重塑粘质黄土冻胀敏感性试验分析

以沈哈高速铁路沿线具有代表性的粘质黄土为研究对象,在室内冻胀试验条件下,研究了非饱和含水量、冷端温度、冻结速率、补水条件以及冻融循环对土体冻胀特性的影响。试验表明,在封闭系统下,非饱和土体冻胀系数随含水量增大而增大,且最终趋于一个稳定数值;封闭系统下,随着冷端温度的降低,含水量较大土样的冻胀系数逐渐减小,含水量较小土样的冻胀系数逐渐增大;开放系统下,土样冻胀系数随冻结速率的减小逐渐增大,且增幅越来越大;外界补水条件下,土体冻胀量增加显著,但随含水量的增大,其影响逐渐减弱;干密度较小土样的冻胀总变形随冻融循环次数的增加呈指数递减的趋势,干密度较大土样则呈指数增加的趋势。     

饱和粉土冻胀过程试验研究及数值模拟

 为了研究温度梯度、冻结速率、土样高度和竖向压力对冻胀的综合影响,在不同试验条件下对饱和粉土进行了一系列一维土柱冻胀试验,试验中测量土样温度、竖向压力、冻胀量和补水量。试验结果表明：在温度场处于准稳态时,冻胀速率与温度梯度成正比例增长关系,冻胀速率随着竖向压力增大而线性减小;定义比冻结速率为冻结速率与土样高度之比,当温度场处于瞬态时,冻胀速率随比冻结速率成幂函数增长,增长率随着比冻结速率增大而减小。提出了综合考虑温度梯度、冻结速率、竖向压力和土样高度的冻胀计算公式,在此基础上推导了考虑水分迁移的冻土热扩散方程,提出了模拟冻胀和温度变化的数值模拟方法,通过一个数值算例计算了冻结过程中的温度和冻胀量变化,并与实测结果对比验证了该方法的可靠性。     

辽西地区风积土冻融特性与冻融过程结构性演变规律试验研究

 介绍新型冻融试验装置及其应用情况,并利用该试验装置对辽西地区典型风积试样在不同温度梯度、荷载和水分补给条件下进行冻融试验,从而揭示风积土冻胀与融沉规律。试验结果表明：随着温度梯度的增大,最大冻胀量和融沉值有不同程度的减小;无论在封闭系统还是在开敞系统中,外力的存在都对冻胀起到抑制作用,融沉值也会随着荷载的增大而减小;外界水分的补给会使最终冻胀量和融沉值都显著增加。风积土在冻融作用下产生冻胀融沉变形,宏观力学性质和微观结构也发生相应的变化,主要是由于土的结构性得以改变,也就是冻融作用破坏了土颗粒间的连接力,同时使土颗粒得以重新排列。将冰率作为结构性参数,针对不同水分场和应力场对辽西地区风积土冻融过程结构性演变规律进行研究。     

单向冻结时开放条件下饱和砂岩冻胀试验及THM耦合冻胀模型

为研究寒区岩石在梯度温度场中补水条件下的冻胀变形规律,进行了单向冻结时开放条件下饱和砂岩冻胀试验。试验结果表明,单向冻结时开放条件下饱和岩石冻胀过程中,沿冻结方向的冻胀位移变化过程可分为冷缩阶段、原位冻胀阶段、分凝冻胀阶段3个阶段。分凝冻胀阶段冻结锋面趋于稳定,冻胀变形持续增长,与时间基本呈线性关系。此外,分凝冻胀阶段补水量换算的迁移水分凝冻胀位移与冻结方向冻胀位移比较接近。随着平均温度梯度增大,分凝冻胀变形速率增大,且分凝冰位置与平均温度梯度线性相关。然后,建立了考虑孔隙水原位冻胀与迁移水分凝冻胀的THM耦合冻胀模型。模型中,孔隙水原位冻胀计算基于未冻水含量,并引入约束系数表征岩石骨架对孔隙水冻胀约束程度;迁移水分凝冻胀计算基于分凝势理论,水分迁移速率与冻结缘处的温度梯度成正比。模型计算结果与试验结果对比表明,建立的THM耦合冻胀模型能够比较准确地计算单向冻结时开放条件下饱和岩石冻胀位移,并能够模拟出分凝冻胀时分凝冰层引起的位移突变及分凝冰位置,可用于寒区冻胀敏感性岩石开放条件下冻胀变形计算。     

