Rules for confidence intervals of permeability coefficients for water flow in over-broken rock mass

Based on the steady-state seepage method, we used the Mechanical Testing and Simulation 815.02 System and a self-designed seepage instrument for over-broken stone to measure seepage properties of water flows in three types of crushed rock samples. Three methods of confidence interval in describing permeability coefficients are presented: the secure interval, the calculated interval and the systemic interval. The lower bound of the secure interval can be applied to water-inrush and the upper bound can solve the problem of connectivity. For the calculated interval, as the axial pressure increases, the length of confidence interval is shortened and the upper and lower bounds are reduced. For the systemic interval, the length of its confidence interval, as well as the upper and lower bounds, clearly vary under low axial pressure but are fairly similar under high axial pressure. These three methods provide useful information and references for analyzing the permeability coefficient of over-broken rock.

A Review of Test Methods for the Determination of the Permeability Coefficient of Gravelly Soils Used for Embankment Dams

The factors influencing the permeability coefficient of gravelly soils used for the development of embankment dams(core wall)are analyzed.Such factors include(but are not limited to)soil size,anisotropy,density and boundary effects.A review of the literature is conducted and new directions of research are proposed.In such a framework,it is shown that gravelly soil with controlled density and vertical stress should be used to optimize the measurement of the vertical and horizontal permeability coefficients,respectively.

THEORETICAL MODEL OF EFFECTIVE STRESS COEFFICIENT FOR ROCK/SOIL-LIKE POROUS MATERIALS

Physical mechanisms and influencing factors on the effective stress coefficient for rock/soil-like porous materials are investigated, based on which equivalent connectivity index is proposed. The equivalent connectivity index, relying on the meso-scale structure of porous material and the property of liquid, denotes the connectivity of pores in Representative Element Area （REA）. If the conductivity of the porous material is anisotropic, the equivalent connectivity index is a second order tensor. Based on the basic theories of continuous mechanics and tensor analysis, relationship between area porosity and volumetric porosity of porous materials is deduced. Then a generalized expression, describing the relation between effective stress coefficient tensor and equivalent connectivity tensor of pores, is proposed, and the expression can be applied to isotropic media and also to anisotropic materials. Furthermore, evolution of porosity and equivalent connectivity index of the pore are studied in the strain space, and the method to determine the corresponding functions in expressions above is proposed using genetic algorithm and genetic programming. Two applications show that the results obtained by the method in this paper perfectly agree with the test data. This paper provides an important theoretical support to the coupled hydro-mechanical research.

Evaluation of Coefficient of Permeability on Contaminated Granitic Residual Soil

This paper reports laboratory research carried out on natural and contaminated granitic residual soil from Covilha region （Portugal） to evaluate the coefficient of permeability in accordance with Darcy＇s law. The soils are contaminated with hydrocarbon （Benzene Toluene Etilbenzene and Xilenes elements） and leached of urban solid waste. The specimens remained saturated and the consolidation and swelling are substantially completed at different effective confining pressures （25 to 400 kPa） before the measurements are performed at different hydraulic gradient to determine the effect of the void ratio, fabric and contamination of soil.

Hydraulic characteristics and vegetation performance of the Yellow River sediment modified by biochar

The Yellow River sediment(YRS)is an important potential soil resource for the mine land reclamation and ecological restoration in the arid regions of northern China.However,it has the shortcomings of poor water-holding capacity and needs to be modified urgently.Therefore,two types of biochar,namely rice husk biochar(RHB)and coconut shell biochar(CSB),were utilized in this study to modify the YRS and compared with rice husk ash(RHA).Some engineering properties of the modified YRS(MYRS),including pore structure,water retention,permeability,and vegetation performance,were investigated by considering the effects of biochar types and dosages.Results showed that the addition of the three materials decreased the bulk density of the YRS and increased the volume of extremely micro pore(d<0.3µm),as well as the effective porosity and capillary porosity,thus contributed to an increase in the water-holding capacity of the sediment.Among the three conditioners,RHB is optimal choice for improving the water-holding capacity of YRS.Furthermore,the effect becomes more pronounced with increasing application rates.With the addition of the three materials,the permeability coefficients of MYRS gradually decreased,while the water retention rate during evaporation significantly increased.The pot experiment showed that the three conditioners all had significant promoting effect on the growth of oats.In particular,compared to plain soil,the total biomass of oats grown for 21 days increased by 17.46%,32.14%,and 49.60%after adding 2%,4%,and 8%RHB,respectively.This study introduces a new approach for using YRS as planting soil in arid and semi-arid areas of China to facilitate mine ecological restoration.

