Underground Singapore 2011
Influence of various modelling assumptions in numerical analysis for deep braced excavation K.K. Loh & C.C.M. Kho CPG Consultants Pte. Ltd., Singapore
ABSTRACT: The advancement in geotechnical software has made modelling soil-structure interaction a common practice in understanding practical geotechnical engineering problems, especially in the design of braced earth retaining structures for deep excavation. However, with no standard guidelines or recommendations on how to model such geotechnical problems in practice, such analyses are subject to various assumptions made by the consulting agencies. The influence of assumptions such as the interface elements between structure and soil, wall permeability and input parameters, are studied on an earth retaining wall system comprising steel soldier-pile wall with sheetpile lagging using two popular software, PLAXIS version 8.6 and SAGE CRISP version 5.3. The results are presented and discussed. Pitfalls that Sage Crisp users may not be aware of are also presented. 1 INTRODUCTION Deep braced excavation is a common method employed in the construction of major underground structures in densely built-up areas. In the design of earth retaining support systems in such geotechnical engineering circumstances, it is standard practice to use 2D finite element (FE) software to derive maximum wall bending moments and strut forces. While the advancement of geotechnical software has provided powerful tools in modelling soil-structure in deep braced excavation, the results they produce are still subject to modelling assumptions made by their users. Thus, for a particular geotechnical problem, a wide range in the results can be obtained by different users of the same software. In this paper, two popular geotechnical 2D FE software, PLAXIS version 8.6 and SAGE CRISP version 5.3, are used to simulate a common deep braced excavation using steel soldier-pile wall with sheetpile lagging as the earth retaining and support system. The standalone PLAXIS is a 2-stage analysis software based on the establishment of a pore pressure distribution using a separate steady seepage analysis, and then using that pore pressure distribution for the solution of the stresses. On the other hand, SAGE CRISP is a fully coupled loading-consolidation (Biot) analysis software. The problem is simplified so that uncertainties that are common to the real circumstance, such as actual groundwater regime during each stage of construction, and variability of soil permeability with depth and with respect to soil anisotropy, are not dealt with. Instead the objective is to see the range of maximum wall bending moments and strut forces attainable by the practicing geotechnical engineers by only varying common assumptions made regarding soil-wall interface and wall permeability, and the capability of the software to model such problem. The output obtained from the two mentioned software will be compared and discussed. Additionally, certain pitfalls that designers should be wary of when simulating the drained behaviour of a deep braced excavation in SAGE CRISP are highlighted.
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