Effect of chemical additives on the consolidation behaviours of mini-PVD unit cells –from macro to micro

The mechanism of adding NaCl, CaCl₂ and small amounts of cement/lime (1–4% by dry weight) into clay slurry for mitigating the effect of apparent clogging around PVDs during vacuum consolidation was investigated by mini-PVD unit cell consolidation tests as well as microstructure observations via scanning electron microscopy (SEM) imaging. The consolidation test results indicated that for the specimens with CaCl₂, cement and lime additives the rate of consolidation increased considerably, while the effect of the NaCl additive was limited. The SEM images show that for the specimen without additive, there were obvious localized deformation-induced microstructure anisotropies. For specimens with CaCl₂ and cement/lime additives, the microstructures of the soils tended to be isotropic. The additives tested reduced the thickness of the diffusive double layer around the clay particles and promoted the formation of a more stable flocculated microstructure, therefore mitigating the effect of apparent clogging.     

Experimental study on the clogging effect of dredged fill surrounding the PVD under vacuum preloading

Clogging effect surrounding prefabricated vertical drains (PVDs) is a typical problem when vacuum preloading is applied to a dredged fill foundation. A large-scale model test was designed to clarify the cause and mechanism of the clogging effect, and the basic physical and mechanical parameters of the soil in the clogging zone were tracked during the test. The results demonstrated that a clogging zone was formed around the PVD in the early stage of improvement with conventional vacuum preloading, and the boundary of the clogging zone was approximately 0.2–0.4 of the boundary radius. The clogging zone surrounding the PVD was formed because of the overall movement of the soil toward the PVD under the high vacuum pressure gradient, rather than fine particle migration. The soil in the clogging zone exhibited permeability anisotropy and equivalent ‘smear’ effect. The permeability ratio (k_h/k_v) was less than 1, and the ratio of horizontal permeability coefficients at the test distances of 45 cm and 10 cm were 9.6 at a depth of 20 cm and 8.9 at a depth of 80 cm. An analysis of the microstructure of the soil in the clogging zone demonstrated that the clay particles tended to be vertically oriented. The re-orientation of the clay particles reduced the horizontal permeability coefficient and led to the permeability anisotropy of the soil in the clogging zone. Thus, decrease in the horizontal permeability coefficient and equivalent ‘smear’ effect of the soil in the clogging zone affect the consolidation of dredged fill, which leads to the clogging effect. The permeability anisotropy also slightly affects consolidation.     

砂井径向排水的不均匀固结效应及简易量化方法

软土的不均匀固结问题普遍存在而且未受到足够的重视，不均匀固结会带来“淤堵”问题，影响单元体径向排水速率。文中利用数值方法研究砂井单元体的径向固结，首先通过分析单元体内部的土体孔隙比和渗透系数的径向分布特征，论证砂井径向排水的不均匀固结特性；对单元体平均固结度的数值模拟结果和理论解析结果进行比较，指出不均匀固结效应对孔压消散速率的影响，并提出可以表征不均匀固结效应影响程度的“等效涂抹效应”的概念（Ke）；最后，文章进一步研究Ke与土体的压缩性、渗透性和应力条件之间的关系。基于文中的研究结果，提出利用土体基本的物理力学指标和工程设计参数对“等效涂抹效应”（Ke）进行计算的简易量化方法。     

细粒迁移及组构变化对黏土渗透性影响的试验研究

为进一步探究水化学环境下土体渗透性的变化规律及作用机理,对饱和重塑压实黏土样在不同水化学条件及渗流路径下进行了一系列变水头渗透试验,分析了不同浓度的NaCl溶液在不同渗流路径下对饱和黏土渗透性的影响。结果表明：不同干密度试样的渗透性随浓度增加展现出较大的差异,干密度为1.40 g/cm³时,渗透系数呈先升后降,而干密度为1.50 g/cm³时,渗透系数不断降低;逆转渗流方向,试样渗透系数发生突变;孔隙盐溶液浓度周期性变化,试样渗透性不可逆。基于核磁共振（NMR）分析技术,测试了土体内部孔隙结构分布随孔隙盐溶液浓度的变化。最后基于上述试验结果从细粒迁移产生的孔隙堵塞效应和组构改变引起的孔隙封闭效应两个方面,解释了孔隙盐溶液浓度变化对黏土渗透性的影响。     

Clogging effect of prefabricated horizontal drains in dredged soil by air booster vacuum consolidation

The typical problem that occurs when the conventional vacuum preloading (CVP) method is applied to dredged soil is that the drainage is impeded, permeability is progressively reduced, and clogging occurs in prefabricated horizontal drains (PHDs). This paper proposes a method in which air booster through PHDs under vacuum preloading to reinforce newly dredged soil, and thereby solve the clogging problem. To evaluate the proposed method, several experiments were conducted using soils with variable degrees of consolidation and the effect on clogging determined. Furthermore, the proposed method was compared to the CVP method considering several variables monitored during consolidation and after consolidation. The results indicate that the proposed method is superior to the CVP as it more effectively alleviates the clogging problem. These findings could provide design criteria for dredged soil consolidation in similar projects and for scientific research and engineering practice.     

