Study On The Metro Induced Ground Vibrations And Isolation Measures

With the development of rail traffic in metropolis, Metro induced environmental ground vibrations are becoming more and more serious. Vibrations have affected the living and working conditions of human being, the safety of building structures and the normal performance of precision instruments. And especially in recent years, many universities and scientific institutions buy their precision instruments in the research work. Worries about the impacts on their precision instruments from Metro induced ground vibrations have consequently arisen because of the valuableness. Therefore an investigation on the Metro induced ground vibrations and the corresponding isolation measures has become one of the most important tasks. In this dissertation, the characteristics of Metro induced ground vibrations and various isolation barriers were studied. The main contents include:(1) Based on field tests, the characteristics of Metro induced ground vibrations and the impacts of ground vibrations on precision instrument were discussed. It is found that there exists an "equal vibration frequency" for ground vibrations. Ground vibrations smaller than " equal vibration frequency" have higher impacts on precision instruments. When bigger than "equal vibration frequency", impacts on precision instrument decrease with increasing distance from line center. Ground vibrations bigger than 100 Hz in frequency domain have no impacts on precision instrument. Superposition of Metro induced ground vibrations and ground traffic causes the response within 6-30 Hz prone to affect precision instrument.(2) Parameters in numerical simulation were analyzed and then the dominant frequency and micro-ground vibrations were successfully simulated through finite element method. Results show that the acceleration of ground vibration is not all the time being attenuating with increasing distance, but showing some fluctuations. High frequency components of ground vibrations fall off a bit faster than the low ones. The frequency of spectrum peak acceleration becomes lower with increasing distance, frequency band of ground vibration becoming narrow.(3) 2-Dimentional modeling was worked out regarding Metro load and the effects of various isolation barriers were analyzed. Besides, combination isolation barriers were conceived and preliminarily studied. Results show that:(a) the major isolation frequency by underground concrete diaphragm wall is 6-16Hz. Only when the depth reach Metro vibration source can diaphragm wall present obvious isolation effect within its major isolation frequency range; and at that depth, variation of barrier thickness has little influence on its isolation effect. (b) Barriers with soft material have better isolation effect on high frequencies than those with rigid material. While barriers with rigid material will be more efficient in isolation of low frequencies; but the isolation range right behind barrier is much smaller than those with soft material. (c) Combination isolation barrier can effectively improve isolation effect of single barrier. Hence, ground vibrations bigger than 6 Hz can be totally isolated by barriers to mitigate the impacts of Metro induced vibrations on precision instrument.(4) 3D-numerical simulation of ground vibration was carried out based on ground vibration measurements. The isolation behaviors of barriers such as isolation effects and the variation of isolation zone were studied. Results show that:the barrier isolation zone is not simply restricted to the region right behind barrier. At both ends of the barrier there can have some isolation effects. When the length of barrier increases along longitudinal direction, the isolation zone will further enlarge consequently, and isolation effect improves obviously. The degree of enhanced isolation effect with increasing length of barrier relies on different frequency components.