Finite element analysis of basal heave stability for braced excavations in clays

Abstract

The basal heave stability of braced excavations in clay soils is one of the key considerations in both design and during construction. The finite element method with shear strength reduction (SSR) is increasingly being used for estimating the safety factor for geotechnical engineering problems. In this paper, results of basal heave stability for deep narrow braced excavations in terms of factor of safety (FS) and stability number (Ns) using two-dimensional finite element method with SSR are presented. Total stress finite element simulations are carried out to parametrically analyse the effects of system stiffness, undrained shear strength and excavation geometry on the base stability of braced excavations in different clay soils. For the cases considered, the study shows that the system stiffness has minimal influence on the factor of safety against basal heave. The stability number Ns calculated from finite element analysis is independent of the undrained shear strength of clays. Also, the Ns value decreases almost linearly as the ratio of the depth to the width of the excavation increases. The failure zone from finite element analysis is more extensive than that proposed by Terzaghi. Comparison of the factor of safety between the calculations from conventional methods and from finite element analysis indicate that the modified Terzaghi's method that considers the wall embedment depth are in close agreement with the results based on the plane strain finite element analyses.

Estimating the Basal Heave Stability of Narrow Braced Excavations

“…To consider the effect of B, the whole braced excavation was taken for analysis during the simulation, instead of only half due to symmetry [2,10,15,33,34,37,39]. In this study, plane strain analyses [10,15,37,38,40] were adopted to explore the configuration of failure surfaces and evaluate the basal heave factors of safety with different excavation widths. H and D of all cases analyzed are 8 m and 6 m, respectively, and B is the only variable.…”

“…Estimating the basal heave stability of braced excavation is quite significant for design. Up to the present, it can be performed by using the limit equilibrium method (LEM) [2,[8][9][10][11][12][13][14][15][16][17][18][19], limit analysis method [9,[20][21][22][23][24], probabilistic method [2,10,[25][26][27][28][29][30][31][32], and finite element method (FEM) with shear strength reduction technique (SSRT) [2,10,15,20,[33][34][35][36][37][38][39][40].…”

“…e limit equilibrium method can be divided into two categories [15,19,21,24]: the bearing capacity method [8,9,13,16,17] and the slip circle method (SCM) [11,16,18,19]. e stability models proposed by Terzaghi [17] and Bjerrum and Eide [8] are based on bearing capacity theory [2,15,19,32,40]. For these two types of methods, factors of safety (F s ) are both defined as the ratio of the resistance over the load [15,34].…”

FEM Optimisation of Seepage Control System Used for Base Stability of Excavation

“…They stated that the thickness of the soft soil layer between the excavation base and hard stratum, the ratio of the depth to the width of the excavations, the penetration depth of the wall below the excavation base and its stiffness all have a significant influence on the base stability of excavations. Zhang and Goh (2016) [15] used the two-dimensional FEM with shear strength reduction using the commercial software Plaxis 2011to analysis the basal heave stability of deep narrow braced excavations in terms of stability number and factor of safety. The results indicate that the stability number is independent of undrained shear strength and the system stiffness has a small influence on the factor of safety against basal heave.…”