Exploring the feasibility of prestressed anchor cables as an alternative to temporary support in the excavation of super-large-span tunnel

Excavating super-large-span tunnels in soft rock masses presents significant challenges.To ensure safety,the sequential excavation method is commonly adopted.It utilizes internal temporary supports to spatially partition the tunnel face and divide the excavation into multiple stages.However,these internal supports generally impose spatial constraints,limiting the use of large-scale excavation equipment and reducing construction efficiency.To address this constraint,this study adopts the“Shed-frame”principle to explore the feasibility of an innovative support system,which aims to replace internal supports with prestressed anchor cables and thus provide a more spacious working space with fewer internal obstructions.To evaluate its effectiveness,a field case involving the excavation of a 24-m span tunnel in soft rock is presented,and an analysis of extensive field data is conducted to study the deformation characteristics of the surrounding rock and the mechanical behavior of the support system.The results revealed that prestressed anchor cables integrated the initial support with the shed,creating an effective“shed-frame”system,which successively maintained tunnel deformation and frame stress levels within safe regulatory bounds.Moreover,the prestressed anchor cables bolstered the surrounding rock effectively and reduced the excavation-induced disturbance zone significantly.In summary,the proposed support system balances construction efficiency and safety.These field experiences may offer valuable insights into the popularization and further development of prestressed anchor cable support systems.

Prestressed tensioning method and its application to testing anchor stress of anchor cables for side-slope stabilization

The anchor stress extent of a prestress anchor cable project has a direct relation with the project safety and performance. Prestressed tensioning method is a kind of nondestructive testing method, by which a reverse stretching load is applied on the external exposure section of anchor cable under construction or in service, and then the elongation variation of stress bars is measured to determine the anchor stress. We elaborated the theory and testing mechanism of prestressed tensioning method, and systematically studied key issues during the prestressed tensioning process of anchor cable by using physical model test, including the composition of tension stress-elongation curve, the variation of anchor stress, the compensation of locked anchor stress, and the judgment of anchor stress, and verified the theory feasibility of prestressed tensioning method. A case study on slope anchor cable of one highway project was conducted to further discuss on the test method, operation procedures and judgment of prestressed tensioning method on obtaining anchor stress, and then the test data of three situations were analyzed. The result provides a theoretical basis and technical base for the application of prestressed tensioning method to the evaluation of construction quality and operation conditions of anchor cable project.

Study on damage-stress loss coupling model of rock and prestressed anchor cable in dry-wet environment

The loss of anchoring force is one of the problems to be solved urgently.The anchorage loss is a key factor causing the failure of anchoring engineering,so it is crucial to study the time-dependent variation of anchoring force.Alternating dry and wet(D-W)conditions have a significant effect on deformation of rock.The anchoring system is composed of anchoring components and rock mass,and thus rock deformation has a significant impact on the loss of anchoring force.Quantifying rock deformation under the effects of D-W cycles is a prerequisite to understanding the factors that influence loss of anchoring force in anchor bolts.In this study,we designed an anchoring device that enabled real-time monitoring of the variation in strain during D-W periods and rock testing.Nuclear magnetic resonance(NMR)measurements showed that under D-W conditions,the increment in porosity was smaller for prestressed rock than unstressed rock.The trends of prestress loss and strain variation are consistent,which can be divided into three characteristic intervals:rapid attenuation stage,slow attenuation stage and relatively stable stage.At the same stress level,the rate of stress loss and strain for the soaking specimen was the highest,while that of the dried specimen was the lowest.In the same D-W cycling conditions,the greater the prestress,the smaller the strain loss rate of the rock,especially under soaking conditions.The characteristics of pore structure and physical mechanical parameters indicated that prestress could effectively suppress damage caused by erosion related to D-W cycles.The study reveals the fluctuation behavior of rock strain and prestress loss under D-W conditions,providing a reference for effectively controlling anchoring loss and ideas for inventing new anchoring components.

Reliability analysis of prestressed anchors in rock slopes of open-pit mines

Prestressed anchor cables are widely used for slope reinforcement,but the loss of prestress makes it difficult to guarantee the reinforcement effect.Anchor cable prestress degradation was considered as a stochastic process,and the probability density function of this process was established using gamma theory and impact theory respectively.Combined with the failure threshold,the probability density was integrated to find the time-dependent reliability of the anchor cable.Based on the monitoring data of the prestress degradation of the anchor cable,parameters in the probability density function were solved by the maximum likelihood method,and the time-varying reliability and service life of the anchor cable were obtained analytically.The applicability of two degradation theories,gamma stochastic process and impact theory,was compared.The results showed that the probability density curves of both degradation models were normally distributed and the error of reliability results did not exceed 0.06.The life prediction results of the gamma stochastic process were closer to the actual life of 400 h than the 500 h of the impact theory,and the probability curves of the anchor cable life also indicated that the impact theory overestimated the service life probability of the anchor cable.Taking the anchor cable reinforcement within the slope of the Dagushan open-pit mine as an example,and the results verify the feasibility of using gamma theory to predict the degradation of anchor cables and provides theoretical support for prevention of the degradation of anchor cables in the slope of an open-pit mine under the action of external forces.

