Limit equilibrium analysis for stability of soil nailed slope and optimum design of soil nailing parameters

Reinforcement of slopes using soil nailing can effectively improve slope stability, and it has been widely used in upgrading cut slopes. Based on the assumptions of stresses on the slip surface, a new method for analyzing the stability of a slope reinforced with soil nails was established in the limit equilibrium theory framework, by considering that slope sliding occurs owing to shear failure of the slip surface, which subjects to Mohr–Coulomb(M–C) strength criterion. Meanwhile, in order to easily analyze the stability of a soil nailed slope in actual engineering and facilitate optimum design of parameters for soil nailing, factor of safety(FOS) contour curve charts were drawn on the basis of the established linear proportional relationship between the spacing of soil nails and slope height, and the length of soil nails and slope height. Then, by analyzing and verifying the results obtained from classic examples, some conclusions can be got as follows: 1) The results obtained from the current method are close to those obtained from the traditional limit equilibrium methods, and the current method can provide a strict solution for the slope FOS as it satisfies all the static equilibrium conditions of a sliding body, thus confirming the feasibility of the current method; 2) The slope FOS contour curve charts can be used not only to reliably analyze the stability of a soil nailed slope, but also to design optimally the parameters of soil nailing for the slope with a certain safety requirement.

Stress Relaxation of Grouted Entirely Large Diameter B-GFRP Soil Nail

One of the potential solutions to steel-corrosion-related problems is the usage of fiber reinforced polymer （FRP） as a replacement of steel bars. In the past few decades, researchers have conducted a large number of experimental and theoretical studies on the behavior of small size glass fiber reinforce polymer （GFRP） bars （diameter smaller than 20 ram）. However, the behavior of large size GFRP bar is still not well understood. Particularly, few studies were conducted on the stress relaxation of grouted entirely large diameter GFRP soil nail. This paper investigates the effect of stress levels on the relaxation behavior of GFRP soil nail under sustained deformation ranging from 30% to 60% of its ultimate strain. In order to study the behavior of stress relaxation, two B-GFRP soil nail element specimens were developed and instrumented with fiber Bragg grating （FBG） strain sensors which were used to measure strains along the B-GFRP bars. The test results reveal that the behavior of stress relaxation of B-GFRP soil nail element subjected to pre-stress is significantly related to the elapsed time and the initial stress of relaxation procedure. The newly proposed model for evaluating stress relaxation ratio can substantially reflect the influences of the nature of B-GFRP bar and the property of grip body. The strain on the nail body can be redistributed automatically. Modulus reduction is not the single reason for the stress degradation.

Pull-out tests and slope stability analyses of nailing systems comprising single and multi rebars with grouted cement

The pull-out capacities for soil nailing systems comprising of one single 29 mm diameter(type A) and four 16 mm diameter(type B) rebars with grouted cement were examined.A field test and numerical analysis for the type A and type B systems were carried out to investigate the pull-out capacities and the slope stability reinforcement efficiency in soil and rock slopes.The results of the pull-out tests show the mobilized shear force and load transfer characteristics with respect to soil depth.The load-displacement relationship was examined for both type A and type B systems.Slope stability analyses were carried out to study the relationships between soil and nail reinforcement and bending stiffness as well as combined axial tension and shear forces.Factors of safety were calculated in relation to the number of nails and their outside diameters.Both soil and rock slopes were included in this evaluation.

A case study on behaviors of composite soil nailed wall with bored piles in a deep excavation

A complete case of a deep excavation was explored. According to the practical working conditions, a 3D non-linear finite element procedure is used to simulate a deep excavation supported by the composite soil nailed wall with bored piles in soft soil. The modified cam clay model is employed as the constitutive relationship of the soil in the numerical simulation. Results from the numerical analysis are fitted well with the field data, which indicate that the research approach used is reliable. Based on the field data and numerical results of the deep excavation supported by four different patterns of the composite soil nailed wall, the significant corner effect is founded in the 3D deep excavation. If bored piles or soil anchors are considered in the composite soil nailed wall, they are beneficial to decreasing deformations and internal forces of bored piles, cement mixing piles, soil anchors, soil nailings and soil around the deep excavation. Besides, the effects due to bored piles are more significant than those deduced from soil anchors. All mentioned above prove that the composite soil nailed wall with bored piles is feasible in the deep excavation.

