人工配制生活垃圾强度特性的超大型直接剪切试验

将生活垃圾分为易降解材料、难降解材料和不可压缩的固体材料3大类,其比例为50%、15%、35%。在环境土工实验室,人工配制该比例的垃圾,采用超大型直接剪切试验仪器,进行了生活垃圾强度特性及抗剪强度参数的研究。研究表明,生活垃圾的强度与压缩时间和剪应变有关,且符合库仑定律,得到了初始孔隙比为2.9的生活垃圾在不同压缩时间及不同剪应变下的抗剪强度参数,黏聚力为9.69~15.64kPa,内摩擦角为18.73~35.53。     

Shear strength characterization of municipal solid waste at the Suzhou landfill, China

The current practice of slope stability analysis for a municipal solid waste (MSW) landfill usually overlooks the dependence of waste properties on the fill age or embedment depth. Changes in shear strength of MSW as a function of fill age were investigated by performing field and laboratory studies on the Suzhou landfill in China. The field study included sampling from five boreholes advanced to the bottom of the landfill, cone penetration tests and monitoring of pore fluid pressures. Twenty-six borehole samples representative of different fill ages (0 to 13 years) were used to perform drained triaxial compression tests. The field and laboratory study showed that the waste body in the landfill can be sub-divided into several strata corresponding to different ranges of fill age. Each of the waste strata has individual composition and shear strength characteristics. The triaxial test results showed that the MSW samples exhibited a strain-hardening and contractive behavior. As the fill age of the waste increased from 1.7 years to 11 years, the cohesion mobilized at a strain level of 10% was found to decrease from 23.3 kPa to 0 kPa, and the mobilized friction angle at the same strain level increasing from 9.9° to 26°. For a confinement stress level greater than 50 kPa, the shear strength of the recently-placed MSW seemed to be lower than that of the older MSW. This behavior was consistent with the cone penetration test results. The field measurement of pore pressures revealed a perched leachate mound above an intermediate cover of soils and a substantial leachate mound near the bottom of the landfill. The measurements of shear strength properties and pore pressures were utilized to assess the slope stability of the Suzhou landfill.     

城市固体废弃物的剪切强度机理及本构关系

根据城市固体废弃物（简称MSW）的大三轴固结排水剪试验结果,延伸岩土工程中研究土体的概念和方法,研究MSW的剪切强度机制。MSW与大变形相关的强度特性主要是由于含有大量的纤维状成分所致,在剪切过程中试样总的抗剪能力由剪切面上的摩擦力和纤维状加筋相的拉力所产生。MSW的摩擦强度主要由3部分构成：纤维状成分与土颗粒之间的摩擦,纤维状成分之间的摩擦,和土颗粒之间的摩擦。基于MSW的剪切强度特性,采用邓肯-张本构模型来描述MSW的应力-应变关系。     

新鲜生活垃圾压缩与直剪联合测定试验研究

在环境土工实验室人工配制了新鲜生活垃圾,采用大型压缩与直剪联合测定仪器,对96个新鲜垃圾试样进行了压缩变形、剪应力及剪应变联合测定试验,研究了垃圾的压缩变形对强度的影响及规律。试样的初始孔隙比分别为2.1、2.5、2.9,试样竖向压力分别为25、50、100、200 kPa,每级压力作用下的时间分别为0、0.25、0.5、1、2、6、12、24 h。试验研究表明,（1）不同压缩时间下剪应力与剪应变的关系符合双曲线模型,模型参数a在0.028～0.144之间,参数b在0.012～0.024之间;（2）不同竖向压力及不同剪应变限值下抗剪强度与竖向压缩应变（压缩时间）的关系可以拟合成多项式的形式,得到了参数的拟合值及相关系数,相关系数在0.63～0.98之间;（3）不同初始孔隙比的抗剪强度与竖向压力的关系符合库仑定律,得到了不同剪应变限值的抗剪强度参数,黏聚力在11.1～34.2 kPa之间,内摩擦角在 11.2°～30.6°之间。     

