Pre-existing Shear Surfaces Research Articles

Effects of overconsolidation on the reactivated residual strength of remoulded deep-seated sliding zone soil in the Three Gorges Reservoir Region, China

Due to the landslide-prone geological conditions, many landslides have occurred in the Three Gorges Reservoir Region (TGRR), most of which are slow-moving ancient landslides characterized by preexisting shear surfaces. In this study, with the purpose of making full use of reactivated residual strength to prevent and treat the slow-moving landslides, such as designing anti-slide piles, the Huangtupo landslide as a typical reactivated ancient landslide in the TGRR was selected to study the overconsolidation (OCR) effect on the reactivated residual strength of deep-seated sliding zone soil by conducting a series of laboratory ring shear tests. It was found that after a short rest of one day, the reactivated residual strength increased with the increase of OCR under a given normal effective stress, but it was lost after a small shear displacement. The overconsolidation effect on reactivated residual strength at a lower stress was more prominent than that at a higher stress. Very coarse sand with a size of 1–2 mm inside the remoulded deep-seated sliding zone soil sample exerts direct control on the reactivated residual strength, and it significantly influences the overconsolidation effect on reactivated residual strength. The results could provide a reliable scientific basis for improving the stability analysis and reinforcement measures of the slow-moving landslides in the TGRR, as well as can be applied to similar slow-moving landslides in other areas.

Creep properties of clastic soil in a reactivated slow-moving landslide in the Three Gorges Reservoir Region, China

Most slow-moving landslides in the Three Gorges Reservoir (TGR) region of China are characterized by pre-existing shear surfaces. The large deformation within the shear zones usually gives rise to clastic soil formation. The creep properties have large influence on the kinematic feature of landslides. In this paper, we report an in-situ direct shear creep test carried out in the shear zone of a reactivated slow-moving landslide in the TGR region. Correspondingly, some laboratory ring shear creep tests are carried out to interpret the movement pattern of this landslide. The shear zone soil exhibits similar non-attenuating creep responses in both the in-situ direct shear and laboratory ring shear creep tests. At the same stress level, however, the in-situ direct shear creep test yields a larger rate of creep displacement due to shearing along the landslide direction. In the ring shear creep tests, at the prepeak stage, the critical creep stress that triggers creep failure is slightly lower than the peak shear strength but much larger than the residual strength; at the postfailure stage, the critical creep stress of the shear-zone soil is equal to the residual shear strength. The rate-dependent residual shear strength may account for the stepwise movement pattern of the landslide.

Comparison of site-specific and empirical correlations for drained residual shear strength

Analyses of landslides, slopes and embankments often require reliable drained residual shear strength estimates for cohesive deposits involving pre-existing shear surfaces. Laboratory tests to measure residual shear strength can be costly and time consuming. Therefore, empirical correlations based on more readily available index properties such as liquid limit and clay-size fraction appear as an attractive alternative in practice. This paper presents residual strength and index properties for 25 natural samples, predominantly claystone and siltstone, collected from a single geologic setting over an approximately 0·25 km2 site. The data show substantial variability in residual strength against any single index parameter considered in a site-specific correlation. Comparisons of these data, along with three additional data sets, generally indicated notable variability among data sets and relatively poor fit to published empirical correlations. Empirical correlations for residual strength may be useful in approximating the expected range of residual strength. However, given the significant variability in the data, both within and between data sets, empirical correlations should not entirely replace carefully performed site-specific laboratory tests measuring residual strength, when reliable values are required.

Discussion of “Correlations for Fully Softened Shear Strength Parameters” by B. A. Castellanos, T. L. Brandon, and D. R. VandenBerge, This Article Was Published in Geotechnical Testing Journal, Vol. 39, No. 4, 2016. [DOI: 10.1520/GTJ20150184

Abstract In this paper, the authors criticize the use of a torsional ring shear device for measuring the fully softened strength (FSS) of fine-grained soils based on problematic ring shear data from Castellanos (2014). This is unfortunate and necessitated taking important time and energy to write this discussion. I have over 26 years of commercial soil testing experience while managing the Woodward-Clyde Consultants and AECOM Technical Services Laboratory in Santa Ana, California. I started performing Bromhead torsional ring shear tests to measure the residual strength (RS) of shear surfaces for landslides in 1991. As Professor Stark expanded his shear strength correlations and the geotechnical engineering practice embraced the use of FSS in stability analyses, I started using the Bromhead torsional ring shear device for FSS testing in 2009. The RS and FSS tests are performed in accordance with ASTM D6467, Standard Test Method for Torsional Ring Shear Test to Determine Drained Residual Shear Strength of Cohesive Soils, and ASTM D7608, Standard Test Method for Torsional Ring Shear Test to Determine Drained Fully Softened Shear Strength and Nonlinear Strength Envelope of Cohesive Soils (Using Normally Consolidated Specimen) for Slopes with No Preexisting Shear Surfaces, test methods, respectively.

