ABSTRACT
This paper presents the results from a finite element study on the effect of the rate of shotcrete setting on surface settlements induced by a tunnel constructed using the New Austrian Tunnelling Method (NATM) in a typical soft ground condition in Singapore. The variation of Young’s Modulus of shotcrete with time that was simulated using the error function curve. A control case of constant Young’s modulus was also analyzed. In addition, four different excavation sequences were simulated. It was found that the maximum achievable stiffness of the shotcrete has significant effect on the surface settlements. For low stiffness shotcrete, the settlements tend to be large. The analyses revealed that the incorporation of variation of Young’s modulus of shotcrete with time has very little impact on the computed surface settlements. This is due to the fact that typical shotcrete mixtures used in NATM are likely to achieve stiffness close to their maximum stiffness within a few hours after placement. The ground stand-up time for the assumed ground conditions was found to be more than 2 days. However, due to the presence of faults and fissures and pockets of weak or relatively permeable material, it is likely to be much shorter in the field.