短龄期城市固体垃圾直剪试验及应力位移模型

城市固体垃圾（MSW）的力学行为是环境岩土工程研究中的一个主要课题。进行了1、4、7 d共3组短龄期的MSW试验室直剪试验,每组试验分别考虑3种不同密度。试验结果表明,MSW试样的直剪破坏仍遵循摩尔-库仑破坏准则。密度较大试样的抗剪强度在1～7 d龄期内持续增长;密度较小试样的抗剪强度在1～4 d龄期内增长,在4～7 d龄期内降低。MSW试样的摩擦角随龄期的发展单调增加,但黏聚力随龄期先增大而后减小。分析了传统剪应力-位移模型的不足,提出了一个新的复合正切-指数曲线模型,该模型具有良好的数学性质,较传统模型有更广泛的适用性。试验数据拟合表明,新模型能很好地描述MSW的剪应力-位移过程,相关系数均大于0.99。其结果可供相关填埋场设计及工程数值分析参考     

污泥掺入生活垃圾后的力学特性试验研究

填埋是污泥消化处置的方法之一,由于污泥的工程特性较特殊,填埋会引发很多环境岩土工程问题。考虑到填埋工程会进行分层碾压铺填,在标准击实试验的击实功下制备试样,通过污泥与垃圾土混合后的直接剪切试验、渗透特性试验,考虑淋滤液和降解产气压力共同影响,进行边坡稳定分析。结果表明：污泥和垃圾混合后的强度比垃圾土的强度低,比污泥的强度高,改善了污泥的强度特性;污泥掺入垃圾后的渗透系数与垃圾土的渗透系数相当,没有劣化垃圾土的渗透特性,但比污泥的渗透系数有显著提高;随着污泥掺量的增加,边坡安全系数会先提高后降低,因此,应结合试验和稳定计算结果,确定实际工程的污泥掺入比;考虑降解产气对边坡稳定的影响,安全系数会降低约15%～20%。     

中高龄期模型固体废弃物三轴试验与强度特性分析

为揭示中、高龄期城市固体废弃物（MSW）的力学特性,通过向石英砂、高岭土混合物中加入不同质量比的草炭人工配制模型固废试样,开展了三轴固结排水剪切和固结不排水剪切试验研究。试验结果表明,中高龄期模型固废均表现出持续硬化的应力应变特性,固结排水试验中中龄期模型固废的体变略大于高龄期,固结不排水试验中高龄期模型固废的孔压略高于中龄期。采用15%轴向应变对应的剪应力描述其抗剪强度,并根据Skempton有效应力原理,通过对比固结排水试验和固结不排水试验结果,得出了固废的孔隙水压力折减系数和抗剪强度参数,孔隙水压力折减系数随龄期增加而增大,高龄期模型固废得到的黏聚力低于中龄期,高龄期固废得到的内摩擦角则高于中龄期。     

颗粒形态对砂土抗剪强度影响的试验和离散元数值模拟

为研究颗粒形态对砂土抗剪强度的影响，制备了5组不同粒径的球形石英砂和颗粒形态不规则的实验室标准砂，并进行一系列的直剪试验；离散元法能有效地模拟砂土的离散性和不均匀性，基于高性能离散元分析软件MatDEM，通过二次开发，建立直剪试验三维模拟器，对直剪试验过程进行了数值模拟，并对比分析了试验和数值模拟结果。数据表明:(1)石英砂试样的自然休止角与其抗剪强度有着正相关性；(2)球形石英砂较实验室标准砂能更快达到峰值强度，但其值相对较低；标准砂存在着一定的黏聚力；(3)在直剪过程中，直剪盒中部颗粒的位移较周围颗粒不同，形成了明显的剪切带，并且球形石英砂试样的剪切带内颗粒的位移更小；(4)砂土直剪试验中配位数与剪应力有着良好的正相关性。本文结果表明，在相同的荷载条件下不规则颗粒形态的石英砂的抗剪强度要明显大于球形石英砂。     

Rock joint modeling using a visco-plastic multilaminate model at constant normal load condition

Rock joints play an important role in the behavior of rock masses under normal and shear loading conditions. Numerical simulation of the behavior of jointed rock masses is not an easy task due to complexities involved in the problem such as joint roughness, joint shear strength, hardening and softening phenomenon and mesh dependency. In this study for modeling purposes, a visco-plastic multilaminate model considering hardening and softening effects has been employed. For providing the necessary data for numerical simulation, a series of laboratory experiments have been carried out on regular tooth-shape asperities made by gypsum, under constant normal load conditions. Shear stress–shear displacement and normal displacement–shear displacement of artificial joint specimens are simulated using the proposed numerical model at constant normal load condition (CNL). The results indicate the capability of the model for simulating rock joints behavior in both strength and deformation field. Although the numerical model has been developed for simulating the behavior of artificial joints, the concept of the method can also be used for natural rock joints.     

