对“钢筋(煤石干石)混凝土网格式加筋土挡土结构强度特性与试验研究”讨论的答复

雷胜友博士对笔者在《岩土工程学报》1999年第 5期上发表的“钢筋 (煤石干石 )混凝土网格式加筋土挡土结构强度特性与试验研究”一文 (以下简称原文 )的讨论 ,是有一定意义的。在原文中提出了钢筋混凝土网格式加筋土挡土结构这种非柔性筋材 ,由于不仅可以承受拉应力 ,而且可以承受剪应力 ,与一般柔性筋材加筋土只增加粘聚力Δｃ ,而视加筋与不加筋具有相同的内摩擦角 φ 不同 ,非柔性筋材加筋土与不加筋相比 ,加非柔性筋材加筋土不仅增加粘聚力Δｃ ,而且也增加了内摩擦角Δφ 。加筋土体相对于无筋土体而言 ,由于增加了Δｃ ,Δφ ,使加筋…     

对“钢筋(煤石干石)混凝土网格式加筋土挡土结构强度特性与试验研究”讨论的答复

雷胜友博士对笔者在《岩土工程学报》1999年第 5期上发表的“钢筋 (煤石干石 )混凝土网格式加筋土挡土结构强度特性与试验研究”一文 (以下简称原文 )的讨论 ,是有一定意义的。在原文中提出了钢筋混凝土网格式加筋土挡土结构这种非柔性筋材 ,由于不仅可以承受拉应力 ,而且可以承受剪应力 ,与一般柔性筋材加筋土只增加粘聚力Δｃ ,而视加筋与不加筋具有相同的内摩擦角 φ 不同 ,非柔性筋材加筋土与不加筋相比 ,加非柔性筋材加筋土不仅增加粘聚力Δｃ ,而且也增加了内摩擦角Δφ 。加筋土体相对于无筋土体而言 ,由于增加了Δｃ ,Δφ ,使加筋土体增加了一个附加围压 Δσ3ｆ 。根据极限平衡条件 ,推导出     

关于“钢筋(煤石干石)混凝土网格式加筋土挡土结构强度特性与试验研究”的讨论

自 1968年法国工程师发明了加筋土技术以来 ,加筋土工程因具有多种优势而风靡全世界 ,它被广泛地应用于公路、铁路、港口、市政、民航等大型土木工程。 加筋土是由抗拉强度比较高的加筋材料及填料 (土 )组成 ,加筋材料分为柔性材料和非柔性材料 ,对于柔性筋的加筋土强度 ,一般认为其强度线平行于素土的强度线 ,其粘聚力比素土多一个增量。对于非柔性筋加筋土的强度 ,在文献 [1]中作过介绍 ,在强度表达式的推导过程中 ,其小主应力增量的计算分两步进行 ,第一步假设加筋土相对于不加筋土而言 ,只是粘聚力有提高 ,第二步认为加筋土相对于不加筋土而言 ,只是内摩擦角有提高 ,将两步计算所得小主应力增量取代     

关于“钢筋(煤石干石)混凝土网格式加筋土挡土结构强度特性与试验研究”的讨论

自 1968年法国工程师发明了加筋土技术以来 ,加筋土工程因具有多种优势而风靡全世界 ,它被广泛地应用于公路、铁路、港口、市政、民航等大型土木工程。 加筋土是由抗拉强度比较高的加筋材料及填料 (土 )组成 ,加筋材料分为柔性材料和非柔性材料 ,对于柔性筋的加筋土强度 ,一般认为其强度线平行于素土的强度线 ,其粘聚力比素土多一个增量。对于非柔性筋加筋土的强度 ,在文献 [1]中作过介绍 ,在强度表达式的推导过程中 ,其小主应力增量的计算分两步进行 ,第一步假设加筋土相对于不加筋土而言 ,只是粘聚力有提高 ,第二步认为加筋土相对于不加筋…     

