ABSTRACT
The first long distance NATM road mountain tunneling in homogeneous soft mudstone’s ground condition In Davao City, Philippines using mechanical excavation method would be presumed to be a smooth project. However, the existence of high overburden and soft ground, lowering the ground’s competence factor well below 1, presented a huge tunneling challenge. The designed tunnel support, DI type according to Japan Highway Standards, was observed to be insufficient to withstand the stresses from the ground and hence, large deformation of the tunnel supports was observed at the high overburden tunnel section. This paper will discuss the impact of soft ground and high overburden on the designed tunnel support and the countermeasures that were applied. The countermeasures included the estimation of the ground’s geotechnical parameters by a back-analysis of one selected tunnel section and re-assessing ground deformation with the more realistic soft ground condition’s geotechnical parameters, to determine appropriate tunnel support.
