隧道开挖围岩土压力拱效应分析

采用弹塑性理论应力反馈算法,研发材料本构模型子程序,将土的应力路径本构模型嵌入大型有限元软件ABAQUS,通过对三轴压缩和三轴拉伸试验条件的数值模拟,验证了子程序的精度。利用二次开发后的ABAQUS对隧道开挖过程进行了三维有限元分析,从不均匀变形、应力重分配、应力路径和地表沉降规律四个方面研究了由重力引起的大主应力改变产生的土压力拱效应。研究结果表明:隧道开挖过程中,围岩土体中的应力路径变化非常复杂,并受几何位置和开挖进程影响;隧道埋深较浅时,土压力拱作用至地表,尚未完全发挥支撑作用,表现为隧道上方土体的整体沉陷变形;隧道埋深较深时,土压力拱的作用未影响至地表,充分发挥了支撑作用,表现为隧道上方土体的塌落变形;土压力拱的作用范围受隧道埋深影响不大,约为隧道半径的3倍。     

盾构隧道垂直土压力松动效应的颗粒流模拟

通过对比室内三轴试验和颗粒流程序双轴数值试验结果,确定了颗粒流模拟砂土的细观参数;通过对室内挡板下落试验的颗粒流数值模拟,验证了颗粒流模拟土拱效应的可行性。在此基础上对盾构隧道垂直土压力的松动效应进行了颗粒流模拟,分析了不同盾尾空隙、不同埋深、不同直径和不同围岩时作用在管片上的土压力、土体位移和土体颗粒接触力的变化情况。结果表明,土拱效应主要发生在隧道上部1～2倍隧道直径的范围内,隧道顶部土体通过土拱效应可大幅度减少作用在隧道上的土压力。     

A new method to calculate wind profile parameters of the wind tunnel boundary layer

This paper introduces a new method to process wind profile data of simulated atmospheric boundary layer flows in the wind tunnel so as to obtain the two important wind profile parameters—the surface roughness length z₀ and the friction velocity u_*. Instead of using the wind speed profile, the turbulent intensity profile of the turbulent surface layer, which is measured with a single probe hot-wire anemometer, is used to calculate the surface roughness length z₀. Then, the calculated surface roughness length z₀ is substituted into the mean wind speed profile of the constant flux layer to calculate friction velocity u_*. From our results this method is better than the simple regression method using the wind speed profile, which has been widely used.     

Response of sandy soil to the volume losses at the tunnel face level

Recent research does not fully identify the causes of surface volume loss that occur at the tunnel level in sand, as related to the excavation of a pressurized-face tunnel boring machine (PTBM). This paper focuses on the face stability of PTBM tunneling, and the response of sandy soil to the inadequate face pressure is evaluated. Data presented are from tests of a model using reduced-scale PTBM tunneling operations in dry sand. Ground displacement and the evolution of ground stress were analyzed. The relationship between the load factor at the face and the volume loss at surface was examined. The effects of soil density and tunnel depth were considered. An empirical formulation was provided allowing determination of the expected volume loss as a function of load factor (LF), cover-to-diameter ratio (C/D) and soil density (I_d). Thus providing a valuable basis for establishing relationship between volume loss associated with tunnel face advancement and face pressures in sandy soil. The results show that instability of the face is accompanied by the triggering of secondary deformation mechanisms at the surface. The volume loss at surface is complicated by the combined effects of soil arching and the dilative/contractive behavior of the soil. The variation of volume loss is sudden prior to face collapse, especially at dense sand, which is different from the progressive variation of volume loss in clays. The size of destressed zone ahead of cutter head is less dependent on C/D in loose soil than in dense soils.     

Active control for a benchmark building under wind excitations

The tall building proposed in the benchmark problem is a concrete office tower, which is wind sensitive due to its slenderness. The wind loads acting on the building are defined by the results of wind tunnel tests conducted at the Sydney University and an active mass driver can be designed for installation on the top floor in order to reduce the structural response under strong wind gusts. In this paper, the control strategy presented for the third generation benchmark control problem for wind-excited buildings (J. Eng. Mech. (1999), submitted for publication) is the simultaneous treatment of both H₂ performance criteria and H_∞ attenuation constraints that demonstrates design tradeoffs and can be applied to the AMD system based on acceleration feedbacks. This control problem can be formulated by linear matrix inequalities in terms of a common Lyapunov function. Solving linear matrix inequalities is a convex optimization problem and efficient interior-point algorithms are now available to solve this problem. Simulation and design results demonstrates that, decreasing H_∞ attenuation constraint can be used to reduce the structural response under wind excitations at the expense of increasing H₂ performance index.     

