Drained Shear Strength Parameters Research Articles

Geological and engineering geological characteristics of surface alluviums in the Gorgan city

Engineering properties of soils and the 3D modeling of geological formations are widely used in site investigations and the preparation of geological hazard maps. The present study was conducted to characterize the engineering geological properties of the young surface alluviums of the Gorgan city (Iran) to a depth of 25 m and 3D modeling of their geology using boreholes data. To this end, after determining the location of the available boreholes on the aerial map of Gorgan, four hypothetical cross-sections were considered in the North-South and East-West directions. Then, the borehole data were marked on each section and their 2D geological cross-sections were manually drawn using correlation of the similar layers. In the next step, by expanding the information of these sections, a 3D geological model of Gorgan city was prepared using a conceptual-observational method. According to the evidence from the boreholes and field observations, the depositional environment of Gorgan alluviums was an alluvial fan created by the Ziarat River. Additionally, in terms of engineering characteristics of alluviums, the Gorgan subsurface soils can be divided into four engineering units, including upper clay unit (UCU), middle gravel unit (MGU), lower clay unit (LCU), and sandy unit (SU), which share the same engineering characteristics. Finally, the results of tests performed on samples from different depths were employed to calculate the engineering geological characteristics of each unit, including Atterberg limits, compressibility, undrained shear strength, and drained shear strength parameters.

ANÁLISE 2D E 3D DA ESTABILIDADE DA BARRAGEM DE FUNDÃO

RESUMO: A falha da Barragem de Fundão, o maior desastre de seu tipo no mundo, foi estudada por Morgenstern et al. (2016) através de abordagens bidimensionais. A investigação realizada não apontou mecanismos de instabilidade de taludes como sendo as causas do desastre. No entanto, o formato complexo da face da barragem, com regiões côncavas e convexas, não foi considerado devido às limitações das ferramentas empregadas. Este trabalho apresenta análises bidimensionais (2D) e tridimensionais (3D), teoricamente mais rigorosas e adequadas para a geometria da barragem. As análises de estabilidade do maciço foram realizadas utilizando o Método Geral das Fatias e das Colunas (GLE), por meio do software SVSLOPE. Foram realizadas buscas por superfícies críticas 2D e 3D, ao longo de todas as zonas da estrutura. Considerando-se parâmetros de resistência drenada, observou-se que Fundão se encontraria em condições de elevada segurança, sendo o menor Fator de Segurança igual a 1,744. Valores próximos de 1,0 foram obtidos considerando rejeitos na condição não drenada, supostamente menos representativa das condições reais de campo. Os Fatores de Segurança 3D se mostraram muito próximos dos 2D, porém com variações significativas nas regiões côncava e convexa. Desta forma, conclui-se que o formato próximo à região do recuo exige maior atenção e análises 3D podem identificar as regiões críticas.
 ABSTRACT: The failure of the Fundão Dam, the largest disaster of its kind in the world, was studied by Morgenstern et al. (2016) using two-dimensional approaches. The carried out investigation did not point to mechanisms of slope instability as the causes of the disaster. However, the complex shape of the dam and its concave and convex regions were not originally considered due to limitations of the analysis tools employed. This paper presents two-dimensional (2D) and three-dimensional (3D) analyses that are more rigorous and well suited for the Fundão dam geometry. Slope stability analyses were accomplished using the General Limit Equilibrium Method of Slices and Columns (GLE) using the SVSLOPE software. Critical slip surfaces were searched in 2D and 3D, across the entire dam face. Drained shear strength parameters led to Factors of Safety that would indicate fairly stable conditions, with a lowest Factor of Safety of 1.744. Values close to 1.0 were obtained considering undrained conditions of the tailings, which are supposedly less representative of field conditions. The 3D factors of safety were very close to the 2D values, but with noticeably higher variations near the concave and convex regions. This leads to the conclusion that the dam shape near the setback region requires closer examination by means of 3D analyses that enable the identification of critical zones.

Drained shear strength parameters of silty sand grouted by colloidal silica

A deep understanding of the behavior and specifications of grouting materials between soil particles is required to increase the applications of various grouting technologies. Out of the different grouts that can be injected in fine-grained soils, colloidal silica has been selected in this study due to its unknown effects on fine-grained soils. In this study, the effects of colloidal silica on silty sand with different amounts of silt were studied, and the strength parameters (c, ϕ) of stabilized samples under long-term static loading was evaluated. A total of 42 uniaxial compression tests and 75 consolidated-drained triaxial tests were done on the samples. Uniaxial compression tests were conducted to determine the most suitable age of the samples for testing, which was three days. The results of the triaxial tests indicate an increase in the drained cohesion parameter in all samples, while changing the drained internal friction angle in silty sands decreases.

