Effective Friction Angle Research Articles

Effect of fiber reinforcement on shear strength and void ratio of soft clay

In this study, a series of multi-stage drained reverse direct shear tests were carried out on soft clay samples reinforced with 0.25% and 0.50% polypropylene fibers of 6 mm, 10 mm and 19 mm in length. Tests were carried out at different normal effective stresses and cumulative horizontal shear displacement of 1.17 times of the sample width. Results showed an increase of the shear strength with the increase of fiber content and length. However, the rate of improvement was capped with the normal effective stress applied during the shearing stage. At a high normal effective stress, the shear strength of the fiber-reinforced soft clay approached that of the unreinforced clay regardless of the amount of fiber inclusion. The rate of shear strength improvement decayed with the number of shear cycles. Fiber reinforcement also resulted in a reduction of the compressibility of the soft clay at consecutive consolidation and shear stages. Although the effective internal friction angle of the soft clay was not altered significantly with the fiber reinforcement, the effective cohesion of the soft clay improved significantly as much as 6.4 and 8.5 times with the inclusion of 0.25% and 0.50% of 10 mm long fibers, respectively.

The Behavior of Bushehr Carbonates Sand in the Persian Gulf Under Different Intermediate Principal Stresses

The design of geotechnical structure acting under multi-axial loading requires knowledge about the soil behavior under intermediate principal stress. Even though conducting the numerous experiments and numerical researches about the behavior of common standard silica soils under 3-D loading, few investigations are made for soils with the dilative behavior. This paper aims to study the behavior of Bushehr carbonate sand from a north shelf of Persian Gulf under various intermediate principal stresses by conducting the hollow torsional shear tests. The test result implies that the plane strain condition occurs when an intermediate principal parameter (b) is equal to 0.3. Under this condition, soil gets the maximum value in ultimate negative pore water pressure, ultimate deviatoric strain, effective frictional angle and the radial strain becomes close to zero. Maximum radial strain and minimum deviatoric strain occur under the triaxial condition where the b is 0. Besides, under b = 0 condition, soil has the lowest value of ultimate negative pore water pressure and effective friction angle. The considerable effect of intermediate principal stress on the soil behavior is found in samples packed with loose state.

Effective Stress Strength Parameters of Clays from DMT

Abstract In this study, an established effective stress limit plasticity solution for piezocone penetration tests (CPTu) in clay developed by the Norwegian Institute of Technology (NTH) for evaluating the effective stress friction angle (ϕ′) is extended to flat plate dilatometer tests (DMT) readings. A nexus between CPTu and DMT is built through spherical cavity expansion solutions for undrained penetration to link the cone tip resistance (qt) and shoulder porewater pressure (u2) from CPTu to the measured contact pressure (p0) and expansion pressure (p1) obtained from DMT in soft to firm intact clays. Data from 49 paired sets of CPTu-DMT soundings in a variety of clays are used to support and validate the links. A variety of soils ranging from lean to plastic clays and clayey silts from marine, alluvial, lacustrine, deltaic, and glaciofluvial origins are subjected to flat DMTs, and the measurements are utilized to evaluate the effective stress friction angle (ϕ′). Data from 46 clays are compiled to examine the DMT-interpreted ϕ′ values in comparison with laboratory benchmark ϕ′ values obtained from CAUC or isotropically consolidated undrained triaxial compression tests on high-quality samples. An approximate inversion of the theoretical solution is derived to allow ϕ′ profiles to be determined with depth. Four well-documented DMT examples, including two natural clay sites and two chamber test series (one pressurized and the other unpressurized), are presented to illustrate the NTH procedures.

On the variability of the effective friction angle of Saint Lucian soils: investigations through a laboratory database

AbstractFor assessment of slope stability in data‐scarce regions prone to natural hazards, modelling relies, to a large degree, on estimates of the effective friction angle. Using a database, comprising soil data from Saint Lucia in the Eastern Caribbean, both simple regression and multiple linear regression analysis were performed. These analyses correlate various basic soil parameters with the effective friction angle measurements contained in the database. The developed statistical relationships are then employed for the estimation of the effective friction angle for use in a slope stability simulation scenario. These analyses show the narrowing of the expected range of results for the slope factor of safety when more soil parameters are used in the estimation of effective friction angle.