An alternative model for evaluating sustainable urbanization

In recent years, there has been rapid urbanization worldwide, resulting in both benefits and problems. Sustainable urbanization has become an important aspect in promoting sustainable development. Existing studies have introduced various methodologies to guide urbanization towards sustainable practices. The application of these methods has contributed to improving urban sustainability. To further support the effective applications of the principles of sustainable urbanization, a tool is needed to evaluate whether a particular process of urbanization is sustainable. In this paper, we introduce an alternative model for evaluating sustainable urbanization by investigating the relationship between urbanization and urban sustainability. The practice of sustainable urbanization is defined as a dynamic process that enables urban sustainability to improve or to maintain a certain level of practice. By employing this definition, we introduce a sustainable urbanization elasticity coefficient e_SU, which is defined by two parameters: urbanization velocity (V_μR) and urban sustainability velocity (V_μS). The sustainability of an urbanization process is measured by the value of e_SU or read from the V_μR-V_μS coordinate. A case study demonstrates the application of the measure e_SU and the V_μR-V_μS coordinate. The proposed model is an effective tool to help policy makers understand whether the urbanization processes they support are sustainable and thus whether to correct practices. The model also allows comparison of different urbanization practices and thereby encourages the sharing of successful experiences.     

Behavior of Ground and Response of Existing Foundation Due to Tunneling

An apparatus has been developed to model the excavation of a tunnel in the laboratory. With this apparatus, 2D model tests are carried out to investigate the surface settlement and the earth pressure brought about by the tunneling. Finite element analyses using an elastoplastic subloading t_ij model are also conducted. The influence of volume loss on the surface settlement and the earth pressure, due to the shallow tunnelling, is illustrated based on the model tests and the corresponding numerical analyses. It is revealed that the surface settlement troughs and the earth pressure distributions around shallow tunnels depend on both the volume loss and the crown drift of the tunnel. The effect of the interaction between the tunneling and existing nearby foundations is also demonstrated in this paper. For existing foundations, the building loads control the surface settlements and the zone of deformation during the tunnel excavation. The behavior of the foundations depends on the deformation mechanism of the ground during the tunnel excavation. The induced axial force and bending moments in the piles of a piled raft are investigated numerically, and it is shown that the axial force changes due to the stress relaxation of the ground. Bending moments are induced in the piles at a lower value of soil cover due to the differential settlement of the piled raft.     

考虑尾隙的盾构隧道土压力离散元数值分析

采用改进的离散元商业软件PFC2D对砂土层中管片周围的土压力进行了大量的数值模拟,重点研究了埋深、尾隙等因素对管片周围土压力分布的影响。研究结果表明:尾隙存在时,隧道顶部的土体产生了不均匀沉降,导致压力拱效应的产生;隧道顶部至地表出现转动加剧区域;应力路径变化过程表明,隧道周围土体经历了先卸载后加载的变化过程;在应变路径变化曲线上可将管片周围土体的变形划分为三个变化阶段。     

Experimental study on face instability of shield tunnel in sand

Face stability is critical for ground settlement and construction safety control in shield tunneling. In this paper, a series of 3D large-scale model tests with a tunnel of 1 m diameter were conducted in dry sand for various cover-to-diameter ratios C/D = 0.5, 1, and 2 (i.e., relative depth; C is the cover depth and D is the diameter of tunnel). Each test provided a measurement of the support pressure and the ground settlement with the advance of face displacement. The evolution of soil arching during face failure was investigated by monitoring the redistribution of earth pressure in front of the face in the test case of C/D = 2. In the displacement-controlled face failure tests in the medium density sands, the support pressure dropped steeply to the minimum value, then increased to a steady state with the continuing increase in the face displacement. Relationships between the support pressure and face displacement for various cover depths were also verified by the numerical analysis using the finite difference program, FLAC^3D (Itasca, 2005). The limit support pressure increases with the increase of the relative depth C/D and then tends to be constant. A significant rotation of principal stress axes in the upward arches in the soil during face failure was found in the tests. A two-stage failure pattern is proposed based on the observation of earth pressure. The theoretical and empirical formulas for estimating limit support pressure were verified by the tests results.     

