Short-term structural response of a model shaft linear as a baseline to inform a simulated shaft breakout in clay

The growth of underground infrastructure that is being driven in major cities by the increase in population and lack of surface space, requires access between the ground surface and the underground space. In urban environments this can be for a range of applications from transportation to service systems (such as water management). In tunnelling, new sections of permanent works access are achieved via the construction of a connection between a new shaft and the tunnel known as “shaft breakout”. This construction process involves the formation of an opening in the side wall of the shaft near its base, which will disturb the loading and supporting equilibrium of the shaft. Currently, there is limited published knowledge about the behaviour of the shaft lining and the ground adjacent to the opening during the breakout. This results in conservative design approaches and substantial temporary works. To assist with this, baseline results are presented from geotechnical centrifuge modelling and finite-element modelling designed to simulate a shaft breakout. The shaft breakout comprises an opening situated in the wall of the cylinder fitted with a plug that can be removed. The model shaft consists of a preformed instrumented cylindrical liner of diameter of 101.6 mm and depth of 254.0 mm located in a sample of overconsolidated Speswhite kaolin clay. At the test acceleration of 118 times Earth’s gravity, this represents a 12 m diameter shaft sunk to a depth of 30 m. The structural response of a freestanding continuous shaft and the same shaft embedded in clay and tested at 118g is presented. The stress distribution in the shaft prior to the breakout is compared to the redistributed stresses following the construction of shaft breakout providing confidence in the instrumentation used and its interpretation.