Effective Shear Strength Parameters Research Articles

Reinforcement and mud-pumping benefits of geosynthetics in railway tracks: Numerical analysis

Laboratory tests were conducted on track materials to evaluate constitutive parameters for three different sets of constitutive relationships, namely, non-linear analysis, straight analysis and coupled analysis. The model test results of finite element analyses using various constitutive relationships are compared. The coupled analysis provided a better prediction of the measured results compared to the non-linear and straight analyses. A detailed parametric study of a prototype track was then performed using the coupled analysis to evaluate the effects of geosynthetics on track reinforcement and mud-pumping reduction.Geogrid reinforcement was found to significantly reduce tie displacement only at low subgrade modulus values and effective subgrade shear strength parameters. Geogrid reinforcement was equally effective at reducing tie displacement within the subballast thickness range of 450–1000 mm. High excess pore water pressure coupled with low effective cohesion gives rise to mud-pumping problems in silty soil subgrades. The provision of geotextiles at the subgrade surface facilitates quick in-plane drainage and dissipation of pore water pressure. Thus, excess pore water pressure was observed to be lower in a geotextile-stabilized track compared to that in an unreinforced track, indicating reduction in mud-pumping potential in the former.

Unified Method for Estimating the Ultimate Bearing Capacity of Shallow Foundations in Variably Saturated Soils under Steady Flow

AbstractThis study presents a unified method for estimating the ultimate bearing capacity of shallow foundations resting on variably saturated soils under a steady flow. The effective stress approach originally employed for predicting the bearing capacity of saturated soils is extended by incorporating a suction stress-based representation. A closed-form equation is used to define the suction stress characteristic curve, capturing changes in the effective stress because of varying soil saturation and matric suction. The proposed method uses the classic effective shear strength parameters and two fitting parameters to represent the soil water characteristic curve. The method can be used for different soil types, various degrees of saturation, and different surface flux boundary conditions. The results from the proposed formulation were compared, and good agreement was observed against three sets of experimental results from model footing and plate load tests. A parametric study was performed, and the ultim...

Assessment of stability problems at southern engineered slopes along Mersin-Tarsus Motorway in Turkey

This paper describes the cut slope stability problems of southern engineered slopes along 57 km long Mersin-Tarsus Motorway. While the northern slopes were slipping during and following construction, the stability problems occurred at southern slopes after heavy rains in December 2001. The most recent slope stability problem took place on chainage at the km 10+800 embankment on 9 April 2009. About 3.62 × 106 m3 soil materials have currently slipped at 60 different location. After the occurrence of slides, different remedial methods (reshaping slopes, soil improvements, etc.) have been applied to stabilize the slipped slopes. The slope stability studies were carried out using back analysis to determine the slope failure mechanisms and to estimate effective shear strength parameters. Pore water pressures increase following intense rainfall events and cause reduced resistance to shear strength at the engineered slopes. In addition, the affects of the static and dynamic parameters to analyse the state of the slope after excavation were investigated and possible remedies to improve the state (toe rockfill and retaining wall) were assessed for motorway slopes through determining the slope stability with water parameter and seismic loading, separately and together. The results of the stability analysis have exhibited that a good drainage system and retaining wall prevents the motorway slope slides.

Shear strength and dilative characteristics of an unsaturated compacted completely decomposed granite soil

Shear strength and dilative characteristics of a re-compacted completely decomposed granite (CDG) soil are studied by performing a series of single-stage consolidated drained direct shear tests under different matric suctions and net normal stresses. The axis-translation technique is applied to control the pore-water and pore-air pressures. A soil-water retention curve (SWRC) is obtained for the CDG soil from the equilibrium water content corresponding to each applied matric suction value for zero net normal stress using a modified direct shear apparatus. Shear strength increases with matric suction and net normal stress, and the failure envelope is observed to be linear. The apparent angle of internal friction and cohesion intercept increase with matric suction. A greater dilation angle is found at higher suctions with lower net normal stresses, while lower or zero dilation angles are observed under higher net normal stresses with lower suctions, also at a saturated condition. Experimental shear strength data are compared with the analytical shear strength results obtained from a previously modified model considering the SWRC, effective shear strength parameters, and analytical dilation angles. The experimental shear strength data are slightly higher than the analytical results under higher net normal stresses in a higher suction range.

