Retrogressive Evolution Research Articles

Airborne combined photogrammetry—infrared thermography applied to landslide remote monitoring

The combination between two remote surveying methods is presented herein as a pioneering approach for landslide airborne monitoring. The survey of an active landslide by UAV-based RGB photogrammetry and infrared thermography, sided by the knowledge of the field condition, allowed increasing the scientific experience on the remote sensing of slope instability phenomena by analyzing multiple aspects related to the evolution of key slope features. In this research, the state and distribution of activity of a landslide was monitored by matching data arising from three-dimensional models of the slope, built by exploiting the aerial RGB photogrammetric technology, and thermal outcomes, resulting from the airborne application of infrared thermography principles. In this frame, thermal anomalies detected during different monitoring campaigns allowed recognizing peculiar features along the unstable slope that could be related to specific kinematic elements involved in the landslide activity. Forming cracks, developing scarps, wet terrain portions, and loose material are some of the elements that could be located by integrating thermal outcomes with Digital Surface Models of the slope. Thanks to the different thermal behavior of such elements, strengthened herein by a novel approach of thermal data processing (i.e. the study of thermal slope profiles), the lateral and retrogressive evolution of the studied movement was first hypothesized and then verified in field. Achieved results show that the location of thermal anomalies well corresponds to field structures, which sometimes are hardly detectable by in situ or RGB surveys, thus suggesting the high potential of the methodological approach developed for this study. The scientific validity of presented data gains relevance thanks to the positive field validation. This paves the way to further studies aimed at implementing the infrared aerial survey of landslides, which surely could bring benefits to practical applications in terms of survey speed and spatial coverage, especially in areas characterized by bad field logistics.

Simulating retrogressive slope failure using two different smoothed particle finite element methods: A comparative study

Various smoothed particle finite element methods (SPFEMs) have been developed to simulate large deformation problems, but their efficiency and accuracy in simulating progressive landslides in sensitive clays have remained unclear. In this study, a series of numerical analyses are carried out to investigate the development of retrogressive landslides by two SPFEMs (an edge-based strain smoothing PFEM, ESPFEM, and a node-based strain smoothing PFEM, NSPFEM) in view of their outstanding performance in large deformation analysis. A strain-softening Mohr–Coulomb (MC) model is adopted to simulate the behaviour of sensitive clays during landslide, assuming a Poisson's ratio of 0.49 to ensure undrained conditions. The influence of mesh density on the development of retrogressive failure is evaluated for two SPFEMs. Numerical analyses of three mesh sizes (0.2 m, 0.15 m and 0.12 m) are carried out sequentially, with all results demonstrating that (1) the spread retrogressive landslides with horsts and grabens can be achieved by both SPFEMs with the adopted soil model, (2) run-out and retrogression distances decrease as mesh density increases for both methods, (3) retrogressive collapse occurs earlier for ESPFEM but is delayed for NSPFEM with increased mesh density, (4) NSPFEM allows faster calculations and reduces mesh dependency problems when compared with ESPFEM and (5) the increase of shape factor can accelerate retrogressive evolution of landslides. Finally, a real landslide in sensitive clay at Sainte-Monique, Quebec, is simulated to demonstrate ESPFEM's computational efficiency and accuracy.

Discontinuity Characterization of Rock Masses through Terrestrial Laser Scanner and Unmanned Aerial Vehicle Techniques Aimed at Slope Stability Assessment

Stabilization projects of rock masses cannot be performed without a proper geomechanical characterization. The classical approaches, due to logistic issues, typically are not able to cover extensively the areas under study. Geo-structural analysis on point cloud from terrestrial laser scanning and photogrammetry from unmanned aerial vehicles are valid tools for analysis of discontinuity systems. Such methodologies provide reliable data even in complex environmental settings (active cliffs) or at inaccessible sites (excavation fronts in tunnels), offering advantages in terms of both safety of the operators and economic and time issues. We present the implementation of these techniques at a tuff cliff over the Santa Caterina beach (Campania) and at the main entrance of Castellana Caves (Apulia). In the first case study, we also perform an integration of the two techniques. Both sites are of significant tourist and economic value, and present instability conditions common to wide areas of southern Italy: namely, retrogressive evolution of active cliffs along the coast, and instability at the rims of natural and/or artificial sinkholes. The results show the reliability of the data obtained through semi-automatic methods to extract the discontinuity sets from the point clouds, and their agreement with data collected in the field through classical approaches. Advantages and drawbacks of the techniques are illustrated and discussed.