Flow and Thermal Properties of Stevia Powder

Abstract Stevia (Stevia rebaudiana Bertoni) has recently received a lot of attention as a sweetener due to its taste and low calorific value. Flow and thermal properties of foods play a significant role in the quantitative analysis of unit operations in the food industry. However, there are no published data available on flow and thermal properties of stevia powder. Powder Flow Tester and KD2 Pro Thermal Properties Analyzer were used to determine the flow and thermal properties of stevia powder, respectively, at different moisture contents (4.96%, 9.68%, 13.99%, 20.08%, and 25.79%, w.b.). Mean angle of internal friction of stevia powder ranged from 41.13° to 46.3°. The mean effective angle of internal friction ranged from 47.8° to 52.5° and the mean flow index ranged from 0.27 to 0.48. Mean thermal conductivity of stevia powder ranged from 0.091 W·m-2·K-1 to 0.115 W·m-2·K-1. Mean thermal diffusivity ranged from 0.103 mm2·s-1 to 0.121 mm2·s-1 and mean volumetric specific heat ranged from 0.865 MJ·m-3·K-1 to 1.019 MJ·m-3·K-1. Polynomial regression models were developed to predict flow and thermal properties of stevia powder using moisture content of stevia powder.

Analysis of passive earth pressure modification due to seepage flow effects

Using an assumed vertical retaining wall with a drainage system along the soil–structure interface, this paper analyses the effect of anisotropic seepage flow on the development of passive earth pressure. Extremely unfavourable seepage flow inside the backfill, perhaps due to heavy rainfall, will dramatically increase active earth pressure while reducing passive earth pressure, thus increasing the probability of instability of the retaining structure. A trial and error analysis based on limit equilibrium is applied to identify the optimum failure surface. The flow field is computed using Fourier series expansion, and the effective reaction force along the curved failure surface is obtained by solving a modified Kötter equation considering the effect of seepage flow. This approach correlates well with other existing results. For small values of both the internal friction angle and interface friction angle, the failure surface can be appropriately simplified with a planar approximation. A parametric study indicates that the degree of anisotropic seepage flow affects the resulting passive earth pressure. In addition, incremental increases in the effective friction angle and interface friction angle both lead to an increase in passive earth pressure.

Internal Friction Angle of Metal Powders

Metal powders are components with multidisciplinary usage as their application is very broad. Their consistent characterization across all disciplines is important for ensuring repeatable and trouble-free processes. Ten metal powders were tested in the study. In all cases, the particle size distribution and morphology (scanning electron microscope—SEM photos) were determined. The aim of this work was to inspect the flow behavior of metal powders through another measured characteristic, namely the angle of internal friction. The measured values of the effective internal friction angle in the range 28.6–32.9°, together with the spherical particle shape and the particle size distribution, revealed the likely dominant mode of the metal particle transfer mechanism for stainless steel 316L, zinc and aluminum powder. This third piston flow mechanism is described and illustrated in detail. The angle of internal friction is mentioned as another suitable parameter for the characterization of metal powders, not only for the relative simplicity of the determination but also for gaining insight into the method of the movement of individual particles during the flow.

Undrained behavior and shear strength of clean sand containing low-plastic fines

This study presents experimental tests to understand the undrained behavior of sand containing various fines contents. The specimens were prepared by the wet tamping method. The consolidated undrained triaxial shear tests were carried out by sands mixed with amounts of fines in ranging from 0 to 60%. The results showed that the deviator stress quickly reaches the peak value with an axial strain in a range of 0.5 to 2%, and then, the value drops significantly with further increases in the axial strain, the pore water pressure of all the sand-fines mixtures rapidly increases as the axial strain reaches a value in a range from 1 to 2% and then slowly increases and reaches a stable state when strain is greater than 8%. Peak deviator stress gradually decreases with an increasing fines content from 0 to 40%, thereafter, the peak deviator significantly increases with further increases in the fines content up to 60%, irrespective of confining pressure values using in these tests. Finally, the effective internal friction angles are remarkably greater than the total friction angles for various sand-fines mixtures.

