Slope Stability Back Analysis Research Articles

The role of a bentonitic layer on slope stability in bedded limestone: the case study of the December 2004 Ca' Madonna Quarry rock slide (Umbria-Marche Apennines, Central Italy)

This work describes and analyses the December 2004 rockslide that affected the working face of the Ca’ Madonna quarry (Municipality of Urbania, Province of Pesaro and Urbino, Italy), where limestones are extensively quarried for aggregates. The landslide occurred along an unsurveyed, approximately 10-cm thick clayey intra-bed layer that mainly consists of smectite-group clayey minerals. Laboratory analyses indicate that it is very active and shows a high capacity to absorb water until reaching a swelling pressure of up to 1200 kPa. Mining activities reduced lithostatic load and facilitated rainwater infiltration down to this bentonitic layer, which led to fully softened conditions with a resulting severe reduction in the shear strength parameters. Under such conditions, the slope stability back-analysis was performed by the limit equilibrium method, considering a planar rock slope failure with tension cracks and with different water level heights. The results show that a small increase of water pressure was sufficient to exceed the factor of safety. Stratigraphically, this layer can be correlated with the Lower Campanian Bentonitic Layer already reported in other sites of Central Italy. Therefore, the potentially great spatial extent of this layer and the possibility that the reported conditions may occur in other similar settings, make it important to identify its presence in order to prevent analogous situations.

Hazard assessment of rainfall-induced landslides: a case study of San Vicente volcano in central El Salvador

The San Vicente volcano in central El Salvador has a recurring and destructive pattern of landslides and debris flows occurring on the northern slopes of the volcano, and in recent memory, there have been at least seven major destructive debris flows. There has been no known attempt to study the inherent stability of these slopes and determine the factors that might lead to slope instability. Past events on the volcano were used to perform a 2D slope stability back analysis and to estimate the unknown model parameters. This analysis confirmed that the surface materials of the volcano are highly permeable and have very low shear strength. Additionally, the analysis provided insight into the groundwater table behavior during a rainstorm. Slope geometry, rainfall totals and initial groundwater table location were found to have the greatest effect on stability. A methodology is outlined for creating a stability chart to be used during rainfall events for monitoring slope stability. This chart could be used by local authorities in the event of a known extreme rainfall event to help make evacuation decisions. Finally, recommendations are given to improve the methodology for future application in other areas as well as in central El Salvador.

Lanslide hydrogeological susceptibility of Maierato (Vibo Valentia, Southern Italy)

A large landslide occurred at Maierato (Vibo Valencia District), Southern Italy, on 15 February 2010, when rapid failure was produced after several days of preliminary movements. The landslide can be classified as a rotational slide with flowing of the mass. It occurred within a larger deep-seated gravitational movement area and was preconditioned for failure by the intrinsic geological weakness of the area. Actually, the in situ survey showed the presence of several ancient movement surfaces all around the urban area. The landslide-triggering factor was the increase in the groundwater flow, consequent to a period of heavy and prolonged rainfall. The aim of the paper is to explain the mechanism of occurrence of the February 2010 composite landslide, in order to assess the landslide hydrogeological susceptibility in the whole urban area of Maierato. For this reason, the present paper deals with: (1) identification of the landslide mechanism and triggering factors (2) slope stability back-analysis using a finite-difference-based shear strength reduction method, and (3) 3D groundwater flow modelling extended to the whole urban area both in pre- and post-failure conditions. The results show that the February 2010 composite landslide was triggered by a water table increase of about 15 m uphill the landslide scarp. This hydrogeological condition could be reached after the period of heavy and prolonged rainfall preceding the landslide event. Finally, based on the groundwater modelling in post-failure condition, a landslide hydrogeological susceptibility map was drawn for the all urban area of Maierato, showing the presence of extended areas exposed to hazard.

An interdisciplinary investigation of a recent submarine mass transport deposit at the continental margin off Uruguay

Assessing frequency and extent of mass movement at continental margins is crucial to evaluate risks for offshore constructions and coastal areas. A multidisciplinary approach including geophysical, sedimentological, geotechnical, and geochemical methods was applied to investigate multistage mass transport deposits (MTDs) off Uruguay, on top of which no surficial hemipelagic drape was detected based on echosounder data. Nonsteady state pore water conditions are evidenced by a distinct gradient change in the sulfate (SO42−) profile at 2.8 m depth. A sharp sedimentological contact at 2.43 m coincides with an abrupt downward increase in shear strength from ∼10 to >20 kPa. This boundary is interpreted as a paleosurface (and top of an older MTD) that has recently been covered by a sediment package during a younger landslide event. This youngest MTD supposedly originated from an upslope position and carried its initial pore water signature downward. The kink in the SO42− profile ∼35 cm below the sedimentological and geotechnical contact indicates that bioirrigation affected the paleosurface before deposition of the youngest MTD. Based on modeling of the diffusive re-equilibration of SO42− the age of the most recent MTD is estimated to be <30 years. The mass movement was possibly related to an earthquake in 1988 (∼70 km southwest of the core location). Probabilistic slope stability back analysis of general landslide structures in the study area reveals that slope failure initiation requires additional ground accelerations. Therefore, we consider the earthquake as a reasonable trigger if additional weakening processes (e.g., erosion by previous retrogressive failure events or excess pore pressures) preconditioned the slope for failure. Our study reveals the necessity of multidisciplinary approaches to accurately recognize and date recent slope failures in complex settings such as the investigated area.

