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A Three-Dimensional Numeric Forecasting of Groundwater Flow Within Square Cofferdams

Congress: 2008
Farid BOUCHELGHOUM 1, N BENMEBAREK 2, S BENMEBAREK 3 1Doctorant et chercheur, ‘Sciences Hydrauliques’, Université de Biskra - Algérie Unité de Recherche en Hydraulique – LRGCB Mail : 2 Maître de Conférences, Université de Biskra - Algé

Keyword(s): Three numerical modelling, Square deep excavation, Piping, Groundwater, Boiling, Heaving.
AbstractA water presence in soil represents a factor of hindrance or interest according to the engineer's preoccupations. In some cases, a problem is about exploitation of resources by withdrawals of underground water; and in others it can be about exhausting an excavation before underground works. Frequently, real configurations are such (pregnant collected) that the problem cannot be considered as in two-dimension. A flow toward excavation is not then to parallel nets but to convergent nets, what introduces a supplementary head loss. The hypothesis of square cut off wall constitutes an approach convenient of these three-dimensional groundwater flows. The study and comparison of different results of bibliography especially achieved in two dimensions put in evidence: imprecision of some relations and some abacuses; a definition often very fuzzy of application domain. In particular, crosschecks between these methods, to their borders are often bad in very useful zones for practical applications. However, using numerical two-dimensional simulations by Flac2D code, Benmebarek and al [2005] have shown that a soil dilation angle has a significant influence on failure mechanism shape. For a dilating material, a rectangular prism failure by heaving is obtained. This study is devoted therefore to three-dimensional effect of problems met with, dewatering on deep excavation of square shape excavation in permeable soils and urbanised sites in the vicinity of existing buildings. In the first part, we deal with problems of dewatering excavations protected by cut-off walls: pumping flow forecasting and head loss. The numerical study in three-dimensional of these groundwater flows (realised with Flac3D code) has permitted to propose a set of simplified methods for evaluation of flow out-skirting cut off walls. The second part is devoted to study a piping phenomenon that these groundwater flows can induce and that appears in shape of subsurface erosion or bulk heave.
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