Design of deep excavations in urban environments

In the design and construction of deep excavations in urban environments, the engineer's goal is to minimise the cost of the excavation's support system while ensuring that the system is not so flexible that it gives rise to soil displacements that adversely affect adjacent surface and buried structures. Current design and construction approaches are largely empirical and can be unduly conservative in sensitive design scenarios, especially when dealing with granular or layered soil conditions. Numerical modelling, while an important and powerful tool in many design situations, can be costly, requires considerable training to implement and interpret, and can be impractical for many projects. There is a clear need for rational and logical semi-empirical design methods to bridge the gap between these two general design approaches. This thesis contributes to more rational and quantified design processes in a number of aspects. First, semi-empirical methods are proposed for improved estimates of loads on excavation support systems. A method to estimate lateral soil pressures on flexible retaining systems constructed in frictional soils is now provided through semi-empirical approximations based on parametric numerical modelling of excavations. Rational design methods are also provided to account for pressures on the embedded portions of excavation support structures. In addition, the effect of temperature on stresses in bracing steel used in the indeterminate soil-structure system can be rationally assessed using geotechnical exploration data and new analytical equations. Second, current practices for determination of bending moments in excavation support wall structures are examined with respect to their general applicability and the application of “load reduction factors” commonly used to obtain more economical designs. Third, current methods of closed-form and analytical calculation methods for determination of the overall stability of internally braced excavations are assessed and recommendations made for consistent and rational design practice. Fourth, better predictions of ground deformations in granular soil deposits can now be made by use of design charts rather than relying on relatively crude “envelopes” of published data. The effect of pre-loading of horizontal support systems (struts or anchors) has been analysed and quantitatively evaluated and design charts are provided such that the effect of pre-loading can be applied and quantified for a variety of soil types and support system stiffness conditions. Fifth, the extent to which excavation-induced ground deformations may damage adjacent structures can now be better predicted using a new method of damage prediction based on strain superposition principles. The strain superposition method of damage prediction is directly linked to degree of damage criteria. Successful practical design of deep excavations, however, must effectively integrate these issues, among others. This thesis uses parametric analytical and numerical studies, examination of detailed case histories, and compilation of published case data, and proposes new and integrated approaches to problems related to design of excavation support systems generally, and particularly for those in urban environments.