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1.
The published literature has revealed conflicting results regarding the effect of low plastic fines fraction (Ip?≤?5.0%) on the mechanical behavior of sandy soils. For this reason, the use of different sample initial structures as (initial relative density approach, global void ratio index approach, etc.) could explain these different mechanical responses of granular materials. Thus, it is necessary to evaluate the quantitative aspect of the low plastic fines effects on the undrained monotonic response of sand-silt mixtures using the global void ratio approach. To achieve this goal, an experimental testing program through controlled monotonic triaxial tests was carried out on reconstituted saturated Chlef sand containing from 0 to 50% silt with an interval of 10% at three global void ratios (e?=?0.64, 0.66 and 0.68) and subjected to constant confining pressure (σ'3?=?100 kPa). The different samples were reconstituted using two different preparation techniques: DFP and MT. The obtained results show that the low plastic fines content appears as a very relevant parameter in the characterization of the mechanical response of sand-silt mixture samples reconstituted at constant global void ratios, where the steady state shear strength and instability shear strength decreased with the increase in low plastic fines content up to the limiting fines contents (Fc?=?40% and Fc?=?10%) considering both studied initial structures (Dry funnel pluviation and Moist tamping), respectively. Beyond these thresholds fines contents, a reverse trend was observed for all parameters under study. Moreover, the test results indicate that the brittleness index, flow potential (Vf), friction index, equivalent void ratio (e*) and equivalent relative density (Dr*) could be considered as reliable parameters in the prediction of the mechanical behavior of the silty sand soils under study. 相似文献
2.
In this study, an alternate approach to establish the e-log p relationships for clayey soils within a vertical pressure range of 10–1,000 kPa is discussed. Skempton’s compression index
equation correlating the liquid limit, w
L, and the compression index, C
c, and the reported equation correlating the void ratio at liquid limit, e
L, and the void ratio at a vertical pressure of 100 kPa, e
100, by Burland ( 1990), were used to establish the e-log p relationships for several reconstituted normally consolidated clayey soils. Consolidation test results of 13 clayey soils
covering a sufficiently wide range of liquid limit were selected from the literature. Also, consolidation tests were carried
out on two highly expansive soils in this study. A comparison of the experimental consolidation test results with the calculated
e-log p relationships in the current study indicated that in general, the agreements between the calculated relationships and the
experimental results are good. The agreements were found to be slightly better for soils with liquid limits less than about
70%. A comparison of the calculated e-log p relationships in the current study with that determined following methods suggested by Nagaraj and Srinivasa Murthy ( 1983) and Burland ( 1990) showed that all the three methods yielded very similar results for soils with liquid limit less than 70%. For soils with
liquid limits greater than 70%, the difference between the e-log p relationships calculated in this study and that following Burland ( 1990)’s method was insignificant, whereas Nagaraj and Srinivasa Murthy ( 1983)’s method slightly over-predicted the void ratios at larger vertical stresses. 相似文献
3.
The ranges of initial void ratios that can be achieved for numerical samples with the same grading as actual sand under gravity are explored using three-dimensional discrete element method (DEM). A corrected compressible accumulation model is proposed to predict the packing density of sand. Compared with the measured results, the errors of the prediction results are small. For three-dimensional DEM samples, when the ratio of sample size to particle size L/ R is equal to or larger than 30, the maximum and the minimum void ratios are minimally affected by the sample size L. The maximum void ratios and the minimum void ratios of numerical samples with spherical particles are much smaller than those of actual sands. The generation method for numerical samples with non-spherical particles is proposed based on the theory of CPM. The variations of void ratios of numerical samples with non-spherical particles are wider than actual sands. So it can meet all the need of DEM simulation on the void ratio of actual sand. 相似文献
4.