Effects of Fines Content on Hydraulic Conductivity and Shear Strength of Granular Structural Backfill

For cast-in-place abutments and retaining walls, structural backfill is preferred to be free-draining, which generally implies less than 5% fines content. This fines content is expected to eliminate the need to design for hydrostatic pressures. The availability of high-quality structural backfill with naturally low fines content is declining. This situation warrants an evaluation of whether granular backfill materials with greater than 5% fines content could be successfully used in practice. Flexible wall, hydraulic conductivity tests on a granular structural backfill with 0%, 5%, 10%, 15%, 20%, and 25% nonplastic fines content were conducted at 41, 83, and 124 kPa (6, 12, and 18 psi) confining pressures followed by consolidated drained triaxial compression tests for obtaining associated drained shear strength parameters of these gradations. The 15.2-cm (6-in.) diameter specimens were prepared at optimum moisture content and 95% of maximum standard Proctor density. To enable a comparison with respect to modified Proctor maximum densities, modified Proctor tests were also performed for all base soil–fines content mixtures. The experimental results were compared with relevant studies found in the literature. This research indicates that a nonplastic fines content up to 10% may be justified in structural backfill specifications for retaining walls and abutments.

Laboratory shear strength parameters of cohesive soils: variability and potential effects on slope stability

The study quantified laboratory geotechnical variability by analyzing compacted alluvial fine-grained soils commonly used to backfill exhausted quarries in Central Italy. Repeating the test 4 times and combining the relevant results, 256 pairs of friction angle and cohesion values were obtained, with mean values of 27.0° ± 0.8° and 19.22 ± 4.08 kPa, respectively in the shear stress range 100–250 kPa. The influence of the variability of laboratory drained shear strength parameters on the long-term stability of artificial saturated slopes was examined, taking into account both deterministic and probabilistic approaches. In many cases, when the deterministic approach indicates stability, the probabilistic one indicates a certain probability of instability, even for the lowest slope height (5 m) and a slope angle higher than 15°.

Effect of Sand Columns on the Undrained Load Response of Soft Clays

When sand columns are used as vertical drains in soil improvement schemes, the possible reinforcing role that these columns can play in regards to improving the bearing capacity is usually neglected in design. The objective of this paper is to evaluate the degree of improvement in the mechanical properties of soft clays in practical applications involving the use of sand drains or sand columns in clayey soils. For this purpose, 32 isotropically consolidated undrained triaxial tests were performed on normally consolidated kaolin specimens. The parameters that were varied were the diameter of the sand columns, the height of the columns, the type of columns (geotextile encased versus nonencased), and the effective confining pressure. Test results indicated that sand columns improved the undrained strength significantly even for area replacement ratios that were less than 18%. The increase in undrained strength was accompanied by a decrease in pore pressure generation during shear and an increase in Young's modulus. The drained shear strength parameters were found to be relatively unaffected by the sand column reinforcement, except for fully penetrating columns with high area replacement ratios.

Drained Shear Strength Parameters for Analysis of Landslides

This paper presents recommendations for selecting the type and magnitude of drained shear strength parameters for analysis of landslides. In particular, the importance, existence, and use of the cohesion shear strength parameter is reviewed. For slope stability analyses, it is recommended that the shear strength be modeled using a stress dependent failure envelope or a friction angle that corresponds to the average effective normal stress acting on the slip surface passing through that particular material instead of using a combination of cohesion and friction angle to represent soil shear strength. Other recommendations for stability analyses include using an effective stress cohesion of zero for residual and fully softened strength situations. To facilitate selection of shear strength parameters for landslide analyses, empirical relationships for the drained residual and fully softened strengths are updated from the previous empirical relationships presented by Stark and Eid. Finally, the paper presents torsional ring shear test results that indicate that pre-existing shear surfaces exhibit self-healing that results in increased shear resistance. The magnitude of healing appears to increase with increasing soil plasticity, and this increase could have implications for the size, timing, and cost of landslide remediation.

Drained-strength parameters from direct shear tests for slope stability analyses in overconsolidated fissured residual soils : Borden, R H; Putrich, S F Trans Res RecN1089, 1986, P102–113

An experimental investigation of the drained shear strenth of soils from three sites within the Durham Triassic Basin of North Carolina is described. Multistage direct shear tests conducted on specimens trimmed from block samples were utilized to evaluate the postpeak shear strength of these materials. In general, the observed reduction in strength with accumulated displacement beyond the peak strength was in good agreement with data repored in the literature for other overconsolidated soils. The residual friction angle for the soils tested was found to follow the trend of decreasing friction angle with increasing plasticity index. These results support the need for utilizing drained shear strength parameters and effective stress analyses for predicting the sability of slopes cut in overconsolidated residual soils. In a companion paper in this Record a case study is presented of a well-documented slope failure within the Durham Triassic Basin that indicates the appropriateness of using noncircular failure geometrics and suggests the applicability of utilizing postpeak strengths obtained from drained direct shear tests.

The Modified Borehole Shear Device

Abstract The standard borehole shear device (BHSD) as developed by Handy and Fox (1976) was modified to include pore-water pressure and load transducers, a motorized pulling mechanism, and different types of shear plates. With the modified device it is possible to monitor the drainage conditions during both the consolidation and shear phases of the test. Undrained and drained shear strength parameters have been measured within boreholes augered into a soft sensitive clay deposit.