Dynamic Properties of Municipal Solid Waste in Bioreactor Landfills with Degradation

Bioreactor landfills are operated to enhance refuse decomposition, gas production, and waste stabilization. The major aspect of bioreactor landfill operation is the recirculation of collected leachate back through the refuse mass. Due to the presence of additional leachate and accelerated decomposition, the characteristics of Municipal Solid Waste (MSW) in bioreactor landfills are expected to change. About 50% of the continental United States comes under the designated seismic impact zone. The federal regulations have focused increase attention on seismic design of solid waste fills, and have mandated that the solid waste landfills located in the seismic impact zones should be designed to resist the earthquake. Accordingly, assessment of dynamic properties of landfills is one of the major geotechnical tasks in landfill engineering. In order to understand the changes in dynamic properties of bioreactor waste mass with time and decomposition, four small scale bioreactor landfills were simulated in laboratory and samples were prepared to represent each phase of decomposition. The state of decomposition was quantified by methane yield, pH, and volatile organic content (VOC). A number of Resonant Column (RC) tests were performed to evaluate the dynamic properties (stiffness and damping) of MSW. The test results indicated that the normalized shear modulus reduction and damping curves are significantly affected by the degree of decomposition. The shear modulus increased from 2.11 MPa in Phase I to 12.56 MPa in Phase IV. The increase was attributed to the breakdown of fibrous nature of solid waste particles as it degrades. Therefore, considering MSW properties to be uniform throughout the bioreactor landfill is not a reasonable assumption and the shear modulus reduction curves should be evaluated based on the degree of MSW decomposition, rather than the sample composition itself.     

扩建城市垃圾填埋场的地震稳定性分析

结合现场剪切波速试验、室内常规及动三轴试验结果,给出了扩建城市垃圾填埋场地震稳定性的有限元分析方法,并分析了扩建垃圾填埋体沿填埋体内部圆弧滑动面、新老填埋体交界面及底部和背部衬垫系统的地震稳定性。当填埋场中渗滤液水头较低时,扩建填埋体最易发生沿新老填埋体交界面的滑动。与总应力法计算结果相比较发现,总应力法只适合分析输入地震动较小的情况,而当输入地震动较大时,由于它不能考虑动孔压上升所导致的MSW软化而使得其计算的扩建填埋体沿圆弧滑动面的安全系数偏保守。另外,采用Newmark法分析了不同输入地震动时不同横、竖向扩建方案的永久位移,分析表明：扩建填埋体沿新老填埋体交界面的地震永久位移的对数近似与屈服加速度同最大水平加速度的比值呈线性关系,并给出了其永久位移的拟合公式。     

Study on soil–cement grout interface shear strength of soil nailing by direct shear box testing method

An important design parameter in cement-grouted soil nailed structures is the shear strength at the interface between the grouted nail and the surrounding soil. Both field and laboratory pull-out tests are normally used to investigate this interface shear strength. However, these tests have some limitations. In this study, direct shear box tests are adopted to investigate the interface shear strength behaviour between a completely decomposed granite (CDG) soil and a cement grout plate. Tests were carried out in a large direct shear test apparatus over a range of constant normal stress, soil moisture content, and soil–cement grout interface surface waviness. The laboratory test procedures are briefly described and the main test results are presented, followed by a discussion of the shear behaviour of the soil–cement grout interface. The interface shear behaviour is compared with the shear strength behaviour of the same soil tested under comparable conditions. It is shown that the shear stress–displacement behaviour of the soil–cement grout interface is similar to that of the soil alone. The test results indicate that the interface shear strength of the CDG and cement grout material depends on the normal stress level, the soil moisture content, and the interface surface waviness.     