Long-term coupled effects of hydrological factors on kinematic responses of a reactivated landslide in the Three Gorges Reservoir

A reservoir-induced landslide is a common geomorphological problem in the TGR (Three Gorges Reservoir) region, China. This study considers the Quchi landslide as an example of a large-scale partially reactivated ancient landslide in the TGR region; the possible kinematic responses and corresponding triggering mechanisms are investigated by analyzing eight-year monitoring data sets, field surveys, observations, and laboratory tests. The reactivated zone is primarily located in the south-western part of the ancient landslide, and it mainly moves along the deep sliding surface. The long-term monitoring data sets show that the presence of faster movements from June to September (rainy season), followed by slower deformation in the remaining nine months of the hydrological year. Both strong rainfall and drawdown during the low period of RWL (reservoir water level) are important hydrological factors responsible for the occurrence of faster movements. An RWL of 160 m a.s.l. (above sea level) during drawdown is considered the key water level for the initiation of faster movements. The motion pattern is primarily retrogressive, and the displacement velocity increases towards the toe. At present, the stability of the reactivated zone is indeterminate; a multistage failure with significant retrogressive evolution can possibly occur in the future. The findings in this study can help in managing reservoir landsides and designing landslide reinforcements in the regions of the word with geological settings similar to that of the TGR region.

Morphological Variability of Submarine Mass Movements in the Tectonically–Controlled Calabro–Tyrrhenian Continental Margin (Southern Italy)

The analysis of high resolution morpho–bathymetric data on the Calabro Tyrrhenian continental margin (Southern Italy) enabled us to identify several morphological features originated by mass–wasting processes, including shallow gullies, shelf–indenting canyons and landslides. Specifically, we focus our attention on submarine landslides occurring from the coast down to −1700 m and affecting variable areas from thousands of square meters up to few tens of square kilometers. These landslides also show a large variability of geomorphic features which seems strictly related to the physiographic/morphological domains where the landslide formed. Tectonically–controlled scarps and canyon flanks are typically characterized by several coalescent and nested landslides, with diameters ranging from hundreds to a few thousands of meters. Canyon headwalls are commonly characterized by a cauliflower shape due to an array of small (diameters of tens of meters) and coalescent scars. In all these sectors, disintegrative–like landslides dominate and are generally characterized by a marked retrogressive evolution, as demonstrated by their morphology and comparison of repeated bathymetric surveys at the canyon headwall. Only in the lower part of tectonically–controlled scarps, a few cohesive–like and isolated landslides are present, indicating the main role of slope gradients and height drop in controlling the post–failure behavior of the mobilized material. Open slopes are generally characterized by large–scale (diameters of thousands of meters) and isolated scars, with associated landslide deposits. A peculiar case is represented by the Capo Vaticano Scar Complex that affected an area of about 18 km2 and is characterized by an impressive variability of landslide morphologies, varying also at short distance. The large extent and variability of such scar complex are thought to be associated with the occurrence of a mixed contouritic–turbidite system. By integrating the high–resolution morpho–bathymetric dataset with the results of previous studies, we discuss the main factors controlling the variability in size and morphology of submarine landslides developed in a tectonically–controlled setting and provide preliminary considerations on their potential geohazard in a densely populated coastal area.

