无水砂卵石地层开敞式盾构开挖土体移动的细观机理研究

Appropriate shield choice is the key to success or failure of shield tunnel construction.Sand and gravel is a special stratum for tunnel construction.Currently close shield machine with cutter wheel is mainly used in sand and gravel stratum,especially earth pressure balance shield.The soil of sand and gravel has big particle,its viscosity is low and particles contacts are rigid.The soil will have serious wear on the shield cutter wheel and cutters,so the shield machine needs to be frequent downtime and cutter replacement during tunneling.It will produce higher risk and poor economy and efficiency.Therefore it is essential to consider shield choice carefully in sand and gravel soil.Practice has proved that dense dry/less water sand and gravel soil has better stability.It will take full advantage of the soil's stability with open shield machine.Then the cutter wear can be avoided,the downtime can be reduced and the economy is improved.Model tests and field tests will be developed in the article for physical and mechanical properties and the face stability of sand and gravel soil.As a calculation tool,particle discrete element method will be used to launch numerical calculation and theoretical analysis.The soil's meso-strength,contacts among the particles and interlocking mechanism and meso-mechanism of deformation by excavation and unloading will be discussed thoroughly.Then impacts of the particle meso-fabric on strength and stability of the tunnel face will be analyzed.When necessary,supporting plates or grids are assemblied.Supporting degree for the tunnel face is also explored in terms of meso-mechanism.As a result,scientific basis and theoretical supports will be provided for open shield machine choice in sand and gravel stratum.

盾构机选型正确与否是盾构隧道施工成败的关键。砂卵石是隧道施工遇到的一种特殊地层，目前砂卵石地层隧道施工大多选用带刀盘的闭胸式盾构（尤以土压平衡盾构居多）。砂卵石具有颗粒大、粘性低、颗粒之间为刚性接触等特点，开挖时对刀盘、刀具磨损严重，需频繁停机换刀，是施工风险高发点，且经济性不好。因此，需慎重考虑砂卵石地层的盾构选型问题。实践证明，密实的无水（或少水）砂卵石地层自立性较好，选用开敞式盾构可以充分利用地层的自稳性，避免刀具磨损，减少停机次数，提高经济性。本项目将针对砂卵石地层的物理力学性质及开挖面稳定性开展模型试验与现场测试，运用颗粒离散元进行数值计算与理论分析，对砂卵石地层的细观强度、颗粒之间的接触及咬合机理、开挖卸荷变形的细观机理进行深入探讨，并分析细观组构对其强度和开挖稳定性的影响，从细观机理说明开挖面所需支撑度（挡板或网格），从而为砂卵石地层选用开敞式盾构提供科学依据和理论支持。

砂卵石具有颗粒大、粘性低、颗粒之间为刚性接触等特点，目前砂卵石地层隧道施工大多选用闭胸式盾构。掘进时刀盘、刀具磨损严重，停机换刀频繁，施工风险高，经济性不好。因此，开展了砂卵石土体的细观特性及开敞式盾构开挖土体破坏的细观机理等方面的研究，主要有：.（1）砂卵石土体细观特征性研究.颗粒形状、颗粒组构、摩擦特性、含水量、临界孔隙比、密实度、剪胀性等对砂卵石土体的强度、稳定性具有重要的影响。颗粒体系具有颗粒→力链→体系的多尺度结构，颗粒骨架是颗粒相互挤压形成的复杂接触网络，反映了颗粒排布空间位置的关系。不同的颗粒级配、颗粒体积份额、颗粒表面摩擦系数等会产生不同的接触网络和传力特性，决定着砂卵石颗粒体系的宏观力学行为。.（2）砂卵石土体成拱特性细观研究。运用颗粒离散元和活动门试验分析砂卵石介质的运动及其相互作用，通过细观特性分析，得到了拱体土颗粒之间以及土颗粒与结构之间接触力的分布规律，得知拱体区域土颗粒以“楔紧” 、重组以及定向排列等方式，自发形成拱体以抵抗和传递外力。.（3）砂卵石土体开敞式盾构开挖细观研究。对土体移动细观机理、松散程度、地层变形、土体移动规律及破坏形式进行了研究与分析，得出了砂卵石土体移动程度与范围、开挖面可能的失稳形式。地层与开挖面土体破坏呈渐进性破坏特征，隧道纵断面上土体破坏面呈对数螺旋线形状。地层与地表位移值，隧道中心线正上方最沉降大，向两侧逐渐减小，呈沉降槽分布。竖向位移、水平位移与砂卵石的颗粒组构、密实度及开挖面支撑度有关。北京地铁6号线开敞式盾构现场测试结果，与试验结果相比，在规律和趋势上具有较好的一致性。.本项研究对于摸清砂卵石地层隧道开挖土体内在变形与破坏机理具有重要的意义，为砂卵石地层选用开敞式盾构提供了科学依据和理论支持。