The influence of the grain-size distribution and soil structure on the unsaturated shear strength of loess sediments in Belgrade, Central Serbia

There is a negative pore water pressure or matric suction in the zone above the ground water level in silty loess soil, which can be as deep as 5-10 m in the Belgrade area. This primary characteristic of unsaturated soil, i.e., matric suction, should be included in laboratory testing and geotechnical analyses. Direct shear or triaxial testing of unsaturated soil are very expensive and time-consuming and require specially modified equipment. Instead, the prediction of unsaturated shear strength using the soil water characteristic curve, SWCC, and the effective shear strength parameters c' and ?' is a widely accepted practice. In this study, constitutive soil-water characteristic curves were obtained from the results of experimental testing by draining saturated soil samples under different pressures. This testing was performed for the first time in Serbia in a 15 bar pressure plate extractor according to ASTM standards. The laboratory testing included natural samples of loess sediments with the original macroporous structure and loess sediments with a destroyed soil structure. The influence of the grain-size distribution and natural soil structure on the unsaturated shear strength of Belgrade loess sediments above the ground water level was also evaluated. The obtained results are in accordance with the results from other investigations.

Studying The Influence Of Compacting Method On The Engineering Properties Of Expansive Clayey Soil

This study deals with the effect of two methods of compaction (static compaction (S.C.) and dynamic compaction (D.C.)), on the engineering properties of high plasticity clayey soil (CH) selected from Mosul city. Results showed that maximum dry density of soil compacted by static method is higher than that compacted by dynamic method. But the inverse was obtained for optimum moisture content. Unconfined compressive strength, effective shear strength parameters (ć, ǿ), splitting and California Bearing Ratio (CBR) for soil compacted by dynamic method has higher values than that compacted by static method. Swelling pressure and free swell tests gave higher values under static compaction method. But the values of compression index and coefficient of consolidation using dynamic compaction method gave higher values. On the other hand, the results of crumb, slackening and pinhole tests showed that the internal erosion of soil compacted by dynamic method are less than that compacted by static method. From filter paper method and soil water characteristic curves showed that compaction methods had no effect on soil suction. Finally, statistical models between engineering properties for soil compacting by two methods were obtained.

An Expedite Method to Predict the Shear Strength of Unsaturated Soils

An expedite method to predict the shear strength of unsaturated soils is proposed and tested against many soils of different origins and subjected to different test types. The method uses an empirical hyperbolic function that has been successfully used to fit experimental data. The parameters of this function are obtained considering effective shear strength parameters from saturated soil and from test results of air dried samples tested without the need of suction control. Air dried samples can be, alternatively, replaced by test results of samples tested at suction larger than the maximum suction expected in the problem under analysis. The good agreement between the estimate functions and the experimental data shown by both alternatives makes them promising and reliable to estimate unsaturated shear strength parameters for preliminary purposes.

Energy-Based Modeling Approach for Debonding of FRP Plate from Concrete Substrate

For concrete beams and slabs strengthened with bonded fiber reinforced plastic (FRP) plates, plate debonding from the concrete substrate is a common failure mode. In this paper, the debonding process is modeled as the propagation of a crack along the concrete/adhesive interface, with frictional shear stress acting behind the crack tip. Crack propagation is taken to occur when the net energy release of the system equals the interfacial fracture energy. The analysis is first performed for the special case with constant shear stress along the debonded interface, and then for the general case with slip softening in the debonded zone. From the results, a direct correspondence between energy-based and strength-based analyses can be established for arbitrary softening behavior along the interface. Specifically, through the proper definition of an effective interfacial shear strength, the conventional strength-based approach can be employed to give the same results as the much more complicated energy-based analysis. Also, based on the relation between the effective shear strength and other material parameters, it is possible to explain the very high interfacial shear stresses observed in experimental measurements. As an application example, distribution of plate stress and interfacial shear stress for the linear softening case is derived. The model results are found to be in good agreement with experimental measurements, showing that the simple linear softening model can describe the debonding process in real material systems.

