Effectiveness of geogrid reinforcement in improvement of mechanical behavior of sand-contaminated ballast

Vertical stiffness and shear strength of ballasts are significantly degraded when contaminated with sands. There is a lack of solutions/studies related to strengthening ballast against sand contamination. Addressing this limitation, a comprehensive laboratory investigation was made on effectiveness of geogrid reinforcement for improvement of mechanical properties of sand-contaminated ballast. To this end, large-scale direct shear tests as well as plate load tests were conducted on geogrid-reinforced ballast samples prepared with different levels of sand contamination. The obtained results indicate that geogrid reinforcement considerably improves shear strength and vertical stiffness of contaminated ballast. A bandwidth was obtained for contamination levels in which ballast reinforcement is effective. Through examining geogrid with different aperture sizes and locations in the ballast layer, the best performance conditions of geogrid reinforcement were derived. The results were used to propose an effective method of ballast reinforcement and an efficient ballast maintenance approach in sandy areas.     

The applicability of Darcy''s law in determining the water permeability of geotextiles

Water conductivity characteristics of geotextiles are mostly given as a permeability coefficient or as a permittivity value. This is only valid in case of laminar flow. Theories and models, developed to describe flow through porous media and geotextiles in order to delineate the laminar flow zone for which Darcy's law can be applied, are treated and discussed. These models are not suitable to delimit the laminar flow zone because some parameters are difficult to define. Measurements of discharges for a range of hydraulic losses allow the flow regime to be determined. In most cases there is no laminar flow and the calculated permeability values are not exact. Therefore, the water conductivity characteristic of geotextiles is better expressed asa discharge for a given hydraulic loss.     

水力梯度对黏土渗透性影响的试验研究

渗透性是土体的重要工程性质之一,其受孔隙率、颗粒粒径及流体性质等诸多因素影响,但通常认为与水力梯度无关。现采用研制的刚性壁固结渗透装置,对经不同固结压力固结的黏土试样,进行先逐级施加水压、后逐级卸除水压条件下的固结和渗透试验。试验结果表明,水压加载时,受渗透力的影响,孔隙比和渗透系数均随水力梯度增大而减小,理论分析验证了递增水力梯度下渗透系数测试值变化趋势的合理性。水压卸载时,孔隙比不变,但渗透系数随水力梯度减小而继续减小,主要与颗粒堵塞和弱结合水相关,水力梯度较大时弱结合水处于流动状态,水力梯度递减后,弱结合水处于黏滞状态,孔隙导水能力下降,渗透系数降低,基于试验数据的线性拟合式显示降幅可达初始值的一半。     

Performance Evaluations of Pump-and-Treat System Using Advection-Dispersion Analysis: Effects of Clay Layer on Remediation Duration

To evaluate the effects of a clay layer on the performance of pump-and-treat remediation, an advection-dispersion analysis assuming non-sorption was calculated by changing five parametric variables, namely, the length of the clay layer, the thickness of the clay layer, the hydraulic conductivity of the clay layer, the effective molecular diffusion coefficient of the contaminants, and the contamination duration. The simulation procedure consisted of firstly contaminating the analysis domain, and then cleaning up this contaminated domain by a pump-and-treat system in order to consider strictly the contamination conditions of a clay layer at a real contaminated site. From the calculation results, the pump-and-treat remediation was found to be effective if at least one of the following two conditions was satisfied, namely, the hydraulic conductivity of the clay layer was greater than 5.0×10^-5 cm/s or the thickness of the clay layer was less than 3 m. In addition, the contamination duration was an important parameter in evaluating the applicability of this pump-and-treat remediation to contaminated sites. For cases in which the contamination duration was short, the contamination conditions of the clay layer easily became heterogeneous such that the remediation duration was much longer than the contamination duration. It is necessary to note that contaminated sites may not always be remedied in a short period of time, even if the sites are discovered early.     