Investigation of fiber effect on the mechanical behavior of municipal solid waste by different shearing test apparatuses

This paper embraces the results of a study on the mechanical behavior of municipal solid waste, MSW, and the effect of shearing mechanism on its mechanical response. Totally, 9 drained direct shear tests and 9 drained triaxial tests have been performed on fresh MSW samples with fiber contents of 0, 6 and 12%. Each apparatus represents a different shearing mechanism. The composition and compaction condition for MSW samples in each fiber content, employed in both shearing test devices, were the same. In triaxial tests, since the shear plane was cutting through the fibers, with increase in the fiber content, an increasing trend of strength was observed; on the contrary, this trend was decreasing in direct shear tests because fiber alignment was parallel to the shear plane. Test results show that the effect of fibers in the triaxial tests depends on the level of imposed axial strain on the samples as well as the confining stress. Based on the findings of this paper, hardening behavior of MSW is attributed to reinforcement action of fibers and particle compressibility.     

Permeability of Municipal Solid Waste in Bioreactor Landfill with Degradation

Bioreactor landfills are operated for rapid stabilization of waste, increased landfill gas generation for cost-effective energy recovery, gain in landfill space, enhanced leachate treatment, and reduced post closure maintenance period. The fundamental process of waste stabilization in bioreactor landfill is recirculation of generated leachate back into the landfills. This creates a favorable environment for rapid microbial decomposition of the biodegradable solid waste. In order to better estimate the generated leachate and design of leachate recirculation system, clear understanding of the permeability of the Municipal Solid Waste (MSW) with degradation and the factors influencing the permeability is necessary. The objective of the paper is to determine the changes in hydraulic properties of MSW in bioreactor landfill with time and decomposition. Four small-scale bioreactor landfills were built in laboratory and samples were prepared to represent each phase of decomposition. Then, the changes in hydraulic properties of MSW in bioreactor landfill with time and decomposition were determined. A series of constant head permeability tests were performed on the samples generated in laboratory scale bioreactor landfills to determine variation of permeability of MSW with degradation. The test results indicated that the permeability of MSW in bioreactor landfills decreases with decomposition. Based on the test results, the permeability of MSW at the first phase of degradation was estimated as 0.0088 cm/s at density 700 kg/m³. However, with degradation, permeability decreased to 0.0013 cm/s at the same density, for MSW at Phase IV.     

垃圾土蠕变-降解特性的室内试验研究

通过室内蠕变-降解对比试验,研究了城市固体废弃物（MSW）的长期变形过程,详细地分析了MSW应变、沉降速率以及孔隙比随时间的变化规律。试验结果显示,考虑有机物降解情况下沉降速率与时间在双对数坐标下呈线性关系;不同的生物降解条件下MSW的变形特性表现出不同的规律。研究表明,MSW的蠕变变形是应力与有机物生物降解耦合作用的结果,采用室内蠕变-降解试验可以很好地模拟垃圾土的沉降变形过程,为室内进一步研究垃圾土的蠕变降解特性提供了很好的依据。     

The influence of temperature on mechanical characteristics of Boom clay: The results of an initial laboratory programme

The influence of temperature increase on the classical mechanical properties of a soil, such as shear strength and elasticity modulus, is a major concern in civil engineering works. This influence becomes even more important in the design of an underground repository for radioactive high-level waste. A laboratory programme on Boom clay samples has, therefore, recently been set up. The testing equipment and initial results are presented. Deviatoric stress was found to decrease with increasing temperature—a phenomenon that should be further investigated.     

Three-dimensional stability analysis of the Kettleman Hills landfill slope failure based on observed sliding-block mechanism

Previous studies of the Kettleman landfill slope failure of 1988 had concluded that the failure, which slid along the underlying liner interfaces with low shearing resistance, occurred as a result of attaining the critical waste fill height. These studies showed, however, some discrepancies in regard to the adopted material strength data as well as the computed factors of safety. Based on the observed sliding-block mechanism, a 3D analysis model was established herein which allowed for variations in material strength mobilization within the sliding mass and at the slip surface. With a careful consideration on the interface strength data, results of forward analysis for the pre-slide slope generally showed better agreement with the field observations. Results of backward analysis for the post-slide slope indicated a consistency in the estimated material strength with the laboratory test data. The current study also showed slightly higher computed 3D factors of safety than the associated 2D values, in both pre-slide and post-slide cases.     