Numerical modelling of pullout of helical soil nail

An investigation into the pullout response of helical soil nail using finite element subroutine Plaxis 2D is presented.The numerical modelling of actual pullout response is achieved by axisymmetric and horizontal loading condition.The effect of varying number of helical plates,helical plate spacing and helical plate diameter is studied to understand the pullout capacity behaviour.The failure surfaces for various helical soil nail configurations and their pullout mechanisms are also analysed and discussed.The pullout capacity is found to increase with increase in number of helical plates.The helical plate spacing ratio（s/D;） and diameter ratio（D;/D;） are found to increase the pullout only up to a critical value.The response of helical soil nail using axisymmetric finite element simulation is found similar to the uplift behaviour of helical piles and helical soil anchors.In the absence of literature regarding numerical modelling of helical soil nail,simulation results are validated with uplift responses of helical piles and soil anchors.A good agreement in their comparative study for pullout response is also observed.

Comparative study on pullout behaviour of pressure grouted soil nails from field and laboratory tests

Pullout resistance of a soil nail is a critical parameter in design and analysis for geotechnical engineers. Due to the complexity of field conditions, the pullout behaviour of cement grouted soil nail in field is not well investigated. In this work, a number of field pullout tests of pressure grouted soil nails were conducted to estimate the pullout resistance of soil nails. The effective bond lengths of field soil nails were accurately controlled by a new grouting packer system. Typical field test results and the related comparison with typical laboratory test results reveal that the apparent coefficient of friction （ACF） decreases with the increase of overburden soil pressure when grouting pressure is constant, but increases almost linearly with the increase of grouting pressure when overburden pressure （soil depth） is unchanged. Water contents of soil samples at soil nail surfaces show obvious reductions compared with the results of soil samples from drillholes. After soil nails were completely pulled out of the ground, surface conditions of the soil nails and surrounding soil were examined. It is found that the water content values of the soil at the soil/nail interfaces decrease substantially compared with those of soil samples extracted from drillholes. In addition, all soil nails expand significantly in the diametrical direction after being pulled out of ground, indicating that the pressurized cement grout compacts the soil and penetrates into soil voids, leading to a corresponding shift of failure surface into surrounding soil mass significantly.

Pullout of soil nail with circular discs: A three-dimensional finite element analysis

An internal failure mode for a soil-nailed system consists of failure at nail heads,slope facing,nail strength,along groutesoil interface and pullout failure.A better understanding of pullout of soil nail thus becomes important to assess the stability of a soil-nailed system.In the present study,an investigation into the pullout behaviour of soil nail with circular discs along the shaft has been carried out by a threedimensional finite element analysis using Abaqus/Explicit routine.A total of 67 simulations have been performed to accurately predict the pullout behaviour of soil nail.The soil nail under study has circular discs along its shaft varying in numbers from 1 to 4.The pullout of this soil nail in a pullout test box has been simulated with a constant overburden pressure of 20 kPa acting on the nail.The pullout load edisplacement characteristics,stresses around soil nail and failure mechanism during pullout are studied.Variations of dimensionless factors such as normalised pullout load factor and bearing capacity factor have been obtained with different combinations of parameters in terms of relative disc spacing ratio,anchorage length ratio,embedment ratio,diameter ratio and displacement ratio.From the results of analyses,it is found that nail with more circular discs requires higher pullout load.There are critical relative disc spacing ratio and diameter ratio which significantly affect the pullout behaviour of nail.