The influence of design parameters on clogging of stormwater biofilters: A large-scale column study

A large-scale laboratory study was conducted to test the influence of design and operating conditions on the lifespan of stormwater biofilters. The evolution of hydraulic conductivity over time was studied in relation to a number of key design parameters (media type, filter depth, vegetation type, system sizing, etc). The biofilters were observed to clog over time, with average hydraulic conductivity decreasing by a factor of 3.6 over the 72 weeks of testing. The choice of plant species appears to have a significant effect on the rate of decrease in permeability, with plants with thick roots (e.g. Melaleuca) demonstrating an ability to maintain permeability over time. Other species studied, with finer roots, had no such beneficial effects. As expected, small systems relative to their catchment (and thus which are subjected to high loading rates) are more prone to clogging, as increases in hydraulic and sediment loading can lead to extremely low hydraulic conductivities. Sizing and the appropriate choice of vegetation are thus key elements in design because they can limit clogging, and therefore, indirectly increase annual load treated by limiting the volume of water bypassing the system.     

An analytical solution for contaminant extraction from multilayered soil using PVD-enhanced system

Prefabricated vertical drains (PVDs) have been employed to enhance the in situ remediation of contaminated fine-grained soils. Subsurface heterogeneity can interfere with the distribution and extraction of contaminants during the remediation process. In the present study, an analytical solution for contaminant extraction from multilayered soil using a PVD-enhanced system is developed based on an equivalent planar two-dimensional model. The analytical solution is derived using a procedure that combines the Laplace transform, eigenfunction method and numerical Laplace inversion. The validity and accuracy of the solution are verified by comparison with an existing analytical solution and the results obtained using a numerical model. The effects of several key parameters on the performance of a PVD remediation system in a triple-layered contaminated soil are evaluated. The results indicate that the remediation efficiency for clay layers decreases with increasing hydraulic conductivity or thickness of the sand layer. The PVD remediation system may be unfeasible for contaminated sites with high subsurface heterogeneity caused by permeability contrast. The remediation process for clay layers can be accelerated by increasing the clay vertical dispersivity.     

竖向应力作用下GCL的膨胀特性和渗透性能

近年来,土工织物膨润土垫(GCL)被越来越多地应用到各种防渗工程之中,它的防渗有效性也成为了设计人员和研究人员所关注的焦点。GCL的防渗有效性包括渗透性能、吸附能力和内部剪切强度三个方面。通过水化膨胀试验和渗透试验研究了GCL在竖向应力作用下的膨胀特性和渗透性能,并分析了正应力和加压水化顺序的影响。试验结果表明:(1)随着竖向应力的增大,GCL的膨胀量不断减小,而GCL的渗透系数则出现先减小后略有增大的规律;(2)水化加压顺序对GCL的膨胀量和渗透系数均有影响;(3)在实际工程应用中,GCL铺设完成后在堆载之前最好完全水化,这样能够大大提高GCL的防渗有效性。     

Cesium-adsorption capacity and hydraulic conductivity of sealing geomaterial made with marine clay,bentonite, and zeolite

Great amounts of soil and waste contaminated with radioactive cesium have been generated due to the decontamination work after the Fukushima Daiichi Nuclear Power Plant accident. The aim of this study is to develop a sealing geomaterial for use at the disposal facilities of the soil and waste constructed in the maritime environment. The geomaterial consists of marine clay, bentonite, and zeolite. The hydraulic conductivity and cesium-adsorption performance of the geomaterial were examined through laboratory tests with different proportions of bentonite and zeolite added to marine clay. It was concluded that the hydraulic conductivity could be reduced to the required level by increasing the amount of bentonite and that the cesium-adsorption capacity could be enhanced by increasing the amount of zeolite.     

