The behaviour of a propped retaining wall: results of a numerical experiment

Abstract

The design of propped retaining walls is currently based on approximate limit equilibrium calculations. A factor of safety is used to ensure adequate stability and to restrict soil and wall movements to acceptable levels. No distinction is made for the type of construction, whether excavated or backfilled, or of the stress state in the soil prior to construction. In this Paper the finite element method is employed to investigate the influence of type of construction and of initial stress in the soil on the behaviour of single propped retaining walls. An elasto-plastic constitutive law is used to model the soil behaviour and a rigid prop is assumed to act at the top of the wall. The results of the investigation indicate that for excavated walls in soils with a high initial K0 value prop forces and wall bending moments greatly exceed those predicted by current design methods. In addition large soil and wall movements are experienced even at shallow depths of excavation. The behaviour is dominated by the vertical unloading caused by the excavation process and large movements still occur even if the wall is fully restrained from horizontal movement. For backfilled and excavated walls in soils with a low K0 value the analyses indicate that the displacements are much smaller in magnitude and that the approximate limit equilibrium calculations produce conservative values of prop force and bending moments. The results of the investigation have serious practical implications for diaphragm or secant pile walls installed in stiff over-consolidated clay with high K0 values. A présent la construction des murs de souténement étayes est basée sur des calculs approximatifs de l'équilibre limite. Un coefficient de sécurité est appliqué pour assurer une stabilité suffisante et pour réduire les mouvements du sol et du mur à des niveaux accepta-bles. On ne fait aucune distinction entre une construction excavée et une construction remblayée, et l'ttat de contrainte dans lequel le sol se trouve ne joue aucun rôle. Dans la méthode décrite dans cet article des éléments finis sont utilisés pour étudier l'influence du type de construction et de la contrainte initiale dans le sol sur le comportement de murs de soutènement étayés individuels. On se sert d'une loi constitutive élasto-plastique pour modéliser le comportement du sol, en admettant qu'un étai rigide est disposé en haut du mur. Les résultats de cette étude indiquent que dans le cas de murs excavés dans des sols avant une valeur K0 initiale élevée les forces dans les étais et les moments de flexion dépassent largement ceux prédits par les méthodes couramment employées dans la construction. De plus, des mouvements de sol et de mur considérables sont trouvés même à de petites profondeurs d'excavation. Le comportement est régi par le déchargement vertical causé par le travail d'excavation et des mouvements considérables ont encore lieu, même si le mur est complètement empêché de faire aucun mouvement horizontal. En ce qui concerne les murs remblayés et excavés dans des sols ayant une valeur K0 réduite les analyses indiquent que les déplacements sont beaucoup moindres et que les calculs approximatifs d'équilibre limite donnent des valeurs minimales des forces dans les étais et des moments de flexion. Les résultats de l'étude ont des conséquences pratiques importantes pour les mur type parois ou pieux ou sécantes installés dans des argiles surconsolidées rigides ayant des valeurs K0 élevées.

Numerical studies of anchored sheet pile wall behavior constructed in cut and fill conditions

“…Factor of safety is applied to the passive pressures during the determination of lateral earth pressures [20,21]. The safety factors are used to take into account the uncertainties in soil conditions, the method of stability analysis, the loading conditions, as well as to restraint soil movements to an acceptable level [3].…”

“…The Mohr-Coulomb constitutive model for soils has been commonly used in finite element modeling of retaining wall behavior [3,7,9,[13][14][15]24,25,29,30,32]. There are more sophisticated constitutive models available in PLAXIS and used by researchers to model complex soil behavior, i.e.…”

“…They investigated the application of the finite element method to anchored sheet pile walls and compared their results to the ones obtained from the free earth support method. Potts and Fourie [3] used the finite element method to investigate the effect of initial soil stress conditions on diaphragm walls constructed in stiff over-consolidated clays. The use of the finite element method for the analysis of various anchored or braced retaining structures in the past [4][5][6][7][8][9][10][11][12][13][14][15] has been mainly for the excavation cases.…”

Modelling of earth and water pressure development during diaphragm wall construction in soft clay

“…The results confirm the observations by Symons and Carder [17], who recognized by field measurements an earth pressure reduction of about 20-30% during the installation of diaphragm wall panels in stiff clay. Taking into account that wall deflections due to pit excavation particularly depend on the magnitude and distribution of the earth pressure prior to the excavation process [18], the assumption that the earth pressure at rest is valid will consequently lead to an incorrect prediction of wall movements. Due to the lower earth pressure, the development of stress during diaphragm wall construction as well as the final earth pressure distribution will be different in particular with soft soil deposits.…”

The influence of the construction process on the deformation behaviour of diaphragm walls in soft clayey ground

“…However, conventional calculations do not consider the construction process of the retaining wall itself and choose the earth pressure at rest as initial condition (wished-in-place models, wip, [2], see Figure 1(a)). Actually, the wall movements and settlements due to the excavation process strongly depend on the in situ stress level close to the retaining structure prior to the excavation [3]. A disregard of the changed pressure conditions due to the construction of the wall will consequently lead to a loss of accuracy in the results.…”