Non-plastic Silt Research Articles

Vegetation Succession and Impacts of Biointrusion on Covers Used to Limit Acid Mine Drainage

A cover with capillary barrier effects (CCBE) was constructed in 1998 on the abandoned Lorraine mine tailings impoundment to limit the generation of acid mine drainage. The Ministry of Natural Resources and Fauna of Quebec (MRNF) is responsible for the site and for all restoration works on it, including CCBE construction. The CCBE is made up of three layers: a 0.3-m layer of sand used as a support and capillary break layer; a moisture-retaining layer with a thickness of 0.5 m (this layer is constructed of a nonplastic silt); and a 0.3-m sand and gravel layer on top. The main objective of the CCBE is to maintain one (or more) of the layers at a high degree of water saturation to impede oxygen migration and acid generation. Vegetation succession on the Lorraine CCBE results in an improvement in soil conditions, leading to the installation of deep-rooted species, which could represent a risk to CCBE long-term performance. Hence, the characterization of vegetation succession is an important aspect of the monitoring strategy for the Lorraine CCBE. Species occurrence was documented, and depth of tree roots was measured by excavation on a regular basis. Eight functional groups of plants were identified; herbaceous plants were the most abundant ecological plant groups. Tree ring counts confirmed that tree colonization started the year of CCBE construction (1999). Of the 11 tree species identified, the most abundant were poplar (Populus spp.), paper birch (Betula payrifera Marsh.), black spruce (Picea mariana Mill.), and willow (Salix spp.). Significant differences in occurrence related to environmental conditions were observed for most functional groups. Root excavation showed that tree roots exceeded the depth of the protective layer and started to reach the moisture-retaining layer; in 2008, root average depth was 0.4 m and the maximal root depth was 1.7 m.

Repeatability Evaluation of Instrumented Column Tests in Cover Efficiency Evaluation for the Prevention of Acid Mine Drainage

Instrumented column tests are often used to assess the effectiveness of methods to prevent acid mine drainage (AMD). These tests are seldom duplicated, and this lack of duplicate can cast some doubt about their repeatability and reliability. This paper provides an analysis of column test studies (with duplicates) performed with a commonly used methodology. The paper presents the analysis of two multi-layer covers with capillary barrier effects in which the water retention layer was made of a non-plastic silt and of two cover scenarios involving a single-layer low sulphide cover. The first study compared the cover performance to limit AMD when different cover materials are used, while the second study compared the thickness of covers on cover performance. Statistical comparison between duplicates was made using available geochemical data, hydro-geotechnical data, and gas concentrations. Student’s t-test statistical tools and analysis of variances were used to determine the repeatability of the data. The results indicate that a good reproducibility of the column tests can be achieved with a good set-up methodology and rigorous control of the boundary conditions.

Effects of Non-Plastic Silts on Liquefaction Potential of Solani Sand

Loose saturated cohesionless soils are most susceptible to liquefaction, however there are strong historical evidence suggesting that soils containing fines such as silty sands are also prone to liquefaction during earthquakes. The liquefaction of silty sands has been observed in a number of recent case studies. This paper presents the effects of fine silts on liquefaction potential of sandy soil. Tests have been conducted on the vibration table at different accelerations and pore water pressure is measured. During the lab investigation, locally (Roorkee, India) available Solani Sand and Dhanauri Silt have been used. The soil samples have been prepared by varying silt content and the initial relative density. The results of the study performed are used to clarify the effects of non-plastic fines content on the Solani sand. As the silt content increases, the number of cycles required to produce maximum pore water pressure increases. For a particular level of excitation, rate of pore water pressure generation is maximum at critical silt content. It is observed that critical silt content to generate maximum pore water pressure is different for different accelerations. Further, effect of silt content is very much dependent on relative density.

Cyclic Pore Pressure Generation, Dissipation and Densification in Granular Mixes

Knowledge of cyclic load induced pore pressure generation, post-liquefaction dissipation and volumetric densification characteristics of sands, silty sands, and silts are important for the analysis of performance of loose saturated granular deposits in seismic areas. This article presents results from an experimental study of these characteristics for such soils containing 0 to 100% non-plastic silt. Pore pressure generation characteristics are studied using undrained cyclic triaxial tests. Pre- and post-liquefaction compressibility and coefficient of consolidation, and post-liquefaction volumetric densification characteristics are determined from consolidation data prior to cyclic tests and pore pressure dissipation tests following undrained cyclic tests. Effects of fines content on these characteristics compared to those of clean sands are examined in the context of intergranular void ratio and intergranular contact density concepts.