Long-runout mechanism and landsliding behaviour of large catastrophic landslide triggered by heavy rainfall in Guanling, Guizhou, China

A large catastrophic landslide was triggered by a heavy rainfall on 28 June 2010 in Guanling, Guizhou, China. The landslide buried two villages and killed 99 people along the runout path. The landslide involved the failure of about 985 000 m3of sandstone from the source area, with a runout of about 1.4 km over a total vertical distance of about 420 m. To understand the possible long-runout mechanism and behaviour of the landslide, a detailed field survey of the landslide was conducted and samples were taken from the runout path. The shear behaviour of the sample based on a series of ring shear tests was examined, and numerical simulation of the landsliding behaviour by using a numerical runout model (DAN-W) was performed in which the shear strength obtained by ring shear tests was used. The experimental results reveal that the residual shear strength measured along the pre-existing shear surface is independent of the shear displacement rate under partially drained conditions, suggesting that the relationship between shear and normal stresses obeys the frictional model. A bulk basal friction angle of 14.4° at the base of the moving mass was then obtained from the test results. The simulated results show that the selected rheological model and parameters based on ring shear tests could provide the best performance in simulating the landslide. Therefore, it is expected that the model and parameters could improve the precision of hazard zonation for areas with geological, topographical, and climatic features similar to the Guanling landslide area.

Progressive shear-surface development in cohesive materials; implications for landslide behaviour

The aim of this study was to investigate mechanisms of progressive shear surface development using a series of specialised triaxial cell tests. Intact and remoulded samples of Gault Clay from the Ventnor Undercliff on the Isle of Wight in southern England were subjected to pore pressure reinflation (PPR) testing in a triaxial cell, in which failure is generated by increasing pore pressure under a constant total stress state. In addition, a novel very long term (>500days) creep test was undertaken, in which the sample eventually failed at a constant stress state below the failure envelope.The experiments showed that undisturbed samples of the Gault Clay failed in a brittle manner, generating a linear trend when plotted using the Saito technique. On the other hand, remoulded samples showed ductile behaviour, as indicated by a non-linear Saito trend. A number of otherwise identical PPR tests were conducted in which the rate of increase in pore water pressure was varied. These tests showed that strain rate generated at any point in the PPR tests depended on both the effective stress and the rate of change of effective stress. The latter is important because a change in stress generates a change in strain. Thus, whilst tests at different rates of change of effective stress are similar when plotted in q–p′ space and in strain–p′ space, they are markedly different in strain rate–p′ space.The long term creep test failed when the stress state had been constant for over 80days. This mechanism was reminiscent of creep rupture, occurring below the failure envelope defined in the conventional experiments.We conclude that first time failure in the Gault Clay is a progressive mechanism dominated by the development of micro-cracking, which leads to strain localisation and the development of one or more shear surfaces at failure. Whilst this mechanism may usually occur in response to a change in stress, the study indicates that failure can develop progressively. In the remoulded Gault Clay shear strains cannot localise along a singular shear surface.The results provide new insight into the mechanisms of landslide movement operating within the Ventnor landslide complex and indicate that present movements are likely to be occurring on a pre-existing shear surface. The lab tests suggest that this material is unlikely to undergo catastrophic failure.

