活性MgO碳化固化土的干湿循环特性试验研究

碳化固化技术是一种利用二氧化碳对搅拌有活性氧化镁的土体进行碳化,以达到快速提高强度的低碳搅拌处理软土的创新技术。通过室内试验研究干湿循环对碳化固化土物理力学特性的影响,并与相同掺量下水泥固化土进行对比。结果表明:活性Mg O固化粉土碳化3 h试样的无侧限抗压强度可达5 MPa,粉质黏土碳化24 h试样可达2.6 MPa;干湿循环后碳化固化土的干密度降低,而水泥土干密度基本不变;6次干湿循环后粉土碳化试样的无侧限抗压强度仍然能达到4 MPa以上,为水泥固化粉土强度的2倍,具有较好的抗干湿循环性能;经过6次干湿循环后,粉质黏土碳化试样的残余强度仅为35%,而水泥固化粉质黏土降到65%,表明固化粉质黏土的抗干湿循环性能均较差,且粉质黏土碳化试样的抗干湿循环能力不及水泥固化粉质黏土试样。通过X射线衍射(XRD)、电镜扫描(SEM)及压汞试验(MIP)测试表明干湿循环对粉土碳化试样的累计孔隙影响不大,因此粉土试样仍然具有比较大的密实度来保证试样强度;粉质黏土碳化试样因孔隙增加明显而变得疏松,因此强度显著降低。     

活性氧化镁碳化加固粉土微观机理研究

基于活性MgO-CO2碳化固化法,开展了不同活性MgO掺量、碳化时间和初始含水率条件下的碳化粉土的无侧限抗压试验、X-射线衍射(XRD)、扫描电镜(SEM)和压汞试验(MIP)等,以通过无侧限抗压强度、化学成分、结构形貌和微观孔隙等特征来揭示碳化固化粉土的微观机理,最后提出活性MgO碳化固化粉土微观机理模型。结果表明：活性MgO碳化固化粉土的无侧限抗压强度在12h内随MgO掺量和碳化时间增加而增加,随初始含水率增加而降低|碳化后含水率显著降低,且强度与含水率间呈很好的线性负相关关系。碳化生成的水化镁式碳酸盐,包括棱柱状水菱镁石、薄片状球菱镁石/水碳镁石和纤维碳镁石,这些碳化产物对应的XRD峰强随MgO掺量和碳化时间增加而增加,随初始含水率增加而降低|存在最优MgO掺量和初始含水率,使碳化土孔隙体积最小。碳化土无侧限抗压强度与水菱镁石晶体对应的XRD峰强呈正相关关系。水菱镁石的生成促进了强度增长,与其他碳化产物共同促进土体孔隙减小。     

活性MgO碳化固化土的渗透特性研究

碳化固化法是一种低碳搅拌处理软土的创新技术,碳化反应生成的产物能有效降低固化土的孔隙率。在已有研究的基础上,着重研究碳化土的渗透特性,以利于其工程应用。采用室内渗透试验,系统研究了活性MgO掺量、碳化时间、初始含水率和CO_2通气压力对MgO碳化粉土和碳化粉质黏土渗透系数的影响规律,并与相同固化剂掺量、相同初始含水率下水泥固化土的渗透系数进行了对比。结果表明:活性MgO碳化土的渗透系数随MgO掺量的增加而降低,与相同掺量下水泥固化土的渗透系数处于同一数量级;MgO碳化粉土的渗透系数明显大于碳化粉质黏土的渗透系数;当MgO碳化粉土和粉质黏土碳化6.0 h时,相应的渗透系数达到最小(10-6);通气压力对MgO碳化土的渗透系数影响不大,在通气压力为200 k Pa时渗透系数较小。因此,活性MgO碳化土具有与水泥固化土相近的抗渗性能,有良好的推广应用前景。     

Mechanical performances and microstructural characteristics of reactive MgO-carbonated silt subjected to freezing-thawing cycles

