| In the recent years,with the acceleration of urbanization in Yinchuan City,many deep foundation pit projects have emerged.When the prestressed anchor bolt supporting structure is applied and there is a weak soil layer,the shear strength and bearing capacity of the soil are low,and the anchor bolts are often arranged densely,thus the stress superposition of the anchor group will occur,which will cause the group anchor effect,and it will inevitably reduce the safety of the foundation pit supporting structure.With the continuous renovation of the scale and volume of foundation pit engineering,accidents and local collapses of foundation pit engineering occur from time to time.In deep foundation pit engineering,dewatering wells are often needed.After dewatering,the hydraulic gradient increases,and the permeability makes the soil subsidence.In addition,with the continuous increase of industrial water use,the groundwater level in Yinchuan City keeps decreasing,which leads to the decrease of pore pressure in soil and the increase of effective stress.It also causes the settlement of soil and the change of soil properties.In addition,with the enhancement of environmental effect,the theoretical calculation is quite different from the actual one.The HS-Small constitutive model can consider the stress history of soil,stress release under different working conditions,fluid-solid coupling and strain-related stiffness.In the numerical analysis of deep foundation pit engineering,it can give more reliable results and displacements.Combined with theoretical and empirical judgment,it greatly improves the safety of foundation pit supporting design.Moreover,there is lack of relevant research on numerical analysis of group anchor effect in China.Therefore,it is particularly important to carry out numerical analysis of small strain of deep foundation pit and study on group anchor effect.Taking the deep foundation pit engineering of underground garage in Ningxia Hui Autonomous Region as the research object,this paper adopts the numerical calculation method,and uses the small strain constitutive model of Midas GTS NX,a professional geotechnical software,to calculate excavation-dewatering process of the deep foundation pit with soil nail and anchor combined support(hereinafter referred to as nail-anchor combined support)and the effect of diameter d,length L,spacing S and setting depth h of anchor on group anchors.Based on the numerical analysis of the influence,the original design form of foundation pit support is optimized.The main results are as follows:(1)The numerical model of excavation-dewatering process of deep foundation pit is constructed by HS-Small.The results show that with the increase of excavation depth,the horizontal and vertical displacement of the top of the foundation pit increases gradually,and the deformation of the sunny corner of the top of the foundation pit is larger.Therefore,the supporting structure of this part should be completed in time during the construction process,and soil nails should be appropriately encrypted and arranged.The axial force of bolt increases with the increase of excavation depth.The deformation control ability of the combined support structure with nail and anchor is better,and the deformation control ability of the lower bolt i5 better than that of the upper bolt.There is a great difference in the deformation between the interface of nail-anchor combined support and soil nailing support.In the nail-anchor combined support,the axial force of soil nailing shows a slight S-shaped curve,and the maximum value of the axial force is about 3/4 of the length of soil nailing.It is different from the distribution law of large axial force in the middle and small axial force at both ends in the simple soil nailing support structure,but the distribution law of the axial force is consistent.The axial force in the middle or near of soil nailing is larger than that at both ends.(2)The numerical analysis of group anchor effect shows that the group anchor effect can be reduced by increasing the length L,diameter d,spacing S and setting depth h of anchor,but there is a limited value,and there are two boundary values for the reduction of group anchor effect when the spacing S of anchor is increased.The non-monotonic relationship between group anchor effect and the reduction of group anchor effect first decreases,then increases and then decreases.Increasing the anchor spacing S in the foundation pit support can effectively reduce the group anchor effect.It 1s more reasonable to choose the anchor spacing S of 2m.Increasing the setting depth h is beneficial to reduce the group anchor effect,but if the setting depth h is large,the anchor rod should be set on the upper part of foundation pit to prevent the deformation of the upper part from being too large.(3)Based on the numerical analysis of deep foundation pit and the study of group anchor effect,the original design form is optimized.The results show that the overall stability of the optimized structure meets the requirements of the code.Appropriate infilling arrangement of soil nails at the sunny cormer of foundation pit can effectively reduce the deformation of sunny comer,improve the stability of soil at the sunny comer and effectively prevent soil damage.The anchor arrangement spacing of 2m can not only reduce the group anchor effect,increase the safety reserve of supporting structure,effectively control the horizontal deformation of the pit top with relatively low anchor axial force,but also reduce the eeonomic investment and speed up the eonstruction progress.The results also verify the validity of the above research results.(4)On the basis of establishing the numerical model correctly,the numerical simulation and monitoring results are eompared and analyzed,and a set of parameters table of deep foundation pit engineering in Yinchuan City is inverted by using the eonstitutive numerieal analysis,so as to provide a basis for the numerical analysis and design of similar projects. |
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