Modelling of hydraulic deterioration of geotextile filter in tunnel drainage system

The discharge capacity of a tunnel drainage system generally decreases with time because of the hydraulic deterioration of the geotextile filter. Hydraulic deterioration restricts groundwater flow into a tunnel and increases water pressure resulting in detrimental effects on the tunnel lining. Hydraulic deterioration of tunnel drainage system is unique in terms of clogging materials, deterioration mechanism, and flow conditions. Current studies and models investigating the clogging mechanism and hydraulic deterioration are not directly applicable to the geotextile filter of the tunnel drainage system. In this study, a theoretical model of the hydraulic deterioration of tunnel geotextile filter has been proposed considering the mechanical and hydraulic behavior of blinding, clogging and squeezing. A parametric study was carried out to evaluate the performance of the model. An experimental study has been conducted to investigate the clogging behavior of the tunnel drainage system and validate the theoretical model. Several types of clogging materials were selected: cement-leaching calcium oxide, calcium carbonate, iron oxide, and bentonite. Agglutinated clogging was mainly observed during the short-term testing. The findings suggest that the in-plane permeability of the geotextile filter decreased by approximately 90%. The proposed model corroborated the experimental results.     

Polymer impact on filter blinding during alum sludge filtration

This paper describes an experimental investigation into the factors which control clogging phenomenon of the filter medium during alum sludge filtration under conditions of excess polymer dosing. Preliminary studies showed that the hydraulic resistance of the filter medium was associated with the amount of polymer trapped, but was not uniquely determined by the quantity of polymer trapped in the filter. Subsequent investigations showed that filter blinding can arise from the interaction of residual polymer and a colloidal fraction. When either of these components was missing, the trapping efficiency of the filter was low. In particular, the study demonstrated that polymer coatings on the filter did not have any significant impact on the hydraulic resistance. Evidence indicated that clogging and polymer trapping was particle size driven, the aggregate size being dependent on the polymer dose. Diagnosis of trends in the filter resistance suggested that they were being influenced by polymer/colloidal solids ratio through its effects on particle size.     

Analytical solution on vacuum consolidation of dredged slurry considering clogging effects

This paper aims to establish an analytical model on vacuum consolidation of dredged slurry treated by PVD with special considerations on the clogging effects. A series of laboratory tests have been conducted to characterize the temporal/spatial variations of the clogging zone and the vacuum pressure in soil. Based on the nonlinear relationship between the compressibility, the permeability and the void ratio (e-p and e-k curves), an analytical model incorporating the clogging zone was proposed and solved considering the nonlinear attenuation of the vacuum pressure. The solution was verified and validated by comparing with published data and field test results, respectively. Parametric studies reveal that the clogging significantly slows down the consolidation rate, and thus reduces the final degree of consolidation of the dredged slurry.     

Laboratory assessment of factors affecting soil clogging of soil aquifer treatment systems

In this study the effect of soil type, level of pre-treatment, ponding depth, temperature and sunlight on clogging of soil aquifer treatment (SAT) systems was evaluated over an eight week duration in constant temperature and glasshouse environments. Of the two soil types tested, the more permeable sand media clogged more than the loam, but still retained an order of magnitude higher absolute permeability. A 6- to 8-fold difference in hydraulic loading rates was observed between the four source water types tested (one potable water and three recycled waters), with improved water quality resulting in significantly higher infiltration. Infiltration rates for ponding depths of 30 cm and 50 cm were higher than 10 cm, although for 50 cm clogging rates were higher due to greater compaction of the clogging layer. Overall, physical clogging was more significant than other forms of clogging. Microbial clogging becomes increasingly important when the particulate concentrations in the source waters are reduced through pre-treatment and for finer textured soils due to the higher specific surface area of the media. Clogging by gas binding took place in the glasshouse but not in the lab, and mechanical clogging associated with particle rearrangement was evident in the sand media but not in the loam. These results offer insight into the soil, water quality and operating conditions needed to achieve viable SAT systems.     

