CHAPTER 5

BEHAVIOUR OF DEEP EXCAVATIONS BEFORE FAILURE

5.1 Introduction

A second series of centrifuge tests was conducted to study the behaviour of an excavation under working condition before failure has occurred. Three tests denoted as Ts4, Ts5 and Ts6 were carried out. Due to the computer hardware corruption, the data of test Ts6 cannot be fully presented. The particulars of the tests are given in Table 3.4. Main differences between the first and second test series are:

(a) The marine clay in the second test series was subjected to 24 hours of high-g consolidation compared to 8 hours in the first series. As shown in Figure 3.7, a soil profile in which stiffness increases nearly linearly with depth is obtained as compared to the relatively uniform marine clay strength obtained in the first test series. A lightly over consolidated soil near the surface was obtained by reducing the surcharge from 55 kPa to 20 kPa.

(b) The strength of the lower stiff clay ranges from 250 kPa to 272 kPa for the second test series which is considerably higher than the 152 kPa to 187 kPa range for the first test series.

(c) The maximum excavation depth for this test series is 50 mm instead of 150 mm for Test Series 1. This is to ensure that the prefailure behaviour of excavation up to a depth of 5 m can be investigated in some details. Normally, such depth is near the position of constructing the slab or installing the struts so that it is a critical stage in a excavation work. In addition, the long term behaviour of an excavation was also monitored