A Novel ISSA–DELM Model for Predicting Rock Mass Permeability

In pumped storage projects,the permeability of rock masses is a crucial parameter in engineering design and construction.The rock mass permeability coefficient(K)is influenced by various geological parameters,and previous studies aimed to establish an accurate relationship between K and geological parameters.This study uses the improved sparrow search algorithm(ISSA)to optimize the parameter settings of the deep extreme learning machine(DELM),constructing a prediction model with flexible parameter selection and high accuracy.First,the Spearman method is applied to analyze the correlation between geological parameters.A sample database is built by comprehensively selecting four geological indexes:burial depth,rock quality designation(RQD),fracture density characteristic index(FD),and rock mass integrity designation(RID).Hence,the defects of the sparrow search algorithm(SSA)are enhanced using the improved strategy,and the initial input weights of the DELM are optimized.Finally,the proposed ISSA–DELM model is employed to predict the permeability coefficient of rock mass in the entire study area.The results showed that the predictive performance of the model is superior to that of the DELM and SSA–DELM.Therefore,this model successfully provides insights into the distribution characteristics of rock mass permeability at engineering sites.

Water inflow forecasting for tunnel considering nonlinear variation of permeability coefficient

To assess the water inflow which is more suitable to the actual conditions of tunnel,an empirical correlation about the permeability coefficient changing with depth is introduced.Supposing that the surrounding rock is heterogeneous isotropy,the formula for calculating water inflow of tunnel with the nonlinear variation of permeability coefficient is deduced.By the contrast analysis with the existing formulas,the presented method has the similar value to them;moreover,the presented method has more simple form and easy to use.Due to parameter analysis,the water inflow decreases after considering the nonlinear variation of permeability coefficient.When the attenuation coefficient a>0,the water inflow increases first till reaches the maximum at a certain depth,then decreases and is close to 0 finally if deep enough.Thus,it is better to keep away from the certain depth where it is with the maximum water inflow for safe operation and economical construction,and reduce the water damage.Based on the analysis,the radius of tunnel has less impact on the amount of water inflow,and the water inflow just increases by 6.7% when the radius of tunnel increases by 1 m.

Experimental Study on Clogging Characteristics of Silt Soft Soil in Nansha District of Guangzhou

The silt soft soil in Nansha District of Guangzhou was the softest soft soil in China. It had the characteristics of high natural water content, high compressibility, long consolidation time, and complex layered distribution of soil layers. These characteristics formed the clogging characteristics of silt soft soil, which greatly increased the construction difficulty and hindered the construction progress. Therefore, based on the basic physical and mechanical properties of silt soft soil in Nansha District of Guangzhou, this paper evaluated the clogging characteristics of three silt soft soil areas in Nansha District of Guangzhou through long-term permeability test, and carried out scanning electron microscope test to explore the influence of different parameters and microstructure on the clogging difficulty of silt soft soil. The results showed that the silt soft soil Zone I and Zone II (shallow layer) in Nansha District of Guangzhou were divided into slight siltation levels, and the silt soft soil Zone III (deep layer) was mild siltation level. Large pores were widely distributed in shallow silt soft soil, while the continuity of large pores in deep silt soft soil was poor. The migration of fine particles that failed to establish contact with surrounding particles in the soil blocks the small pores of seepage and thus produces siltation.

Seepage-stress coupling constitutive model of anisotropic soft rock

To provide a seepage-stress coupling constitutive model that can directly describe the seepage-stress coupling relationship, a series of one-dimensional seepage-stress coupling tests on two kinds of soft rock (argillaceous siltstone and brown mudstone) were performed by using an MTS-815.02 tri-axial rock mechanics test system, with which the stress-strain curves according to the seepage variation were obtained. Based on the experimental results and by employing Hooke's law, the formulation of the coefficient of strain-dependent permeability was presented and introduced to establish a coupling model. In addition, the mathematical expression and the incremental formulation for coupling model were advanced, in which five parameters that can be respectively determined by using the experimental results were included. The calculated results show that the proposed coupling model is capable of simulating the stress-strain relationship with considering the seepage-stress coupling in the nonlinear elastic stage of two kinds of soft rock.

