Relations of calculated and actual settlement of a thawing foundation bed

Conclusions 1. The dependence of relative compression of thawing soil on pressure is nonlinear, and it is in agreement with views on the nature of deformation of thawing soil. A dominance of natural pressure in the total load on a foundation bed permits one to use the method of unit summation in calculating settlement of nonlinearly deformable soil. Such calculations, as compared with the linear formulas employed in SNiP, give better agreement with actual settlement.2. In calculating settlement of thawing foundation beds, we need differential consideration of soil compressibility with depth, since the combination of genetic features of frozen ground and the increasing natural pressure with depth may create a different inhomogeneity in the foundation bed.Yakutniproalmaz, Mirnyi. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 18–21, January–February, 1975.     

Investigation of the compressibility of thawing soils by means of a pressure meter

Conclusions 1. The three-dimensional compressibility anisotropy of a thawing soil is determined by the cryogenic constitution of the frozen soil and affects primarily the value of the coefficient of thawing.2. The generalization of the law of compressive compaction of a thawing soil leads to stress-strain relations containing a parameter which characterizes the initial compressive deformations in the absence of an external load. The concept of the coefficient of thawing as a volumetric strain leads to equations in which the component of initial compression is present independently of the sign of the strain. The first soil model corresponds to collapse and to brittle fracture, and the second, to volumetric contraction of the soil.3. The model of a structurally unstable thawing soil corresponds closely to the nature of its deformations and permits formulating the pressure-meter problem for a geometrically anisotropic thawing soil.4. Pressure-meter tests permit determining all the deformation characteristics of a thawing soil.Yakutniiproalmaz. Moscow Civil Engineering Institute (MISI). Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 17–21, July–August, 1975.     

冻土融沉系数的经验确定方法

冻土融沉系数是计算冻土融化后沉降量的重要参数,也是多年冻土融沉性分类的一个指标.本文根据345个冻土原状样品融沉压缩试验数据资料,对细砾土、砂土、粉土、粘性土、泥炭化粘性土和泥炭质土这六类土,分别得出了估计其融沉系数a0的回归分析方程.同时还简单论述了采用超塑含水量(w-wp)因子估计山区粘性土融沉性的效果及原因,分析...     

An observational method for consolidation analysis of the PVD-improved subsoil

Surcharge preloading together with prefabricated vertical drains (PVDs) have been widely used to improve properties of thick clay deposits. To assess the performance of soil improvement works, the average degree of consolidation needs to be estimated. A curve fitting formula is proposed in this paper to simulate the degree of consolidation versus a non-dimensional time factor relationship. The proposed formula fits the theoretical consolidation solutions well with a regression coefficient R² larger than 0.9996 and an error of less than 1.2%. Based on the formula, a modified Asaoka's observational method is proposed to predict the ultimate settlement and calculate the coefficient of consolidation using field settlement monitoring data. The effectiveness of the proposed observational method has been verified using some well-documented case histories. Comparisons between the proposed method and the Asaoka's method indicate that the proposed method will give a less than 1.0% higher ultimate settlement than that by the Asaoka's method and the proposed method is able to predict the c_h value with the consideration of both vertical and horizontal flow through the ratio of time factor in horizontal and vertical direction ν_hv.     

基于动态压缩模量的路基工后沉降计算研究

基于太沙基单向固结理论,结合土体变形计算公式,推求饱和土压缩模量的表达式.研究饱和土体压缩模量随固结度、固结系数和排水路径的变化规律,并分析了压缩模量的时间效应及其在路基工后沉降预估中的应用研究.结论表明：1）固结初期,压缩模量随固结度增加呈缓慢线性增长;固结度U ＞60％时,压缩模量增长速度加快;固结后期,压缩模量增加很少;2）固结时间相同,压缩模量随固结系数增加而增加;固结系数Cv＜5000 cm2/a时,压缩模量几乎不变;3）双面排水时的压缩模量增长速度远大于单面排水;4）压缩模量随时间呈近似线性增长,且存在上限值;利用动态压缩模量可有效预估路基工后沉降.     

