经编土工格栅施工损伤现场试验研究

经编土工格栅因施工损伤导致强度折减现象较为严重,但国内现场试验较少。比较了双向拉伸经编格栅在4种填料下压实后的纵横向拉伸强度、伸长率、应力应变曲线,对施工损伤导致的强度折减影响因素进行了分析,并给出了相应的折减系数,为设计人员提供了适合国内施工工艺、施工器械、经编格栅产品的施工损伤折减系数的参考值。     

长期荷载作用下土工格栅蠕变特性的试验研究

为探讨长期荷载作用下土工格栅的蠕变特性,在不同的外加荷载和环境温度的各种组合条件下,进行土工格栅的室内蠕变试验,以此获得格栅的蠕变关系曲线、载荷-应变等时曲线及拉伸模量的变化特征,并进行综合对比分析。根据试验与分析发现:荷载水平、环境温度和材料生产工艺是影响土工格栅长期蠕变特性的重要因素。进而,采用时温叠加原理,对于某一给定环境温度下确定土工格栅长期强度的经验估算模式和蠕变强度折减系数。试验结果与理论分析为土工格栅加筋结构长期工作性能的分析与评价提供参考依据。     

Geogrid reinforcement in landfill closures

This paper illustrates the use of geogrids in landfill closures from the perspective of both cover soil stability and subsidence-induced liner support. Both applications have as their objective the avoidance, or at least the minimization, of tensile stresses in the geomembrane. Case histories regarding each application are provided.

The important design consideration of long-term allowable strength of geogrids is also addressed. Factors such as geogrid creep, installation damage, chemical degradation, biological degradation and structural connections are discussed since all should be considered in arriving at a proper allowable design strength.     

柔性与硬性地工格网在砂土与风化泥岩围压下之力学特性

地工格网（以下称格网）用於加劲土壤时，除考虑无围压下的张力行为之外，围压下之力学性质更是设计考量的重点。实际工程应用而言，基於经济考虑，期以现地土壤作为回填材料。本研究分别以拉出、围压抗张与直剪三种试验来探讨格网放土壤中之力学行为；并利用凝聚性泥岩与非凝聚性细砂作为回填材料，评估两种回填材料对加劲成效之影响。结果显示，柔性格网之肋条在拉出过程中易扭曲，造成主应力面旋转的现象，以致拉出阻抗大放硬性格网；围压下格网抗张的应力-应变行为可分为三阶段，即束制阻抗期、张力发展期与破坏期。束制阻抗期大都於3％应变内即已完成；在低围压情况拉出阻抗达20％～60％之拉出强度（相同应变），在高围压下达150％。由直接剪力试验结果可以预测：（a）格网／泥岩加劲结构-低围压时，剪力破坏面应通过格网／泥岩之界面；而高围压时，剪力破坏面应通过泥岩上体。（b）格网／细砂加劲结构-低围压与高围压下剪力破坏面应通过格网／细砂之界面。     

Bearing force mobilisation in pull-out tests on geogrids

This paper presents a study on the mobilisation of bearing forces in geogrids subjected to pull-out solicitation. A theoretical model incorporating the effects of interference between grid bearing members on grid pull-out behaviour is presented and used for the interpretation of the results of large-scale pull-out tests on grids with varying geometrical and mechanical properties. The results obtained in this study show the influence of parameters such as free reinforcement length, test speed and interference between members on the pull-out response of geogrids. It is also shown that the load–displacement curve obtained in pull-out tests is not sufficient for an accurate investigation of soil–grid interaction and bearing force degradation mechanisms must be incorporated in the analysis of grid pull-out response if accurate predictions of pull-out strength and grid deformations are to be made.     

On residual strength of high-performance concrete with and without polypropylene fibres at elevated temperatures

Cubes of 100×100×100 mm³ and cylinders of 100×100×515 mm³ were designed and fabricated with C50, C80 and C100 high-performance concrete (HPC) mixed with and without polypropylene (PP) fibres, respectively. These specimens were heated in an electric furnace, approximately following the curve of ISO-834, with a series of target temperatures ranging from 20 to 900 °C. No explosive spalling was observed during the fire test on HPC specimens with PP fibres, whereas some spalling occurred for HPC specimens without PP fibres. The relationship between the mass loss and the exposure temperature was investigated. In addition, the heated and cooled cubes and prisms were tested under monotonic compressive loading and four-point bending loading, respectively. The degradation of both the residual compressive strength and the residual flexural strength was analyzed. Furthermore, the effects of PP fibres on the residual mechanical strength of HPC specimens at elevated temperatures were also investigated. Finally, a fire-resistance design curve relating the residual compressive strength to temperature, as well as a design curve relating the residual flexural strength to temperature, were proposed based on the statistical analysis of the test data.     

