Effects of silica fume on the geotechnical properties of fine-grained soils exposed to freeze and thaw

Both the landfill liner and cover systems are the most important parts on a waste disposal landfill site. These systems are generally constructed using compacted fine-grained soils. It is known that the strength and permeability are particularly affected by freezing and thawing cycles in the cold regions. The aim of this study is to reduce the effects of freezing and thawing cycles on the strength and permeability. To modify the fine-grained soils, silica fume generated during silicon metal production as very fine dust of silica from a blast furnace and historically considered a waste product has been used as a stabilizer. The natural fine-grained soils and soil–silica fume mixtures have been compacted at the optimum moisture content and subjected to the laboratory tests. The test results show that the stabilized fine-grained soil samples containing silica fume exhibit high resistance to the freezing and thawing effects as compared to natural fine-grained soil samples. The silica fume decreases the effects of freezing and thawing cycles on the unconfined compressive strength and permeability. We have concluded that silica fume can be successfully used to reduce the effects of freezing and thawing cycles on the strength and permeability in landfill liner and cover systems constructed from compacted fine-grained soils.     

Experimental investigation of matric suction in compacted fine-grained soils

This paper presents the results of laboratory tests performed to study the effect of compaction properties on the development of matric suction in four different soils (three soils were classified as silty clay and one was classified as silty sand). The soils were compacted at the optimum water content (OWC) and wet and dry of optimum according to the standard Proctor procedure (ASTM D698-12). For each sample, matric suction measurements were made using the filter paper technique. When the silty clay soils were compacted at a water content less than the OWC, a significant increase in matric suction was observed. For all the soils tested, matric suction values were very low when the compaction water contents were greater than the OWC. Plots of matric suction vs. compaction water content [ranging from (OWC-2%) to OWC] were developed for the compacted silty clays. The test results indicate that there is a direct relationship between the compaction properties (i.e. compacted dry unit weight and water content) and the matric suction values retained in compacted silty clays. Preliminary relationships were proposed between the matric suction of compacted silty clays and water content based on the data collected in this paper.     

Evaluation of the compactness of subbase and base geomaterials by using stiffness

Stiffness of soil that is defined as ratio of load to deflection depends on some physical properties such as compactness and water content. The purpose of this research study is to use stiffness for estimating compactness of granular geomaterials that are used as subbase and base courses of roads. In this research study, the stiffness of soil samples was determined according ASTM D6758-08 test method by using electro-mechanical device. The dry density of the compacted soil was determined by sand-cone method in the field. More than 80 laboratory and in situ tests were performed. The obtained stiffness data was used to establish a relationship between stiffness, compaction ratio and water content. The results showed that there is a strong relationship between the stiffness, compaction ratio and water content. Also, the result of validation showed that there is good correlation between in situ and laboratory tests results.     

Characterising the elastic behaviour of fine-grained subgrade soils under traffic loading

Resilient modulus is a measure of the elastic behaviour of subgrade soil under traffic loading. The design thickness of a pavement structure and its predicted performance are highly dependent on subgrade modulus. Characterisation of subgrade resilient modulus involves conducting advanced repeated loading triaxial testing that requires special equipment and technical experience that are not available in many soil laboratories. The objectives of this paper were to characterise the resilient modulus of typical fine-grained subgrade soils and develop design inputs for the Mechanistic-Empirical Pavement Design Guide. The resilient modulus of subgrade soil samples was tested at different levels of moisture content. Results of the laboratory testing were used to develop prediction models for resilient modulus as function of physical properties of the subgrade soil and stress state. The proposed models were compared to those developed under the Long-Term Pavement Performance programme. The results showed that the proposed models provided more reliable predictions with lower root mean square error.     

