Effects Of Dry Density Research Articles

Experimental Study on the Influence of Soil Structure Index on Loess Permeability

In order to study the effects of dry density and initial water content on the permeability characteristics of loess, the saturated permeability tests of undisturbed loess and remolded loess were carried out in Lanzhou New Area. The anisotropy of undisturbed loess under two factors is analyzed and compared, and the permeability difference between undisturbed loess and remolded loess is also analyzed. The results show that the structure of loess has an important impact on the permeability characteristics, and the dry density is an important structural parameter. The natural structure of undisturbed loess has obvious anisotropy. Under different pore conditions and water contents, it will show the anisotropic behavior of permeability. The influence of water content on permeability is mainly reflected in the influence on the properties of cement between soil particle skeletons. Under different dry density conditions, the influence of water content on permeability is different. When the pore structure is relatively compact, the influence of water content is more obvious. In practical engineering, these two factors need to be comprehensively considered in order to accurately describe the permeability distribution of loess. This study can provide some theoretical guidance for the construction of relevant loess projects.

Moisture–Conductivity Calibration for Electrical Imaging of Horticultural Substrate

Electrical imaging studies of laboratory- and small-scale plant root zones are gaining increasing attention. However, for essential moisture–conductivity calibrations on numerous substrate columns with variability in dry density, the use of one conventional strategy is relatively laborious or complicated. Thus, in this work, a relatively convenient calibration method is presented, and the objective is to identify its feasibility and potential to assess the effects of factors (e.g., volumetric water content (VWC), and dry density) on conductivity and establish necessary moisture–conductivity curves for porous materials (e.g., soils and substrates). In the method, with a specially designed fixture, variable VWCs, dry densities and related complex conductivities of the samples can be easily acquired through static compaction. The results show that the in-phase conductivity (or magnitude of conductivity) increases with the increasing VWC or dry density, primarily owing to the increase in the dominant pore water connectivity. Moreover, the effect of dry density on conductivity is relatively smaller than that of VWC. Thus, for the substrates at dry densities with certain variability, good power law relations (R2 ≥ 0.99) between in-phase conductivity (or magnitude of conductivity) and VWC at different frequencies can be established. Overall, the proposed approach is practical, promising, and relatively time- and labor-saving.

Soil-water characteristics of weathered crust elution-deposited rare earth ores

The permeability of the weathered crust elution-deposited rare earth ores directly affects the efficiency of in-situ leaching. The soil-water characteristic curve (SWCC) is an important constitutive relation for calculating the permeability of ore body, which is related to many factors. Soil-water characteristic tests of rare earth ore samples considering different factors were carried out by using the pressure plate instrument. Effects of dry density, particle size and solution leaching on water holding behavior and the mechanism were investigated. The experimental observations indicate that with the decrease of dry density, the pore ratio increases gradually, and the saturated water content increases. Under the same matric suction, the water content decreases gradually with the increase of particle size, thus decreasing water holding capacity of ore accordingly. In the same water content, matric suction is inversely proportional to particle size. Under the same matric suction, the water content of ore samples after leaching is less than that of the ore samples before leaching, indicating that solution leaching can decrease water holding capacity of ore.

Measurement and Uniform Formulation of Soil-Water Characteristic Curve for Compacted Loess Soil with Different Dry Densities

To investigate the effect of dry density on the soil-water characteristics of compacted soil, loess used as filling in the land-making project of the Yan’an new district was collected and compacted to five initial dry densities of 1.40, 1.50, 1.60, 1.70, and 1.80 g/cm3, respectively. The soil-water characteristic curves (SWCCs) of all specimens in the range of 0–105 kPa were measured using the filter paper method. The measured data were fitted using the Fredlund and Xing equation for each initial dry density. The SWCCs have obvious differences in a suction range below 100 kPa and overlap when the suction range is higher. This suggests that the SWCC of compacted soil is independent of the initial dry density in the high suction range, but the correlation with the initial dry density exists in the low suction range. Therefore, the correlation functions of the parameters in the Fredlund and Xing equation with respect to the initial dry density were regressed, respectively. By substituting these functions into the Fredlund and Xing equation, the state surface function of θ w − ψ − ρ d was obtained and can reflect the SWCCs of all densities of the filled soil to support the further investigation of the unsaturated behavior of compacted soil.

