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A method directly using rainfall records to predict a slope's potential instability is devised. The method consists of three sequential steps: identifying the critical suction stress (pore water pressure when soil is saturated) profile of a given slope, developing a rainfall intensity–duration threshold curve for the slope, and using rainfall record to determine if the threshold is reached (failure occurs) or not (no failure). It innovatively uses a slope's strength parameters and slope angle to develop the critical suction stress (tensile) or compressive pore water pressure profile where, at each depth within the slope, the effective stress reaches the failure state. Hydromechanical numerical modeling is then conducted under various rainfall intensities to identify their corresponding duration for slope failure, thus, the rainfall intensity–duration threshold curve of the slope. Two previously well-documented and studied rainfall-induced slope instability cases; one near the town of Edmonds, Washington State, and the other near the village of Rüdlingen in northern Switzerland are used to validate the method. Excellent predictions of the slope failure depth and timing are demonstrated, indicating the effectiveness of the proposed method. Because the suction stress–based rainfall intensity–duration curve is characteristic of a given slope and it can be determined a priori, the method provides a practical way to conduct real-time rainfall monitoring and predict instability for a specific slope, and a pathway to forecast instability of natural slopes in a region. [ABSTRACT FROM AUTHOR]

In general, the value of the parameter HT ht in Eq. (9) ranges from 1.0 to 1.2, as suggested by the discussers. Substituting Eq. (7) into Eq. (1) of the discussion, the extended critical state line equation can be expressed as HT ht (8) where HT ht = critical state parameter; HT ht = deviator stress (Pa); and HT ht = undrained shear strength (Pa). [Extracted from the article]

Infiltration-induced landslides are common in mountainous and hilly areas of the world. When they occur near transportation corridors, they can impact public safety, impede transport of goods and people, and damage transportation infrastructure. This work presents a study of the hydrological behavior and its effects on the stability of an active landslide located on an embankment along Interstate-70 west of the Eisenhower Tunnel in central Colorado, USA. Groundwater dynamics were monitored for three years; two piezometers were installed near the head of the slide and one piezometer was placed near the toe. The hydrological observations at this site are unusual in that water table positions beneath the westbound shoulder of the highway (upslope) varied twice as much as water table positions beneath the eastbound shoulder (downslope), only 30 m distant horizontally. To better understand the factors controlling these observed differences, observations of the stratigraphy and the geomorphology of the watershed beyond the landslide body were incorporated into a conceptual model tested using numerical simulations of two-dimensional, variably saturated groundwater flow. Results from the numerical simulations calibrated against field measurements and a seasonally varying stability analysis of the site show that the large observed differences in the water table positions over the short horizontal distance are likely due to a combination of (1) the large size of the watershed that allows a significant amount of infiltration of snowmelt into the hillslope, (2) the contrast of hydrological properties of soils in the watershed, and (3) the changes in steepness of the dip of the bedrock below the slide. These three factors control the direction, speed, and amount of groundwater flow traveling through the slope. It is also shown that the seasonal hydrology of the site is a key factor in the stability of the slope, where most of the observed displacement occurs during the early summer season. Variations in the water table level within a year resulting from low snow years compared to variations from high snow years can be as much as 100%. Finally, it is important to consider the large contributing area of the watershed when evaluating the hillslope hydrologic conditions and remediation options. [ABSTRACT FROM AUTHOR]

To experimentally validate a recently developed theory for predicting the stability of cut slopes under unsaturated conditions, the authors measured increasing strain localization in unsaturated slope cuts prior to abrupt failure. Cut slope width and moisture content were controlled and varied in a laboratory, and a sliding door that extended the height of the free face of the slope was lowered until the cut slope failed. A particle image velocimetry tool was used to quantify soil displacement in the x-y (horizontal) and x-z (vertical) planes, and strain was calculated from the displacement. Areas of maximum strain localization prior to failure were shown to coincide with the location of the eventual failure plane. Experimental failure heights agreed with the recently developed stability theory for unsaturated cut slopes (within 14.3% relative error) for a range of saturation and cut slope widths. A theoretical threshold for sidewall influence on cut slope failures was also proposed to quantify the relationship between normalized sidewall width and critical height. The proposed relationship was consistent with the cut slope experiment results, and is intended for consideration in future geotechnical experiment design. The experimental data of evolution of strain localization presented herein provide a physical basis from which future numerical models of strain localization can be validated. [ABSTRACT FROM AUTHOR]

The unified effective stress equation for soil under both saturated and unsaturated conditions is ([5]) HT ht (1) where HT ht soil's suction stress characteristic curve (SSCC) being a function of water content HT ht . The liquid limit (LL) is a soil property index in the Unified Soil Classification System, originally devised by Swedish scientist Albert Atterberg in 1911 and later refined by Casagrande's percussion cup test. Under the unified effective stress Eq. (1), the LL is the undrained shear strength equal to the prevailing suction stress of HT ht when a soil is fully saturated. [Extracted from the article]

This paper extends Culmann's vertical-cut analysis to unsaturated soils. To test the extended theory, unsaturated sand was compacted to a uniform porosity and moisture content in a laboratory apparatus. A sliding door that extended the height of the free face of the slope was lowered until the vertical cut failed. Digital images of the slope cross section and upper surface were acquired concurrently. A recently developed particle image velocimetry (PIV) tool was used to quantify soil displacement. The PIV analysis showed strain localization at varying distances from the sliding door prior to failure. The areas of localized strain were coincident with the location of the slope crest after failure. Shear-strength and soil-water-characteristic parameters of the sand were independently tested for use in extended analyses of the vertical-cut stability and of the failure plane angle. Experimental failure heights were within 22.3% of the heights predicted using the extended theory. [ABSTRACT FROM AUTHOR]

Slope-stability analyses are mostly conducted by identifying or assuming a potential failure surface and assessing the factor of safety (FS) of that surface. This approach of assigning a single FS to a potentially unstable slope provides little insight on where the failure initiates or the ultimate geometry and location of a landslide rupture surface. We describe a method to quantify a scalar field of FS based on the concept of the Coulomb stress and the shift in the state of stress toward failure that results from rainfall infiltration. The FS at each point within a hillslope is called the local factor of safety (LFS) and is defined as the ratio of the Coulomb stress at the current state of stress to the Coulomb stress of the potential failure state under the Mohr-Coulomb criterion. Comparative assessment with limit-equilibrium and hybrid finite element limit-equilibrium methods show that the proposed LFS is consistent with these approaches and yields additional insight into the geometry and location of the potential failure surface and how instability may initiate and evolve with changes in pore water conditions. Quantitative assessments applying the new LFS field method to slopes under infiltration conditions demonstrate that the LFS has the potential to overcome several major limitations in the classical FS methodologies such as the shape of the failure surface and the inherent underestimation of slope instability. Comparison with infinite-slope methods, including a recent extension to variably saturated conditions, shows further enhancement in assessing shallow landslide occurrence using the LFS methodology. Although we use only a linear elastic solution for the state of stress with no post-failure analysis that require more sophisticated elastoplastic or other theories, the LFS provides a new means to quantify the potential instability zones in hillslopes under variably saturated conditions using stress-field based methods. [ABSTRACT FROM AUTHOR]