Incorporation of Surrounding Rock Layer Presence in the Design of Underground Cut and Cover Box Structure

Abstract

In all underground works, ground conditions play a predominant role in finalizing the construction methods and the practicality of different designs. History of underground structures is filled with different instances where sudden encounters of unanticipated ground conditions produced delay due to changes in construction methodology and design, resulting in huge increase of project duration and costs. Hence, it is crucial to adopt an innovative practice beyond the existing conventional methods in design/construction having site-specific consideration and design parameters. It is also a fact that innovative technical solutions can save a project from long standstills. A cut and cover station of Chennai Metro Project Phase II has been referred for the present case study. As per ground conditions, top 11–12 m is medium to dense sand layer followed by 3–4 m soft rock layer which ends at hard rock layer. As per track alignment plan, half of the station depth is situated in sand layer and rest in rock layer. Construction of underground structure in such mixed ground condition is quite difficult, time consuming and uneconomical with ongoing conventional methods. Therefore, separate consideration was taken for such case. Instead of full height diaphragm wall as the earth retaining structure as well as part of station main body structure, diaphragm wall with shear pin embedded in rock strata in combination with a stitch wall has been adopted. For lower depth diaphragm wall, a reinforced concrete stitch wall is provided as a connection between diaphragm wall and station bottom slab using open excavation method. Entire sequence of construction has been analyzed in finite element-based numerical model for checking of structural design, stability, ground deformation, etc. The case study of present paper may be helpful for practicing engineers as a good reference for similar situations and to simplify the design.KeywordsUnderground structureCut and cover stationGround conditionDiaphragm wall