Estimation of resilient behavior of crushed waste rocks using repeated load CBR tests

Crushed waste rocks (CWR), generated from mining operations, have been widely used for the construction of mine haul roads because of their low-cost, high-strength, and availability. Resilient modulus is a necessary input to mechanistic design for mine haul roads, but its determination usually requires advanced and complex equipment such as repeated load triaxial (RLT) test. Repeated load CBR (RLCBR) tests are simpler and more often available, and were therefore investigated in this study to estimate CWR resilient behavior under low and high stress levels. RLCBR equivalent modulus calculated using various equations were validated using RLT resilient modulus. The MR-θ model showed good representation and prediction of CWR resilient behavior under both low and high stress levels. A new equation was also proposed to predict more precisely the equivalent modulus of CWR based on RLCBR tests. Results showed that the impact of loading frequency and waveform was limited, while the measured equivalent modulus increased significantly with contact stress (especially when contact stress < 10%). The effect of dry density and soaking on equivalent modulus was negligible, but specimen drying resulted in a marked increase of the equivalent modulus. The good agreement between RLCBR equivalent modulus and RLT resilient modulus indicates that RLCBR tests could be an effective alternative to RLT tests to estimate resilient behavior of CWR used in mine haul roads.

Effects of dry density and water content on compressibility and shear strength of loess

Investigation on the compressibility and shear strength of compacted loess is of great importance for the design and operation of engineering infrastructures in filling area. In this study, the mechanical behaviors of Yan'an compacted loess are investigated at various dry densities and water contents by conducting one dimensional compression and direct shear tests. And the elastic compressibility, plastic compressibility, yield stress and strength are obtained from the experiments. Results show that when water content increases, plastic compressibility parameter increases, but yield stress decreases. However, the increase of dry density leads to a decrease in plastic compressibility parameter but an increase in yield stress. In addition, elastic compressibility parameter is found to be a constant which is irrelevant to water content and dry density. As for strength, cohesion and internal friction angle is directly proportional to dry density, but inversely proportional to water content. Moreover, the mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) tests were also performed to observe the pore size distribution and microstructure of the specimens. Finally, by using results of MIP and SEM tests, the compressibility and strength behaviours of Yan'an compacted loess are explained from the perspective of pore-size distribution and microstructure.

Experimental study on permeability characteristics and radon exhalation law of overburden soil in uranium tailings pond.

The permeability characteristics of overburden soil have a significant effect on radon exhalation in uranium tailings pond. To understand this change, the effects of dry density, particle size range, and moisture content on the gas permeability and radon exhalation were studied by correlation analysis method. According to the mechanism of radon generation, the permeability characteristics of overburden, and the weather conditions, a theoretical model of radon exhalation was constructed and an experimental setup was designed. A series of tests on different properties of overburden soil were conducted by uranium tailings permeameter, and the radon exhalation rate of corresponding overburden surface was measured by local static method. Results show that the permeability of overburden decreased, which led to the decrease of radon exhalation: (1) At the same moisture content, with the increase of dry density, both the gas permeability and radon exhalation rate decrease. (2) At the same moisture content, with the increase of the particle size range, both the gas permeability and radon exhalation rate increase. (3) At the same dry density, with the increase of moisture content, the gas permeability decreases, while the radon exhalation rate increases slightly at first and then decreases.

Effect of Water Content and Dry Density on Mechanical Properties of Compaction Loess

Loess landslides in irrigation districts are common geological disasters in the world. Long-term watering will reduce the soil mechanical properties of the loess, leading to frequent occurrence of landslides. In this paper, using shear strength and compressibility test to explore the effect of dry density and water content on the mechanical properties of loess, The results show that the lower the water content, the greater the effect of dry density change on cohesion and at the same dry density, the internal friction angle of compacted loess decreases with increasing water content; the compressive strain of each test sample increases nonlinearly with the increase of vertical pressure, and the magnitude of the increase gradually decreases. Therefor water content and dry density play an important role in mechanical properties of compaction loess.

Effects of dry density on shear behavior and particle breakage for slate rockfill material

Dry density is known to be one of the primary factors affecting the behavior of granular materials. This study investigates dry density effects on shear behavior and particle breakage for rockfill materials. To achieve this, we conducted large-scale consolidated drained tests followed by sieve analysis for slate rockfill. Our results demonstrate that the stress–strain characteristics of the slate investigated depend on both dry density and confining pressure. Based on our observation of non-linear shear strength envelopes, we propose a failure criterion incorporating the effects of dry density to describe shear resistance. Our results also demonstrate that the peak friction angle increases with increasing dry density but decreases with increasing confining pressure. Moreover, the secant modulus depends on both dry density and confining pressure and is closely related to the void ratio after consolidation. Finally, our sieve analysis data indicate that dry density has a minor effect on particle breakage for this slate material.