经编和玻纤格栅加筋粘性土的三轴试验研究

对经编格栅和玻纤格栅加筋粘性土进行不固结不排水的三轴压缩试验。试验结果表明,在粘性土体上布置格栅筋材,都能提高土体强度,但不同的筋材,其加筋效果是不一样的,经编格栅加筋土的加筋效果要优于玻纤格栅加筋土。加筋层数越多,加筋效果越好;随着加筋土应力增加,加筋土抵抗变形的作用才能得到更充分发挥,土体加筋效果更明显。不同筋材的加筋土,其粘聚力与内摩擦角的变化规律不一致;玻纤格栅和经编格栅加筋粘性土的加筋效果与砂土不同,不仅表现在粘聚力的增加上,还表现在内摩擦角的增加上。加强筋条结点连接的牢固性,能够提高加筋效果。     

土工格栅加筋土体强度形成机理研究

从试验研究和理论分析两方面推导了土工格栅加筋土体的强度形成机理.研究表明:当土体致密,加筋材料与土体结合紧密,且筋材的极限抗拉强度较低时,筋材发生拉断破坏,与加筋前相比,加筋复合体的内摩擦角不变,内聚力有较大的提高;而当土体较疏松,筋材的抗拉强度较高时,筋材发生摩擦型(拔出)破坏,加筋复合体的内聚力变化不大,而内摩擦角有较大的提高.     

紫茎泽兰筋材改良昔格达土物理力学试验研究

针对攀西地区特殊昔格达土质,利用广泛分布的紫茎泽兰做改良土筋材,对昔格达紫茎泽兰加筋土进行不固结、不排水的三轴压缩试验,研究不同加筋量及不同围压下加筋土的σ-ε关系曲线和抗剪强度,并分析其影响因素。试验结果表明:昔格达紫茎泽兰加筋土的抗剪强度和抵抗变形的能力较素土有显著提高;内摩擦角φ变化相对较小,相对变化率绝对值在40.13%以内,但黏聚力增长较大,最高可达17.03倍;无筋素土试样大多以剪切破坏为主,而掺入紫茎泽兰筋材后,试样的破坏主要为鼓胀破坏;抗剪强度增加量与紫茎泽兰掺入量的比例呈非线性关系,且并非掺入量越大越好。说明紫茎泽兰筋材对土体存在补强的作用,同时试验得出黏聚力c值是影响加筋土抗剪强度的主要因素。     

基于蒙特卡罗法的加筋路堤稳定可靠性分析

软土路基上加筋路堤的稳定性可以采用圆弧滑动极限平衡法进行分析,加筋路堤的破坏可以归纳为圆弧内加筋的破坏、圆弧外加筋的破坏以及加筋路堤的整体破坏,建立了计算加筋路堤稳定系数K的3个不同的稳定系数表达式,运用蒙特卡罗法对设计参数随机变量与加筋土路堤稳定可靠指标的关系进行分析,通过数值计算发现密度越小,黏聚力、内摩擦角、筋材抗拉强度和筋土摩擦角越大,加筋土路堤稳定性越好,因此,在加筋土路堤可靠性设计中建议采用密度小、黏聚力和内摩擦角大的填料以及高强度筋材,并且重点考虑内摩擦角的变异水平。     