Combining field data with infrared thermography and DInSAR surveys to evaluate the activity of landslides: the case study of Randazzo Landslide (NE Sicily)

Infrared thermography and DInSAR surveying methodologies have been integrated and compared to study the state and distribution of activity of one of the largest landslides of northeastern Sicily, Italy, which is characterized by a slow retrogressive evolution. The Randazzo Landslide was triggered by heavy rainfalls in 1996 and its activity caused the disruption of a strategic transportation corridor, along with the formation of a landslide dam in the Alcantara Valley. After more than 20 years, the slope is still affected by instability and worrying signs of reactivations (e.g., deformation and cracking of road pavement) were surveyed in the field. The application of infrared thermography during the landslide survey allowed detecting peculiar features according to the different thermal pattern of the elements occurring along the slope. In particular, areas with different surface temperature were associated to vegetated spots, steep portions, bare sectors, and contact between different lithologies. Incipient portions and ancient landslide bodies were located, proving the utility of this experimental approach to the surveying of unstable slopes. Information on the entity, in terms of velocity, of the recent landslide displacements were provided by a DInSAR monitoring, which allowed ascertaining the presence of movements affecting several sectors of the landslide, mainly matching with the areas highlighted by thermal images. Achieved outcomes represent a scientifically relevant datum providing important information on the surveyed landslide from the risk management point of view and proving the utility of integrating infrared thermography and DInSAR methodologies for the study of complex movements.

Towards a complete understanding of the triggering mechanism of a large reactivated landslide in the Three Gorges Reservoir

The primary aim of this study is to research the characteristics of a slide motion and its relationships with environmental triggers, with reference to the Quchi landslide, which is a large, slow-moving, reactivated landslide in the Three Gorges Reservoir region. Since 2006, this instability has been involved in a slow-moving process with clearly visible signs (ground fissures, cracking of houses and road). The landslide consists of 18 Mm3 rock masses threatening Quchi Town and a road that is crucial for local transportation. For civil protection purposes, a continuous monitoring system has been put into use since 2009. Combining four years of meteorological and hydrological data with GPS displacement measurements and in situ observations, allows a detailed identification of the key processes involved. The deformation is thought to be closely related to precipitations and fluctuating reservoir levels: slope movements have a highly distinctive pattern that is characterized by high deformation starting in June and lasting into September, when the reservoir level drops and rainfall increases, followed by a rapid transition to constant deformation until the next wet season. In order to investigate in detail the influences of rainfall and fluctuating water levels on the hydromechanical behavior of the landslide, a series of two-dimensional numerical simulations are performed using the Universal Distinct Element Code (UDEC). The modeling reveals that precipitation mainly mobilizes the slope materials on the upper part of the slope, while the other trigger factor—fluctuating water levels—destabilizes the foot of the slope into a multistage failure with a remarkable retrogressive evolution. The slide is most likely to collapse in a complex pattern characterized by en masse sliding on the upper part of the slope and a retrogressive rupture at its foot.

Geomorphological and structural patterns of the scarp‑related steep dry valleys in limestone ridges: A case study from South Carpathians, Romania

Scarp retreat is a common natural process which drives the evolution of landscape in monoclinal or horizontally bedded structures. The NW‑facing, retreating limestone slope of the Vânturarița‑Buila Massif, a mountain ridge in the South Carpathians, is incised by steep dry valleys (SDVs), parallel to each other and perpendicular to the ridge crest. We performed morphometric and structural analyses on these features in order to estimate the geomorphic control on the scarp retrogressive evolution and their relations to tectonics. Longitudinal profiles display most of their relevant morphometry. Average slope commonly ranges between 35° and 50°, often exceeding 60° on certain segments. Lengths reflect the variability of exposed caprock thickness, and changes in slope show the structural control of bedding on scarp morphology. Spatial distribution and development of the steep dry valleys, as well as the scarp retreat process, is controlled by tectonic lineaments. We performed a comparative orientation analysis on fractures of extensional origin, dextral strike‑slip faults, major faults, and steep dry valleys. Results show preferred orientations on the NW‑SE and NE‑SW. Variability in size and morphology of the amphitheater‑shaped scarp sectors depends on the exposed caprock thickness. Speleothems formed in crevice‑type caves and unearthed by scarp retreat were found along some of the steep dry valleys. This study shows that combined geomorphological and structural analysis of steep dry valleys from alpine limestone ridges can provide valuable information on the evolution and morphology of the retreating scarp they develop on.

Author’s reply to letter from Dr. Ahmed Al-Badr on: “Trends in the aetiology of urogenital fistula: a case of ‘retrogressive evolution’?”