Filters managed in zoned dams are designed according to criteria based on the grain size distribution of both filter and eroded soil. However, the constriction size distribution of the filter is the key parameter which governs the filter retention process of flowing eroded particles. To assess the filter efficiency regarding eroded particles, several filters and base soils are tested in a vertical cell with a configuration coupling erosion and filtration processes. For setting the boundary condition of eroded particles at the filter inlet, hole erosion test (HET) was performed on the base soil. The investigation of the evolution of filter behavior shows that the void ratio and the grain shape are of a great influence on filter efficiency. A new approach of filter clogging was proposed by evaluating a damage index which is affected by various parameters such as the ratio D15/d85 and the size of eroded particles. An approach linking the geometrical parameters (damage index) to the hydraulic conductivity leads to an estimation of the filter performance which provides a more quantifiable and realistic criterion. The results indicate that even existing criteria were not met; the tested filters remain efficient as regards to experimental data. An analytical approach based on constrictions size distribution was used and pore reduction was matched with experimental results. 相似文献
5.
Liquefaction of soils is a natural phenomenon associated with a dramatic loss of the soil shear strength in undrained conditions due to a development of excess pore water pressure. It usually causes extensive damages to buildings and infrastructures during earthquakes. Thus, it is important to evaluate extent of influential parameters on the liquefaction phenomenon of soils in order to clearly understand the different mechanisms leading to its triggering. The soil gradation is one of the most important parameters affecting the liquefaction phenomenon. In this context, a series of undrained compression triaxial tests were carried out on eighteen natural loose (Dr = 25%) sandy samples containing low plastic fines content of 2% (I p = 5%) considering different extreme sizes (1.6 mm ≤ D max ≤ 4 mm and 0.001 mm ≤ D min ≤ 0.63 mm) and two mean grain size ranges (0.25 mm ≤ D 50 ≤ 1.0 mm) and (1.0 mm ≤ D 50 ≤ 2.5 mm). The initial confining pressure for all tests was kept constant (P′ c = 100 kPa). The obtained test results indicate that the mean grain size (D 50) and extreme grain sizes (D max and D min) have a significant influence on the undrained shear strength (known as liquefaction resistance) and appear as pertinent factors for the prediction of the undrained shear strength for the soil gradation under study. The undrained shear strength and the excess pore water pressure can be correlated to the extreme grain sizes (D max and D min) and the mean grain size (D 50) of tested wet deposited samples. 相似文献
6.
The shear modulus at small-strain, G max is a maximum value of shear modulus for a given stress state and void ratio, and is a key parameter to evaluate the dynamic response of geotechnical structures. However, the laboratory testing procedures for determining G max are time-consuming, cumbersome and require elaborate equipment especially for unsaturated soil samples. A semi-empirical model is proposed in this paper that can be used to estimate the variation of G max with respect to matric suction for non-plastic sandy soils (i.e. I p = 0 %). The proposed model uses the Soil–Water Characteristic Curve ( SWCC) and the shear modulus at saturation condition along with two fitting parameters ζ and ξ. The proposed model permits estimation of the variation of G max with respect to matric suction over different zones of the SWCC (i.e. boundary effect, transition, and residual zones) for various non-plastic sandy soils. The fitting parameters ζ and ξ required for the proposed semi-empirical model can be estimated from simple relationships derived from the grain size distribution curve. 相似文献
7.
Void ratio has been used as a state variable for predicting the liquefaction behaviour of soils under the critical state, sometimes also referred to as the steady state, framework. Recent publications show that void ratio may not be a good parameter for characterising sand with fines because the steady state line (or curve) in the e-log( p′) space moves downward with increase in fines content until it reaches a threshold value referred to as the threshold fines content (TFC). Recently, an alternative state variable, referred to as the equivalent granular void ratio, has been proposed to resolve this problem. To calculate this alternative state variable, an additional parameter ‘ b’ is needed. This parameter ‘ b’ represents the fraction of fines that actively participate in the force structure of the solid skeleton. However, predicting the ‘ b’ value is problematic. This paper examines the factors affecting the ‘ b’ value based on published work on binary packing. This leads to a simple semi-empirical equation for predicting the ‘ b’ value based on fines size and fines content. The proposed equations were evaluated with published data sets. Then, the concept of an equivalent granular steady state line is proposed. This concept was used to predict the location of SSLs for sand with different fines content from either the SSL of clean sand or the SSL of sand with a given fines content. The predictions agree well with experimental results. 相似文献
8.