The characteristics of reactive magnesia(MgO)-carbonated silt in respect to long-term stability have not been well understood in severely cold climate despite the usage of reactive MgO in enhancing the engineering performances.Under the binder content of 15% and initial water content of 25%,MgO-admixed silt specimens were carbonized for 3 h and 6 h and then subjected to different numbers of freezingthawing(F-T) cycles.After different F-T cycles,the physico-mechanical properties of MgO-carbonated silt were analyzed in comparison with Portland cement(PC)-stabilized silt through physical and unconfined compression tests.Besides,a series of micro tests on MgO-carbonated specimens was performed including X-ray diffraction(XRD),scanning electron microscopy(SEM) and mercury intrusion porosimetry(MIP) tests.The results demonstrate that both mass change ratio and moisture content of carbonated/stabilized silt decrease,and these values of MgO-carbonated silt are significantly lower while the density is higher compared to PC-stabilized silt.The strengths and moduli of MgO-carbonated silt are still two times higher than those of PC-stabilized specimens and the strength change ratio of keeps above0.8 after F-T cycles.There is no visible transformation between nesquehonite and dypingite/hydromagnesite,although the XRD peaks of nesquehonite decrease and the bonding and filling effects weaken slightly.After 6 and 10 F-T cycles,the pore-size characteristics changed from a unimodal distribution to a three-peak and bimodal distribution,respectively.The total,macro and large pore volumes increase obviously while the medium and small pore volumes decrease except for intra-aggregate pore.The findings show better F-T durability of MgO-carbonated silt,which would be helpful for facilitating the application of MgO carbonation in the soil treatment.     

Hydraulic properties of single fractures grouted by different types of carbon nanomaterial-based cement composites

In underground projects such as nuclear waste disposal facilities, the requirement of controlling the permeability is very important, and common grouting materials are not suitable. Therefore, creating a composite material to be used for the grouting in engineering projects is essential. The present study characterizes the efficiency of several kinds of carbon grouting composites based on nanomaterials, in terms of the hydraulic properties of single fractures in granites using a rock permeability test system. The pore structure and distribution of the carbon nanomaterials in the grouting pastes were investigated using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The results reveal that the hydraulic properties of grouting samples can be well described by linear Darcy’s law. These grouting composites can decrease the equivalent permeability (K_g) by 31.8–81.0% in different confining pressures. The enhancing effect of the graphene oxide (GO)/multi-walled carbon nanotube (MWCNT)–based ordinary Portland cement (OPC) grouted sample is the best, improving the permeability resistance by 240.3–315.0%. The MIP test proves that mixing carbon nanomaterials will improve the pore characteristics. The GO/MWCNT-OPC grouting presents the optimal decrease in all total intruded volume, average pore diameter and porosity, dropping by 33.8, 40.7 and 22.9%, respectively. Both the average pore diameter and porosity of grouting pastes have zero-intercept linear correlations with the equivalent permeability and can predicate permeability-related properties with considerable accuracy. The SEM images further confirmed that carbon nanomaterials can not only refine the pore structure of grouting materials through the superior properties of the MWCNTs to bridge micro-sized cracks and the interfacial adhesion of GO but also reinforce the flow resistance of the interface between the grouting material and the granite. The findings of this study can help guide the implementation of carbon nanomaterial-OPC grouting in engineering projects.

     

Chemical interaction and hydraulic performance of geosynthetic clay liners isothermally hydrated from silty sand subgrade

The effects of the silt aggregation, compaction density, and water content of the subgrade on the hydration of five different geosynthetic clay liner (GCL) products is reported based on a series of laboratory column experiments conducted over a six-year period. GCLs meeting typical specifications in terms of minimum hydraulic conductivity and swell index are hydrated to equilibrium from the same subgrade soil with sufficient cations to cause cation exchange during hydration. It is then shown that the GCL bentonite granularity and GCL structure can have a significant (~four orders of magnitude) effect on hydraulic conductivity under the same test conditions (from 8 × 10⁻¹² m/s for one GCL to 6 × 10⁻⁸ m/s for another GCL product). The effect of subgrade water content on the hydraulic performance of GCLs are not self-evident and quite dependent on the bentonite granularity, GCL structure, and permeant. Varying the subgrade water content from 5 to 16% and allowing the GCL to hydrate to equilibrium before permeation led to up to 5-fold difference in hydraulic conductivity when permeated with tap water and up to 60-fold difference when the same product is permeated with synthetic municipal solid waste leachate. When permeated with synthetic leachate, increasing stress from 70 kPa to 150 kPa led to a slight (average 37%; maximum 2.7-fold) decrease in hydraulic conductivity due to a decrease in bulk void ratio. It is shown that hydraulic conductivity is lower for GCLs with a scrim-reinforced geotextile, and/or with finer bentonite. It is shown that selecting a GCL based on the initial hydraulic conductivity and swell index in a manufacturers product sheet provides no assurance of good performance in field applications and it is recommended that designers pay more attention to selection of a GCL and preparation of the subgrade for important projects.     

