The role of polyurethane foam compressible layer in the mechanical behaviour of multi-layer yielding supports for deep soft rock tunnels

The polyurethane foam(PU)compressible layer is a viable solution to the problem of damage to the secondary lining in squeezing tunnels.Nevertheless,the mechanical behaviour of the multi-layer yielding supports has not been thoroughly investigated.To fill this gap,large-scale model tests were conducted in this study.The synergistic load-bearing mechanics were analyzed using the convergenceconfinement method.Two types of multi-layer yielding supports with different thicknesses(2.5 cm,3.75 cm and 5 cm)of PU compressible layers were investigated respectively.Digital image correlation(DIC)analysis and acoustic emission(AE)techniques were used for detecting the deformation fields and damage evolution of the multi-layer yielding supports in real-time.Results indicated that the loaddisplacement relationship of the multi-layer yielding supports could be divided into the crack initiation,crack propagation,strain-hardening,and failure stages.Compared with those of the stiff support,the toughness,deformability and ultimate load of the yielding supports were increased by an average of 225%,61%and 32%,respectively.Additionally,the PU compressible layer is positioned between two primary linings to allow the yielding support to have greater mechanical properties.The analysis of the synergistic bearing effect suggested that the thickness of PU compressible layer and its location significantly affect the mechanical properties of the yielding supports.The use of yielding supports with a compressible layer positioned between the primary and secondary linings is recommended to mitigate the effects of high geo-stress in squeezing tunnels.

Aluminum foam as buffer layer used in soft rock tunnel with large deformation

The squeezing deformation of surrounding rock is an important factor restricting the safe construction and long-term operation of tunnels when a tunnel passes through soft strata with high ground stress.Under such soft rock geological conditions,the large deformation of the surrounding rock can easily lead to the failure of supporting structures,including shotcrete cracks,spalling,and steel arch distortion.To improve the lining support performance during the large deformation of squeezed surrounding rock,this work selects aluminum foam with densities of 0.25 g/cm3,0.42 g/cm3 and 0.61 g/cm3 as the buffer layer material and carries out uniaxial confined compression tests.Through the evaluation and analysis of energy absorption and the comparison of the yield pressure of aluminum foam with those of other cushioning materials and yield pressure support systems,the strength,deformation and energy absorption of aluminum foam with a density of 0.25 g/cm3 meet the yield pressure performance requirements.The numerical model of the buffer layer yielding support system is then established via the finite element analysis software ABAQUS,and the influence of the buffer layer setting on the lining support is analyzed.Compared with the conventional support scheme,the addition of an aluminum foam buffer layer can reduce the stress and deformation of the primary support and secondary lining.The maximum and minimum principal stresses of the primary support are reduced by 13%and 15%,respectively.The maximum and minimum principal stresses of the secondary lining are reduced by 15%and 12%,respectively,and the displacement deformation of the secondary lining position is reduced by 15%.In summary,the application of aluminum foam buffer layer can reduce the stress and deformation of the primary support and secondary lining,improve the stress safety of the support and reduce the deformation of the support.

Mechanism of high-preload support based on the NPR anchor cable in layered soft rock tunnels

The control of large deformation problems in layered soft rock tunnels needs to solve urgently.The roof problem is particularly severe among the deformation issues in tunnels.This study first analyzes the asymmetric deformation modes in layered soft rock tunnels with large deformations.Subsequently,we construct a mechanical model under ideal conditions for controlling the roof of layered soft rock tunnels through high preload with the support of NPR anchor cables.The prominent roles of long and short NPR anchor cables in the support system are also analyzed.The results indicate the significance of high preload in controlling the roof of layered soft rock tunnels.The short NPR anchor cables effectively improve the integrity of the stratified soft rock layers,while the long NPR anchor cables effectively mobilize the self-bearing capacity of deep-stable rock layers.Finally,the high-preload support method with NPR anchor cables is validated to have a good effect on controlling large deformations in layered soft rock tunnels through field monitoring data.