从流动到可塑状态软黏土的一维固结特性试验研究

软土在固结过程中含水率的减小将导致其物理状态从流动态进入可塑态,为研究物理状态变化对软黏土的压缩性和渗透性的作用影响,采用饱和重塑黏土,配制高于液限含水率和接近液限含水率的试样,分别开展不同压力作用下的一维固结试验,依据位移和孔压测试数据,对比分析了固结过程中的位移发展、孔压消散、压缩系数以及渗透系数的变化特性。结果表明:在流动状态下,位移快速发展,而孔压却存在明显的滞消现象;进入可塑状态后,压缩系数和渗透系数的对数随孔隙比的变化关系均发生明显的转折。这意味着,在软土固结问题的理论分析中,应考虑物理状态的变化对土体压缩性和渗透性参数变化规律的影响。     

低渗透薄互层砂岩油藏大型压裂裂缝扩展模拟

 对低渗透薄互层砂岩油藏的大型压裂开发时,由于低渗透薄互层砂岩油藏中隔层岩层比较薄、强度低,对裂缝高度方向上的扩展抑制作用较小,往往会出现裂缝长度与高度之比小于4,导致只考虑缝内流体一维流动的拟三维裂缝扩展模型不再适用。根据低渗透薄互层砂岩油藏大型压裂的特点,在适当假设的基础上,应用线弹性断裂理论,建立考虑缝内流体沿着裂缝高度和长度方向流动的拟三维裂缝扩展模型,并用解析法得到压裂过程中裂缝扩展尺寸和缝内流体压力的精确解;利用Visual Basic计算机语言编制二维流动的拟三维裂缝扩展模型求解程序,并对某低渗透薄互层砂岩油藏压裂过程中裂缝扩展情况进行了求解分析。计算结果表明：二维流动的拟三维裂缝扩展模型能够很好地预测低渗透薄互层砂岩油藏大型压裂过程中裂缝的扩展,可以满足工程需求。     

The stress-strain-permeability behaviour of clay rock during damage and recompaction

Characterisation and understanding of the stressestrainepermeability behaviour of a clay host rock during damage and recompaction are essential for prediction of excavation damaged zone and for assessment of its impact on the repository safety. This important issue has been experimentally studied in triaxial compression tests on the Callovo-Oxfordian clay rock in this study. The samples were sequentially loaded by(1) hydrostatic precompaction to close up sampling-induced microcracks,(2)applying deviatoric stresses to determine damage and permeability changes, and(3) recompression along different loading paths to examine reversibility of the damage. The critical stress conditions at the onset of dilatancy, permeability percolation, failure strength, and residual strength are determined. An empirical model is established for fracturing-induced permeability by considering the effects of connectivity and conductivity of microcracks. The cubic law is validated for the variation of permeability of connected fractures with closure. The experiments and results are also presented and discussed.     

冻结状态青藏粉质黏土的渗透系数测量研究

粉质黏土在青藏高原多年冻土地区广泛分布,为了充分认识其在冻结状态下的渗透性质,开展了一系列渗透试验测量了在-0.6℃~0.1℃条件下含水率为50%的青藏粉质黏土的渗透系数。结果显示:在高负温区间内,冻结粉质黏土的渗透系数处于8.22×10~(-11)~7.19×10~(-9) cm/s之间;另外,渗透系数与土温之间呈现出幂函数关系,土温越接近于0℃,渗透系数增大越快;冻土的渗透系数表现出随着未冻水体积含量的增加而快速增大的趋势。渗透性的存在可测意味着高温冻土在外荷载下会存在固结作用。     

