This paper employs Discrete Element Method (DEM) simulations to investigate the influence of relative density on soil arching within a plane-strain active trapdoor scenario. For varying relative trapdoor depths, DEM simulations illustrate the key influence of dilatancy on displacement and strain fields and on stress rotation and trapdoor pressure, confirming that shear bands develop at the trapdoor depending on the soil’s dilation angle. The interplay between dilatancy and soil cover governs the arching phenomenon and the ground deformation mode; the significance of relative density is also highlighted by its effects on the principal stress rotation and ground reaction curves. To predict the minimum trapdoor pressure, we propose a Limit Equilibrium Method (LEM) solution that considers the type of failure mechanism (trapezoidal or triangular) and the lateral earth pressure as a function of the soil’s dilatancy and stress arching shape; this approach coincides with Terzaghi’s soil pressure concept at the critical state. LEM predictions of minimum and ultimate (or terminal) trapdoor pressure, and of shear deformation modes, are validated with our DEM results and with literature results. Finally, the impact of effective stresses and relative density on deformation patterns and design charts that quantify the minimum trapdoor pressure is discussed.

Triaxial tests were employed to investigate palm-fiber-reinforced sand under consolidated drained conditions in this study. Sixteen series of triaxial tests were carried out to investigate the properties of palm-fiber-reinforced sand. One series of pure sand was also employed for comparison. The deviator stress, stress path, shear strength, volume change, void ratio, and enhanced coefficient of fiber-reinforced sand were studied with different fiber lengths varying from 8 mm to 20 mm and fiber contents varying in mass from 0.3% to 0.9%. The test results indicate that palm fibers were beneficial for enhancing the shear strength of the sand. Compared to the peak shear strength increase of about 10% to 20%, the critical shear strength increased much more, by a little over 100%. Therefore, the fibers played a key role in enhancing the critical shear strength of the sand but not the peak shear strength of the sand. The addition of fiber to sand resulted in prolongation of the axial strain required to reach the critical void ratio and improved the sand’s ability to resist larger deformations, enhancing its toughness. Furthermore, the critical shear strength of the sand was positively correlated with both fiber content and fiber length, and the axial strain required to reach the critical shear strength increased with increasing fiber content and length. This study provides valuable experimental data and serves as a reference for temporary reinforcement in geotechnical engineering.

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The earth pressure balanced shield supplies a passive support for the tunnel face. However, the slurry shield offers an active support for the tunnel face with slurry (liquid medium). The key work for slurry support is to choose a suitable slurry and a support pressure to form a slurry cake and maintain its integrity. Based on the theory of soil fracturing and the wedge-prism model, the upper and lower bounds of slurry support pressures are supplied. Slurry support pressures are analyzed with consideration of hydrostatic pressures, soil properties, tunnel diameters and earth covers. It is suggested that the lower bound of the support pressure is mainly affected by hydrostatic pressure and soil friction, especially by the hydrostatic pressure. The slurry support pressure can be usually set as 20 kPa more than the hydrostatic pressure; the upper bound of support pressure is soil fracturing (seepage failure) pressure, mainly affected by hydrostatic pressure and earth cover. The soil fracturing (seepage failure) pressure can be increased by increasing the thickness of earth cover. And then the ranges of slurry support pressure can be also enlarged. However, the ranges of slurry support pressure can only be moved but not enlarged by increasing the hydrostatic pressure. The range length of slurry support pressure is also affected by the soil friction rather than other factors mentioned above. Considering the setting of slurry pressure in shield cutter replacement condition, the ratio of earth cover to tunnel diameter should be larger than 0.8~1.0

Aiming at the current situation of continuously promoting the construction of crossriver(sea) tunnel,the importance of the excavation face stability to shield tunneling was analyzed.Whilst the analysis on excavation face stability of shield tunnel was carried out from methods,relevant theories and problems for solution.Through the review of the development of excavation face stability of slurry shield tunneling at home and abroad,the limitation of classical theory and model test of the excavation face instability was discussed.The relevant researches on the criterion and model test of slurry fracturing instability was introduced.Whilst the effect of filter cake and cutter disc on the stability of the excavation face was analyzed.Finally,the outlook of the development direction of the excavation face stability was provided,especially for the slurry shield tunneling with high water pressure and large diameters.The research mainly discussed the excavation face stability of slurry shield from the classical theory,slurry fracturing,filter cake,involving other problems,such as rational depth of earth cover.The results show that it is difficult to investigate the excavation face stability under the interaction between shield and soil,on account of the complexity of the slurry shield and the diversity of the strata.There have been no universal evaluation criteria of dynamic film formation of slurry and few researches on supporting effect of cutter disc on excavation face.Especially under the high water pressure and complex strata,the problem of excavation face stability referring to crossing fields needs indepth investigation.Only in this way can it apply to the associated works.

苏州轨道交通2号线穿越古城区,从480幢各类房屋的正下方或侧面通过,其房屋多为无基础或简单条形基础,易受变形影响发生开裂、崩塌等;再加上2号线穿越粉土和粉砂层,地下水位较高,透水性强,盾构穿越时由于土体扰动极易发生流砂或板结现象,使开挖面失稳.虽然盾构隧道施工技术随着盾构性能的改进有了很大发展,但在上述特殊情况下成功穿越近邻的大量建筑物依然是全新的挑战.文章通过掘进速度,以及浆液材料和注浆量现场试验,确定了盾构穿越房屋时的影响范围,提出了盾构通过时的地表沉降控制标准,并给出了掘进速度、浆液材料组成与配比、注浆量及注浆时机的控制标准,为2号线的安全穿越提供了技术保障.

