砂土介质注浆渗透扩散试验与加固机制研究

设计了一套可视化砂土介质恒压注浆渗透扩散与加固模拟试验装置。选用普通硅酸盐42.5水泥（OPC）、超细硅酸盐水泥（MC）、超细硫铝水泥（MSAC）及自主研发材料（EMCG）,对砂土多孔介质进行了注浆渗透扩散与加固试验。研究了细砂土体不同浆液、注浆压力工况下扩散距离、注浆量随时间变化规律,以及不同浆液、砂样级配及注浆压力对加固效果影响。采用极差分析法确定了加固效果主控因素,获得了加固体宏观破坏模式,通过SEM分析了岩-浆界面微观加固模型,揭示了不同材料加固效果差异的本质原因。研究结果表明：注浆材料与颗粒质量分数主导着细砂土体可注性,注浆压力对可注性提高程度不大,EMCG可注性最强,MC、MSAC次之,OPC最差;在完全可注情况下,砂样级配越细,加固体强度改善效果越明显;EMCG材料加固体7、28 d强度明显高于MSAC、MC、OPC加固体;EMCG的7 d加固体强度达到28 d参数70%以上;注浆材料主导着注浆加固效果,EMCG对不同级配砂土体注浆加固效果显著优于其他水泥类材料,OPC最差;注浆后EMCG浆-岩界面生成的致密C-S-H凝胶体能够有效提高胶结强度。最后从注浆材料选型、注浆过程控制、钻孔布置方面对砂土层渗透注浆设计方法提出了工程治理改进建议。     

Factors Affecting Engineering Properties of Microfine Cement Grouted Sands

An experimental investigation was conducted in order to evaluate the influence of distance from the injection point and of parameters pertinent to the cement, the suspension and the sand on the effectiveness of microfine cement grouts. Three different cement types, each at three different gradations having nominal maximum grain sizes of 100, 20 and 10 μm, were used. Grouting effectiveness was evaluated by injecting suspensions with water to cement (W/C) ratios of 1, 2 and 3, by weight, into five uniform sand fractions with different grain sizes and eight composite sands with different gradations, using a specially constructed apparatus. Unconfined compression and permeability tests were conducted on the resulting grouted sand specimens, after curing for 28 and 90 days. Microfine cement grouted sands obtained unconfined compression strength values of up to 14.9 MPa and permeability coefficients as low as 1.3 × 10^?6 cm/s or by up to 5 orders of magnitude lower than those of clean sands. The W/C ratio and the bleed capacity of suspensions as well as the effective grain size and the permeability coefficient of sands are very important parameters, since they affect substantially the grouted sand properties and are correlated satisfactorily with them. The strength and permeability of grouted sands can increase, decrease or remain constant with distance from the injection point depending on the easiness of suspension penetration into the sands. The improvement of grouted sand properties with increasing distance from the injection point is consistent with the observed increase of the cement content of grouted sands.     

Effect of Natural Zeolite and Cement Additive on the Strength of Sand

It is widely known and well emphasized that the cemented sand is one of economic and environmental topics in soil stabilization. In some instances, a blend of sand, cement and other materials such as fiber, glass, nano particle and zeolite can commercially be available and effectively used in soil stabilization especially in road construction. In regard to zeolite, its influence and effectiveness on the properties of cemented sands systems has not been completely explored. Hence, in this study, based on an experimental program, it has been tried to investigate the potential of a zeolite stabilizer known as additive material to improve the properties of cemented sands. A total number of 216 unconfined compression tests were carried out on cured samples in 7, 28 and 90 days. Results show unconfined compression strength and failure properties improvements of cement sand specimens when cement replaced by zeolite at optimum proportions of 30 % after 28 days due to pozzolanic reaction. The rate of strength improvement is approximately 20–78 and 20–60 % for 28 and 90 days curing times respectively. The efficiency of using zeolite has been enhanced by increasing the cement content and porosity of the compacted mixture. The replacement of cement by natural zeolite led to an increase of the pH after 14 days. Chemical oxygen demand (COD) tests demonstrate that the materials with the zeolite mixture reveal stronger adsorptive capacity of COD in compare to cemented mixture. Scanning electron microscope images show that adding zeolite in cemented sand changes the microstructure (filling large porosity and pozzolanic reaction) that results in increasing strength.     