Behavior of geogrid–reinforced sand and effect of reinforcement anchorage in large-scale plane strain compression

This paper presents experimental investigations on the behavior of geogrid–reinforced sand featuring reinforcement anchorage which simulates the reinforcement connected to the wall facings in numerous in-situ situations. A series of large plane strain compression tests (the specimen 56 cm high × 56 cm wide × 45 cm long) was conducted. Standard Ottawa sand and 4 types of PET geogrids exhibiting 5% stiffness in the range of 750–1700 kN/m were used in this study. The specimens were tested by varying the relative density of sand, confining pressures, geogrid types, and reinforcement-anchorage conditions. Experimental results indicate that relative to unreinforced specimens, both anchored and non-anchored geogrid reinforcements can enhance the peak shear strength and suppress the volumetric dilation of reinforced soil. The studies on anchorage revealed that anchoring the reinforcement can restrain the lateral expansion of reinforced specimens, resulting in a substantial increase in shear strength and a reduction in volumetric dilation. The strength ratios of non-anchored specimens appeared to be insensitive to the reinforcement stiffness, whereas the strength ratios of the anchored specimens increased markedly with increases in soil density, reinforcement stiffness, and system deformation (i.e., axial stain). Geogrid anchorage contributed a large percentage of the total shear-strength improvement, nearly 3-times more than the contribution of the soil–geogrid interaction in non-anchored specimens. Lastly, an analytical model was developed based on the concept that additional confinement is induced by reinforcement anchorage, and the predicted shear strength of the anchored soil was verified based on the experimental data.     

土工格栅施工损伤现场足尺试验研究

对两种不同规格型号的聚酯纤维单向土工格栅分别开展上设中粗砂保护层、上下均设保护层和无保护层 3 种工况的施工损伤现场足尺试验,共进行了 6 场试验,通过室内拉伸试验得到不同工况下格栅经施工损伤后的拉伸强度,给出了不同工况下格栅的施工损伤系数。通过对不同方式保护效果的对比分析,发现：在格栅上表面与粗颗粒填料间设置 10 cm 厚中粗砂保护层,对降低格栅筋材施工损伤程度以及提高其可用强度有明显效果;上设保护层和上、下均设保护层两种工况格栅的施工损伤系数较接近,说明格栅施工损伤主要源自上层填料的影响;填料粒径是影响格栅施工损伤程度的主要因素,而筋材自身技术规格的差异对施工损伤系数取值的影响较小。     

土工格栅界面摩擦特性试验研究

土工格栅与土的界面作用特性直接影响着加筋土挡墙的安全与稳定性。因此,土工格栅与填料的界面技术指标在加筋土挡墙的设计中至关重要。本文在从试验方法、加载方式、试验箱侧壁边界效应和尺寸效应、填料厚度、压实度以及筋材夹持状况等几方面分析土工格栅界面摩擦特性影响因素基础上,进行了土工格栅在砂砾料和粘性土中的拉拔试验和直剪试验。试验结果表明:土工格栅与砂砾料接触面抗剪强度较高,而与粘土接触面抗剪强度很低;对于加筋土挡墙拉拔力较大的层位,应选用刚度大的土工格栅和砂砾料为填料。直剪摩擦试验不适合确定土工格栅接触面的抗剪强度。该试验结果对土工格栅加筋土挡土墙的设计具有重要的参考价值。     

Geogrid connections

Connections can have a significant impact on the performance of geosynthetic systems whether their intended function is separation or reinforcement. When reinforcement is the function, the connection must be considered as a limiting strength factor and evaluated by the same performance standards as the reinforcement material alone. If used in critical structures with design lives of more than 1 or 2 years, the geosynthetic and its connections must be evaluated for durability limitations such as creep, site damage and chemical degradation. Geogrid connections typically fall into this category.