Optimization of construction scheme and supporting technology for HJS soft rock tunnel

For a soft rock tunnel under high stress in jointed and swell soft rock （HJS）, two construction schemes pilot-tunneling enlarging excavation and step-by-step excavation were optimized using FLAC20, and the deformation effects of the two construction schemes were verified by field tests. Based on engineer- ing geological investigation and mechanical analysis of large deformations, the complex deformation mechanisms of stress expansion and structural deformation of the soft rock tunnel were confirmed, and support countermeasures from the complex deformation mechanism converted to a single type were proposed, and the support parameters were optimized by field tests. These technologies were proved by engineering practice, which produced significant technical and economic benefits.

Study on the new shell bolting and shotcrete support and its application in soft rock tunnels

A new kind of tunnel support was put forward on the basis of the anchor spraysupport principle. The mechanics of the new three-dimensional steel bar shotcrete liningsupport was studied and the structure's internal forces were analyzed. The model experiment was done relying on the industrial test. The conclusion of numerical calculationsproved that the ANSYS program is reasonable and creditable. It was compared to otherkinds of support that are commonly used in soft rock tunnels. The technique and economiccontrasts of the typical tunnel with support three-dimensional steel bar were completed.

Study on the Deformation Mechanism of a Soft Rock Tunnel

The large deformation of soft rock tunnel is one of the key problems to be overcome in the tunnel construction stage.In the present study,the deformation mechanism of a representative tunnel and some related countermeasures are investigated using field tests and engineering geological analysis.Owing to the scarce performances of methods based on other criteria such as small pipe spacing,anchor bolt length and steel frame spacing,a new support scheme is implemented and optimized.Results show that shear failure and bedding sliding are produced under high horizontal stress conditions.The low strength of the surrounding rock results in the uneven convergence of both sides of the tunnel.With the aforementioned new support scheme,however,most of such problems can be mitigated leading to good stability properties and ensuing economic advantages.

Experimental study on performance of spring damping support structure system for large deformation tunnel in soft rock

According to the convergence confinement theory,it is an effective measure to control the large deformation of high ground stress in fractured soft rock tunnels by using yielding support.The yielding support can be classified as either radial or circumferential yielding support.Circumferential yielding support is achieved by transforming radial displacement into circumferential tangential closure without compromising the support capacity of the primary lining support structure.Based on this,and inspired by the design principle of dampers,a yielding support structure system with spring damping elements as its core was developed,based on the connection characteristics of steel arches in highway tunnel,which can provide increasing support resistance in the yielding deformation section.Then the mechanical properties of spring damping elements were obtained through indoor axial pressure and flexural tests.In addition,according to these results with numerical calculations,the yielding support structure system with embedded spring damping elements can reduce the internal force of the support structure by approximately 10%and increase the area of the plastic zone of the surrounding rock by 11.23%,which can fully utilize the self-bearing capacity of surrounding rock and verify the effectiveness of circumferential yielding support.Finally,the spring damping support structure system was designed with reference to the construction process of the tunnel excavated by drilling and blasting method,and the transformation of the spring damping element to spring damping support structure was achieved.Based on field test results,surrounding ground pressure for the yielding support optimization scheme was reduced by 40%and more evenly distributed,resulting in the successful application and a reduction in the construction cost of large deformation tunnels in soft rock.

中小断面引水隧洞软弱围岩钻爆开挖超挖控制技术研究

超挖现象是软岩隧洞钻爆开挖的一大通病,对工程的安全、质量、成本和进度等会产生严重影响。以某中小断面引水隧洞钻爆开挖为例,首先描述其爆破设计和开挖效果,然后结合爆破参数和地质条件分析超挖的主要影响因素,最后从技术和管理层面提出优化爆破参数、改善地质条件、动态精细管理三大控制技术,建立以“周边孔、辅助孔、超前支护、超前注浆”为优化改善对象的递进式超挖控制技术。通过现场实践对比研究了不同技术下的超挖深度和各自贡献。结果表明:超挖控制无需同时对多个因素进行优化改善,可采用以“周边孔、辅助孔”为优化对象的爆破参数优化控制技术,一般优先通过该方法进行超挖控制;在爆破参数优化后仍无法控制超挖的情况下,可采用以“超前支护、超前注浆”为改善对象的地质条件改善控制技术,和爆破参数优化控制技术形成“递进式超挖控制技术”;按照超挖控制贡献大小依次为超前注浆、超前支护、周边孔和辅助孔,但地质条件改善成本较高,在满足工程要求的前提下,建议优先选用爆破参数优化控制技术。

