Study On Foot-lock Bolt Force In Soft Surrounding Rock And Pre-ling Structure Bearing Capacity

ABSTRACT:According to statistics,30%to40%of our country’s7500km railway tunnels which is under construction are soft surrounding rock tunnels, and a larger proportion of railway tunnels which are designing and planning are soft surrounding rock tunnels. The feature of soft surrounding rock is large deformation, fast deformation speed and the deformation is difficult to control and so on. With the increasing number of tunnels which is built in soft surrounding rock, the accident have occurred at times owing to lack of arch foundation bearing capacity.Through theoretical analysis, model test and numerical calculation method, this paper analyses the foot-lock bolt mechanism, the comparison of different foot-lock micropiles in pre-lining structure and pre-ling structure bearing capacity. The main work and the results achieved are as follows:1. Although foot-lock bolt in soft surrounding rock tunnel project has been used a lot, the force mechanism and load transfer mechanism is not clear. By analyzing the interaction mechanism of foot-lock bolt, steel arc and surrounding rock, this paper put forward foot-lock bolt mechanic computation model. According to the model, we calculate the force conditions of foot-lock bolt based on entrusted project and compare it with measured values. Then the paper study force changes of different elastic rock resistant coefficient, different bolt diameter, different bolt thickness, different length and different bolt angle, at last we give suggestions of choosing reasonable foot-lock bolt parameter.2. In regard to soft surrounding rock tunnel in pre-ling method, we use different models of load structure method to calculate arch foundation load owing to avoid shortage of arch foundation bearing capacity. We use foot-lock bolt mechanic computation model to calculate different specification foot-lock micropile and suggest to choose T76L-1bolt (outer diameter is76mm, thickness is7mm).3.We determine similar conditions of model test according to chamber and tunnel size. According to geotechnical test results, surrounding rock similar material used2mm sand, iron, clay and wood, pre-lining model material used gypsum slurry which water cream ratio equals1.1.4.We use MIDAS GTS to carry on numerical simulation corresponding to model test, and we find the most unfavorable parts of pre-ling structure is the inner side of the dome and arc by comparing the experimental and numerical model results. Owing to test conditions,pre-ling structure can’t be loaded into structural damage, so we use MIDAS GTS to predict. When the applied load is22.56Kpa, the tensile stress of the inner side of the dome(51.3Kpa) achieves tensile plaster model intensity values and cracks begin to appear. If now we consider pre-ling structure achieve ultimate limit state, the prototype pre-lining structure can withstand height of47.5m soil column (=1.19×40m) when arch parts are fully handled, the height of soil column has exceeded deep and shallow buried tunnel boundary depth.