Non-plastic Silt Research Articles

Engineering Geological Evaluation Of A Proposed Landfill Site At Aba-Kulodi, Near Ibadan, Southwestern Nigeria

Evaluation of a location at Aba-Kulodi adjacent to kilometre 8 along the Ibadan / Ile-Ife expressway, Southwestern Nigeria was carried out to determine its suitability or otherwise as a landfill site. Two Vertical Electrical Soundings (VES) 30.00m apart were executed to obtain subsurface information on depth to bedrock and possibly the water table. Disturbed and undisturbed soil samples were collected from each of the two trial pits established at vertical interval of 0.50m up to a depth of 3.00m. The disturbed samples were subjected to grain size distribution and consistency limits tests while consolidation and permeability tests were carried out on undisturbed samples. The VES results revealed 3 geo-electric layers with depths to bedrock of between 14.30m and 17.60m at VES 1 and VES 2 respectively. The depths to water table measured from an existing nearby well were 8.10m and 7.00m during the dry and rainy seasons respectively, suggesting that the water table lies within the second geo-electric layer. The soil samples taken from the depth of 3.00m in the two pits are generally well-graded non-plastic sandy clayey silt, a possible indication of no compressibility. The coefficients of permeability obtained for samples from pits 1 and 2 ranges from 1.85 x 10–4mm/s to 3.91 x 10-6mm/s and 1.91 x 10-4mm/s to 2.91 x10-6mm/s respectively. The coefficient of permeability obtained for samples taken from the 3.00m depth in the two pits are significantly higher than the minimum value of 10-6mm/s recommended for soil that should form the base of landfills. The coefficient of volume compressibility at the sampling pits 1 and 2 range from 1.83 to 7.69 x10-3 m2/KN but generally increases with applied pressure (31.2 - 498.9 KN/m2) and decrease with depth (0.5 to 3.0m). Statistical treatment of the values of the coefficient of compressibility of soil samples from the depth of 3.00m in the two test pits shows no significant variation, an indication that there is no likelihood of differential settlement if the site is used for disposal of solid wastes. If in-situ compaction and lining are done, the location can be used as a landfill site. KEY WORDS: Landfill, Plasticity, Compressibility, Permeability, Compaction. Global Journal of Geological Sciences Vol.3(1) 2005: 25-33

Direct Simple Shear Testing for Post-Cyclic Degradation in Stiffness of Nonplastic Silt

Abstract Using a sequential procedure of cyclic and postcyclic direct simple shear tests, the stiffness degradation characteristics of nonplastic silt are studied during and immediately after cyclic loading by direct simple shear (DSS) tests. The results from cyclic and postcyclic DSS tests were interpreted using methods modified from those adopted for cyclic and postcyclic triaxial tests. In particular, the effect of initial static shear stress (ISSS) on postcyclic behavior is considered for nonplastic silt. The findings obtained from the sequential DSS testing on nonplastic silt are: (i) during cyclic loading the tendency of the stiffness to decrease with increasing normalized pore pressure is different for plastic and nonplastic silts, (ii) since the relation between equivalent stiffness ratio and single amplitude shear strain for nonplastic silt does not filt the Hardin-Drnevich Model, new modified relation is needed, (iii) the larger the ISSS is, the faster the decrease in stiffness ratio becomes, and (iv) the decrease of stiffness ratio for nonplatic silt is less marked than that for plastic silt when plotted against the normalized pore pressure after cyclic load.

Stiffness degradation and shear strength of silty sands

Soils behave nonlinearly from very early loading stages. When granular soils contain a certain amount of fines, the degree of nonlinearity also changes, as stiffness and strength characteristics vary with fines content. Hyperbolic stress–strain models and variations of these models are often used for description of the nonlinear behavior. A modified hyperbolic stress–strain relationship is used in this paper for representing the degradation of the elastic modulus of silty sands. The model is based on two modulus degradation parameters that determine the magnitude and rate of modulus degradation as a function of stress level. Realistic representation of soil behavior using this nonlinear relationship requires estimation of the degradation parameters as a function of silt content and relative density DR. A series of triaxial test results on sands containing different amounts of nonplastic silt were analyzed with this purpose. Relationships between the degradation parameters and cone penetration test (CPT) cone resistance qcare also proposed.Key words: hyperbolic model, silty sands, triaxial tests, modulus degradation, stress–strain response, shear strength, Gmax.

