The influence of the degree of saturation on compaction-grouted soil nails in sand

A series of large-scale model tests was conducted on compaction-grouted soil nails to study the influence of the degree of saturation on the soil response to compaction grouting and pull-out. The experimental results show that the initial degree of saturation of the soil strongly influences the grout injectability, thus the formed diameter of grout bulb. Subsequently, the diameter of the grout bulb alters the pull-out force, with larger grout bulbs generating higher pull-out forces and exhibiting greater hardening behaviour. Interestingly, the initial pull-out forces are the same for the same grouting pressure, regardless of the initial degree of saturation and the subsequently grout bulb. In addition, some of the main factors influencing the pressure grouting and pull-out of the soil nail, as the initial degree of saturation varies, are as follows. First, the variations in the soil pressure and density with the initial degree of saturation are similar to that of the volume of grout injected, and the compression of the soil induced by pressure grouting exhibits a similar evolution with the initial degree of saturation at different locations. Second, the initial degree of saturation of the soil sample plays a dominant role in the change in suction during pressure grouting and pull-out of soil nail. Third, the horizontal soil pressure derived from the pull-out of soil nail propagates closely in the soil sample of lower initial degree of saturation. The vertical soil pressure induced by the vertical soil dilation and squeezing effect varies in accidence with the initial degree of saturation and the grout bulb.

     

The compaction effect on the performance of a compaction-grouted soil nail in sand

In this study, two series of physical modeling experiments, with and without a grouting process, were conducted under different grouting pressures to study the effect of compaction grouting on the performance of compaction-grouted soil nails. In addition, a hyperbola-based model was proposed to describe the variation of the pullout forces with and without grouting. Some of the main conclusions drawn are as follows. First, the compaction effect initially influences the mobilized pullout force, but not the final stage of pullout; the large difference between the two series of tests in regard to the pullout force at the initial stage led to the first part of this conclusion. However, the final pullout force results of the tests, both those with and those without grouting, were similar. Second, once the soil condition changes, the compaction effect on the performance of a soil nail depends on the grouting pressure rather than the diameter of the grout bulb. Third, the difference in the soil response (i.e., vertical dilatancy and the vertical and horizontal squeezing effects) derived from the compaction grouting effect will result in the initial difference in the increased rate of the pullout force between the tests with a grouting process and those without. Finally, a hyperbola-based model was proposed to describe the variation of the pullout force of the model tests with and without grouting, through which the pullout force is available of prediction for the given diameter of grout bulb and pullout displacement.

     

土钉抗拔试验过程中钉周土体应力变化研究

土钉的抗拔强度是土钉支护结构设计需要用到的重要参数。抗拔强度的大小取决于拔出过程中作用于土钉表面的法向应力的大小。通过室内模型抗拔试验以及三维有限元分析,研究了钻孔灌注型土钉在钻孔、注浆及拔出过程中土钉杆周围土体中的应力变化情况。研究发现,对于钻孔灌注型土钉,孔周土体中的应力在钻孔结束时几乎全部释放掉;对于低压灌浆土钉,灌浆结束到浆体硬化的过程中土钉杆周围土体中的应力没有得到有效恢复;拔出过程中,由于土体的剪胀变形受到约束,土钉周围土体中的应力随着拔出位移的增加逐渐增加,从而导致抗拔应力的增加,说明土的剪胀性对钻孔灌注型土钉来说非常重要,对于低剪胀性土,需要通过增加灌浆压力等方法对钻孔过程中释放掉的应力进行补偿     

Influences of overburden pressure and soil dilation on soil nail pull-out resistance

Soil nailing is the most popular technique for stabilizing newly formed and existing sub-standard slopes in Hong Kong because of its economic and technical advantages. The nail–soil interface shear resistance is an important parameter in design of soil nailed structures. A three-dimensional finite element model was established and used for simulating soil nail pull-out tests. The finite element model was verified by comparing simulated results with measured data. The agreement between the experimental and simulated results in terms of both average pull-out shear stress and stress variation was very good. Using this finite element model, a parametric study was carried out to study the influences of the overburden pressure and soil dilation angle on the soil nail pull-out resistance. The simulated peak pull-out resistance was not directly related to the overburden pressure, which was coincident with the observations in laboratory pull-out tests. The simulated pull-out resistance increased significantly with the increase in dilation angle of the shearing zone. This analysis indicated that the constrained dilatancy of the nail–soil interface and the soil surrounding the nail contributed a lot to the development of peak pull-out resistance.     