Resilient modulus–moisture content relationships for pavement engineering applications

In recent years, there has been an increased interest in determining the influence of moisture changes on the resilient modulus (M_R) of subgrade soils beneath pavement structures. Efforts have also been made to develop mathematical models that predict the change in M_R values with moisture. These models are expected to account for seasonal variations in subgrade moisture content. This study evaluates the variation of resilient modulus with post-compaction moisture content of soils in the State of Oklahoma and the State of Pennsylvania. A series of specimens was compacted at optimum moisture content and then tested for resilient modulus; other series of specimens were prepared at optimum moisture content and then either wetted or dried prior to M_R testing. Employed wetting and drying procedures are time-efficient in developing the M_R–moisture relationships. Results showed that M_R–moisture content relationships varied with soil types and M_R values varied inversely with changes of moisture content. In addition, an M_R–moisture model predicting the variation of resilient modulus with moisture contents is proposed. This model can be used to predict changes in the bearing capacity of pavements due to seasonal variations of moisture content.     

Systematic back-calculation protocol and prediction of resilient modulus for MEPDG

A research focusing on the characterisation of representative local material properties was conducted to facilitate the full implementation of the Mechanistic-Empirical Pavement Design Guide for roadway designs in Wyoming. As part of the test program, falling weight deflectometer deflection data were collected from 25 test sites in Wyoming for back-calculation of subgrade resilient modulus. Also, subgrade materials from these test sites were sampled for laboratory resilient modulus measurement in accordance with the AASHTO T 307. The back-calculation is a user-dependent procedure and produces a non-unique resilient modulus estimation. To alleviate this limitation, this paper focuses on the recent development of a systematic back-calculation protocol for subgrade resilient modulus using MODCOMP6 software. The protocol is intended for use on a flexible pavement with a crushed base. The proposed procedure discusses pre-analysis checks, seed modulus adjustment, pavement structure adjustment and program termination criteria. A correlation study was conducted to correct back-calculated resilient modulus to laboratory-equivalent values. The results conclude that a non-zero intercept linear regression model provides a better correlation than the widely used zero intercept linear regression model. Furthermore, better correlations are achieved when the back-calculated resilient modulus of a lower subgrade layer and resilient modulus measured at higher laboratory test sequences Nos. 11 to 15 are considered. The non-zero model based on M_r test sequence No. 14 and lower subgrade layer yields the best correlation. For the zero model, a C-factor of 0.645 based on M_r test sequence No. 15 and lower subgrade layer yields the best correlation.     

Possible estimation of resilient modulus of fine-grained soils using a dynamic lightweight cone penetrometer

Resilient modulus is an important parameter to characterise the resilient behaviour of pavement materials. Resilient modulus can be determined in the laboratory from repeated load triaxial test and is defined as the ratio of deviator stress to recoverable strain. Inherently, it is a challenge to perform repeated load triaxial tests as a routine basic test due to its complicated, time-consuming and expensive procedure; hence, several empirical approaches to estimate the resilient modulus from other soil mechanical properties – California bearing ratio, unconfined compressive strength or physical properties – have been proposed. This study has investigated the application of a dynamic lightweight cone penetrometer for the estimation of the resilient modulus in the laboratory and field conditions for some Victorian fine-grained subgrade soils. The results show the possibility to estimate the resilient modulus of fine-grained soils using the dynamic lightweight penetration index at any moisture content (MC) from optimum MC to soaked conditions.     

Strength properties of fly ash based controlled low strength materials

Controlled low strength material (CLSM) is a flowable mixture that can be used as a backfill material in place of compacted soils. Flowable fill requires no tamping or compaction to achieve its strength and typically has a load carrying capacity much higher than compacted soils, but it can still be excavated easily. The selection of CLSM type should be based on technical and economical considerations for specific applications. In this study, a mixture of high volume fly ash (FA), crushed limestone powder (filler) and a low percentage of pozzolana cement have been tried in different compositions. The amount of pozzolana cement was kept constant for all mixes as, 5% of fly ash weight. The amount of mixing water was chosen in order to provide optimum pumpability by determining the spreading ratio of CLSM mixtures using flow table method. The shear strength of the material is a measure of the materials ability to support imposed stresses on the material. The shear strength properties of CLSM mixtures have been investigated by a series of laboratory tests. The direct shear test procedure was applied for determining the strength parameters Phi (angle of shearing resistance) and C(h) (cohesion intercept) of the material. The test results indicated that CLSM mixtures have superior shear strength properties compared to compacted soils. Shear strength, cohesion intercept and angle of shearing resistance values of CLSM mixtures exceeded conventional soil materials' similar properties at 7 days. These parameters proved that CLSM mixtures are suitable materials for backfill applications.     