Determination of Shear Strength and Three-dimensional Yield Strength for the Hoek-Brown Criterion

Most currently used techniques for analysing the stability of near surface structures, such as rock slopes, are based on the application of the effective Coulomb shear strength parameters cohesion c′, and the angle of friction φ′ on some known or anticipated shear surface subjected to an effective normal stress σ′ n . The most widely used of these techniques are the variants of the method of slices and related upper bound techniques. If the Hoek-Brown criterion is to be used to model the strength of near surface fractured rocks, it is necessary to determine equivalent Coulomb shear strength parameters for the specified level of effective normal stress. Calculation of the equivalent Coulomb parameters for the Hoek-Brown criterion for cases when a ≠ 0.5 is not a straightforward matter. A simple procedure for calculating instantaneous values of c′ i and φ′ i has been developed based on spreadsheet calculations and the application of a numerical optimisation routine. This procedure can also be applied to calculating the Hoek-Brown envelope plotted in shear stress/normal stress space. A simple closed form solution for c′ i and tan φ′ i has also been developed for the special case when a = 1. A three-dimensional version of the Hoek-Brown criterion has been developed by combining it with the Drucker-Prager criterion. This new yield criterion has been implemented by numerical solution of the governing equations. A simplification of this three-dimensional yield criterion has been developed by introducing an intermediate principal stress weighting factor. Comparison with published results demonstrates that this simplified criterion has the capacity to model the results of true triaxial tests for a range of different rock types over a wide range of stress levels. The new three-dimensional yield criterion has the advantage that its input parameters can be determined from routine uniaxial compression tests and mineralogical examination.

Improved use of pulverised fuel ash as general fill

In the UK, the introduction of a tax on landfill and the tightening of planning consents for disposal provided powerful incentives for the use of industrial by-products such as pulverised fuel ash (PFA) in construction. PFA (known as fly ash in some parts of the world) has a long history of successful use as general fill, including applications in environmentally sensitive locations. However, the current commonly used Specification for Highway Works treats PFA as a manufactured material, and applies an end-product specification for compaction. This form of specification is restrictive as it requires that the maximum dry density and optimum moisture content be determined for each consignment of PFA delivered to site, a process that can take up to two days. In addition, the Specification requires testing to determine effective shear strength parameters under fully saturated conditions for embankment design. No guidance is given on whether peak or post-peak parameters are to be selected for design purposes or, indeed, on appropriate factors of safety. As a result, most engineers test PFA in the fully saturated condition, select post-peak effective shear strength parameters for design, and apply a conservative factor of safety. As the testing protocol represents a worst-case scenario the application of such a conservative approach can be questioned. A method specification is more commonly applied to natural fill materials used as general fill. Historical evidence shows that PFA has been placed using a method specification, and that those fills still function as designed. Thus the introduction of a method-based specification for the use of PFA as general fill would make possible its greater use and thus reduce the UK-wide need for landfill.

Slope Failure in Weathered Claystone and Siltstone

This paper describes the failure of an 8.5-m-high fill slope constructed over weathered claystone and siltstone in Martinez, Contra Costa County, in northern California. The failure occurred about 5 months after construction. Several piezometers and inclinometers were installed to measure the pore pressures and the location of the failure surface(s), respectively. The failure surface was observed just below the interface between the fill and the natural materials. Factors of safety of the failed slope calculated using laboratory-derived effective shear strength parameters were on the order of 1.58–1.95. Reduction factors were necessary to reduce the shear strength values and to obtain reasonable agreements with the field observations. A class “C” prediction of the location failure surface and the factor of safety using the reduced shear strength values indicated results that were consistent with the field observations. This paper highlights the importance of exercising extreme caution when evaluating the ...

