Elimination of temporary propping on large underground transportation projects has been achieved through the implementation of a technique called the observational method. Using a progressive modification approach, propping trials are conventionally undertaken which allow an incremental reduction in the amount of propping during excavation, and contingency steelwork props are specified in order to maintain an acceptable level of safety. Extension of this technique to smaller projects where propping trials and contingency steelwork props are not feasible represents a novel and potentially important development in the use of the technique. This thesis describes the successful implementation of such an approach during construction of the new Airside Road Tunnel at Heathrow Airport, where substantial cost and time savings helped to improve project delivery. The main findings are summarised as follows: (a) Adapting the progressive modification approach required a carefully controlled excavation sequence, in which soil was removed in a series of individual layers or bays, combined with robust monitoring of wall movements during the critical phases. The innovative development of specifying a concrete blinding strut as the main contingency measure in place of the steel props is thought to be the first time this has been done in an observational method application. (b) Based on the results of analyses undertaken and observed wall movements, there is an argument that in certain situations provided the risk can be adequately controlled (e.g. by using the observational method), designers may not need to be as cautious when estimating the partial drainage of the clay during construction. By placing greater reliance on the undrained shear strength in the short-term more economical designs may be possible. (c) Smaller than expected wall deflections indicate that three-dimensional effects were significant on the ART project. By adopting a controlled excavation sequence that increased the three-dimensional effects in the short-term, wall movements in the corners were shown to be 40 to 50% less than at the centre of the structure.