Study Of Construction Safety Control Technology On Shield Tunneling Under Cobble Stratum Approaching Pile Foundations Of Railway Bridge

The shallow cut method is widely used for construction of city metro. It may greatly reduce the impact on groud traffic. However, with some problems arising from approaching to existing buildings or structures, its development is more restricted and becoming a key issue. So for ensuring the safe construction of digging, it is very necessary to study the safety control technology for tunnel construction approaching to pile foundations.Relying on the engineering examples of Civil Works Contract No.2 of Phase â…¡ Works of Chengdu Metro Line No.2 (West Line) as a research object, this paper introduces the status of the tunnel, adjacent bridges and geological conditions, describes the adjacent conditions between the tunnel and the railway bridges and mainly makes researches on safety control measures focusing on:(1) safety evaluation on the protection measures and reinforcement plan for shield tunneling through railway bridge in close range; (2) research on safety control technology for shield tunneling through adjacent railway bridge; (3) data analysis on shield tunneling through adjacent railway bridge; (4) information construction technology for shield tunneling through adjacent railway bridge.During the construction of the shield tunneling near the West Circular Railway Bridge, the following three protection measures for safety control are adopted:(1) making manual digging piles (or with drilling piles)+reinforced beams for piers before the shield passes through, reinforcing the soil mass while the shield passes by the top side, and drilling grout holes for tracking on the ground surface; (2) influence process control on shield tunneling through corresponding bridge piers; (3) using the drilled grout holes above the tunnel to track timely and grouting, or by jacking grouting inside the tunnel.A numerical model is established by using ANSYS finite element software during the proof process to make settlements comparison for the railway bridge piers after completion of shield tunneling and decide whether reinforcing and supporting measures to be taken. It is shown that the reinforcing and supporting measures can effectively reduce the impact of shield tunneling on railway bridge and ensure the safety control of bridge pier settlement within standard range. After the existing pile foundations of bridge piers are reinforced and supported, two times of excavations have been made by shield tunneling for both the left and right line of the tunnel under the piers of the West Circular Railway Bridge. The bridge pier settlements are with in the normal range through monitoring and the cumulative change curve for settlement changes gently in the subsequent monitoring. The slope change for piers is also within the allowable range of specification. The cumulative change curve for the central point of the top of the tunnel is relatively mild and it is also mild in the subsequent monitoring. Therefore, it is considered that, after the pile foundations of bridge piers are reinforced and supported, there are no obvious and uneven settlement changes for the bridge piers by two times of excavations by shield tunneling underneath. The reinforcing and protection plan is safe and effective.It is a problem for construction by shield tunneling crossing existing pile foundations of railway bridge nearby. Because of the large traffic volumes for railway bridges and mainly the sandy and gravel stratum, if the tunnel construction is improper, serious economic loss and bad social impact may be brought out. In order to ensure the safety of railway transportation and the tunnel itself during construction by shield tunneling, it is significantly necessary to make research of safety control technology on shield tunneling approaching pile foundations of railway bridges. In addition, as for the bored tunneling approaching pile foundations, some achievements have been made by domestic and foreign researchers. However, few research had been made about construction safety control technology of shield tunneling approaching bridge pile foundations under sandy cobble stratum in area of Chengdu. Therefore this paper has certain reference effect for tunneling approaching bridge pile foundations in the Chengdu Metro construction.