Research On Catastrophe Mechanism Of Soil Nail–Prestressed Anchor Composite Retaining System In Construction Phase

Soil nail–prestressed anchor composite retaining system has been widely applied in the excavation engineering because of its advantages of low cost, good capability of controlling deformation and convenience in information construction since it appeared in the 1980 s. In this composite retaining system, soil nail becomes to be stressed as soon as horizontal displacement of foundation pit occurs, which is a part of passive retaining structures; however, prestressed anchor limits the deformation of foundation pit through imposing initial tension on the side of foundation pit, which belongs to active supporting structures. The two elements are different in the aspect of function mechanism, which will lead to a gap in construction phase, an emergency of catastrophe is more likely due to minor cohesion and larger gravity when the composite retaining system being encountered rainfall infiltration.At present, the findings on the soil nail–prestressed anchor composite retaining system at home and abroad mainly focus on inner forces of the retaining members and deformation of foundation pit, and there is a serious lack of full-scale field tests restricted by sites and funds. The studies on the whole stability are limited to the factors of safety and reliability analysis, and the researches on catastrophe mechanism of the composite retaining system is so rare. Therefore, study on catastrophe mechanism of soil nail–prestressed anchor composite retaining system in construction phase will play an important role in improving dynamic reliability of retaining structure, ensuring construction safety of foundation pit and reducing the project cost.The purposes of the thesis are to study construction mechanics behaviours, working properties and catastrophe mechanism of the soil nail-prestressed anchor composite retaining system, and the analysis method of catastrophe theory of the composite retaining structure is established, which focuses on investigating deformation and damage of soil reinforced by soil nail, debonding failure of prestressed anchor and the whole stability of the composite system considering the rainfall infiltration and overloading in construction phase through multiple research means including site test, numerical simulation and theory analysis. The main work and research findings are as follows:(1) Firstly, prototype test on soil nail–prestressed anchor composite retaining structure is carried out, which is based on a certain deep foundation pit in Zhengzhou. The influence of the unfavourable circumstances such as previous soil disturbance, unloading, overexcavation, delayed tensioning of prestressed anchor and continuous rainfall on stress-time relationship curves of soil nails are analyzed; some concepts including “procedure effect”, “vicinity effect” and “top effect” are put forward; “time effect” of inner forces of retaining elements and deformation of foundation pit during the process of excavation and supporting is discussed.(2) Secondly, numerical simulation on the working mechanism of soil nail–prestressed anchor composite retaining structure is performed, which is based on a certain typical example. When analyzing, deformation characteristics of foundation pit, stress properties of retaining elements and influences of prestressing force on axial forces of soil nails and stress of soil are presented after validity of model being verified, furthermore, the design concept of “weakening soil nails and strengthening prestressed anchors” is proposed; the traditional strength reduction method is modified considering plastic yielding of supporting elements and a method of determining critical failure plane according to distribution of displacement contours is put forward; the changing rule of the critical failure plane is developed through changing magnitude of prestressing force, the length of unbounded part and the location of prestressing force.(3) Thirdly, mechanics analysis on the composite retaining system is developed. A simplified mechanical model is established and stress analysis of soil mass in slip surface is suggested based on the design concept of “weakening soil nails and strengthening prestressed anchors” considering reinforcement mechanism of soil nailed retaining to obtain equivalent parameters of reinforcement; load increment produced in the process of prestressing and excavation is calculated according to sliding wedge theory and increment method, and is assigned between the reinforcement and prestressed anchor; bar system FEM for beam in specification is improved and the development regularity of axial forces of soil nails according to the relationship between load and displacement is revealed combining construction characteristics of soil nail–prestressed anchor composite retaining structure.(4) Lastly, catastrophe analysis on soil nail–prestressed anchor composite retaining structure is developed. The reinforcement is regarded as a kind of “equivalent material” based on the design concept of “weakening soil nails and strengthening prestressed anchors”; a constitutive model of reinforcement is set up according to the additional stress method; damage variable is acted as instability criterion of reinforcement by introducing the concept of energy dissipation and releasing; non-linear shear slip model is derived from the relationship of load-displacement based on the pulling out experiment of anchor considering anchorage effect of anchor; total potential energy of the system is obtained according to the theory of elasticity based on the assumption of taking anchor as an elastic body; when adopting catastrophe theory, critical criterion of anchor is established and instability analysis is developed by simplifying the energy function of anchor to the normal form of cusp catastrophe model; the power equilibrium equation is established considering the effect of the rainfall infiltration to cohesion and gravity of soil and the whole stability of the composite retaining system is analyzed.