Hydraulic conductivity of a glacial clay till liner

The study of water flow through soils is an important aspect of soil mechanics. Past experience with glacial clay till deposits investigated in central and southern Saskatchewan has demonstrated that in situ saturated hydraulic conductivities vary from about 10^?6 to 10^?9 cm/s. In some applications and regulatory references, an accepted practice in the past is to assume that the field hydraulic conductivity value for a soil liner will be one order of magnitude (i.e. ten times) greater than the confirmed laboratory hydraulic conductivity value for a remoulded sample composed of the same soil. In fact, experience has shown that measured as-built field hydraulic conductivity values for a soil liner can be several orders of magnitude greater than the laboratory value if the liners are poorly constructed and, therefore, would not be compliant with regulatory criteria. The reliability of artificially applying any correction factor to a laboratory value to obtain an assumed field value is, therefore, questionable. This paper describes field and laboratory investigations that were conducted to characterize the general properties and the hydraulic conductivity of a glacial clay till soil liner. For this example case study, laboratory measurements of the hydraulic conductivity for remoulded soil samples were undertaken using a flexible wall permeameter (FWP). Air-entry permeameter (AEP) tests were performed on a soil test pad prior to construction and then on the completed soil liner immediately after construction to measure the as-built hydraulic conductivity. Both the FWP and AEP tests resulted in comparable hydraulic conductivity values that were in the order of 10^?8–10^?9 cm/s. A preceding and preliminary empirical approach was also found to be reasonable for purposes of determining soil suitability and estimating the “achievable” hydraulic conductivity value. The preliminary estimate was determined to be conservatively greater than the values determined by both the FWP and AEP test methods.     

Influence of the quality of fine fractions on engineering-geological properties of crushed aggregate

Fine fractions of aggregate crushed from Finnish Precambrian crystalline bedrock were studied by determinining the particle size distribution with a Sedigraph particle size analyzer, specific surface area using the nitrogen adsorption method and porosity with a mercury porosimeter. According to abrasion resistance (Los Angeles) and brittleness (Swedish impact) values, all the materials studied were suitable as road aggregate. Roads where material with high specific surface area were used however soon proved to suffer from frost damage. No correlation between specific surface area and values of abrasion and brittleness were found. The specific surface area also proved to depend on the quality rather than the quantity of the fine fractions. Presence of clay minerals tends to increase the sp.s.a. Thus, in addition to the quantity also the quality of fine fractions must be analysed in road aggregate material.     

Improving the abrasion resistance of hydraulic-concrete containing surface crack by adding silica fume

Abrasion damage generally results from constant friction and impact of waterborne silt, sand, gravel, rocks, ice, and other debris on concrete surfaces during the operation of a hydraulic structure. In this study, a waterborne abrasion over a large area of the test slab was developed to investigate the influence of surface cracks on the abrasion–erosion resistance of concretes, with variable surface crack type and silica fume content. The test results concluded that: (1) the abrasion rate is higher for water flow impinging directly on the crack than that above the crack; (2) increased crack width reduces the abrasion resistance of the tested concrete; (3) the abrasion rate increased with the angle of the crack to the water flow; and (4) the abrasion resistance increased with the addition of silica fume for concrete having a surface crack. These findings and may be of interest to engineers designing concrete hydraulic structures.     

Laboratory assessment of healing of fractures in rock salt

Tension-induced fractures and fractures formed by saw-cut surfaces and polished surfaces were prepared in salt specimens. A series of gas flow testing was performed to monitor the changes of the fracture permeability under quasi-static loading (0.7–20 MPa) for up to 120 h. Healing tests under static loading were carried out under both dry and saturated conditions. The results suggest that the primary factors governing the healing of salt fractures are the origin and purity of the fractures, and the magnitude and duration of the fracture pressurization. Inclusions or impurities significantly reduce healing effectiveness. The hydraulic conductivity of fractures in pure salt can be reduced permanently by more than 4 orders of magnitude with applied stresses of 20 MPa for a relatively short period. In most cases the reduction of salt fracture permeability is due to fracture closure, which does not always lead to fracture healing. The closure involves visco-plastic deformation of the asperities on both sides of the salt fracture, while the healing is related to the covalent bonding between the two surfaces. Fracture roughness and brine saturation appeared to have an insignificant impact on the healing process.     

