| The engineering background of subject is the construction of a new tunnel of second-stage construction of Beijing subway line 8 which located at the Huoying station. Beijing subway line 13 from Huoying station to Lishuiqiao station was under-crossed by the new tunnel. On the basis of consulting a large number of relevant literatures, the methods of indoor test, theoretical analysis, field test and numerical calculation were selected as the main research methods. With these research methods, the mechanics performance of soil under cyclic loading and impact loading was studied firstly. Secondly, the influence on foundation and existing subway which caused by tunnel excavation was studied theoretically. At last, the character of interaction between foundation and under-crossing tunnel was studied with the methods of field test and numerical calculation. The research can be a good guidance to the construction of underground tunnel, and also provide theoretical basis for reasonable design of similar projects.The main research results are as follows:(1) In order to study the dynamic characteristics of soil under cyclic loading, the dynamic triaxial test was designed and implemented. On the design of the test, the interval time between adjacent trains was considered which is different from the majority of existing research. The test on the quaternary clay and silt in the Beijing area was implemented with using different dynamic stress amplitude. The conclusion is as follow:(a) With the increase of the amplitude of dynamic stress, the quaternary silt and clay in Beijing area has a maximum stress before damaging. After the maximum value of the stress, the stress of the silt attenuates rapidly, while the stress of clay maintains the maximum until damage. (b) With the increase of the number of loading times, the centre of the hysteretic loop continuously moving with the increase of strain, and this shows that deformation accumulation occurred in the soil. (c) The deformation accumulation of silt is in three stages. In the first stage, deformation increases rapidly and strain increases quickly, but the growth rate decreases. In the second stage, the increase rate remains a constant speed. In the third stage, the growth rate decreases and tends to zero; Strain tends to remain a stable value. The deformation of surrounding soil is large at an early stage after the construction of under-crossing tunnel, and this should be focused on. (d) If the stress intensification of clay does not occur, the deformation accumulation will increase at a constant speed. But if the stress intensification happened, there will be a turning point on the deformation rate; the deformation speeds up after the point of stress intensification, and this is adverse for the stability of soil. (e) The dynamic elastic modulus of soil keeps decreasing with the increase of axial strain. This means that the strain has softened. This is same as the exiting research results. When the dynamic stress is small which means that soil samples is not damaged in the experiment, dynamic elastic modulus increases slightly with the increase of vibration frequency. But when the dynamic stress is large which means that soil samples is damaged in the experiment, dynamic elastic modulus decrease with the increase of vibration frequency. This phenomenon is different from exiting research. The possible explanation is that the interval that considered in the test loading has changed the way the soil particle rearrangement, and this leads to the occurrence of this phenomenon, (f) The dynamic damping ratio of soil shows a gradually decrease trend with the increase of vibration frequency. The dynamic damping ratio of soil remain unchanged when the vibration frequency arriving at a certain number of times. For each 24 times vibration loading, dynamic damping ratio shows an increasing trend with the increase of vibration frequency. This phenomenon is different from exiting research. The possible explanation is that the interval that considered in the test loading has changed the way the soil particle rearrangement, and this leads to the occurrence of this phenomenon.(2) In order to study the dynamic characteristics of soil under impact loading, the split hopkinson pressure bar test with passive confining pressure was designed and implemented. The test on the quaternary clay and silt in the Beijing area was implemented with using different impact velocity. The conclusion is as follows:(a) The elastic segment of soil is not obvious, and there is no obvious yield process. (b) Stress also increases with the increase of strain rate, and there is a strong strain rate effect, (c) Compared with clay, the stress-strain curve of silt is relatively flat; the maximum stress is less than the maximum stress of clay.(d) The confining pressure has nothing to do with strain rate. (e) Under impact loading, the dynamic yield strength of soil increase with the increase of strain rate; the dynamic yield strength of clay is greater than that of silt. (f) Compared with concrete, rock and coal, the relaxation time of soil which reflects the low strain rate effect, which its representative letter is θ1, is similar to that of these materials, and they are all between 0.3s-1 and 0.4s-1. This shows that there is a low strain rate response for rock, concrete, coal, and soil. The relaxation time of soil which reflects the high strain rate effect, which its representative letter is θ2, is greater than that of concrete, rock, coal and other materials. This shows that sensitivity of soil material of high strain rate is stronger than that of these materials. For silt and clay, the value of θ1 has little difference; the value of θ2 of silt is greater than clay. The longer the relaxation time under the high strain rates, the more energy in the material. So after the impact loading, the silt produced damage, but the clay was not destroyed.(3) The influence on foundation and existing subway which caused by tunnel excavation was studied theoretically. The track and embankment was simplified as an Euler-Bernoulli beam, and the train loading was simplified as a series of moving axle loads. The uniformly distributed load coming from the surface of foundation was obtained. Under this uniformly distributed load, and according to three basic equations of elastic mechanics and Laplace transform, the analytical solution of elastic foundation was obtained. By using the continuity conditions of deformation and displacement, the analytical solution of layered elastic foundation was obtained. Secondly, based on the Laplace transform for time, the physical equations of viscoelastic model was put forward which using the expression of flexibility coefficient, and the analytical solution of single-layer viscoelastic foundation under impact loading was obtained. By using the continuity conditions of deformation and displacement, the analytical solution of layered viscoelastic foundation was obtained. Finally, considering the train loading and impact loading at the same time and using continuous elastic method, the influence of excavation on existing tunnel was analyzed in theory.