Experimental study on a dredged fill ground improved by a two-stage vacuum preloading method

The vacuum preloading method has been wisely chosen among many ground-improvement methods considering the time limit of many projects and the characteristics of reclaimed soil. However, the loss in vacuum with soil depth, the clogging around prefabricated vertical drains (PVDs), and the deteriorative consolidation of the deep soil layer, among other factors, create a large challenge to vacuum preloading for dredged marine clay fill. Thus, this study proposes a two-stage vacuum preloading method and focuses on its feasibility and effectiveness. Contrasting laboratory tests are performed in two identical experimental tanks with dredged marine clay fill from the Wenzhou land reclamation site in China. In one tank, the one-stage vacuum preloading method is used to serve as a baseline for this study. In the other tank, use of the two-stage vacuum preloading method is proposed for consolidation; it comprises two stages. In the first stage, the dredged marine clay fill is conditioned by vacuum preloading using half of the PVDs, where the dissipation of the excess pore water pressure tends to be steady. In the second stage, vacuum preloading is activated using all the PVDs. The results show that a better consolidation effect is achieved with the proposed method in terms of the settlement, vacuum pressure, pore water pressure, water content, vane shear strength, and soil particle microstructure after soil consolidation.     

Effects of pressurizing timing on air booster vacuum consolidation of dredged slurry

Air booster vacuum preloading is a newly improved method applied in land reclamation projects. Highly pressurized air can provide an additional pressure difference between the prefabricated vertical drain (PVD) and injection point, thereby increasing the hydraulic gradient and generating small fractures that can improve the soil permeability and the transmission efficiency of the vacuum pressure. However, with a premature activation time, the pressurized air can create air channels connected to the PVD, which may drastically decrease the vacuum pressure. With a delayed activation time, the strength of the dredged clay may be too high to permit fractures, thus limiting the permeability improvement. In this study, soils with degrees of consolidation (DOCs) of 0%, 40%, 60%, and 80% were selected for testing the efficacy of initial booster activation times in four tests. The results show that the pressurizing groups were more effective in improving the consolidation of soils, and the best effect of the use of air booster is obtained when soil has been consolidated to a DOC of 60%. The lower soils of the pressurized groups showed greater increase rates than those demonstrated by conventional vacuum preloading.     

High-Strengthening of Cement-Treated Clay by Mechanical Dehydration

A technique called the cement-mixing and mechanical dehydration method (CMD) as one of recycling techniques for soft clay slurry is developed. In order to evaluate the effectiveness of the CMD for increasing the strength of soft clay, a series of unconfined compression tests and several durability tests were performed together with the literature review of unconfined compressive strength in cement-treated soils. Moreover, a series of constant strain rate consolidation tests were also performed to evaluate the effects of cement content and dehydration speed on the permeability of cement-treated clay. The following conclusions are obtained: 1) Literature review and theoretical considerations on the shear strength of cement-treated soils show that an additional treatment for the purpose of increasing the density of cement-treated specimen is effective for increasing the shear strength of cement-treated soil. 2) The mechanical dehydration of soft clay with high pressure is accelerated by cement mixing, where the coefficient of consolidation of cement-treated clay increases as the cement content increases. 3) The high-strength specimen having the unconfined compressive strength of more than 20 MPa can be created from soft clay treated by the CMD with the cement content of over 20% and the dehydration pressure of 20 MPa.     

基于泊肃叶定律的土体渗透系数估算模型

 渗透性影响土体工程地质特性与力学行为,要对渗透系数进行理论估算相当困难。为应用泊肃叶定律建立土体渗透系数的估算模型,引入等效渗透管径概念并推导其关系式,利用针对性渗透试验对模型合理性与适用性进行校验。结果表明：土体内部渗透通道可借助等效性原理给予等效细管化;等效渗透管径的关系式,能够考虑土体密实度、结合水含量和颗粒级配、排布方式、几何形状等;模型的计算结果与试验数据较为一致,在误差控制方面表现良好。基于泊肃叶定律的土体渗透系数估算模型,各关键参量具有相对严格的物理背景,物理含义较为明确;可能适用的土体粒径范围为0.002～0.500 mm,大致覆盖上至中砂粒、下至粗粘粒包含的区间。     

Assessment of clay soils and clay-rich rock for clogging of TBMs

Shield tunnel drives in both, overconsolidated clays soils and sedimentary rock with clay minerals, frequently suffer extremely from hindrance due to clogging. The problem of clogging can have varying degrees, from locally blocked disk cutter housings to completely clogged excavation chambers with a blocked cutterhead. The clogging risk depends on multiple mineralogical and soil-mechanical parameters. The authors have more than 15 years of experience in practice and scientific research with the problem of clay clogging of TBMs. The paper shows the occurrence and risk for clogging in varying ground types and for different machine components, summarizes the available characterization methods for the stickiness of the ground, discusses a variety of laboratory tests for the assessment of clogging potential, explains a recently developed diagram for the assessment of clogging risks for all types of tunnelling machines, and introduces a new testing scheme for the evaluation of sedimentary rocks regarding clogging.     