Constraining pre-instrumental earthquake parameters from slope stability back-analysis: Palaeoseismic reconstruction of the Güevéjar landslide during the 1st November 1755 Lisbon and 25th December 1884 Arenas del Rey earthquakes

Slope stability back-analysis performed for landslides known to have been triggered by an earthquake can provide additional constraints on the size and location of pre-instrumental seismic events. This paper reconstructs the pre-earthquake conditions of a major landslide located in the Granada Basin –the Güevéjar landslide, which was triggered twice by the 1755 Lisbon and 1884 Arenas del Rey earthquakes. For each case the minimum seismic acceleration needed to trigger the instability has been calculated, and from this datum the most likely magnitude and epicentral location have been inferred for each event. The results suggest that the moment magnitude of the 1755 Lisbon earthquake was at least 8.5 and that it was located as far as 580 km from the landslide. For the 1884 Arenas del Rey earthquake, the moment magnitude was at least 6.5 and it was located within 55 km of Güevéjar. These results indicate the Ventas de Zafarraya Fault as the seismogenic source of the event. Apart from these two major events, other significant historical earthquakes which took place in the Granada Basin are also analysed in the paper. It is confirmed that none of these was able to reactivate the landslide, not even the 1806 Pinos Puente earthquake. The Güevéjar landslide is stable under present-day conditions, but it could be reactivated by an earthquake as small as M w = 4.7 directly at the site.

The January 10, 1997 Pozzano landslide, Sorrento Peninsula, Italy

From 1997 to 1999, a huge number of slides, often turning into extremely rapid debris-earth flows, repeatedly affected the late Quaternary volcaniclastic deposits mantling the carbonate slopes of Campania region, Italy. The Sorrento Peninsula was the epicentral district of the 1997 regional slope-instability crisis. Some hundred shallow mass movements took place during January 1997 in this area. These were the last episode of a long series of slope failure events dating back to mid-18th century. Results from geological and geomorphologic surveys are presented. Landslide mechanism and triggering factors are analysed for the most important mass movement, which occurred during the January 9–11, 1997, regional event. On January 10, 1997, at about 8:15 PM, a rainfall-induced debris slide-debris flow occurred at Pozzano (province of Naples), mainly affecting the 79 AD pyroclastic products. Following a J-path, the landslide destroyed a private house and invaded the State Road no. 145. This event resulted in four deaths, 22 persons injured and road closure for about 2 months. There was less than 200 mm of rainfall in the 72-h period prior to the landslide, although intense precipitation had occurred during a preceding 4-month period. However, the slope failure event was not preceded by an extreme short-term antecedent rainfall, as already noticed in previous landslides of this type in Campania. Finally, following a preliminary geotechnical characterization of volcaniclastic soils, a slope-stability back analysis was carried out, which adopted the classical infinite slope scheme. This analysis gave further evidence of the role played by pore pressure in reducing the overall shear strength of pyroclastic soils.

Back Analysis Technique for Slope Stabilization Works of Embankment Landslide Due to Foundation Instability

ABSTRACT This paper deals with the overall stability of an embankment foundation failure that lies at km 40 + 700 of the new Irbid-Amman Highway in Jordan. Slope stability back analysis was carried out for the slope to assess the conditions at time of failure, and estimate most representative shear strength parameters of foundation materials. Slope stability analysis was also carried out for proposed remedies. Probabilistic seismic hazard analysis was carried out for the landslide site. Peak Ground Acceleration (PGA) value of 0.2 g was estimated for design. This corresponds to a 90% probability of non-exceedence in a 50 year design life of the highway. Pseudo-static slope stability analysis was also carried out. The study concluded that the landslide movement occurred within the foundation colluvium material. It resulted primarily from the excessive load of the embankment and excess piezometric pressures generated within the slope. The most feasible remedial measure to stabilize the landslide area was removal of existing failed embankment down to the top of sandstone layer, and reconstruction (using imported free-drainage rockfill) of a split level embankment together with the construction of surface and subsurface drainage system. These measures were successfully implemented in the field.

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.