An artificial neural network for groutability prediction of permeation grouting with microfine cement grouts

The use of microfine cements in permeation grouting has been growing as a strategy in geotechnical engineering because it usually provides improved groutability (N). One of the major challenges of using microfine cement grouts is the ability to estimate the N within a reasonable level of error. The suitability of traditional groutability prediction formulas, which are mostly based on the grain-size of the soil and the grout, is questionable for semi-nanometer scale grout. This study first investigated the accuracy of the current formulas; we found that the accuracy ranges from 45% to 68%, a level that is not adequate for practical engineering. An alternative approach, based on a Radial Basis Function Neural Network (RBFNN), was developed. RBFNN provides a prediction with a 95.8% accuracy within a short time frame. Several parameters were considered in our proposed network; besides the grain-size of the soil (D₁₀/D₁₅), other important parameters included the void ratio (e), the fines content (FC), the uniformity coefficient (C_u), the coefficient of gradation (C_z) and the water-to-cement ratio (w/c). A total of 240 in situ data samples were collected to support the training and testing of the network. After finding a good correlation between the field observation and the RBFNN output, it was concluded that RBFNN is a suitable and reliable tool to predict the outcome of permeation grouting when microfine cement grout is used.     

Laboratory determination of mechanical and hydraulic properties of chemically grouted sands

The purpose of this paper is to investigate mechanical and hydraulic properties of sands treated with mineral-based grouts through the results of a laboratory test programme consisting of unconfined compression tests (UCS), triaxial bender element tests (BeT) and constant flow permeability tests in triaxial apparatus. An improved apparatus was set up for obtaining high quality, multiple grouted specimens from a single column. Two selected natural sands having different grain sizes were grouted with two mineral-based silica grouts, resulting in different levels of improvement. The behaviour of the sands treated by mineral grouts, in terms of strength, initial stiffness and permeability, was compared with that exhibited by more traditional silicate grouts. The results of this study indicate that sands treated with mineral grouts result in higher strengths, higher initial shear modulus and lower permeability values than the sands treated with the silicate solution. The effect of grout type, effective confining pressure, and sand particle-size on small-strain shear modulus of grouted sand specimens was evaluated. Based on test results, the small strain shear modulus increment from treated to untreated specimens has been correlated with the unconfined compressive strength, obtaining a unique relationship regardless of grout type and grain-size of tested sands.     

石佛寺水库改性粉细砂筑坝试验研究

采用室内试验和现场原型试验,对辽河石佛寺水库库区内掺入水泥后粉细砂的特性进行了研究。试验结果表明,随着水泥掺入量的增加,粉细砂的凝聚力和内摩擦角增大,渗透系数减小。当水泥的掺入量为2 % 时,抗剪强度指标和渗透系数已明显改善,而且在最不利的情况下满足对渗透变形的要求。粉细砂掺入水泥后,其性能得到了很大改善,用于修筑拦河主坝是完全可行的。     

Groutability of cement‐based grout with consideration of viscosity and filtration phenomenon

The groutability depends on the properties of the grout, its injection processes, and on the mechanical properties of the soil formation. During the process of pouring cement‐based grouting into a porous medium, a variation with time occurs in the viscosity of grout suspension. In addition, the particle filtration phenomenon will limit the expansion of the grouted zone because cement particles are progressively stagnant within the soil matrix. In this paper, a closed‐form solution was derived by implementing the mass balance equations and the generalized phenomenological filtration law, which can be used to evaluate the deposition of cement‐based grout in the soil matrix. The closed‐form solution relevant to a particular spherical flow was modified by a step‐wise numerical calculation, considering the variable viscosity caused by a chemical reaction, and the decrease in porosity resulting from grout particle deposition in the soil pores. A series of pilot‐scale chamber injection tests was performed to verify that the developed step‐wise numerical calculation is able to evaluate the injectable volume of grout and the deposition of grout particles. The results of the chamber injection tests concurred well with that of the step‐wise numerical calculation. Based on the filtration phenomenon, a viable approach for estimating the groutability of cement‐based grout in a porous medium was also suggested, which might facilitate a new insight in the design of the grouting process. Copyright © 2009 John Wiley & Sons, Ltd.     

