Performance of Vertical Shafts Constructed In Swelling Shale Using Finite Element Method

ABSTRACT: Micro-tunneling technique is employed to construct pipelines and tunnels in different types of ground including swelling shales. However, these shales exhibit time-dependent deformation behavior, generally known as rock swelling that causes internal stresses in the underground structures in addition to those caused by the in-situ stresses. These combined stresses may exceed the allowable tensile or compressive strength of the concrete forming the pipeline, the tunnel liner and the walls of vertical shafts, which may result in cracks in these structures. Vertical shafts are used to initiate construction in micro-tunneling technique, then they are utilized as manholes for inspection and monitoring purposes of the constructed pipeline or tunnel. This paper investigates the induced stresses in vertical shafts caused by the time-dependent deformations of swelling shales from south-western Ontario region. The induced stresses in vertical shafts are calculated numerically using the finite element method and the computer program PLAXIS-2D. The vertical shaft is simulated using an axi-symmetrical model to accurately predict its behavior. The time-dependent deformation parameters of the host rock were derived from experimental test program that followed the theoretical model proposed by Lo and Hefny (1996) and they are used to define the rock swelling model of PLAXIS-2D. The results of the performed numerical analyses in this research may aid in adopting the suitable construction procedure to decrease the induced stresses in the walls of vertical shafts or it may lead to adjust their structural design, if needed.