Ground Behavior Due to Tunnel Excavation with Existing Foundation

Two-dimensional (2D) and three-dimensional (3D) model tests of tunnel excavation with nearby existing foundation are carried out to investigate the influence of the existing foundation due to the interaction between ground and the existing structures. Three types of foundations: flat foundation, group-pile foundation and piled raft are considered. 2D and 3D finite element analyses using subloading t_ij model are also conducted. The deformation mechanism and distribution of earth pressure during tunnel excavation in the ground with nearby foundation are found to be different from those of green field condition. Surface settlement trough due to tunnel excavation in the ground with existing foundation does not follow the usual pattern of a Gaussian distributive curve, which can be observed in the case of green field. Especially, in the case of pile foundation, D_p, the distance between pile tip and tunnel is an important factor for the ground deformation and surface settlement. For a short distance D_p, although the length of pile is long, the ground deformation is concentrated at a place near the front pile and the rotation of foundation becomes larger. The maximum surface settlement in the case of existing foundation is also larger than those in the case of green field. Due to the existing foundation, unsymmetrical distributions of earth pressure occurred at the bottom of the ground due to tunnel excavation, both in model tests and numerical analyses. The earth pressure at the crown of tunnel in the case of existing foundation is almost the same as those in the case of green field. The arching at the shoulder of tunnel in the case of existing foundation, however, is much larger than those in the case of green field due to the dead load exerted on the foundation. The numerical results agree well with the results of the model tests.     

Soil deformation induced by Double-O-Tube shield tunneling with rolling based on stochastic medium theory

The prediction of soil deformation during tunneling is very difficult for Double-O-Tube (DOT) shield tunnel construction, especially for the shield rolling. According to the characteristics of DOT shield tunneling and rolling, a calculation model of soil deformation due to tunneling-induced ground loss was established. Based on the stochastic medium theory, the theoretical solutions of soil deformations considering the rolling of DOT shield machine were derived by polar coordinate transformation and multi-subdomain integral method. The predicted surface settlement from the proposed solution is better agreement with the observed data than those obtained by the two previous methods (namely the equivalent excavated-area method (EAM) and the simple superstition method (SM)). In addition, only ground surface settlement can be estimated under no rolling of DOT shield machine using the two previous methods, while this proposed solution owns great progress in solving the subsoil deformation and the influences of rolling. In order to further study the influence of DOT shield rolling angle on soil deformation under different engineering conditions, the parameter sensitivity analyses regarding the tunnel depth h, the ground loss parameter ɛ and the influence zone angle β₀ were extensionally discussed.     

Polyaxial strength criterion and closed-form solution for squeezing rock conditions

In this paper,a nonlinear strength criterion is proposed using the average of intermediate(σ_2) and minor(σ_3) principal stresses in place of σ_3 in Ramamurthy(1994)'s strength criterion.The proposed criterion has the main advantages of negligible variation of strength parameters with confining stress and ability to link with conventional strength parameters.Additionally,a new closed-form solution based on the proposed criterion is derived and validated for Chhibro Khodri tunnel.Further,analytical solutions including Singh's elastoplastic theory,Scussel's approach,and closed-form solutions based on conventional and modified Ramamurthy(2007) criteria are compared with the results of proposed approach.It is shown that the in situ squeezing pressure predictions made by the proposed approach are more accurate.Also,a parametric study of the present analytical solution is carried out,which displays explicit dependency of tunnel stability on internal support pressure and tunnel depth.The influence of tunnel geometry is observed to be dependent on the applied support pressure.     

