| In recent years, the construction of China’s traffic tunnel has made unprecedented development, China has become the largest country in construction scale and speed of tunnel and underground engineering in the world. With the rapid development of the scale and speed of tunnel construction, the tunnel is inevitable to pass through the complex conditions in the process of construction, such as high stress, weak broken surrounding rock. At the same time, the increasing traffic flow has an urgent demand for super section tunnel, such as the double-way eight lanes. A large number of tunnels, which have the characteristics of large cross section, high stress, long tunnel line, extremely weak, will be built under complex geological conditions. Design and construction specifications are inadequate in super section tunnel under such kind complex conditions. The deformation and failure mechanism of surrounding rock in the construction process are not clear, traditional support methods can’t meet the needs of reality demand, these problems which can affect the safety and stability of the tunnel are highlighted. The failure control mechanism and supporting technology of super section tunnel with soft surrounding rock should be studied deeply. High strength support technology of underground engineering confined concrete has been applied in the mine engineering with deep complex condition, the control effect of surrounding rock has been verified. At present, the research on the control technology of confined concrete in super section tunnel with weak surrounding rock is still at an initial stage. On this background, this paper mainly carries out the following researching work, and some useful conclusions are achieved.(1) Model test research on deformation failure and control mechanism in super section tunnel construction processBased on the field engineering geological conditions of Long ding tunnel, the geological mechanics model contrast test of construction process in the super section tunnel has been carried out under the excavation method of CRD. The surrounding rock displacement, the stress evolution law and force characteristics of support component are analyzed with or without of high strength support. Deformation failure mechanism of surrounding rock and the control mechanism of surrounding rock in the construction process of super section tunnel are revealed, the correctness of theoretical analysis and numerical calculation is verified, it provides the basis for the design and construction safety of the high strength primary support in the super section tunnel.① In the process of tunnel construction, the original rock stress is continuously released, the vertical stress of vault presents a general trend of decreasing; the excavation disturbance range of working face is about 1.1 times as well as the diameter of tunnel; the stress change process of different parts is not synchronized in the same excavation section, stress release first appears in the vault generally; the vault stress release rates with high strength support and non-support conditions are 87.2%,81.7% and 94.7%,94.5% respectively after the right tunnel and left tunnel excavation completed, high strength support has a certain inhibitory effect on the stress release of rock mass, which is favorable for controlling the deformation of surrounding rock.② Internal measuring point displacement of model shows stepwise changing, CRD excavation method suddenly arises the phenomenon of "double spot ladder suddenly increasing" in early excavation stage, when monitoring section is away from the working face in the late stage, this phenomenon basically disappears. Near point displacement of left and right vault with high strength support is reduced by 32.8% and 34.5% separately compared the non-support condition, far point displacement is reduced by 59.3% and 50% separately, small spacing is reduced by 31.3% and 36.8% separately, surrounding rock displacement of strength support is overall lower than that condition of non-support, surrounding rock control effect is obvious.③ The overall arch centering is compressed, shows a state of asymmetric distribution, measuring point stress increasing extent of different parts is far different from laws within excavation disturbance range. When it’s far from disturbance range, only the force of various measuring points increases, the overall distribution pattern remains unchanged; affected by the disturbance effect of right tunnel advanced excavation acting on rock mass of small spacing side, asymmetric features of the left tunnel arch force distribution is more obvious, stress mainly is concentrated in the small spacing side.④ Compared with non-support condition, the high strength support has an obvious influence on the control effect under the overload effect, when the simulated buried depth reached 700m, the vault and two shoulders of non-support tunnel are met wide destruction, and the whole stability is lost. At same time, high strength support is still playing the role of bearing capacity, in addition to the vault overall sinking, there is no obvious failure phenomenon of surrounding rock. The vault displacement of high strength support tunnel is reduced by 77.2% compared with non-support condition, and the control effect is more significant under the condition of high ground stress.(2) Study on failure mechanism and high strength supporting technology of super section tunnel with weak surrounding rockNumerical experiments of soft rock control mechanism applied to super section tunnel have been systematically carried out under the three excavation methods that include whole section, CRD and twin side heading methods. The change laws of tunnel surrounding rock deformation, plastic zone development and supporting components force, are contrasted and analyzed under six types of supporting schemes and different excavation methods, which include excavation without support, bolt support, H-beam arch support, SQCC arch support, H-beam and bolt support, SQCC arch and bolt support. The high strength control mechanism of super section tunnel with weak surrounding rock is researched.①Under the different conditions of stress levels and strength grades of surrounding rock, different excavation methods, vault displacement of different supporting schemes and control effects of surrounding rock plastic zone are contrasted and analyzed, quantitative evaluation index of vault displacement control rate and maximum plastic strain control rate of surrounding rock is built. Vault displacement control rate and the maximum plastic strain control rate of bolt support, H-beam arch support and SQCC arch support are 54.6%,66.8%,81% and 40.6%, 60.6%,82.4% respectively. The control effect of high strength support for weak broken surrounding rock deformation and plastic range is better than conventional supporting method.