基于固结试验的膨胀土地基变形预测方法

由广西膨胀土的固结试验,确定了广西膨胀土的体积变形指标。对依托工程膨胀土进行了现场静力触探试验,根据比贯入阻力随深度的变化曲线特征,确定了膨胀土活动区深度。给出了膨胀土裂隙开展深度的理论解,膨胀土裂隙开展深度的计算值与静力触探试验确定的膨胀土活动区深度和现场观测的裂隙开展深度基本一致。最后介绍了基于固结试验基础上的膨胀土地基变形计算方法,并在试验基础上给出了膨胀土地基变形的计算结果。以膨胀土地基变形量作为膨胀土地基的分类指标,对膨胀土地基进行了分类。分类结果对膨胀土基础选型具有重要意义。     

铁路路基下膨胀土地基浸水响应现场试验

在工程建设中,膨胀土大都属于非饱和土范畴。对高速铁路无砟轨道路基而言,膨胀土的胀缩变形可能加剧线路的不平顺性,影响高速铁路的正常运营。为研究铁路路基荷载下非饱和膨胀土土层在人工浸水后的变形特征,结合云桂铁路建设,设计并开展了铁路原型路基荷载下膨胀土地基现场浸水试验,并监测了从路基填筑开始到人工浸水结束时膨胀土地基与路基本体变形及浅层土水分的时程变化。试验结果揭示了膨胀土地基浸水饱和后的极限相对膨胀量、膨胀变形沿路基横向与地基深度的分布规律以及地基表面膨胀变形沿路基本体的衰减特征。基于试验成果,初步提出了以路基表面膨胀变形为0作为控制标准确定路基临界填高的设计思路。现场试验的设计原则与实施方法也可为今后研究铁路路基下膨胀土地基胀缩特性提供参考。     

低矮路堤下膨胀土地基现场浸水试验研究

 为研究低矮路堤下中–强膨胀土地基浸水饱和后的变形特性,依托云桂高速铁路建设,通过人工浸水方式开展不同高度等尺寸路基现场浸水试验。试验采用砂孔、砂槽及砂垫层等方式多面浸水,并同步观测路基填土期、稳载期及人工浸水期地基表面与路基表面的变形。研究结果表明：从路基填筑开始到人工浸水结束,膨胀土地基的变形曲线呈“S”型分布。沿路基横断面方向,地基表面的膨胀变量呈“锅底”型分布,路基表面的膨胀变量呈“V”型分布,地基表面的相对膨胀量随路基填高的增加而呈线性递减。通过对比不同路基填高的相对膨胀量表明,地基表面的相对膨胀量均大于路基表面的相对膨胀量,地基表面的膨胀变形沿路基本体呈衰减变化。在试验成果的基础上,初步提出以路基表面膨胀变形为0作为控制标准确定路基临界填高的设计思路。现场试验的设计原则与实施方法也可为今后研究铁路路基下膨胀土地基胀缩特性提供参考。     

Construction and field measurement of high-speed railway test embankment built on Indian expansive soil “Black Cotton Soil”

The railway embankment applied to high-speed railways is required to have high performance in terms of strength and deformation characteristics. Especially in the case of railway embankments that support slab tracks, the allowable settlement is very small. There are two technical challenges in constructing high-speed rail embankments to support slab tracks in India. The first challenge is dealing with problematic black cotton soil (BCS), which is widely distributed in India but very unusual soil in Japan. The second challenge is posed by the strict deformation requirement in the construction of the embankments. In this study, a 6 m-high test embankment was constructed on BCS in India. The deformation of the embankment and changes in water content were measured over a period of 18 months. In the construction of the test embankment, two different BCS countermeasures were applied. The results of the tests on this embankment were compared with those from an embankment without countermeasures. Complicated deformation behaviors, including settlement and the uplift of embankment, were observed in the section without countermeasure. However, in the embankment with cement-mixed gravelly soil (CGS) slab improvement with geosynthetics, the much lower amplitude of embankment deformation is evidence of the effectiveness of this countermeasure. The cohesive non-swelling soil (CNS) layer applied immediately below the embankment to reduce the water content fluctuation of BCS was not effective enough for use for high-speed railway embankment. Besides determining the technical challenges for the BCS countermeasures, the results of this study confirmed that a high-performance embankment can be constructed with Indian embankment material by performing sufficient compaction management.     