极高地应力软岩隧道非对称变形机理及支护优化研究

针对极大断面公路隧道施工中出现的非对称大变形问题,考虑高地应力第一主应力与隧道轴线关系、层状软岩夹层与互层状态、掌子面软岩空间不对称、地下水等因素,基于对工程地质条件、围岩与支护结构失效及破坏特征的分析,结合岩样物理力学特性室内试验研究及地应力实测情况,探究了非对称大变形形成机理并提出针对性的支护结果优化方案。结果表明:高地应力层状软岩隧道围岩不对称变形是在岩层倾角α、最大水平主应力与隧道轴线夹角β和岩层夹角γ、围岩岩性和地下水综合作用下的大变形,围岩不对称部位由以上几种因素共同决定;当主应力σ1与隧道轴线既不垂直也不平行时,会产生挤压性偏压构造水平地应力,使隧道横断面侧向受力不对称,发生偏压性非对称大变形;通过改变锚杆的布设方式、提高超前注浆小导管的长度和刚度、喷射临时封闭、在防水板与喷射砼间增加高密度橡塑海绵板缓冲层等措施,可以有效的减少变形量,防止围岩因开挖扰动而松动和坍塌。

考虑湿度应力的深部软岩隧道大变形控制研究

为探明深部高地应力软岩隧道大变形产生机理,制定适应大变形控制措施,以月直山隧道为工程依托,首先明确求解围岩形变压力与松动压力的Kastner与Caquot公式,基于岩体弹塑性力学与连续介质理论建立围岩湿度应力解析解,采用收敛-约束法绘制出3种围岩应力作用下围岩与初期支护特征曲线,分析断面变形过程中围岩与支护结构相互作用规律及3种应力占比演化规律。分别以混凝土与型钢钢架作为二衬支护结构对月直山隧道围岩稳定性进行计算分析,明确考虑湿度应力与松动压力条件下隧道二衬最优支护时机与支护参数,以支护结构安全系数FS评判出最优支护方案并对隧道软岩大变形进行治理。结果表明:当围岩径向位移达到550 mm时,仅考虑形变压力Pi时围岩对支护结构的压力P仅为0.813 MPa,考虑湿度应力Pw与松动压力Pa时P增大为1.372 MPa,湿度应力与松动应力总占比达40.7%,仅考虑形变压力而设计的支护结构不满足围岩稳定性要求;根据“卸压支护”理念,确定以“位移释放峰值upeak=0.325 m”与“间距d=0.7 m”作为增设第二层钢架的最佳支护时机与支护参数,现场二次钢架设置24天后使围岩变形收敛于50.8 mm,围岩大变形得到控制,研究成果可为今后相关隧道工程设计与施工提供参考。

基于总安全系数法的隧道支护“强度-变形-时机”一体化设计方法研究

为解决宜兴高铁兴山东隧道含炭质页岩段大变形问题,基于总安全系数法理论体系,以围岩变形和多层结构协同承载控制为目标,建立适用于主动和被动支护体系的支护“强度-变形-时机”一体化设计方法。在强度满足承载要求的前提下,通过调整支护构件本身的变形能力和支护时机,使支护可变形能力大于设计支护力下围岩必需的变形量(防止支护刚度过大),并小于最小支护力下围岩必需的变形量(防止支护刚度过小),从而使整体支护结构按设计意图以最经济的方式“工作”。采用该设计理论,设计“双层初期支护+可缩钢架方案”和“预应力锚索主动支护方案”2种支护体系,提出各分项支护构件的施作时机和安全控制要求,并在现场开展试验。试验结果表明:1)2种支护方案均有效实现了设计意图,解决了隧道含炭质页岩段大变形问题。2)采用双层初期支护+可缩钢架方案时初期支护安全系数最小值为1.83,隧道最大水平收敛为588 mm,满足设计要求;采用预应力锚索主动支护方案时围岩最大水平收敛为372 mm,锚索最大轴力为310 kN,满足设计要求。