Study on range interval distance of prestressed anchor bars using update backpropagation neural network

Taking the practical reinforced engineering of a reinforced soil retaining wall as an example, which located in Shandong Province and set on 104 national highway, the stress spread behaviors of the anchor bars in the preforced proceeding were tested. According to the test data, and by use of the update backpropagation (BP) algorithm neural network(NN), the test method and it’s mechanism were studied by the network, then the learning results show the mean square error(MSE) only at the 2 55% level, and the proof testing results show the MSE at 4 38% level (the main aim is to build a NN directly from the in situ test results (the learning phase)). Ipso facto, the learning and adjustment abilities of the NN permit us to develop the test data, subsequently, 36 test data were acquired from the NN. By use of the provide data, as well as the failure situation and carried loading capacity of the retaining wall, finally, the choice the reasonable range interval distance of prestress cement grouting anchor bars were carried out, and the result was 2 m×2 m.

ANALYSIS ON INCREMENTAL COHESION OF SURROUNDING ROCK DUE TO PRESTRESSED CABLE ANCHOR

A new method for determining the incremental cohesion △Cm of surrounding rock due to prestressed cable anchor is presented, and the formulas for △Cm are deduced and △Cm distributions also are discussed, based on the two anchorage effects, one is the effect with the prestressed vaIue △σ3 of cable anchor improving the stress state of surrounding rock and increasing the surrounding rock strength, the other is the fully encapsulated effect. The determined incremental cohesion △Cm is subiected to the model test and field measurement in the references, and coincides well with those tested results. The formulas for △Cm can be used in designing supoport parameters and related numerical analyses of prestressed cable anchor.

Deformation mechanism of rock mass and prestressed anchor cable support technology of Haidong soft rock tunnel

The Haidong Tunnel is one of the four soft rock tunnels of the Central Yunnan Water Diversion Project(CYWDP),where large deformation hazards of soft rock occur frequently,which seriously affect construction safety.The effect of highly prestressed anchor cable support was studied based on the active support test in the No.3 branch tunnel of Haidong Tunnel.Firstly,the geological conditions and failure causes were analyzed on the basis of the results of geological survey,in-situ test,and rock laboratory test.Then,the Mohr circle form of the highly prestressed anchor cable active support theory for the support of bedded rock mass was given in combination with the excavation compensation method.It is considered that the prestress active compensation value required for the bedded rock mass is larger than that for the homogeneous rock mass.The deformations of rock mass under both passive and active supports were analyzed by numerical simulations.Furthermore,the'pressure bubble'mechanical model for anchor cable support of bedded rock mass in Haidong Tunnel is given.Field monitoring results show that the highly prestressed anchor cable support can control rock mass deformation well,with a maximum deformation of about 200 mm.The prestressed anchor cable is effective in the bedded stratum,which makes the stress of rock mass uniform and reduces the risk of failure of steel arches due to local bias.Meanwhile,the expansion of plastic zone was efficiently controlled,which is of positive significance for the overall stability of rock mass.

Hydrogeochemical modelling of corrosive environment contributing to premature failure of anchor bolts in underground coal mines

Anchor bolts are commonly used throughout underground mining and tunnelling operations to improve roof stability.However,premature failures of anchor bolts are significant safety risks in underground excavations around the world due to susceptible bolt materials,a moist and corrosive environment and tensile stress.In this paper,laboratory experiments and hydrogeochemical models were combined to investigate anchor bolt corrosion and failure associated with aqueous environments in underground coal mines.Experimental data and collated mine water chemistry data were used to simulate bolt corrosion reactions with groundwater and rock materials with the PHREEQC code.A series of models quantified reactions involving iron and carbon under aerobic and anaerobic conditions in comparison with ion,pH and pE trends in experimental data.The models showed that corrosion processes are inhibited by some natural environmental factors,because dissolved oxygen would cause more iron from the bolts to oxidize into solution.These interdisciplinary insights into corrosion failure of underground anchor bolts confirm that environmental factors are important contributors to stress corrosion cracking.

