Probabilistic analysis of immersed tunnel settlement using CPT and MASW

Abstract

Settlement data of the Kiltunnel and the Heinenoordtunnel show that immersed tunnels in the Netherlands have been experiencing much larger settlement than expected when designing the tunnels causing cracks in the concrete and leakages in the joints. Settlements of 8 - 70 mm have been measured at the Kiltunnel and of 7 - 30 mm at the Heinenoordtunnel while settlements in the range of 0 - 1 mm were expected. Both sites are investigated through non-invasive geophysical site investigation method MASW (Multichannel Analysis of Surface Waves) for each 2.5 meter along the length of the tunnel and invasive site characterisation method CPT's (Cone Penetration Tests).The settlement of immersed tunnels is similar to that of a shallow foundation. It can be modelled using the Mayne equation which uses the small strain shear stiffness and the degradation of secant stiffness based on the load compared to the ultimate bearing resistance. The initial settlement can not have caused the settlements that were measured, because the initial settlement have already occurred at this time. The 5% and 95% boundaries of the creep settlements for both tunnels is much smaller than the measured settlements. To see if cyclic loading has an effect on the settlement of the middle section the effect of thermal expansion and contraction of the elements and the cyclic loading of the tides is examined.The expansion of the Gina Gaskets between the flat element and the elements at an angle is between -3 and 3 mm. This expansion and contraction of the Gina Gasket causes a vertical load on each of the edges of the middle element of 268 kN. This means that it does have a small influence on the settlement of both tunnels measured at these joints but does not explain all occurred settlement. For an indication of the settlement of the Heinenoordtunnel and the Kiltunnel under the loading of the tides a few calculations have been performed using an assumed lower bound of 40% and an upper bound of 90% of loading of the tides. At the Heinenoord the lower bound scenario gave results between 4.99 - 7.51 mm, which means that the measured values of around 7 mm are within the range of the results, while the upper bound scenario gave results between 10.05 and 15.57 mm which is larger than the measured values. At the Kiltunnel at location 21 and 23 the lower bound is more accurate, at location 19, 25 and 27 the upper bound is more accurate while for location 29 the measured settlement is somewhere in between the two bounds. At location 19 even the 95 % of the upper bound (21.34 mm) is much smaller than the measured settlement (around 34 mm). The difference in shear wave velocity is not that large and can not explain the large difference in settlement while the information from the CPT's is so limited that it is not possible to determine if there is a difference between location 19 and the other locations.