Unstable Soil Mass Research Articles

A practical approach for predicting landslide retrogression and run-out distances in sensitive clays

In the present study a novel approach of practical interest is proposed to perform a predictive analysis of the landslides in sensitive clays. Specifically, this approach allows a prediction of the landslide triggering along with the consequent distances of retrogression and run-out. To this end, a simple limit equilibrium solution is proposed to deal with the first two steps (prediction of the landslide triggering and the subsequent retrogression distance). A calculation code based on the material point method (MPM) is instead employed for evaluating the run-out distance of the unstable soil mass that was defined in the previous step. Operative values of the undrained shear strength are also suggested for each of the aforementioned steps. The proposed method is very attractive from a practical viewpoint because it is simple to use, not time-consuming and requires few geotechnical parameters of easy experimental determination as input data. The method is applied successfully to several well-documented case studies available in the literature, demonstrating also its reliability for engineering applications. Three of these landslides are analysed in more detail to illustrate the application of the present method to real cases.

Estimation of Shallow Landslide Susceptibility Incorporating the Impacts of Vegetation on Slope Stability

This study aimed to develop a physical-based approach for predicting the spatial likelihood of shallow landslides at the regional scale in a transition zone with extreme topography. Shallow landslide susceptibility study in an area with diverse vegetation types as well as distinctive geographic factors (such as steep terrain, fractured rocks, and joints) that dominate the occurrence of shallow landslides is challenging. This article presents a novel methodology for comprehensively assessing shallow landslide susceptibility, taking into account both the positive and negative impacts of plants. This includes considering the positive effects of vegetation canopy interception and plant root reinforcement, as well as the negative effects of plant gravity loading and preferential flow of root systems. This approach was applied to simulate the regional-scale shallow landslide susceptibility in the Dadu River Basin, a transition zone with rapidly changing terrain, uplifting from the Sichuan Plain to the Qinghai–Tibet Plateau. The research findings suggest that: (1) The proposed methodology is effective and capable of assessing shallow landslide susceptibility in the study area; (2) the proposed model performs better than the traditional pseudo-static analysis method (TPSA) model, with 9.93% higher accuracy and 5.59% higher area under the curve; and (3) when the ratio of vegetation weight loads to unstable soil mass weight is high, an increase in vegetation biomass tends to be advantageous for slope stability. The study also mapped the spatial distribution of shallow landslide susceptibility in the study area, which can be used in disaster prevention, mitigation, and risk management.

Liquefaction-induced flow-like landslides: the case of Valarties (Spain)

This paper examines a flowslide involving a glacial deposit of low-plasticity silty sand triggered by a karstic spring after a rainfall period. The work aims at explaining the triggering, propagation and kinematics of flow-like landslides in a unique framework. In particular, a material point method open-source code, able to solve coupled hydro-mechanical problems for saturated/unsaturated soils, was developed. Laboratory and field experiments revealed a liquefaction potential of the mobilised material. To simulate such potential, a recent liquefaction model (Ta-Ger), validated so far at a laboratory scale, was selected, extended to unsaturated conditions, implemented and calibrated. The analysis indicates a complex behaviour of the moving mass and explains the mechanisms developing sequentially in the flowslide. The impact of the upper unstable soil mass against the soil at lower elevations is a key phenomenon to generalise soil liquefaction in the entire slope. Patterns of soil velocity and displacements are far from being a uniform flow of liquefied material. The model developed is a powerful tool to interpret flowslides involving a saturated and unsaturated soil profile. The paper includes sensitivity analyses and discusses the discrepancies observed in the run-up of the flowslide climbing on the opposite slope of the valley.

Platform for integrated monitoring data management applied to an instrumented slope in southern Brazil

The monitoring of slopes and geotechnical structures is fundamental for guaranteeing safety and improving the knowledge of soil behavior. This study evaluated the use of a platform for integrated monitoring data management applied to an instrumented slope. The slope, known as “Morro do Boi”, is located in BR-101 highway, in Santa Catarina state, Brazil. The instrumentation was installed in 2012 during the execution of a stabilization system designed to contain an unstable soil mass, and allows the verification of positive and negative pore pressure, deformations of the soil mass and the precipitation. In addition, the structure, composed by passive anchors and flexible metal mesh, is monitored with crackmeters, strain gages and load cells. Until May 2018, the historical data comprising 6 years of monitoring was automatically registered by a datalogger hourly, with manual download. On June 2018, the remote dispatch of the data was implemented and the management of this information is done through a web-based platform. This tool has been extremely useful for the ease of visualization, data management and logistical efficiency, allowing the creation of a rapid alert system in cases of variation of some parameters that could trigger landslide.