Modelling of cement suspension flow in granular porous media

A theoretical model of cement suspensions flow in granular porous media considering particle filtration is presented in this paper. Two phenomenological laws have been retained for the filtration rate and the intrinsic permeability evolution. A linear evolution with respect to the volume fraction of cement in the grout has been retained for the filtration rate. The intrinsic permeability of the porous medium is looked for in the form of a hyperbolic function of the porosity change. The model depends on two phenomenological parameters only. The equations of this model are solved analytically in the one‐dimensional case. Besides, a numerical resolution based on the finite element method is also presented. It could be implemented easily in situations where no analytical solution is available. Finally, the predictions of the model are compared to the results of a grout injection test on a long column of sand. Copyright © 2005 John Wiley & Sons, Ltd.     

Deposition and mobilization of functionalized multiwall carbon nanotubes in saturated porous media: effect of grain size,flow velocity and solution chemistry

Carbon nanotubes (CNTs) are widely manufactured nanoparticles which are utilized in a number of consumer products, such as sporting goods, electronics and biomedical applications. Due to their accelerating production and use, CNTs constitute a potential environmental risk if they are released to soil and groundwater systems. It is, therefore, essential to improve the current understanding of environmental fate and transport of CNTs. The current study systematically investigated the effect of solution chemistry (pH and ionic strength) and physical conditions (collector grain size and flow rate) on the deposition and mobilization of functionalized multiwall carbon nanotubes (MWCNTs) using a series of column experiments under fully saturated conditions. A one-dimensional convection–dispersion model including collector efficiency for cylindrical nanoparticles was used to simulate the transport of MWCNTs in porous media. It was observed that an increase in pH resulted in increased mobility of MWCNTs. However, the transport of MWCNTs was strongly dependent on ionic strength of the background solution and a critical deposition concentration was observed between 3 and 4 mM NaCl concentration, with more than 99 % filtration of MWCNTs at 4 mM. The finer sand grains were able to filter a significant amount of MWCNTs (15 % more than coarse sand) from the inflow solution; this was likely caused by grain-to-grain straining mechanisms in the finer sand. A decrease in pore water velocity also led to more deposition of MWCNTs due to lowering of the kinetic energy of the particles. The results from this study indicated that a weak secondary minimum existed under unfavorable conditions for deposition, but the particles were trapped at both primary and secondary minimum.     

Preparation of Uniform Sand Specimens Using Stationary Pluviation and Vibratory Methods

Relative density is an important state parameter that influences the soil behavior. Preparation of sand specimens with uniform density is critical during large-scale laboratory testing in geotechnical engineering. In this study, the details of a stationary air pluviation device used to prepare uniform sand specimens in a large-size test chamber with dimensions equal to 900 mm × 900 mm × 1000 mm (in length, width, and depth) are provided. The proposed device is found to be simple to construct due to presence of only two diffuser sieves with an ability to produce uniform sand beds in a reasonably quick time. Prior to construction of full-scale pluviation device, a scaled-down model of the device with plan dimensions equal to 300 mm × 300 mm is fabricated to perform calibration studies. The range of densities of two gradations of Indian Standard sand (IS Grade II and IS Grade III) obtained using this device for various heights of fall of sand particles and passing through different opening sizes are provided. Relative density in the range of 53–99 % and 74–99 % is achieved for IS Grade II and III sands, respectively. The spatial uniformity in densities is also assessed, and the coefficient of variation (COV) in the density is found to be less than about 7 %. In addition to pluviation method, uniform sand beds are also prepared using pneumatically-operated vibratory method. The target relative density of sand bed is achieved by adjusting the pressure of compressed air inside the vibrator, and the maximum relative density of IS Grade II and Grade III sands from vibratory method is found to be higher than that from pluviation method for the range of pressures chosen in the study.     