Effects of Silt Content and Void Ratio on the Saturated Hydraulic Conductivity and Compressibility of Sand-Silt Mixtures

The hydraulic conductivity, the coefficient of consolidation, and the coefficient of volume compressibility play major roles on the pore pressure generation during undrained and partially drained loading of granular soils with fines. This paper aims to determine how much these soil parameters are affected by the percentage of fines and void ratio of the soil. The results of a large number of flexible wall permeameter tests performed on 60 specimens of two poorly graded sands with 0, 5, 10, 15, 20, and 25% nonplastic silt are presented and discussed. Hydraulic conductivity measurements were done at effective confining stresses of 50–300 kPa. The evaluation of the data shows that the hydraulic conductivity and the coefficient of consolidation of sands with 25% silt content are approximately two orders of magnitude smaller than those of clean sands. The coefficient of volume compressibility of the sand-silt mixtures is affected in a lesser degree by void ratio, silt content, and confining stress. The influence of the degree of saturation on the laboratory-measured k values is also discussed.

Linear Regression Models for Predicting Liquefaction during Cyclic Triaxial Testing

Abstract Cyclic triaxial tests are commonly used in research and engineering practice to evaluate soil liquefaction potential and the factors that influence it. During testing it is necessary to estimate the level of loading to be applied to the specimen. This can be a difficult task unless the engineer has previously acquired a reasonable amount of experience with this type of testing. In order to provide guidance in selecting an appropriate level of loading, a series of four linear regression models have been developed to predict liquefaction in sands and soils with nonplastic silts. The models were separated by the soil type and the method of specimen preparation. These models were developed using the data from over 750 tests collected from the author’s files and the literature. The validity of each model was assessed by examining the statistical parameters of the model, an analysis of residuals, and predictions made using additional data obtained from the literature.

Assessment of the Axial Load Response of an H Pile Driven in Multilayered Soil

Most of the current design methods for driven piles were developed for closed-ended pipe piles driven in either pure clay or clean sand. These methods are sometimes used for H piles as well, even though the axial load response of H piles is different from that of pipe piles. Furthermore, in reality, soil profiles often consist of multiple layers of soils that may contain sand, clay, silt or a mixture of these three particle sizes. Therefore, accurate prediction of the ultimate bearing capacity of H piles driven in a mixed soil is very challenging. In addition, although results of well documented load tests on pipe piles are available, the literature contains limited information on the design of H piles. Most of the current design methods for driven piles do not provide specific recommendations for H piles. In order to evaluate the static load response of an H pile, fully instrumented axial load tests were performed on an H pile (HP 310 × 110) driven into a multilayered soil profile consisting of soils composed of various amounts of clay, silt and sand. The base of the H pile was embedded in a very dense nonplastic silt layer overlying a clay layer. This paper presents the results of the laboratory tests performed to characterize the soil profile and of the pile load tests. It also compares the measured pile resistances with those predicted with soil property- and in situ test-based methods.

Dynamic compaction of saturated sands and silty sands: theory

Dynamic compaction (DC) with pre-installed wick drains is an emerging soil improvement technique for densification and liquefaction mitigation of saturated low-permeability loose sands containing non-plastic silts. The compaction process induces excess pore pressures, possible soil liquefaction, and concurrent densification. Wick drains relieve pore pressures and enhance densification. This paper presents a theoretical model for spatial distribution of the energy dissipated in the soil during dynamic compaction, a pore pressure model based on energy principles to estimate the spatial distribution of pore pressures induced during dynamic compaction, and a coupled consolidation model for pore pressure dissipation and soil densification. It also presents a numerical scheme to implement the theoretical models to simulate dynamic compaction at a site and obtain post-improvement soil densities or penetration resistances. A sample set of simulation results are presented. While this paper is limited to the theoretical development of the models, a second companion paper presents the results from this numerical model applied to a few case histories, and a third companion paper presents a methodology for application of the results for liquefaction mitigation design using DC in silty soils supplemented with wick drains.

Liquefaction Resistances of Unsaturated Non-Plastic Silt

It has been pointed out that there are two possible mechanisms that enhance liquefaction resistances of unsaturated sand. The first mechanism is where air in a partially saturated sand mass plays a role of absorbing generated excess pore pressures by reducing its volume. Okamura and Soga (2005) derived the influential factors of the liquefaction resistance for partially saturated sand from theoretical consideration and effects of the factors were examined through a series of triaxial tests on a clean sand. They found a unique relationship between liquefaction resistance ratios and the potential volumetric strain, which allows the estimation of the liquefaction resistance for partially saturated sand. The second is the matric suction of unsaturated sand which increases the effective stress and thus the strength of the soil mass. In this study two series of cyclic triaxial tests on non-plastic silt were carried out to observe the liquefaction resistance in both mechanisms. In the first series, a top cap with an accumulator tank was used to study the effect of compressibility of pore fluid on the liquefaction resistance. The empirical relationship derived by Okamura and Soga is found to be valid even for the silt provided that the matric suction is negligible. In the second test series an ordinary cap was used. The liquefaction resistance increased linearly with the matric suction, with the increasing ratio being higher than that for the net stress. A unique linear relationship is found between the normalized liquefaction resistance and the matric suction. Results are summarized in the form which can be easily applied to evaluate the liquefaction resistance of a partially saturated soil.

