Deep Excavations in Taipei Basin and Performance of Diaphragm Walls

Abstract

Since movements of diaphragm walls are reduced by the presence of existing underground structures in the vicinity of excavation, comparison of the observed wall deflections with the results obtained by using two-dimensional analyses may lead to erroneous conclusions. Similarly, additions to diaphragm walls, such as buttresses, station entrances, ventilation shafts, etc., will also tend to reduce wall deflections. It is thus recommended to compare the results of two-dimensional analyses with the upper envelopes, designated as “reference envelope”, of a family of wall deflection paths of the same geometry of excavation and the same characteristics of the retaining system.

Inclinometer readings obtained at Shandao Temple Station of the Bannan Line of Taipei Metro were studied to establish the relationship between wall deflections and depth of excavations. The results are verified by numerical analyses using computer program PLAXIS. Reference envelopes were developed for the T2 Zone for estimating maximum wall deflections and charts were established for correcting inclinometer readings to account for the movement at diaphragm wall toes. It has been found that the width of excavation has significant influence on wall deflections and toe movements. It has also been found that consolidation of the Songshan Formation due to the drawdown of groundwater in the Jingmei Formation reduce the movements of diaphragm wall toes.