纤维加筋非饱和黏性土的剪切强度特性

纤维加筋是近些年发展起来的一种土质改良技术,系统掌握纤维加筋土的力学性质对评价纤维加筋土工程的稳定性和进一步推广该技术在工程中的应用具有重要意义。为了了解纤维加筋的剪切强度特性,以非饱和黏性土为研究对象,以聚丙烯纤维为加筋材料,在控制含水率和干密度条件下开展了一系列直剪试验。借助扫描电镜,从微观的角度探讨了纤维的增强机理,并对相关宏观力学性质进行了分析。结果表明：纤维加筋能有效提高土体的抗剪强度,且抗剪强度随纤维掺量的增加而增加;相对于内摩擦角,纤维对黏聚力的增强效果要明显得多;纤维加筋土的抗剪强度随含水率的增加而减小,随干密度的增加而增加;总体上,低含水率和高密实度条件有利于发挥纤维的拉筋效果,提高纤维对强度的贡献。此外,纤维加筋在提高土体峰值剪切强度的同时,还能增加土体破坏时对应的应变及破坏后的残余强度,改善土体的破坏韧性。由扫描电镜分析可知,单根纤维一维拉筋作用和纤维网三维拉筋作用是纤维加筋土的主要增强机理,增强效果则取决于纤维-土界面力大小;剪切面上的纤维在剪切过程中呈现拔出和拉断两种失效模式。     

单层立体加筋砂土性状的三轴试验研究

针对加筋土中传统的筋材布置特点,提出了立体加筋土的概念,并设计了采用轴对称布置的单层立体加筋砂土的试验方案,进行了48组镀锌铁皮和橡胶板两种筋材的单层立体加筋砂土的室内三轴试验,探讨了不同立体加筋方式、不同围压作用下应力–应变及强度变化规律。通过试验结果对比,分析了立体加筋砂土同传统水平加筋砂土之间应力–应变关系和强度指标的差异规律,竖向筋高度对立体加筋砂土的强度影响,单侧和双侧布置竖向筋材等两种布筋方式对立体加筋砂土强度的影响,以及不同变形模量的加筋材料对立体加筋土强度的影响。试验结果表明:立体加筋砂土的强度随竖向筋的高度增加而增大;立体加筋不仅能提高砂土的粘聚力,同时也能增加砂土的内摩擦角,尤其是双侧立体加筋砂土;在竖向筋总高度相同时,双侧立体加筋形式比单侧立体加筋能更有效提高砂土的强度。     

Effect of discrete fibre reinforcement on soil tensile strength

The tensile behaviour of soil plays a significantly important role in various engineering applications. Compacted soils used in geotechnical constructions such as dams and clayey liners in waste containment facilities can suffer from cracking due to tensile failure. In order to increase soil tensile strength, discrete fibre reinforcement technique was proposed. An innovative tensile apparatus was developed to deter- mine the tensile strength characteristics of fibre reinforced soil. The effects of fibre content, dry density and water content on the tensile strength were studied. The results indicate that the developed test apparatus was applicable in determining tensile strength of soils. Fibre inclusion can significantly in- crease soil tensile strength and soil tensile failure ductility. The tensile strength basically increases with increasing fibre content. As the fibre content increases from 0% to 0.2%, the tensile strength increases by 65.7%. The tensile strength of fibre reinforced soil increases with increasing dry density and decreases with decreasing water content. For instance, the tensile strength at a dry density of 1.7 Mg/m^3 is 2.8 times higher than that at 1.4 Mg/m^3. It decreases by 30% as the water content increases from 14.5% to 20.5%. Furthermore, it is observed that the tensile strength of fibre reinforced soil is dominated by fibre pull-out resistance, depending on the interracial mechanical interaction between fibre surface and soil matrix.     

Required strength of geosynthetics for reinforced 3D slopes in cohesive backfills with tensile strength cut-off

In extant studies, most of the stability analyses of geosynthetic-reinforced slopes focused on two-dimensional conditions using the Mohr-Coulomb (M-C) failure criterion to describe the strength of backfills. However, in reality, all failures of slopes indicate a somewhat three-dimensional (3D) feature, and the M-C criterion is observed to overestimate the tensile strength of cohesive soils. To partially remedy this shortcoming, the concept of tensile strength cut-off is adopted to include the actual tensile strength of backfills in the yield envelope, and a kinematic approach is presented to evaluate the required strength of geosynthetics for 3D reinforced slopes in cohesive backfills. A 3D rotational mechanism of collapse that is associated with the strength envelope with tension cut-off is developed. The amount of required reinforcement is evaluated and listed as a dimensionless coefficient. The results indicate that the inclusion of the 3D effect and soil cohesion can lead to substantial savings in terms of the reinforcement to be made. In addition, a higher amount of reinforcement is required when the effect of tension cut-off is considered; this effect is more distinct for backfill with a higher amount of cohesion.     