膨胀土胀缩机理及其在控制隧道沉降中的应用

因黄土的膨胀特性导致的隧道地表沉降和变形坍塌事故屡见不鲜。为探究浅埋段膨胀土隧道地表沉降和变形控制技术,结合在建太兴铁路TXXS-2小河沟段膨胀土隧道施工,采用现场监测、理论分析和现场试验研究相结合的方法,对膨胀土的特性、胀缩机理及其影响因素进行研究,结合监控量测数据结果,提出地表沉降控制技术。通过对膨胀土微结构特征进行研究,引用了膨胀土的判别函数式,对膨胀土的膨胀潜势进行了分类,根据膨胀潜势情况指导施工;研究了膨胀土强度特性,建立了强度与含水量、干密度、饱和度的关系,并对有效蒙脱石含量对强度的影响进行研究,理论上分析了晶体胀缩的根本机理;根据对膨胀土胀缩特性进行研究,分析了隧道的膨胀潜势等级和隧道特殊地质特征,通过现场土工试验,确定了小河沟隧道胀缩影响因素及其曲线关系;通过对现场施工进行监控量测,结合关系曲线和现场情况,提出了控制地表沉降的处治措施,指导现场工程施工。     

膨胀土用作路基填料的分类指标体系研究

交通部西部"膨胀土路基设计、加固与施工技术研究"项目课题组在不同地区用不同胀缩等级膨胀土直接修筑了多段路堤实体工程已取得成功经验,为促进这种处治新技术的推广并保证路基的强度和稳定性,必须建立与之配套的填料分类指标体系。为此,依据室内大量的膨胀土路用性能试验结果,归纳总结已有的实体工程修筑经验,遵循继承与创新相结合的原则,探讨采用改进的CBR试验强度、CBR膨胀量以及稠度三项指标建立膨胀土路堤填料分类指标体系,并对三种典型膨胀土开展了相关指标的试验验证。结果表明:该体系真正反映了膨胀土的本质特征及作为填料的工程特性,且指标参数的获取测试简便、快捷,能大大拓宽膨胀土用作填料的使用范围,可供编制路基新规范和工程师们进行膨胀土路堤设计与施工时参考。     

Effect of different types of wetting fluids on the behaviour of expansive soil during wetting and drying

This paper presents the results of an experimental investigation into the mechanical behaviour of an expansive soil during wetting and drying cycles. The experimental tests were conducted in a modified oedometer under two different surcharge pressures (10 and 20 kPa). During the tests, the samples were inundated with different types of wetting fluids (distilled water, saline water and acidic water). The volumetric deformation, void ratio and water content of the samples were determined during cycles of wetting and drying. The results show that the swelling potential increases with an increasing number of wetting and drying cycles. The effect of the distilled water on the swelling potential is not the same as that of the saline water or the acidic water, particularly for different surcharge pressures. The variations in void ratio and water content show that, at the equilibrium condition, the wetting and drying paths converge to nearly an S-shaped curve. This curve consists of a linear portion and two curved portions, and the majority of the deformation is located between the saturation curves of 90% and 40%.     

软土地基加筋石灰土路堤离心模型试验数值模拟

 建立以离心试验几何尺寸的有限元数值模型,模拟变加速度加载下软土地基加筋石灰土路堤中的位移、土压力、孔隙水压力和加筋拉力随时间的变化规律,并与离心模型试验结果进行比较;同时,采用该数值模型计算了不加筋、加1,2层筋时路堤和地基位移情况。计算结果表明,加筋路堤沉降量、土压力、孔隙水压力和加筋拉力的计算值与离心试验实测值吻合很好或基本一致,表明该数值模型是合理的;不加筋路堤的中心沉降量和坡脚下地基水平位移比加1层筋时明显大一些,两者在加速度为100.0 g时地面坡脚处的水平位移差值达近2 mm,而加2层筋时位移与加1层筋接近。     

膨胀土中单桩性状的模型试验研究

桩基是一种有效处理膨胀土的方法,膨胀土中桩基的工作性状会受到膨胀土浸水膨胀的影响。通过大型模型试验分析了浸水对膨胀土中单桩承载特性的影响,研究了浸水过程中单桩的上升和胀切力的变化规律。试验结果表明:由于膨胀土浸水软化作用,膨胀土中桩的侧摩阻力和承载力明显降低,而由于膨胀作用,桩中产生胀切力而使桩上升。     