Effect of depositional method on the undrained behavior and microstructure of sand with silt

It has been known for many years that depositional method plays an important part in the results of laboratory testing of clean sands. In this paper, undrained triaxial compression tests were performed on specimens composed of Nevada sand with 20% non-plastic silt content using a variety of depositional methods that include slurry deposition, water sedimentation, air pluviation, mixed dry deposition, and dry funnel deposition. It was found that there was a marked difference in the undrained behavior even though the density and stress conditions were identical. The conclusion was that the soil fabric was responsible for this result. Using the scanning electron microscope, a method was developed to examine and quantify the microstructure of silty sand. These results appear to indicate that there are internal particle structures in sand with silt and their relative quantities correlate with the observed macroscopic undrained behavior.

Permeability and Consolidation Characteristics of Compacted Fly Ash

Detailed testing regarding geotechnical utilization was carried out on fly ash from the Dadri thermal power station. The chemical composition and physical properties of the fly ash were determined. The morphology of the fly ash particles was studied by scanning electron microscopy. An X-ray diffraction test was also conducted. The geotechnical testing program included the classification tests, the compaction test, the consolidation test, and the permeability test. The influence of the head loss across the specimen, the effective stress, and the void ratio on the permeability was studied. The paper explains the details and results of the tests, especially those of the consolidation and permeability behaviors. The results showed that the coefficients of permeability and consolidation of the compacted fly ash were comparable to those of nonplastic silts. The variation in head loss across the specimen had only a small influence on the coefficient of permeability. Even at a high effective stress there was no appreciable reduction in the coefficient of permeability. The consolidation and permeability characteristics of the Dadri fly ash have been compared with those of some other Indian fly ashes. The geotechnical utilization of the Dadri fly ash has been explained through illustrative examples.

Liquefaction Resistance of Clean and Nonplastic Silty Sands Based on Cone Penetration Resistance

Liquefaction of granular soil deposits is one of the major causes of loss resulting from earthquakes. The accuracy in the assessment of the likelihood of liquefaction at a site affects the safety and economy of the design. In this paper, curves of cyclic resistance ratio (CRR) versus cone penetration test (CPT) stress-normalized cone resistance qc1 are developed from a combination of analysis and laboratory testing. The approach consists of two steps: (1) determination of the CRR as a function of relative density from cyclic triaxial tests performed on samples isotropically consolidated to 100 kPa; and (2) estimation of the stress-normalized cone resistance qc1 for the relative densities at which the soil liquefaction tests were performed. A well-tested penetration resistance analysis based on cavity expansion analysis was used to calculate qc1 for the various soil densities. A set of 64 cyclic triaxial tests were performed on specimens of Ottawa sand with nonplastic silt content in the range of 0–15% by ...

A Reconciliation of the Effects of Non-Plastic Fines on the Liquefaction Resistance of Sands Reported in the Literature

The literature presents several seemingly contradictory reports concerning the effects of non-plastic (i.e., silty) fines content on the liquefaction resistance of sands. These seemingly contradictory trends were reconciled in light of the results of a recent study by the authors that linked cyclic resistance and relative density. It was shown that the trend of decreasing cyclic resistance with increasing silt content reported in the literature could be explained by considering the soil's relative density. The same argument was made for the trend of decreasing and then increasing cyclic resistance with increasing silt content. The concept that cyclic resistance is controlled by the sand skeleton void ratio of the soil was also reconciled with the results of the authors’ previous study. The trend of increasing cyclic resistance (without a corresponding initial decrease) with increasing silt content that has been reported in the literature does not appear to occur in non-plastic silts.