Behavior of cemented sands—I. Testing

This paper is accompanied by a study on constitutive modelling issues of cemented sands. The concentration here is on experimental issues related to the triaxial testing of cemented sands. A preliminary investigation is performed aiming to identify potential effects of specimen size and slenderness on the stress–strain–strength characteristics of cemented sands. A comprehensive experimental study follows where clean sand specimens, as well as specimens with 2, 4 and 6 per cent cement content, are tested. The aim of the study is to examine the effects of cement content and confinement on the shear strength, stiffness, softening and dilation characteristics of cemented sand. © 1997 John Wiley & Sons, Ltd.     

Methane-derived carbonate cements in barrier and beach sands of a subtropical delta complex

Highly negative δC¹³ values, ?18 to ? 40%., for carbonate cements found in Recent barrier and beach sands of the Mississippi River Delta complex strongly suggest that considerable carbon is furnished to the CaCO₃ cements by either chemical or biological oxidation of CH₄. These cemented sands are commonly found on beaches of the Chandeleur barrier island chain and other sites along the Louisiana coast where Holocene sands are rapidly transgressing over highly organic marsh deposits. Generation of CH₄ from underlying anoxic marsh sediments, followed by vertical migration and oxidation to CO₂ in the porous overlying sand, appears to be the unique set of conditions regulating this process of carbonate cementation.     

Influence of grain size gradation of sand impurities on strength behaviour of cement-treated clay

In nature, soils are often composed of varying amounts of clay, silt and sand. Variation of the percentage of these compositions can affect the final strength of the soils when stabilised with cement. In this study, focus was placed on clayey soils with different gradation of sand impurities up to 40% in mass. An extensive study of such clayey soils treated with cement was investigated. For the results, it is noted that water:cement ratio was a major influence of strength development of cement-treated clayey soils. In contrast, the soil:cement ratio was found to have minor effects on the strength development. The presence of sand impurities has a significant reduction on the strength development of the cement-treated clayey soil mixture due to more free water available for hydration. The use of free-water:cement ratio is adopted which was shown to be capable of adjusting for such change in amount of free water and water holding capacity of the clay which is determined with Atterberg’s liquid limit tests. The effects of gradation (fine, coarse and well-graded) of the sand impurities were found to affect strength development minimally, owing to similarities in their liquid limits when mixed with clay. Ordinary Portland cement (OPC)-treated clayey soils produced a more rapid gain in strength but lower final strength at 28 days of curing as compared with Portland blast furnace cement (PBFC). This is found to be persistent for different gradation of sand impurities. A linear correlation can be established based on the log of the unconfined compressive strengths developed at different curing age, with slopes of these linear trends found to be similar for PBFC and OPC-treated clayey soil specimens. Finally, a strength prediction model comprising of these findings is developed. The parameters adopted in this model coincide with values proposed by past studies, thereby validating the robustness of the model. The practical benefits from this study offer a quality control scheme to forecast long-term performance of cement-treated clayey soils as well as optimise cement dosage in cement stabilisation to produce a more cost-effective and less environmental-invasive usage of the technology in geotechnical applications.