现场运行近50年水工混凝土性能的试验研究

试验研究了龄期近50年混凝土的材料性能.混凝土的抗压强度增长趋势符合对数曲线的关系;劈拉强度与抗压强度存在良好的内在关系,且泊松比的数值与普通混凝土的基本一致;过水水工混凝土的碳化层强度要低于未碳化混凝土的强度,这与碳化混凝土的孔径、总孔隙率及最可几孔径变大,孔结构状况变差有关;长龄期未碳化混凝土的抗渗能力与混凝土的设计强度比较匹配,并没有随着强度的增长而提高,孔结构压汞测试及电镜观察结果揭示了其中的原因.     

CO2碳化矿渣-CaO-MgO加固土效能与机理探索

CO₂碳化联合工业固废协同加固土技术是旨在替代传统水泥固化方法的新型技术尝试。以工业废料——矿渣为主要材料,辅以活性MgO和CaO形成矿渣-CaO-MgO固化体系,将固化土料均匀搅拌制样后进行CO₂碳化试验。通过无侧限抗压强度、扫描电镜和X射线衍射等试验,探究固化剂掺量、配比、碳化时间和初始含水率等因素对碳化加固土效果的影响。结果表明：CO₂碳化对土体加固具有明显改良效果,碳化24 h试样抗压强度最高可提升25.77倍;碳化试样抗压强度与固化剂掺量（6S4L0M除外）、活性MgO占比呈正相关;碳化试样强度随碳化时间先增加后趋于平缓（或略微下降）、最佳碳化时间为6 h左右,随初始含水率增加而先增加后下降、最佳含水率为16%左右;活性MgO碳化效能明显优于活性CaO,矿渣中低活性CaO并不能显著改良自身碳化性能。CO₂碳化作用促使碳酸盐晶体（CaCO₃、MgCO₃）生成,晶体发育程度与碳化时间、固化剂掺量及活性等因素有关;碳酸盐晶体有效填充试样内部孔隙并黏结土颗粒,形成整体骨架结构使试样强度得以大幅提升。     

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。     

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。     

基于土水特征曲线预测城市固体废弃物（MSW）非饱和渗透系数研究

基于对人工配制的城市固体废弃物（ MSW ）的土水特征试验,研究了不同有机含量、不同孔隙比下 MSW 的非饱和渗透系数,得到了 MSW 非饱和渗透系数随有机含量和孔隙比变化的定量关系式。研究结果表明在双对数坐标系下, MSW 的气相和液相非饱和渗透系数和吸力几乎为线性关系;对于相同的吸力作用,孔隙比越大,其液相和气相非饱和渗透系数也越大;总体上液相非饱和渗透系数的变化范围为 10 ^{- 11} ～ 10 ^{- 5} cm/s ,气相非饱和渗透系数的变化范围为 10 ^{- 5} ～ 10 ^{- 2} cm/s 。与有机含量的变化相比,孔隙比的变化对 MSW 非饱和渗透特性的影响较为明显。公式验证表明,利用该方法预测已知成分和孔隙比的 MSW 的非饱和渗透系数是可行的。     

Ageing Effects on Consolidation Properties—Based on the Site Investigation of Osaka Pleistocene Clays—

Soil samples were recovered from the Osaka basin, where Holocene as well as Pleistocene marine clays are thickly deposited. It is found from X-ray diffraction analysis that the component of clay minerals for these day layers is almost identical through all depths. 'This fact suggests that the origin of these sediments has not been drastically changed through at least more than 1,000,000 years. Using data obtained by oedometer tests, this paper examines ageing effects on consolidation properties. It is found that void ratio for the Pleistocene clays keeps large even under high in situ effective overburden pressure (p_vo′). Salinity of pore fluid decreases with depth and becomes nearly zero at depths deeper than 70 m. However, the change in salinity does not give any influences on the index properties, suggesting that the relatively large void ratio for the Pleistocene clays is caused by other reasons than leaching. It is also found that the Pleistocene clays reveal no particular differences in consolidation properties, but fundamentally similar to those of Holocene clays'. There is no distinguished difference in the overconsolidation ratio (OCR), non-linearity of the e-log p' relation after yield consolidation pressure including the maximum compression index (C_cmax) and the hydraulic conductivity (k). The only prominent difference in the Osaka Pleistocene clays, compared to the Holocene clay, is relatively small void ratio due to the large p_vo′.     

