Importance of FEM analysis in arriving at structural system to deal with large lateral force due to creeping slope - A case study of Shongtong Karcham HEP (450 MW), India

Geological surprises, encountered during construction of water resource projects, sometimes open up issues quite unexpected. One such is the case of Shongtom Karcham HEP (450 MW), India, wherein detailed geological investigations during construction stage led to the identification of creeping in right bank slope of the barrage structure. The creeping slope has been estimated to be a sliding mass with an extent of 1km into hill with varying depth (80-220m) and width in plan (1km at hill top which flares to 3km at river edge). Subsequent to detailed deliberations and studies of alternative proposals for the barrage from techno-economic aspects, the alternative finalized is the one in which the existing barrage complex is to be modified to make it capable to safely transfer the large creeping force onto competent left bank or into the foundation.

FE modeling of barrage complex along with its foundation has been carried out in MIDAS FEA-NX. Considering computational limitations, three separate FE models (one each for the intake-apron floor, the barrage and the stilling basin) along with their foundation and interface (defined using stiffness parameters) have been modeled. Re-designing the barrage complex to transfer such huge creep forces, which are estimated of the order of 17-20MN/m, without adversely impacting the hydraulics of the barrage, was an unprecedented task. Poor foundation conditions at barrage site further added to the complexities of the issue. Iterative analysis of the FE models for various load combinations of loads (hydrostatic, creep and uplift, seismic) were carried out. Based on the numerical studies, a design concept of barrage complex with highly stiff structural components which are capable of transferring unusual creeping forces was arrived at. Allowing limits of displacement and adequacy of various structural components, stresses in concrete portions, base pressures of the apron floor and hydraulics of the flow were critical parameters in arriving at the finally adopted concept of the barrage complex. Resizing of semi-gravity wall, introduction of monolithic horizontal struts, thickening of apron floor, introduction of divide wall in upstream apron, foundation improvement provisions, introduction of piling towards right bank, cable anchors below power intake, construction sequence and delineation of critical points for instrumentation and monitoring are major aspects leading to the unique structural form for the barrage complex.

The project, which otherwise was stalled due to the rare issue of creeping slope, is a typical example wherein capabilities of FEM analysis have been aptly utilized to arrive at an unconventional but technically viable solution in a short time span. The paper discusses this journey of the numerical studies carried out for the project, with specific emphasis on the interpretation of the results that led to the finally adopted structural form for the barrage complex.