Study On Soil Disturbance And Its Classified Settlement Due To EPB TBM Excavation

With the fast development of the urban rail transit network,it is inevitable that the construction of underground tunnels gets close to or through the existing buildings or structures.One of the main concerns during the construction is to control the deformation of the affected existing building or structures.Tunnel Boring Machine(TBM)tunnelling method,as one of the main methods used in tunnel construction,disturbs the surrounding soil to deform to some extent during the excavation,which consequently induces the deformation of the existing building.Therefore,attenuating or decreasing the disturbance and deformation of the surrounding soil caused by TBM is the key to control the deformation of the existing building or structures.The current study systematically investigates the disturbance and classified settlement characteristics of the surrounding soil caused by TBM,which directly related to and have significant implications for the safe construction of urban rail transit network and successful control of the deformation of excavation affected surrounding buildings within safe range.Taking Beijing Subway Line 14 "Fangzhuang station to Shilihe station" section as the in-situ research site,with the use of the field test,theoretical analysis and numerical simulation,the soil disturbance and classified settlement characteristics in the whole process of TBM construction are systematically summarized and analyzed.The following conclusions can be drawn through this study:1.Construction of the settlement monitoring system,data collection and monitoring(1)The field test of the whole process of the soil classified settlement caused by TBM is realized by measure of the single hole.(2)The test needs to combine the settlement values measured at single points and convert them to the corresponding settlement values at different depths in a hole.The “space-time transformation” is proposed and applied,i.e.,it needs to make full advantage of the space-time effects,the reference to the dynamic position of the TBM cutterhead,and the longitudinal horizontal distances between the points and cutterhead as the coordinate value.The data are selected when the distances are same.The distances are from the cutterhead to the points at the different depths.So the displacements of a single hole at the different depths are obtained.(3)To make use of “the real-time monitoring system of TBM construction”,the data processing method is proposed,which takes the average of construction parameters for a set of 17 rings as those for a single generalized cross-section.Besides,multiple regression analysis is performed to eliminate the difference of the measured data.Through this operation,the credibility of “space-time transformation” is improved.2.Characteristics of the ground settlement and applicability of the calculated method(1)The ground settlement-time history curves and the law of the transverse settlement are analyzed.The change law of the ground settlement is obtained.The progressive process of the ground settlement is divided into five phases.The two important progressive phases,that have great impact on the formation of the ground transverse settlement trough,are respectively the processes when TBM passes within and through the points.(2)The transverse influence range of the ground settlement is analyzed.Through the two methods which are the curve fitting and settlement variation per unit length,the reasonable range of the ground settlement is 2.5i in the clay and sand strata above the tunnel,here i is the horizontal distance from the tunnel axis to the inflection point of the curve.(3)The applicability and the parameters of the existing formulas used for calculation of the ground settlement are discussed.The superposed effect of two sequentially constructed tunnels of the ground settlement is analyzed and the position where the maximum settlement happens is proposed in account of such effect.The main reason,which is about the settlement of 2nd tunnel is smaller than that of 1st tunnel,is discussed.3.Characteristics of the soil classified settlement and revision of its calculated model(1)The measured data and the law of the soil classified settlement are summarized and analyzed,and the important phases in the whole process of the soil classified settlement are studied.The result suggests,in the clay and sand strata,the soil classified settlement mainly occurs within 1m above the tunnel.Compared with the upper soil,the classified settlement proportion is higher within 1m above the tunnel when TBM passes within the points.The effective measures should be taken to control the classified settlement of this phase.The synchronous grouting has great influence on the settlement when TBM passes through the points.(2)The applicability of the existing model for predicting distribution of soil disturbance region is discussed,based on which,a new prediction model is proposed to analyze the soil disturbance range ahead of the cutterhead.(3)Based on the monitoring results with high accuracy,a method is proposed,through which the disturbance range of the whole process of TBM construction is divided and the specific distribution of each region is defined.The disturbance range is divided into three zones,respectively significant disturbance zone,transition zone and settlement affected zone.(4)The superposed effect on the classified settlement in the disturbance range of two sequentially constructed tunnels is analyzed.The laws of the soil classified settlement at positions with different distance to the tunnel are discussed in view of the superposed effect.And the position where the maximum settlement happens is given.4.Preliminary study on the methods to calculate soil deformation(1)The space classification of EPB TBM construction is studied.The influence factors of the soil deformation are discussed.The significant factors of the settlement are the change in the soil original stress state and ground loss(the amount of the actual soil excavation is in excess of the designed value).(2)Assumed that the soil is not drained,the calculated formula of the soil deformation induced by the thrust of TBM is proposed in the normal EPB TBM construction.Three kinds of the calculated models are proposed according to the different forms of the friction.The formulas for calculating the deformation caused by the grouting pressure are proposed.The deformations are respectively caused by the vertical and horizontal components of the grouting pressure.The soil deformation law caused by those factors is analyzed by the corresponding programs coded in MATLAB.(3)The two-dimensional and three-dimensional solutions for soil deformation occurring in strata investigated,which is induced by ground loss,are proposed in present study.The given formula,which takes account of the effect of the Poisson's ratio and the soil non-equivalent radial movement,are considered,allowing for the calculation of soil settlement during the proceeding of TBM construction.(4)The calculated formula for the soil deformation is derived,considering influence factors including the thrust of TBM,friction between the shield and soil,grouting pressure and ground loss.The deformations induced by the friction,vertical component of the grouting pressure and ground loss should get more attention.5.Numerical simulation analysis of the soil settlementThe finite element model for calculating the soil classified settlement is established,taking characteristics of EPB TBM construction into consideration.The contours of the soil classified settlement of two tunnels are compared via numerical simulation.Through the comparison,the laws of the soil classified settlements are discussed,which contain soil settlement occurring at the the same depths in different construction phases and that at different depths in the same construction phases.Based on this,ground settlement and soil classified settlement datasets,respectively obtained from numerical simulation and field tests,are compared.And it shows that numerical simulation produced results are consistent with the in-situ measured data,which implies that numerical simulation can be used for prediction of the soil settlement,serving as a supplement to the field test.