CIRI-CIRI KEJURUTERAAN TANAH BAKI GRANIT DI SEMENANJUNG MALAYSIA

Pengetahuan tentang kelakuan dan korelasi antara ciri-ciri tanah adalah penting dan berguna bagi rekabentuk dan ramalan pembinaan. Objektif utama kajian ini adalah untuk menentukan sifat-sifat kejuruteraan tanah baki granit di Semenanjung Malaysia. Tanah baki granit di Semenanjung Malaysia boleh dikelaskan sebagai tanah liat hingga pasir berkelodak dengan keplastikan rendah sehingga tinggi. Kandungan lembapan semulajadi bertambah dengan kedalaman dan kandungan lempung dan ianya berjulat di antara 10% - 40 %. Nilai graviti tentu didapati berjulat di antara 2.50 dan 2.74. Indeks mampatan, pekali pengukuhan dan pekali kebolehtelapan masing-masing berjulat 0.11– 0.42, 1.53 – 49.64 m2 tahun-1 dan 1.1 x – 9.86 x 10-9 ms-1. Nilai kejeleketan berkesan pula antara 0 – 36.4 kNm-2 manakala sudut geseran berkesan di antara 16º – 31º. Beberapa kolerasi di antara ciri-ciri kejuruteraan tanah baki dapat ditentukan dan boleh dijadikan panduan dalam rekabentuk awalan bagi pembinaan struktur

Shear strength of granular materials based on fractal fragmentation of particles

The Mohr-Coulomb criterion has two shear strength parameters commonly known as the cohesion and the friction angle. The peak shear strength of granular materials obtained from direct shear tests has common non-linear strength envelopes due to particle crushing. The effective cohesion continues to increase while the effective friction angle decreases with the increase of effective stress in direct shear tests. Particle crushing of granular materials in direct shear tests leads to a power-law distribution of fragment sizes, and is accurately characterized by the fragmentation fractal dimension drawn from the power exponent. Based on the fractal model for particle fragmentation, a power function is advocated to represent the shear strength criterion of granular materials involving only two constant parameters. The exponent of power-law shear strength criterion is found to express using the fragmentation fractal dimension. The proposed power-law shear strength criterion is validated with experimental data of municipal solid waste incineration (MSWI) bottom ashes from direct shear tests.

Investigation of scour effects on lateral behaviors of suction caisson

ABSTRACTThe use of suction caissons can reduce the development costs of offshore wind energy and has broad application prospects. However scour around marine foundations is inevitable, it gravely affects the stability of marine engineering. Therefore, there is an urgent need to study the weakening effects of scour on suction caisson. In this study, the variation trends of remaining soil parameters (the effective unit weight and the peak effective friction angle) after scour are examined with consideration of the dilatancy and stress history of sandy silt. It is found that the parameters of shallow soil change considerably after scour, and the larger the scour depth, the greater is the change in the parameters. However, the deep soil is less affected. On the basis of these findings, scour effects on the ultimate moment capacity of suction caisson are studied. The ultimate moment capacity is found to greatly reduce under scour, and its calculated value is larger than the actual value when the effects of dilatancy and stress history are ignored. To simplify calculation, it is feasible to replace the ultimate moment capacity when both dilatancy and stress history are considered with that when only dilatancy is considered.

Determination of the presence of microstructure in a soil using a seismic dilatometer

This article shows an approach for investigating the presence of microstructure in a soil based on the results obtained by seismic dilatometer (SDMT). Three main parameters are considered: intermediate dilatometer test (DMT) parameters material index (ID) and horizontal stress index (KD) and small strain stiffness (G0) obtained by shear wave velocity (Vs) measurements. The approach is engineering-oriented since it is simple, fast, inexpensive, and based directly on in situ measurements. We found that with increasing microstructure (cementation and aging) the ratio of measured to estimated G0 increases, which could be used to improve interpretation of empirically determined geotechnical parameters from DMT results. Applications of a DMT-based, CPT-based and Vs-based correlations for determination of effective peak friction angle of sands, with and without presence of microstructure, are evaluated. Finally, we propose an approach to reducing the influence of microstructure on K0–KD correlation in a clay.