In an effort to study the relation of fabrics to the critical states of granular aggregates, the discrete element method (DEM) is used to investigate the evolution of fabrics of virtual granular materials consisting of 2D elongated particles. Specimens with a great variety of initial fabrics in terms of void ratios, preferred particle orientations, and intensities of fabric anisotropy were fabricated and tested with direct shear and biaxial compression tests. During loading of a typical specimen, deformation naturally localizes within shear bands while the remaining of the sample stops deforming. Thus, studying the evolution of fabric requires performing continuous local fabric measurements inside these bands, a suitable task for the proposed DEM methodology. It is found that a common ultimate/critical state is eventually reached by all specimens regardless of their initial states. The ultimate/critical state is characterized by a critical void ratio e which depends on the mean stress p, while the other critical state fabric variables related to particle orientations are largely independent of p. These findings confirm the uniqueness of the critical state line in the e ? p space, and show that the critical state itself is necessarily anisotropic. Additional findings include the following: (1) shear bands are highly heterogeneous and critical states exist only in a statistical sense; (2) critical states can only be reached at very large local shear deformations, which are not always obtained by biaxial compression tests (both physical and numerical); (3) the fabric evolution processes are very complex and highly dependent on the initial fabrics. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
9.
This paper revisits the earth pressure coefficient at rest K
0 of granular materials, with the focus being placed on the variation of K
0 with the internal friction angle, density and compressibility of soils. Following laboratory tests that are carried out to
determine K
0 of two granular materials, the experimental data are interpreted using the original hypoplasticity model for sand proposed
by von Wolffersdorff [ 49]. K
0 is generally a function of void ratio, stress level and the critical state friction angle; it can be alternatively related
to the compressibility of soil. The results show that Jáky’s equation may still be considered as a reasonable representation
of K
0 for granular soils statistically, even though it may not be able to reproduce the experimental data of a specific soil. 相似文献
10.
This paper investigates the effect of model scale and particle size distribution on the simulated macroscopic mechanical properties, unconfined compressive strength (UCS), Young’s modulus and Poisson’s ratio, using the three-dimensional particle flow code (PFC3D). Four different maximum to minimum particle size ( d max/ d min) ratios, all having a continuous uniform size distribution, were considered and seven model (specimen) diameter to median particle size ratios ( L/ d) were studied for each d max/ d min ratio. The results indicate that the coefficients of variation (COVs) of the simulated macroscopic mechanical properties using PFC3D decrease significantly as L/ d increases. The results also indicate that the simulated mechanical properties using PFC3D show much lower COVs than those in PFC2D at all model scales. The average simulated UCS and Young’s modulus using the default PFC3D procedure keep increasing with larger L/ d, although the rate of increase decreases with larger L/ d. This is mainly caused by the decrease of model porosity with larger L/ d associated with the default PFC3D method and the better balanced contact force chains at larger L/ d. After the effect of model porosity is eliminated, the results on the net model scale effect indicate that the average simulated UCS still increases with larger L/ d but the rate is much smaller, the average simulated Young’s modulus decreases with larger L/ d instead, and the average simulated Poisson’s ratio versus L/ d relationship remains about the same. Particle size distribution also affects the simulated macroscopic mechanical properties, larger d max/ d min leading to greater average simulated UCS and Young’s modulus and smaller average simulated Poisson’s ratio, and the changing rates become smaller at larger d max/ d min. This study shows that it is important to properly consider the effect of model scale and particle size distribution in PFC3D simulations. 相似文献
11.
The minimum and maximum void ratios, corresponding to states of densest and loosest packing, of 111 systematically prepared mixed graded sand samples are determined with Method 2A and Method B according to ASTM D 4253 and ASTM D 4254 standards, respectively. Although, those standards are applicable to soils that may contain up to at most 15% fines content, for the harmony of the results even for samples having fines content greater 15%, no other methods are used throughout the experimental program. The test results show that 3rd degree polynomial equations derived from existing experimental data, as a function of packing material percentage, are quite reasonable ( R2 ≈ 1.0) to estimate the variation of minimum and maximum void ratios for the sands considered. Furthermore, error analysis of the limit void ratios is also carried out. It is found that predictability of maximum void ratio for the soil types and gradations considered is more accurate than that of minimum void ratio. 相似文献
13.
