A coupled analysis of cavity and pore volume changes for pulse tests conducted in soft clay deposits

This paper presents a new interpretation method for pulse tests, a field permeability test that allows for rapid measurements of hydraulic conductivity k in aquitards. This new method applies to soft clay deposits. To initiate a pulse test, a known volume of water is injected into the sand filter of a monitoring well isolated by a packer. The resulting pressure increase yields an outward movement of the sand filter cavity wall. After presenting the usual interpretation methods and their limits, this paper proposes a new interpretation method based on a coupled analysis of the pressure and displacement fields in the soil using the Biot–Darcy formulation. A series of analytical and numerical non‐dimensional velocity graphs, normalized plots of the mean hydraulic head difference versus its rate of change, are given. For a linear elastic material, these type curves show relatively small variations with the sand filter aspect ratio and the Poisson ratio of the tested clay. The type curves are also found to be independent of the clay compressibility (m_v) and k, an important result. A series of pulse tests conducted in a soft marine clay deposit near Montreal, Canada, are interpreted with the proposed method. The hydraulic conductivity values calculated from these tests are closely correlated with independent estimates obtained using long‐duration variable‐head tests. Compared with previous interpretation methods, the proposed method allows soil volume changes to be reconciled with cavity expansion phenomena and the range of type curves available for the interpretation of test data to be constrained. Copyright © 2013 John Wiley & Sons, Ltd.     

On the Study of Pile Driving Formula Based on Blow Counts

Accuracy of predicting pile capacities by pile driving formulas have been investigated. Five test piles were driven up to a depth of about 9 m of clay deposit and the penetrations due to final blows were recorded. The pile bearing capacity of each pile was predicted using 6 different pile driving formulas and the predicted pile capacity was compared with measured pile capacity from the pull up tests. Hiley formula, Modified Engineering News Record (ENR) formula, Janbu formula, Dutch formula, Danish formula, and Gates formula were used. The performance and accuracy of each formula was evaluated and the correlation coefficient of each pile driving formula was determined for a more accurate pile capacity prediction. Methods used to evaluate the performance of each formula were; (1) the best fit line for Q _p versus Q _m (2) cumulative probability for Q _p/Q _m and (3) the arithmetic mean and standard deviation for Q _p/Q _m. From the study, it was found that using Dutch formula provided the most accurate pile capacity estimate compared to the other formulas with an average of 7% deviation from value obtained from the field pull up test. It was followed by the Danish formula, Janbu formula, Hiley formula, Modified ENR formula, and Gates formula. The ability to predict the accuracy of estimating pile capacity using an appropriate method is very important and valuable to contractors, developers, geotechnical engineers, and manufacturers.     

人工内分泌网络模型在水文地质参数研究中的应用

传统方法在计算含水层渗透系数的过程中通常面临着不均匀系数C难以确定、对研究区水文地质条件较为依赖等困难。通过分析和概括内分泌系统独特的信息处理功能,构建了人工内分泌网络模型。在详细介绍模型算法和计算步骤的基础上,将其应用到华北平原滹沱河冲洪积扇前缘,根据孔隙度及粒度分布特征对含水层渗透系数进行预测,并与传统的经验公式Beyer法和Slichter法进行对比。计算结果表明：Beyer法的预测精度最低,其相对误差主要集中于0.078~1.342;Slichter法的预测精度有所提高,但对于渗透系数极端值的预测仍存在较大误差,相对误差主要集中于0.046~0.643;人工内分泌网络模型具有较高的预测精度,其相对误差主要集中于0.006~0.420。与传统经验公式相比,人工内分泌网络模型具有较高的计算精度和良好的通用性,且能够直接计算出含水层的垂向渗透系数,无需后期的数据验证。     

土柱变密度溶质运移试验推求水平及垂直方向弥散度研究

基于溶质运移对流弥散理论的变密度海水入侵模型广泛用于海水入侵研究,而水动力弥散系数是影响模型模拟效果的关键性参量之一。利用传统土柱溶质运移试验结合旁侧抽水,采用数值反演法成功获取了水平及垂直两方向弥散度。相对于传统方法,在不增加试验复杂度的前提下,同时推求了不同方向的弥散度,提高了试验效率,节约了试验成本,可广泛用于测定水动力弥散系数等参数。     

Determination of hydraulic conductivity of sand-bentonite mixtures for engineering purposes