Consolidated-undrained triaxial testing of Opalinus Clay: Results and method validation

Specific equipment and procedures developed for geomechanical testing of hydrocarbon caprocks were adopted to conduct truly undrained triaxial tests with Opalinus Clay. The amount of pore pressure development during consolidation, and the resulting effective stress, is managed by equilibrating the samples in vacuum desiccators of different relative humidities (vapor equilibration technique) prior to assembling into the test apparatus. A drained consolidation test was first conducted to determine the appropriate strain rate for consolidated-undrained (CU) triaxial testing. Opalinus Clay samples were then consolidated in the triaxial rig to mean effective stresses in the range from 3 to 52 MPa and eventually sheared. Within the explored stress range elastic and pore pressure coupling parameters were found to be stress dependent. The different stress paths to peak indicate a transition from overconsolidated to rather normally consolidated state, yet failure was in all tests dilatant, i.e. associated with a drop in pore pressure and strain-softening (more so at low effective stress). Accurate pore pressure monitoring enabled the discrimination of different deformation stages during deviatoric loading. In terms of Mohr–Coulomb strength parameters, transition from peak to post-peak strength is manifested by a reduction in the effective cohesion whereas the effective friction angle remains nearly constant. The robustness of the CU testing methodology is demonstrated by (i) diagnostic analyses, (ii) inconsistency of CU tests with two CU tests deliberately loaded faster to explore the effect of strain rate, and (iii) consistency of CU tests with two consolidated-drained tests. Finally, test results of two caprock shales are also shown for comparison. The caprocks are of similar basic properties as the Opalinus Clay and stem from a large data base of tests conducted using the same methodology.

Hydraulic and mechanical characteristics of compacted sand–bentonite: tyre chips mix for its landfill application

Compacted sand–bentonite mixtures are generally placed at waste dumping places as a means of avoiding contaminant movement to reduce or remove the potential for groundwater pollution. Owing to their almost impermeable nature and high pollutant adsorption tendency, bentonite generally treated as a liner material to avoid subsurface contamination. Cracks are formed in the landfill liner due to the desiccation, large interlayer shrinkage and differential settlement of bentonite. Fibres are commonly added to sand–bentonite mixture to act as a reinforcing material to eliminate the development of tensile cracks. Waste tyre chips are derived from the tyre retreated technique, and it can be included as short fibre in sand–bentonite mixture for the use as landfill barrier material. Hydraulic conductivity and shear strength are the two important parameters of the liner material, which are considered for hydraulic, and stability analysis in landfill application. In this context, consolidation and consolidated undrained tests were performed to study the hydraulic and mechanical behaviour of sand–bentonite mixture mixed in a proportion of 80:20 and mixed with 0, 5, 10 and 15% of waste tyre chips. From the consolidation test, swelling pressure and swelling potential reduced maximal at 15% of tyre chip content. Hydraulic conductivity and negative pore water pressure altered significantly with the application of tyre chips. Effective internal angle of friction enhanced from 24.9° to 30.5° and critical state parameter improved from 0.738 to 1.429 with the presence of 15% of tyre chips. Test result suggested that maximum of 10% of tyre chips can be used as a landfill liner and 15% tyre chips would be benefited for landfill cover.

Cohesion and tensile strength estimation from incomplete shear analysis data

The Yield Loci Curves (YLC's) of compacted cohesive powder specimens, from which its Flow Function (FF) is derived, are frequently described by the Warren Spring equation whose parameters are the cohesion (C), tensile strength (T) and curvature index (n). Ideally, for each pre-consolidation level, T should be determined independently with a special instrument, and C and n extracted from the experimental yield loci measurements by nonlinear regression using the Warren Spring equation as a model. There are situations, however, where direct determination of T is not a feasible option, and the number of experimental yield stress data, limited by logistic considerations, can be insufficient for meaningful regression. In such cases, T, C and n can be estimated with a freely downloadable interactive Wolfram Demonstration, with which three (the theoretical minimum) or more experimental yield loci are matched with a generated YLC using the Warren Spring equation as a model. To obtain a match, one moves T, C and n sliders on the screen using the calculated Mean Squared Error (MSE) as a guide and for fine-tuning. At least in principle, once T, C and n have been estimated in this way, they can be used to calculate the corresponding principal stresses and effective internal friction angle. The method is demonstrated with published experimental shear data on wetted glass beads and limestone powder.