注浆体对土钉受力特性的影响研究

基于土钉抗拔力学模型的解析解,结合现场土钉抗拉试验工程实测数据,分析了水泥注浆体弹性模量Eg及钉土剪切变形系数K的变化对土钉受拉力学特性的影响。结果表明,随着Eg的增大,最大荷载下钉头位移逐渐减小,浅部钉土间相对位移和剪力有所减小,而深部则有所增大。Eg越大,钉土间相对位移和剪力沿钉身的变化越小,各部分侧阻分布趋于均匀。随着K的增大,最大荷载下土钉沿钉长各处的相对位移均减小,且减小比率逐渐降低,注浆对土钉的影响逐渐趋弱。相同变化率下K对土钉荷载-位移曲线的影响要大于Eg,而对钉土间剪力的影响则不及Eg的影响大。     

Pull-out behaviour of wood bolt fully grouted by PS-F slurry in rammed earth heritages

This article presents a laboratory and field investigation of pull-out resistance of wood bolts in rammed earth heritages. The laboratory testing involved axial tensile tests of four grout specimens prepared in specially designed moulds using PS-F (Potassium Silicate solution-Fly ash) slurry to investigate the performance of anchors. The field testing involved pull-out tests of three wood anchors installed in rammed earth heritages by gravity grouting. Experiments reveal that the failure mode is the pull-out of bolt from grout and average skin friction resistance of bolt–grout interface is approximately 0.34MPa. Load–displacement curves and elastic–residual displacement derived from cyclic loading indicate such anchor system has strong ductility with small elastic deformation and large residual deformation. Bond collapse firstly occurs at the loaded end and then propagates towards the full bonded length. Bond stress distribution is not uniform along the bonded length. Maximum bond stress distributes at the range from 0.3 m to 0.4 m. Emergence of compressive strain in the interface reveals such anchor system enjoys the advantages of both tensile and compressive anchor types, which significantly differs from results from other regular bolts. The research conclusion makes scientific senses to traditional material and craft in rammed earth heritages.     

A simple mathematical model for soil nail and soil interaction analysis

Soil nails have been widely used to stabilize slopes and earth retaining structures in many countries and regions, especially, in Hong Kong. The analysis of the interaction between a soil nail and the surrounding soil is of great interests to both design engineers and researchers. In this paper, authors present a simple mathematical model for the interaction analysis of a soil nail and the surrounding soil considering a few key factors which are soil dilation, bending of the soil nail, vertical pressure, and non-linear subgrade reaction stiffness. The lateral subgrade reaction between the soil and the soil nail is assumed to obey a hyperbolic relation. Reported test data in the literature are used to verify the present model. The contributions of the soil-nail bending on the pull-out resistance are evaluated in two case studies.     

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.     

Numerical and experimental studies of the mechanical behaviour for compaction grouted soil nails in sandy soil

A model test was conducted for a newly developed soil nail, the result of which was compared to that of a 3D finite element method (FEM) simulation. The shape angle (β) and friction coefficients (μ) contributed to the difference in pull-out force between the model test and the simulation. Further verifications were conducted, during which the pull-out mechanisms of the soil nail for different β and μ were analysed. It was found that both β and μ have a positive influence on the pull-out force; β only affects the increase rate, while μ influences the overall pull-out force level. Three components were found to govern the pull-out force, with the component applied to the expanded cement bulk surface accounting for approximately 80% of the total. This study proves that the optimal approach for increasing pull-out force is to enlarge the diameter of the cement bulk rather than extend the nail length.     

Pull-out behaviour of steel grid soil reinforcement embedded in silty sand

This paper investigates the pull-out behaviour (particularly the bearing resistance) of a steel grid reinforcement embedded in silty sand using laboratory tests and numerical analyses. It is demonstrated that the various common analytical equations for calculating the bearing component of pull-out resistance give a wide range of calculated values, up to about 200% disparity. The disparity will increase further if the issue of whether to use the peak or critical state friction angle is brought in. Furthermore, these equations suggest that the bearing resistance factor, N_q, is only a function of soil friction angle which is not consistent with some design guidelines. In this investigation, a series of large scale laboratory pull-out tests under different test pressures were conducted. The test results unambiguously confirmed that the N_q factor is a function of test pressure. A modified equation for calculating N_q is also proposed. To have more in-depth understanding of the pull-out behaviour, the tests were modelled numerically. The input parameters for the numerical analysis were obtained from laboratory triaxial tests. The analysis results were compared with the experimental results. Good agreement between experimental and numerical results was achieved if the strain-softening behaviour from peak strength to critical state condition was captured by the soil model used.     