Discrete Element Modeling for fouled railroad ballast

Fouling refers to the condition of railroad ballast when voids in this unbound aggregate layer are filled with relatively finer materials or fouling agents commonly from the ballast aggregate breakdown, outside contamination such as coal dust from coal trains, or from subgrade soil intrusion. This paper aims to investigate the effect of fouling on ballast strength and stability. Fouling mechanism was first explained followed by investigation of mechanical properties of fouling agent. Coal dust was chosen as the fouling agent due to its poor mechanical properties and it had been reported to mostly decrease aggregate assembly strength comparing to other fouling agents. An image-aided Discrete Element Modeling (DEM) approach was introduced to simulate the coal dust fouled track field settlement performance. To that end, DEM model parameters for clean and coal dust fouled ballast were validated by matching the DEM direct shear box simulation results to the laboratory shear box testing results for both samples. By assigning laboratory calibrated model parameters to the “half-track” ballast sample generated in DEM, effects of different fouling percentage as well as different fouling locations on ballast settlement performance are studied. Results from the “half-track” DEM simulation revealed that fouling could lead to unfavorable track distresses such as “hanging tie”. Further, shoulder fouling scenario was proven to be critical in track maintenance.     

有纺土工织物覆土条件下的渗透特性试验研究

分别配制不同孔隙比的粉砂、标准砂和黏土试样,采用自主研制的一套多功能渗透试验装置,开展了一系列纯土和有纺土工织物覆土条件下的渗透试验,对比分析了这两种条件下渗透系数的差异,并探讨了有纺土工织物与土共同作用下的渗透机理。结果表明,有纺土工织物对于土体渗流略有一定的抑制作用,表现为覆土条件下的渗透系数略小于纯土的渗透系数,但是对于粉砂,当其孔隙比比较大、细砂颗粒的含量相对较多时,细砂颗粒则有可能在渗流作用下通过有纺土工织物孔隙而产生流失,使得覆粉砂条件下的渗透系数略大于纯粉砂的渗透系数。     

Excavation-induced relaxation effects and hydraulic conductivity variations in the surrounding rocks of a large-scale underground powerhouse cavern system

Located in the middle reach of Yalong River in China, the Jinping-I Hydropower Station consists of a large-scale cavern system for water conveyance and power generation. Compared to other typical large-scale underground powerhouse cavern systems in Southwestern China, the construction site is characteristic of higher in situ geostresses, lower uniaxial compressive strength (UCS) and poorer quality of the surrounding rocks, resulting in greater depth of the disturbed zone. In this study, the excavation-induced relaxation effects and their impacts on the hydraulic conductivity variations and seepage behaviors in the surrounding rocks of the Jinping-I underground powerhouse caverns were assessed with site characterization data and numerical simulations. The excavation-induced disturbance zones around the caverns were modeled using the plastic yield zone predicted with an equivalent elasto-plastic model and a constant deviatoric stress criterion based on the Hoek–Brown parameters of the surrounding rocks, respectively. The predicted results agree rather well with the disturbed zones detected by the in situ acoustic wave velocity measurements and borehole TV images. The excavation-induced permeability changes in the surrounding rocks were characterized with a strain-dependent hydraulic conductivity model that accounts for the development patterns and deformation behaviors of the critically-oriented fractures. The seepage behaviors with consideration of the permeability changes in the surrounding rocks were modeled with a variational inequality method at a steady state, and the numerical results imply the significance of proper characterizations of the excavation-induced disturbance effects and permeability changes in better understanding the groundwater flow and its controlled effect in the surrounding rocks.     

粘土岩饱和-非饱和渗流应力耦合模型及数值模拟研究

岩体中饱和-非饱和渗流、应力耦合行为对工程岩体的强度和稳定性有十分重要的影响。基于粘土岩实验室非饱和渗流试验的结果，将考虑塑性应变硬化的非饱和渗流、应力耦合模型、Hoek—Brown非饱和渗流、应力耦合模型应用于模拟某粘土岩竖井，研究其在开挖、通风、衬砌支护及长期运营期围岩内渗流从初始饱和→非饱和→近饱和的过程，以及竖井的非饱和流动区域大小对水力学参数（Biot固结系数、饱和度与毛细孔隙压力、水相的相对渗透系数与饱和度）的敏感性。结果表明：水力学参数对非饱和渗流的影响区域非常显著，尤其是水相的相对渗透系数与饱和度的关系。力学模型的差异对非饱和区的影响几乎可以忽略不计。