Non-Darcy flow seepage characteristics of saturated broken rocks under compression with lateral constraint

Using an MTS816.03 test system and self-designed seepage apparatus, seepage tests of saturated broken rocks were conducted, and the influence of lithology, axial stress, grain size distribution and loading rate on seepage characteristics was analyzed. The results show that： （1） Under the same axial stress （12 MPa）, the permeability of different lithologic samples increases in the order： gangue 〈 mudstone 〈 sandstone 〈 limestone. The permeability of gangue is 3 magnitudes lower than that of limestone. The absolute value of the non-Darcy coefficient β increases in the order： limestone 〈 sandstone 〈 mudstone 〈 gangue. The non-Darcy coefficient β of limestone, which is positive, is 5 magnitudes lower than that of gangue. （2） With increasing axial stress, the permeability of saturated broken sandstone decreases, and the absolute value of the non-Darcy coefficient β increases. After the axial stress exceeds 12 MPa, the curves of permeability and non-Darcy coefficient β all tend to be stable. （3） With increasing Talbol power exponent, the permeability increases, and the absolute value of the non-Darcy coefficient β decreases. （4） With increasing loading, the permeability increases, and the absolute value of the non-Darcy coefficient β decreases. When the loading rate is 0.5 kN/s, the non-Darcy coefficient β is positive.

土石坝高喷桩防渗墙抗压强度与渗透系数相关性及分布模型研究

防渗墙的抗压强度与渗透系数之间的相关性及其分布模型是影响土石坝防渗墙强度及大坝渗透可靠性的关键因素。收集了9座土石坝高喷桩防渗墙检测数据,采用AIC准则识别抗压强度与渗透系数的最优边缘分布类型与构造相关联合分布模型的最优Copula函数,Bootstrap方法模拟识别结果的统计不确定性。结果表明:抗压强度和渗透系数之间存在显著负相关性、主要服从威布尔分布,构造两参数相关非正态联合分布的最优Copula函数为Frank Copula,识别结果差异主要来源于防渗墙施工质量导致的频率分布差异。研究结果可为土石坝高喷桩防渗墙强度可靠度与坝体/坝基渗透稳定可靠度分析提供简单、有效的分布模型。

全吸力范围内高填方渠堤土持水特性及渗透系数预测研究

土体的持水特性能够反映土体的强度、渗透和变形,而土体的渗透系数是影响高填方渠堤的重要因素之一。以南水北调中线工程某高填方渠堤土体为研究对象,通过开展持水特性试验,获得了全吸力范围内膨胀土、改性土和渠基土的土-水特征曲线,并基于VG模型和FX模型对全吸力范围内这3种土体的渗透系数进行预测。结果表明:①在全吸力范围内,3种土体的含水率、饱和度和孔隙比均随吸力的增大而减小;在含水率-吸力曲线中,脱湿曲线与吸湿曲线在吸力小于200 MPa时存在明显的滞回现象。②受膨胀土的胀缩性与干湿路径影响,膨胀土和渠基土的孔隙比与吸力的脱湿曲线均低于吸湿曲线,改性土由于水泥的改性效果,其孔隙比与吸力的脱湿曲线与吸湿曲线几乎重合。③采用VG模型和FX模型拟合出3种土的土-水特征曲线,拟合度均大于0.95,并结合统计传导率模型对3种土的渗透系数进行了预测,改性土土体渗透性在全吸力范围内显著大于膨胀土和渠基土的渗透性。

环境化学条件对软土电渗排水影响的试验研究

电渗排水在软土地基加固方面具有较大的应用前景。为研究土体环境化学条件对软黏土电渗排水过程的影响,开展了一系列室内试验,从电渗系数、排水量、排水速率、含水率分布等方面进行了对比分析,深入探讨土体pH值和盐度对电渗排水过程的影响机理。试验结果表明:土体pH值对土体的电渗系数和zeta电位均有明显影响,电渗系数随着pH值的升高而升高,通过测量zeta电位可以准确获取电渗系数;随着土体初始pH值的升高,电渗排水试验后阳极附近土体的含水率逐渐降低,阴极附近土体的含水率逐渐升高;在对较酸性土体的电渗试验中则出现了阴极附近土体排水固结较快的现象,对土体的整体排水过程产生不利影响;随着土体盐度的升高,电渗系数和电渗总排水量降低,能耗增大。低盐度、偏碱性的环境有利于提高电渗加固效果。

