Laboratory measurement of fine-grained soil fluid conductivity

Advective fluid flow, which depends directly on hydraulic conductivity, controls contaminant migration through soils in many cases. The state of practice in laboratory measurements of hydraulic conductivity of soils was reviewed and found to be extremely variable, with the result that measurements on a given soil material often give variations of several orders of magnitude. An experimental program was completed using compacted clay soils to study several facets of laboratory testing practice, and a suitable test procedure for consistent results was developed. Recommendations on several important aspects of procedure are presented.     

Effect of dry density,soil texture and time-spatial variable water content on the soil thermal conductivity

Study of the heat transfer process in saturated and unsaturated soils requires, basically, a relationship between thermal conductivity and the characteristics of the soil, such as water content, dry density and texture of the soil. This study intends to produce a generic model that can predict soil thermal conductivity with the help of easily measurable parameters. The proposed model is first calibrated using measured thermal conductivities from literature data. In order to validate the proposed model the predicted thermal conductivity of this proposed model as well as existing ones are compared with the measured thermal conductivity in literature for different soils. Validation of the proposed model was also performed on our experimental results obtained for a compacted Misillac sand and in-situ clay loam soils. The results show an average of 15% improvement in prediction accuracy for the proposed model compared to the existing models, considering all soil textures. Moreover, we perform a model to estimate thermal conductivity over time throughout the profile of soil in the context of seasonal variation of temperature. The proposed model shows an important effect of heterogeneity on the thermal conductivity variations of a double layered soil.     

A new generalized soil thermal conductivity model for sand–kaolin clay mixtures using thermo-time domain reflectometry probe test

As the demand of exploitation and utilization of geothermal energy increases, more geothermal-related earth structures occur recently. The design of the structures depends upon an accurate prediction of soil thermal conductivity. The existing soil thermal conductivity models were mostly developed by empirical fits to datasets of soil thermal conductivity measurements. Due to the gaps in measured thermal conductivities between any two tested natural soils, the models may not provide accurate prediction for other soils, and the predicted thermal conductivity might not be continuous over the entire range of soil type. In this research, a generalized soil thermal conductivity model was proposed based on a series of laboratory experiments on sand, kaolin clay and sand–kaolin clay mixtures using a newly designed thermo-time domain reflectometry probe. The model was then validated with respect to k _dry–n (thermal conductivity of dry soils and porosity) and k _r–S _r (normalized thermal conductivity and degree of saturation) relationships by comparing with previous experimental studies. The predicted thermal conductivities were found to be in a good agreement with the experimental data collected from both this study and the other literatures with at least 85% confidence interval. It is concluded that the proposed model accounts for the effects of both environmental factors (i.e., moisture content and dry density) and compositional factors (i.e., quartz content and soil type) on soil thermal conductivity, and it has a great potential in predicting soil thermal conductivity more accurately for geothermal applications.     

砾石地层冻土热物理特性研究

地铁建设中时有穿越富含砾石的地层,需使用冻结法施工,而砾石地层的热物理性质是冻结法设计的重要依据。为研究人工冻结砾石土热物理特性,通过自制试验仪器,开展了砾石土冻结温度、导热系数和容积热容量测定方法及其特性研究,并与黏土、粉土等典型土层进行对比分析。结果表明:砾石土冻结温度曲线变化符合常规变化规律,且冻结温度为-0.21 ℃,高于黏土、粉土等;砾石土容积热容量与黏土、粉土等相近,随颗粒粒径增大,常温和冻结状态下土体容积热容量比值减小,其中砾石土的比值为1.19;砾石土导热系数较大,冻结状态下达3.89 W/(m·k),是常温状态下的1.65倍,符合颗粒粒径对导热系数的影响规律。砾石土导热系数可按各组成物质的导热系数及其相应的体积比推算。     

基于细观模拟的软土导热系数数值预测模型

热环境控制是城市地下空间安全运行的有力保障,科学预测地下围岩的导热性能是地下空间环控系统热负荷评估的基础。软土是地下空间开发中一类常见土体,现有模型主要适合于预测中低含水率范围内土体导热系数的变化,而对于高含水率软土,合适的导热系数预测模型较少。基于细观模拟,本文提出了一种能有效预测软土导热系数的数值模型。该模型除了能够反映含水率、干密度等常规因素影响外,还可考虑矿物组成以及粒径分布等的影响。最后,通过与苏通GIL管廊工程中20个软土样的实测导热系数进行对比以验证数值模型的可靠性。首先借助激光粒度分析仪和X射线衍射分析测试了矿物组成和粒度分布,代入模型进行数字建模并通过细观导热模拟得到导热系数预测值。导热系数模拟预测值与实测值对比结果显示:模拟预测值基本在实测值±20%范围内,验证了本文模型的可靠性,表明了该模型在预测高含水率软土导热系数方面的潜力。此外,该模型还可以直观地展示土内各处局部热流的分布特征,这为深入认识土体导热行为的机理奠定了基础。本文研究可为软土以及土体导热系数的预测评价提供新的思路和方法。     

