ABSTRACT
This paper explores the pioneering construction of the mountain tunnel for a new road that forms part of the Davao City Bypass Construction Project in the southeastern region of the Philippines. The 2.3-kilometer-long tunnels traversing the Late Miocene to Early Pliocene-aged Masuhi Formation is characterized by interbedded mudstone and sandstone rocks, with sporadic conglomerate beds, that exhibit unfavorable geological characteristics, making the ground extremely difficult for tunneling. This study focuses on the complex engineering and geological challenges encountered during excavation of the 10-meter diameter tunnel through soft rock with compressive strength between 0.3-2.4 MPa, under an overburden reaching up to 150 meters. The discussion highlights the behavior of the rocks encountered, detailing instability problems attributable to very weak rock material, significant water ingress, and extensive squeezing ground conditions within the tunnel section. The results of monitoring of deformation of the tunnel support throughout the construction process are also presented.