Feasibility of saline soil reinforced with treated wheat straw and lime

Saline soils in inshore areas have characteristics that are problematic to engineering such as salt expansion, dissolution and water absorption; therefore, these soils cannot meet the requirements of strength and anti-deformation in construction. A method of reinforcing saline soil with wheat straw and lime was investigated in this study. Specifically, the feasibility of using wheat straw treated with an SH (modified polyvinyl alcohol) agent as reinforcement and the compaction and strength of saline soil reinforced with wheat straw and lime were investigated. The results indicated that wheat straw treated with SH agent is suitable for use as a reinforced material owing to its higher corrosion resistance, tensile force and elongation. Additionally, reinforcement with wheat straw and lime was found to have a positive effect on the mechanical properties of soil. Reinforcement with wheat straw fiber enhanced the strength of soil during the early curing period and reduced the brittle failure problem associated with lime soil. Reinforcing soil with wheat straw and lime is an effective method for improving soil in the geotechnical field.     

Required strength of geosynthetics in a reinforced slope with tensile strength cut-off subjected to seepage effects

The Mohr-Coulomb (M-C) yield criterion is found to overestimate the tensile strength of cohesive soils. By introducing the concept of tensile strength cut-off, the M-C criterion is modified to reduce or eliminate the tensile strength from the criterion. In this study, a new approach is proposed to investigate the stability of geosynthetic-reinforced slopes in cohesive soils subjected to seepage effects by means of the kinematic approach of limit analysis. The distribution of pore-water pressure is obtained using the numerical modeling software package, FLAC3D. A kinematically admissible failure mechanism is discretized to incorporate the results from the numerical simulation. The strength of geosynthetics required for maintaining the slope stability is evaluated from the work-energy balance equation. An optimization routine is used to seek out the maximum value among all possible results. Design charts providing the normalized required reinforcement under different parameters are plotted for a parametric study and convenient use in engineering. The obtained results show that less reinforcement is required in the presence of soil cohesion, and that the inclusion of the effect of tensile strength cut-off leads to a more conservative solution, which is more obvious in the presence of seepage effects.     

浅埋盾构隧道端头土体稳定性极限平衡分析

 盾构始发与到达时易引发端头土体失稳破坏,是盾构隧道施工中的高风险环节。针对浅埋盾构隧道,提出一种直线和对数螺旋线组合的土体滑动模式,运用极限平衡分析方法,对端头土体的稳定性进行分析,推导稳定系数计算公式,并对其影响因素进行分析。端头土体的稳定性取决于土体自身的强度、加固体的厚度和加固强度及洞门直径。端头土体自身的内摩擦角和黏聚力越大,稳定系数越大;加固体的强度越大,稳定系数越大;加固体厚度越大,端头土体稳定性越好;但随着洞门直径的增大,端头土体稳定系数逐渐降低。随后得出端头土体的加固厚度的计算方法和流程,分析其影响因素,并给出工程算例。将该方法应用于南京地铁盾构隧道端头土体稳定性分析及加固厚度计算,结果令人满意,被工程所采纳。该研究成果和方法可为同等条件下盾构隧道端头土体稳定性分析及加固设计提供借鉴。     

