Diaphragm Wall Embedment Depth Design In Road Underpass Tunnel Project In Singapore

This paper describes the process for diaphragm wall embedment depth design in a single level underpass project in Singapore. The proposed underpass is located at an extremely heavy traffic junction/interchange. The existing Northeast Line MRT (NEL) tunnels run directly beneath the underpass for much of its length. The existing Woodsville Flyover is in close proximity to the proposed underpass structure. Other major constraints include an adjacent existing deep sewerage tunnel and residential/commercial buildings.

Different design conditions were considered in the diaphragm wall embedment depth design, which include soil retaining walls in bottom up construction and load bearing elements in top down construction and permanent condition. Finite element models (FEM) using geotechnical software Plaxis were implemented for earth retaining structure design and damage assessment for the impacts on the adjacent existing tunnel/flyover/building. Adequate wall embedment depths were to be verified by hand calculations in terms of geotechnical capacity of the diaphragm wall, floatation resistance, toe kick-out stability, basal heave stability and blowout failure issues. The proposed embedment depths in design drawings were reviewed by using predrilled holes results during the construction stage.