Pollution of the environment due to leakage from waste repositories is a well-known and wide spread problem. Emphasis has therefore been put on design of liners for such repositories, focusing on hydraulic conductivity and its variation with time, liner composition, water content, compaction etc. The paper addresses the hydraulic conductivity of sand/bentonite mixtures, especially the variation of the hydraulic conductivity as a function of bentonite content, compaction and degree of saturation. In order to better understand the variation of the hydraulic conductivity of a sand–bentonite mixture a new parameter k ₁ has been proposed. The parameter reflects the amount of bentonite per pore volume and can easily be calculated based on the amount of bentonite and the dry density of the soil mixture. Thereby, the hydraulic conductivity can be predicted as a function of different degres of compaction. This method can be used for engineering purposes to predict the hydraulic conductivity at an early stage of a design to get an idea of the required design and hence, cost.     

基于分层假设的Green-Ampt模型改进

传统Green-Ampt入渗模型由于假设过于简化而限制了计算精度,针对这一问题,在分层假设的基础上对模型进行了改进:将均质入渗土壤分为饱和层、过渡层和干土层,采用Richards模型分析了不同入渗条件下各层的厚度以及各层内含水率和导水率的变化规律,结果表明,入渗条件一定时,过渡层占湿润层(包括饱和层和过渡层)比例会随湿润层厚度的增加线性减小,积水深度和土壤初始含水率分别会影响这一线性关系的纵轴截距和斜率,而土壤饱和导水率则对其没有明显影响;过渡层内土壤含水率分布为椭圆形曲线,而导水率随深度增加线性减小。在此基础上,通过引入湿润层的等效导水率并相应给出锋面吸力的计算方法,对传统Green-Ampt模型进行了改进。结果显示,改进后模型计算精度显著提高。     

Experimental study about the influence of cyclic load on the hydraulic conductivity of clay

The hydraulic conductivity k, one of the most important engineering properties of soft clay, plays a great role during the whole life cycle of underwater tunnel. Therefore, it is necessary to systematically study the responses of k to the dynamic load under the background of the great development of geotechnical engineering in the world. In this study, a series of seepage tests after cyclic loading were conducted on reconstituted kaolin clay using a modified hollow cylinder apparatus. The influence of cyclic load on the permeability characteristics of soft clay was illustrated in two aspects. The cumulative axial deformation of clay induced by cyclic loading resulted in the smaller hydraulic conductivity of the specimens, and also, the dynamic load reconstructed the microstructure of clay and made the number of large pores getting decreased and the small pores increased. There was a positive correlation between the deformation of soil and the change of hydraulic conductivity, but the reconstruction effect was irregular with the frequency of dynamic load. The measured k values got affected at the beginning, this phenomenon appropriately explains the positive correlation between the number of cycles of dynamic load and the change of hydraulic conductivity.

     

The consolidation characteristics of an unsaturated compacted soil

The compacted soils are extensively employed as mineral liners or a sealing system, constructed under municipal solid waste and other containment hazardous materials to prevent or minimize the leakage of pollutant liquids and gases into ground water and sublayers. This article presents a detailed experimental study on settlement characteristics, which are required for the construction of a sealing system. For the experimental study where a compacted unsaturated soil taken from Adana in Turkey has been used, a special equipped consolidation cell has been designed and constructed. The matric suction adopted as independent stress-state variables has been controlled by using the axis translation technique. The consolidation tests have been performed under constant matric suction. Furthermore, the pressure plate tests for various net vertical stresses have been conducted. The test results have indicated that the settlement characteristics of a compacted soil as mineral liners and covers are highly affected by matric suction. The water retention capacity of mineral liners, an important factor for hydraulic conductivity, also changes with the vertical load. The compressibility decreases as the matric suction increases. It has been observed that there exists an exponential relationship between the compressive indexes (C _t, D _t) and matric suction.     

土壤饱和导水率空间预测的不确定性分析

当土壤转换函数应用于土壤水力性质估计时,对于预测值的不确定性往往容易被忽视。为了有针对性地提出减少这种不确定性的方法和措施,提高土壤转换函数的实际应用能力,以两种现有的土壤转换函数(Vereecken和HYPRES模型)为例,将其应用于山东省平度市土壤饱和导水率的空间预测,并利用拉丁超立方抽样(LHS)方法对预测结果的不确定性进行了分析。结果表明,饱和导水率空间预测的不确定性主要来源于土壤基本性质的空间插值误差和土壤转换函数自身的预测误差。当Vereecken模型应用于饱和导水率空间预测时,预测结果的不确定性主要由土壤基本性质空间插值误差所决定,土壤转换函数预测误差的影响较小,而HYPRES模型则是受二者的双重影响。     

分形渗透模型在饱和冻土中的应用

冻土中的渗透系数对于评估冻土工程中的水,热和溶质迁移至关重要。以往研究表明,渗透系数主要依赖孔隙结构,经常被描述为孔径大小和孔隙率,但是这两个参数并不能充分地表征孔隙结构。为加强对孔隙结构的描述,引用分形理论研究了冻土中的渗透系数。基于非均匀毛细管束模型和分形理论,提出了饱和冻土中渗透系数的分形模型,并提出通过土体冻结特征曲线获取冻土中孔径分布的理论方法。为了验证分形模型的有效性,对已有实验数据进行分析。分析表明,分形渗透系数模型是毛细管分维、最大孔径、黏度和迂曲度的函数,孔径分布变化是导致冻土渗透系数变化的根本原因。通过对比,计算值与实测值吻合较好。结果表明分形模型可以较好的预测冻土中的渗透系数,研究结果可为冻土渗透机理研究提供参考。     

确定田间土壤水力传导率的分形方法

采用了一种简便易行的分形方法来间接估计水力传导率函数,即根据土壤粒径分布曲线确定孔隙表面分形维数,将其作为分形水力传导率模型的参数来预测整个压力水头范围内的水力传导率.利用UNSODA数据库中217个样本的实测资料对模型进行检验,结果表明分形方法预测的水力传导率其精度明显高于根据水分特征曲线利用统计孔径分布模型的估计结果.     

The Effect of Heavy Tamping on Structured Residual Clay Site

This paper examines the effect of heavy tamping (dynamic compaction) on highly porous structured residual clayey soil. The aim of this study is to analyse the feasibility of this technique when applied on lightly bonded residual soil sites, which are commonly found in tropical and subtropical regions. This soil has some interesting characteristics, such as high fine grain soil percentages (56% clay and 22% silt), a plastic index of 11%, high porosity (initial void ratio of 1.21), high hydraulic conductivity (about 10^?5 m/s) and a high stiffness at small strains (E?=?49.2-MPa). The research involves field [Cone Penetration Test (CPT) and the dynamic compaction] and laboratory (triaxial tests, characterization and hydraulic conductivity) investigation. According to laboratory tests, the void ratio decreased to 0.96, hydraulic conductivity decreased to 2.8?×?10^?7 m/s, the effective peak friction angle (?′) increased from 30.5° (in natural conditions) to about 35.5°, and the triaxial stiffness at small strains decreased to E?=?20-MPa due to dynamic compaction. CPT results have shown an improved depth in which CPT tip strength (q_t) increased from nearly 650-kPa to an average of 1700-kPa and CPT sleeve friction (f_s) increased from approximately 50-kPa to about 130-kPa. Horizontal displacements were observed up to about 4.0 m of depth (approximately the same depth at which CPT results showed soil improvement). It was concluded that heavy tamping reduces soil voids and substantially increases soil strength, but also breaks soil structure and decreases soil stiffness. It is thus not a suitable ground improvement solution for highly porous structured residual clayey soil.

     

天然软土地基路堤临界高度一种计算方法研究

考虑地表硬壳层应力扩散作用、硬壳层的抗剪强度、中间主应力 对地基承载力的影响,从实际情况下土的静止侧压力系数出发,依据极坐标表示的弗拉曼公式和统一强度理论,推导出天然软土地基临塑与临界荷载公式,提出天然软土地基上路堤临界高度的一种计算方法。结合算例,探讨了软土层和硬壳层静止侧压力系数 、中间主剪应力系数 取不同值对路堤临界高度 的影响。结果表明： 与 的改变对路堤临界高度 影响很大,新的理论公式更能反应软土地基承载力的实质,对覆有硬壳层的软土地基上路堤的修筑具有一定的指导意义。     

Estimation of stream water quality parameter using regime channel theory

For predicting and forecasting fate of non-conservative pollutants downstream from source using advection–dispersion–decay equation (ADDE), estimation of three parameters; mean flow velocity (U), longitudinal dispersion co-efficient (D _L) and decay rate co-efficient (λ), is required a priori. In this three parameters model, estimation of D _L holds difficulties and draws interest towards it. The empirical formulae use the field and experimental data of channel and flow characteristics to estimate D _L. In this paper, an innovative approach has been proposed towards the estimation of D _L using regime channel concept. Having known discharge of flow and silt factor of the riverbed material, hydraulic parameters of the channel can be determined theoretically, which in turn can be used to estimate D _L appropriately making use of a suitable empirical formula.     

磁共振探测估算含水层渗透系数的原位试验研究

通过一个完整的磁共振探测（MRS）、钻探、抽水试验过程,开展了MRS估算含水层渗透系数的原位试验研究。在分析MRS估算渗透系数准确性的基础上,系统剖析了引起准确性差异的主要因素,并对进一步提高估算效果提出了建议。结果显示：（1）采用现阶段常用的参数设置,与抽水试验计算值相比,MRS估算渗透系数的差值为抽水试验计算值的17.59%。（2）MRS推断的含水层顶板、底板埋深与钻孔揭示的信息相比,差值分别为4.11m、1.03m,表明其对估算渗透系数准确性的影响较小。（3）CP值是影响估算准确性的重要因素,其值为10-9数量级符合大多数地层的特点。另外,通过在已知渗透系数的钻孔附近进行MRS,从而获取CP参考值,应用该值估算的渗透系数准确性高于常用的参数设置。（4）指数a、b设置方面,应用Seevers公式（a=1、b=2）的估算效果优于Kenyon公式（a=4、b=2）。本成果有助于提高MRS估算渗透系数方法在野外条件下的适用性。     

Evaluation of Combined Base Soil Erodibility and Granular Filter Efficiency

Filters managed in zoned dams are designed according to criteria based on the grain size distribution of both filter and eroded soil. However, the constriction size distribution of the filter is the key parameter which governs the filter retention process of flowing eroded particles. To assess the filter efficiency regarding eroded particles, several filters and base soils are tested in a vertical cell with a configuration coupling erosion and filtration processes. For setting the boundary condition of eroded particles at the filter inlet, hole erosion test (HET) was performed on the base soil. The investigation of the evolution of filter behavior shows that the void ratio and the grain shape are of a great influence on filter efficiency. A new approach of filter clogging was proposed by evaluating a damage index which is affected by various parameters such as the ratio D₁₅/d₈₅ and the size of eroded particles. An approach linking the geometrical parameters (damage index) to the hydraulic conductivity leads to an estimation of the filter performance which provides a more quantifiable and realistic criterion. The results indicate that even existing criteria were not met; the tested filters remain efficient as regards to experimental data. An analytical approach based on constrictions size distribution was used and pore reduction was matched with experimental results.

     

Short Note: Effective Hydraulic Conductivity and Transmissivity for Heterogeneous Aquifers

Regional scale models of groundwater flow and transport often employ domain discretizations with grid blocks larger than typical scales of field data. For heterogeneous formations, this difference in scales is often handled by using effective (upscaled) parameters. We investigate the problem of upscaling hydraulic conductivity and transmissivity from a small scale of measurement to a larger scale of grid blocks. Transmissivity statistics is expressed in terms of statistics of hydraulic conductivity, and expressions for the effective (upscaled) hydraulic conductivity K _eff and transmissivity T _eff for steady state flow in confined heterogeneous aquifers are derived by means of stochastic averaging and perturbation analysis. These expressions reveal that the commonly used relation T _eff = BK _eff, where B is the confined aquifer thickness, is not generally valid.     

Evaluation of hydraulic conductivity of gap-graded granular soils based on equivalent void ratio concept

Gap-graded granular soils are used as construction materials worldwide, and their hydraulic conductivity depends on their relative content of coarse and fine grains, initial conditions, and particle shape. In this study, a series of constant head hydraulic conductivity tests were performed on gap-graded granular soils with different initial relative densities, fine contents, and particle shapes. The test results show that the hydraulic conductivity decreases with an increase in fine fraction and then remains approximately constant beyond the “transitional fine content.” The role of the structural effect on the hydraulic conductivity is different from that on the mechanical properties (such as stiffness and shear strength). This can be attributed to the degree of filling within inter-aggregate voids, disturbance of soil structure, and densified fine bridges between coarse aggregates. The equivalent void ratio concept was introduced into the Kozeny–Carman formula to capture the effect of fines (aggregates) on the “coarse-dominated” (“fine-dominated”) structure, and a simple model is proposed to capture the change of hydraulic conductivity of gap-granular soils. The model incorporates a structural variable to capture the effect of fines on “coarse-dominated” structure and coarse aggregates on “fine-dominated” structure. The performance of the model was verified with experimental data from this study and previously reported data compiled from the literature. The results reveal that the proposed model is simple yet effective at capturing the hydraulic conductivity of gap-graded granular soils with a wide range of fine contents, initial conditions, and particle shapes.