Thermal analysis of vaulted roofs

In hot arid regions, cooling buildings by passive techniques is very important regarding energy saving and the need to keep clean the environment. In such areas, domed and vaulted roofs are widely used for centuries, such as in the Middle East region and central part of Iran. In this article analysis is made to explore east–west direction of wind flow around north–south vaulted roofs and flat roof buildings. Combined convection and solar radiation over the roofs is considered to studying thermal performances of vaulted roofs and comparing their heat transfer with flat roofs. Two-dimensional RNG k–? turbulence model is incorporated to predict turbulent flow field as well as separation and recirculating patterns around the vaulted roofs and flat roof buildings. Solar radiation distribution over the roofs is determined based on an appropriate model applicable to hot arid regions of Iran. Pressure differences above the vaulted roof are compared with flat roof for various rim angles and different wind speeds. Heat transfer to the building with respect to time is determined for a certain inside ceiling design temperature, various wind flows and vault shapes, and results are compared with corresponding flat roof. It was found that daily average heat flux for all vaulted roofs, except vaulted roof of rim angle 180° is less than flat roof and it reduces further by increasing wind speed.     

盾构施工引起地面长期沉降的理论计算研究

 对盾构施工引起的隧道轴线上方地面工后沉降进行研究,结果表明,土体开挖卸荷引起应力释放,产生初始超孔隙水压力,其分布呈三角形。假定衬砌不排水、土体为单面排水、压缩层厚度为隧道覆土厚度,采用太沙基一维固结理论,得到地面工后固结沉降的理论计算公式。假定地面长期沉降主要由施工期间沉降和工后固结沉降组成,进而得到地面长期沉降的理论计算公式。算例分析结果表明：该方法的预测值与实测值非常吻合;上海软土地区盾构隧道施工引起的地面长期沉降相当显著,最终地面沉降量在80 mm以上,固结沉降占总沉降量的80%～90%;按最小覆土深度5 m计算,需要2 a以上地面沉降才能最终稳定。     

盾构法隧道施工引起的土体变形预测

 理论分析表明,不同土质条件下盾构法隧道施工引起的土体移动模型有区别。基于盾构法隧道统一土体移动模型,假定土体不排水,采用N. Loganathan等提出的研究方法,通过对Verriujt计算公式进行修正,推导得到盾构施工过程中由于土体损失引起的土体变形二维解,该方法适用于施工阶段。算例分析表明：所给出方法的计算结果与实测值较吻合,适用于从流塑～坚硬状态的所有黏性土。Loganathan公式只适用于流塑状态的黏性土,当土质较硬时,计算所得到的土体沉降要比实测值小;盾构施工引起的隧道上方土体沉降从地面向下呈非线性增大,在隧道顶部达到最大,离隧道越近,增长越快;隧道周围土体产生向隧道侧的水平位移,从地面向下逐渐增大,在略高于隧道轴线附近达到最大值,再逐渐减小直到0。离隧道越近,土体水平位移越大。     

上海软土深埋盾构施工引起的土压时效规律分析

针对上海软土地区深埋盾构开挖所引起的土压力时效性发展规律,选取4倍直径埋深盾构,布设土压力全断面长期监测点,获取盾构开挖阶段及后期固结蠕变阶段的土压力数据,以得到深部地层的土压时效变化规律.通过现场试验可得,盾构开挖所采取的土仓压力按照理论静止土压力取值时,刀盘周围会形成半径为1～1.5D的被动土拱效应作用区域,土拱范...     

Sorption of ionized and neutral emerging trace organic compounds onto activated sludge from different wastewater treatment configurations

The objective of this study was to examine sorption of a suite of 19 trace organic contaminants (TOrCs) to activated sludge. Compounds examined in this study included neutral, nonionized TOrCs as well as acidic TOrCs which may carry a negative charge and basic TOrCs which may carry a positive charge at the pH of wastewater. These TOrCs were evaluated to examine how sorptive behavior might differ for TOrCs in different states of charge. Additionally, multiple sludges from geographically and operationally different wastewater treatment plants were studied to elicit how solid-phase characteristics influence TOrC sorption. Characterization of sludge solids from 6 full scale treatment facilities and 3 bench-scale reactors showed no significant difference in fraction organic carbon (f_oc) and cation exchange capacity (CEC). Sorption experiments demonstrated that sorption of TOrCs also exhibits little variation between these different sludges. Organic carbon normalized partition coefficients (log K_oc) were determined as a measure of sorption, and were found to correlate well with octanol-water partition coefficients (log K_ow) for nonionized TOrCs, and log D_ow for anionic TOrCs where log D_ow is greater than 2. These data were used to construct a linear free energy relationship (LFER), which was comparable to existing LFERs for sorption onto sludge. No trend in sorption was apparent for the remaining anionic TOrCs or for the cationic TOrCs. These data suggest that predicting sorption to activated sludge based on K_ow values is a reasonable approach for neutral TOrCs using existing LFERs, but electrostatic (and likely other) interactions may govern the sorptive behavior of the charged organic chemicals to sludge.     