动静组合加载下颗粒迁移与沉积特性试验研究

悬浮颗粒在路基中迁移和沉积运动规律研究对揭示翻浆冒泥病害孕育、发展与致灾机制具有重要的理论意义和应用价值。通过开展动-静组合荷载下分层砾石-砂粉土柱翻浆试验,探究了动荷载加载强度、静荷载间歇时长、以及动静组合加载重复次数等对颗粒迁移和沉积行为的影响规律,并结合试样动水力响应特征分析了诱发颗粒迁移的驱动机制。试验结果表明：颗粒在动水力作用下发生悬浮迁移而在静荷载段受重力作用发生沉降,从而造成砾石层内泥浆浊度呈现波动型增长特征。随动静组合加载重复次数增加,每一个重复周期内的颗粒悬浮增量趋于降低,且悬浮颗粒逐渐堵塞砾石孔隙并在一定程度上抑制翻浆发展。提高动载频率和动应力会进一步降低砂粉土表层的内部稳定性,但相比于增大平均动应力,提高加载频率对增大颗粒迁移量和竖向迁移距离更加显著。     

冻结法施工中的冻土特性试验研究

为获得冻结法施工中土体冻胀融沉特性规律,以某地下联络通道工程为原型,根据相似理论,进行了水平冻结模型试验。结果表明,冻胀融沉过程中,土体温度先迅速降低后升高,维持在0℃一段时间后,继续缓慢升高至室温;土压力值先增加后减小,其中,竖向土压力值随深度的增加而增大,相同埋深下,距冻结管越近,水平土压力值越大;土体融化固结沉降值明显大于冻胀位移值,土体竖向位移较水平位移变化显著。积极冻结期内土体温度降低速率变慢,且埋深越大、距冻结孔越近,土体温度降低越快、降幅越大;无侧限土体压力值先增加后减小,侧限土体压力值则逐渐增大,全封闭土压力值变化率更显著。     

列车荷载下螺杆桩复合地基动力特性及承载性状试验研究

通过进行模拟列车荷载下铁路螺杆桩复合地基的室内模型试验，测试在列车多运作模式下桩–土系统的动应变、动土压力、累积位移特征；分析螺杆桩–土系统在列车荷载下的动力响应时频域特性，并进一步探讨在不同时速条件下的桩–土受力变形程度的相关性。基于线性拟合方法建立桩土动应力比与列车运行时速的经验关系。结合桩–土荷载分担关系、桩体动轴力及桩侧动摩阻力的综合考虑，揭示列车动荷载下螺杆桩–土的相互作用关系及荷载传递机制。结果表明：(1)在列车动载作用下，螺杆桩螺纹段变形程度大于直杆段，尤其在变截面处易产生变形破坏。(2)在普速、快速、高速行驶条件下优势频率分别集中在2～6,8～11,14～16Hz，随着列车时速的增大，主导频率趋近于列车自振频率，频带宽度减小。(3)螺杆桩复合地基的动力特性与列车运行时速存在高度相关性。(4)螺杆桩复合地基加固作用下，铁路地基临界速度在280 km/h左右，相对普通桩加固地基提高了29.62%。(5)列车运行时速越快，在桩土复合地基中土体承载发挥作用变大，而桩体承载发挥作用减小，造成桩土协同作用性能降低。研究成果对铁路螺杆桩复合地基的设计优化及列车运营时速的规划具有一定参...     

冻融与荷载作用下土体内部孔隙水压力、水分变化规律及其模型试验研究

 在季节冻土区,反复冻融作用导致土体孔隙水压力发生变化,进而导致土体中水分的迁移;对土体施加静荷载,土体内部应力场发生变化,土体内的水分和孔隙水压力亦发生变化。通过模型试验,研究无荷载和荷载两种条件下土体内部孔隙水压力和水分场随冻融循环作用的变化规律,试验结果表明：在荷载和无荷载两种条件下,孔隙水压力在初期呈负值稳定变化,然后快速增大,最后呈周期性变化。在无荷载条件下,随着土体深度的增加,孔隙水压力增大,水分含量减小;在荷载条件下,土体上部和中部位置处的孔隙水压力和水分含量都大于荷载两侧。在一个冻融周期内,土体内部孔隙水压力和水分含量都随温度的升高而增大,随温度的降低而减小,而且孔隙水压力和水分含量随温度的变化都具有滞后性。     