A methodology to determine thermal conductivity of soils from flux measurement

Determination of thermal properties of soils (viz., thermal resistivity, thermal conductivity, thermal diffusivity and heat capacity), which primarily influence heat migration through the soil mass, is essential in situations where geomaterials are relentlessly subjected to higher temperatures and temperature variations. These properties of the soil mainly depend upon its type, mineralogy, particle size and gradational characteristics, density and water content. In this context, earlier researchers have determined thermal conductivity of soils by employing a thermal probe (a line heat source), which works on the principle of transient method (TM) of heat conduction. However, this methodology cannot be employed for establishing the heat flow (read thermal regime) through the soil. Hence, development of an alternate technique, which facilitates quantification of temporal and spatial variation of the heat flux and temperature in the soil mass, becomes essential. With this in mind, a methodology to determine thermal conductivity of soils by employing the concept of thermal flux measurement (TFM) has been developed and its details are presented in this paper. Results obtained from the TM and TFM have also been critically evaluated for the sake of validation and generating more confidence in the proposed methodology.     

浅层岩土室内、外热物性测试的相关性

对于浅层岩土热物性参数测试中常用的实验室和现场热响应试验方法,结合上海某工程的实际情况,提出一种基于室内试验的岩土综合热物性参数确定方法。首先,根据室内、外试验测试结果的差异,选取地层厚度、含水率、密度及渗透系数作为影响二者热物性参数测试差异的主要因素,使用层次分析法确定各影响因素的权值,并按权值大小修正室内热物性参数测试结果。然后,分别模拟室外现场热响应试验和修正后的室内热物性参数以及实际地层的传热过程,得出三者在热量传导能力之间的差距分别为1.2%、1.1%及2.3%。最后,提出埋管深度和导热系数的乘积可代表岩土层的换热能力,且计算出修正后的室内热物性参数对应的均一导热系数与现场热响应试验测出的综合导热系数分别为1.832 W/(m·℃)和1.778 W/(m·℃)。     

青藏高原含砂砾石土壤导热率实验研究

土壤导热率是土壤的基本物理参数之一,也是陆面模式的重要输入量,对研究土壤热传输、水热耦合运移有重要意义。青藏高原由于独特的地理环境备受学者关注,但目前常用的土壤水、热属性参数化方案仅仅考虑了沙土、粉土和黏土,就砂砾石重要性的认识还不足,很少有模式模拟砾石对青藏高原多年冻土和高寒草地的影响。采用便携式热导仪（KD2 Pro,DECAGON,美国）测量了青藏高原玛多和北麓河两地典型土壤在冻结和未冻结状态下不同水分条件时的土壤导热率,分析了砂砾石含量对土壤孔隙度的影响及冻结和未冻结状态下,不同水分条件下砂砾石含量对土壤导热率的影响。结果表明：当含水量高于某一阈值时,含水土壤冻结状态下的导热率高于未冻结状态下的导热率;土壤含水量对土壤导热率影响显著,导热率随着含水量的增加而增大,在含水量较小时变化更明显;砂砾石含量比重多的土壤孔隙度较小,且砂砾石含量越大的土壤在冻结状态下导热率高。以上结果表明,砂砾石对土壤导热率有显著影响,在将来的模式模拟研究中必须考虑砂砾石对土壤热属性的影响,进而提高土壤水热过程模拟的精度。     

Finite element analyses of rate-dependent thermo-hydro-mechanical behaviors of clayey soils based on thermodynamics

Clayey soils in the vicinity of energy geostructures may be exposed to long-term periodic thermal cycles. The creep and consolidation behaviors of the clayey soils can be both rate-dependent and temperature-dependent, and the underlying physical mechanisms are merely investigated theoretically. In this study, based on the theory of thermodynamics, a fully coupled thermo-hydro-mechanical (THM) finite element (FE) program for saturated soils is developed for this purpose. The FE formulation accounts for the combined effect of rate and temperature through the novel concept of granular temperature. Simulations of THM coupled validation cases and a series of experimental observations on the soft Bangkok clay are carried out. The obtained numerical results exhibit good agreement with analytical solutions and laboratory measurements. It is found that three fundamental physical mechanisms contribute to the irreversible thermal contraction observed for normally consolidated and lightly overconsolidated clays under drained thermal cycles: (1) the thermal creep excited by mass exchange from adsorbed water to free water; (2) the mechanical creep induced by confining stresses; and (3) the increase in granular packing caused by the thermal expansion of soil particles. The thermal contraction generally stabilizes within a few thermal cycles, as a result of the noticeable reduction in the thermal creep rate. It is further demonstrated that the transient heat transfer and the heating rate can greatly influence the deformation of clays subjected to thermal cycles.