Deformation Characteristics of Mono Strand Anchor Head

This paper investigates experimentally the deformation characteristics of the mono strand anchor head used in prestressed concrete bridge. Since the strains measurable in the anchor head are the axial and hoop strains, the deformation characteristics of the anchor head are examined by measuring these strains at various positions and according to the jacking force exerted on the pre-stressing strand. Moreover, the possibility to estimate the jacking force acting in the tendon based upon the measured strains and the corresponding error are evaluated.

Computing of the Anchor by the Method of Three-Dimension Point-Radiate Infinite Elements

On the basis of the one-dimension infinite element theory, the coordinate translation and shape function of 3D point-radiate 8-node and 4-node infinite elements are derived. They are coupled with 20-node and 8-node finite elements to compute the compression distortion of the prestressed anchorage segment. The results indicate that when the prestressed force acts on the anchorage head and segment, the stresses and the displacements in the rock around the anchorage head and segment concentrate on the zone center with the anchor axis, and they decrease with exponential forms. Therefore,the stresses and the displacement spindles are formed. The calculating results of the infinite element are close to the theoretical results. This indicates the method is right. This article introduces a new way to study the mechanism of prestressed anchors. The obtained results have an important role in the research of the anchor mechanism and engineering application.

Applied on bolting-cable anchor support of full-seam roadway in weaker thick coal seam

The designing method and the supporting mechanism of both bolt and small cable anchor for full seam roadway in the weaker thick coal seam are systematically analyzed, and the construction technology and the supporting results are briefly summarized.

Dynamic calculation model and seismic response for frame supporting structure with prestressed anchors

A dynamic calculation model of frame supporting structures with prestressed anchors for the slope stability is proposed. The frame and soil are closely contacted in the role of prestressed anchors and they cannot be separated along the whole slope. The lateral displacement of frame and soil is nearly in phase. The movement characteristic satisfies the theory of elastic foundation beam. The frame is treated with elastic foundation beam in this model. The influence of prestressed anchors is simplified as linear spring and damped system related with velocity. Under the condition of horizontal earthquake excitation, the equation of vibration response is established by using the model of dynamic Winkler beam and the analytical solutions are obtained for simple harmonic vibration. This method is applied to a case record for illustration of its capability, in order to verify the method, 3D nonlinear FEM (ADINA) is used to analyze the seismic performance of this case, the comparative results show that the design and the analysis are safe and credible by using the proposed method. The calculation model provides a new way for earthquake analysis and seismic design of slope stability supported by frame structure with prestressed anchors.

基于Barton-Bandis准则的锚固边坡稳定性分析

结合岩石节理面非线性Barton-Bandis破坏准则,探讨了将Barton-Bandis破坏准则参数转化为线性Mohr-Coulomb破坏准则抗剪强度参数的两种常用方法,通过实例分析了各方法的优劣。推导了加锚单岩石节理面控制的平面滑动岩体边坡抗滑稳定性安全系数计算式,开展了锚固参数分析。研究表明:相对于采用等效线性拟合法或切线等效法获取Mohr-Coulomb抗剪强度参数而言,直接应用Barton-Bandis破坏准则计算节理面控制的岩石边坡稳定性更为直观和简便;随着结构面基本摩擦角、结构面粗糙度系数和结构面壁面有效抗压强度的增大,边坡安全系数逐步提高,且结构面基本摩擦角和粗糙度系数对边坡安全系数的影响程度更为显著;锚索锚固力越大,边坡抗滑稳定性越好,而锚索设置角度越大,边坡抗滑稳定性越低,锚索角度设置不当将明显减小锚固效应的有效性。

High prestress truss cable support principle and its application in large cross section coal roadway

With the enlarge of cross section of roadway, the radius of plastic area and broken area increase, and the tensile stress and shear stress distributing in roof coal-rock layers relevantly increase, which induce support effect not obvious for ordinary bolt（cable）. While bounding point and support structure of the truss cable is in vertex angle of roadway, and supplies coal-rock layers in bounding area with the horizontal and vertical pressure, so it settles the support problems in large cross section coal roadway. From the point of view of mechanics, gave emphasis on the invalid mechanics of ordinary bolt （cable） in large cross section coal roadway and supported mechanics of prestress truss cable. The author successfully used this technique in Wuyang Mine, and had the huge economic efficiency and the social benefit.

Calculation of prestressed anchor segment by 3D infinite element

Based on 1D infinite element theory,the coordinate transformation and shape function of 3D point-radiation 4-node infinite elements were derived.They were coupled with 8-node finite elements to compute the compressive deformation of the prestressed anchor segment.The results indicate that when the prestressed force acts on the anchor segment,the stresses and displacements in the rock around the anchor segment are concentrated in the zone center with the anchor axis and are subjected to exponential decay.Therefore,the stresses and the displacement spindles are formed.The calculation results of the infinite element are close to the theoretical results.

