Consolidated-undrained Triaxial Compression Tests Research Articles

Diatom-induced impact on shear strength characteristics of fine-grained soils

Diatomaceous soils, composed of diatom microfossils with biological origins, have geotechnical properties that are fundamentally different from those of conventional non-diatomaceous fine-grained soils. Despite their high fines content, diatomaceous soils typically exhibit remarkably high shear resistance, approaching that of sandy soils. However, the exact role that diatoms play in controlling the mechanical properties of fine-grained soils and the underlying mechanisms remain unclear. In light of this, the shear strength response of diatomaceous soils was systematically investigated using consolidated undrained triaxial compression tests on diatom–kaolin mixtures (DKMs) with various diatom contents and overconsolidation ratios. The micro- and nano-scale structures of the soil samples were characterized in detail using scanning electron microscope (SEM) and atomic force microscope (AFM) to interpret the abnormal shear strength parameters of diatomaceous soils. The results indicated that the presence of diatoms could contribute to significantly higher strength, e.g. the friction angle of DKMs was improved by 72.7% to 37° and the value of undrained shear strength tripled with diatom content increasing from 20% to 100%. Such significant improvement in soil strength with diatom inclusion could be attribute to the hard siliceous skeleton of diatoms and the interlocking between particles with rough surfaces, which were quantitatively analyzed by the surface roughness parameters with AFM. Furthermore, a conceptual model established based on the macro-mechanical tests and microscopic observations portrays a microstructural evolution of soils with increasing diatoms. The microstructure of soils was gradually transformed from the matrix-type to the skeletal one, resulting in a continual augmentation in shear strength through mutual interactions between diatom microfossils. This paper provides new insights into the multi-scale structural properties of diatoms and significantly advances our understanding of the mechanical behavior of diatomaceous soils.

Effect of polyurethane foam adhesive on the static mechanical properties of municipal solid waste incineration bottom ash (IBA)

Municipal waste incineration bottom ash (IBA) is a by-product of municipal waste incineration and is characterized by possessing non-cohesive and enriching in irregular particles. Associated with particles breakage, using such problematic materials as filler initiates the degradation of structure durability and mechanical properties in geotechnical engineering unless an appropriate improvement technique is adopted. In this study, the effect of Polyurethane Foam Adhesive (PFA) as a cementing agent was investigated on the mechanical behavior of IBA by a series of unconfined compressive strength and consolidated-undrained triaxial compression tests. The effect of PFA contents, PFA gelling time and confining pressures on the mechanical properties of IBA was also investigated. With the addition of PFA, the mechanical properties of IBA were significantly improved and the improvement effect increased with the dosage of PFA and the gelling time. It was observed that increasing PFA contents led to higher shear strength, deformation resistance and suppression of strain-softening performance. Furthermore, SEM (scanning electron microscopy) images showed that PFA wrapped around the surface of IBA particles and filled in the pores among the IBA particles. These demonstrate in the form of improving internal friction angle and cohesion of IBA, especially the cohesion which ultimately resulted in significantly improving in overall shear strength of PFA-improved IBA. This study offers an efficient way to improve IBA, and provides references for improving the recycling utilization rate of IBA.

Shear Strength of Coal Combustion Product by Vane Shear

The objective of this study was to evaluate the shear strength of a coal combustion product (CCP) by using the vane shear test. A series of small-scale vane shear (SS-VS; diameter = 12.7 mm and height = 25.4 mm) and large-scale vane shear (LS-VS; diameter = 25 mm and height = 50 mm) laboratory tests were conducted. Undrained and drained strength envelopes were determined using consolidated undrained triaxial compression tests, and drained strength was checked via consolidated drained direct shear. In addition, effects of (1) rate of vane rotation, (2) time delay between vane insertion and beginning rotation, and (3) elapsed time under the final vertical effective stress before shear were evaluated via SS-VS. Vane shear strength results were represented in terms of peak shear strength and the initial horizontal effective stress acting on the vertical-oriented failure surface during rotational shear. Both SS-VS and LS-VS yielded shear strengths that plotted between the drained and undrained triaxial strength envelopes. Shear strength of CCP increased with increasing time under load, which was potentially attributed to diagenesis. Peak shear strength in SS-VS was reached after approximately 72 hours of elapsed time under constant vertical effective stress. Peak shear strength in SS-VS and LS-VS compared favorably to the drained strength measured via direct shear testing.