Evaluation of tunnel face stability by transparent soil models

Accurate estimation of tunnel face support pressure is necessary for economical and safe shield tunneling in cohesionless soils. This paper presents measurements of tunnel face support pressure and associated soil movements obtained using a transparent soil model that simulates shield tunneling in medium dense saturated sand. The use of a transparent soil surrogate permits measuring the internal soil deformations within the model soil. Soil deformations associated with various face support pressures are presented for 4 cover-to-diameter (C/D) ratios. Failure is found to be sudden with sand flowing into the tunnel leading to a prismatic wedge in front of the tunnel face and a vertical chimney of soil above. A minimum support pressure was achieved with support pressures as low as 10 ± 1% of the effective vertical stress at the tunnel axis. The stability of the tunnel face was related to the coefficient of active earth pressure with C/D ratio having a small effect on the magnitude of required pressure at collapse.     

Deformation and failure characteristics and fracture evolution of cryptocrystalline basalt

Cryptocrystalline basalt is one of the two major types of rocks exposed in the super large-scale underground powerhouse in Baihetan hydropower station in China.The rock of this type shows various sitespecific mechanical responses(e.g.fragmentation,fracturing,and relaxation) during excavation.Using conventional triaxial testing facility MTS 815.03,we obtained the stress-strain curves,macroscopic failure characteristics,and strength characteristics of cryptocrystalline basalt.On this basis,evolution of crack initiation and propagation was explored using the finite-discrete element method(FDEM) to understand the failure mechanism of cryptocrystalline basalt.The test results showed that:(1) under different confining stresses,almost all the pre-peak stress-strain curves of cryptocrystalline basalt were linear and the post-peak stresses decreased rapidly;(2) the cryptocrystalline basalt showed a failure mode in a form of fragmentation under low and medium confining stresses while fragmentation-shear coupling failure dominated at high confining stresses;and(3) the initial strength ratio(σ_(ci)/σ_f,where σ_(ci)and σ_f are the crack initiation strength and peak strength,respectively) ranged from 0.45 to 0.55 and the damage strength ratio(σ_(cd)/σ_f,where σ_(cd) is the crack damage strength) exceeded 0.9.The stress—strain curve characteristics and failure modes of cryptocrystalline basalt could be reflected numerically.For this,FDEM simulation was employed to reveal the characteristics of cryptocrystalline basalt,including high σ_(cd)/σ_f values and rapid failure after σ_(cd),with respect to the microscopic characteristics of mineral structures.The results showed that the fragmentation characteristics of cryptocrystalline basalt were closely related to the development of tensile cracks in rock samples prior to failure.Moreover,the decrease in degree of fragmentation with increasing confining stress was also correlated with the dominant effect of confining stress on the tensile cracks.     

Monitoring the role of aceticlasts in anaerobic digestion: activity and capacity

Aceticlastic methanogens are seen as a key to digester capacity and stability. This paper develops and applies an assay to measure digester stability by measuring the maximum aceticlastic methane production rate (V_max,ac). The V_max,ac in combination with acetate concentrations was found to be an effective digestion monitoring tool to indicate process upsets. At steady state, thermophilic, first stage and short SRT digesters generally had a greater V_max,ac than mesophilic, second stage or long SRT digesters. The ratio of the V_max,ac to the plant aceticlastic methane production rate, termed the Acetate Capacity Number (ACN), is a measure of the excess capacity of the digester. Either V_max,ac or ACN can be used to estimate the capability to handle higher organic loading rates. Monod modeling was used to predict V_max,ac, ACN and maximum VS loading rates for mesophilic and thermophilic digestion and for staged digesters to better understand expected digestion capacity and stability.