棕榈和麦秸秆加筋上海粘土的无侧限抗压强度研究

为改良上海粘土的强度和变形特性,分别采用棕榈和麦秸秆加筋上海粘土,以提高土的强度和抗变形性能。试验以质量加筋率和加筋长度作为影响因素,分析比较两种加筋土的无侧限抗压强度。试验结果为：①棕榈加筋土的无侧限抗压强度高于麦秸秆加筋土,两种加筋土均高于素土。②棕榈的适宜加筋条件为：质量加筋率1．0％,加筋长度的平均值15mm;四分之一圆管状麦秸秆的适宜加筋条件为：质量加筋率0．3％～O．4％,加筋长度15mlTl。③棕榈加筋土的应力应变曲线表现为应变强化型,麦秸秆加筋土的应力应变曲线表现为应变软化型。试验结果表明,棕榈与麦秸秆均适宜作上海地区粘土的加筋材料,并且棕榈加筋效果优于麦秸秆。     

Laboratory determination of clay-geotextile interaction

Current design regulations preclude the usage of cohesive backfills in reinforced soil structures regardless of whether the reinforcement is metallic or polymer fabric. The main reasons for this are: firstly, cohesive materials can be expansive; and secondly, the maximum bond strength between the reinforcement and the clay is normally not expected to be more than the undrained strength of the clay, giving no advantage. However, low-plasticity (so-called semi-cohesive) soils are not expansive and could be used in reinforced soil structures provided the reinforcement can give an increase in strength. A large number of shearbox and pull-out tests have been carried out to investigate which are the major factors governing the clay-geotextile interaction in both undrained and drained conditions. Woven and non-woven fabrics and meshes were used in the tests. The results have shown that the shearing strength of clay can be increased by properly selected geotextile reinforcement in both undrained (short-term) and drained (long-term) loading. It has been also shown that the pull-out resistance of the geotextile reinforcement is essentially proportional to the normal stress and for high transmissivity geotextiles or for geogrids it is limited by the tensile strength and relaxation of the material. The low transmissivity however is also a factor obstructing the development of high pull-out resistance in undrained conditions. The results indicate that geotextile reinforced cohesive backfill might be a viable alternative in reinforced soil structures if good-quality granular backfill material is not readily available.     

团聚体大小对填筑土强度影响的试验研究

为了了解团聚体大小对填筑土强度的影响,对不同改性填筑土的12组土样进行了一系列的无侧限抗压和剪切试验。结果表明:当团聚体的平均粒径小于3.5mm时,素土、纤维土和石灰土的粘聚力随粒径的增大而减小,内摩擦角随粒径的增大而增大;当团聚体的平均粒径大于3.5mm时,随粒径的增大,素土和石灰土的粘聚力明显增大且内摩擦角明显减小,纤维土的粘聚力和内摩擦角则变化不大;在受压破坏时,素土表现为应变软化的塑性破坏,纤维土表现为应变硬化的塑性破坏,石灰土则表现为完全的脆性破坏;素土、纤维土和石灰土的无侧限抗压强度随团聚体粒径的增大而降低。在试验结果的基础上,对团聚体大小的影响机理进行了分析和讨论。试验结论对进一步认识和掌握填筑土的工程性质具有重要意义。     

麦秸秆加筋滨海盐渍土的抗剪强度与偏应力应变

与土工格栅加筋土不同,植物纤维土在加筋补强机制与抗变形性能上具有自身的特点,这可通过纤维土的三轴UU压缩试验进行研究。由麦秸秆纤维加筋滨海盐渍土的三轴抗剪强度及其偏应力应变曲线表明:①与盐渍土相比,麦秸秆纤维土的黏聚力大幅提高,内摩擦角的增幅很小。②加筋长度比0.16质量加筋率0.25%为适宜的纤维加筋条件。③纤维土的偏应力应变曲线与盐渍土的变化形式相近,均呈应变硬化型。轴向应变较小时,加筋长度比和质量加筋率对主应力差的影响较小,纤维加筋作用微弱;随轴向应变的增大,两者的影响加大,纤维加筋作用才逐渐发挥出来。④相互交织的麦秸秆纤维对土形成了空间约束作用,使得4个围压的纤维土的破坏应变均大于盐渍土的。麦秸秆纤维加筋不仅提高了土的黏聚力,还增强了土的抗变形能力。