Influence of mineral precipitation on the performance and long-term stability of Fe 0 -permeable reactive barriers: A review on the basis of 19 Fe 0 -reactive barrier sites

A comparative evaluation of the monitoring results from 19 Fe⁰-permeable reactive barriers (PRBs) is presented which considers changes of groundwater composition within the Fe⁰-PRBs, the type and distribution of mineral phases that precipitate, porosity reductions, extent of the load of inorganic groundwater constituents and the overall performance. The findings show that the changes in groundwater composition within the PRBs are extremely similar, independent of flow velocities, residence times, and concentrations of inorganic substances. In addition, the type and distribution of mineral precipitates do not show great variances with the largest mineral accumulations within the first 10 to 30 cm from the entrance face of the PRBs. From these comparisons it can be concluded that loads of dissolved inorganic species higher than 200 kg?·?a^–1?·?m^–2 limit the long-term performance of Fe⁰-PRBs up to a few years due to porosity losses caused by mineral precipitation, independent of the type of construction. Moreover, detailed calculations of porosity and hydraulic conductivity decreases of the PRBs at the Rheine and Tübingen sites show that the influence of mineral fouling on the hydraulic permeability of a PRB can only be estimated by accounting for the interaction between reduction in porosity, permeability and flow rate. The findings suggest that it is possible to pre-estimate the porosity and permeability development for a potential PRB.     

水工碳化混凝土微观试验研究

采用压汞法研究近50年龄期的水工混凝土的微观结构。试验表明,混凝土碳化层较未碳化层孔径增大,总孔隙率变大,最可几孔径变大,使得混凝土碳化层强度低于未碳化层强度。     

饱和高庙子膨润土的渗透特性

在核废料深层地质处置工程中,膨润土的渗透特性是需要考虑的重要因素之一。对浸水饱和的高庙子钙基膨润土压实试样进行压缩试验,利用时间平方根法测定压缩试验中每级荷载下的固结系数,然后用太沙基一维固结理论计算土的饱和渗透系数。试验研究表明:在弹塑性阶段,计算所得饱和渗透系数随竖向应力的增大而减小,在双对数坐标下,计算渗透系数与竖向应力呈线性关系;孔隙比对渗透性起着主导作用,在半对数坐标下,计算渗透系数随孔隙比的减小而线性减小,由该线性关系得到的某一孔隙比下的计算渗透系数与公开发表的相同孔隙比的高庙子钙基膨润土的渗透系数接近。研究结果及相关文献证明应用固结理论方法间接推算膨润土饱和渗透系数的方法是可行的,该方法适用于低渗透性黏土的饱和渗透系数的量测。     

The Effect of Fines on Critical State and Liquefaction Resistance Characteristics of Non-Plastic Silty Sands

Monotonic and cyclic triaxial tests were carried out on sand-silt mixtures for the investigation of the effect of fines content on their critical state and liquefaction resistance characteristics. Both the undrained and the drained monotonic tests produce a unique critical state line for each tested mixture, which moves downwards with increasing fines content up to a threshold value of 35% and then upwards. At a given void ratio and mean effective stress, the liquefaction resistance ratio decreases with increasing fines content up to the same threshold value of 35%, and increases thereafter with further increasing fines content. However, at a given intergranular void ratio, defined as the ratio of the volume of fines plus voids to that of sand particles, liquefaction resistance ratio increases with increasing fines content up to the threshold value. The threshold fines content value, which is an important parameter in determining the transition from the sand dominated to the silt dominated behaviour of sand-silt mixtures, is related to their particle packing. An expression is proposed for the estimation of the threshold fines content as a function of the mean diameter ratio, d₅₀/D₅₀, and the void ratio. The results, presented herein, also show that for each tested mixture the liquefaction resistance ratio is related to the state parameter and that this relation is influenced by the effective stress level and fines content. The results on the sand-silt mixtures are supported by similar results on natural silty sands.     