海积软土渗透性变化及固结分析

如何提高软土地基固结沉降计算的准确性是当前急需解决的难题之一。海积软土性状特殊,具有固结变形量大,固结过程中压缩性、渗透性发生较大变化的特性。针对海积软土,研究其固结过程中渗透固结性质的变化规律是提高其地基固结沉降计算准确性的关键。本文以深圳前海湾海积软土为例,通过试验和分析,对目前常用的软土渗透性计算公式是否适用于海积软土进行了检验,提出了适合海积软土渗透系数计算的经验公式,在此基础上进行了考虑固结系数变化时的一维固结计算,用试验数据对计算得到的固结曲线的合理性进行了验证,并将计算结果与工程中基于Terzaghi固结理论的分析结果进行了比较。试验结果和计算结果表明,所提出的渗透系数计算公式对海积软土合理,方法实用;海积软土固结过程中固结系数变化较大,应发展考虑固结系数变化时的固结分析方法;工程中进行海积软土固结分析时应根据实用固结压力情况及不同的试验方法合理地取用固结系数。     

浙江近海软黏土次固结系数研究及应用

我国室内固结试验中把次固结沉降曲线作为线性处理,然而,大量的实验数据证明,测定周期的不同次固结曲线呈非线性特征,故次固结系数为变数。通过我国近海典型海相软黏土——浙江平阳港次固结试验为验证实例,提出一种适用于工程的分级加载、延时校正及保持试样结构原状的土工快速测试法。对工程中试验曲线分析及研究表明:我国近海区域软黏土次固结系数与固结压力、压缩指数以及测试时间存在着一定的相关性,次固结系数与固结压力变化呈先增、后降、趋稳的现象,次固结系数峰值与土体深度等有关,C_a/C_c比值为常数。实验中所得到的经验值可作为邻近工程固结沉降量预估,所创方法为填补国家标准规范次固结试验的空白提供有益参考。     

渗透特性及填筑速率对路基固结的影响

计算软土路基沉降的一般方法没有反映路基渗透系数的变化以及填筑过程的时间效应影响,而大多采用同一渗透系数和一次加载的方式分析路基沉降。笔者采用ABAQUS有限元软件在考虑软粘土渗透系数随孔隙比变化经验公式以及不同填筑速率的基础上分析了软土路基的固结沉降,并进一步研究了填筑阶段路基内竖向有效应力随时间的变化。结果表明:(1)固结随时间的变化跟填筑过程有相关性;(2)渗透系数随深度非线性分布对路基的固结影响不可忽略;(3)加快填筑速率使得竖向有效应力增大的速率加快,沉降也加快;(4)填筑速率对固结沉降的影响主要集中在填筑阶段,对工后沉降影响不大。     

长江口软土的次固结特性试验研究及应用

对长江口淤泥质软土进行一系列固结实验,研究了荷载、超固结比、土性指标等对软土次固结系数的影响,得出了该区域软土的次固结特性及不同土层的次固结系数。试验结果表明:次固结系数与荷载有关,且随着荷载的增大先增大后减小,最后趋于平稳;次固结系数与最大固结压力有关,且随着超固结比的增大而减小;次固结系数与压缩指数的比值近似为常数0.031;次固结系数与初始孔隙比及初始含水率呈线性关系。提出了一种考虑主次固结同时发生的沉降计算方法,并通过所得次固结系数对横沙围堤进行了沉降预测,与现场监测吻合较好。所提出的计算方法更符合工程实际需要,具有很好的实用价值。试验所得的相关参数及结论对研究长江口软土特性及后期工程的沉降预测具有一定参考意义。     

Investigations of negative skin friction on piles and suggestions on its calculation