On the critical particle size of soil with clogging potential in shield tunneling

Shield tunneling is easily obstructed by clogging in clayey strata with small soil particles. However, soil clogging rarely occurs in strata with coarse-grained soils. Theoretically, a critical particle size of soils should exist, below which there is a high risk of soil clogging in shield tunneling. To determine the critical particle size, a series of laboratory tests was carried out with a large-scale rotary shear apparatus to measure the tangential adhesion strength of soils with different particle sizes and water contents. It was found that the tangential adhesion strength at the soil–steel interface gradually increased linearly with applied normal pressure. When the particle size of the soil specimen was less than 0.15 mm, the interfacial adhesion force first increased and then decreased as the water content gradually increased; otherwise, the soil specimens did not manifest any interfacial adhesion force. The amount of soil mass adhering to the steel disc was positively correlated with the interfacial adhesion force, thus the interfacial adhesion force was adopted to characterize the soil clogging risk in shield tunneling. The critical particle size of soils causing clogging was determined to be 0.15 mm. Finally, the generation mechanism of interfacial adhesion force was explored for soils with different particle sizes to explain the critical particle size of soil with clogging risk in shield tunneling.     

上海粘土的准超压密特性

室内试验成果表明,上海粘土的准先期固结压力较天然有效上复压力大得多。本文根据临界状态原理来讨论准先期固结压力对土特性的影响。通过室内三轴试验测定出上海粘土的正常固结线、临界状态线及天然强度线,并给出各线的表达式。从土的固结、强度、应力-应变关系及孔隙压力的变化规律说明天然地基具有轻度的准超压密特性。最后,我们认为在进行此类地基的沉降计算及稳定分析时,应考虑准先期固结压力的作用,以提高设计水平。     

粘性土层井孔抽水试验的参数识别

粘性土层属于弱透水层,其水流、溶质运移和地球化学过程对于水文地质条件和环境变化具有重要意义.通过抽水试验来获取粘性土原位水力学参数是对粘性土水文地质问题进行研究的必要手段,但由于其渗透性很小,不能采用和常规含水层一样的方法求参.粘性土中的单孔抽水试验与一般含水层中的微水试验具有较大的相似性,因此可以借用微水试验的计算公...     

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。     

一种现场测定地基固结系数与渗透系数的方法

地基固结系数及渗透系数是地基计算及设计中的关键参数。本文利用IFCOBAT系统测试地基原位水平固结系数和渗透系数,采用考虑塑性区内外孔压系数不同的圆柱形孔洞扩张理论和一维径向固结方程模拟滤头周围的初始超静孔隙水压力分布及其消散过程,在渗透系数分析中,同时考虑了针头对流经水的阻力和滤头及容器内水柱的影响。现场测试结果与薄壁土样的室内试验结果的比较分析表明,现场测定的固结系数和渗透系数与室内试验结果接近。另外,还讨论了两种测试结果之间差别的原因及其合理性。     

Effect of pressurization frequency and duration on the consolidation of a dredged soil using air booster vacuum preloading combined with prefabricated horizontal drains

The prefabricated horizontal drain (PHD) clogging problem often results in poor drainage and consolidation effects. To solve this problem, this paper adopts air booster vacuum preloading combined with PHDs to treat dredged soil. Laboratory model tests and microscopic tests are used to investigate the improvement effect of this method on dredged soil consolidation at different pressurization frequency and duration. The test results indicate that pressurization can effectively relieve clogging and improve the consolidation efficiency. The frequent pressurization is conducive to the consolidation at the same air-boost ratio, and the longer pressurization duration has an obvious promoting effect in the later stage of consolidation at the same pressurization frequency. Furthermore, pressurization can significantly alleviate the PHDs clogging problem has also been demonstrated from the micro perspective. These findings could provide an important reference for the treatment of dredged soil by air booster vacuum preloading combined with PHDs.