Effects of Silt on Three-Dimensional Stress–Strain Behavior of Loose Sand

An experimental study on the effects of nonplastic silt on the 3-D drained behavior of loose sand was performed employing a true triaxial testing apparatus. Lab experiments were performed on clean sand and on sand containing 20% nonplastic silt. Results indicate the failure stress levels and the overall trends of the stress--strain behavior were similar for both sands. However, the volume change behavior is significantly influenced by presence of silt. The silty sand exhibited higher degrees of volumetric contraction during shearing than the clean sand. Relative density was used as the basis of comparison. The development of a shear band appears to have caused failure in all true triaxial testing performed, except in triaxial compression. This form of instability appears to increase its influence on the experimental results as the participation of intermediate principal stress increases. The formation of shear bands also appears to coincide with the cessation of contractive volumetric strain.

Monotonic and Cyclic Liquefaction of Very Loose Sands with High Silt Content

The results from an experimental study on sands with high nonplastic silt content are presented. Drained and undrained triaxial compression tests, undrained cyclic triaxial tests, and drained/undrained instability tests were performed on specimens of loose Nevada sand with 40% silt content. The behavior was observed to be somewhat different from previously published tests with sands at lower silt content. The greater silt content appears to provide a more volumetrically contractive response throughout the entire stress-strain curve. However, some aspects of the response were similar to sands with lower silt content. Monotonic undrained tests indicated “reverse” behavior, i.e., static liquefaction occurred at low confining pressures and increasing dilatant volume-change tendency was observed with increasing confining pressure. Analyzing the results using the concepts of steady state resulted in a unique steady-state line only when undrained tests were sheared from the same isotropic compression line. When specimens of different initial densities were tested at the same initial confining pressures, the resulting steady-state points did not fall on the same steady-state line.

EFFECT OF SAMPLE PREPARATION ON LIQUEFACTION AND POST-LIQUEFACTION CHARACTERISTICS OF A NON-PLASTIC SILT

本論文は, 前報同様, 非塑性シルト供試体に対する応力制御繰返し三軸試験によって, シルト質土の液状化特性, 液状化後の体積変化特性および剛性劣化特性を調べた結果を報告したものである. まず, これらの特性は供試体作製法 (すなわち, 結果的に得られる供試体初期構造) の影響を強く受けることを明らかにした. 次いで, 液状化後の体積変化特性と剛性劣化特性については, ともに過剰間隙水圧とせん断ひずみをパラメータとした, 実用に供する可能性のある簡便な予測モデルに定式化した.

Steady-State Concepts and Static Liquefaction of Silty Sands

The results from an experimental study on silty sands are presented and evaluated in view of the framework of critical-state or steady-state soil mechanics. Drained and undrained compression tests were performed on Nevada sand containing nonplastic silt. The drained and undrained steady-state lines diverged. This divergence was caused by the tendency for static liquefaction at low pressures. Results from undrained tests with different initial void ratios produced different steady-state lines. Unique steady-state lines may therefore not always exist for silty sands. Denser specimens liquefied at low confining pressures, while looser specimens at higher confining pressures showed stable behavior. Thus, a unique relation between void ratio and confining pressure, namely the steady-state line, may not represent the behavior of loose silty sands. This occurs because silty sands exhibit “reverse” behavior characteristics; more dilative behavior is observed with increasing confining pressure under undrained shearing. Load and deformation control testing was performed to evaluate the effect of strain rate. Compressibility was examined as an alternative measure of liquefaction potential.

Cyclic undrained behavior of silty sand

Laboratory cyclic triaxial tests were performed to investigate the effect of fine content on the pore pressure generation in sand. Strain-controlled, consolidated undrained tests have been performed with a cyclic shear strain range of 0·015-1·5%. These tests were carried to 1000 cycles or to initial liquefaction, which ever occurred first. Triaxial tests were performed on pure sand silt specimens and specimens with silt additions of 10, 20, 30, and 60% by weight. Two types of silt, a non-plastic silt and a low plasticity silt (PI ≃ 10) were used as control materials. The main parameters varied in this study were the amount of silt, the plasticity index of silt, and the void ratio where the observed parameter was the pore pressure generation. For all silt contents, silt plasticity and the number of loading cycles have no significant effect at strain levels below 0·01%. Therefore, threshold strain for silty sands have approximately the same value as sands. For both non-plastic and low plasticity silts, there is a significant increase in the generated pore pressure at high strain levels.

