Experimental Study On Dynamic Response Of Ground Fissure-tunnel-stratum Structure Under Variable Frequency Multi-level Vibration Load

Ground fissures,as one of the typical geological disasters in Xi’an,have a significant impact on underground linear projects such as subways.At present,many scholars have done a lot of research on the force mechanism of the tunnel on the ground fissure site,the transmission law of train vibration load,and the response characteristics of the rock and soil around the tunnel to vibration.Among them,in terms of train load,most scholars choose to simplify it to uniform speed,and set up multiple sets of different fixed-frequency excitation loads in the test to correspond to different train speeds.The research on variable-frequency excitation loads is relatively relatively small.less.And we know that the frequency of the exciting load is not static when the train is actually entering the station at a variable speed.On the whole,it should show a gradual decrease trend as the train speed decreases.Therefore,combining the deformation and failure mechanism of the tunnel and the law of vibration propagation in the ground fissure site,in-depth analysis of the dynamic response of the rock and soil around the tunnel under the variable frequency excitation load has great guiding significance for the construction of the subway tunnel.This paper takes the Xi’an Metro Line 2 tunnel orthogonally crossing the ground fissure as the engineering prototype,and innovatively adds variable frequency excitation load to the model test and numerical simulation to simulate the speed change process of the train.The main research contents and results are as follows:(1)A fixed-frequency excitation load was applied in the model test,and the law of vibration propagation was analyzed by summarizing the vibration acceleration and the distribution law of dynamic additional earth pressure along the longitudinal direction of the tunnel under the active state of the ground fissure.The results show that the propagation of vibration is closely related to the compactness of the soil around the tunnel.Based on this,it can be divided into three areas: compaction,looseness,and void.(2)Apply a variable frequency excitation load in the model test,summarize the change law of the vibration acceleration of the rock and soil around the tunnel with the excitation frequency under different settlement conditions,and analyze its response characteristics in different excitation frequency bands.The results show that the greater the vibration response of rock-soil masses under various subsidence states during fixed-frequency excitation,the greater the frequency of vibration response during variable-frequency excitation tends to midand high-frequency excitation,and the greater the vibration response during fixed-frequency excitation.Smaller,the more the vibration response degree of each excitation frequency interval tends to be the same during variable frequency excitation.(3)Use ABAQUS to establish a numerical model to analyze the displacement of the rock and soil around the tunnel under variable frequency excitation load,the dynamic additional earth pressure and the change law of vibration acceleration.The results show that the sensitivity of vibration acceleration to variable frequency excitation load is much greater than dynamic additional earth pressure,and the propagation of vibration mainly has two modes:excitation point propagation and overall tunnel propagation.(4)Simulate the activity of ground fissures by controlling the settlement of the bottom wall of the ground fissure,summarize the contact earth pressure of the top and bottom of the tunnel and the strain distributed along the longitudinal direction of the tunnel,and analyze the mechanism of the tunnel’s stress and deformation under the active state of the ground fissure.Mode and its effect on the vibration response of surrounding soil.The results show that as the settlement progresses,the tunnel force model gradually transitions from the transverse simply supported beam force model to the transverse cantilever beam,and the ultimate failure of the tunnel is a typical bending failure,and the failure point is located near the ground fissure.After the lining is damaged,the response of the rock and soil to vibration is significantly reduced,and excessive ground fissure activity may loosen the track inside the tunnel,resulting in a significant vibration amplification effect in the site.