②The change regularities of arch and anchor stress are analyzed in all kinds of contrastive tests, stress evaluation index of arch, stress discrete index and usage rate index of bolt strength are built, the supporting arch stress and its uniformity and mechanical properties of bolt are analyzed and evaluated quantitatively. Under the condition of full section excavation method with the biggest stress of support component, the steel confined concrete arch stress evaluation index is higher than H-beam, its stress discrete index is less than H-beam arch. When surrounding rock strength grade is A9 and supporting schemes are bolt support, H-beam and bolt support and SQCC and bolt support, the usage rate of bolt strength utilization are 100% (broken),76.8% and 61.8% respectively, the steel confined concrete arch support can withstand larger load than H-beam arch and its uniformity is better, under the combined effect with bolt, strength reserve of anchor increases and the support potential is greater.(3) Theoretical research on the bearing characteristics of confined concrete arch centersBased on the mechanical calculation model of confined concrete arch with arbitrary section numbers and various stiffness, internal force calculation formulas of multi-center arch are proposed, strength bearing criterions of different components types are built, the influence rules of different factors acting on the internal force and strength bearing capacity are discussed, stability bearing capacity calculation formulas of the two hinge and fixed multi-center arch are deduced on the basis of arch instability characteristics, the stability weighted model of multi-center arch is built, the influence rules of different factors acting on stable critical load are analyzed.①Based on calculation models of different types of five-center, six-center and circular confined concrete arch, internal force calculation formulas of arch with arbitrary section numbers and various stiffness are deduced, the influence rules of different factors acting on the internal force are discussed, the factors include load, lateral pressure coefficient, node rigidity ratio, bending stiffness, node location and so on. When the load and lateral pressure coefficient increase linearly, the internal force is also linearly increased, load fluctuation only change the value of internal force, the lateral pressure coefficient has an effect on internal force value and shape of internal force diagram, The load and lateral pressure coefficient perform a great effect on the internal force.on bearing capacity calculation formula of square steel confined concrete and profiled bar component, ultimate bearing capacity criterions of SQCC150x8, SQCC180x10, I22b and H200x200 member are established. The influence rules of various factors acting on strength bearing capacity and failure location of five-center, six-center and circular arch are contrasted and analyzed. In the case of the same amount of steel, the bearing capacity of SQCC150x8 is 1.4-1.9 times higher than I22b, SQCC180×10 is 1.8-2.4 times higher than H200x200. Result shows that confined concrete arch possesses higher strength capacity than steel arch.③Aiming at the instability characteristics of underground engineering arch, theory formula of the double hinge and fixed multi-center arch unstable failure are deduced. Based on the embedded degree of arch foot in engineering practice, weighting coefficients are determined. Stability weighting model of multi-center arch, which can meet the stability bearing capacity of project requirements, is built. The stable bearing capacity changing rules of the two hinge and fixed multi-center arch with different radius ratio and node stiffness ratio are compared and analyzed.(4) Comparative study on components property of confined concreteAiming at five basic components of I-beam, H-beam, U-beam, steel reinforced concrete and SQCC, mechanical property tests of axial compression, pure bending and eccentric compression are carried out, the calculation parameters of theoretical analysis are obtained. The numerical test method is determined and the numerical calculation parameters are retrieved. The basic mechanical properties of various kinds of components and the reinforcing mechanism of key parts are defined, bending strength criterions of basic components are built.①By contrasting axial compression tests of basic components which have similar steel content, bearing capacity of SQCC short column is increased by 32.3%-73.1% than the other three kinds of short columns. Meanwhile, the bearing capacity don’t appear the decreasing phenomenon like steel short columns under axial compression, reflecting that the square steel confined concrete have better strength and bearing capacity in late stage.② The obvious stress concentration phenomenon is generated near the filling port of confined concrete component, the filling port is firstly squashed and then arises whole roll unstable failure, it becomes critical failure site. Three different ways of reinforcing test are carried out, the strength of the side bending steel plate reinforced short column filling port and the nearby position has been greatly improved, the stress concentration degree of short column is obviously reduced, the ultimate bearing capacity is improved by 45.6% compared with the opening filling port, its reinforcing effect is best. The influence rules of the length and thickness of side bending steel plate acting on the reinforcement characteristics of the filling port are confirmed through experiment, and targeted engineering proposals are given.③ Though pure bending mechanical property tests of different types of components and nodes, pure bending mechanical parameters, such as M - △φ curves of moment and curvature increment, ultimate bending moment and flexural rigidity, are contrasted and analyzed, the ultimate bending moment of SQCC components has been improved significantly than section steel and steel reinforced concrete. Four stages of M -△φ curve are defined, which include elastic increase (OC), elastic-plastic increase (CD), plastic increase (DE) and plastic stability (EF), the process of mechanical behavior in the pure bending deformation of the component is confirmed; The influence rules of parameters of flange thickness, bolt type and core concrete strength acting on node mechanical property are obtained.