Author’s reply to letter from Dr. Ahmed Al-Badr on: “Trends in the aetiology of urogenital fistula: a case of ‘retrogressive evolution’?”

Genesis of chromium-rich kyanite in eclogite-facies Cr-spinel-bearing gabbroic cumulates, Pohorje Massif, Eastern Alps

Natural kyanites with Cr2O3 contents >1 wt% are very rare and known only from high- P environments, for example, eclogite-facies gabbroic cumulates containing Cr-spinel from the Pohorje Massif, Eastern Alps, Slovenia. In these rocks, turquoise-colored Cr-rich kyanites are present in two different textural types. A first type has formed as blocky crystals of several hundred micrometers in size around clusters of small drop-like Cr-spinels. This kyanite shows a highly irregular Cr distribution and may contain up to 15.6 wt% Cr2O3, which is one of the highest Cr2O3 contents reported so far. A second type is present as part of reaction coronas around large red-brownish Cr-spinel and forms deep-blue needle-like crystals that rarely exceed 100 μm in size. This kyanite contains up to 8.2 wt% Cr2O3 and is associated with Cr-rich corundum (≤9.1 wt% Cr2O3) and Cr-Al-rich pargasite (≤3.9 wt% Cr2O3). The formation of kyanite around Cr-spinel droplets is interpreted to be the result of increasing P-T conditions during prograde metamorphism where Cr-spinel and plagioclase or quartz react to Cr-kyanite, ±garnet, ±omphacite. In contrast, the formation of kyanite associated with Cr-rich corundum and Cr-rich pargasite within coronas around Cr-spinel occurred in an early stage of the retrogressive evolution of the gabbroic cumulates at eclogite-facies conditions of ~2.5 GPa and 750–800 °C triggered by the influx of H2O-rich fluids. The driving force for developing these coronas was an increase in the chemical potential of silica caused by the infiltrating hydrous fluid phase. P-T estimates using matrix mineral assemblage place the peak metamorphic conditions close to the quartz/coesite transition with temperatures in the range of 750–810 °C and pressures of ~2.9 GPa.

Trends in the aetiology of urogenital fistula: a case of 'retrogressive evolution'?

It has long been held as conventional wisdom that urogenital fistulae in low-income and middle-income countries are almost exclusively of obstetric aetiology, related to prolonged neglected obstructed labour, whereas those seen in high-income countries are largely iatrogenic in nature. There is, however, a growing perception amongst those working in the field that an increasing proportion of urogenital fistulae in low-income and middle-income countries may be iatrogenic, resulting from caesarean section. Recent studies suggest that adverse patterns of care may also be emerging in high-income countries; an increase in the risk of both vesicovaginal and ureterovaginal fistulae following hysterectomy has been reported, concurrently with the reduction in overall use of the procedure. These apparent secular trends are discussed in the context of evolution of practice, teaching and training in obstetrics and gynaecology.

Origin of an enigmatic regional Mio-Pliocene unconformity on the Demerara plateau

The Demerara plateau, located offshore French Guiana and Suriname, is part of a passive transform continental margin particularly prone to develop slope instabilities, probably in relation to the presence of a free distal border along its steep continental slope. Slope failure occurred at different periods (Cretaceous to Neogene) and shows an overall retrogressive evolution through time. Upslope these failure headscarp, an enigmatic regional Mio-Pliocene unconformity has been discovered through the interpretation of new academic and industrial datasets. The aim of this work is to describe and understand the origin of this surface. Our analysis shows that this unconformity is made of a series of valleys that cross-cut sedimentary strata. Each one of these valleys has a short lateral extent and is closed along two perpendicular directions, which suggests that it could correspond to a highly meandering system, or to some sub-circular depressions. The infill of these features is equivalent to the regional stratigraphic strata found outside the structures, but in a subdued position. This seems to imply that the structures have originated by a local loss of sediments at their base or by sliding processes. Furthermore, these depressions intersect each other through time, while migrating progressively downslope. We discuss a series of hypotheses that try to explain the onset and evolution of these depressions forming the Mio-Pliocene unconformity (Canyons? Slope failures? Contourite moats? Hydrate pockmarks?). Having established that these structures are depressions formed by collapse, and have many similarities with structures recently described in the literature as pockmarks associated with gas hydrate dissolution, we favor this hypothesis. We propose that these hydrate pockmarks form with a mass failure that was triggered by fluid-overpressure development at the base of the hydrate stability zone.