Load Testing of a Closed-Ended Pipe Pile Driven in Multilayered Soil

Piles are often driven in multilayered soil profiles. The accurate prediction of the ultimate bearing capacity of piles driven in mixed soil is more challenging than that of piles driven in either clay or sand because the mechanical behavior of these soils is better known. In order to study the behavior of closed-ended pipe piles driven into multilayered soil profiles, fully instrumented static and dynamic axial load tests were performed on three piles. One of these piles was tested dynamically and statically. A second pile served as reaction pile in the static load test and was tested dynamically. A third pile was tested dynamically. The base of each pile was embedded slightly in a very dense nonplastic silt layer overlying a clay layer. In this paper, results of these pile load tests are presented, and the lessons learned from the interpretation of the test data are discussed. A comparison is made of the ultimate base and limit shaft resistances measured in the pile load tests with corresponding values predicted from in situ test-based and soil property-based design methods.

Effect of depositional method on the undrained response of silty sand

The results from a laboratory experimental study on silty sand are presented. Undrained triaxial compression tests were performed on Nevada sand containing nonplastic silt. All specimens underwent monotonic loading after isotropic consolidation to 25 kPa. Various depositional techniques were used to create specimens to compare their stress–strain behavior within three separate density ranges and three different silt contents. As density increased, the effect of the depositional method on the undrained behavior decreased. The influence of the depositional method on specimen behavior also was found to increase with silt content, particularly at lower densities. These findings therefore are of special interest with regard to laboratory testing for the purpose of characterizing liquefaction potential, where loose, silty sands may be encountered. A description and analysis of five different depositional techniques is provided. These methods include: dry funnel deposition, water sedimentation, slurry deposition, mixed dry deposition, and air pluviation. The results of the undrained testing are analyzed using void ratio after consolidation and silt content as the bases of comparison.

Effect of depositional method on the microstructure of silty sand

The results from a laboratory experimental study on silty sand are presented. A scanning electron microscope (SEM) was used to investigate the effect of depositional method and silt content on the grain contact structure of silty sand. Triaxial specimens containing Nevada sand with various quantities of nonplastic silt were isotropically consolidated to 25 kPa and then preserved through epoxy impregnation. Specimens were formed using both dry funnel deposition and water sedimentation. A procedure was developed that allowed the microstructure to be quantified in terms of potentially stable and unstable grain contacts. As silt content increased, the percentage of stable grain contacts decreased. Specimens formed by dry funnel deposition contained a higher percentage of potentially unstable grain contacts than those reconstituted by water sedimentation, possibly reflecting a more compressible particle structure. This effect became more pronounced as silt content increased. A description of the specimen preservation technique as well as the methodology used to both prepare and analyze SEM specimens for grain contact analysis are presented.

Cyclic strength of a natural liquefiable sand stabilized with colloidal silica grout

This paper summarizes the experimental results of a series of cyclic simple shear tests on liquefiable silty sand with and without sample improvement with colloidal silica grout. The objective of the paper is to evaluate the effectiveness of colloidal silica grouting in reducing the liquefaction potential of natural silty sand. Colloidal silica was selected as a stabilizing material due to its low viscosity, wide range of gel times, nontoxicity, and low cost. The soil tested in this experimental program is a poorly graded sand with 11.5% of nonplastic silt from the Port of Lázaro Cárdenas, México. Colloidal silica treated and untreated sand specimens show different pore pressure response and deformation behavior under cyclic loading in simple shear tests. The results indicate that, for a given initial relative density and initial effective vertical stress, liquefiable silty sand specimens stabilized with colloidal silica grout generally exhibit significant gain in liquefaction resistance compared with untreated specimens. It was also found that the colloidal silica grout reduces considerably the rates of pore pressure generation and shear strain of the silty sand specimens subjected to cyclic loading.