Thermo-mechanical and moisture absorption properties of fly ash-based lightweight geopolymer concrete reinforced by polypropylene fibers

An experimental investigation on the thermo-mechanical and moisture absorption properties of lightweight geopolymer concrete prepared with fly ash, NaOH, sodium silicate and Polypropylene Fibers (PF) is presented in this study. The effects of dry density, NaOH, PF, aggregates and hydrophobic agent on the compressive strength, thermal properties and moisture absorption were studied. Results indicate that thermo-mechanical properties of Fly ash-based Lightweight Geopolymer Concrete (FLGC) strongly depend on the dry density, NaOH, PF and aggregates contents. The increase in dry density and fine aggregate contents resulted in higher compressive strength and thermal conductivity. NaOH within mass ratio of 0–10% is able to enhance thermo-mechanical properties. The optimal compressive strength was achieved when the length and content of the PF was 12 mm and 0.5% respectively. Meanwhile, PF in the range of 0–1% can also increase thermal conductivity and enhance moisture absorption. The increase in coarse aggregate ranging from 0 to 15% led to reduced dry density and thermal conductivity and enhanced moisture absorption, but did not affect compressive strength. Interestingly, the decrease in fine aggregate with the same content had the opposite impact to the moisture absorption in comparison to the coarse aggregate. However, the moisture absorption can be considerably weakened by surface waterproofing treatment which makes the enhanced thermal performance durable. Therefore, the FLGC reinforced by PF has excellent thermo-mechanical properties and can also be engineered to be an environmentally friendly and durable thermal insulation material with the assistance of waterproofing treatment.

The Shear Strength of the Nature Loess Joint: A Case Study in Shaanxi Province

Abstract Shear strength of the loess joint has significant influence on the stability of loess engineering with joints. “Joint” is the weak structural plane of loess, and its shear strength determines the stability of loess. In this study, a series of loess joint samples was collected from Binxian County, Liquan County, and Zichang County in Shaanxi Province. The shear strength of the loess joint was determined by means of the direct shear test. The influence of the water content on the shear strength parameters of the loess joint was investigated by means of tests. The results showed that initially the internal friction angle varied slightly with the water content but decreased very quickly as the water content surpassed the threshold value of 14 %. The relationship between shear strength and water content can be expressed as an exponential function. This study devised a new empirical formula that determines the peak internal friction angle by observing the effects of dry density, water content, shear strength, and joint surface morphology.

Gas breakthrough tests on saturated GMZ01 bentonite using RCP technique with consideration of dry density effect

In this study, gas breakthrough tests were conducted on GMZ01 bentonite specimens with dry densities 1.3, 1.5 and 1.7 Mg/m3 under rigid boundary conditions using the residual capillary pressure (RCP) technique. Prior to the gas breakthrough tests, water permeability tests were performed for determining intrinsic water permeability and establishing full water saturation for the follow-up gas breakthrough tests. The intrinsic water permeability measured ranges between 11 and 150 nDarcy (1.10×10-20 and 1.50×10-19 m2). As dry density increases, the residual capillary pressure differences of the initially water-saturated bentonite specimens recorded increase from 0.12 to 0.61 MPa. Meanwhile, the time to breakthrough was recorded to increase sharply as dry density increases. In addition, the maximum effective gas permeability decreases from 8.91×10-18 to 8.92×10-19 m2 as dry density increases from 1.3 to 1.7 Mg/m3. These results indicate that the capillary drainage in the bentonite specimen is highly related to the capillarity effects in the largest pores. The higher dry density, the smaller flow channels, resulting in more difficult for gas phase to dispel water from the interconnected flow pathways and a better sealing capacity with a higher residual capillary pressure.

Experimental investigation of dry density effects on dielectric properties of soil–water mixtures with different specific surface areas

The dielectric constant of soil is used to estimate its water content in a range of applications. Unlike the widely known effect of water content on the soil dielectric constant (consistent direct proportionality), only a limited number of studies have reported the effects of soil dry density, however, with equivocal results. This paper, therefore, investigates the effects of dry density or degree of compaction on the dielectric constant of five different soil types. The results of the experimental work for the soils ranging from sand to Bentonite clay with distinct specific surface areas were evaluated based on the use of two simple mixture models (De Loor and Birchak). The effects of dry density on the soil dielectric constant were found to be soil type dependent. This is demonstrated by the experimental data and further proven by the modified De Loor model. The behavior is shown to be defined by the changes in the free water, bound water, and solid particle volume fractions, ultimately controlled by the soil specific surface area. The dielectric constant changes from being directly proportional to dry density to inversely proportional at a threshold specific surface area of between 122 and 147 m2/g. Supported by the experimental observations, parametric analysis has revealed that the range for the dielectric constant of bound water was found to be 9–37, while the geometrical parameter α in the Birchak model was found to be 0.4–0.8.