Study of preexisting shear surfaces of reactivated landslides from a strength recovery perspective

Residual shear strength is generally considered in the design of preventive measures for slopes consisting of preexisting shear surfaces of large-scale landslides. Recent research suggests that the preexisting shear surface of a reactivated landslide can regain strength with the passage of time, which might also be considered in designing the slope stability measures. In this study, three reactivated landslide soils were tested in a ring shear apparatus for the discontinued shear periods of 1, 3, 7, 15, and 30days with the following main objectives: (i) to understand the strength recovery behavior of landslide soils in a residual state of shear after as long as 30days of discontinued shear, (ii) to understand the comparative pattern of strength recovery in highly plastic and less plastic soils, and (iii) to understand the mechanism involved in strength recovery at a residual state of shear. The results indicate that recovered strength measured in the laboratory is hardly noticeable after a rest period of 3days, but recovered strength is lost after a small shear displacement. This paper primarily focuses on the effect of strength recovery from residual strength on preexisting shear surface soils and the mechanisms behind it.

Strength Recovery from Residual-State of Shear on Soils

Residual shear strength is used for design and repairs on slopes containing pre-existing shear surface. Some recent research suggested that the pre-existing shear surface of landslides can gain strength with time. In this study, torsional ring shear strength recovery tests on different soils using rest periods of up to 30 days are performed at the effective normal stress of 100 kN/m2. Test results show that recovered strength measured in the laboratory is slightly noticeable after a rest period of 3 days, but recovered strength is lost after a very small shear displacement. This paper mainly focuses on the strength recovery phenomenon from the residual-state of shear on different soils based on torsional ring shear test results and the mechanisms behind it.

Residual Shear Strength Measured by Laboratory Tests and Mobilized in Landslides

Drained residual shear strength measured by multiple reversal direct shear or ring shear tests has been successfully used for over four decades for stability analyses of reactivated landslides in stiff clays and clay shales; A body of literature has accumulated in recent decades, claiming that “healing” or “strength regain” is realized in time on preexisting slip surfaces already at residual condition. In other words, the shear stress required to reactivate a landslide is claimed to be larger than the drained residual shear strength determined using laboratory tests. This article presents (1) a comparison of secant residual friction angle determined from laboratory tests and secant mobilized friction angle back-calculated for reactivated landslides; (2) explanations that field evidence used to claim “healing” can be attributed to alternative factors, and the laboratory evidence on “strength regain” upon reshearing is the result of either the testing apparatus or testing procedure, or is inapplicable to stiff clays and shales; and (3) laboratory aging test results, which show no “strength regain” on preexisting shear surfaces at residual condition.

Shear Strength in Preexisting Landslides

Drained residual shear strength is used for the analysis of slopes containing preexisting shear surfaces. Some recent research suggests that preexisting shear surfaces in prior landslides can gain strength with time. Torsional ring and direct shear tests performed during this study show that the recovered shear strength measured in the laboratory is only noticeably greater than the drained residual strength at effective normal stress of 100 kPa or less. The test results also show that the recovered strength even at effective normal stresses of 100 kPa or less is lost after a small shear displacement, i.e., slope movement. An effective normal stress of 100 kPa corresponds to a shallow depth so the observed strength gain has little, if any, impact on the analysis of deep landslides. This paper describes the laboratory strength recovery testing and the results for soils with different plasticities at various rest periods and effective normal stresses.

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.

A large landslide in Zigui County, Three Gorges area

On 13 July 2003, a landslide with a volume of approximately 20 × 106 m3 occurred on the left bank of the Qinggan River, Zigui County, Hubei Province, China. As a result, 14 people died and 10 people are missing. A landslide dam was formed, blocking the Qinggan River. A channelized diversion was constructed for prevention of upstream flooding and damage downstream caused by dam breaching. The landslide was a typical translational rockslide in weathered shale and sandstone, a block of which slipped down along the bedding plane. It is estimated that the landslide was caused by the combined effect of the following factors: (i) a bedding plane between incompetent weathered shale and competent sandstone that daylights at or is shallowly buried at the bottom of the Qinggan River; (ii) a thin layer of clayey soil acting as the slip plane that is likely a preexisting shear surface; (iii) excavation of the shale as raw material for brick fabricating on the slope; and (iv) a combination of prolonged rainfall and reservoir impounding.Key words: landslide, landslide dam, rainfall, impounding, Three Gorges area.