Mechanical fatigue behavior in treated/stabilized soils subjected to a uniaxial flexural test

The use of in situ fine-grained soils treated with lime and/or hydraulic binders as subgrade in common civil engineering infrastructures is a sustainable upgrading process for natural materials with low mechanical performances. In the case of land transport projects, the lack of knowledge on mechanical fatigue behavior in these materials leads either to empirical oversized design of the layers made with these materials or to their rejection. However, the development of a relevant test now enables us to accurately measure the mechanical fatigue performances of treated soils. First, sample preparation appears to explain most fatigue performances, not sample mineralogy. Second, based on original results on three treated soils and previous results from the literature, it seems that a behavior law governs these performances. Finally, a simple classification tool shows that these materials can be considered within the entirety of transport infrastructures from subgrade layers to subbase layers.     

水平渗流作用下无黏性土接触冲刷细颗粒起动机理分析

为了在细观层面上研究水平渗流作用下无黏性土接触冲刷的临界水力条件,构建了粗-细粒土层接触面上细粒土的两种冲刷起动模型。根据细粒土滑动和滚动两种失稳模式进行受力分析,并采用层流假设对流动切应力进行分析。提出了上部为粗粒土、下部为细粒土的无黏性土接触冲刷水平临界水力梯度的理论计算公式,依据试验中接触面可动细粒土起动位置的特征对公式进行了简化,并通过试验数据将本文公式与已有经验公式进行了对比,验证了其适用性。研究结果表明:水平临界水力梯度受粗-细粒土的粒径比和可动细粒土粒径的影响较大,受粗粒土孔隙率影响较小。该值与粒径比和粗粒土孔隙率成负相关,与可动细粒土粒径成正相关。当粗-细粒土粒径比不大于10时,接触冲刷不发生;当粒径比大于10且小于50时,临界水力梯度取决于粒径比和可动细粒土粒径的影响;当粒径比大于50时,临界水力梯度受可动细粒土粒径的影响较大。该文提出的公式中,可动细粒土的粒径值d_i由接触面上细粒土粒径分布特征决定,在计算时建议其取值不大于细粒土特征粒径d₅₀;当细粒土粒径分布特征未知时,可取细粒土的特征粒径d₁₅。     

Strength characteristics of dune sand stabilized with lime-silica fume mix

While more than half the land surface of Iraq consists of deserts covered mainly with sand dunes, little research has taken place to study the characteristics and the behaviour of sand dunes. The growth of economy, demography and building activities in Iraq necessitates carrying out geotechnical investigations for the dune sand. The purpose of the present work is to assess the suitability of sand dunes as subgrade layer for carrying roads and rail foundations. An extensive laboratory testing programme was carried out to study the geotechnical properties and the behaviour of sand dunes. Sand dune samples were collected from a region in Baiji area in Salah-Aldeen governorate, North of Iraq, in situ field density of the soil was measured by sand-cone test. The tests include moisture content, classification tests, compaction tests, relative density and direct shear test. Chemical tests and X-ray diffraction analyses were also carried out. Silica fume (SF) and lime-silica fume (L-SF) mix have been used for stabilising and their effects on the sand dunes were investigated. A grey-coloured densified SF is used. Four percentages are used for lime 0, 3, 6 and 9% and four percentages are used for SF 3, 6, 9 and 12% and the optimum percentage of SF is mixed with the percentages of lime. Several tests are made to investigate the soil behaviour after adding the lime, and SF. It was found that L-SF caused an increase in the angle of friction ? and cohesion c. Higher cohesion was reached; 10 kPa with higher percentage of 6% L + 12% SF. In addition, the angle of internal friction increases with increasing the maximum dry density, where the values of the angle of internal friction ranged between 35° and 41°.     