The paper presents a physically-based constitutive model for unsaturated soils that considers the bonding effect of water menisci at inter-particle contacts. A bonding factor has been used to represent the magnitude of the equivalent bonding stress, defined as the bonding force per unit cross-sectional area. The average skeleton stress is employed to represent the effect of average fluid pressures within soil pores. Based on an empirical relationship between the bonding factor ζ and the ratio e/ es (where e and es are void ratios at unsaturated and saturated states, respectively, at the same average skeleton stress), we propose an elasto-plastic constitutive model for isotropic stress states, and then extend this model to triaxial stress states within the framework of critical state soil mechanics. Because only one yield surface is needed in the proposed model, a relatively small number of parameters are required. Comparisons between experimental data and model results show that, in most cases, the proposed model can reasonably capture the important features of unsaturated soil behavior. 相似文献
14.
State parameter defined using void ratio, e, and the steady-state line has been shown to be effective in predicting the undrained behaviour of sand. However, steady-state
line for sand with fines is dependent on fines content. To overcome this problem, the concept of equivalent granular void
ratio, e*, has been well investigated. However, the conversion from e to e* has been essentially a back-analysis process. A methodology for converting e to e* without the need of a back-analysis process was first presented. The concept of equivalent granular state parameter, ψ*, defined in terms of e*, and equivalent granular steady-state line was then developed. An extensive experimental study was conducted to investigate
whether ψ* can capture the effects of fines content, and thus can be used to correlate undrained behaviour of sand–fines mixtures without
the need of separately considering the effects of fines content. This study suggested that the effective stress path and deviatoric
stress–strain responses in undrained shearing can be correlated with the ψ* value at the start of undrained shearing irrespective of fines content. 相似文献
15.
This paper presents an advanced constitutive model for unsaturated soils, using Bishop’s effective stress ( σ′) and the effective degree of saturation ( Se) as two fundamental constitutive variables in the proposed constitutive model. A sub-loading surface and a unified hardening parameter ( H) are introduced into the σ′– Se modelling framework to interpret the effects of initial density on coupled hydro-mechanical behaviour of compacted soils. Compared with existing models in the literature, the main advantage of the proposed model that it is capable of modelling hydro-mechanical behaviour of unsaturated soils compacted to different initial densities, such as the dependence of loading–collapse volume on initial void ratio and density effect on the shearing-induced saturation change. The proposed model requires 13 material parameters, all of which can be calibrated through conventional laboratory tests. Numerical studies are conducted to assess the performance of the model for a hypothetical soil under two typical hydro-mechanical loading scenarios. The proposed advanced unsaturated soil model is then validated against a number of experimental results for both isotropic and triaxial conditions reported in the literature. 相似文献
16.
This article deals with the effect of grain crushing on shear localization in granular materials during plane strain monotonic compression tests under constant lateral pressure. The grain diameter and the initial void ratio were stochastically distributed using a spatial correlation. To describe the mechanical behavior of cohesionless granular materials during a monotonic deformation path in plane strain compression, we used a micropolar hypoplastic constitutive model that is able to describe the salient properties of granular bodies including shear localization. The model was extended by introducing changes to the grain diameter with varying pressure using formulae from breakage mechanics proposed for crushable granulates. The initial void ratios and grain diameters took the form of correlated random spatial fields described by both symmetric and nonsymmetric random distributions using a homogeneous correlation function. The field realizations were generated with the help of an original conditional rejection method. A few representative samples of the random fields selected from the generated set were taken into account in numerical calculations. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
17.
