The stability of shallow tunnels and underground openings in cohesive material

Abstract

A basic engineering decision to be made in designing a tunnel in soft clay is whether or not the tunnel can be excavated without internal support. The safety of constructing a shallow heading in soft clay can be assessed in terms of the fluid support pressure which may be required to maintain stability. This can be estimated by means of the lower and upper bound theorems of plasticity. The Paper considers three different shapes of shallow underground opening relevant to tunnelling and upper and lower bound stability solutions are derived for collapse under undrained conditions. Solutions are also derived for assessment of the risk of blow-out failure caused by excessively high fluid pressures. Conditions are considered under which local collapse can occur, independent of the cover above the tunnel. Une décision fondamentale doit être prise lorsque l'on veut réaliser un tunnel dans de l'argile molle; il s'agit, en effet, de décider si un support interne sera ou non nécessaire pour l'excavation du tunnel. La sécurité de la construction d'une galerie peu profonde dans de l'argile molle peut être évaluée en fonction de la pression du fluide support qui pourrait être nécessaire au maintien de la stabilité. Cette estimation peut se faire à l'aide des théorèmes à limites inférieure et supérieure. L'article envisage trois formes différentes d'ouverture souterraine peu profonde intéressant le creusement de tunnels, et des solutions de stabilité à limites inférieure et supérieure sont trouvées pour l'affaissement dans des conditions non drainées. Des solutions sont également trouvées en ce qui concerne l'évaluation du risque de rupture par éruption due aux pressions excessives du fluide. Les conditions dans lesquelles un affaissement local peut se produire sont envisagées, indépendamment de la couverture située audessus du tunnel.

Centrifuge model test on the face stability of shallow tunnel

“…The face stability at shallow depth is a popular topic of research of the last decades and until today [3,8,16,19,21,23,26,27,35]. Among others, Leca and Dormieux [15] proposed a failure criterion for the tunnel face in cohesive and frictional soil.…”

Undrained End Bearing Capacity of an Improved Soil Berm in an Excavation

“…One interesting observation is that the failure mechanism proposed above for the analysis of a berm used to restrain a retaining wall is similar to that proposed by Davis et al (1980) for the collapse of a plane strain tunnel, who used the failure mechanism shown in Fig. 7 to determine the upper bound solution.…”

“…This also supports the present approach to isolate Nc and Nq. Figure 14 shows that the value of N c from the proposed upper bound solution is larger than the corresponding Nc from the FE analysis, while the FE solution lies between the upper bound and lower bound solutions proposed by Davis et al (1980). The main reason the upper bound solution of N c is higher than that of the FE solution is likely to be due to the assumption of full mobilisation of the undrained shear strength on the slip planes for the upper bound analysis.…”

“…2) As upper bound solution would overestimate the end bearing capacity provided by the improved soil berm, a modiˆed upper bound solution which combines the results of the upper bound solution and FE analysis was then developed to improve the estimation of qb. The end bearing capacity factor Nc presented here lies between the upper bound and lower bound solutions provided by Davis et al (1980). 3) In the FE analysis, a procedure for estimating qb is introduced.…”

“…According to Davis et al (1980), this solution is also applicable to the case of a blow-out failure for a plane strain tunnel, though the direction of the movement of the failure body would be reversed. In essence, this type of failure is similar to the failure of the end bearing capacity of an improved berm.…”