The construction of diaphragm wall panels can cause the stress change and soil movements in adjacent ground. In this paper, the construction sequence of a typical diaphragm wall panel in saturated soft clay is simulated with a 3D finite element program. The soil is assumed to behave as an isotropic linear elastic/Mohr–Coulomb plastic material with a soil–water coupled consolidation response. Influence of the pore water pressure is concerned to consider the consolidation behavior of the saturated soft clay. The analysis shows that the changes in effective horizontal stress and pore water pressure during diaphragm wall installation depend on arching mechanism and permeability. The variation in stresses and movements of ground computed by the coupled consolidation analysis and the total stress analysis are compared. Influences of the permeability coefficient on the installation effects are discussed by parametric studies. Finally, a case study of a diaphragm wall construction in Shanghai, in which the ground settlements were monitored, is presented to illustrate the prediction procedure of coupled consolidation analysis.

Numerical analysis of the installation effect of diaphragm walls in saturated soft clay

Advanced finite element analysis of deep excavation case histories

Abstract Numerical modeling of deep excavations becomes a standard practice in modern geotechnical engineering. A detailed numerical model for a given case is able to reproduce major effects of soil-structure interaction by taking into account any kind of drainage conditions, strong stiffness variation due to effective stress and strain changes, creep and cracking, when reinforced concrete is used as a structural material, but also interface effects between subsoil and structure. Calibrating soil constitutive models is one of the most difficult tasks and due to several sources of uncertainty there is no one unique set of the data that should be used in numerical predictions. Lack or incompleteness of experimental data, significant mismatch between laboratory and field tests is an another source of difficulty. Contrary to several simplified methods, that are usually limited to two dimensions, numerical models allow a full 3D analysis in which many simplifications can be eliminated. This paper is devoted to the problem of in situ stress disturbance caused by diaphragm wall installation in overconsolidated quaternary sandy clays and its influence on final wall deformations.

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Influence of diaphragm wall installation in overconsolidated sandy clays on in situ stress disturbance and resulting wall deformations

Abstract The diaphragm wall and the open caisson represent two main competitive technologies used in the construction of underground objects. In modern times, diaphragm walls are primarily applied for large-size objects, with open caissons being preferred in the case of small-sized ones. Currently, objects of this type are designed mainly for sewage treatment plants and detention reservoirs. Their construction involves highly labour-intensive processes. During the execution of works unforeseen negative effects are observed to occur. During the underground objects construction the most common phenomena are: deviations from the vertical (tilt), sagging, sinking below the designed level, cracking, scratches or leakage through the wall. The purpose of the paper is to classify undesired risk factors emerging in the process of underground objects construction and selection of the optimal technological and material solution for municipal facilities. The implementation of this task involved the selection of Multi-Criteria Decision Making methods, taking into account the cause-effect rating, as the mathematical apparatus. The Ratio Estimation in Magnitudes or deciBells to Rate Alternatives which are Non-DominaTed (REMBRANDT) method was applied. The research proved that it is possible to analytically assess unforeseen risk factors conducive to emergency situations during the implementation of underground objects, using the REMBRANDT method.

Risk of emergency situations of underground objects in the perspective of multi-criteria decision making

Comparative study of contiguous pile wall and steel sheet pile wall as shore protection system using numerical technique

Deep excavations in soft soil deposits frequently cause large wall deflections and surface ground settlements, which possibly compromise the serviceability of neighbouring structures. Therefore, ground movements have to be estimated by the use of numerical methods already at the design stage of the retaining structure. Conventional finite element models choose the earth pressure at rest as initial conditions at the beginning of the excavation process. However, in the case of a diaphragm wall, the installation of the wall panels incorporating the trench excavation under slurry support and the subsequent pouring of the panels precedes the pit excavation. In order to evaluate the influence of the diaphragm wall construction on wall movements and settlements due to the pit excavation, a three-dimensional coupled pore water flow/stress-strain finite element model of the TNEC-excavation pit in Taipei has been generated. The model comprises the stepwise construction of five adjacent 5 m long wall panels as well as the simulation of the subsequent excavation of the pit. For each panel, the trench excavation is modeled by removing the respective elements inside and placing distributed loads, which represent the hydrostatic slurry pressure. Afterwards, the loads are increased to the adopted bi-linear fresh concrete pressure and are finally replaced by additional finite elements simulating the concrete. The increasing stiffness of the fresh concrete due to aging is considered by a suitable evolution of the Young's modulus and the Poisson ratio. A visco-hypoplastic constitutive model is used to describe the clayey soil deposits taking into account the viscous behavior of the ground. After the completion of all the panels, the effective lateral stress adjacent to the diaphragm wall shows an oscillating distribution and strongly differs from the initial earth pressure at rest. This development is mainly affected by the distribution of fresh concrete pressure. At lower depth, the concrete pressure exceeds the total earth pressure at rest in normally- to slightly over-consolidated soil deposits, which results in a stress concentration in the middle in conjunction with an unloading besides the panel during the pouring process. At greater depth however, the concrete pressure is lower than the initial total earth pressure. Therefore, a stress increase in the middle of the panel takes place and a horizontal arching mechanism causes a stress increase besides the trench. Taking this different pressure condition during the subsequent pit excavation into account, the calculated wall deflections and the surface ground settlements differ from those of a conventional finite element model neglecting the installation process of the wall. However, the difference of wall movements is less than 5%, so the influence of the panel construction is of minor interest for the considered TNEC-pit. One reason might be the numerous strutting levels of the top-down construction, which generally limit the deflections. On the contrary, the settlements increase up to 20% and fit the measurements obviously better than those ones calculated by a conventional FE-model. Especially in the case of an adjacent structure in urban areas, it can be expected that the impact of the panel installation process will even increase the settlements and that a neglect of this effect will lead to a loss of accuracy of the predicted ground movements. (A). "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.

T Triantafyllidis

Papers

Influence of the construction process on the deformation of diaphragm walls in soft clayey ground