Geogrid connections are accomplished by either overlapping or mechanical connections. Overlap connections must be designed and constructed to assure proper overlap lengths. Overlap length is governed by soil interaction behavior. Geogrid-soil interaction is measured by pull-out testing and pull-out performance is a function of geogrid dimensional stability. Overlaps can be secured by a variety of tyiing techniques but these ties are not efficient for load transfer. Geogrid Bodkin Joints are the preferred mechanical connection for assuring full load transfer between adjacent layers of some geogrids.

This paper discusses geogrid connections, the techniques used, and the factors that affect their performance.     

动荷载作用下土工格栅应变软化特性

土工格栅的强度衰减特性对加筋路堤和加筋挡墙的稳定性有重要影响。对塑料土工格栅进行了应力控制式单向循环拉伸试验,研究了循环拉力、预拉力、加载频率等对格栅应变软化及变形的影响。试验结果表明,随着循环拉力、预拉力的增加,格栅的累积应变增大,软化指数增大,强度减弱;荷载振动频率的减小也会产生类似的结果。通过对试验数据的分析,总结了格栅应变软化的规律,并将其引入改进的Iwan模型中,建立了能描述循环拉伸荷载作用下土工格栅的拉力应变关系的模型,通过将模型计算结果与试验结果的对比,验证了模型的正确性。     

Strength enhancement of clay by encapsulating geogrids in thin layers of sand

Large size direct shear tests (i.e.300 × 300 × 200 mm) were conducted to investigate the possibility of strength enhancement of clays reinforced with geogrids embedded in thin layers of sand. In this paper test results for the clay, sand, clay–sand, clay–geogrid, sand–geogrid and clay–sand–geogrid samples are presented and discussed. Thin sand layers with thicknesses of 4, 6, 8, 10, 12 and 14 mm were used to quantify their effect on the interaction between the clay and the geogrids. In this regard effects of sand layer thickness, normal pressure (i.e. confinement) and transversal members of geogrids were investigated. All the tests were conducted using saturated clay with no drainage allowed. Test results indicate that provision of thin layers of sand for encapsulating the geogrids is very effective in improving the strength and deformation characteristics of saturated clay. Maximum strength enhancement was derived at an optimum sand layer thickness of 10 mm which proved to be independent of the magnitude of the normal pressure used. For a particular sand layer thickness, increasing the normal pressure resulted in enhanced strength improvement. Results also showed that removal of the geogrid transversal members resulted in reducing the strength of the reinforced samples by 10% compared to geogrids with transversal members. Encapsulating geogrids in thin layers of sand not only will improve the performance of clays if used as backfill it would also provide drainage paths preventing pore water pressure generation on saturation of the backfill.     

Properties of geosynthetics exhumed from a final cover at a solid waste landfill

Samples of geocomposite drain (GCD), geomembrane (GM), and geosynthetic clay liner (GCL) were exhumed from a final cover at a solid waste landfill to assess their condition after 4.7–5.8 yr of service. Permittivity of the GCD diminished by a factor of 3.9, but the transmissivity was higher than published by the manufacturer. Ply adhesion of the GCD diminished by a factor of 2.0. Geonet ribs in the geocomposite drain (GCD) contained a light coating of fines and plant roots, but there was no evidence of significant clogging. The geotextile on the upper surface of the GCD met the commonly used criterion for filtration (AOS < 0.6 mm for adjacent soil with <50% fines), indicating that this filtration criterion was satisfactory. Tensile yield strength of the GM diminished by a factor of 1.2, but the melt flow index was unchanged and the oxidation induction time exceeded the manufacturer's specification. GCD-geomembrane interface strength appeared unchanged. Four GCL samples had hydraulic conductivities 1000–10,000 times higher than the hydraulic conductivity measured during construction, whereas hydraulic conductivity of the other seven samples was practically unchanged. Based on these observations, the following reduction factors are suggested for installation damage and near-term service conditions (<6 yr) for the geosynthetics used at this site: GCD permittivity or transmissivity – 4.0, GCD ply adhesion – 2.0, geomembrane tensile strength – 1.5, and GCD-geomembrane interface friction – 1.0. No recommendation is made regarding a factor for the hydraulic conductivity of GCLs.     