Seismic analysis of nailed vertical excavation using pseudo-dynamic approach

An attempt has been made to study the behavior of nailed vertical excavations in medium dense to dense cohesionless soil under seismic conditions using a pseudo-dynamic approach. The effect of several parameters such as angle of internal friction of soil （φ）, horizontal （kh） and vertical （kv） earthquake acceleration coefficients, amplification factor （fa）, length of nails （L）, angle of nail inclination （a） and vertical spacing of nails （S） on the stability of nailed vertical excavations has been explored. The limit equilibrium method along with a planar failure surface is used to derive the formulation involved with the pseudo-dynamic approach, considering axial pullout of the installed nails. A comparison of the pseudo-static and pseudo-dynamic approaches has been established in order to explore the effectiveness of the pseudo-dynamic approach over pseudo-static analysis, since most of the seismic stability studies on nailed vertical excavations are based on the latter. The results are expressed in terms of the global factor of safety （FOS）. Seismic stability, i.e., the FOS of nailed vertical excavations is found to decrease with increase in the horizontal and vertical earthquake forces. The present values of FOS are compared with those available in the literature.

循环张拉作用下新型多节泡土钉抗拔性能现场试验研究

新型多节泡土钉具有拉拔硬化特征,相比传统土钉,其在土质边坡中抗拔性能更优。为了探究实际工况条件下新型多节泡土钉的边坡加固性能,并与传统土钉进行比较,开展7组新型多节泡土钉及传统土钉现场循环张拉试验,研究注浆工艺、节泡间距及埋深对土钉抗拔力及其发挥速率的影响规律,进而探明了每一循环下土钉弹、塑性拉拔位移占比,并分析其内在机理。试验结果表明:循环张拉作用下,注浆工艺对土钉的抗拔力影响显著,采用改进式压密注浆工艺和直接压密注浆工艺的土钉抗拔力远远超出传统注浆工艺,且初期的拉拔硬化现象更明显。采用改进式压密注浆工艺的土钉抗拔力发挥速率最快,直接压密注浆工艺次之,传统注浆工艺最小。从总体趋势上看,新型多节泡土钉抗拔力发挥速率在降低,传统土钉抗拔力发挥速率在升高。采用改进式压密注浆工艺虽然不能提升土钉弹性拉拔位移的占比,却能显著提高极限抗拔力;节泡间距对多节泡土钉抗拔力和抗拔力发挥速率影响较小,主要受到土钉周围土体强度的影响;埋深影响多节泡土钉的极限抗拔力,埋深越大,极限抗拔力和初始抗拔力发挥速率越大,同时抗拔力发挥速率随拉拔位移递减的趋势也更加明显。研究成果对提高新型多节泡土钉加固性能和优化工程应用提供指导。

深基坑支护理念演进和设计方法改进剖析

基于我国30多年建设实践,分析现有深基坑支护临时性理念、与主体结构相结合理念实施的优点和不足,提出促进基坑支护工程高质量发展的永久化理念。结合济南某基坑工程,阐释三个理念的逐次提升和相应支护结构的计算内容,定义适应与主体结构相结合理念、永久化理念的结构岩土化、岩土结构化设计方法;重点分析土钉墙与抗浮锚杆相结合的复合土钉墙的设计,以及抗浮锚杆与外伸支撑、水平楼板永久支护结构中抗浮锚杆的力学性状与耐久性,分析了永久支护对于地下室外墙受力机制的影响,阐释深基坑支护理念演化方向和与之适应的设计改进内容。对于反思我国岩土与结构设计管理体制,促进基坑与基础设计理论和方法进步,推动“新发展理念”在基坑工程的实践具有导向价值。

不同含水量粉土与土钉界面的剪切特性

通过控制含水量的粉土与土-钉界面拉拔剪切室内试验,研究不同土体含水量及上覆压力时,钉-土界面的剪切应力与剪切位移关系、界面剪切强度及表观摩擦系数的变化规律。试验结果表明:钉-土界面剪切应力随上覆压力的增加而显著增加,峰值剪切应力对应的拉拔位移也逐渐增大,且剪切应力达到峰值后出现软化现象;钉-土界面的剪切应力与土体含水量呈负相关;峰值、残余剪切应力均随着上覆压力的增大近似呈线性增加,随着土体含水量的增大而显著下降;峰值应力与残余剪切应力的差值随着土体含水量的增加而减小,随着上覆压力的增加而增大。研究成果可为土钉拉拔承载性能研究提供参考。