The presence of lateritic soils occurs in tropical and subtropical regions. The improvement of lateritic soils that are not suitable for a particular purpose through techniques that combine modification of grain size through the insertion of sand, incorporation of Portland cement and densification through compaction is seen as an alternative. In this context, a dosage method to use a local lateritic soil as construction material in a most rational way reducing the economic and environmental impacts related to this activity is still missing. Therefore, the current research aims to evaluate the performance of a lateritic soil via modification of grain size through the insertion of sand, incorporation of Portland cement and densification through compaction. For this, unconfined compression, and durability (wetting and drying) tests were carried out on specimens of compacted clayey gravel lateritic soil, whose granulometry was modified by the insertion of distinct amounts (from zero to 45%) of weathered sand, treated with distinct Portland cement contents (from 4 to 10%), molded at different dry unit weights (from 16.8 to 20.1 kN/m3) and cured for 7 and 28 days. Results of the mechanical tests have shown the significant influence exerted by cement content and dry unit weight of the blend, followed by curing time and finally sand insertion. Satisfactory correlations between the response variables (qu and ALM) and the adjusted porosity/cement index (η/Cv) were obtained. Furthermore, an innovative approach to replacing the laborious durability test is proposed. 相似文献
18.
A barrier system based on the hydraulic trap design concept for a landfill was proposed. To study the field scenario in which
a clay liner is underlain by a granular layer functioning as a secondary leachate drain layer, a laboratory advection–diffusion
test was performed to investigate factors controlling the transport of contaminants in a two-layer soil system. The soils
used for this study were Ariake clay and, the underlying layer, Shirasu soil from the Kyushu region of Japan. Potassium (K +) was selected as the target chemical species with an initial concentration of 905 mg L −1. The effective diffusion coefficients ( D
e) of K + for Ariake clay and Shirasu soil were back-calculated using an available computer program, Pollute
V 6.3. Values of D
e derived from this experiment are consistent with previously published ones. The Ariake clay has lower D
e than the Shirasu soil. The hypothesis that mechanical dispersion can be considered negligible is reasonable based on both
the observation that the predicted values well fit the experimental data and the analyses of two dimensionless parameters.
Parametric analyses show that transport of K + through soils is controlled by advection–diffusion rather than diffusion only, whereas at low Darcy velocity (i.e., ≤10 −9 m s −1), transport of K + will be controlled by diffusion. Applications of the test results and parametric analysis results in practical situations
were reviewed. 相似文献
19.
最小孔隙比是确定岩土体的密实程度与孔隙特征的有效物理指标,如何快速有效地确定岩土体的最小孔隙比,可为岩土体的固结与稳定提供可靠参数。多数估算细粗混合材料最小孔隙比的模型参数与细粗粒径比一一对应,导致估算困难。在分析尾矿粒度组成、沉积规律和固结稳定的基础上,以8种不同粗细粒径,7～9种不同细粒含量尾矿为试验对象,拟合得到不同粒组尾矿最小孔隙比分布模型参数的函数关系;基于混合尾矿颗粒的粒组特征,给出了确定参数幂函数关系的指数值。分别采用模型参数要求粒组范围内的其他6组岩土材料和非粒组范围内的3组岩土材料进行验证。结果表明,考虑粒组分类影响下的模型,参数简单,对不同材料的最小孔隙比估算准确率较高,给出的最小孔隙比的分布规律合理,可为岩土工程领域最小孔隙比估算提供可靠的计算方法。 相似文献
20.
It is universally known that residual soils behave very differently from sedimentary soils. While the latter is widely known as cross-anisotropic, little is known regarding the strength anisotropy of residual soils. This study presents how the inherent anisotropy affects the strength of natural granite residual soils under generalized conditions, where intact specimens were carefully prepared and sheared under triaxial compression, extension, simple shear, and hollow cylinder torsional shear tests. The strength of natural residual soil, in terms of ultimate stress ratio M and undrained shear strength Su, is found to be significantly anisotropic in a different way from normally consolidated clays with the maximum strength obtained under triaxial compression and the minimum under simple shear or at intermediate principal stress direction. As a result, the existing method failed to measure the anisotropy degree of the studied soil. Two parameters were proposed accordingly to quantify the anisotropic strength under general conditions, taking the special strength anisotropy pattern and cohesive-frictional nature of GRS into account. The proposed parameters enable the direct comparison of strength anisotropy among soils. This study serves as a data set to better understand residual soils regarding their anisotropic behaviors under generalized conditions. Although specific to granite residual soils in China, this study is expected to be more widely applicable to other weathered geomaterials. 相似文献
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