     

Transport and mobilization of multiwall carbon nanotubes in quartz sand under varying saturation

In this study, a series of sand packed columns were used to investigate the mobility of multiwall carbon nanotubes (MWCNTs) in unsaturated porous media under unfavorable conditions for deposition. The flow through column experiments were designed to assess water content, flow rate, and grain size effect on the mobility of MWCNTs. It was found that variation in water content had no significant effect on retention of MWCNTs until it was lowered to 16 % effective saturation. Thick water films, high flow rate, and repulsive forces between MWCNTs and porous media made MWCNTs highly mobile. Different porous media grain sizes (D ₅₀ = 150–300 μm) were used in this study. The mobility of MWCNTs slightly decreased in finer grain sands, which was deemed to be an effect of increase in surface area and the number of depositional sites, in combination with low-pore water velocity. However, physical straining was not observed in selected fine-grain sands and aspect ratio of MWCNTs had low impact on mobility. Variations in pore-water velocity were produced by both changes in water saturation and in flow rate. At high pore-water velocities, the MWCNTs were generally mobile. However, for the combination of low-pore water velocity with either low water saturation or small grain size, some retention of MWCNTs was observed. Hence, low velocity in combination with flow through smaller pores increased MWCNT deposition.     

Strength and Stiffness Properties of Green Lightweight Fill Mixtures

Approximately 250 million tires are discarded each year in the United States and 20 million in Canada. Finding sustainable ways to dispose these tires continues to be a problem throughout the world not only in North America. Disposal issues along with a continuing increase in tire production have resulted in an increase in tire stockpiles, 30 % of these tires end up occupying valuable landfill space. Tire Derived Aggregates (TDA) made from scrap tires have a compacted dry unit weight of 6.9 kN/m³ and a specific gravity of 1.06 making their use as lightweight fill material especially for highway embankments over soft soils very advantageous. Using TDA as fill material not only provide a new construction material but also help to provide sustainable solution for several environmental and economic problems. Previous studies have shown that the addition of Tire Derived Aggregates (TDA) to sand results in a fill material which is characterized by its higher shear strength compared to that of 100 % sand. While there have been several studies investigating various properties and situations involving TDA’s, there remain areas that still require exploration. The effect of soil/aggregate gradation on shear strength is one of particular interest. All of the research conducted to date investigated only the sand gradation mixed with uniform TDA gradation. None of the conducted research investigated the interaction between the gradation of the two materials and its effect on the performance of the fill mixture. The main goal of this paper is to gain an understanding of this interaction in aim of producing better fill mixtures with enhanced characteristics.     

Heavy metal exposure from co-processing of hazardous wastes for cement production and associated human risk assessment

This study was carried out to determine the concentration of heavy metals (Cd, Ni, Pb, Cr, Ni and Zn) in ordinary Portland cement (OPC) produced from the co-processing with hazardous waste in comparison with OPC produced using natural raw materials. The results showed that the concentration of heavy metals in cement produced from natural raw material was in the order of Zn > Pb > Cr > Ni > Cu > Cd. Zn and Cd were the highest and the lowest concentrations, respectively, in cements produced from the co-processing activity. The difference between heavy metals concentrations in OPC produced with and without co-processing was found to be statistically significant. The concentration of heavy metals in the cement produced is generally factory dependent. The human risk assessment associated with the heavy metals for non-carcinogenic and carcinogenic risks has been evaluated. The calculated hazard index (HI) and total lifetime cancer risks (LCR) were lower than the acceptable threshold reference values, HI < 1 and LCR < 1 × 10^?4, respectively. Thus, it is anticipated that there is no potential of non-carcinogenic and carcinogenic risks for both adult and children. However, the findings indicated that there is a need for regulatory monitoring. The exposure pathway for both non-carcinogenic and carcinogenic risks are both in the order of ingestion > dermal > inhalation.     

Carbonate cementation of some Pleistocene temperate marine sediments

Carbonate cementation of some carbonate and quartz sands in three raised beaches of temperate origins was investigated. The carbonate of the cements was found to have been derived from the dissolution of skeletal debris. The sandstones, so produced, now possess only low-magnesium calcite, but the original sediments, like adjacent modern beach and blown sands, probably contained low-magnesium calcite, aragonite and some high-magnesium calcite, all of skeletal origin. In meteoric water the dissolution has occurred of all carbonate within minute, tubulelike, volumes of sand. Concurrent deposition in adjacent volumes of sand of low-magnesium calcite formed cements that are irregularly nodular or uneven on a small scale. Aragonite within the minute nodules has been replaced paramorphically by low-magnesium calcite. Additional local carbonate cements were formed at later dates, around and within solution pipes.     