深基坑斜直交替钢管桩复合土钉墙数值模拟分析

依托福建某医院的基坑工程实例,采用数值模拟方法,研究斜直交替钢管桩复合土钉墙支护结构的变形特征及其稳定性。结果表明:坡面水平位移最大变形出现在基坑“腰部”,设计时应适当加强此位置的变形控制;斜直交替钢管桩复合土钉墙支护体系安全可行,其较双排直立钢管桩复合土钉墙及单一土钉墙有更好的变形控制能力和更高的稳定性。

考虑土性参数空间变异性的珠三角软土地区土钉支护结构可靠度分析

如何定量定性描述软土土性参数空间变异性对准确确定软土土性参数和合理评价软土地区土钉支护结构稳定性至关重要。采用既有统计分析方法定量描述空间变异性通常需要大量数据,难以满足。本文基于随机场理论模拟软土土性参数空间变异性,建立了一套考虑土性空间变异性的软土地区土钉支护结构可靠度设计方法,最后以MATLAB软件包为平台进行计算程序的开发与工程实例研究。分析结果表明,本文采用随机场方法模拟软土土体固有空间变异性及其在土钉支护结构可靠度分析中的应用具有合理性与普适性。

GFRP筋土钉支护钉头的锚固性能

土钉钉头为土钉墙整体结构中的薄弱部位,为研究土钉钉头锚固性能,以一种装配式柔性面层GFRP筋土钉墙为例,通过室内钉头锚固性能试验及三维数值模拟对外径32 mm的中空GFRP筋钉头的极限抗拉强度、钉头变形及破坏规律进行研究。研究结果表明,该GFRP筋钉头极限承载力在240~290 kN之间,当螺母拧紧时螺纹副的应力主要分布在螺纹牙前3环,其中以钉头第1环螺纹牙应力集中现象尤为明显;此外,采用螺纹展开法建立钉头螺纹牙的力学模型,得出了第1环螺纹极限剪切强度与钉头极限荷载之间的关系,并通过计算得出该GFRP筋钉头极限承载力为244.54 kN,与试验结果相符。通过室内钉头锚固性能试验的脆性破坏特点,确定GFRP筋钉头安全系数在1.8~2.0之间,并判定此种GFRP筋安全荷载在125~135 kN之间。

不同注浆工艺下土钉的现场拉拔试验

针对传统注浆工艺下土钉注浆控制难、抗拔性能不佳的问题,开展不同注浆工艺的土钉现场抗拔对比试验,探究传统注浆工艺、改进式压密注浆工艺以及分步改进式压密注浆(预注浆+改进式压密注浆)3种工艺下土钉抗拔性能及荷载传递规律,并将多节泡土钉抗拔力现场试验结果与理论模型计算结果进行对比分析。研究结果表明:注浆工艺对土钉的抗拔力影响显著,改进式压密注浆工艺和分步改进式压密注浆工艺下的土钉抗拔力是传统注浆工艺下的2倍以上,且拉拔初期的硬化现象更明显;改进式压密注浆工艺下土钉的轴力在节泡段变化显著,节泡端抗力占总抗拔力的75%左右;分步改进式压密注浆工艺下土钉轴力后段变化趋势与改进式压密注浆工艺下的土钉轴力变化趋势相似,其前段变化趋势与传统注浆工艺下土钉轴力的变化规律一样,呈线性减小,土钉端反力与侧摩擦力基本相等,且出现抗拔力软化时对应的拉拔位移最大;在3种注浆工艺下,土钉侧摩擦力都是靠近拉拔端最先屈服,且直径越大的节泡段提供的侧摩擦力越大。抗拔力理论模型能够基本反映不同注浆工艺下土钉在现场应用的抗拔规律,可为多节泡土钉的现场应用提供理论参考。