     

黏土矿物对软土结合水特征及力学性质影响的定量分析

黏土矿物对软土结合水特征及力学性质的影响尤为重要,但国内研究多侧重于宏观物理力学性质层面。在微观层面探讨黏土矿物对软土工程性质的影响为此研究重点,利用X射线衍射仪(XRD)测得各地软土的矿物成分,并用同步热分析仪对各地软土进行热重分析(TG)和微商热重法分析(DTG)。试验结果显示,软土的黏土矿物含量与其吸附水含量正相关,其中蒙脱石含量对软土吸水性影响巨大;软土的TG与DTG曲线与其所含黏土矿物类型、含量有关,失重曲线在一定程度上可以体现软土中黏土矿物类型,DTG曲线中的失重谷对软土中的矿物类型起到一定的指示作用。     

结构性软土地基施工扰动定量分析

绝大多数软土具有一定的结构性,地基处理施工扰动会改变软土地基的工程性质,受到扰动的软土地基的力学参数却难以通过室内试验模拟准确测定。从理论和原位试验两方面分别给出了由于地基处理施工引起的结构性软土地基扰动程度的定量计算方法,并通过实际工程计算得出了设置砂井或排水板的软土地基的施工扰动度在30 %左右,定量计算得到的扰动度可用于扰动土地基的沉降及稳定性计算。     

基于DTS技术的菏泽地层导热性研究

为了确定山东省菏泽市某地区的地下热物性参数,本文基于线源热传导理论,将DTS技术与现场热响应试验（TRT）相结合,利用测温光缆对现场单U和双U地埋管加热（或冷却）过程的钻孔温度进行了分布式测定,对该地区每一地层的导热系数进行了计算。试验结果表明:利用DTS技术可在现场热响应试验中准确可靠地获得各个地层导热系数分布,评价各个地层的热交换能力;渗流对地层导热性具有显著的提高作用;双U管试验的测试结果比单U管试验所获得的结果更准确;受热量累积效应的影响,换能试验时间越长,所得到的导热系数越小,并逐渐趋于一稳定值。本研究成果可为地源热泵设计提供基本依据。     

天津滨海地区深部黏土层弹塑性变形特征与地面沉降关系研究

利用在滨海新区施工的2眼全取芯钻孔（G2和G3）,通过原状土样工程特性指标测试、固结压力试验、0-P0反复加、卸荷试验及地面沉降分层标监测数据分析等,系统阐述了滨海地区深部黏性土层弹塑性变形特征与地面沉降的关系。结果表明：天津滨海地区100 m以浅主要为欠固结土层;100～400 m土层处于超固结和微超固结状态,主要是由过去地下水超量开采造成的;400 m以下土层以正常固结为主。G2和G3孔不同层位黏性土层在反复加、卸荷试验过程中表现出塑性变形量逐渐减小,而弹性变形量几乎不变,与反复加、卸荷次数无关,表明黏性土层在水位反复升降条件下,逐渐变为以弹性变形为主。黏性土层这种特性显示,在地下水位反复升降多次后,黏性土层将会逐渐变成弹性体,在水位恢复时,将产生同步回弹,对防治地面沉降具有重要意义。分析弹塑性变形与黏性土层深度、天然含水率和黏粒含量的相关性发现：弹性变形量与黏性土层深度、天然含水率及黏粒含量呈正相关性;塑性变形量与深度相关性不明显,与天然含水率和黏粒含量呈负相关性。     

天津地区地层热物性特征及影响因素分析

为分析天津地区地层热物性参数的地区分布特征和平面分布规律,布置88个勘查孔、现场采集1 076个岩土样、室内分析热导率、比热容、热扩散系数等地层热物性实验,对测试数据进行统计分析。结果表明:天津市岩土体热导率在1.26~1.70 W/(m·K),比热容在2 050~2 090 J/(kg·K),热扩散系数在0.45×10-6~0.74×10-6 m2/s。同一地区不同岩性的比热容,黏土最大、粉砂最小,热导率刚好相反,热扩散系数与比热容规律相同;不同地区同一岩性的热物性参数差别不大。天津市比热容与热导率呈现大致相反的趋势,热导率高值区位于蓟县、宝坻和宁河的东部、武清西部、静海南部以及滨海新区的中部地区;比热容高值区位于蓟县、宝坻和宁河西部、武清东部、静海北部以及滨海新区大部分地区。     