Closure to “Liquefaction Susceptibility Criteria for Silts and Clays” by Ross W. Boulanger and I. M. Idriss

The authors’ study provides valuable insight into the stress-strain behavior of fine-grained soils silts and clays under monotonic and cyclic undrained loading. The distinct behavioral features of sands versus clays under the aforementioned types of loading are first identified with regard to the shape of the hysteresis loops, the value of excess pore-water pressure ratio, and the magnitude of developed strains. Then, a classification of fine-grained soils into two classes of materials is proposed, namely, “sand-like” materials that are susceptible to liquefaction in the classical meaning of the term , and “clay-like” materials that are susceptible to cyclic softening. In addition, the authors emphasize the critical importance of using the appropriate procedure for evaluating the cyclic strength for each class of material. Following the establishment of the above framework, the authors review a database of test results compiled from the literature and found that the plasticity index, PI, of fines is the single most important parameter for distinguishing between the two types of behavior. They propose the use of a limit value of PI 7 as follows: 1. for PI 7, the material is susceptible to liquefaction i.e., exhibits “sand-like” behavior and its liquefaction resistance should be evaluated using the available procedures for sands; 2. for PI 7, the soil behaves as a ‘clay-like’ material and its behavior should be analyzed using the procedures available for evaluating the cyclic strength of cohesive soils. A transition zone of plasticity index from 3 to 8 is proposed by the authors, who also suggest discontinuing the use of the Chinese criteria of liquefaction susceptibility. Finally, the authors discuss the applicability of the liquefaction criterion proposed in the paper to soils containing less than 50% fines coarse-grained materials . They argue that as long as the amount of fines is greater than a threshold value, FCthr which is the limit above which the fine grains constitute the loading-carrying matrix of soil , their criterion could be extended to fines content FC 50%. The discussers have recently presented results of cyclic triaxial tests on saturated mixtures of sand with different amounts of nonplastic silt Xenaki and Athanasopoulos 2003 . The tests were conducted to investigate the effect of fines content on the liquefaction resistance of sand-silt mixtures. In view of the authors’ study and other recently published data, the discussers would like to present some related data not included in their previous publication and compare them to the findings reported in the paper.

Modeling sand response using two-surface plasticity

This paper presents a two-surface plasticity constitutive model based on critical-state soil mechanics and describes a practical process for the determination of its parameters. Determination of the constitutive model parameters can be done in a hierarchical manner, starting with the model parameters that have the most bearing on sand behavior and that can be determined using routine experimental procedures. Most parameters can be determined through simple curve fitting through experimental data points. The constitutive model is calibrated against experimental data for Toyoura sand, clean Ottawa sand and mixtures of Ottawa sand with non-plastic silt. The model simulates closely the mechanical response of sands under various loading conditions and predicts both drained and undrained behavior of sands at small and large strains using the actual small-strain shear modulus, as measured in resonant column or bender elements tests, along with realistic values of Poisson’s ratio. Performance of the model in simulating sand response is demonstrated for a variety of initial states and loading conditions.

Study on seismic behavior of nonplastic silt by means of ring-shear apparatus

The influence of static shear stress on undrained cyclic behavior of nonplastic and low-plasticity silts has been studied by means of undrained cyclic torque-controlled ring-shear tests. The cyclic and post-cyclic behavior of silty soils assumed on sliding surface were investigated to assess the liquefaction potential and cyclically induced deformation of silty slopes. Six different initial static shear stresses corresponding to slope angles from 0° to 25° were examined. To better understand undrained cyclic behavior of silt governed by a change in clay content, three different mixtures were achieved by mixing of nonplastic silt with 0%, 10%, and 20% of commercially available clay. These tests were conducted to simulate field conditions prior to earthquake with initial static shear stresses corresponding to slopes and those with no initial static shear stresses of level grounds. The gradual loss of mobilized undrained cyclic shear resistance after failure and pore water buildup in relation to a number of cycles was observed. The undrained response of the soil to cyclic shear stress loading with the constant amplitude revealed the significant effect of the initial static shear stress on the excess pore water pressure generation and post-failure shear resistance. Test results showed that an increase in the initial static shear stress at the given initial effective normal stress is associated with an increase of mobilized shear resistance at its peak state; however, the actual resistance to liquefaction diminished for both nonplastic and low-plasticity silts. During both cyclic and post-cyclic stages of loading, distinctly different types of shear deformation were identified. In order to evaluate mobility of landslides, a modified conventional brittleness index for seismic loading, \(I_{{\text{BS}}} = {{\left( {S_{{\text{peak}}} - S_{\text{u}} } \right)} \mathord{\left/ {\vphantom {{\left( {S_{{\text{peak}}} - S_{\text{u}} } \right)} {\left( {S_{{\text{peak}}} - S_{{\text{initial}}} } \right)}}} \right. \kern-\nulldelimiterspace} {\left( {S_{{\text{peak}}} - S_{{\text{initial}}} } \right)}}\), was proposed and used to characterize unlimited deformation of silts.

