Evaluation of design parameters of near embankment underground tunnel structure

Abstract

Now-a-days, traffic congestion is a major problem in many cities. Due to excessive pressure on overland roadways, underground subway or roadway is regarded as an alternative solution to accommodate this huge traffic. For this reason, construction of a suitable underground tunnel is a good alternative. Because the urban tunnel construction is normally carried out in difficult and crowded city areas, one of major design and construction challenges lies in minimizing the induced ground movement, as it poses potential threats to nearby buildings and utilities along the tunnel alignment. Most of the previous works were done to study the impact of the tunnel excavation on adjacent buildings which is of major interest for tunneling construction in urban areas, due to the high interaction between tunneling and existing structures. In context of the previous studies, present study aims at investigating the design parameters (such as thickness, position) of an underground tunnel which is constructed in such a zone where a residential area is being developed far away from the business center, for example, near any river embankment, where new land is being developed using sand filling taken from the dredged materials from the bed of the rivers. The response of the nearby structures (displacement, bending moment, shear force) will not only be dependent on the construction sequence of the tunnel, but also will be dependent on the change in water level near the embankment as these embankments are subjected to a certain water head difference due to difference in water levels on both sides of the embankment causing seepage of water through the embankment and generating uplift forces in adjacent structures. Such phenomena may also induce lateral displacements to nearby tunnels, buildings and if overlooked, can cause severe structural damage at their base. In this particular study, a simple 2D plain strain model has been developed in PLAXIS-2D. Various scenarios have been simulated by construction of tunnel and varying the water level on one side of the embankment. The behavior of both nearby tunnel and building has been observed in terms of generated maximum displacement, bending moment and shear force. Simulated results show both increase and decrease in the parameters mentioned above for varying thickness of tunnel, vertical distance of center of tunnel from existing ground level, distance between tunnel center and center of structure and varying water head difference on both sides of the embankment. Model results in this research, incorporating some of these variations showed that a tunnel away from the embankment, just below the center of the road or any heavily loaded building, having lining thickness of 0.380m at 6.5m below ground level is adequate considering economy of tunneling and vertical displacements generated in nearby structures.