寒区渠基黏土热参数最优概率分布

为分析寒区渠基黏土热参数的随机分布特征及概率分布模型,以寒区渠基黏土的导热系数为样本,结合经典分布拟合法、多项式逼近法、最大熵法和正态信息扩散法,分别对寒区渠基黏土热参数的概率分布规律进行了研究。首先通过分析热参数的离散性,并比较概率分布曲线、拟合检验值和累计概率分布值,对不同方法描述热参数随机性的优劣进行了评价;然后,基于寒区渠基黏土热学参数对温度的敏感性,提出了一个可以达到理想拟合精度的寒区渠基黏土热参数概率推断的区间取值标准。研究结果表明：寒区渠基黏土的热参数具有随机变量的特征;正态信息扩散法可以描述热参数样本的随机波动性;在4种方法中,正态信息扩散法的拟合精度最高。使用3.5σ法,将[μ-3.5σ,μ+3.5σ]（μ为随机变量的均值,σ为标准差）作为概率函数推断时的取值区间,同时考虑偏度的影响,可使得累计概率值达到1.000 0的精度,能够较准确地推断热参数的概率分布函数。     

Analysis on variation characteristics of geothermal response in Liaoning Province

Due to energy shortage and increasing environmental awareness, resources in shallow underground space have been rapidly exploited and utilized. So that studying variation characteristics of geothermal response in gneiss is necessary for effective and rational use of underground heat. Based on field test of thermal response in gneiss under hydrogeological survey project carried out in shallow geothermal energy development zone in Liaoning Province, this thesis analyzes mathematical statistics of geothermal response characteristics in main gneiss of Laoning Province. The initial formation temperature ranges from 10.80℃ to 15.80℃ according to field test. The statistical results show that in the condition of natural water content, the average thermal conductivity of Quaternary loose rocks comes as clay silty silty fine sand medium sand coarse sand gravelly sand. This order is consistent with thermal conductivity characteristics of gneiss obtained in the laboratory. Formation temperature recovery in different strata follows as granite medium sand clay. This order is opposite to the absolute value of temperature recovery curve slope of corresponding lithology, which shows that the stratum with higher temperature recovery rate has lower temperature recovery curve slope.     

陕西关中盆地粉质黏土热物理性质研究

关中盆地浅层地热能资源十分丰富,但目前开采程度仍有待提高。土壤的热物理性质在很大程度上决定了其工程特性,对浅层地热能在开发利用过程中所涉及的基本理论和开发方法等具有较大影响。通过室内试验对西安市、咸阳市、渭南市和宝鸡市范围内的粉质黏土的热物理性质开展研究,对试验数据进行了分析,进而揭示了粉质黏土比热容、导温系数和热导率的宏观变化规律。结果表明: 粉质黏土的热导率为0.82~2.65 W/(m·K),导温系数为0.001 28~0.004 86 m²/h,比热容为0.77~1.53 kJ/(kg·K); 随着含水率的增大,热导率整体呈减小趋势,且数值分布范围缩小; 比热容总体随导温系数的增大而减小; 导温系数随热导率的增大呈线性增大,且相关系数均较高。研究可为工程实践过程中开发利用浅层地热提供一定的理论基础。     

不同遗址土的热物理参数研究

为了研究干密度、含水率和NaCl含量对遗址土热劣化的影响,采用Hot Disk热常数分析仪对良渚遗址、交河故城遗址和通古斯巴西古城遗址的遗址土重塑样进行了热物理参数的测试,分别确定了遗址土的导热系数、热扩散系数和体积比热随干密度和含水率的变化关系。对通古斯巴西古城遗址重塑样掺入了不同含量的NaCl,研究了含盐量对遗址土重塑样导热系数的影响。结果表明,当含水率与干密度增加时,3种遗址土的导热系数均呈线性增长,热扩散系数和体积比热也随着含水率与干密度的增加而增大;相同条件下3种遗址土的热物理参数存在差异,良渚遗址的较大,交河遗址的较小,通古斯巴西古城的居中总体上与土中SiO2含量成正相关关系。当干密度与含水率一定时, 掺入NaCl增加了遗址土的导热系数。     

上海黏性土与水泥混合后强度增长特性试验研究

对于使用水泥与上海黏性土进行混合加固的土体,其强度增长特性与水泥含量、加固土的初期pH值、养护时间有关。以上海4类黏土为研究对象,对加固土的强度增长特性进行了试验研究;探讨了加固土的养护时间、水泥含量、初期pH等与加固土强度的关系。试验结果表明,当上海黏土中水泥含量或者加固土的初期pH值大于某一临界值时,水泥加固土的强度将迅速增加,对于上海黏土,该临界pH值为11.7,对应的水泥含量为17%～20%。但当水泥含量达到一定值后,它对土体的pH值的影响开始变小,而且水泥土的强度趋于稳定的时间变长。