膨胀土填筑公路路堤的物理处治技术

化学改良是以往膨胀土地区筑路研究和倡导(尤其是国外)的主要技术措施,但因工艺复杂、施工设备要求高、不经济而无法推广。我国西部膨胀土地区筑路往往采取弃土换填,造成土地资源浪费、水土流失生态环境破坏,同样导致工程投资大为增加。为此,在实施交通部西部膨胀土项目研究中,以广西南宁–友谊关高速公路为主要依托开展试验,同时总结在不同地区用不同类型和胀缩等级的膨胀土成功修筑多段路堤实体工程的经验,首次提出膨胀土填筑路堤的物理处治技术。通过实体工程现场跟踪监测、室内大型模型试验以及评价膨胀土路用性能的改进试验,对经物理处治后路堤土体含水率的变化、强度以及总体稳定性进行分析评价,论证了这一新技术的合理、有效性。在此基础上总结提出膨胀土路堤物理处治的技术要领及方法供工程参考。     

改良膨胀土的干湿循环特性试验研究

干湿循环作用对改良膨胀土的工程性质存在重要的影响,而目前这方面的研究成果很少。以掺石灰和粉煤灰改良的典型合肥膨胀土为研究对象,通过系统的室内试验,研究在干湿循环作用下,改良膨胀土的膨胀性、界限含水量、颗粒分布以及无侧限抗压强度等方面的变化规律,深入探讨干湿循环作用对改良膨胀土工程性质的影响。结果表明,改良膨胀土的膨胀量、液限、塑性指数、黏粒含量随干湿循环次数的增加而增大,而塑限、粉粒含量以及无侧限抗压强度则随干湿循环次数的增加而减小。     

刚性桩加固高速公路软基土拱效应现场试验研究及其与解析解的比较

用刚性桩处理高速公路软基能减小路堤沉降,提高路堤稳定性,缩短工期。土拱对路堤的承载变形性状有重要影响,到目前为止,对路堤中土拱效应的研究还不深入。本文结合某高速公路刚性桩处理软基试验段,实测了路堤填筑过程中及填筑结束后一段时间内桩帽、桩间土土压力及相应的沉降,分析了桩土沉降差、路堤高度、桩间距、桩帽大小等因素对土拱效应的影响,并与现有的几种土拱效应计算方法进行了比较。结果表明:①土拱效应的发挥程度与桩土沉降差密切相关;②当路堤较高,桩间距较小,桩帽较大时,桩体荷载分担比较大;③根据Hewlett&Randolph及陈云敏改进的土拱效应计算方法所得到的桩体荷载分担比与实测结果比较接近。     

Full-scale model tests of load transfer in geogrid-reinforced and floating pile-supported embankments

Well-designed field full-scale model tests were carried out to enhance the understanding of geogrid-reinforced and floating pile-supported (GRFPS) embankments constructed on medium compressibility soil (MCS). Two comparative test sections of GRFPS embankments with and without pile caps were built over silty clay with medium compressibility for field monitoring, over 25 months. The heavily instrumented embankments produced comprehensive high-quality data. Settlement, earth pressure, and geogrid strain measurements during embankment filling stages and the postconstruction placement stage were conducted. The influence of pile cap installation on the differential deformation and load transfer behaviour of the GRFPS embankment was evaluated. The results demonstrate the installation of pile caps can significantly improve the evolution characteristics of the stress increment ratio on the pile, facilitating a change in load sharing of the pile top from a “softening” feature to a “hardening” feature. The state of the “arching structure” heavily depends on the relative displacement. After the maximum arching is formed, although the subgrade load continuously increases, the arching enters the damage and recovery state, and the transfer of the overburden load increment is largely conducted by the tensioned membrane effect.     

改良膨胀土填料的综合试验

从室内配比试验、填筑试验、质量检测、动态试验及变形观测等角度对膨胀土改良后作为路堤填料的适宜性进行了系统研究。对膨胀土改良后作填料的的设计、填筑施工及质量检测方法、标准提出了建议。通过近4年的开通运营证明合宁铁路膨胀土地区采用改良后的膨胀土作为路堤填料是成功的。     

Practical seismic fragility estimation of Japanese railway embankments using three seismic intensity measures

This study proposes a practical fragility estimation equation for Japanese standard models of railway embankments using the peak ground acceleration, peak ground velocity, and Arias intensity. The analytical models were implemented as unreinforced and geosynthetic-reinforced embankment models. A sensitivity analysis of the seismic fragility estimation of the embankment models was conducted on various embankment heights, average values of friction angles in the backfill soil, and tensile strengths of the primary reinforcement. Consequently, a unique formula for the fragility function in the presence of different seismic intensities was successfully presented. The parameters of the fragility function were successfully estimated using commonly used design parameters, such as the embankment height, average value of the friction angle of the backfill soil, and average value of the tensile strength. Additionally, another sensitivity analysis using different seismic databases was conducted to explore the effect of the seismic database on the fragility curve estimation of railway embankments. As a result, using different seismic databases, different fragility curves were obtained. These results highlight the importance of checking the sensitivity of the seismic database when developing the fragility curve.     