冻融循环后高温冻结粉土在循环荷载下的动力特性试验研究

对经过10次冻融循环的青藏人工冻结粉土试样在-1℃恒温状态下进行了3Hz和5Hz两种加载频率、不同动应力幅值的单轴循环动荷载压缩试验。试验结果表明:冻融循环后高温冻结粉土的累积应变随着振动次数的增加呈增长的趋势,在相同的振次时,动荷载应力幅值越大,累积应变越大;频率越高,临界动应力越小;初始阶段应变速率随时间的增大而减小,但随着振动时间的增加,破坏型曲线的应变速率达到一个最小值后开始增大,稳定型曲线的应变速率则在一定幅度的范围内波动;在振动时间相同时,动应力幅值越大、动荷载振动频率越高,应变速率越大;动应力幅值越大,破坏应变越大,破坏时间越短;同一动应力幅值下,频率越高,破坏时间越短。     

重载列车作用下桥墩-桩基竖向动荷载特性试验研究

为探明大轴重、长编组和高密度重载列车作用下桥墩和桩基承台顶的动荷载特性,开展不同车速和轴重条件下的桥墩-桩基体系竖向动荷载响应现场试验。分析桥墩动荷载曲线的时域、频域特征,采用Kolmogorov法对动荷载幅值进行正态性检验,建立列车参数与动荷载频域特征的联系;基于3σ原则对不同敞车作用下的动荷载特征值(均值、均幅值和均峰值)进行统计,结合实测数据建立重载列车作用下的墩顶动力系数计算公式,并提出墩顶和桩基承台顶的荷载谱公式及其参数取值。结果表明:动荷载响应在时域上可分为快速增加,周期波动和快速衰减3个阶段,在频域上服从倍频规律和幅值调制效应,0～5Hz频段激振对动荷载起控制作用;动荷载幅值服从正态分布,动荷载均峰值和振动能量沿墩身衰减明显,动荷载均值和均峰值随轴重增大而增大,均幅值随轴重的增加而减小;建议采用φ=1+0.0045v计算墩顶动力系数,采用三角级数式拟合桥墩荷载谱效果显著。研究成果可为重载列车作用下桩基的长期承载变形性状及环境振动评估的研究提供关键参数和可靠检验样本。     

土的冻胀机理研究

一个多世纪以来人们对有关土的冻胀机理问题进行了大量的实验和研究,提出了很多假设,取得了很大的成果。文中从土的冻胀特性入手提出了"起始冻胀力"、"冻胀-抽吸力"等新概念,并提出了冻胀力才是冻结过程中水分迁移的驱动力的新概念。     

几种典型土料冻胀敏感性试验研究

对三种典型土料进行不同含水量、饱和度和干密度条件下的一系列闭式冻胀模拟试验。研究了土的η-ω、η-γd关系,并对其进行线性分析,得出试验土料的冻胀规律。根据所得η-ω关系式,得出各种土料相应于各冻胀性分级的界限含水量值。进而分析各种土料的冻胀敏感性,并与有关文献进行了对比分析。     

Hypothesis Concerning Distribution of Normal Forces of Frost Heaving across the Lower Surface of Solidly Frozen Soil Layers beneath Foundations

Existing schemes of interaction between heaving soils and immovable foundations are analyzed. Individual deficiencies are noted for use of results of these investigations in determining the uplift of shallow foundations on heaving soils, and the possibility of determining additional forces in foundations subject to nonuniform heaving of soil. A hypothesis regarding the distribution of normal forces induced by frost heaving against the lower surface of solidly frozen layers of soil beneath foundations with allowance for their strength and observance of equilibrium between the loads on the foundations and the normal forces induced by frost heaving, and also an engineering method for their determination are proposed.     