Leaching Effects on Properties of Cemented Sands in Kuwait

The effect of leaching on the properties of cemented sand deposits in Kuwait was examined by laboratory tests on samples taken from two sites with different cementation levels. The cementing agents identified include carbonates, sulfates, and chlorides. Tests included basic properties, consolidation, and consolidated undrained triaxial tests with pore pressure measurements. All tests were performed on natural and leached specimens. Leaching was carried out in the laboratory under low pressure using fresh tap water. The results indicate increased compressibility and a reduction of the effective shear strength parameters c′ and ϕ′ due to leaching. The effect is more pronounced for weakly cemented sands compared with moderately to strongly cemented sands. The effect of leaching depends on several factors including the soil type, relative density, grading characteristics, degree of cementation, and the amount of cementing agents and their solubility in water.

Analysis and remedies of landslides of cut slopes due to the presence of weak cohesive layers within stronger formations

Several cases of instability of cut slopes along major highways in Jordan were reviewed in this study, in some detail. Emphasis was placed on the Amman-Irbid highway, but some cases along the Na'ur-Dead Sea highway were also reviewed, with the aim of establishing a wider database of case-studies and examining all possible mechanisms and factors influencing stability. The study showed that major cut slope failures were caused by the presence of weak cohesive layers (mainly clayey marl) interbedded within mostly stronger formations, in addition to the steep cutting angles and unfavorable dip of strata combined with relatively high piezometric surface brought about by poor surface and subsurface drainage. No remedies were implemented to stabilize major cut slope failures (at km 39+200, 44+300, and 56+400) along the Irbid-Jerash-Amman highway. The three major cut slope failures require extensive remedial work and probably advanced geotechnology, which is expected to be expensive. To investigate the influence of various parameters on slope stability, several analyses were performed in addition to back analyses to determine shear strength parameters; parametric sensitivity studies were also performed on some cases. Data was obtained from previous investigations by local and international firms and were screened and modified where needed before being used in stability analyses. Considerable difference between back analysis and test result values for shear strength parameters were observed in many cases. Failure surfaces and mechanisms were accurately depicted in most cases, allowing back analyses to be performed with considerable confidence. The effective residual shear strength parameters for clayey marl needed for stability analysis were found to be: c r ′=5–18 kPa, and φ r ′=13°–18°. The relation between total annual rainfall and occurrence of landslides was investigated; it was shown that all landslides occurred in years of very high rainfall, with values always exceeding 400 mm. The probability of exceeding the average total annual rainfall was found to be approximately 0.42 for the three gauging stations considered.

A comparative study of slope stability methods and mitigative design of a highway embankment landslide with a potential for deep seated sliding

During the very exceptional 1991/1992 winter of heavy rainfall in Jordan, slope stability problems were encountered at an embankment at Station 47 + 300 along Irbid-Amman Highway. Geological and geotechnical investigations were conducted at the landslide area. Parametric slope stability analysis and back analysis were carried out for the embankment at the time of instability, using both Modified Bishop and Modified Janbu limiting state equilibrium methods (incorporated in the computer program PCSTABL5M (Achilleos, E., 1988. User Guide for PCSTABL5M. Joint Highway Research Project, Report JHRP-88/19, School of Civil Engineering, Purdue University, West Lafayette, IN, USA)), in order to determine with confidence the most representative values of regional shear strength parameters, explain the failure mechanism, assess the condition at the time of failure, and investigate all possible failure mechanisms through different foundation layers. Furthermore, slope stability analysis was carried out for mitigative design options. In the analysis, different conditions of piezometric water level were considered. Moreover, circular and sliding “Block” failure surfaces at shallow and deep seated levels were investigated. Analysis showed that the main causes of instability were: excessive load of the embankment, constructing the embankment on colluvial deposits, and the ingress of water into the embankment, and the underlying colluvial material, resulting in its softening. The landslide movement occurred mainly within the colluvial deposits. The most likely location of the slip surface was at the embankment-colluvium contact. Analysis also showed a potential for a deep seated sliding along a marl layer that governed the remediation procedures which should be implemented to stabilize the area. The best estimates of effective shear strength parameters of the materials encountered at the landslide area were as follows: for colluvium, cohesion ( c′)=0.0 and angle of friction ( gft')=25.0°, and for Marl, c′=25 kPa, and gft'=13.0°. The most appropriate remedial measure for road design and stabilization of the sliding area was found to be a combination of vertical reduction in embankment height and horizontal realignment. This remediation was implemented in the field successfully.