考虑锚杆预应力作用下隧道围岩力学分析及参数设计

隧道工程中锚杆设计多依靠经验法和类比法,为研究预应力锚杆支护体系下围岩的力学效应,建立预应力锚杆与围岩力学耦合模型,考虑围岩开挖损伤效应,通过弹塑性力学理论推导出隧道开挖后围岩应力分布及径向位移解析表达式,采用有限差分软件FLAC3D进行算例验证并分析预应力锚杆参数对围岩状态的影响。研究结果表明:理论解析和数值模拟中最大切向应力和围岩塑性区半径分别为4.77 MPa、15 m和4.49 MPa、15.86 m,理论解析与数值模拟所计算隧道围岩应力分布、位移基本一致,在Ⅳ、Ⅴ级围岩地层中验证了理论解析解的合理性;随着预紧力增大,处于塑性区的围岩径向位移及塑性区半径逐渐减小,弹性区基本不受影响;当锚杆自由段长度位于塑性区时,随着锚杆长度增加,围岩塑性区径向位移及塑性区半径逐渐减小。在实际工程中应尽量增大锚杆预紧力,锚杆长度增加对围岩位移以及塑性区的发展控制效果有限,应控制锚杆自由段长度在塑性区内。

带锚板预应力螺纹钢筋抗拔性能试验研究

目的为了探讨直径25 mm、强度等级PSB1080的带锚板预应力螺纹钢筋在有效锚固长150,200,250 mm的钢筋混凝土厚板或基础中拔出时的锚固性能,方法以有效锚固长度、套管有无、锚板宽度和构造钢筋数量为试验参数,对7个试件进行抗拔性能试验研究。研究试件的破坏特征,分析荷载-位移曲线、极限抗拔承载力、试件开裂荷载和荷载-应变曲线。结果结果表明:试件抗拔承载力随着锚固长度的增加而增加,当有效锚固长度为250 mm时,预应力螺纹钢筋均能达到屈服;混凝土开裂荷载随着有效锚固长度、锚板宽度和套管的增设而增加,构造钢筋数量对面层混凝土开裂荷载的影响并不明显;采用套管试件的面层混凝土开裂荷载比未采用套管试件提高了2.66倍;根据荷载-位移曲线可知,在锚板上增设构造钢筋可以防止带锚板预应力螺纹钢筋拔出时承载力急剧下降;基于试验结果和相关文献结果,对比锚固区混凝土锥体破坏试件的试验值与计算值可知,Nilforoush建议式和日本JSCE规范式9.2.11分别比试验值低19%和10%。结论增设套管可以有效提高开裂荷载;使用日本JSCE规范式9.2.11计算抗拔承载力更接近试验值。

风力发电塔基础预应力锚栓T型法兰连接系统计算方法研究

由于锚固区域基础刚度的影响,风力发电塔基础预应力锚栓组件受力不同于塔筒钢管间锻造法兰连接系统。基于Petersen计算方法的原理,推导出适用于预应力锚栓式风力机基础的计算方法,建立包含锚栓组件在内的单区隔和整体有限元模型研究基础约束刚度的空间效应。通过对单区隔和整体有限元计算所得锚栓拉力的对比发现,基础约束刚度的空间效应可忽略不计;通过对整体有限元模型计算结果和工程算法计算结果的对比,验证了工程算法的精度。

动载作用下锚固体应力波传递及破坏特征研究

为研究动载作用下锚固体的应力波传播规律及破坏特征,利用分离式霍普金森杆(SHPB)对锚固体试件开展了不同预应力及不同冲击次数的冲击试验。研究结果表明:随着预应力增加,锚固围岩中应力波峰值逐渐增加,衰减速度逐渐减小,增大锚杆预应力,锚固自由端拉伸波峰值明显减小;随锚固体试件预应力增加,锚固体轴力损失越快,轴力损失率呈二次线性增加的趋势;预应力较小时,锚固自由端围岩产生裂缝较多,多次冲击下裂缝距离较为接近;随着预应力的增加,自由端头破坏程度减弱;随着预应力增加,拉拔最大位移量越来越小,未受动载作用的原始试件拉拔后未出现明显的破坏;动载作用后,随着预应力增大,锚固端头发生一定程度的劈裂破坏,预应力持续增加劈裂破坏程度越来越弱。

复杂条件下超深基坑三排桩锚索坑内支护设计及施工

基于星河雅宝四A地块项目设计应用了一种三排桩锚索坑内支护结构,根据其特点针对性地建立了相应的模型,并进行了支护结构抗倾覆稳定以及抗滑移分析计算。经现场实践,其各项监测数据均满足设计要求,应用情况良好,为复杂条件下超深基坑因周边环境影响提供了一种坑内支护的设计形式。