Effect of geosynthetic inclusion on the bearing ratio of two-layered soil

In this article, the load-settlement characteristics of unreinforced and reinforced two-layered soil during the loading process are investigated. A series of bearing ratio tests was performed on a granular soil as the base layer overlaying a cohesive soil as the subgrade layer. Three reinforcing conditions (unreinforced, reinforced with nonwoven geotextile, and reinforced with geogrid) at the interface of layers, with four compaction moisture contents (CMCs) of the subgrade layer and three thicknesses of the base layer for both soaked and non-soaked conditions are considered. The results show that the CMC of the subgrade layer has a significant effect on the behavior of two-layered soil, such as swelling amount and the efficiency of the reinforcements. Reinforcing with geogrid resulted in a considerable increase in strength of the soaked samples due to adhesion between geogrids and clayey subgrade layer. For nonwoven geotextiles, strength of the two-layered soil decreased at shallow penetration depths due to reinforcements; and as the penetration increased in depth, the strength also increased. Also, it was found that with decreases in base layer thickness, the test variable's value (i.e., CMC), and the type of geosynthetic reinforcement have significant effects on the behavior of two-layered soil.     

Effect of repeated compaction of tablets on tablet properties and work of compaction using an instrumented laboratory tablet press

The repeated compaction of Avicel PH101, dicalcium phosphate dihydrate (DCP) powder, 50:50 DCP/Avicel PH101 and Starch 1500 was studied using an instrumented laboratory tablet press which measures upper punch force, punch displacement and ejection force and operates using a V-shaped compression profile. The measurement of work compaction was demonstrated, and the test materials were ranked in order of compaction behaviour Avicel PH101?>?DCP/Avicel PH101?>?Starch?>?DCP. The behaviour of the DCP/Avicel PH101 mixture was distinctly non-linear compared with the pure components. Repeated compaction and precompression had no effect on the tensile fracture strength of Avicel PH101 tablets, although small effects on friability and disintegration time were seen. Repeated compaction and precompression reduced the tensile strength and the increased disintegration time of the DCP tablets, but improved the strength and friability of Starch 1500 tablets. Based on the data reported, routine laboratory measurement of tablet work of compaction may have potential as a critical quality attribute of a powder blend for compression. The instrumented press was suitable for student use with minimal supervisor input.     

The effect of grain size on small fatigue crack growth in pure titanium

The growth behaviour of small fatigue cracks has been studied in both fine- and coarse-grained versions of a pure titanium under axial loading at stress ratio, R, of −1. The growth behaviour and its statistical properties in a coarse-grained version of a different pure titanium have also been investigated under rotating bending (R = −1), and the results obtained were compared with those of a fine-grained version of this titanium in a previous report. Under both loading conditions, small cracks grew faster than large cracks. As the growth data were plotted in terms of the effective stress intensity factor range ΔK_eff (after allowing for crack closure, the growth rates could be well correlated with large-crack data in a large-crack regime. In a small-crack regime, however, small cracks still grew faster than large cracks. Small cracks in coarse-grained material showed higher growth rates than those in fine-grained material owing to a much smaller effect of microstructure such as grain boundaries and crack deflection. Stage I facets were observed in all the specimens tested, and their depths were less than the maximum grain size estimated by the statistics of the extreme values, but the distribution of stage I facet depths approximately corresponded to the maximum value distributions of grain size of the materials. The growth rates of small cracks followed log-normal distributions independent of grain size. The coefficients of variation, η, of growth rate in coarse-grained material were smaller than those in fine-grained material. The η values were significantly large at a/d 3 (a = crack depth, D = grain size), indicating that the relative size of microstructurally small cracks was not dependent on grain size.     

Compacted snow as a pavement material for runway construction

An analysis of snow compaction and hardness has been carried out to assess the prospects for preparation of a compacted snow runway for wheeled aircraft on Law Dome near Casey, Antarctica.Snow which successfully duplicates the in situ surface snow on the Law Dome has been made in the laboratory. Compaction, unconfined compression and California Bearing Ratio (CBR) tests have been performed on this laboratory-made snow.The CBR value of compacted snow depends strongly on its density. A pavement CBR value of not less than 10 is required by a wheeled C130 aircraft and this was achieved at a snow density of 0.6 Mg m^?3. If allowance is made for the scatter in the experimental data, then this requirement corresponds to an average CBR of about 15, and initially this can be achieved at a snow density of about 0.63 Mg m^?3 for temperatures around ?5°C to ?10°C. The CBR strength of compressed snow increased with time after compaction. The temperature of the compacted snow did not influence the CBR strength of the snow as strongly as the density. However, snow is more easily compacted to high densities at higher temperatures.The pavement thickness required for C130 operation depends on the pavement CBR, the subgrade CBR and the acceptable wheel settlement. Calculations indicate that for a wheel settlement of 20 mm, a pavement CBR of 10 and a subgrade CBR of 3, a minimum pavement thickness of about 0.5 m is required.     