土钉支护边坡物理模型试验研究进展

为提高工程扰动与自然因素联合作用下边坡稳定性,厘清不同加载条件下边坡土钉支护方式的作用过程及其特点,选取适用于实际工况的最优支护模式及布设工艺,保障边坡结构系统的本质安全。首先,在静力、振动和离心3类受力作用下系统阐述土钉支护物理模型试验过程与结果;其次,对比分析典型边坡支护技术与新型边坡支护方式的特点及适用性;最后,通过复杂环境效应下的物理模型试验,探究海底边坡、台风暴雨边坡及高速远程滑坡3类特殊边坡的变形破坏机制。结果表明:土钉在各类边坡支护中均具有良好的适用性,但对于复杂特殊环境下边坡支护的可靠度与安全系数仍有待提高。实际中需融合新材料新工艺开展组合支护结构研究;同时,加强物理模型试验中的相似材料选取、观测手段创新、特殊环境模拟研究,提高监测数据的高还原度,实现边坡系统的全过程安全保障。

土钉墙+桩锚式组合的深基坑支护技术研究

以石家庄市某深基坑项目为案例背景,通过研究土钉墙+预应力锚索组合的支护形式,将理论与施工现场实际相结合,同时对常见的施工质量问题进行分析并提出相应的解决措施,以期为相关工程提供参考经验。

深基坑支护方案设计及开挖数值模拟分析

随着城市建筑地下室建设规模的扩大,对深基坑的开挖和支护的技术要求也越来越高。本文以某深基坑工程为例,建立了相应的数值模型。利用FLAC3D软件分析了基坑在连续开挖过程中的变形和稳定性。根据数值模拟的结果,预测了可能失去稳定性的薄弱位置。提出了一个土钉加固方案来保持基坑的稳定性,采用索结构单元来模拟土与土钉的相互作用。计算结果表明,最大的水平位移发生在基坑壁的中间部分,最小的水平位移发生在基坑壁的角落部分。土钉支护通过分散土体的内应力,显著减少了基坑的变形。通过对数值模拟结果和基坑变形监测数据的对比分析,验证了模拟结果的可靠性。提出的加固方案有利于保证广场基坑的安全,可为类似基坑工程的加固提供经验。

中空GFRP筋土钉抗拔性能研究

中空GFRP筋土钉是一种空心厚壁土钉,具有质量轻、易切割、耐腐蚀等优点。目前,对中空GFRP筋土钉的研究相对较少,对其抗拔性能和抗拔机理尚不清楚。依托北京地铁12号线北岗子站基坑工程,对中空GFRP筋土钉进行了现场抗拔试验,并采用FLAC3D数值模拟探究了不同拉拔力下中空GFRP筋土钉应力和位移沿钉长的分布规律,分析了不同内径和长度对应力分布和抗拔力的影响,对中空GFRP筋土钉的抗拔机理进行了分析。研究结果表明,拉拔过程中土钉的位移和轴力沿钉长呈指数衰减,剪应力在距钉头1.5 m处达到峰值,在距离钉头3.5 m处降至零;增大内径使土钉轴力沿钉长衰减更快,同时可提高其抗拔承载力,建议外径为32 mm的中空GFRP土钉内径取为15 mm;增加土钉长度不仅可以在一定范围内提高其抗拔力,还可使土钉轴力分布更均匀;中空GFRP筋土钉的破坏过程可分为弹性、塑性和破坏三个阶段,其破坏形式表现为拔出。

深基坑土钉墙与桩锚联合支护结构数值模拟与优化

为解决现有基坑开挖施工中存在支护桩设置不合理,造成地表沉降过大,影响施工质量的问题,开展深基坑土钉墙与桩锚联合支护结构数值模拟与优化研究。结合深基坑土钉墙与桩锚联合支护结构数值模拟得到的结果,对支护桩间隔距离、支护桩嵌固深度进行优化。采用优化后的施工方案可以实现对基坑地表沉降量的有效控制,提升工程整体施工质量。