④Though eccentric compression tests of various types of components, the N-um curve of axial force with mid-span deflection and N-M curves of components are obtained. N-M curve is consistent with the theoretical calculation results, the envelope range of SQCC is bigger than steel reinforced concrete and steel reinforced concrete is bigger than section steel; the bending strength criterions of basic components are obtained by fitting N-M curve, section steel is linear equations, steel reinforced concrete is two linear piecewise equations, steel confined concrete is piecewise equations of straight line and quadratic curve, confined concrete has better mechanical properties.(5) Experimental study on mechanical properties of confined concrete archA large scale mechanics test with ten trusses of six-center and circular arches, which include different types for square steel confined concrete arches, circular steel confined concrete, I-beam and U-beam, is carried out by transforming large mechanical test system of confined concrete arch applied to underground engineering. Combined with numerical contrast experiment, deformation failure mode, internal force and stress distribution characteristics, critical damage location, ultimate bearing capacity are researched deeply under different loading conditions, which include balancing pressure and eccentric pressure. Compared with the theoretical calculation, the correctness of the theoretical calculation is verified.① Ultimate bearing capacityA. For six-center arch:ultimate bearing capacity of SQCC150×8 arch is increased by 84.6% and 38.2% respectively than I22b and I22b-C arch under balancing pressure, and increased by 78.1% and 42.8% respectively under eccentric pressure; ultimate bearing capacity of SQCC 180x10 arch is increased by 105% and 72.7% than H200x200 and H200x200-C arch under balancing pressure, and increased by 71.5% and 61.5% under eccentric pressure.B. For circular arch:bearing capacity of SQCC 150x8-C40 arch is increased by 4.6%,72.2% and 74.9% respectively than CC, I22B and U36 arch under eccentric pressure, and increased by 10.0%,409.8% and 414.5% under balancing pressure.Compared with section steel arch, the bearing properties of square steel confined concrete has been greatly improved; and its bearing capacity with the condition of balancing pressure is much higher than the eccentric pressure effect.②Deformation failure mode and critical damage locationA. For six-center arch:under the effect of whole balancing pressure, the bottom of arch firstly appears failure and depression into arch’s internal position, the top and both sides of arch don’t appear strength damage, the whole shape of arch gradually change to shape of "heart"; under the effect of balancing pressure and eccentric pressure with bottom fixed, the arch’s overall shape becomes flat, strength failure firstly appears in the both sides and foot of arch.B. For circular arch:under the effect of balancing pressure, both SQCC and CC arch don’t cause damage when the indoor test loading reached 8000kN. Under the effect of eccentric pressure, four types of arch show that overall shape becomes ellipse, strength failure happens in both sides and top of arch.③The affection regularity of thickness and length of steel pipe, strength of the core concrete, vertical pressure ratio (the value of vertical stress divided by horizontal pressure) and other parameters acting on mechanical properties of five-centered, six-centered and circular arch are researched, relation equation of each factors and bearing capacity is obtained by fitting, the bearing mechanism of confined concrete arch is confirmed.(6) The design method of confined concrete arch and its applicationBy researching on deformation of weak surrounding rock and control mechanism applied to super section tunnel, combined with mechanics property analysis of component and arch of confined concrete, design methods of confined concrete arch is proposed:based on field geological conditions and tunnel design requirements, the size of external load and the mode of action are confirmed, the bearing capacity, cross section selection and node parameters are calculated, which are based on the establishment of confined concrete arch theory, overall design scheme is proposed, meanwhile, the numerical calculation is used to check the scheme.Design method of SQCC arch of super section tunnel with complex condition is proposed, which is based on field measured data, the arch type is selected through theoretical calculation, design scheme is proposed and the scheme is checked and optimized by numerical simulation.For the surrounding rock control problem of national typical soft rock mining area of Liang Jia coal mine of Longkou Mining Group, the high strength confined concrete support scheme is determined by using the proposed design method, field tests have been conducted, the maximum deformation is only 37mm after 120 days, failure phenomena of traditional support under this kind of geological condition are solved successfully, the good control effect of surrounding rock is achieved, and the correctness of design method is verified. According to the fault broken zone and surrounding control problem of Longding tunnel in Jinan connection line of Beijing-Shanghai high speed, which is the first one-way four lane with super section tunnel in Shandong province, SQCC180x10 arch is designed as primary support, it enhances higher bearing capacity compared with H200x200 section steel arch by 86.9%. Checked by numerical simulation, its vault settlement is only 2.6cm, stress of arch and bolt is far less than the yield strength, the design scheme is reasonable and feasible, and meets the requirements of construction.Through the research all above, the deformation failure of weak surrounding rock in super section tunnel and the high strength control mechanism are confirmed, the bearing behavior and mechanical properties of different section shapes of confined concrete arch are mastered, calculation theory and design method of confined concrete arch of super section tunnel with weak surrounding rock are proposed, it provides guidance and reference for the control system and design theory of confined concrete surrounding rock in super section tunnel. |
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