采动下淋水软岩巷道围岩变形破坏特征

在采动影响及淋水浸蚀双重作用下,软岩巷道围岩变形破坏严重,且淋水使锚固剂失效,造成锚杆及锚索锚固力衰减,承载能力下降,严重影响工作面安全生产。针对该问题,采用物理相似模拟实验、数值模拟实验与现场实测相结合的研究方法,分析了淋水软岩巷道围岩在采动影响及不同支护参数条件下的破坏情况,揭示了采动下淋水软岩巷道围岩变形破坏特征并给出了合理的支护参数。实验结果表明:①采动影响下软岩巷道围岩变形破坏形态具有非对称性,且在淋水条件下,巷道围岩破坏范围进一步增大。②锚索长度从9 m增加至10 m,巷道围岩破坏范围减小了4.3%;使用12.2 m长锚索支护顶板时,两帮破坏范围大于使用9 m锚索与10 m锚索,主要原因为12.2 m锚索贯通至顶板含水层,导致水体渗流加剧,巷道围岩进一步软化;使用9.2 m长锚索+4.2 m短锚索联合支护时,相较前3种支护参数,巷道围岩破坏范围减小,水体渗流情况明显改善。现场应用结果表明,采用9.2 m长锚索+4.2 m短锚索联合支护时,顶板下沉量、两帮变形量、底板鼓起量比采用原支护参数时分别减小77.5%,65%,76.5%,有效提高了巷道围岩抵抗变形破坏的能力。

软土地层隧道围岩分区初期支护技术研究

软土地层通常具有较低的承载力和较高的压缩性,隧道开挖过程中极易发生变形和坍塌。为了控制围岩的变形和破坏,确保施工安全,提出软土地层隧道围岩分区初期支护技术。通过合理规划围岩压力监测点,采集压力数据;利用简化的锚杆、钢架及岩体结构面,构建初期支护本构模型;计算锚杆与钢架的最佳间距和长度,并以此为依据设计复合支护结构,实现围岩分区初期支护。结果表明:所提方法隧道沉降位移较小、塑性区分布面积较小。体现了支护技术的有效性,具有较好的应用价值。

西(宁)成(都)铁路高地应力节理发育软岩地层隧道支护体系研究

为保证西成铁路高地应力节理发育软岩大变形隧道安全修建,采用现场勘察、室内试验、地应力测试、工程类比、数值模拟等方法展开研究,结果表明,西成铁路与毗邻兰渝线地质条件类似,均以二叠系、三叠系板岩与砂岩为主,层理清晰、节理裂隙发育,地应力以水平主应力为主导,水平主应力在10~30 MPa之间,竖直主应力在4~13 MPa之间,有较强水平构造应力作用,兰渝线控制围岩变形主要采用增加预留变形量、自进式长锚杆、钢花管加固、增加初支及衬砌刚度、钢纤维喷射混凝土等技术措施。在此基础上针对西成铁路节理发育软岩大变形主要发生在垂直节理倾角方向上的特点,提出首先增大矢跨比,然后增大钢拱架刚度,对于Ⅲ级大变形在垂直节理方向设置预应力锚索的非对称大变形隧道支护体系,该支护体系相比传统支护体系能够减小13.8%~21.2%软岩变形,对于控制节理发育软岩变形效果良好。

软岩隧道NPR锚索主动控制关键技术

为解决横断山区软岩隧道的大变形问题,本文以云南省昌宁至保山高速公路昌宁隧道为工程背景,采用理论分析、现场试验等方法,从作用机理、支护方案、施工工艺等方面对软岩隧道开展了高预紧力恒阻大变形(NPR)锚索主动支护及关键技术研究。结果表明:被动支护下,隧道围岩节理裂隙发育、黏土矿物含量高、围岩强度低、微观结构存在细小溶蚀孔、结构疏松等是昌宁隧道围岩发生变形破坏的主因;提出了“NPR锚索+PR锚索+“W”钢带+高强柔性网”主动支护技术,并通过现场松动圈测试等试验,确定了NPR锚索、PR锚索等的支护参数;针对极破碎围岩及含水软岩的特殊环境,提出了“先喷后支+复喷”和“锚固端下缘封堵+钢绞线端头加丝”NPR锚索主动支护优化技术;现场试验结果表明,NPR锚索主动控制技术能够有效控制软岩隧道围岩大变形,最大变形量由原来的2125 mm控制到300 mm以内,保障了隧道支护结构的安全。