The effect of pressure-grouted soil nails on the stability of weathered soil slopes

Pressure-grouted soil nails have been increasingly used for stabilizing slopes. The pullout resistance of a soil nail is the main factor for reinforcing the slope stability. In this study, a two-dimensional axisymmetric finite element model is developed to simulate the pullout behavior of a pressure-grouted soil nail. This model is verified with field pullout tests result of a pressure-grouted soil nails by comparing with gravity-grouted soil nails. Based on the analysis, a three-dimensional finite element model is proposed for stability analysis of a slope reinforced with pressure-grouted soil nails using the shear strength reduction method. A series of numerical slope stability analyses for a slope composed of weathered soil are performed to investigate the effects of grouting pressure on the slope stability and the behavior of the soil nails. Special attention is given to the installation effect of a pressure-grouted soil nails. It is found from the result of this study that the pressure-grouted soil nails increase the safety factor by fifty percent in a slope by increasing the stiffness of the nailed slope system. Numerical analysis results confirm the fact that the pullout resistance of a soil nail is the main factor for stabilizing slopes rather than the shear resistance of the soil nail.     

An Innovative Post Grouting Technique for Soil Nails

This study presents an innovative technique of executing soil nails called sectorized post grouting (SPG). The most utilized technique of soil nail grouting is gravity grouting, with the literature reporting advances in pressurized grouting. Although obtaining higher pullout resistance of soil nails, pressurized grouting, mostly done in single-stage grouting, does not compensate for exudation and its use in higher nail lengths is difficult. Thus, a technique has been developed that compensates for exudation, with easier application in lengthier nails. The technique was qualitatively assessed to evaluate its surface roughness and later applied in seven real soil nailing works, where it could be quantitatively assessed. The results show that sectorized post-grouted nails obtained greater pullout resistances than gravity grouted and single-stage grouted nails. Although similar improvement was found in tube-à-manchette (TAM) grouted nails, this method presents lower economic efficiency than sectorized post grouting. The pullout resistance results obtained in this study can be utilized in future soil nailing works executed utilizing SPG.

     

软土地基注浆扩散过程数值模拟研究

廊沧高速公路K132+177～K132+258软土地基主要由低液限中、高压缩性黏土层组成,属于滨海沉积类型,采用静力注浆方法进行了加固处理,并在现场分别对全孔一次性和自上而下分段式注浆施工方式进行了试验及室内研究。基于该路段软土地基地基物理力学性质和注浆试验地表变形观测资料,利用FLAC3D软件对该软土地基在不同注浆方式（全孔一次性注浆、自上而下分段式注浆）和在0.5、1.0 、2.0 MPa注浆压力作用下浆液在土层中的扩散过程进行了分析。其结果表明,在相同注浆压力作用下,自上而下分段式注浆效果比全孔一次性注浆效果好,而注浆压力的增大对浆液在土层中的扩散是有利的,但压力过大会导致地面冒浆,应根据现场实际情况选择合理的注浆压力     

上覆压力和剪胀作用下土钉抗拔的有限元模拟

灌浆土钉在边坡加固工程中有着广泛的应用。土钉在不同条件下的抗拔机制仍是进一步研究的课题。通过建立一个三维有限元模型,研究土钉在不同上覆压力和剪胀作用下的土钉抗拔行为。采用摩尔-库仑模型模拟花岗岩土,土与土钉界面采用“接触对”和库仑摩擦模型描述。模型可以模拟土钉发生较大抗拔位移下发生滑移破坏的过程。模型参数通过三轴试验结果进行校正。通过与大尺寸室内抗拔试验结果相比较,验证了模型的有效性。模型展现了土钉在钻孔后和抗拔过程中土钉周围的土压力的变化。最后通过参数分析,研究了上覆压力和剪胀对土钉抗拔力的影响。结果显示,当上覆压力较小时,剪胀角对抗拔应力的影响较小。随着上覆压力的增加,剪胀角对抗拔应力的影响变得更加明显。     

Numerical modelling of soil nails in loose fill slope under surcharge loading

A plane–strain numerical model has been developed to mimic a nailed loose fill slope under surcharge loading. The model has been used to back-analyse a field test that was conducted to examine the behaviour of soil nails in loose fill slopes under surcharge loading. Incremental elasto-plastic analyses coupled with pore water diffusion have been performed to study the internal deformation, water content redistribution in the soil, and the performance of the soil nails during and after the application of surcharge loading. The model parameters describing the mechanical and hydraulic properties of the nailed slope were obtained from field or laboratory tests. Different modelling techniques and boundary conditions for mimicking soil–nail interaction in loose fill material have been examined. Comparisons between numerical predictions and field measurements demonstrate that a new interfacial model, denoted as the embedded bond–slip interface model, is more suitable for mimicking the interfacial behaviour. Despite the simplicity of the numerical model, the predicted responses are in close agreement with the field test results, in particular the mobilisation and distribution of nail forces in response to surcharge loading. Both the numerical and the field test results suggest that soil nails are capable of increasing the overall stability of a loose fill slope for the loading conditions considered in this study. The increase in confining stress along the soil nails near the surcharge area is central to the overall stabilising mechanism. On the contrary, the nail forces mobilised near the nail heads are much smaller, indicating that the beneficial effect of having a structural grillage system at the slope face is limited for the range of surcharge pressures considered in this study.     