Damaging Landslides of Cut Slopes Along Major Highways in Jordan: Causes and Remedies

Several cases of instability of cut slopes along major highways in Jordan are reviewed in this study, in some detail. Emphasis was placed on the Amman-Irbid Highway, but some cases along the Na9ur-Dead Sea Highway were also reviewed, with the aim of establishing a wider data base of case studies and examining all possible mechanisms and factors influencing stability. This study shows that major cut slope failures are caused by the presence of weak cohesive layers (mainly clayey marl) interbedded within other, stronger formations, in addition to the steep cutting angles and unfavorable dip of strata combined with a relatively high piezometric surface brought about by poor surface and subsurface drainage. Sections II and III of the Amman-Irbid Highway suffer from an alarmingly high level of instability in cut slopes where more than 50 percent of the entire route length was found to have a high level of instability. To investigate the influence of various parameters on slope stability, several analyses were performed, in addition to back analyses, to determine shear strength parameters; in some cases, parametric sensitivity studies were also performed. Data was obtained from previous investigations by local and international firms, screened and modified where needed, before being used in the stability analyses. Considerable differences between back analysis and test result values for shear strength parameters were observed in many cases. Failure surfaces and mechanisms were accurately depicted in most cases, allowing back analyses to be performed with considerable confidence. The effective residual shear strength parameters for clayey marl needed for the stability analysis were found to be: c r ′ = 5 to 18 kPa, and (ϕ r ′ )/ = 13° to 18°. A classification system was developed for factors influencing landslides based on the case studies considered, and a stability grading scheme was also introduced. This system was applied to several existing cases along the Amman-Irbid Highway and the resulting assessments were in agreement with the observed stability conditions.

Mitigative Design for Colluvial Slope Instability, Wadi Es-Sir Sewage Treatment Plant, Jordan

Lagoons were constructed within colluvial deposits in an area of 140,000 m2 as part of Wadi Es-Sir Sewage Treatment Plant Project in Jordan. The site has been excavated into steps and risers to construct the lagoons. A major noncircular slope failure occurred in October 1993 on the upper slope of the middle lagoon, resulting in an interruption of construction. Field and laboratory investigations were conducted to identify various geomorphologic zones, stratigraphy and geotechnical properties of all materials encountered at the site. Slope stability back analysis of failed slope was carried out using the 2-D limiting equilibrium Simplified Janbu method. Also, stability analysis was carried out for existing and improved conditions for all lagoons. Dry, partially saturated and fully saturated conditions were considered in the analysis. Pseudostatic stability analysis of the slopes was carried out considering a horizontal bedrock Peak Ground Acceleration (PGA) value of 0.2 g, evaluated at the site using a line source probabilistic seismic hazard assessment model incorporated in the computer program FRISK (McGuire, 1978). The study showed that the failure occurred through colluvial material due to the removal of a large amount of such materials near the toe of the slope during construction of the lagoon. The location of the slip surface was between 8 to 12 m in depth. Effective residual shear strength parameters along the failure plane were found to be: angle of friction (ϕ′) = 17°, and cohesion (c′) = 0.05 kg/cm2. Stability conditions for slopes at all lagoon sites were found to be unacceptable. Recommended measures to stabilize the slopes include replacing (partially or fully) the existing colluvial materials with free-drained rockfill to reach acceptable factors of safety under design conditions. These measures were implemented at the site.