植被根系含量对膨胀土持水和渗透特性的影响

为探究植被根系含量对膨胀土持水和渗透特性的影响规律,选取南宁市膨胀土地区的膨胀土,掺入狗牙根根系制备不同含根率的试样,进行室内压力板仪试验和变水头渗透试验,采用Van Genuchten-Mualem模型预测不同含根率的膨胀土非饱和渗透系数。结果表明:掺入根系后,膨胀土持水能力减弱,根系掺入增加膨胀土内部大孔隙的比例,含根率越高,大孔隙体积占总孔隙体积比例越大,持水能力相较于纯土下降的幅度越大;饱和状态下,膨胀土渗透系数随含根率增大而增大;非饱和状态下,低吸力阶段(0~25 kPa)的膨胀土渗透性随含根率的增大而增大,随着基质吸力增加到高吸力阶段(200~1 000 kPa),根系优势流效果减弱,含根膨胀土渗透性逐渐趋近低于纯土;细观结果表明根系的掺入使膨胀土内部产生贯通裂缝,是影响膨胀土持水和渗透能力的关键因素。研究进一步揭示了植被根系对膨胀土增渗作用的机理和规律,为综合评价植被防护膨胀土边坡效果提供参考。

考虑多孔介质渗透率变化的宾汉流体渗透注浆扩散模型

渗透注浆是一种广泛应用于岩土多孔介质加固的方法,但已有基于浆液流变方程和平衡方程的研究,侧重分析浆液本身性质如黏度时变性、滤过效应、自身重力等对渗透注浆扩散范围的影响,尚未考虑浆液在孔隙内的流动阻力对多孔介质渗透率的影响,导致现有理论解析结果与试验测试结果相差较大。因此,基于黏性流体的运动方程、连续方程和平衡方程,引入浆液在孔隙内的流动阻力对多孔介质渗透率的影响,推导获得多孔介质渗透率随介质孔隙特性、注浆施工参数和注浆扩散半径的函数解析表达式,据此建立考虑多孔介质渗透率变化的浆液扩散控制方程。与已有理论结果对比表明,考虑浆液流动阻力影响的理论结果更接近试验结果,可以验证该理论方法的有效性。参数分析结果表明:考虑浆液在孔隙内的流动阻力时,岩土多孔介质渗透率随注浆时间的增加呈现先迅速降低后缓慢上升趋势,随注浆压力的增大呈现逐渐下降趋势;对于工程中常见的符合宾汉流体性质的水泥浆液材料,当注浆时间小于5 min时,多孔介质渗透率变化是影响浆液扩散的主控因素,超过7 min后,浆液黏度时变性成为主控因素。研究成果能为岩土多孔介质渗透注浆工程实际中注浆参数选取和质量控制起到良好的理论指导意义。

改性黄泛区松散粉砂土的渗透试验研究

黄泛区地下室周围回填粉沙土存在回填不密实,极端降雨雨水渗入地下室四周引起地下室上浮,通过降低松散粉砂土的渗透系数防止雨水渗入,采用水玻璃、碳酸氢钠、苯丙乳液对松散粉砂土进行改性。开展凝胶时间、变水头渗透试验、浆液入渗研究。结果表明:混合浆液中各组分与水的比例:水玻璃为5∶45和15∶45、苯丙乳液为5∶45和15∶45、碳酸氢钠为(0.8~1.5)∶45时凝胶时间大于10 min能凝结成整块凝胶体符合注浆改性要求。改性粉砂土渗透系数随水玻璃用量增加而减小,水玻璃与苯丙乳液和碳酸氢钠和水的比例为15∶15∶1.2∶45时,粉砂土的渗透系数为1×10^(-8)cm/s。土体含水量对浆液渗透距离影响显著,改性施工宜在春天进行;水玻璃的用量对浆液渗透距离改变变化显著,距离需求可通过调节水玻璃用量来实现。浆液由上到下进行渗透,改性土渗透系数由1×10^(-8)cm/s逐渐增大到接近素土的渗透系数1×10^(-4)cm/s。通过电镜微观分析得出:浆液主要通过在土体表面形成阻水凝胶层和在土体内凝胶体填充土颗粒间孔隙且土颗粒间相互黏结起到阻水作用。