Liquefaction‐Induced Lateral Spreading of Mildly Sloping Ground

Dynamic centrifuge modeling was used to study the behavior of two mildly sloping ground models subjected to earthquake loading and liquefaction. The first model consisted of a uniform layer of saturated sand, and the second consisted of a layer of sand overlain by a relatively impermeable layer of nonplastic silt; both model slopes were constructed at an angle of 2.6°. Time histories of acceleration, excess pore‐water pressure, and displacement are presented with photographs to illustrate the mechanisms of failure. In both tests approximately 0.8 m of prototype lateral displacement was measured at the surface. In the uniform sand model this lateral displacement was distributed throughout much of the layer; however, in the layered model displacement was concentrated along the interface between soil layers. The concentration of displacement is consistent with a reduction in the sliding resistance between the silt and the sand layers due to the redistribution of voids at the interface. This possibility of redistribution of voids indicates that care must be taken when using undrained strengths in the postliquefaction analysis of layered slopes.

Characterization of lime sludge for engineering applications

Chemical composition and engineering properties of 20 samples of lime sludge, produced from the lime calcining process, were determined in order to evaluate its potential applications as an engineering construction material. The results indicate that lime sludge is composed mainly of calcium (51.48%), silica (8.30%), and aluminum (3.24%). Iron, magnesium, and sulfur each make up less than 3% by weight, while zinc, chromium, selenium, lead, cadmium, barium, and silver collectively make up less than 0.01% of the total composition. The loss on ignition (LOI) is 34.4% by weight of the total composition. Engineering tests show that lime sludge can be considered as a uniformly-graded, nonplastic silt (ML) with an average liquid limit of 48, average natural water content of 90%, average maximum dry density of 64 pcf (1.03 Mg/m 3), average optimum water content of 50%, and an average friction angle of 27 degrees. The average permeability and unconfined compressive strength of samples compacted to at least 90% of maximum dry density are 2 × 10 −5 cm/sec and 38 psi (0.26 MPa), respectively. Because of its high natural water content, low density, and poor strength characteristics, lime sludge in its natural state is considered unsuitable for engineering construction. However, when lime sludge was mixed with varying percentages of a silty soil, it was found that the density of the admixtures increased and the permeability decreased with an increase in soil content. This suggests that lime sludge-soil admixtures may have potential for such engineering applications as daily landfill cover, hydraulic barriers, and structural fill material.

Evaluation of geotextile/soil filtration characteristics using the hydraulic conductivity ratio analysis

The evaluation of the flow properties and filtration characteristics of geotextiles used in conjunction with soils is a complex problem encountered in many engineered systems. This paper examines changes in the hydraulic conductivity of geotextile/soil composites resulting from permeation of water under controlled conditions. The evaluation of the filtration characteristics of the geotextile/soil composite was performed using both natural and prepared soil specimens. The prepared soil specimens were composed of clean, uniform sand and pure, nonplastic silt. The analyses were performed in a modified triaxial permeability device so that the state of stress, stress history, void ratio, drainage path, soil structure and other variables affecting the hydraulic conductivity could be measured or controlled. The hydraulic conductivities of several combinations of geotextiles and soils were measured. The hydraulic conductivity ratio, or ratio of the hydraulic conductivity of the soil/geotextile composite to that of the soil, was computed for each specimen.

Bearing Capacity and Settlement of Pile Foundations

Conventional bearing capacity of driven and bored piles in sand and nonplastic silt is limited to short piles above the critical depth of penetration. The bearing capacity of longer piles can be estimated from the limiting values of the point resistance and skin friction using either the friction angle of the soil or preferably the results of static and standard penetration tests directly. For driven and bored piles in clay and plastic silt, conventional bearing capacity theory using the undrained shear strength of the soil represents mainly the failure condition at the pile points. The positive and negative shaft resistance some months after pile installation is governed by the drained shear strength of remolded soil and can be estimated from skin friction factors, provided the earth pressure coefficient at rest of the deposit is known. The ultimate load of pile groups and the settlement of pile foundations are examined.