Deformation Control Of Existing Tunnels Under Multi-line Overlapped Shield Tunneling And Moving Train Loads

In recent years, more and more subway tunnels are being constructed to meet the increasing demands of transportation in densely populated urban areas. It is common that the newly-built subway tunnels pass over or below the existing tunnels. However, the formation of subway network inevitably brings the case of shield tunnels crossing the adjacent existing tunnels at different locations to shield tunnel excavation. Theoretical researching of multi-line overlapped tunnels drops behind practical engineering. Considering the complicated project of four-line overlapped tunnels in Shanghai metro construction, in which Metro Line 11 below-shield and above-shield crosses the existing Metro Line 4 tunnels successively, laboratory model tests and finite element numerical simulations, combined with in-situ monitoring were employed to study the deformation control of the existing tunnels under multi-line overlapped shield tunneling and moving train loads. The main works in this dissertation were as follows:(1) The system variables of ‘soil-shield tunneling-moving train load’ were determined, and then dimensional analysis method was used to deduce the similarity relations. Discharge and injection of the fluid was adopted to simulate the process of excavation unloading, ground loss and grouting during the shield tunneling. The influences of ground loss and grouting on the surface settlement were discussed.(2) Based on the construction of short-distance multi-line overlapped shield tunnels, a three-dimensional finite element model was established to simulate the soil excavation, the support pressure of tunnel face, grouting pressure, segments assembling and grouting hardening through adding or removing the elements, changing the modulus and applying the pressure. According to the distribution law of earth pressure around the existing tunnels, the shield operation parameters were set to change with the advance of the EPB shield machine. Particular attention was paid to the effect of the chamber earth pressure and grouting pressure on the deformation of the existing tunnels during the below-shield tunneling as well as the loading scope and its value during the above-shield tunneling.(3) Moving axle loads, excitation force function and deterministic formulas for vibration acceleration of track were employed to calculate the metro train load time-history in Shanghai area. Considering the interface characteristics between the segments and the surrounding soils as well as other segments and the effect of longitudinal joint bolts, a three-dimensional dynamic finite element model of ‘track bed-lining-soil’ system was constructed to compare the dynamic responses on shield tunnel structures generated by three different calculation methods. The results can be used to guide the choice of moving train load in the laboratory model tests.(4) A new testing method was introduced to apply moving-axle loads of a subway train on the track structure which consists of rails, sleepers and the track bed in a model shield tunnel. In order to investigate the dynamic responses of the shield tunnel subjected to moving-axle loads, a series of laboratory model tests were conducted in a 1/40 scale model tunnel(160 mm in diameter) which was buried in the sandy soil. The influences of the axle load, the wheel speed and the cover depth of the shield tunnel on the vertical displacement and acceleration of the lining were presented and discussed.(5) The different crossing sequence tests of two newly-built tunnels were designed based on the project of two shields going across the existing tunnel from below and above successively. In the model tests, the deformation of the existing tunnels, which is generated by alternating discharge of the fluid with newly-built tunnel and the moving-axle loads in the existing tunnel, was comparatively analyzed.(6) The ratio of vertical displacement relative change of the existing tunnel to the outer diameter of the newly-built tunnel is chosen as the index for disturbance evaluation of the existing tunnel during the shield tunnel crossing construction. Based on the vertical displacement monitoring data of the existing tunnel below or above the newly-built tunnel in Shanghai soft ground, the ratio value was determined to evaluate the micro-disturbance of the existing tunnel. The micro-disturbed control measures of the existing tunnel were summarized to relieve the disturbance caused by shield tunneling. Then, the more favorable crossing sequence of the newly-built tunnels was discussed under the condition of the newly-built tunnels’ centerlines coincide with each other or not. In the project of four-line overlapped tunnels, the crossing sequence of below-shield first and then above-shield is more favorable for controlling the deformation of the existing tunnels.In the last part, the obtained conclusions in this dissertation were summarized, and the shortcomings and the prospect were discussed for further studies in this field.