泥水盾构掘进过程中,泥水压力是其重要的掘进参数.特别是在过江越海隧道修建过程中,泥水压力设定不当易发生泥水劈裂、喷发到江底引发江水倒灌等工程事故,如Heinenoord第二隧道.为防止泥水劈裂的发生,结合已有研究成果给出了地层劈裂抗力的估算方法、测定原理和现场测定方法.讨论了控制泥水压力、提高隧道上覆土层强度、增加隧道覆土厚度和提高泥水的黏性和比重等防止泥水劈裂的方法,并指出控制泥水压力具有实时、有效和经济的特点.在此基础上,结合工程施工中的一般条件和特殊条件,对泥水压力的设定范围进行了研究.研究结果表明:①一般掘进条件下,使用地层静止土压力作为泥水压力的设定上限是安全的,不会发生泥水劈裂地层现象.②在盾构穿越建筑物等超载条件下,由于局部超载的存在导致其泥水支护压力增加,此时可以运用地层劈裂抗力的余量,适当提高泥水容重和支护压力.③在特殊情况下,如盾构带压换刀等,需要提高泥水压力时,宜使用所提供的现场泥水劈裂仪对地层的劈裂抗力进行精确测定后设定泥水压力上限.

泥水盾构掘进时,在江中冲槽段等复杂条件下,泥水压力很难设定,稍有不慎就会发生泥水劈裂,甚至导致泥水喷发危及工程安全。为观察并研究劈裂伸展现象及其力学机制,在研制现场泥水劈裂仪并进行现场试验的基础上,建立泥水劈裂伸展模型,给出防止泥水劈裂的泥水压力上限设定的计算方法。研究表明:泥水劈裂伸展的力学特性不仅与地层性能有关,而且与所选用的泥浆特性有关。实际工程中,在地层性能不易改变的条件下,增加泥水重度、提高泥水黏性有利于阻止劈裂进一步伸展。劈裂发生后出现的泥水压力降低现象将有利于阻止泥水劈裂进一步伸展。在劈裂发生后,不能因为泥水压力下降而急忙增加泥水压力,这样不但泥水压力不能增加,反而会导致泥水劈裂的迅速伸展,加快泥水喷发的发生。绝大多数情况下,在泥水劈裂伸展的过程中泥水和地层的组合抗劈裂能力会逐渐变小。因此,预防泥水盾构开挖过程中的泥水喷发问题,要从预防泥水劈裂入手,对泥水压力的上限进行控制。

With the advantages of little soil disturbance, good continuity of the test data and good prediction of mechanical properties of in-situ soil, flat dilatometer tests have been used in more and more projects. But the rate of pressurization and other factors bring errors to the test results in flat dilatometer tests, and it's difficult to evaluate the error and give the emendatory results. The pore water pressure cannot be measured directly by the flat dilatometer tests. Thus, the micro-displacement and pore pressure senors are installed on the flat dilatometer to make it change into the pore pressure and displacement flat dilatometer. It can regulate the results by itself and measure pore pressure directly. The principle, method and procedure of the pore pressure and displacement flat dilatometer tests are introduced. The differences between the pore pressure and displacement flat dilatometer tests and the flat dilatometer tests are also given. The results measured in the pore pressure and displacement flat dilatometer tests are more accurate and stable by comparing with these of the flat dilatometer tests. The in-situ lateral earth pressures in the test field of Weisan Road are measured in the pore pressure and displacement flat dilatometer tests for the first application.

For constructing slurry shield tunnels under rivers or seas,it's a difficult technical problem to maintain the stability of the excavation face while preventing slurry fracturing. There are some laboratory investigations of slurry fracturing. But,due to small sizes and different boundary conditions,the results cannot be directly applied to the projects. In-situ slurry fracturing instrument is developed on the basis of theoretical analysis,and the test procedures of fracturing and the relevant method of confirming fracturing pressure are introduced. The in-situ slurry fracturing test is carried out on the rive-crossing tunnel under construction in Nanjing. The results indicate that:(1) as the fracturing process is a mutation process,the stratum fracturing model can predict fracturing pressure by means of the total stress method,which agrees with actual working condition. The theoretical results show that the fracturing pressure of tunneling model,which is almost equal to the lateral earth pressure,is smaller than that of the in-situ slurry fracturing test. It's efficient to increase fracturing pressure by increasing the slurry viscosity,however,the effect is limited; and(2) the upper limit value of slurry pressure set for preventing slurry fracturing is given according to the results of the in-situ slurry fracturing test. The in-situ slurry fracturing test is performed by shield machine after originating,and it may verify the accuracy of the prediction model to some extent.

In order to study coupled groundwater flow and heat transfer in the near-field of high-level nuclear waste repositories, a meter-scale physical model with regular fractures has been constructed, using rock blocks taken from the preliminary potential Beishan site in China's Gansu province, to conduct experiments of fracture water flow and heat transfer. As a preliminary theoretical study of the physical modeling, this paper presents a numerical modeling analysis of the fractured rock physical model using an equivalent continuum model. Under the prescribed conditions, the calculations reveal: heat conduction and fracture water flow turn quickly from uncorrelated in the early phase of heating to fully coupled at later times; while the standstill fracture water essentially behaves through heat storage and conduction, the fracture water flow can induce heat convection between water and the rock matrix as well as heat advection; if the fracture water flux is kept constant, then variation of the fracture aperture would exert little influence on the system; if the fracture water velocity is kept constant, then the effect of the variation of the fracture aperture would be significant; higher heat intensity would cause more convection heat transfer from fracture water flow and larger water pressure increase, and the latter effect would be much more significant when the temperature exceeds 100.; the model system approaches to steady state after about 7 days of heating, with the amount of heat flushed out due to fracture flow equaling the effective heat input.