Conclusions The investigations showed that negative frictional forces appear at the time of occurrence of continuous settlement of the soil relative to the pile, but for their maximum development this settlement should be relatively large, of the order of 5 cm and more. In addition, it was established that the negative frictional forces act only during active displacement of the soil surrounding the pile as a result of its consolidation or other causes. After this displacement stops the negative frictional forces practically disappear.The established physical characteristics of the effect of negative frictional forces permitted taking the presence of an excess of the rate of soil settlement over the rate of pile settlement as the criterion of their occurrence. The suggestions on calculating piles worked out with consideration of this criterion allows in a number of cases disregarding the negative frictional forces in the effective design load or taking them to be considerably less than by the previously known calculation methods.Scientific-Research Institute of Bases and Underground Structures. State Planning Institute for General Construction and Sanitary-Engineering Planning of Industrial Establishments, Kiev. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 11–15, July–August, 1974.     

塑料排水板加固软土地基的若干问题

本文介绍了几个典型塑料排水板加固软基的设计、施工实例 ,对软土地基加固设计的固结系数选取及固结系数各向异性的特点、利用实测沉降曲线推算最终沉降量方法、加固深度等问题进行了讨论 ,通过对大量的现场检测资料反分析与实验室结果进行统计分析 ,提出了软土地基加固设计时固结系数选取的经验方法 ,分析了施工因素对固结效果的影响。     

高速公路软土地基次固结系数影响因素研究

就软土次固结系数的影响因素进行了分析,总结了应力水平、应力历史、加荷速率、应力比等因素对次固结系数的影响,以及次固结系数与压缩指数和塑性指数的关系等诸多方面的研究成果,以期对软弱土地区的高速公路地基工后沉降问题提供参考。     

Improving settlement and reinforcement uniformity of marine clay in electro-osmotic consolidation using microbially induced carbonate precipitation

This study explored the effect of microbially induced carbonate precipitation (MICP), a natural bio-geotechnical process, on mitigating the uneven settlement in electro-osmotic consolidation of soft soil. The real-time drainage and settlement were measured, and the control behavior of drainage to settlement was discussed. MICP solution components were also selected as the different additives to determine the control mechanism of MICP in improving settlement and reinforcement uniformity of clay. After the tests, the chemical properties and microstructure were analyzed according to pH, conductivity, and SEM. The addition of MICP solution in clay significantly even reduced the coefficient of settlement variation by 53.2%, and the upper surface profile tended to be uniform. Contrary to control, the coefficient of settlement variation of MICP-treated soil decreased gradually with drainage volume, mainly due to the filling of solid substances such as calcium carbonate, biofilm, and/or calcium hydroxide produced within soil pores. MICP significantly improved the uneven soil reinforcement generated during the electro-osmotic consolidation but resulted in the lower strength near the anode due to the less drainage. The contribution of MICP solution components to the improvement of settlement and strength uniformity obviously varies. Bacterial cells improved the settlement uniformity but had no effect on the strength improvement of soil. The co-existence of Ca²⁺ and bacterial cells maximized the modification effect, which determined the production of mineral precipitation. Microstructure observation proved the formation of calcium carbonate. The results demonstrated that MICP is an effective technique to improve the settlement and reinforcement uniformity of marine clay in electro-osmotic consolidation.

     

从实测沉降过程推算固结系数

按照Terzaghi固结理论,直接从土体压缩时间过程定出控制孔压消散速率的固结系数,暗含固结度U_σ和压缩度U_ε等同的假设。由于土的应力应变关系是非线性的,所以U_σ和U_ε并不等同。在应力增量（△p/p）较大时,特别是对于高压缩性的软粘土来说,上述假设可能引起相当大的误差。本文采用双曲线型和半对数型的压缩曲线,推导出固结度U_σ和压缩度U_ε之间的两种解析关系,并提出从实测沉降过程推算现场土层平均固结系数的方法。经工程检验,证明从双曲线型压缩曲线求出的U_σ和U_ε之间的关系优于从半对数型压缩曲线求出者。最后指出,只须对古典固结理论中的线性化假设作出适当修改,从假设固结过程中土的压缩性m_υ和透水性k不变,改为假设固结系数C_υ或C_h不变（k/m_υ=常数）,则在采用非线性应力应变关系时,仍能运用古典理论进行计算。这样,就可避免那种明显不合理的U_σ和U_ε等同的假设,使实用时的误差大大减小。     

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.     