Correlating Cyclic Strength with Fines Contents Through State Parameters

Attempts have been made in the past few decades to establish an equivalent void ratio indicative of the effective density state of sands that contain fines. The concept of an equivalent void ratio is most useful when the grain size distribution of the sand/silt mixture is similar to that of binary packing. Due to its grain characteristics and the wide range of compressibility when the fines content changes, correlating the cyclic strength to fines content via the equivalent void ratio for Mai Liao Sand (MLS) is not a straightforward procedure. Earlier studies have demonstrated the effectiveness of relating cyclic strength to state parameters for sands. Using a series of cyclic triaxial test results as a database, the authors demonstrated the effectiveness of relating cyclic strength to fines contents (f_c) through a state parameter for MLS with an f_c ranging from 0 to 30% that takes advantage of the equivalent void ratio. In addition to the density and stress states, the state parameter based correlations indirectly reflect the effects of soil grain characteristics. The drawback of requiring a separate critical state line when f_c changes can be circumvented by using a unified critical state line based on the equivalent void ratio.     

Production of MgO-type expansive agent in dam concrete by use of industrial by-products

A novel MgO-type expansive agent in a designed formula has been prepared with industrial by-products—magnesite, serpentine and dolomite calcined at 1150 °C for 1 h. The compositions of expansive agent were studied by X-ray diffractometry (XRD), DSC and scanning electron microscopy (SEM), and the results showed that the main phases were MgO, CaO and C₂S. The hydration of CaO to Ca(OH)₂ is quick and can be completed in 3 days in water at 20 °C, and MgO to Mg(OH)₂ is slow and about 57% MgO hydrates into Mg(OH)₂ in 180 days under the same condition, which is quicker than in water. The hydrations of CaO and MgO generate expansions in early and later age, respectively.     

Variability of Physical and Consolidation Test Results for Relatively Uniform Clay Samples Retrieved from Osaka Bay

Samples collected from both Holocene and Pleistocene layers in Osaka Bay were examined in this study. The objective of this study is to evaluate variations of soil parameters in a sample length of about one meter. The retrieved samples from EL. (elevation) -37.5 to -38.5 m and from EL.-125.5 to -126.5 m with a length of about one meter were divided into every 25 mm long to trim consolidation specimens. Variations of soil parameters obtained from laboratory tests were evaluated. Clay microfabric was also evaluated by scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). The clay layers are relatively homogeneous with variation coefficient CV in a range of 0.025 to 0.12 for the consolidation parameters (e, p′_c, C_c and c_v). CV of void ratio e is only 0.025 to 0.056. In contrast, CV values of the other consolidation parameters are significantly greater than this, but the average is less than 0.1. It can be concluded that CV of soil consolidation parameters for the homogeneous natural clay deposits dealt in this study is less than about 0.1. These variations are mainly influenced by sedimentary environment. Even the soil is said to be homogeneous, because the specimen size for the laboratory tests is sufficiently large in consideration of microscopic heterogeneity obtained from SEM and MIP.     

宽级配砾质土防渗性能研究

宽级配砾质土是一种性能良好的防渗料,然而土体的细粒含量、细粒物理特性、击实功以及颗粒级配等多种因素均对其渗透系数有很大的影响。针对以上因素对宽级配土的渗透系数影响因素展开试验研究。采用不同细粒含量、不同种类细粒部分以及不同级配的宽级配砾质土进行室内变水头渗透试验,探究这些因素对渗透系数的影响规律。试验结果表明:宽级配砾质土的渗透系数随着细粒含量的升高先迅速降低后趋于平稳;随着细粒料液限和塑性指数的增加,宽级配砾质土的渗透系数逐渐降低;以粉质黏土和黏土为细粒料的宽级配砾质土的渗透系数均随击实功的增加呈指数函数形式下降;粗粒料级配连续性越好,宽级配砾质土渗透系数越低。根据大坝心墙渗透系数小于1×10~(-5) cm/s的防渗要求,提出了以界限含水率和细粒料百分比为控制指标的宽级配土防渗料的控制标准。