A simple method to reduce mesh dependency in modelling landslides involving brittle soils

This paper focuses on the numerical simulation of the deformation processes occurring in the slopes when soils with strain-softening behaviour are involved. In these circumstances, a progressive failure may occur with the consequent motion of the unstable soil mass in the post-failure stage. This problem can be only solved using advanced numerical techniques capable also of accounting for the occurrence of large deformations. However, the solution is generally mesh dependent and may be affected by lack of convergence. In this study, the material point method is employed to simulate the occurrence of large deformations, in conjunction with a strain-softening Mohr–Coulomb constitutive model, in which the shear strength parameters are reduced as a function of the accumulated deviatoric plastic strain, and a model parameter controlling the rate of strength decrease. To evaluate this parameter and to reduce the effects of the mesh dependency on the numerical solution, a novel procedure based on the results of direct shear tests is presented. This methodology is completely analytical and requires few parameters with a clear physical meaning and of simple experimental determination. Some simulations are performed to assess the reliability of the proposed procedure.

Analyzing the effectiveness of anti-slide piles for slope stabilization against seismic loading using discrete element method

One of the widely known mitigation measures against slope failure and landslides is pile reinforcement. These vertical piles reduce the slope deformation by providing support for the unstable soil mass behind them and also through the arching effect. In this study, two slopes were created using discrete element method (DEM). The slopes had similar geometries and properties, except that a reinforcement pile was installed in one of them to investigate its effectiveness. The soil grains were modelled using spherical DEM particles and the pile was composed of particles connected to each other by parallel bonds with a proper stiffness to produce the desired macroscale properties. A seismic base excitation was applied to the base of the models and different quantities were recorded during shaking. According to the results, the lateral displacement and ground settlement significantly reduced after the installation of the reinforcement pile. It was also observed that the lateral earth pressure, shear force and bending moment over the pile length substantially increased by the end of the simulation. The observations presented in this paper are qualitatively consistent with the results of the previously published studies.

Introducing expandable mechanical plate anchors for onshore and offshore anchoring

Soil anchors are among the most widely used reinforcements to stabilize unstable soil mass or stabilize and anchor structures built in onshore and offshore regions. Anchors are generally installed in soil and rocky environments by penetration and embedment methods. These methods are classified into two main groups of grouted and mechanical anchors. In the present study, Expandable Mechanical Plate Anchors (EMPLAs), a new generation of mechanical anchors, are introduced and their bearing capacities are evaluated. EMPLAs can be implemented, locked, and activated in the soil more cheaply and with a lower driving force. Previous studies have validated the efficiency of these anchorages based on the experimental test results. Here, it is tried to evaluate the effect of properties of anchor plates such as their shape and curvature on the locking mechanism, as well as the ultimate bearing capacity of reinforcement and drop rate of locking force under a constant pullout speed rate. Furthermore, the ultimate bearing capacity of the anchors is calculated using two methods of 1) the well-known theories for calculating the tensile bearing capacity of plate anchors in cohesionless soil and 2) numerical modeling using finite element software. The results from both methods showed that the plate curvature intensity had a great impact on the plates opening rate and, consequently, faster locking and activation of the reinforcement at the higher embedment depth. Eventually, the effective zone of the soil at the top of the expandable plates during a 90° rotation of the plates and lockage of the reinforcement was monitored using the Particle Image Velocimetry (PIV) technique.

Seismic stability of the excavation fronts in the ancient Roman city of Pompeii

The paper deals with the problem of the seismic stability of excavation fronts in the ancient Roman city of Pompeii, a famous UNESCO World heritage site near Napoli, Italy, buried under 4–6 m of volcanic ashes and pumices during the eruption of Mount Vesuvius in 79 AD. The attention is focused on the Insula dei Casti Amanti, a block of masonry buildings in the city centre partly excavated in recent years. Ground motion recorded on outcropping rock compatible with the seismo-tectonic background and the spectrum specified by the Italian code for the life-safety limit state criteria were used for wave propagation analysis. Both 1-D and 2-D Seismic Response Analyses were carried out to identify both surface (topographic) and valley (morphology) effects. This study allowed to quantify in a detailed manner the seismic demand in the excavation fronts. Re-profiling interventions based on the classical pseudo-static approach would be too invasive for the archaeological site. On the contrary, the alternative Newmark-type approach, based on the evaluation of the permanent rotations of the potentially unstable soil mass, is much more appropriate for a sustainable definition of slope stabilising interventions.