Experimental simulation of boundary condition effects on bentonite swelling in HLW repositories

On the basis of textural and hydro-mechanical characteristics, bentonite has been proven to be an effective buffer/backfill material for long-term containment of high-level radioactive waste (HLW) in deep geological repositories. Herein, the results of experiments performed to investigate the swelling equilibrium limit (SEL) of bentonite under various boundary conditions are presented. A special apparatus was employed to simulate various stress–strain boundary conditions, including constant volume (CV), constant vertical stress (CVS), and constant stiffness (CS). Bentonite samples were prepared with various initial dry densities ranging from 1.5 to 1.7 g/cm3 and vertically stressed to different levels. During wetting, they were subjected to different boundary conditions before the swelling strain or swelling pressure reached equilibrium. Test results indicate that stress–strain boundary conditions have significant effects on the measured swelling strain and swelling pressure of the tested bentonite. More specifically, the relationship between the sequence of swelling pressure under different boundary conditions is CV > CS > CVS, while the relationship between the sequence of swelling strain is CVS > CS > CV. In addition, the characteristics of SEL curves are governed by the initial dry density and vertical stress with the effect of dry density being more significant. Based on these results, several SEL curves were developed to index the effects of boundary conditions on swelling potential of bentonite. They can be used to evaluate the final stress and volume states of bentonite during fluid infiltration under the range of boundary conditions possible in HLW repositories.

The Research on Strength and Deformation Behaviors of Buffer/Backfill Material under High‐Temperature and High‐Pressure Conditions

By using the high‐temperature and high‐pressure triaxial apparatus, 81 undrained triaxial shearing tests of GMZ bentonite were carried out, and the effects of temperature, confining pressure, water content, and dry density on the deformation and strength characteristics of GMZ bentonite were systematically analyzed. Five results are presented in this paper: (1) confining pressure and dry density have a great influence on the failure forms of GMZ bentonite; under the nonconfined pressure condition, the failure forms of the specimen are brittle failure, and it is the same with high dry density, but the specimens with low dry density and high confining pressure exhibit plastic failure; (2) the strength of GMZ bentonite decreases with increasing water content; the position of the deviator stress‐axial strain curve under low dry density rises with the increase of temperature, whereas the location of the curve under high dry density decreases with the rise of temperature; (3) initial Young’s modulus rises with increasing temperature under low dry density, but it is on the contrary under high dry density; with rising water content, initial Young’s modulus generally decreases; (4) Poisson’s ratio rises with rising temperature. The effect of dry density and water content on Poisson’s ratio is insignificant; and (5) the formulae of cohesion and internal friction angle of GMZ bentonite with the changing dry density, water content, and temperature are proposed, respectively. The research results of this paper provide a scientific basis for analyzing the thermo‐hydro‐mechanical coupling characteristics of buffer materials.

Effects of a simulated gap on anisotropic swelling pressure of compacted GMZ bentonite

Gaomiaozi (GMZ) bentonite is a promising buffer material for Chinese high-level radioactive waste (HLW) disposal, and will be either compacted in situ or used as dense prefabricated material when constructing engineered barriers. Uniaxial compaction can induce anisotropy of the bentonite structure, resulting in anisotropic swelling. In this work, a series of swelling pressure tests were performed on compacted GMZ bentonite to investigate the anisotropy of its property and highlight the dry density effect. Two kinds of technological gaps were simulated to analyze the change in anisotropy of properties with limited swelling deformation. The relationship between swelling pressure and dry density was found to be logarithmic, and axial pressure increased more rapidly than radial pressure with dry density. The ratio of radial to axial pressure decreased significantly in the dry density range 1.395 Mg/m3–1.706 Mg/m3; beyond this, it remained at about 0.76. For the swelling volume ratio of less than 16.7%, the stable swelling pressure was controlled by the final dry density. For a larger swelling volume ratio, the axial gap induced a smaller swelling pressure compared with the case of no gap at the same final dry density.

