The front-shunt tunnel was the first tunnel of the Terminal 5 project at Heathrow to be constructed, and was the first section of sprayed-concrete-lined (SCL) tunnel to be constructed using the method known as LaserShell. This innovation represented a significant deviation from the methods previously used in SCL construction. Therefore it was subjected to a careful examination before and during construction using sophisticated 3D numerical modelling and monitoring during construction. The paper presents typical results from surface settlement levelling, inclinometers and extensometers, pressure cells and tunnel lining displacement measurements, and comments on the performance of the methods and instruments used. The paper then presents the methodology and typical results of the numerical modelling, and shows that the predictions of displacements and stresses compared well with the field measurements. In terms of the control of ground deformations and structural safety the tunnel performed well.

A series of geotechnical centrifuge tests to investigate the long-term heave behaviour of basements in over-consolidated clay was performed, where the profiles of slab displacement and underslab pressure were measured simultaneously. The same prototypes were simulated using Plaxis 2D with the clay stratum represented by the small-strain hardening soil model. The results were compared and a good agreement was found in terms of the profiles of slab-soil contact pressures. The data showed that existing semi-analytical methods of long-term heave predictions can be improved by assuming a quadratic distribution of slab-soil contact pressure, and example calculations for the relaxation ratio method and the relative stiffness method are presented.

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Long-term heave of basement slabs is a significant problem in cities with over-consolidated clay, such as London. There is a dearth of data to calibrate the methods commonly used by designers to predict heave displacement and swell pressure. This paper presents results from two centrifuge tests aimed at reproducing the phenomenon of long-term basement heave. Reduced scale models of rectangular basements with different slab thicknesses underlain by over-consolidated clay were tested, to investigate the effect of base slab stiffness on heave behaviour. The centrifuge tests provided measurements of the profiles of vertical displacement, bending moments in the slab, and contact pressure at the slab-soil interface. This is the first geotechnical centrifuge study to provide simultaneous measurements of vertical displacement and swell pressures during long-term basement heave. Whereas the flexible basement underwent significant differential heave and almost complete relaxation of swell pressures, the stiff basement generated large swell pressures and consequently large bending stresses. These results confirm that the prediction of high heave pressures is a self-fulfilling prophecy: a basement slab with high stiffness will beget large swell pressures. The experimental measurements of swell pressure and heave were compared to predictions by a simplified non-linear method of heave calculation. The simplified non-linear method produced acceptable predictions of total heave.