1Laboratory experimental research of loading friction forces acting on the lateral surface of tapered piles in structurally unstable soils

In modern urban conditions, needs arise for the development of sites, which are geotechnically complex and disadvantageous for construction. The peculiarities of such sites, among other things, include the presence of structurally unstable soils that exhibit subsidence deformations when wet or under dynamic and vibration loads due to a decrease in the strength of structural bonds. In this regard, the development and implementation of rational design solutions are of current interest when designing and constructing foundations in the presence of structurally unstable soils. One such solution is the use of bored piles. When designing piles for structurally unstable soils, the action of loading friction forces should be taken into account; these forces can arise if the subsidence rate of the soil mass around the pile exceeds the subsidence rate of the pile itself. The study of the interaction of a structurally unstable soil mass with piles and the corresponding theoretical justification are necessary to clarify the methods for evaluating loading friction forces. In an experimental study, the effect of reduction of loading friction forces depending on the taper rate of the pile was discovered, and the results of laboratory measurements of loading forces were analyzed using various schemes for modeling subsidence deformations.

A performance-based method for the design of drainage trenches used to stabilize slopes

Drainage trenches are widely used as a measure for slope stabilization. Their effect is generally quantified in terms of a safety factor against a possible collapse mechanism of the slope. A more rational and economical design approach consists in admitting the occurrence in some time intervals of limit conditions in the slope, and evaluating the performance of the slope in terms of accumulated permanent displacements of the unstable soil mass. The resulting displacements should avoid the occurrence of significant damage and preserve the serviceability of existing structures. In this context, a new method is proposed in the present paper for the design of drainage trenches used to control the mobility of translational landslides periodically activated by rainfall. This method is based on some analytical equations that relate rain to the groundwater regime as modified by the trenches, and a simple sliding block model to evaluate the landslide displacements. The proposed method also accounts for the delay of the hydraulic response at different depths. The above-mentioned equations are validated by several comparisons with numerical results obtained using a finite element code or published in the literature. Finally, the method is applied to some case studies to show the effectiveness of the drainage trenches to control the landslide mobility owing to rainfall.

The Evolution of Launched Soil Nails: A 30-Year Retrospective

Launched soil nails are a unique remedial technology in the geotechnical construction toolbox. These 20-ft-long, 1.5-in.-diameter nails are installed in a single shot using a compressed air "cannon" at velocities of up to 250 miles/hour, and at rates approaching 250 nails/day. The nails reinforce an unstable or potentially unstable soil mass by transferring the nail’s tensile and shear capacity into the sliding soil. However, at least as interesting as the tool itself, is the story behind the development of launched soil nail technology over the past 30 years.

Analytical Prediction for Piles Used for Slope Stabilization

Many unstable slopes are extensively supported by stabilizing piles. Excavation work, rainfall, and earthquakes can cause lateral soil movement, creating forces that act on the stabilizing piles. We present a new simplified method based on the Winkler elastic model for analyzing the response of flexible piles subjected to lateral soil movement in landslides. The new method can analyze cases in which the movement depth profile of the sliding layer is arbitrary by using a two-stage analysis method. First, the free-field soil movement data of the unstable soil mass are fitted by a polynomial spline using the leastsquares method. Then a system of differential equations for flexible piles subjected to lateral soil movement is presented to describe the behavior of the piles embedded in the slope. The formulation considers the soil-pile interaction to derive the analytical solution via a matrix pattern. Finally, the proposed method is verified by comparing the calculated results with those of three published case studies. The results show a good correlation between the calculated and measured responses of flexible stabilizing piles, verifying the reliability of the proposed analytical solution.

