Stability of unsupported axisymmetric excavations in soft clay

Abstract

The stability of an axisymmetric excavation has been given little attention in the past, even though axisymmetric excavations are common in the construction industry. Analytical upper bound solutions for the failure of an unsupported axisymmetric excavation are presented for conditions where the soil is treated as a rigid plastic material, satisfying the Tresca yield criterion and possessing an uniform undrained shear strength. Wall failure mechanisms are found to be more critical than base failure and combined wall-base failure modes. The width of the failure zone at the surface for the critical wall mechanism is found to be 0·4 times the depth of the excavation. The stability number given by the present analyses is lower than that obtained previously. On ne s'est guère penché dans le passé sur l'étude de la stabilité d'excavations axisymétriques, bien que ce genre d'excavations soit courant la pratique. L'article présente des solutions analytiques de limite supérieure se rapportant à la rupture d'une excavation axisymétrique non étayée pour des conditions dans lesquelles le sol est considéré comme un matériau plastique rigide dont les propriétés sont conformes au critère d'écoulement de Tresca et qui possède une résistance au cisaillement non drainée uniforme. On constate que les mécanismes de rupture de paroi sont plus critiques que les ruptures profondes et les modes combinés de rupture profonde–rupture de paroi. Dans le cas du mécanisme critique de rupture de paroi, on note que la largeur de la zone de rupture en surface est égale à 0·4 fois la profondeur de l'excavation. Le facteur de sécurité au renversement indiqué dans ces analyses est inférieur à celui qui a été obtenu précédemment.

Experimental investigation of reverse end bearing of offshore shallow foundations

“…A gap has greater potential to be formed and remain open the higher the soil strength ratio su/γ'z (Britto and Kusakabe 1982;Coffman et al 2004;Supachawarote et al An experimental investigation of Mana/Gourvenec/Randolph reverse end bearing of offshore shallow foundations 6 2005). Analytical solutions (Britto and Kusakabe 1982) indicate that the minimum strength ratio necessary for the stability of an axisymmetric excavation in soft clay varies from around 0.25 to 0.18 for unsupported excavations with an excavation depth to diameter ratio of 0.1 to 0.5.…”

“…Analytical solutions (Britto and Kusakabe 1982) indicate that the minimum strength ratio necessary for the stability of an axisymmetric excavation in soft clay varies from around 0.25 to 0.18 for unsupported excavations with an excavation depth to diameter ratio of 0.1 to 0.5. A gap is unlikely to form in a deposit with strength ratio less than this value, even under significant displacements.…”

A novel technique to mitigate the effect of gapping on the uplift capacity of offshore shallow foundations

“…While neither case exactly replicates the boundary conditions of this study, the solutions were used to provide a guideline for the targeted shear strength. The theoretical solutions indicated that su/γz > 0.275 for a plane strain cut or gap (Pastor, 1978) and more critical, 0.2 for an axisymmetric excavation with depth to diameter ratio relevant to this study (Britto & Kusakabe, 1982) For this study, a sample with su/γz (su/γ'z in the case of the saturated, inundated samples in this study) > 0.275 was targeted, the highest of the theoretical predictions, to ensure the gaps would form and remain open.…”

“…A lightly over consolidated clay was selected for the study to improve the likelihood of a vertical gap remaining stable once formed, since the ability of a gap to form and stand open increases with increase in the soil strength ratio, su/γ'z (Chen 1975, Pastor 1978, Britto & Kusakabe, 1982). Analytical solutions for the depth that an unsupported excavation can remain stable (i.e.…”

Experimental Study of the Earth Pressure Distribution on Cylindrical Shafts

“…Berezantzev (1958), Cheng and Hu (2005), Cheng et al (2007), Liu andWang (2008), and Liu et al (2009) applied slip line method to study the active/passive earth pressure against cylindrical retaining walls. Britto and Kusakabe (1982;1983), and Pastor and Turgeman (1982) applied limit analysis to study the stability of an axisymmetric excavation in cohesive soil. Kaiser and Wong (1988) combined elastoplastic solution of a circular opening with limit equilibrium to obtain closed-form solutions for the stresses and displacements around a circular shaft taking into account the effect of gravity.…”