Continuous combined Fenton's oxidation and biodegradation for the treatment of pentachlorophenol-contaminated water

Pentachlorophenol (PCP) was studied as a model recalcitrant compound for a sequential chemical oxidation and biodegradation treatment, in a continuous laboratory-scale system that combined a Fenton’s chemical reactor and a packed-bed bioreactor.PCP degradation and dechlorination were observed in the Fenton’s reactor at a residence time of 1.5 h, although no reduction of total organic carbon (TOC) was observed. Both PCP degradation and dechlorination were strongly dependent on the H₂O₂ dose to the chemical reactor. The PCP degradation intermediates tetrachlorohydroquinone and dichloromaleic acid were identified in this reactor. Further treatment of the Fenton’s reactor effluent with a packed-bed bioreactor (operating at a residence time of 5.5 h) resulted in partial biodegradation of PCP degradation intermediates and reduction in TOC, although no further reduction of PCP or dechlorination was achieved in the bioreactor. Increased residence time in the bioreactor had no significant impact on degradation of TOC. Recycle of the effluent from the bioreactor to the chemical reactor increased the TOC degradation, but not the extent of the PCP degradation or dechlorination.A mathematical model of the combined Fenton’s oxidation and biodegradation system supported the experimental results. While the model over-predicted the PCP and TOC degradation in the combined system, it adequately predicted the sensitivity of these parameters to different H₂O₂ doses and recycle rates. The model indicated that high recycle rates would improve TOC degradation.     

Assessing the effects of sodium hypochlorite exposure on the characteristics of PVDF based membranes

Sodium hypochlorite is commonly used as a cleaning agent to remove adsorbed foulants from PVDF based micro/ultra filtration membranes in water and wastewater treatment applications. Although effective for fouling control, extended sodium hypochlorite exposure can affect the physical/chemical characteristics and hinder the treatment performance of these membranes. To assess these effects, PVDF based membranes were exposed to sodium hypochlorite at different concentrations for varying periods of time, and the physical/chemical characteristics of the virgin and sodium hypochlorite exposed membranes were compared. The membranes were characterized based on chemical composition (FTIR and NMR), mechanical strength (yield strength), surface hydrophilicity (contact angle), pore size and porosity (scanning electron microscopy and challenge test), and membrane resistance (clean water permeation test). The results indicated that exposure dose and concentration of the sodium hypochlorite used have significant influence on the membrane characteristics. The impact of sodium hypochlorite exposure on the parameters investigated could be most accurately and consistently correlated to an exposure dose relationship of the form Cⁿt (where, C = concentration and t = exposure time) rather than the Ct relationship commonly used to define the extent of exposure to cleaning agents. For all the parameters investigated, the power coefficient n was less than 1 indicating that time had a greater impact on the changes than did the concentration of the sodium hypochlorite. The results suggest that the use of sodium hypochlorite for chemical cleaning, at concentrations that are higher than those typically used for chemical cleaning would have less of an effect on the characteristics of the membrane materials. Changes in the characteristics were attributed to the oxidation of the hydrophilic additives (HA) present in blended PVDF membranes.     

Hoek-Brown准则中确定地质强度指标因素的量化

 追踪Hoek-Brown准则利用岩块力学参数估算岩体力学参数所采取方法的进展及各种改进措施。指出Hoek创立的最新方法——地质强度指标(GSI)法完全依赖定性描述和个人主观经验的不足,揭示决定地质强度指标的物理因素是岩体结构类型,化学因素是岩石风化状况。引入基于钻孔岩芯不同完整长度的岩体块度指数,以定量地表征岩体结构。分析岩石风化在化学上是原生矿物水解、淋失促使矿物蚀变而导致岩性改变的过程,依此提出表征岩石风化状况的定量指标——岩石绝对风化指数,同时研究推出其计算方法。将岩体块度指数、绝对风化指数在地质强度指标的区间范围概化表上有机结合起来,构成了GSI的定量确定方法。利用该方法估计一公路隧道围岩的变形模量、抗拉、抗压及抗剪强度等力学参数,并计算塌落拱高度。与实测数据对比,其最大误差在14%左右,验证方法的有效性和实用性。     

Bond strength of steel bars embedded in high-performance fiber-reinforced cementitious composite before and after exposure to elevated temperatures