兰新线路基土冻胀特性试验分析

为了研究兰新线路基土冻胀特性,进行了不同含水量、不同压实度的封闭系统冻胀试验和开放系统冻胀试验。试验结果表明,在封闭系统下冻胀量随含水量的增大而增长,随压实度增加而减少;开放系统冻胀量远大于封闭系统冻胀量,且与初始含水量有关;在冻结的过程中,冻土中的水分出现了重分布现象。     

Experimental checking of stability of underground pedestrian crossings on heaving soils

Conclusions 1. Underground crossing structures on natural soil bases may be used also for heaving soils provided their strength and rigidity are increased, taking into account the nonuniform deformations. It is recommended that a value of 0.0015 be used for the limiting allowable coefficient of relative deformation nonuniformity of tunnels.2. If the tunnel is considered as a reinforced concrete underground frame buried in the soil, then for strength, crack-resistance, and deflection design under the normal frost heave force, this force should be assumed to be equal to the soil reaction in the tunnel base under all the permanent and short-duration loads, taken in accordance with SNiP II-D.7-62; also, account must be taken of the friction force caused by the backfill sand on the tunnel lateral surface.3. For freezing and thawing of the soils in the bases of underground crossings, the plastic deformations are insignificant, i.e., the frost heave is practically equal to the settlement under thawing.4. The weak correlation between the coefficient K_n and the heave indicates that measures intended to reduce the depth of freezing (heave) by heating the bottom or by partially replacing the heaving soils by nonheaving under deep seasonal freezing cannot, per se, ensure crack resistance of the crossing structures.5. During freezing of soils in the bases of crossings, processes of heaving and shrink-age occur. For hard and medium hard consistency, with W_PL, the shrinkage may be greater than the heaving, which should be taken into account for determining the depth of the foundations under deep seasonal freezing conditions.Omskgrazhdanproekt. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 10–12, May–June, 1981.     

A dynamic gradient ratio test apparatus

The soil-geotextile filtration mechanism is a complex process which depends on physical compatibility between the geotextile and the soil to be retained. Several methods have been proposed by researchers for assessing the filtration behaviour of soil-geotextile composite systems under steady state conditions. The Gradient Ratio (GR) test is the most commonly used method for measuring filtration compatibility of soil-geotextile systems. This paper describes the design of a modified GR permeability test apparatus to overcome disadvantages associated with traditional GR test devices. The apparatus can perform filtration tests under static and dynamic conditions and can be used to evaluate the filtration compatibility of fine-grained soils with geotextiles. The apparatus is incorporated within a triaxial testing system, hence representative field stress conditions can be applied to test specimens. Some exemplar GR tests performed on coarse and fine-grained soils with a non-woven geotextile are presented in this paper. Unidirectional dynamic loads are applied within the filtration tests to replicate highway traffic loading. Test results show that dynamic loading affects the filtration behaviour at the soil-geotextile interface by increasing the fine particles migration towards the geotextile, but that, for the soil evaluated here, this effect was small.     

Calculation of the optimal thickness of a sandy-gravel cushion in bed soils prone to frost heaving beneath the shallow foundations of low-rise buildings

A method of calculating the optimal thickness of the cushion beneath shallow foundations installed on bed soils subject to various degrees of heaving is cited for low-rise construction. Design parameters are given for the frost heaving of soils; they take into account the load due to the building, the moisture conditions of the soils, and the heaving-deformation diagrams. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 23–27, May–June, 1999.     

地铁双线隧道水平冻结位移场的模型试验

 针对地铁双线隧道水平冻结工程,基于相似理论,建立一套完整的物理模型试验系统。在保证冻结壁强度相似前提下,重点对双线隧道冻结暗挖过程中的冻胀和融沉位移场进行试验研究。试验结果表明,冻结壁的交圈时间与土层冻胀位移规律有着密切联系,先行冻结的上行线隧道由于水分迁移现象较强,使得土层冻胀位移相对较大。试验对比分析冻结壁2种不同解冻方式,发现冻结壁强制解冻条件下土层融沉现象较弱,强制解冻条件下近地表土层融沉位移仅为自然解冻条件下近地表土层融沉位移的47%,说明采用强制解冻方式可减小土层融沉。     

Effect of frost-heave forces of soils on the bearing capacity of pyramidal piles

Conclusions 1. Foundations formed from short pyramidal piles can be used in soils slightly prone to heaving without any restructions placed on bearing capacity, and should never be used in soils highly prone to heaving. 2. The installation of foundations formed from short pyramidal piles 3–4 m long in soils moderately prone to heaving with a heaving modulus m ≤ 7 is possible in principle. In this case, however, the load on the pile should be limited to the value determined by computation as a function of the frost depth of the soil and the assigned length of the pile in conformity with the method outlined in the paper. 3. Since the results obtained and the conclusions drawn apply to piles only of three of the most widely used type sizes, and their investigation was limited only to three experimental sites, questions raised in the paper will require further study. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 6, pp. 8–11, November–December, 1989.