径向让压系统对软岩隧道围岩力学特性影响研究

为处理深部软岩隧道存在的大变形问题,一般采取强支护或支护后进行修复,但收效甚微。在三维地质力学模型的基础上,提出以径向让压为核心的主动支护,先释放围岩应力,后抵抗围岩变形,改善挤压型大变形隧道的力学特性,有效地控制隧道开挖过程中的位移变形。文章以公路隧道典型大变形段为研究对象,建立三维地质力学模型,分析径向让压支护对围岩变形和支护承载能力的控制效果。结果表明:径向让压支护段相对正常支护段能有效控制围岩相对变形,从锚杆变形看出,最大拉应变减小了75.8%,最大压应变减小了67.6%;从围岩与初支接触压力看,将拱底、拱脚压力降低80%,验证了径向让压支护对于围岩塑性区发展和应力释放的有效控制。研究成果可为同类深部软岩隧道围岩控制技术提供数据参考。

基于变形控制的某深埋软岩水工隧洞初期支护参数的选择与验证

大变形下围岩的稳定控制一直是水利水电、公路、铁路等行业的传统难题,尚未形成一套验证软岩大变形等级下初期支护型式及参数选取的经验方法。依据某深埋水工隧洞工程地质情况,选择典型洞段预测挤压变形风险,利用控制变形估算所需支护力,依据规范和相关工程经验确定支护类型与参数的同时,考虑实际开挖过程利用数值仿真分析验算支护的合理性。结果表明,该隧洞具有极大的大变形风险,采用支护力的方法所确定的支护类型和参数基本合理,可将围岩变形有效控制在规范要求的1.0%~3.0%范围,且锚杆长度及支护应力等均未超过极限强度。

顺层偏压挤压性软岩隧道让压预留变形角支护技术研究

针对顺层偏压挤压性软岩隧道的非均匀大变形问题,提出一种基于钢拱架预留变形角的新型让压单元。基于三维精细化建模分析顺层偏压荷载下让压预留变形角支护的承载特性和效果,并应用于中老铁路甘庄隧道,验证新型让压预留变形角支护技术的可行性。研究结果表明:1)顺层偏压荷载下隧道支护结构以弯曲变形为主,传统让压柔性节点在弯矩内力作用下易发生受拉破坏,让压预留变形角能有效释放拱架的弯矩内力,适应更大的弯曲变形;2)顺层偏压荷载作用下,采用让压预留变形角的钢拱架整体应力水平明显降低,拱架整体的承载能力满足强度要求;3)工程现场采用局部扩挖和设置钢拱架让压预留变形角后,隧道变形在断面开挖18 d以后控制在允许范围内,初期支护的应力集中现象减弱,钢拱架主体不再发生弯曲破坏。

高应力软岩隧洞稳定性及支护模拟研究

为分析高应力软岩隧洞的变形特征和支护效应,采用FLAC3D软件模拟滇中引水工程深埋输水隧洞软弱围岩稳定性与支护措施,对毛洞和支护条件下围岩的位移场、应力场和损伤破坏区的分布规律以及衬砌受力状态进行分析和围岩稳定性评价。结果表明:对深部软岩施加锚喷支护可以提高岩体的围压和承载能力,显著减小软弱围岩的变形和损伤破坏程度,但仅依靠锚喷支护时,围岩变形仍然较大。施加永久衬砌时的围岩位移释放率越大,衬砌的内力越小,但围岩的变形量越大。建议对高应力软岩洞室采用全断面锚喷支护,提高初期支护的刚度和强度。可根据不同围岩位移释放率条件下永久衬砌的内力和隧洞的变形特征,结合永久衬砌的承载能力和隧洞的净空要求,确定合适的永久衬砌支护时机。

支护结构参数对深埋软岩隧道影响及优化研究

为控制隧道大变形,保障施工安全,文中依托桂溪隧道,采用数值模拟和非线性规划的方式对隧道稳定性影响较敏感的支护结构参数,如喷射混凝土厚度、注浆加固厚度、锚杆长度进行研究并优化。结果表明,增加喷射混凝土厚度、注浆加固厚度、锚杆长度均能有效降低围岩关键点的变形,减小塑性区体积。但当这些参数过大时,其支护效果均会减弱。基于此,本文采用抽样计算和非线性规划方法对这3个支护参数进行成本及性能组合优化,得到最佳支护参数为厚度24 cm喷射混凝土+3 m锚杆+0.5 m注浆加固厚度施工方案。