(4) Foundation settlement and tunnel structure vibration rule were studied by in-situ test method. The settlement was analyzed using the Peck formula, and the time-frequency characteristics of vibration signals were analyzed using wavelet transform. The conclusion is as follows:(a) The measured settlement data was analyzed by using Peck formula. The results show that the values of surface settlement trough width coefficient, which its representative letter is K, are determined between 0.8055 and 1.4609, and most of the values are greater than 1; the values of the strata loss ratio, which its representative letter is V1, are between 0.7569% and 3.9185%, and the vast majority of the values are below 3%. (b) Based on the settlement law of each monitoring point on the ground surface which is perpendicular to the tunnel axis, ground surface settlements of the excavation section is higher than the part of non-excavation part along with the tunnel excavation; after excavation of a section, ground surface settlement accelerated, and the performance of convex degree of settling tank increased, and strata loss ratio also increased. Judging from the ground surface accumulation settlement law, the two positive lines below the surface settlement generally increases with the increase of time, but the surface settlement on two contact lines decrease firstly and then increase. This shows that the train loading also has influence on the ground surface settlement. (c) The measured vibration velocity in the vertical direction of the tunnel structure is larger than that in the horizontal direction along with the direction of train operation. The measured vibration velocity in the horizontal direction along with the direction of train operation is larger than that in the horizontal direction which is perpendicular to the direction of train operation. In the monitored scope which is 5m away from the two positive lines, vibration velocity decreases with the increase of the distance. The measured frequency are between 13 Hz and 87Hz, and most of them are between 30Hz and 60Hz. (d) Under subway moving loading, the wavelet analysis of the tunnel structure vibration shows that the optimal wavelet is the wavelet of bior3.Nd, which the values of Nd are 1,3,5,7 and 9. The bands which are range from 1 to 5 correspond to the frequency ranges which are from 0 to 62.5Hz. The energy of these bands contained the most of the signal energy. Because the frequency is low frequency which closes to the natural frequencies of the building, the vibration of metro trains should cause enough attention.(5) According to the analysis of measured results on the spot, recommendations of monitoring on foundation settlement and tunnel structure vibration were given, (a) When the buried deep is smaller than 4m, the influence of the buried depth on the maximum settlement is greater than the area of tunnel excavation. When the buried deep is larger than 4m, the influence of tunnel excavation area will be in a dominant position. For the engineering such as underground tunnel which under-crosses existing lines, the influence of the tunnel excavation area and buried depth should be fully considered when developing a construction plan. To the tunnel with same area, settlement control standards and different buried depth, more effective control measures are needed for the tunnel excavation with smaller buried depth, such as using synchronous jack-up technique, timely check rail operations, etc. (b) In view of the particularity of structure vibration that caused by subway moving loading, the vibration in three directions should be monitored, and this is different from the monitoring of blasting seismic wave. The vibration velocity can’t be used as a standard to control tunnel structure vibration, and this should be conducted by the method of combination of speed and frequency. According to the research in this article, the speed and frequency of structure vibration should be focused on special monitoring in the area which within 10m distance to the loading position of train load.(6) The behavior of foundation-undercrossing tunnel interaction was studied by using numerical simulation method. Using small restart of ANSYS/LS-DYNA software, the whole process of tunnel excavation and long-term effects of train loading were simulated. The conclusion is as follows:(a) The ground located in the tunnel axis and the vault of tunnel appear vibration hollow effect. Symmetry about the position of train loading, the velocity of nodes which are located in excavation area is greater than these which are located in unexcavated area. Vibration amplification coefficient is defined as vibration velocity in the excavation area divided by vibration velocity in unexcavated area. The results show that the hollow effect vanished after the excavation. Most half of velocity amplification coefficient peak appear in the position where half of tunnel has excavated. (b) In the process of excavation, the train speed of 60 km/h has the greatest influence on the surface displacement. The train speed of 80km/h has the greatest influence on the displacement of the arch of under-crossing tunnel. The train speed of 80km/h has the greatest influence on displacement after the excavation. (c) In the process of the construction of under-crossing tunnel, buried depth of 4.5m has the greatest influence on displacement. Buried depth of 15m has the greatest influence on the surface displacement after the excavation. Buried depth of 10m has the greatest influence on the displacement of arch after the excavation, (d) Based on the Asa-Jakobsen formula which calculates the concrete material fatigue life, the fatigue life of under-crossing tunnel can satisfy the requirement.This article consists of three innovation points.(1) In order to study the dynamic characteristics of soil under cyclic loading, the dynamic triaxial test was designed and implemented. The distribution rules of the dynamic modulus and cumulative deformation of soil were got which along with the change of shear strain and vibration frequency. In order to study the dynamic characteristics of soil under the impact loading, the SHPB test with passive confining pressure was designed and implemented. The Z-W-T constitutive model was improved by study results.(2) The dynamic response calculation method of elastic and viscoelastic layered foundation under dynamic loading were established. The influence on foundation and existing subway which caused by tunnel excavation was studied theoretically by using continuous elastic method.(3) A new way to calculate and load train loading was proposed. The interaction effect of foundation-undercrossing tunnel system under the action of tunnel excavation and train loading were simulated by using ANSYS/LS-DYNA software. The effects of excavation process, train speed and the vault depth on vibration velocity and displacement were got. |
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