硫酸盐侵蚀水泥改良路基段上拱研究

首先对某高速铁路无砟轨道路基上拱问题进行了资料调研,通过现场变形监测、填料取样试验、矿物成分分析等手段,明确了硫酸盐侵蚀水泥改良填料膨胀是引起路基上拱主要原因,然后结合化学反应机理和室内模拟试验,对发生此类膨胀变形的反应条件、膨胀变形特征进行了分析。研究结果表明,膨胀变形主要发生在路基水泥改良填料层位,该土层具有大量硫酸盐侵蚀水泥产物钙矾石和硅灰石膏发育的特征,路基上拱与硫酸盐侵蚀水泥改良土形成钙矾石和硅灰石膏晶体相关;水泥改良填料中相对潮湿的高PH值碱性环境和石膏等硫酸盐矿物的存在是发生此类膨胀变形的必要条件;硫酸盐侵蚀水泥改良填料将发生持续性膨胀变形,上拱持续时间可达数年,破坏性强。本文研究成果可供类似环境下路基工程的建设及相关研究借鉴。     

Full-Scale Embankment Consolidation Test using Prefabricated Vertical Thermal Drains

In this study the field feasibility of an innovative thermal technique to improve the performance of prefabricated vertical drains (PVD) used in conjunction with the preloading ground improvement method is investigated. For this purpose, two identical 6.0 m high full-scale test embankments for preloading were constructed over the soft Bangkok clay where a conventional PVD system was installed underneath one embankment and a novel prefabricated vertical thermo-drain (PVTD) system was utilized for the other. The PVTD unit consists of a U-tube made of cross-linked polyethylene plastic (PEX) that is attached to a conventional PVD unit. Preheated water at about 90°C is circulated through the attached U-tube to raise the soil temperature underneath the PVTD embankment. The behavior of the two test embankments were compared in terms of excess pore water pressure and consolidation results. The comparison shows the advantage of a PVTD system over a conventional PVD system. The rate of consolidation increases significantly in the PVTD system due to the temperature effect on the hydraulic conductivity. Moreover, the embankment with the PVTD system generates more settlement due to the thermally induced irreversible contraction of saturated normally consolidated soft Bangkok clay.     

A case study of geotextile-reinforced embankment on soft ground

Full-scale test embankments, with and without geotextile reinforcement, were constructed on soft Bangkok clay. The performances of these embankments are evaluated and compared with each other on the basis of field measurements and FEM analysis. The analyses of failure mechanisms and the investigations on the embankment stability using undrained conditions were also done to determine the critical embankment height and the corresponding geotextile strain. The high-strength geotextile can reduce the plastic deformation in the underlying foundation soil, increase the collapse height of the embankment on soft ground, and produce a two-step failure mechanism. In this case study, the critical strain in the geotextile corresponding to the primary failure of foundation soils may be taken as 2.5–3% irrespective of the geotextile reinforcement stiffness.     

引江济淮工程膨胀土水泥改性剂量研究

膨胀土具有遇水膨胀、失水收缩的工程特性,掺加一定量水泥对其进行改性是处理膨胀土的主要方法之一。选取引江济淮工程弱膨胀土,通过对素膨胀土和改性土的自由膨胀率、界限含水率及无侧限抗压强度的试验分析,揭示了水泥掺量和养护日期对引江济淮工程弱膨胀土的物理力学性质的影响。试验结果表明:(1)随掺灰率的增加,改性膨胀土的自由膨胀率、液限、塑性指数均降低,无侧限抗压强度增加;(2)随养护日期的增加,自由膨胀率、液限、塑性指数减小,无侧限抗压强度增大;(3)基于本研究试验结果,同时综合考虑工程实际,建议引江济淮工程弱膨胀土水泥改性剂量为4%。     

中膨胀土路堤包边方案及其试验验证

中膨胀土具有较强的吸水膨胀软化特性，其CBR值低于3％，不能满足路基对填料的强度要求，若用作路基填料必须进行处理。对中膨胀土路堤包边方案进行深入论证，对中膨胀土包边路堤边坡稳定性、路基强度与变形及石灰包边层的保湿作用进行分析。分析结果表明：中膨胀土包边路堤具有较好的浅层稳定性和整体稳定性；包边条件下，填土具有较高的强度和较低的变形，满足路用要求；包边层具有良好的防风化和保湿作用。在此基础上，提出中膨胀土路堤包边方案，并通过现场试验进行验证，试验验证表明，石灰包边方案能经受当地自然条件和气候条件的影响，能保证路基的稳定和安全营运。