泥炭质土非线性固结理论研究

在泥炭质土软基固结过程中,孔隙比和渗透系数会随固结时间增加逐渐变小,导致按Terzaghi一维固结理论计算的软基固结值与实测值存在较大误差,故以滇池湖相沉积泥炭质土为研究对象,进行固结-渗透联合试验,研究泥炭质土e-σ′,k-σ′之间规律,从而得到e-lgσ′,lgk-lgσ′模型,然后引入这两个模型推导出适用于泥炭质土的一维非线性固结理论,并求解出渗压指数C_(k)=1,C_(k)=2两种不同条件下的解析解。根据所取泥炭质土的C_(k)不同,当C_(k)=1时,一维非线性固结微分方程的解析解与Terzaghi一维固结方程解析解比较,仅竖向固结系数存在差异;当C_(k)=2时,一维非线性固结微分方程难以直接得到解析解,可引入激活函数得到非线性固结微分方程的一个特解。研究发现,C_(k)=1时,泥炭质土固结过程中Terzaghi一维固结理论预测沉降速率比实测快,固结完成所需时间短;推导的一维非线性固结理论预测的各时间段固结沉降值和实测值接近,预测固结完成所需的时间误差小,且上覆荷载越大St-t关系曲线与实测值越接近,推导的一维非线性固结理论预测结果越精确。研究结果可为泥炭质土类软基提供一种更为精确的固结计算方法。

干密度和增减湿对压实黄土水力特性的影响

研究非饱和土的水力特性对分析工程建设活动中黄土的渗流和变形问题具有重要意义。以西安市南郊黄土为研究对象,采用滤纸法和饱和盐溶液法测得3种干密度下的黄土增减湿土-水特征曲线,并通过Childs&Collis-George模型预测试验黄土的非饱和渗透系数,探究干密度和增减湿对非饱和渗透特性的影响。结果表明:经历增减湿的土样的非饱和渗透系数曲线应结合土样的含水状态和对应的土-水特征曲线来预测;土体非饱和渗透系数随体积含水率的增大而增大,随基质吸力的增大而减小,在0~5 MPa范围快速衰减4~5个数量级;受瓶颈效应和减湿收缩的影响,土体增湿阶段渗透系数变化幅度小于减湿阶段的变化幅度;渗透系数随干密度增大而减小,干密度从1.719 g/cm^(3)增大到1.834 g/cm^(3),渗透系数减小2个数量级;压实黄土在增减湿阶段的非饱和渗透系数最大可相差3~4倍。

复合水泥固化剂固化泥炭土渗透性研究

针对泥炭土工程性质差、改良难度高的问题,本文提出了一种由超细水泥(UFC)和普通硅酸盐水泥(OPC)组成的复合水泥固化剂(CCS)用于固化模拟泥炭土,并通过渗压试验和X射线衍射(XRD)测试研究UFC对水泥土试样渗透性的影响规律。渗压试验结果表明:CCS掺入量相同时,试样渗透系数k随UFC质量分数的增加而减小,UFC质量分数大于12%时k值下降趋势显著减缓;CCS中UFC质量分数为12%时,k随CCS掺入量的增加而减小,CCS掺入量大于25%时下降趋势明显减缓。XRD测试结果显示,试样中水化硅酸钙(C-S-H)的衍射峰强度随UFC质量分数及养护龄期的增加而增强。相较于OPC,CCS以更少的水泥用量便能达到优异的固化效果,从而实现减碳目标。

基于固结试验的高原湖相沉积泥炭土渗透特性研究

随着中国昆明地区的快速发展,泥炭土地基的处置需求日益增多。为了明确高原湖相沉积泥炭土在固结过程中的渗透特性及其影响因素,通过固结试验方法测定六个场地的渗透系数kv,分析固结压力P、孔隙比e及有机质含量wu等因素对其渗透性能的影响并建立了渗透模型。试验结果表明:在逐级加载条件下,泥炭土的渗透系数kv和孔隙比e随固结压力P增大而减小,P越大,kv和e下降幅度越小,且kv的下降主要发生在主、次固结阶段;泥炭土渗透系数kv随有机质含量wu增加而增大,但kv并非单一受wu的影响,还会受到有机质的分解度的影响,不同分解度的泥炭土其孔隙特征和渗透性能均不同;高原湖相沉积泥炭土的渗透系数可选用lg(1+e)-lgkv渗透模型进行计算,能较好反映出e与kv的变化关系。