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

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.

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  • Authors:
    • SCHAFER, R
    • TRIANTAFYLLIDIS, T
  • Publication Date: 2004-7

Language

  • English

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  • Accession Number: 01011630
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Files: ITRD
  • Created Date: Dec 19 2005 3:22PM