Correlation between Cyclic Resistance and Shear-Wave Velocity for Providence Silts

As an alternative to a field-based liquefaction resistance approach, cyclic triaxial tests with bender elements were used to develop a new correlation between cyclic resistance ratio CRR and overburden stress-corrected shear-wave velocity VS1 for two nonplastic silts obtained from Providence, Rhode Island. Samples of natural nonplastic silt were recovered by block sampling and from geotechnical borings/split-spoon sampling. The data show that the correlation is independent of the soils' stress history as well as the method used to prepare the silt for cyclic testing. The laboratory results indicate that using the existing field-based CRR-VS1 correlations will significantly overestimate the cyclic resistance of the Providence silts. The strong dependency of the CRR-VS1 curves on soil type also suggests the necessity of developing silt-specific liquefaction resistance curves from laboratory cyclic tests performed on reconstituted samples.

Intergrain contact density indices for granular mixes—II: Liquefaction resistance

Whether the presence of non-plastic silt in a granular mix soil impact its liquefaction potential and how to evaluate liquefaction resistance of sand containing different amounts of silt contents are both controversial issues. This paper presents the results of an experimental evaluation to address these issues. Two parameters, namely, equivalent intergranular void ratio (e c)eq and equivalent interfine void ratio (e f)eq, proposed in a companion paper (Thevanayagam, 2007) as indices of active grain contacts in a granular mix, are used to characterize liquefaction resistance of sands and silty sands. Results indicate that, at the same global void ratio (e), liquefaction resistance of silty sand decreases with an increase in fines content (C F) up to a threshold value (C Fth). This is due to a reduction in intergrain contact density between the coarse grains. Beyond C Fth, with further addition of fines, the interfine contacts become significant while the inter-coarse grain contacts diminish and coarse grains become dispersed. At the same e, the liquefaction resistance increases and the soil becomes stronger with a further increase in silt content. Beyond a limiting fines content (C FL), the liquefaction resistance is controlled by interfine contacts only. When C F C Fth, at the same (e f)eq, the cyclic strength of a sandy silt is comparable to the host silt at a void ratio equal to (e f)eq.

Undrained monotonic response of clean and silty sands

A series of undrained monotonic triaxial compression tests were performed on Ottawa sand containing 0%, 5%, 10% and 15% of non-plastic silt. The paper focuses on distinctive states of the monotonic undrained response of sands, namely the critical state, the phase transformation state, the quasi-steady state, and the state of undrained instability (onset of flow liquefaction). Specimens were prepared using slurry deposition and moist tamping to investigate the effect of the initial sand fabric on these states. It was observed that an increase in the non-plastic fines content leads to a downward shift of the critical-state line in the void ratio–mean effective stress space and to an increase of the critical-state friction angle. The present data suggest that the initial fabric of the sand appears to have a significant effect on the undrained behaviour in the early stages of shearing, with its influence vanishing at large strains. Moist-tamped specimens demonstrate considerably larger undrained instability state strength than their slurry-deposited counterparts, and a unique critical state locus is reached by both moist-tamped and slurry-deposited specimens. A mathematical formula that allows the estimation of the mean effective stress at the phase transformation state based on the initial mean effective stress and void ratio is also proposed.

Equivalent granular void ratio for characterization of Singapore's Old Alluvium

Characterization of the Old Alluvium (OA) Formation in Singapore is difficult due to its natural heterogeneity. The OA formation was deposited by a braided river system, and it exhibits a wide range of engineering properties. Its strength and stiffness at a given depth, within a few metres distance horizontally, may differ by an order of magnitude, and this cannot be explained by differences in void ratio alone. The formation consists of sand, silt, and clay particles in varying proportions with no consistent relation with depth. Since the majority of OA is sand mixed with fines (silt and clay), the concept of granular void ratio, which treats silt and clay particles as void, was first introduced to try to characterize the formation. Though the concept of the granular void ratio was more useful than that of the void ratio, it could not take into consideration the different contributions that nonplastic silt fines and plastic clay fines make to the shear strength and thus was found to be inappropriate for a natural soil such as OA. To deal with this problem, the relative contribution of different kinds of fines is incorporated through the concept of equivalent granular void ratio. The merit of equivalent granular void ratio for the characterization of mixed soil formations is demonstrated in this paper through the analysis of results from triaxial tests on in situ OA samples.Key words: natural soil, clayey sand, Singapore Old Alluvium, heterogeneity, granular void ratio.