多排孔注浆引起土体变形与孔压规律试验研究

目前尚未见到软土地区多排孔注浆引起土体变形和超孔压发展规律的系统研究,限制了注浆变形主动控制策略的深化发展。开展了多排孔注浆现场试验,系统研究了单排孔逐孔注浆、多排孔逐排注浆引起土体水平位移和超孔压的变化与叠加规律。研究表明,注浆引起的超孔压随距离衰减迅速,变化规律可用幂函数表达,注浆距离取3 m左右时纠偏效果较好;天津地区超孔压消散较快,有利于注浆控制变形的快速稳定,超孔压随时间的消散规律可用指数函数表达,并提出了计算注浆引起超孔压的经验公式;多排孔逐排注浆时,先行注浆形成的注浆体墙对后续注浆一侧及另一侧土体变形分别具有反力效应和遮挡效应,控制既有建构筑物变形时应遵循“先远后近”逐排注浆的原则,以提高变形控制效果。     

Discussion of “A simple mathematical model for soil nail and soil interaction analysis” by Wan-Huan Zhou and Jian-Hua Yin [Computers and Geotechnics 35 (2008) 479–488]

The paper by W. H. Zhou and J. H. Yin includes a problem regarding the pull-out resistance of a soil nail within dilatant soils. They calculate the pull-out force due to soil dilatancy using cylindrical expansion in an elastic medium. However, the soil in the vicinity of the nail experiences significant local softening behavior. This discussion indicates another method for calculating the normal stress due to soil dilatancy.     

带加强肋单向土工格栅的拉拔试验

在水平-垂直加筋体系研究的基础上,对单向土工格栅设置了加强肋,使其具备立体加筋效果。通过大量的拉拔试验,研究了带加强肋土工格栅加筋的筋-土界面特性。通过对汇总得到的36组拉拔试验数据进行分析,探讨了肋间距与肋厚对极限拉拔阻力的影响情况。试验结果表明：在相同法向应力作用下,带加强肋土工格栅的极限拉拔阻力明显高于普通土工格栅,其极限拉拔阻力随着加强肋肋厚的增加而显著增加,并随着加强肋肋距的增加而逐渐减小。在试验基础上,进一步分析了带加强肋土工格栅与砂土的相互作用机制,探讨了极限拉拔阻力的影响因素,建立了拉拔阻力理论公式,并将试验结果与理论值比较,二者基本吻合。     

钙质砂中单桩轴向抗拔模型试验研究

针对取自南沙群岛的钙质砂,通过室内模型试验对钙质砂中单桩轴向抗拔承载特性进行研究,讨论了地基相对密实度与桩基埋深对于其抗拔承载力的影响特征。结果表明,在一定范围内,增大地基相对密实度和埋深均能显著提高桩基的承载能力;降低相对密实度或埋深不仅会降低其承载能力,也会增加其在同级荷载下产生的变形;模型桩的桩身轴力从桩顶随深度增加而逐渐降低至0;相对密实度的增加不仅能提高极限桩侧摩阻力的大小,还会在一定程度上影响桩侧摩阻力的分布形式;0.1倍的桩径可以看作是模型桩出土破坏的临界位移量。     

岩溶地区土洞地基灌浆处理过程研究

[摘 要] 灌浆是岩溶土洞地基处理的一种常用方法,灌浆压力对灌浆施工的有效性有着至关重要 的作用。针对岩溶地区土洞地基所处地质环境的复杂性,为了科学合理地确定其灌浆施工过程中的灌 浆压力,本文通过传统理论力学分析方法以及数值模拟方法对灌浆过程中灌浆压力变化及其引起的整 个地基应力应变场的变化进行了分析与研究,同时与施工现场监测结果进行了对比研究。研究结果表 明:在灌浆施工过程中,合理适当的灌浆压力既能确保浆液充分填充土洞,又不会破坏原土体结构;通过 对原土体抬升位移的监测,能准确预测与控制施工时的灌浆压力;理论分析与数值模拟结果和现场情况 吻合较好,为灌浆工艺施工提供了可靠的理论依据与指导。