A simplified method for predicting rainfall-induced mobility of active landslides

This paper deals with the landslides that are reactivated by a groundwater level increase owing to rainfall. These landslides are usually characterized by low displacement rate with deformations essentially concentrated within a narrow shear zone above which the unstable soil mass moves like a rigid body (i.e., with a horizontal displacement profile that is essentially constant with depth). In view of this evidence, a new method based on a simple sliding block model is proposed in the present study for a preliminary evaluation of landslide mobility. Unlike other existing methods that provide an evaluation of landslide mobility on the basis of groundwater level measurements, the present method directly relates landslide movements to rain recordings. This possibility constitutes a significant advantage from a practical viewpoint because it allows future displacement scenarios to be predicted from expected rainfall scenarios. In addition, the present method requires a limited number of parameters as input data, many of which can be obtained from conventional geotechnical tests. To evaluate the other parameters involved, an efficient calibration procedure is also proposed. Four case studies documented in the literature are analyzed to assess the capability of the present method to reproduce the main features of the slope response to rainfall. In all these case studies, both groundwater level variations and landslide displacements observed in field are well approximated by the method.

A Simple Approach for Evaluating Slope Movements Induced by Groundwater Variations

Landslides reactivated by groundwater variations are usually characterized by low displacement rate with deformations essentially concentrated within a narrow shear zone above which the unstable soil mass moves like a rigid body. A simplified method was recently proposed by Conte and Troncone [1] for a preliminary evaluation of the mobility of these landslides. In the present study, this method is extended by incorporating a calibration procedure for a direct evaluation of some parameters which are difficult to obtain experimentally. Some case studies are analysed to validate the present approach.

Prediction of landslide run-out distance based on slope stability analysis and center of mass approach

Mitigation of landslide hazard requires the knowledge of landslide run-out distance. This paper presents the application of slope stability analysis and center of mass approach to predict the run-out distance of a rotational landslide model with different soil types. The Morgenstern-Price method was used to estimate the potential sliding zone and volume of landslide material. The center of mass approach used a simple Coulomb friction model to determine the run-out distance. Results of the slope stability analysis showed that the soil unit weight can influence the depth of sliding zone, and the volume of unstable material. The slope model of silty sand and gravel would have the largest volume of unstable mass. From the Coulomb friction analysis, this slope model has higher run-out distance and velocity than other slope models. Thus, the run-out distance will be influenced by soil type and the dimension of unstable soil mass.

A finite element approach for the analysis of active slow-moving landslides

In the present paper, a finite element approach is proposed to analyse the mobility of active landslides which are controlled by groundwater fluctuations within the slope. These landslides are usually characterised by low displacement rates with deformations that are essentially concentrated within a narrow shear zone above which the unstable soil mass moves with deformations of no great concern. The proposed approach utilises an elasto-viscoplastic constitutive model in conjunction with a Mohr-Coulomb yield function to describe the behaviour of the soil in the shear zone. For the other soils involved by the landslide, an elastic model is used for the sake of simplicity. A significant advantage of the present method lies in the fact that few constitutive parameters are required as input data, the most of which can be readily obtained by conventional geotechnical tests. The rest of the required parameters should be calibrated on the basis of the available monitoring data concerning the change in the piezometric levels and the associated movements of the unstable soil mass. After being calibrated and validated, the proposed approach can be used to predict future landslide movements owing to expected groundwater fluctuations or to assess the effectiveness of drainage systems which are designed to control the landslide mobility. The method is applied to back-predict the observed field behaviour of three active slow-moving landslides documented in the literature.

Analytical Method for Predicting the Mobility of Slow-Moving Landslides owing to Groundwater Fluctuations

This paper deals with the active landslides that are controlled by pore water pressure changes owing to groundwater fluctuations. These landslides are usually characterized by low displacement rate with deformations essentially concentrated within a narrow shear zone, above which the unstable soil mass moves as a rigid body. Taking advantage of some original analytical solutions, a method is developed for a preliminary prediction of the landslide mobility. This method is based on a simple sliding-block model and allows the landslide velocity to be readily evaluated once pore water pressure measurements are available. An application to a case study documented in the literature is also shown.

Estimation of Residual Undrained Shear Strength from Gravity Corer Penetration Data

Residual undrained shear strength, s u_res , is an important parameter for analyzing the response of structures buried within potentially unstable soil mass. A framework for estimating s u_res of fine grained sediments from gravity corer penetration has been developed considering viscous drag during free fall of the corer through seawater and cohesive energy loss during sediment penetration. The procedure was used to estimate s u_res using data from a submarine geotechnical investigation in western Canada. Comparison of the results with alternative estimates of s u_res from miniature torvane tests on the gravity core samples and CPTs performed nearby reveals a reasonable agreement.