Interfacial bond strengths of steel reinforcing bars embedded in a high-performance fiber-reinforced cementitious composite (HPFRCC) are investigated in this paper. Out of 303 pullout specimens, 48 HPFRCC and 3 normal concrete specimens were tested without any heat treatment, and 240 HPFRCC and 12 normal concrete specimens were heated at 200, 400, 600, or 800 °C in a furnace for 2 h prior to testing. The effects of bar shape, diameter, and length embedded in HPFRCC on the bond strength of HPFRCC specimens were investigated. The bond strength decreased with the heating temperature and with the size and embedded length of steel bars. It was reduced further when the heated specimens were cooled in water instead of air. It was disaggregated into chemical adhesion and mechanical interlock from a comparative study of plain and deformed bars. The mechanical properties and microstructures of HPFRCC specimens before and after heat treatment were compared to understand the mechanisms of interfacial bonding degradation due to heat treatment.     

露天矿山岩体力学参数估算法的应用研究

露天矿山边坡稳定性分析中岩体力学参数的取值至关重要,Hoek-Brown强度准则可以充分反映岩石的非线性破坏的特点,广泛应用于工程实践;结构面作为岩体的一部分,其对岩体的影响不可忽略;结构面的稳定性由抗剪强度决定,对结构面力学参数的估计同样是评价岩体强度的重要组成部分。利用轮廓曲线法和回弹法量取结构面粗糙度和壁岩强度量化地质强度指标GSI,并对潜在滑移结构面抗剪强度精准取值。结合广义Hoek-Brown强度准则估算和兴矿山II-B边坡岩体力学参数,通过极限平衡法分析其整体稳定性系数;利用Barton-Bandis模型线性拟合,计算结构面抗剪强度,对滑体落岩的稳定性进行分析。计算结果表明,整体边坡安全系数F_s=1.285,稳定性较好;含有潜在滑移结构面的岩体安全系数F_s=0.911,局部存在单平面滑移可能。计算结果符合现场实际情况,经反算法验证后,证明通过该方法估算得到的岩体强度参数及结构面抗剪强度是合理的。     

条形荷载作用下加筋土边坡稳定性分析

建立了用于模拟和分析3个大型室内足尺加筋与不加筋边坡稳定性的数值计算模型。数值计算采用基于强度折减技术的连续介质快速拉格朗日分析方法,分别对条形荷载下的位移响应、节点位移速度向量、塑性区和剪应变速率分布进行计算,获得3个边坡在条形荷载下的极限承载力和双楔体破坏机制,计算结果与试验结果吻合较好,验证了模型的可行性。在此基础上,对影响边坡稳定性的各主要因素进行分析。研究结果表明,经过格栅加固的边坡承载能力和稳定性明显提高,且随加筋层数、格栅刚度和强度的增加而增大;条形荷载越大或荷载位置离坡顶越近,边坡的稳定性越低;土体强度增大,边坡的稳定性明显增加,但土体摩擦角对安全系数的影响比黏聚力更为敏感;此外,顶层筋材埋深与条基荷载宽度比值大小与边坡的安全性密切相关,其最佳比值随加筋层数不同而改变。     

The effect of cement dosage on mechanical properties of concrete exposed to high temperatures

Although concrete is a non-combustible material, it is found that when exposed to high temperatures, such as fire, the physical, chemical and mechanical properties of concrete can drastically change. Thus, it becomes important to assess the relative properties of concrete under high temperatures in order to evaluate and predict the post-fire response of reinforced concrete (RC) buildings and structures. This paper assesses the effects of elevated temperatures and cement dosages on the mechanical properties of concrete. Two concrete mix designs were considered in this research in an attempt to study the effects of cement dosage (250 and 350 kg/m³) on the post-fire response of concrete. Once cast, the test samples were first exposed to elevated temperatures ranging from 100 to 800 °C, and then allowed to cool down slowly to ambient room temperature of 20 °C before being tested to failure. Several tests were then carried out to determine the mechanical properties of the cooled concrete specimens. The test results indicated that at temperature above 400 °C, concrete undergoes significant strength loss when compared to the strength of non-heated concrete. In addition this strength reduction was found to be unaffected by the cement dosages. The experimental results were also compared with current European standard (BS EN 1992-1-2:2004 standard) strength equations and American Concrete Institute standard (ACI 216.1).