地震作用下华丽高速公路金沙江桥华坪岸顺层边坡动力稳定性评价

以华丽金沙江桥华坪岸顺层边坡地震稳定问题为研究对象,在slide下利用Janbu-simplified法计算了其最不利滑动面和安全系数,为模型试验确定了宽900 m×高450 m的试验范围。采用拟静力法,在Phase 2D下利用有限元强度折减法模拟模型边坡,计算了0.00g、0.10g、0.15g、0.20g、0.25g、0.30g、0.35g等7个地震工况的安全系数和位移。对比不同工况下的安全系数、位移与水平地震加速度曲线,结果表明,（1）极限平衡法与强度折减法所得安全系数在1.0附近安全系数离散性最小,远离1.0则安全系数的离散性增大,差异是两类方法极限状态定义的不同造成的。基于安全系数确定的临界水平地震加速度为0.20g;（2）剪应变揭示的边坡破坏模式为桥墩位置拉剪破坏、T2凝灰岩主滑和前缘近于水平剪出破坏,坡体内部的变形响应敏感性为水平位移＞竖向位移＞剪应变。基于变形演化规律确定的临界值为0.15g～0.20g,相应水平位移预警值为10.2 mm,室内模型试验验证了上述分析的合理性;（3）综合确定华坪岸顺层边坡临界水平地震加速度为0.2g,50年超越概率为10%和100年超越概率为2%对应的安全系数分别约为1.2和1.1,华坪岸边坡在现行抗震设防标准下是安全的。当前选择安全系数作为边坡稳定性评价指标,而安全监控实践中一般选择可操作性和预警性强的变形值作为控制指标,所以边坡稳定性评价需要采取安全系数与临界位移相结合的方法。     

A coupled model for prediction of settlement and gas flow in MSW landfills

Prediction of long‐term settlement and control of gas pollution to the environment are two principle concerns during the management of municipal solid waste (MSW) landfills. The behavior of settlement and gas flow in MSW landfills is complicated due to the combined effect of mechanical deformation of the solid skeleton and continuous biodegradation of the waste. A one‐dimensional settlement and gas flow model is presented in this paper, which is capable of predicting time evolution of settlement as well as temporal and spatial distribution of gas pressure within multi‐layered landfills under a variety of operating scenarios. The analytical solution to the novel model is evaluated with numerical simulation and field measurements. The resulting efficiency and accuracy highlight the capability of the proposed model to reproduce the settlement behavior and gas flow in MSW landfills. The influences of operating conditions and waste properties on settlement and gas pressure are examined for typical MSW landfills. Copyright © 2009 John Wiley & Sons, Ltd.     

Physical explanation for the displacement-length relationship of faults using a post-yield fracture mechanics model

A plane strain model for a fault is presented that takes into account the inelastic deformation involved in fault growth. The model requires that the stresses at the tip of the fault never exceed the shear strength of the surrounding rock. This is achieved by taking into account a zone, around the perimeter of the fault surface, where the fault is not well developed, and in which sliding involves frictional work in excess of that required for sliding on the fully developed fault. The displacement profiles predicted by the fault model taper out gradually towards the tip of the fault and compare well with observed displacement profiles on faults. Using this model it is found that both (1) the shape of the displacement profile, and (2) the ratio of maximum displacement to fault length are a function of the shear strength of the rock in which the fault forms. For the case of a fault loaded by a constant remote stress, the displacement is linearly related to the length of the fault and the constant of proportionality depends on the shear strength of the surrounding rock normalized by its shear modulus. Using data from faults in different tectonic regions and rock types, the in situ strength of intact rock surrounding a fault is calculated to be on the order of 100 MPa (or a few kilobars). These estimates exceed, by perhaps a factor of 10, the strength of a well developed fault and thus provide an upper bound for the shear strength of the crust. It is also shown that the work required to propagate a fault scales with fault length. This result can explain the observation that the fracture energy calculated for earthquake ruptures and natural faults are several orders of magnitude greater than that for fractures in laboratory experiments.