Use of Late-Barremian sands from Central Tunisia in white cement clinker

The present work focuses on the ability of Late-Barremian sands from Ouaddada Mountain (Central Tunisia), to produce high-quality white cement clinker. Results indicate that these sands are particularly characterized by their fineness of grains (mean grain-size 90–140 μm), high cleanness (sand equivalent 88–98%), extremely low coloring oxides and volatiles levels (less than 0.03 and 1.9% respectively) and high values of whiteness and lightness indices (more than 86 and 90% respectively). Studies based on Bogue calculation, X-ray diffraction (XRD), burnability of raw mixes, Point Counting and Scanning Electron Microscopy (SEM) with Energy-Dispersive X-Ray Spectroscopy (EDS) methods, reveal that the Late-Barremian sands from Ouaddada Mountain can be used in the production of white Portland clinker by mixing with 70.8–75.6% of local limestones and 3.5–24.8% by weight of common Mediterranean kaolins from Guadalajar province (Central Spain), Tamazert (North-East Algeria), ?ile (North-West Turkey), Sebha (South-West Libya), and Abu Zanima area (West-Central Sinai, Egypt). Synthesized clinkers are composed largely of alite (75–76.2%), belite (12.8–16.5%) and aluminate (7.4–8.9%) with subordinate quantities of ferrite (less than 1%), slightly harder to burn and seem to be much whiter than Mediterranean ones with a whiteness index β more than 86%.     

Effect of fines content and void ratio on the saturated hydraulic conductivity and undrained shear strength of sand–silt mixtures

The hydraulic conductivity represents an important indicator parameter in the generation and redistribution of excess pore pressure of sand–silt mixture soil deposits during earthquakes. This paper aims to determine the relationship between the undrained shear strength (liquefaction resistance) and the saturated hydraulic conductivity of the sand–silt mixtures and how much they are affected by the percentage of low plastic fines (finer than 0.074 mm) and void ratio of the soil. The results of flexible wall permeameter and undrained monotonic triaxial tests carried out on samples reconstituted from Chlef river sand with 0, 10, 20, 30, 40, and 50 % non-plastic silt at an effective confining pressure of 100 kPa and two initial relative densities (D _r = 20, 91 %) are presented and discussed. It was found that the undrained shear strength (liquefaction resistance) can be correlated to the fines content, intergranular void ratio and saturated hydraulic conductivity. The results obtained from this study reveal that the saturated hydraulic conductivity (k _sat) of the sand mixed with 50 % low plastic fines can be, in average, four orders of magnitude smaller than that of the clean sand. The results show also that the global void ratio could not be used as a pertinent parameter to explain the undrained shear strength and saturated hydraulic conductivity response of the sand–silt mixtures.     

Engineering Properties, Microstructure and Strength Development of Lightweight Concrete Containing Pumice Aggregates

Lightweight aggregate concretes are widely incorporated in construction and development. This study presented an experimental investigation on the engineering properties of volcanic pumice lightweight aggregates concretes. Three groups of lightweight concretes: 1—coarse pumice aggregates (2–6 mm), 2—course pumice aggregates (2–6 mm)/sand size pumice aggregates and 3—course pumice aggregates (2–6 mm)/sand fraction were built and the physical/mechanical aspects of them were studied. The results of the compressive strength, density, water absorption, pH and shrinkage showed that these lightweight concretes were affected by the type of aggregates, the cement paste and the interfacial zone between cement and aggregates. One grain size pumice (2–6 mm) showed best compressive strength (65 kg/cm²), density of 0.60 g/cm³, linear shrinkage 0.4 % and high water absorption 29.73 %. Strength minerals represented by calcium-silicate-hydrate (CSH) and calcium-aluminate-hydrate (CAH) leading the concrete strength.