Shear Strength of Coal Combustion Product by Vane Shear

The objective of this study was to evaluate the shear strength of a coal combustion product (CCP) by using the vane shear test. A series of small-scale vane shear (SS-VS; diameter = 12.7 mm and height = 25.4 mm) and large-scale vane shear (LS-VS; diameter = 25 mm and height = 50 mm) laboratory tests were conducted. Undrained and drained strength envelopes were determined using consolidated undrained triaxial compression tests, and drained strength was checked via consolidated drained direct shear. In addition, effects of (1) rate of vane rotation, (2) time delay between vane insertion and beginning rotation, and (3) elapsed time under the final vertical effective stress before shear were evaluated via SS-VS. Vane shear strength results were represented in terms of peak shear strength and the initial horizontal effective stress acting on the vertical-oriented failure surface during rotational shear. Both SS-VS and LS-VS yielded shear strengths that plotted between the drained and undrained triaxial strength envelopes. Shear strength of CCP increased with increasing time under load, which was potentially attributed to diagenesis. Peak shear strength in SS-VS was reached after approximately 72 hours of elapsed time under constant vertical effective stress. Peak shear strength in SS-VS and LS-VS compared favorably to the drained strength measured via direct shear testing. © 2019 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved.

Consolidated Undrained Triaxial Compression Tests and Strength Criterion of Solidified Dredged Materials

Consolidated undrained triaxial compression tests were performed to investigate the shear strength behavior of the solidified dredged materials (SDM). The variation law of deviator stress and excess pore water pressure with the increase of the applied confining pressure was investigated. It is found that the shear strength envelope is consisted of two lines, and there exists a transitional stress on the intersection point. The undrained shear strength develops slightly with the increase of applied normal stress in the preyield state. However, the undrained shear strength increases significantly in the postyield state, and the strength envelope is nearly a straight line with the extension through the origin. Based on the triaxial test data and the binary medium model, a strength criterion considering strength evolution mechanism is proposed and the relevant parameters of the strength criterion were discussed. Comparisons of the predicted results and experimental data demonstrate that the proposed strength criterion can properly describe the strength evolution rules of the SDM.

Investigation of rainfall-induced shallow landslides on the northeastern rim of Aso caldera, Japan, in July 2012

High-intensity rainfall struck the Northern Kyushu area of Japan between 11 and 14 July 2012. This heavy rainfall triggered many shallow landslides, especially on the northeastern rim of Aso caldera, leading to significant loss of life and damage to many villages. One landslide site at Ichinomiya (Kumamoto Prefecture) was selected for detailed study. Field and laboratory investigations were conducted to identify the initiation mechanism of the shallow landslides during heavy rainfall. The thickness of the soil layer was determined using portable dynamic cone penetration tests. The soils became thinner (about 1 m) from the upper to the lower slope. In-situ infiltration tests indicate that hydraulic conductivity of this thin soil layer is low. During heavy rainfall events, most rainfall will transform into surface runoff instead of infiltration. The results of consolidated-undrained triaxial compression tests show that the effective friction angle and cohesion of the soil are 36.9° and 6.3 kPa, respectively. Soil behavior in response to increase in pore-water pressure was evaluated using a pore-water pressure controlled triaxial test. The results indicate that the steep slopes on the northeastern rim of Aso caldera will remain stable under normal rainfall conditions, due to the high shear strength of the soil. The probable initiation mechanism for the shallow landslides on the northeastern rim of Aso caldera suggests that the initial failure process begins with toe erosion caused by surface runoff during heavy rainfall. Due to the loss of the resistance force at the toe of slope, the slope will reach a critical condition when pore-water pressure in the slope also increases.