Field investigations of the deformation of slumping soil under experimental plates

Summary 1. As a result of these investigations, a graphic concept was obtained concerning deformation of slumping soil of natural structure and comparatively high water content (a coefficient of saturation equal to 0.58) in settlement. The boundaries of the deformable zone, obtained by two methods, mainly by means of indicators and by determining the soil density, coincided.2. The character of the deformation of slumping soil in the bed of the plate during settlement is similar to the previously obtained data on slumping of soil under plates [2]. Settling of the plate in the experiment occurred as a result of compaction of the soil within a rather limited deformable zone without upward yielding of the soil.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 7–8, May–June, 1965     

高含水率疏浚泥轴对称大应变固结模型

高含水率疏浚泥在外加荷载作用下通常产生大应变固结变形,不适用于传统的Barron轴对称小应变固结理论。为此,基于Gibson一维大应变固结理论和Hansbo径向固结理论,摒弃小应变假定,考虑高含水率疏浚泥的材料和几何非线性、径竖向渗流等因素,建立了等应变条件下以孔隙比为变量的轴对称大应变固结模型ALSC,Gibson、Hansbo、Kjellman等建立的固结方程是该模型的特例。基于有限差分法,编制了计算程序,进行了ALSC模型与小应变模型的数值模拟,验证了ALSC的有效性。研究结果表明:土体变形较小时,ALSC模型与Barron模型计算的固结度和超静孔压数值基本吻合;土体变形较大时,ALSC模型与"Barron+Terzaghi"理论计算的最终沉降量和固结速率取决于土体的固结参数;当C_c/C_k=1时,ALSC模型的最终沉降量小于"Barron+Terzaghi"理论,但二者固结速率相当;当压缩系数av保持不变,ALSC模型(C_k=1)与"Barron+Terzaghi"相比,最终沉降量大,固结速率慢。     

Experimental thawing of permafrost soils in bases of reconstructed main building of the Chita-I Thermoelectric Power Plant

Conclusions 1. Compaction of thawed soils takes place directly during the thawing process or in the first 10–35 days after it. The thawed soils do not require subsequent compaction and, as regards the load-carrying properties, they are comparable to similar rocks in a naturally thawed state.2. The specific electric energy consumption for thawing 1 m³ of permafrost soils in bases is 25.2 kWh, and it is a fundamental characteristic in the design of industrial thawing of soil bases for determining the spacing between the heaters their capacity, and the thawing period.3. In test thawing, the number of ground mark vertical lines and the number of marks in each vertical line should be determined from the degree of heterogeneity of the frozen soils as regards the collapsibility in plan and section. For permafrost soil conditions similar to those of the construction site of the Chita-I plant, the optimal solution is to lay one vertical line per 100–120 m² of thawing area, and to use in each vertical line not less than one mark per 5 m of section of thawed permafrost soils."Atomteploelektroproekt" Institute, Novosibirsk. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 9–10, September–October, 1986.     

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。     

软土变形时效特性的试验研究

软土变形的重要特征为具有时效性,这将导致软土工程的工后沉降。根据实际工程需要,采用原状土样和扰动土样对软土变形机理进行了一系列室内试验,包括软土的应力–应变特性、固结效应、次固结特性、蠕变特性等。通过试验成果系统分析可得:(1)初始固结度对应力–应变关系的影响;(2)次固结系数与固结压力的关系以及应力历史对次固结系数的影响;(3)次固结系数与压缩指数的关系;(4)排水蠕变和不排水蠕变的变形特征;(5)蠕变变形的影响因素和降低蠕变效应的技术路径等。研究结果表明:软土变形的时效性产生于固结特性和蠕变特性的耦合效应,土体的变形过程实际是固结和蠕变共同作用的过程,任一时刻两种变形在总变形中所占比例取决于多种因素,其中最重要的是应力水平和排水条件。