Comparison of direct shear and simple shear responses of municipal solid waste in USA

Although large-size simple shear (SS) testing of municipal solid waste (MSW) may arguably provide a more realistic estimate of the shear strength (τ ) of MSW than the most commonly used direct shear (DS) testing, a systematic comparison between the shear responses of MSW obtained from the two testing methods is lacking. In this study, a large-size shear device was used to test identical MSW specimens sampled in USA in DS and SS. Eight DS tests and 11 SS tests were conducted at vertical effective stresses of 50–500 kPa. The stress–displacement response of MSW in SS testing was hyperbolic and a maximum shear stress was reached, whereas a maximum shear stress was not reached in most DS tests. The τ, effective friction angle (ϕ ′) and cohesion (c ′) of MSW were obtained from DS and SS tests by using a displacement failure criterion of 40 mm. τ in SS testing was found to be equal to or lower than τ in DS testing with ratios of τ between 73 and 101%. SS testing resulted in higher ϕ ′ but lower c ′ than DS testing. The shear strength parameters were lower than those obtained in previous studies from DS tests at 55 mm displacement.

Approach for Estimating Effective Friction Angle from Cone Penetration Test in Unsaturated Residual Soils

AbstractThis paper proposes approaches to estimate effective friction angle from cone tip resistance with incorporation of matric suction. Cone penetration tests (CPTs) are performed in unsaturated...

Influence of cementation level on the strength behaviour of bio-cemented sand

Microbially induced calcite precipitation (MICP) is used increasingly to improve the engineering properties of granular soils that are unsuitable for construction. This shows MICP technique significant advantages such as low energy consumption and environmentally friendly feature. The objective of the present study is to assess the strength behaviour of bio-cemented sand with varying cementation levels, and to provide an insight into the mechanism of MICP treatment. A series of isotropic consolidated undrained compression tests, calcite mass measurement and scanning electron microscopy tests were conducted. The experimental results show that the strength of bio-cemented sand depends heavily on the cementation level (or calcite content). The variations of strength parameters, i.e. effective friction angle φ′ and effective cohesion c′, with the increase in calcite content can be well evaluated by a linear function and an exponential function, respectively. Based on the precipitation mechanism of calcite crystals, bio-clogging and bio-cementation of calcite crystals are correlated to the amount of total calcite crystals and effective calcite crystals, respectively, and contributed to the improvement in the effective friction angle and effective cohesion of bio-cemented sand, separately.

Probabilistic model of unsaturated slope stability considering the uncertainties of soil-water characteristic curve

Many soil slopes are unsaturated and failure of them can be a major cause of damage to structures. Apart from soil properties, the Soil-Water Characteristic Curve (SWCC) is the backbone of any unsaturated slope analysis. Uncertainties of these effective parameters of unsaturated slopes cause the probabilistic analysis to be more realistic rather than deterministic. In this research, the stochastic analysis of unsaturated slope stability is carried out based on simplified Bishop’s method. The stochastic parameters are the input parameters of SWCC in addition to effective internal angle of friction, effective cohesion and unit weight of soil. Based on the collected results from hundreds of stochastic analyses, the probability of failure is presented as a three dimensional surface. Finally, probabilistic model is developed to model this surface and evaluate the probability of failure as function of safety factor and its correlation of variation.

Feeding properties and behavior of hammer- and knife-milled pine

This work reports the rheological characterization of hammer- and knife-milled lodgepole pine materials as a function of grind size and moisture content (MC). Characterizations included particle size and shape distributions, bulk density, compressibility at 10kPa uniaxial pressure, elastic recovery from 10kPa uniaxial pressure, shear strength, effective angle of internal friction, arching in a custom hopper, and feeding performance in an auger feeder operating separately in volumetric and gravimetric feeding modes. Compressibility and elastic recovery increased as much as 15% and 4%, respectively with increasing MC. Adding 3kPa of pressure increased the minimum hopper outlet width needed to ensure consistent flow (i.e. the critical arching width) for all hopper arching tests, except for the pellets. Interestingly, for materials with smaller particle sizes, the volumetric feed rate tends to decrease as MC increases even though the density of the material increases with MC. Time variability of volumetric feed rate increased dramatically with increasing particle size but the impact of MC was not as clear. For nearly all tests, gravimetric auger feeding resulted in better feeding control and substantially decreased time variability in the material feed rate. Shear tests indicate that material strength generally decreased with increasing particle size; while the impact of MC was not as obvious. Agreement between experiments and predictions of the critical arching width were best for 1.6mm and 6mm grinds and worst for the 13mm grinds. This work also shows that the correlation between experiments and predictions of the critical arching width can be improved in terms of the coefficient of determination (R2) from 0.56 to 0.84 by correlating the respective upper limits of the 95% confidence intervals rather than the mean values.