Behaviour of a test excavation in soft Champlain Sea clay

The field observations made during a sloped excavation in a soft sensitive clay are presented. The test site is located in Saint-Hilaire, in a typical Champlain Sea clay deposit, of 30 m depth. The trench was 8 m deep and 60 × 60 m square and its slopes were at 45, 34, 27, and 18° to the horizontal. The induced pore pressures and total lateral stresses have been recorded with vibrating-wire instruments since the beginning of the excavation. The steepest, 45° slope completely failed 1 day after excavating and the 34° slope, 2 weeks after. The piezometer readings have shown that the groundwater regime was modified proportionally to the changes in total stresses; these modifications took place within a time period of 5 months, after which the heads became more or less constant. The total stress cells indicated significant rotation of the principal axes at the toe of the slopes. Stability analyses using the lowest vane strength profile gave minimum factors of safety of 1.15 for the 45° slope and 1.23 for the 34° slope, thereby underestimating the risks of failure. Back analyses using regional effective shear strength parameters were made and the parameters mobilized at failure were found to be [Formula: see text] and [Formula: see text]. Key words: case record, excavation, soft clay, slope stability, back analysis.

Relationship between undisturbed and remoulded effective stress shear strength parameters

Relationship between undisturbed and remoulded effective stress shear strength parameters

Effective shear strength parameters for stiff fissured clays : Moon, A T Proc 4th Australia-New Zealand Conference on Geomechanics, Perth, Western Australia, 14–18 May 1984VI, P107–111. Publ Barton: Inst of Engineers, 1984

Shear box and triaxial tests have been used to investigate the effective shear strength of a stiff fissured clay of constant mineralogy but variable plasticity. Different residual shearing mechanisms were recognised in the shear box tests with significantly different values of residual strength. The fully softened strength parameters appropriate for the analysis of first-time slides were investigated by both triaxial and shear box tests. The lower plasticity samples had a higher strength than the higher plasticity samples. For the soil tested both the residual and fully softened effective friction angles showed a pattern of dependence on the plasticity. It may be possible to establish similar correlations for other soils if the results reflect different shearing mechanisms caused by grading variations within a soil of constant clay mineralogy. (Author/TRRL)

Yield states and stress–strain relationships in a natural plastic clay: Discussion

The authors are to be congratulated for their very careful and well-performed tests on Winnipeg clays. The results add valuable data and further our knowledge of the yield behaviour of natural clays, and show a well-defined yield locus that can be normalized with respect to the vertical preconsolidation pressure o:,. The results show that the yield behaviour for Winnipeg clays, as for other natural clays, differs to some extent from that of the Cam-clay models. The establishment of the yield locus in this case involved an extensive triaxial testing program that can normally be carried out only for research purposes. Larsson and Sallfors (1981) have previously shown that a very good estimate of the yield locus can be obtained from the results of only two oedometer tests, one on an ordinary vertical sample and one on a horizontally trimmed sample. The method is based on the simple hypothesis that yield will occur when the preconsolidation pressure is exceeded in any direction or when the effective shear strength parameters are mobilized (see Fig. 1). This requires knowledge of the effective shear strength for stresses up to the preconsolidation pressure. In this range the effective shear strength forms a slightly curved envelope, which for many Scandinavian and Canadian clays can roughly be described by a straight line with +' = 30 and ct = 0, or in more detail, by a curve with the end points on the line 4' = 30 and c' = 0, and an average c' = 0 . 0 3 ~ : ~ in the intermediate stress range. The investigated Winnipeg clays differ from these clays in that the dominating clay mineral is smectite and FIG. 1. Simplified yield locus defined by the vertical preconsolidation pressure a:, the horizontal preconsolidation pressure DL,, and an effective angle of friction of 30. Standard plot for stress paths in triaxial tests.