Study Of The Surrounding Rock Stability Of The Large-Span Metro Station With The Construction Of Pillar By Center Drift Method

Because of its low cost, less demolition and don't affect the ground transportation and the environment and so on. Shallow Excavation method is widely used in the construction of the subway station. CRD Method is a common method in the driving tunnel method. It has been more applicated in station construction with demanding settlement control. During the construction with center drift method. Firstly, excavate the center guide hole and make the pillar. Then, excavate both sides of guide hole under the support of the pillar. It can make the large span station converted into the small span excavation with support. Ultimately, it will reduce the impact on surrounding environment during the station excavation. The first of the pillar facility has a crucial role in the construction of center drift method. It can guarantee the safety during the construction of the side of the hole. It is a permanent member of the station in the later operations. Therefore, it has great significance to study the deformation of the force for the excavation of the station with the Construction of pillar by Center Drift Method.The paper relies on the station of Xinxing Street of the NO.1 subway in Urumqi of Xinjiang. The methods of the study are theoretical analysis and numerical simulation. It is based on the Selection of the station excavation. It analyzed the impacts of the timing of pillar on the mechanical effects of the excavation of station with the construction of pillar by center drift method. Then, make a further analysis of the affect of the timing of pillar facility by the roof strata thickNess and the level of the surrounding rock.The contents and results are as follows:(1) It highlights introduct some common construction programs of shallow excavation method. Such as: whole section method, step method, center drift method, CRD method, arch cover method, PBA method and double wall pilot tunnel method.Making analyzing of the characteristics of each power law. Then, make a systemed introduction of the construction of processes, construction characteristics and the applicable scope of these methods. Then make a comparative analysis of the characteristics of these conventional construction methods.(2) Analyze the Engineering geology and the design parameters of the station. Choose the center drift method, arch cover method and CRD method as the excavation method station tentatively. Make the software of Midas/GTS as the mean of numerical analysis. Firstly, choose the weakest rock section of station to establish the plane analysis model. Then, make a comparative analysis of the parameters of deformation of rock, surrounding rock stress and structure force with the three kinds of excavation programs. Get the result that the deformation of arch cover method occurs the increased-sharply phenomenon in the facilities of the arch secondary lining. The deformation of CRD method appears the phenomenon of shift in the construction process. The center drift method has the obvious advantages on the control of rock deformation, rock stress and the structure force. Therefore, determine the center drift method as the best option ultimately, through analyzing the various factors.(3) Choose the center drift method as the station excavation program and stablish the numerical model with the weakest section of the station. Make a comparative analysis of the mechanical effect of the construction with and without pillar in the excavation of station. We can see that the amplitude of the difference of surface subsidence is up to 196%, the amplitude of the difference of vault settlement is up to 159%, the amplitude of the difference of invert uplift is up to 103%, under the two different conditions. Also, the amplitude of the difference of the maximum principal stress of rock is up to 580.27%, the amplitude of the difference of the minimum principal stress is up to 71.98%, the amplitude of the difference of shear stress is up to 170.78%, the amplitude of the difference of the supporting structure axial force is up to 80.2%, the amplitude of the difference of bending moment is up to 51.6% under the two different conditions. Therefore, pillar has a very significant effect on reducing the surrounding rock deformation, rock stress and supporting structure force. It is the extremely important support structure by the center drift method in the station excavation.(4) Select a representative range segment to establish the 3D numerical analysis model. And considered two conditions: one condition is to bulit the pillar after the middle pilot tunnel completed, another is to built the pillar after the whole station section excavation completed. Then, make a comparative analysis on the station excavation with the timing of pillar construction by the two conditions. The results show that, the surface subsidence, rock deformation, rock stress, structure force had some extent increased during the excavation of the both sides of guide holes, because of the lag of pillar construction on the formation conditions. Thus, When the ground condition of th station is serious, it will reduce the disturbance in the station excavation and ensure the support structure force reasonably,if the pillar constucts seasonably.(5) It has the direct relation between the mechanical effect on the station excavation by the time of pillar construction and the ground condition of the staion. Establish the multi-group plane numerical analysis models. Then, Analyze the different variation of mechanical effects in the station excavation with the two conditions of the construcion of pill first and later, while the two factors that the thickNess of the roof strata and the level of the surrounding rock are different. Choosed four cases of roof strata thickNess 8m, 12 m, 14 m, 16 m and three kinds of rock Classification grades ?, ?, ?. Analysis shows: the impact of the timing of pillar construction on the mechanical effects of station excavation are no longer significantly, when the roof strata thickNess is 14 m or rock classification is grade ?.(6) When the subway station excavated by using the center drift method, the pillar will be impacted by blasting vibration with the both sides of tunnel excavation. And it often become to an obstacle that hinder the subsequent construction. Therefore, when the formation condition of the station is good(such as, the conditions of roof strata thickNess ?14m or the level of rock reaches grade ? in this station project). It will avoid the impact of blasting on pillar and obtain satisfactory supporting effect while the pillar constructed after the excavation of full station section comleted.