L'article suivant donne une description de la methode theorique pour le calcul du danger d'un soulevement du fond d'une fouille cadragee dans l'argile. Terzaghi et Tschebotarioff ont deja donne des formules pour un pareil calcul, mais ces formules ne sont valables que pour des fouilles de petite profondeur. Basee sur les formules de la capacite portante des fondations dans l'argile, une nouvelle solution du probleme de soulevement du fond a ete developpee. La profondeur critique d'une fouille est donnee par : D c = N c . S/γ ou s et γ sont respectivement la resistance au cisaillement et le poids d'unite de l'argile, N c etant un coefficient sans dimension et dependant de la forme de la fouille, c'est-a-dire de la proportion largeur-longueur et de la proportion profondeur-largeur. Les valeurs de N c , identiques aux valeurs donnees par Skempton (1951) pour la capacite portante, sont montrees graphiquement dans la Figure 2. La validite de ces formules a ete examinee par comparaison de quatorze fouilles. Dans sept de ces fouilles, un soulevement total du fond a eu lieu, les coefficients theoriques de securite donnant comme valeur moyenne de 0,96. Quatre fouilles, ou on constatait un soulevement partiel du fond, ont fourni une valeur moyenne des coefficients theoriques de securite de 1,11. Les trois dernieres fouilles ont ete accomplies sans evenement speciaux, mais des observations sur place indiquaient que, avec une profondeur agrandissante, on aurait du craindre un soulevement du fond, la valeur moyenne des coefficients theoriques de securite etant de 1,08. Les resultats obtenus ci-dessus montrent, ainsi, que la methode decrite nous permet d'estimer avec une precision suffisante le danger d'un soulevement du fond d'une fouille dans l'argile, et que la methode suffit pour la plupart des usages pratiques.

Stability of strutted excavations in clay

Short-term undrained stability often controls the design of braced excavations in soft clays. This paper summarizes the formulation of numerical limit analyses that compute rigorous upper and lower bounds on the exact stability number and include anisotropic yielding, typical of K0-consolidated clays and bending failure of the wall. Calculations for braced cuts bound the actual failure conditions within ±5%, and highlight limitations of existing basal stability equations. The analyses clarify how wall embedment and bending capacity improve the stability of well braced excavations. Careful selection of mobilized strengths at shear strains in the range 0.6–1.0% are necessary to match the predictions of anisotropic limit analyses with nonlinear finite-element predictions of failure for the embedded walls. Two example applications from recent projects in Boston highlight the practicality of the numerical limit analyses for modeling realistic soil profiles and lateral earth support systems, but also focus atte...

Undrained Stability of Braced Excavations in Clay

Zoned and staged construction of an underground complex in Shanghai soft clay

This paper examines various issues related to analysis and design of deep mixing (DM) technology. The significance of site characterization in cost-effective design is highlighted, along with major design considerations, design principles based on the allowable-stress or limit state approach, and methods to analyse composite ground. In addition, a comprehensive design methodology is outlined, incorporating various modes of external and internal failure and an optimization scheme. Empirical equations and coefficients are provided to be used in the analysis of composite ground. Guidelines and design examples are provided for special applications of DM to support open cuts, to maintain embankment stability, to prevent base heave during excavation and for seepage control for dams and levees using DM cut-off walls. Further discussions are presented of deformation characteristics on the basis of centrifuge and field tests, settlement analysis and finiteelement analysis, coupled with an outline of research needs...

State of the art in deep mixing technology. Part IV:design considerations

Estimate of deformations around urban excavations is a primary concern for designers, contractors, owners, and potentially affected third parties. Significant efforts have gone into the development of empirically based methods to estimate deformations relying on a large number of case histories. The construction of the deep excavations for the central artery/tunnel project provides valuable information on observed deformations due to the construction of these excavations. Lateral deformations and surface settlements for three construction contracts are collected and summarized in a form similar to published empirical charts. The stiff support system used in these braced excavation and the embedment of the wall into stiff strata control deformations to minimal levels. The data show that surface settlements, although small, extend farther away from the excavation than previously reported.

Central Artery/Tunnel Project Excavation Induced Ground Deformations

This paper describes the performance of a tiedback excavation with a soil mixed wall in Boston where underlying marine clay was stabilized by deep soil mixing (DSM) and jet grouting. Lateral and vertical soil displacements, soil strains, and tieback loads are evaluated. The interaction between the retained soil and a reinforced concrete box culvert, supported on drilled shafts, is described. Piezometer measurements behind the wall are summarized. They show pore-water pressures in marine clay substantially less than the hydrostatic pressures assumed in design.

Field Behavior of Excavation Stabilized by Deep Soil Mixing

This paper addresses the base stability of excavations in deep, relatively weak deposits of clay that are supported by earth-anchored tiebacks. Under these conditions there may be the potential for a circular sliding surface to propagate outside the anchor bond zone and daylight to show in the basal portion of the cut. In this paper the analysis of deep rotational stability is described, whereby limiting equilibrium computations are performed for circular sliding surfaces. Dimensionless charts are presented to estimate safety factors and identify the centers of rotation of critical slip circles. The suitability of the method is assessed by means of a case history involving an unstable excavation in Boston marine clay. The soil conditions and ground movement measurements at the site are presented, followed by the results of limiting equilibrium and finite-element analyses. Deep rotational stability is shown to be one of the most important failure mechanisms for tiedback excavations in relatively weak clay, and the dimensionless charts presented in this work are shown to provide a good estimate of the stability difficulties experienced at the case history site.

C. J. O'Donnell

Papers

Field Behavior of Excavation Stabilized by Deep Soil Mixing

Deep rotational stability of tiedback excavations in clay