Dynamic Shear Strength of Improved Soil with B-Type Blast-Furnace Cement

It is attempted to apply the improved soil using cement materials as the hardening agent to the foundation ground of the important buildings for electric power plants. The current design procedure of the improved ground is based on the static strength such as the unconfined compressive strength of the improved soil. However, it is not clear whether the static strength is relevant to seismic designs where the effects of the cyclic loading induced by the earthquake shall be considered. Hence, the authors studied the dynamic strength of the improved soil in comparison with the static strength, by executing a series of the cyclic triaxial tests with different loading frequencies and the consolidated-undrained triaxial compression tests (CUB tests) with different axial strain rates. The tested soil specimens which have the unconfined compressive strength of about 5 MPa were made in laboratory using the mixture of a dredged marine clayey soil, a B-type blast-furnace cement (300kg/m3) and water. Both of the CUB tests and the cyclic triaxial tests showed similar results with respect to the relationships between the strength and the strain rate. The larger the compressive strain rate was, the higher the strength became. It is concluded that the static strength could be used as a sufficiently safe value in seismic designs since the strain rate of the cyclic loading induced by the earthquake is much larger than that of the static test.

Geotechnical properties of municipal sewage sludge

The geotechnical properties of municipal sewage sludge, in particular those pertinent to the handling and landfilling of the material, are presented. Index, drying, compaction, shear strength and consolidation tests were conducted on the material at different states of biodegradation. The organic content and specific gravity of solids were found to be inversely related, with typical organic contents of 50–70% and specific gravity of solids values of 1.55–1.80. The density of the compacted material was low in comparison with mineral soils. Standard Proctor compaction yielded a maximum dry density of 0.56 tonne/m3 at 85% water content. Laboratory vane-shear and triaxial compression tests indicated that, below about 180% water content, the shear strength of the sludge material increased exponentially with reducing water content. Consolidated-undrained triaxial compression tests on the pasteurised sludge material indicated an effective angle of shearing resistance of 32° for the moderately degraded material and 37° for the strongly degraded material. Biogas was produced at rates of up to 0.33 L/day/kg slurry due to ongoing biodegradation and the resulting pore pressure response must be taken into account in any stress analysis. Consolidation tests using the hydraulic consolidation cell, oedometer and triaxial apparatus indicated that the sludge material was highly compressible although practically impermeable, for example the coefficient of permeability for the moderately degraded slurry was of the order of 10−9m/s. However, creep deformation was significant with typical coefficient of secondary compression values of 0.02–0.08 for the compacted material. A more free-draining material was produced at higher states of biodegradation.

The behavior of loose gassy sand

Methane gas, found in loose deltaic deposits, can contribute to the triggering of flow liquefaction of submarine slopes. The behavior of loose gassy sand is studied in the laboratory using monotonic consolidated undrained triaxial compression tests. Samples consisted of reconstituted Ottawa sand prepared to degrees of saturation ranging from 80 to 100%. Gas contents were determined using a time domain reflectometry miniprobe with a series of calibrations relating apparent dielectric constant to degree of saturation. The results indicate that loose gassy sands can strain soften and experience flow liquefaction. If the degree of saturation is lower than the "cut-off" value, for a specific void ratio, flow liquefaction will not occur.Key words: triaxial testing, liquefaction, Ottawa sand, gassy, unsaturated.

