Studies On Dynamic Characteristics Of Water-Rich Loess Tunnel's Foundation Base

To solve the problem of possible mudding and softening of water-rich loess below a tunnel invert under high-speed rail train dynamic loading, a research was conducted and documented in this dissertation. This research utilized the Zhangmao Tunnel through water-rich loess on the Zhengzhou-Xi'an Passenger Dedicated Line (PDL) as a target and considered the construction situation in the field. The research work started with developing measures of drainage and waterproof for the tunnel during construction. Based on the development, a systematic and detailed investigation was performed by means of in-situ experiment, numerical computation and theoretical analysis. The tasks for the research consist of the following items:(1) According to the background of the research topic, an introduction was presented in relatively detail to the research status concerning high-speed rail, high-speed rail tunnel, large cross-section loess tunnel and field forced excitation tests all over the world.(2) Aiming at the construction method for the Zhangmao Tunnel, which is a water-rich loess tunnel, a measure of drainage and waterproof was developed, which includes "sealing excavation surface, concentrating seepage, protecting excavation surface, and strengthening shotcrete." This measure achieved excellent results, which were verified by Light Dynamic Penetration Tests.(3) Using the DTS-1 Dynamic Test System that was specially developed by Southwest Jiaotong University to simulate high-speed rail train loading, field forced vibration tests were conducted in a loess tunnel for the first time. Dynamic characteristics and deformation relationships of a water-rich loess tunnel and surrounding rock under high-speed rail train loading were obtained. They can be summarized as follows.①Before excitation frequency reaches 21Hz, dynamic displacement on the loading surface increases quickly with excitation frequency in a nonlinear fashion. Afterwards the rate of increase slows down and can be regarded linear approximately. Accumulative settlement on the loading surface is less than or equal to 0.32 mm after 2.3 million times of excitation.②Transmission and attenuation law of vertical vibration velocity along depth of tunnel invert can be described as that attenuation in invert is not obvious, there is mutation in a region near the interface of loess and invert concrete, and there is nonlinear quick attenuation in loess under the invert. Inevitability of mutation and corresponding impact factors are explained with the wave theory. In addition, attenuation laws of vertical vibration velocity along the transverse and longitudinal directions of the tunnel cross section are obtained.③When excitation frequency is lower than or equal to 21 Hz, vibration velocity on the loading surface changes very little with the increase of excitation frequency and can be regarded as appreciatively linear increase. When excitation frequency is higher than 21 Hz, vibration velocity on the loading surface increases quickly in a nonlinear relationship with excitation frequency. The maximum vibration velocity on the loading surface is 1.61 mm/s.④By conducting field forced excitation experiments in the loess tunnel, the relationship between dynamic stress and dynamic load time under different frequencies, the relationship between dynamic stress and accumulative number of excitation under the same frequency, the relationship between dynamic stress and depth under different frequencies, the relationship between dynamic stress and number of excitation under the same frequency were obtained. It is found that dynamic stress is neither sensitive to excitation time nor to excitation frequency under the experimental conditions.⑤By conducting field forced excitation experiments in the loess tunnel, the relationship between dynamic strain in the concrete invert backfill and excitation time was obtained under dynamic loading with different frequencies, Dynamic strain is found not sensitive to excitation frequency.⑥By conducting field forced excitation experiments in the loess tunnel, the relationship between excess pore pressure below the invert and excitation time was obtained. Excess pore pressure is found basically unchanged, which indicates that excess pore pressure is not sensitive to excitation frequency. With the maximum value of 0.1 kPa, excess pore pressure can not soften the loess below the invert.⑦Properties of the loess below the invert after excitation experiments have little change, which shows that the water-rich loess below the invert is not mudded or softened and can meet the requirements of high-speed rail for the tunnel foundation.(4) Using the dynamic module of the computer program Plaxis 2D, simulations of the excitation experiments were conducted and dynamic response of the tunnel-foundation system was analyzed with four different frequencies. Furthermore,3-dimension finite element method analysis was carried out and the results indicate that the influence range of excitation along the tunnel longitudinal direction is approximately 18 m on both sides of the loading surface. The numerical analysis results were verified by combining the in-situ measurements and theoretical relationships.(5) Based on the field experiments and numerical analyses, variations of dynamic stress and vibration velocity along the depth was obtained and expression for attenuation of dynamic stress was proposed. The mutation of vibration velocity near the interface between the arch invert and the loess was explained theoretically.(6) Based on the results of the field experiments and numerical analyses, the depth influenced by excitation is approximately 6 m below the loading surface, i.e., the influenced depth is about 3 m below the invert.(7) It is found in the results of the field experiments and numerical analyses that the maximum strain of the concrete invert backfill does not exceed 3μ, which indicates the stress level of the concrete invert backfill is low. Therefore, the backfill material can be optimized for the invert arch. The dynamic stress and vibration intensity at the bottom of the invert arch are very small, thus the thickness of the invert can be also considered to be optimized.(8) Based on elastodynamics and making use of the Biot kinetic equations for two-phase media, dynamic response of the tunnel-foundation system was studied and an analytical solution was obtained for the special conditions. Moreover, importance and necessity for further study were discussed.Results of the research presented in this dissertation illustrate that the water-rich loess below the tunnel invert will not be mudded and softened and the accumulative settlement on the loading surface under 2.3 million times of dynamic excitation is less than 0.32 mm. Therefore, the foundation of the water-rich tunnel is stable and the tunnel is capable of meeting the safety requirement for the high-speed rail's operation.