghi method