Effect of desiccation cracking on the hydraulic conductivity of a compacted clay liner

Despite our best efforts to reduce the waste stream, there will always remain some residues which cannot be further treated and must be disposed in landfills. One critical aspect of landfill construction is the integrity of the landfill liner. Current landfill liner technology includes a composite liner which consists of a FML component and a compacted soil component. The primary characteristic for selecting a soil for use in composite liner construction is that the soil have a saturated hydraulic conductivity of 1 × 10^?7 cm s^?1 or less. In the present study the effects of desiccation cracks on the hydraulic conductivity of the compacted soil were measured. Two soils of diverse mineralogy and typical of soils used for clay liner construction were selected for use. Each was tested in its native state plus after the addition of 30% sand. Laboratory measurements were made of the volumetric shrinkage of each soil. In addition, the hydraulic conductivity was determined using 10 cm diameter fixed wall permeameters. Additional conductivity measurements were made using 60 cm diameter fixed wall double ring permeameters which had been exposed to 0, 1, and 2 periods of desiccation prior to hydraulic conductivity determinations. The data show that laboratory measurements using 10- cm diameter fixed wall permeameters underestimate the hydraulic conductivity of the same soils when packed in large diameter permeameters. It was also found that exposure to two cycles of desiccation resulted in large increases in hydraulic conductivity. The time required to reach a steady outflow volume decreased as the amount of desiccation increased. The hydraulic conductivities of soils which had been allowed to dry were greater than those which were not allowed to dry prior to measurement. The relationship between volumetric shrinkage and the increase in hydraulic conductivity after desiccation indicates that soils which exhibit less than 11% shrinkage in the laboratory, exhibit increases in K of less than a factor of 2 upon desiccation. Clay soils with greater than 11% shrinkage can potentially be amended with sand to decrease the volumetric shrinkage and their response to desiccation.     

The influence of amendments on the volumetric shrinkage and integrity of compacted clay soils used in landfill liners

Clay liners remain an important component of composite liners used in landfill construction. Because their hydraulic integrity is frequently lost due to desiccation cracking, either during construction or shortly thereafter. The present study was initiated to evaluate the effects of common soil additives including lime, cement, and sand on the shrinkage and hydraulic conductivity of compacted clay soils commonly used in clay liner construction. Three soils having predominant clay minerals of smectite, illite and kaolinite were amended with varying amounts of lime, cement or sand; compacted using the Harvard miniature compactor; and the volumetric shrinkage was measured on the compacted samples. Additional samples of each treated soil were compacted according to ASTM 698 and used for measurement of the hydraulic conductivity. The results show that the majority of shrinkage occurs when the samples were dried to 25 °C with little additional shrinkage at temperatures up to 105 °C. The amendments of either 4% lime or 40 to 50% sand resulted in reduced shrinkage and increased hydraulic conductivity. The addition of 3% cement reduced shrinkage by up to 50% and simultaneously reduced hydraulic conductivity by 2 orders of magnitude. Thus, amendment of clay soils having a high shrink-swell potential with Type I Portland cement has the greatest poetential for field application as an amendment to help maintain the integrity and improve the long term performance of compacted clay liners.     

土壤干缩开裂三维模型构建及其参数敏感性分析

为揭示农田土壤干缩裂隙沿深度方向的发展规律及形成机理,该研究改进了Vogel提出的Hookean弹簧模型,通过构建由节点组成的三棱柱状网格结构,并考虑重力的影响,建立了可以模拟裂隙深度的三维农田土壤干缩开裂模型,分析了纵向弹性系数（与土壤沿深度的弹性有关）对模拟裂隙深度的影响,通过Minkowski密度（即面积密度、长度密度和欧拉数密度）量化分析裂隙形态。结果表明：试验与模拟裂隙图像的面积、长度、欧拉数密度及裂隙深度频率的决定系数在0.849～0.959之间,一致性指标在0.965～0.988之间,偏差在0.103～0.189之间,均方根误差在0.005～0.083之间,说明改进后的三维模型达到率定要求,该三维物理模型可以模拟出裂隙沿深度的拓展情况,模拟的表层裂隙形态特征符合自然裂隙的发育规律。敏感性分析中,纵向弹性系数越小,裂隙沿深度方向发育的趋势越明显,深裂缝（土深5～10 cm）的占比越大。研究可为模拟农田土壤裂隙沿纵向的发育和形成提供算法参考。     

粒度组成对红黏土干缩裂隙影响

为了探讨粒度组成对红黏土干缩裂隙发育的影响,从而给红黏土地区的工程建设及减灾防灾提供理论依据。通过室内干燥试验,研究了粒度组成对昆明呈贡红黏土饱和泥浆样的水分蒸发、干缩裂隙的形成和演化、表面干缩裂隙结构形态的影响。结果表明:(1)蒸发曲线分为4个阶段,常速率阶段是最重要阶段;(2)常速率阶段历时随粒径减小而增长,随粒径分布范围变大而减短;(3)蒸发速率与粒径、粒径分布范围的关系不明显;(4)干缩裂隙形成与演化过程分为5个阶段,受粒度组成影响最显著的是裂隙形成阶段;(5)粒度组成影响裂隙形成阶段历时、不同等级裂隙先后衍生关系、开裂曲线的分段性、裂隙网络结构和土块分布;(6)天然粒组样的干缩裂隙产生向上卷曲现象;(7)粒径分布范围越大,分布非均匀性越强,表面裂隙率越大,而粒径的影响不明显;(8)表面裂隙的分形维数随粒径减小而减小。综上,粒径分布范围和分布非均匀性显著影响红黏土的水分蒸发过程,干缩裂隙的形成和演化,表面干缩裂隙结构形态。     

Wasserhaushalt mineralischer Deponiebasisabdichtungen unter dem Einfluß von Temperaturgradienten

Moisture flow in mineral landfill liners under temperature gradients In order to assess the risk of desiccation and cracking of the mineral bottom liner (without plastic liner) of a seven years old warm landfill, water contents were measured and soil hydraulic parameters determined in the lab. No cracks were visible, and a desiccation could not be proven due to the high spatial heterogeneity of the liner. Studies with a newly developed numerical model of coupled moisture and heat transport show that mineral liners might only crack if a plastic liner or very dry waste obstruct the temperature induced vapor transport from the waste to the mineral liner. Whether desiccation occurs under these conditions depends on the distance to groundwater and on the unsaturated hydraulic conductivity functions of the mineral liner and the subsurface. Simulations with unsaturated hydraulic conductivity functions that have been determined by different methods indicate that a new standard method to determine the unsaturated hydraulic conductivity of dense clayey substrates is necessary.     

温度对崩壁红土层土壤干缩裂隙形态影响的定量研究

  目的  探究干燥温度对土壤水分的蒸发速率及其崩壁土壤干缩裂隙的发育过程的影响,为准确认识崩壁土壤裂隙的形态及其对揭示崩岗的发展过程提供理论依据。  方法  研究选取典型崩岗区崩壁的红土层土壤,模拟干缩裂隙发育过程,通过图像处理技术,定量分析裂隙表面参数,探讨温度对崩壁红土层土壤裂隙发育的影响。  结果  温度降低,土壤水分蒸发速率下降,裂隙面积和裂隙总长度增大,裂隙率提高,同时块区面积降低,块区个数以及裂隙条数、节点数显著增加,整个土体呈现出越发破碎的状态。由于裂隙条数的显著增加,低温状态下（40 ℃）裂隙的平均长度小于高温条件（80 ℃）的试样,即低温条件较高温条件土壤裂隙发育数量更多,平均长度更小;土壤裂隙的平均宽度随温度升高呈现增大的趋势;分形维数随着温度的降低呈现增大的趋势。黏粒含量显著影响土壤干缩裂隙发育的温度效应。  结论  在较低温度条件下,崩壁红土层表层土壤水分蒸发时间长,裂隙发育过程更缓慢,但发育程度高,裂隙结构更复杂。     

再生水灌溉对亚热带典型土壤干缩裂缝演变特征的影响

为揭示再生水灌溉对亚热带土壤干缩裂缝及其发育过程的影响,该研究选取红壤、潮土、紫色土、水稻土作为供试土壤,采用再生水原液(RW)及稀释2倍(RW-2)、4倍(RW-4)、6倍(RW-6)4种不同浓度再生水进行持续模拟灌溉,并进行脱湿开裂试验,提取干缩裂缝参数。结果表明:1)再生水灌溉抑制红壤及促进其余3种土壤干缩裂缝形成与发育,除RW-4处理促进作用存在异常外,其余处理呈再生水浓度增高,抑制或促进作用增大;2)再生水灌溉下,红壤干缩开裂过程更为平缓,其余3种土壤干缩裂缝面积发育更集中于前期,低浓度再生水集中作用更大;3)土壤类型与再生水浓度对干缩裂缝具有交互作用,土壤类型对面积密度及其发育过程减缓段变化速率影响较大,再生水浓度对面积发育过程初始段变化速率、减缓段长度、加速段变化速率、长度发育过程变化速率峰值影响较大;4)土壤裂缝面积密度发育程度与含水率的关系可用三直线模型拟合,长度密度及连通性系数发育程度与含水率的关系可用log-logistic模型拟合。研究结果可为亚热带地区再生水水质标准制定及其灌溉制度设计提供参考。     

Influence of in-house produced biochars on cracks and retained water during drying-wetting cycles: comparison between conventional plant,animal, and nano-biochars

Purpose

Biochars produced from different feedstocks (such as wood, pig manure) possess varying physical and chemical properties, which have influence on crack and evaporation rate of biochar-amended soil (BAS). Furthermore, influence of compaction state and drying-wetting cycles on evaporation rate and cracking of BAS has not been investigated comprehensively. The objective of this study was to investigate the effects of biochar types, compaction state of BAS, and drying-wetting cycles on crack propagation and retained water (or evaporation rate).

Material and methods

An animal and plant feedstock-based biochars were produced in-house from pig manure (PM) and wood (W), respectively. In addition, nano structured chalk and wheat biochar (CWB) were also produced. Soil amended with individual biochars was compacted in petri-glass discs at two densities. Disc specimens were subjected to multiple drying-wetting cycles, and evaporation rate of specimens and crack area were monitored throughout the experimental period (70 days). Images were captured after every 24 h and processed using image processing technique to obtain the crack intensity factor (CIF).

Results and discussion

The results show that plant-based W BAS showed the high water retention, i.e., low evaporation rate and low CIF. Furthermore, the crack potential of CW BAS was seen to be higher. In dense compacted soil, maximum CIF% can be reduced from 3.9 to 0.4% for W BAS, from 3.9 to 1.7% for PM BAS, and from 3.9 to 1.6% for CW BAS.

Conclusion

WB was able to resist cracking more efficiently than other types of biochar. Evaporation was found to be minimal for plant-based W BAS at 10% biochar percentage. Higher biochar content in soil was seen to increase the water retention of BAS significantly. Dense state of BAS at high biochar content (i.e., 10%) was effective in reducing evaporation rate and crack progression.

     

Landfill sealing potentials of bottom ashes of sludge cakes

The main objective of this study is to determine if bottom ashes of dried sludge cakes (SC_bash) could be used as landfill cover or liner, a function which is normally performed with clay. Laboratory tests were performed to determine its particle size distribution, Atterberg limits, compaction characteristics, hydraulic conductivity, and shear strength parameters. The effects of desiccation and freeze–thaw on the hydraulic conductivity of SC_bash were also examined. The results of this investigation proved that properly compacted and stabilized SC_bash has the required properties to be used in landfill covers or liners. The SC_bash can be compacted into a dense mass with a low hydraulic conductivity of the order of 1.15×10⁻⁷ m/s. The compacted SC_bash showed also a good resistance to the increase in hydraulic conductivity caused by desiccation and freeze–thaw as compared to other compacted clays. The results also indicated that the compacted SC_bash has greater shear strength as typically expected for compacted clay, and therefore is likely to remain stable on a typical landfill slope designed and based on clay shear strength.     

Modelling cracking stages of saturated soils as they dry and shrink

Cracks that form when clay soils shrink on drying eventually form a network that determines transport properties. I propose and validate a model for (i) analysing the initial cracking stages of shrinking saturated soils, (ii) estimating the minimum dimension of quasi‐brittle cracks capable of developing in such conditions, and (iii) determining relations between the minimum crack dimension and other characteristic dimensions of the soil structure. Shrinkage cracks in soils can be classified on the concept of the minimum quasi‐brittle crack capable of developing at shrinkage. I use the model of developing a shrinkage crack in a semi‐infinite brittle medium with constant relevant properties, desiccating in conditions of shock drying. The model is generalized to the cracking of a saturated clay soil with a limited maximum crack depth. The available data justify the use of constant elastic, strength, diffusivity, and shrinkage properties of clay soil. The critical point of crack development is the existence of the minimum crack capable of developing in the particular conditions. The dimension of the crack is related to the soil properties. The crack goes through stages of delay, jump, stable growth with approximately constant velocity, and then quick decline until it stops. I show that the minimum crack dimension is related to the mean dimension of soil particles, the thickness of an upper intensive‐cracking layer, and the mean spacing of primary cracks at the soil surface.     

Desiccation and cracking behaviour of clay layer from slurry state under wetting-drying cycles

Laboratory tests were conducted to investigate the effect of wetting-drying (W-D) cycles on the initiation and evolution of cracks in clay layer. Four identical slurry specimens were prepared and subjected to five subsequent W-D cycles. The water evaporation, surface cracks evolution and structure evolution during the W-D cycles were monitored. The effect of W-D cycles on the geometric characteristics of crack patterns was analyzed by image processing. The results show that the desiccation and cracking behaviour was significantly affected by the applied W-D cycles: the measured cracking water content θ_c, surface crack ratio R_sc and final thickness h_f of the specimen increased significantly in the first three W-D cycles and then tended to reach equilibrium; the formed crack patterns after the second W-D cycle were more irregular than that after the first W-D cycle; the increase of surface cracks was accompanied by the decrease of pore volume shrinkage during drying. In addition, it was found that the applied W-D cycles resulted in significant rearrangement of specimen structure: the initially homogeneous and non-aggregated structure was converted to a clear aggregated-structure with obvious inter-aggregate pores after the second W-D cycle; the specimen volume generally increased with increasing cycles due to the aggregation and increased porosity. The image analysis results show that the geometric characteristics of crack pattern were significantly influenced by the W-D cycles, but this influence was reduced after the third cycle. This is consistent with the observations over the experiment, and indicates that the image processing can be used for quantitatively analyzing the W-D cycle dependence of clay desiccation cracking behaviour.     

Impact of biochars on swell–shrinkage behavior,mechanical strength,and surface cracking of clayey soil

Swell–shrinkage, cracking and stickiness of expansive clayey soils usually lead to their low yield. Improvement of these poor soil physical properties is a key goal for enhancing the crop productivity of expansive clayey soils. This article presents results of a study on the impact of three biochars produced from wheat straw (SB), woodchips (WCB), and wastewater sludge (WSB) on the swell–shrinkage behavior, mechanical strength, and surface cracking of a clayey soil. The soil was treated with biochars at the rate of 0, 20, 40, and 60 g biochar kg^?1 soil, respectively; and incubated for 180 d in glasshouse. Application of biochars decreased significantly (p < 0.01) the coefficient of linear extensibility (COLE) of the soil, the effect of SB being most prominent. The tensile strength (TS) of the clayey soil was originally 937 kPa, which decreased to 458 kPa, 495 kPa and 659 kPa for 6% SB‐, WCB‐, and WSB‐amended soils, respectively. Shear strength tests indicated that biochars significantly reduced cohesion (c) and increased internal friction angle (θ). Biochar significantly reduced the formation of soil surface cracks, surface area, and length of the cracks. The surface area density of cracks in the 6% biochar‐amended soils decreased by 14% for SB, 17% for WCB, and 19% for WSB, respectively, compared with control. The results suggest that biochar can be used as a soil amendment for improving the poor physical properties of the clayey soil, particularly in terms of reduction in swell–shrinkage, tensile strength and surface area density of cracking.     

Spatial distribution of roots and cracks in soils cultivated with sunflower

The aim of this research was to investigate the spatial distribution of roots and cracks in two clay soils cropped with sunflower under different inter-row spacing in order to identify the optimal management. A latin square experimental design was applied to compare bare soil and soil cropped with sunflower, with three plant densities, obtained by keeping constant the number of plants on the row (3 plants m^?1) and varying the row spacing (0.4, 0.6, 0.8 m). The presence of the crop and the different distance between rows influenced soil moisture content as well as the root spatial distribution and thus the structural features of cracks. Increasingly lower values of moisture were found in both soils as the distance between rows decreased; an opposite trend was observed for both root density and crack size. The volume of cracks in the soil grown with sunflower at 0.4 m row spacing was 201.4 m³ ha^?1, thus 8 times higher than the value on the bare soil and 2.5 times higher compared to the one grown at 0.8 m between rows. Optimal results in terms of root density, soil moisture and crack size were obtained with an inter-row spacing of 0.6 m.     

Effect of artificial soil compaction in landfill capping systems on anisotropy of air‐permeability

Soil air permeability is an important parameter which governs the aeration in soils that significantly promotes the root growth of field and grassland species and leads, in turn, to higher levels of evapotranspiration. The German Landfill Directive (2009) requires a rigid or a minimal shrinking capping system that ensures a high evapotranspiration rate to decrease the infiltration rate through the underlying waste body and therefore the leachate generation. This research is focussed on the questions if compacted glacial till can ensure the required rigidity and if and how air permeability is affected by soil compaction. The objective was to compare air‐filled porosity and the direction‐dependency of air permeability of a capping soil when assuming rigid and non‐rigid conditions considering a shrinkage factor. Intact soil cores were sampled in vertical and horizontal direction in 0.05, 0.2, 0.5, and 0.8 m depths at two profiles of a mineral landfill capping system at the Rastorf landfill in Northern Germany. Desiccation experiments were carried out on differently‐compacted soils and soil shrinkage was measured with a 3D laser triangulation device, while the air permeability was estimated with an air flow meter. The results indicate that the “engineered” soil structure which was predominately platy due to a layered installation, led to a more anisotropic behaviour and therefore to higher air permeability in horizontal than in vertical direction. The compacted installation of the capping system seems to be effective and observes the statutory required more‐or‐less rigid system, otherwise, soil shrinkage would lead to vertical cracks and a more pronounced isotropic behaviour.     

湖南邵阳地区高液限红黏土干缩裂隙演化过程的量化分析

红黏土对环境湿度变化非常敏感,在干燥环境中极易开裂,纵横交错的裂缝网络损害了土壤结构的完整性,很容易诱发红黏土边坡失稳和崩塌,导致农田水利设施的破坏,甚至加剧整个生态环境的水旱灾害。为探究红黏土裂隙的演化规律,该研究采用自制试验装置和三维应变测量系统开展了自然湿热条件下的红黏土泥浆样干燥试验,通过采集土体水分和土表位移、应变和裂隙的变化,定量分析脱湿过程中土体表面裂隙形态和应变场的演变特征,并进一步探讨水分变化对裂隙形态和应变场的影响。结果表明：1）土样表面干缩裂隙的演化一共经历了6个阶段,后阶段裂隙分割前阶段裂隙围成的区域,且不同阶段裂隙的交叉角接近90o;2）裂隙产生初始,裂隙尖端处拉应变约为0.5%,土表面大部分区域处于受拉状态;随着裂隙进一步发展,裂隙周边土体逐渐由拉应变状态向压应变状态转变;当所有裂隙发育完成,裂隙周边土体处于压应变状态;3）裂隙演化阶段与界限含水率有关,当泥浆土样的含水率接近液限时（67.7%）,土体表面裂隙开始发育,裂隙迅速张开和延伸;当土的含水率达到塑限时（28.3%）,裂隙发展的速率逐渐变缓;当含水率小于缩限时（18.8%）,裂隙变化已经很小,裂隙发育接近完成;4）在裂隙演化过程中,早期裂隙的发展持续时间和裂隙宽度均超过后期裂隙;土表不同位置的位移和应变均不相同,土块中心竖向收缩大于边缘竖向收缩,而土块中心位移及应变均小于土块边缘,研究可为红黏土开裂引发的工程地质灾害的预防及治理提供参考。     

The creation of longitudinal cracks in shrinking soils to enhance seedling emergence. Part I. The effect of soil structure

Stand establishment in crusting soils is one of the most critical stages in the production of crops with delicate seedlings. This becomes more difficult in hard‐setting soils of arid lands where dispersion of aggregates due to irrigation results in the formation of a hard layer as the soil dries from the surface downwards and impedes seedling emergence. However, seeds planted near to natural cracks manage to emerge through them. The aim of this study was to improve seedling emergence of irrigated crops in arid and semi‐arid conditions by devising methods to create longitudinal cracks in the vicinity of the seed rows during the subsequent drying phase. Laboratory experiments were conducted in soil boxes containing air‐dried clay soils to investigate the influence of different mechanical effects on the cracking pattern after flooding. Promising results were achieved by compacting a 7–15‐mm wide strip along the seed row. Monitoring the effect of compaction on water release characteristics and tensile strength of soil revealed that the greater water loss of the surrounding non‐compacted soil caused a suction gradient towards the points of lower water content resulting in movement of water and particles towards the drier zones. The compacted soil with a greater tensile strength did not permit the flow of water and particles to the loose soil and a discontinuity occurred. A field experiment in central Iran proved the feasibility of the technique in a semi‐arid area. This was achieved by lightly compacting a narrow strip of soil by applying 17–35 kg on a 22‐mm wide, 558‐mm diameter wheel covered by a layer of flexible rubber which ran over the pre‐compacted soil.     

Compaction of forest soils with heavy logging machinery affects soil bacterial community structure

Soil compaction is widespread but tends to be most prevalent where heavy machinery is used in landfill sites, agriculture and forestry. Three forest sites strongly disturbed by heavy logging machinery were chosen to test the physical effects of different levels of compaction on soil bacterial community structure and soil functions. Community analysis comprised microbial biomass C and T-RFLP genetic profiling. Machine passes, irrespective of the compaction level, considerably modified soil structural characteristics at two soil depths (5–10 cm; 15–20 cm). Total porosity decreased up to 17% in the severe compaction. Reflected in this overall decline were large decreases in macroporosity (>50 μm). Reduction in macroporosity was associated with higher water retention and restricted gas exchange in compacted soils. The strongest effect was observed in the severely compacted wheel tracks where air and water conductivities were reduced permanently to 10% or even lower of the original conductivities of undisturbed soils. Very slow drainage in combination with a dramatically reduced gas permeability led to unfavorable soil conditions in severely disturbed traffic lanes reflecting the changes in the total bacterial community structures at both soil depths. Additionally, microbial biomass C tended to be lower in compacted soil. Our results indicate that the type of severe treatments imposed at these forest sites may have strong adverse effects on long-term soil sustainability.     

调控措施对滨海盐渍土磷素形态及作物磷素吸收的影响

滨海盐渍化土壤存在磷素有效性低的问题。本试验采用根袋法盆栽试验,共设不施磷肥、常规磷肥、磷肥+生物质炭、磷肥+腐殖酸、磷肥+商品有机肥5个处理,分析不同调控措施对非盐渍土、轻度盐渍土和中度盐渍土有效磷含量、磷素形态以及大麦磷素吸收利用的影响。结果表明:①盐碱障碍降低根区内外土壤有效磷含量,表现为非盐渍土轻度盐渍土中度盐渍土。添加生物质炭能显著提高轻度、中度盐渍土根区内外土壤有效磷含量,较常规磷肥对照处理分别提高40.72%、84.80%。②盐碱障碍降低大麦产量,抑制地上部对磷素的吸收,不同调控措施均能促进盐渍土上大麦对磷素的吸收,提高磷肥利用率。轻度盐渍土上不同调控措施的增产效果不显著,中度盐渍土上添加生物质炭处理显著提高大麦产量,较常规磷肥对照处理提高63.20%。③盐碱障碍降低土壤活性无机磷、NaOH-Pi、NaHCO_3-Po、NaOH-Po比例,增加HCl-Pi比例。添加生物质炭处理能显著提高盐渍土活性无机磷比例,提高土壤磷的有效性。添加生物质炭和商品有机肥处理对中度盐渍土上HCl-Pi比例的降低效果优于轻度盐渍土。     

土壤裂隙研究的回顾与展望

土壤裂隙的发育是土壤自身内在属性在外界条件变化时综合作用的结果,与土壤物理、化学及生物性质息息相关,对农田水肥蓄积、坡体稳定、地下水污染等的危害很大。本文首先阐释了土壤裂隙的内涵及其与孔隙之间的区别与联系,而后从形成机制的角度对土壤裂隙进行了分类,并归纳总结了水平裂隙和垂直裂隙的描述方式及裂隙的评价指标体系,接着对干缩裂隙发育的外因及内因做了较深入的讨论,最后对土壤裂隙日后研究工作的重点提出了建议,旨在为土壤裂隙研究提供有益借鉴。