Rate-Dependent Undrained Shear Behavior of Saturated Clay

This paper presents the findings from 25 one-dimensionally consolidated-undrained triaxial compression tests on resedimented Boston Blue Clay using a computer-automated triaxial apparatus with lubricated end platens and a midheight pore-pressure measurement device. Specimens were consolidated to four overconsolidation ratios (OCR) and for each OCR, undrained shear was conducted using four axial strain rates. The findings reveal that the undrained strength rate sensitivity across the two fastest strain rates does not differ with OCR and is equal to about 9 percent. Across the three slower rates, though, increases in OCR cause a consistent decrease in the rate sensitivity that reaches zero at OCR = 8. For high OCR clay, increases in undrained strength are caused by lower shear-induced pore pressures because the effective stress envelope at the peak strength does not change with strain rate. For low OCR clay, increases in undrained strength are caused by lower shear-induced pore pressures and increases in the mobilized friction angle. The authors propose a simple procedure for modeling the rate dependent stress-strain curves of overconsolidated clay.

Effects of anisotropic versus isotropic consolidation in consolidated-undrained triaxial compression tests of cohesive soils : Donaghe, R T; Townsend, F C Geotech Test J, V1, N4, Dec 1978, P173–189

Effects of anisotropic versus isotropic consolidation in consolidated-undrained triaxial compression tests of cohesive soils : Donaghe, R T; Townsend, F C Geotech Test J, V1, N4, Dec 1978, P173–189

Effects of Anisotropic Versus Isotropic Consolidation in Consolidated-Undrained Triaxial Compression Tests of Cohesive Soils

Abstract This investigation examines effects of anisotropic versus isotropic consolidation in consolidated-undrained triaxial compression tests performed on specimens of two clays consolidated from a slurry in large-diameter consolidometers. Shear strength and corresponding deformation characteristics were developed in both normally consolidated and overconsolidated ranges. Test results indicate that volume changes during consolidation are not a unique function of the major principal consolidation stress but instead are related to the mean effective consolidation stress and the deviator stress during consolidation. Consequently, for a given major principal consolidation stress, the undrained strength of anisotropically consolidated specimens is lower than that of isotropically consolidated specimens.

Preparation of Isotropically Consolidated Clay Samples with Random Fabrics

Abstract This paper describes equipment and procedures that can be employed to prepare clay samples with a highly random fabric and a known stress history. By appropriate choice of the clay mineral and proper control of pore fluid chemistry, high water content slurries can be consolidated isotropically to produce large size, relatively homogeneous samples for laboratory use in the study of fabric effects on clay behavior. The fabric of the resulting samples has been identified by the complementary use of scanning electron microscopy, optical microscopy, and X-ray diffractometry, and the isotropy in the mechanical response of specimens trimmed from the sample in perpendicular directions has been determined by consolidation tests, permeability tests, and consolidated-undrained triaxial compression tests. Comparisons in all cases have been made with analogous characteristics of specimens trimmed from anisotropically consolidated samples with a highly oriented fabric, and results have indicated that fabric effects can be significant in some cases.

Quantitative dependence of strength on particle orientation of clay

Consolidated-undrained triaxial compression tests were performed on specimens trimmed from eight different labotatory prepared samples of a kaolin clay. The samples covered a wide range of micro-fabrics, and specimens were trimmed parallel and perpendicular to the direction of preferred particle orientation. The micro-fabric of each sample was identified by the combined use of scanning electron microscopy, optical microscopy, and X-ray diffractometry, the latter of which gave a quantitative index that was correlated with the directional dependence of the ultimate strength determined from the triaxial tests.

True Shear Parameters of Saturated Clays

Ever since Hvorslev proposed a failure criterion incorporating intrinsic parameters, several test procedures have come into practice to determine these ‘true’ strength parameters. Several consolidated undrained triaxial compression tests with pore water pressure measurement were conducted on both montmorillonite and kaolinite clays and the results were analyzed using different existing methods. All the methods through which the data were analyzed fail to assign any unique true strength parameters. Even a particular method yields different values depending upon the initial conditions (stress history, water content) of the sample and stress level during testing. It has been reasoned that these variations are due to the probable differences in fabric between the samples which are involved in various methods. There seems to be a unique linear relationship between tan [Formula: see text] and Cc/pe irrespective of the sample state, stress level, and stress history in both undisturbed and remolded conditions for all the procedures adopted.