Effect of Basalt Fiber on the Strength Properties of Polymer Reinforced Sand

This paper displays an experimental study of the effect of basalt fiber on the strength properties of polymer reinforced sand. Laboratory trials of unconfined compression test (UCS), direct shear test, and tensile test were conducted on the specimens treated with polymer and basalt fiber, and several factors including polymer content, fiber content and dry density of sand that will influence the strength behaviors are investigated in detail. Based on test results and scanning electron microscope (SEM) images, the reinforcement mechanism was analyzed. The results showed that the polymer content, basalt fiber content and dry density of sand had greatly improved the strength behaviors of reinforced specimens. The increase in polymer and fiber content had an active effect on strength characteristics, while the angle of internal decreased slightly. The strength properties were enhanced with the increase in dry density, and the effect of dry density on tensile strength is affected by fiber content. The presence of randomly distributed fibers has formed a spatial fiber-sand net in sand, and the additive of polymer solution formed membrane to enwrap sand particles and connect sand and fibers, thereby formed a stable structure in sand. These structures have increased the bonding and interlocking forces between sand and fibers, and decreased the void ratio of reinforced specimens.

Effect of dry density on the liquefaction behaviour of Quaternary silt

Quaternary silt is widely distributed in China and easily liquefies during earthquakes. To identify the influence of the dry density on the liquefaction behaviour of Quaternary silt, 40 cyclic triaxial liquefaction tests were performed on loose silt (dry density ρd=1.460 g/cm3) and dense silt (ρd=1.586 g/cm3) under different cyclic stress ratios (CSRs) to obtain liquefaction assessment criteria, determine the liquefaction resistance, improve the excess pore water pressure (EPWP) growth model and clarify the relationship between the shear modulus and damping ratio. The results indicate that the initial liquefaction assessment criteria for the loose and dense silts are a double-amplitude axial strain of 5% and an EPWP ratio of 1. The increase in the anti-liquefaction ability for the dense silt is more significant under lower confining pressures. The CSR of loose silt falls well within the results of the sandy silt and Fraser River silt, and the dense silt exhibits a higher liquefaction resistance than the sand-silt mixture. The relationships between the CSR and loading cycles were obtained at a failure strain of 1%. The EPWP development in the dense and loose silts complies with the “fast-stable” and “fast-gentle-sharp” growth modes, respectively. The power function model can effectively describe the EPWP growth characteristics of the dense silt. Finally, based on the liquefaction behaviour of silt, a suggestion for reinforcing silt slopes or foundations is proposed.

Effects of lime treatment on the hydraulic conductivity and microstructure of loess

Lime treatment of loess in foundation engineering modifies the soil structure, leading to changes in mechanical and hydraulic properties of soil, which in turn will affect the flow of water and transport of contaminants in the loess. In light of this, it is essential to identify the dominant effects of different lime treatments on hydraulic conductivity, and to ascertain the optimum lime treatment. For this purpose, we investigated the effects of dry density and lime content on changes in hydraulic conductivity and microstructure of loess in Yan’an City, China. The results indicate that hydraulic conductivity has a log negative correlation with dry density, and lime addition can result in a decrease of hydraulic conductivity of loess at the same dry density. Under a given degree of compaction, however, lime addition can lead to a decrease in dry density due to an increase in flocculation and aggregations. The significant decrease of dry density leads to an increase in hydraulic conductivity when lime content (in mass percentage) is lower than 3%. Nevertheless, when lime content is higher than 3%, the reactions between loess particles and lime will be intensified with an increase in lime content, and become the primary factors affecting pore characteristics. These reactions can further decrease the hydraulic conductivity of lime-treated loess, and the lowest hydraulic conductivity was obtained for lime-treated loess with 9% lime content. The excess lime (above 9% lime content) dramatically increased pore size, leading to a significant increase in hydraulic conductivity. Therefore, 9% is the optimum lime content for loess treatment, and the degree of compaction in engineering should be higher than 95%. In addition, statistical analysis of microstructure of lime-treated loess shows that the distribution trends of macro- and meso-pores coincided with that of saturated hydraulic conductivity, which indicates that lime content affects saturated hydraulic conductivity of lime-treated loess by changing the soil structure, especially the properties of pores larger than 8 µm.

Experimental study of erosion characteristics of soils in homogeneous embankments

The erosion characteristics of soils in homogeneous embankments is the key problem of dam breach which is directly related to the breach formation and the rate of breaching. The Erosion characteristics of several soils in embankment have been researched. On the basis of the erosion test results, the effects of dry density, net shear stress, sediment detachment exponent on the rate of erosion were analyzed. According to the results the sediment detachment exponent should be calculated based on the erosion velocity and net shear stress. It is not convenient for analysis if the value of sediment detachment exponent was set 1 for cohesive soil or 1.5 for non-cohesive soil. It has an definite logarithmic relationship between erosion coefficient and sediment detachment exponent. The critical shear stress has an significant effect on the rate of erosion of soils. The erosion resistance of soils in embankment dams and the dam safety can be evaluated well by using the sediment detachment exponent.