Slope Stability Analyses in Stiff Fissured Clays

Results of torsional ring shear, direct shear, and triaxial compression tests on cohesive soils reveal that the fully softened shear strength is stress-dependent and related to the type of clay mineral and quantity of clay-size particles. An empirical relationship for the fully softened friction angle is presented that is a function of liquid limit, clay-size fraction, and effective normal stress. Studies of first-time slides, i.e., slopes that have not undergone previous sliding, in stiff fissured clay with a liquid limit between 41 and 130% suggest that the mobilized shear strength along the failure surface can be lower than the fully softened shear strength. Recommendations are presented for estimating the mobilized shear strength in first-time slides based on soil plasticity. Soils with a liquid limit greater than 30% exhibit a large difference between the fully softened and residual friction angles. In these soils, the presence or absence of a pre-existing shear surface should be clarified.

Motorway slip report warns ‘go easy’ with clay embankments : Byrd, T New Civ Engr, N516, 11 Nov 1982, P18–20

The final report on the failure of a massive M25 embankment has not blamed a terram separation membrane, although the fabric possibly promoted a soft clay layer. Failure involved a 200 M long section of the batter slope breaking away with secondary rotation of the fill behind. It was concluded that the ground probably contained shear surfaces, that it was saturated with rain at the time of loading and the drainage blanket was not draining freely above it. A loss of strength due to softening of the clay immediately above the separation membrane occurred and possible segregation of the finer size clay particles during migration of moisture through the membrane made the problem worse. Tendency of the original ground to spread all contributed to the collapse. Very careful sampling and evaluation of the top-most layer of the foundation are needed to assess the influence of discontinuities and pre-existing shear surfaces. The effect of rainfall on the natural ground and the effectiveness of any drainage blanket need to be carefully assessed. Also a short break in construction of an embankment on saturated clay should allow local excess pore water pressures to dissipate through fissures and drainage blankets. (TRRL)

First-Time Slides in Over-Consolidated Clays

(a) After a slide has taken place the strength on the slip surface is equal to the residual value. The residual strength is associated with strong reorientation of the clay particles and is represented by an angle of shearing resistance $= which in most clays is considerably smaller than the value of 4’ at peak strength (Fig. 1). (b) First-time slides in slopes in non-fissured clays correspond to strengths only slightly less than the peak. (c) First-time slides in fissured clays correspond to strengths well below the peak. (d) Some form of progressive failure must be operative to take the clay past the peak. This could be simply the result of a non-uniform ratio of stress to strength along the potential slip surface; but probably the fissures play an important role as stress concentrators and in leading to softening of the clay mass. (e) Granted a progressive failure mechanism the limiting strength would be residual, and it is therefore reasonable to express the actual strength at the time of failure as a function of the upper (peak) and lower (residual) limits, i.e. by the residual factor. (f) In natural slopes of London CIay the strength has fallen approximateIy to the residual value. (g) The residual strength obtains on pre-existing shear surfaces, whether these are the result of tectonic shearing or old landslides.

A Shallow Slab Slide in the Lias Clay Near Uppingham, Rutland

Synopsis The landslide is shown to result from movement on an extensive pre-existing shear surface in a 9° Lias clay slope. The slide is of particular interest since the slope angle was close to the observed angle of ultimate stability against land-sliding for the Lias clay in the East Midlands. It follows that this slide provides a test case to check the supposition that, other factors being equal, the lowest angle of unstable slope occurs when the ground water table coincides with ground level. The slide occurred in about 1962; piezometers installed shortly after a further small movement in the winter of 1968–9 showed the phreatic surface to be virtually at the ground surface. Stability analyses indicate that the movement was highly consistent both with the observed pore water pressures and the shear strength parameters of the slip surface. On fait apparaître que le glissement de terrain résulte d'un déplacement se produisant sur une surface de cisaillement étendue existant au préalable sur une pente d'argile liasique de 9°. Le glissement présente un intérêt particulier du fait que l'angle de la pente était proche de l'angle de stabilité finale observé pour pr7évenir les glissements de terrain dans l'argile liasique dans la région de l'East Midlands. I1 en découle que ce glissement fournit un cas type pour vérifier la supposition selon laquelle, tous les autres facteurs étant égaux, l'angle minimum de pente instable se présente lorsque la nappe d'eau souter-raine coincide avec le niveau du sol. Le glissement se produisit vers 1962; des pièzomètres installé peu de temps après un autre petit déplacement au tours de l'hiver 1968–9 montrèrent que la surface phréatique se trouvait virtuellement à la surface du terrain. Des analyses de stabilité indiquent que le déplacement correspondait à la fois aux pressions interstitielles observées et aux paramètres de résistance à la rupture au cisaillement de la surface de glissement.