粉煤灰的路用土工性质试验分析

以阜新发电厂煤粉炉粉煤灰为研究对象,通过室内试验研究了粉煤灰的路用土工性质。该粉煤灰属于含砂低液限粉土,级配不良;粉煤灰的比重为2.27,明显低于一般粘土和砂土。在含水率18%~35%范围内均可获得较为理想的压实密度,表明粉煤灰击实密度对含水率不是很敏感。该粉煤灰渗透性较强,遇水易崩解。粉煤灰压缩系数小,因此路基填成后的沉降量也小,利于路基稳定。粉煤灰的抗剪强度主要依赖于内摩擦角,其内摩擦角远大于一般的粘土。粉煤灰的回弹模量E0值与粘土的回弹模量值差别不大。在相应的压实度条件下,该粉煤灰可用于下路堤填料、上路堤填料和下路床填料。     

粗粒土静止侧压力系数影响因素试验研究

目前,适用于测试粗粒土K₀值的试验仪器及方法较少,关于粗粒土K₀影响因素的研究几乎空白。为研究分析粗粒土K₀影响因素,利用新近研制的大型K₀测试仪对某砂卵砾石料进行了大量K₀试验。通过改变试样的相对密实度D_r0与颗粒最大粒径d_M等初始条件,研究了粗粒土在不同试验条件下K₀的变化规律。试验结果表明:粗粒土的D_r0与K₀存在线性负相关关系;颗粒最大粒径d_M与K₀呈负相关关系,可近似用幂函数表示;K₀随着竖向应力σ'_v的增加呈减小趋势,而且较低竖向应力下这种趋势显著。     

PFC/FLAC coupled simulation of dynamic compaction in granular soils

This paper presents the PFC/FLAC coupled method to simultaneously reveal the macro- and micro-mechanisms of granular soils during dynamic compaction. A good agreement was found between the numerical simulation and model test. By analyzing the soil displacement field, motion of tracer particles, and evolution of local porosity, the dynamic densification process of granular soils was reproduced. The results show that soil deformations under dynamic compaction can be divided into two modes: the punching deformation caused by the wedging effect of a conical core based on the bearing capacity mechanism, and the compaction deformation induced by the propagation of dynamic waves based on the densification mechanism. The dynamic compaction process is composed of two phases: compaction because of the transient impact and compaction because of the vibration of soil particles.     

高碾压混凝土坝智能碾压理论研究

给出了高碾压混凝土坝智能碾压的概念及实现过程,提出了大坝实时监控和智能反馈控制的原理;在实时分析压实指标的基础上,建立了大坝压实质量多元回归模型,进而提出了碾压混凝土坝压实度全仓面实时评价方法;实现了施工过程中碾压机工作性态的自适应调整,确保碾压机速度、激振力、碾压遍数和压实厚度等全过程达标,可有效保证碾压质量,提高施工效率,为碾压混凝土坝建设质量控制提供了一条新的途径。     

Experimental analysis of the effect of some compaction methods on mechanical properties and durability of cement stabilized soil

Cement stabilized soil is usually compacted by different mechanical methods to increase its strength and durability. This paper summarizes the results of an experimental study on the effect of different compaction methods on the performance of stabilized soil. The compaction methods investigated were either static compaction by applying a static pressure using an universal compression testing machine, dynamic compaction by a drop weight method, or static compaction coupled with vibration. All methods were applied on unstabilized soil or cement stabilized soil. The effect of each method of stabilization on compressive strength, shrinkage and water resistance are reported. Dynamic compaction with about 8% of cement content seems to give the best performance for the soil investigated.