Geology, geomorphology and dynamics of the 15 February 2010 Maierato landslide (Calabria, Italy)

On 15 February 2010, as a result of intense and long-lasting rainfalls, a large landslide affected a wide area near the town of Maierato (Calabria, Italy). The studies conducted – including (i) aerial photo interpretation, (ii) geological and geomorphological field investigations, (iii) interpretations of lithology and stratigraphy from borehole data, and (iv) observation of videos filmed during the main diastrophic phases of the landslide and of antecedent Google Street View® images – allowed researchers to reconstruct the geological and tectonic setting of the slope and the internal structure of the landslide with the estimation of the depth of the sliding surface, the triggering mechanisms and its evolution.The analysis of the prelandslide event setting demonstrates that this mass movement is the reactivation of a preexisting landslide of alleged seismic origin, remaining at an incipient stage.The Maierato landslide occurred on a gentle slope made of late Miocene to Plio-Pleistocene clastic and evaporitic sedimentary rocks. The main basal failure surface that developed on the hemipelagic marls has a maximum depth of 50m. The volume of the landslide is ~5million cubic meters.The type of landslide movement is a complex one, consisting of a very rapid slide of rock and earth and of flow of debris and earth. The landslide clearly shows three major types of failure mechanisms: the first type is described as a rapidly moving rotational slide where back-tilted blocks of sediment are preserved; the second type includes a very rapidly moving translational slide of large rock blocks; the third type includes sudden, extremely rapid flow-slides where the slide material is disaggregated while flowing downward along a gentle slope.The slide is a compound one, with a retrogressive evolution and transformation into earth and debris flow during the failure. After the triggering of the landslide, and as a result of the relevant displacement, an important portion of the lower evaporitic unit (Calcare di Base Formation), close to the failure surface, collapsed, thereby undergoing a quick change of its mechanical behavior that became similar to that of a viscous fluid. During the landslide evolution, large rocky blocks consisting of Miocene evaporitic limestones, Pliocene silty clays, and sands were rafted, without severe disturbance, on the destructurated and fluidized limestone. The intense destructuration and the presence of water transformed the limestone (in the lower parts of the unit) into a viscous material that was squeezed out of the landslide mass through the jags between the several rafted rocky blocks and along the natural levees of flow tongue.This event is a rather frequent combination of mass movements made complicated and spectacular by the fluidization of the weak limestone that imparted great dynamics to the movements. Such fluidization is an infrequent phenomenon especially in this geological context.

A process-based model for the definition of hydrological alert systems in landslide risk mitigation

Abstract. The definition of hydrological alert systems for rainfall-induced landslides is strongly related to a deep knowledge of the geological and geomorphological features of the territory. Climatic conditions, spatial and temporal evolution of the phenomena and characterization of landslide triggering, together with propagation mechanisms, are the key elements to be considered. Critical steps for the development of the systems consist of the identification of the hydrological variable related to landslide triggering and of the minimum rainfall threshold for landslide occurrence. In this paper we report the results from a process-based model to define a hydrological alert system for the Val di Maso Landslide, located in the northeastern Italian Alps and included in the Vicenza Province (Veneto region, NE Italy). The instability occurred in November 2010, due to an exceptional rainfall event that hit the Vicenza Province and the entire NE Italy. Up to 500 mm in 3-day cumulated rainfall generated large flood conditions and triggered hundreds of landslides. During the flood, the Soil Protection Division of the Vicenza Province received more than 500 warnings of instability phenomena. The complexity of the event and the high level of risk to infrastructure and private buildings are the main reasons for deepening the specific phenomenon occurred at Val di Maso. Empirical and physically-based models have been used to identify the minimum rainfall threshold for the occurrence of instability phenomena in the crown area of Val di Maso landslide, where a retrogressive evolution by multiple rotational slides is expected. Empirical models helped in the identification and in the evaluation of recurrence of critical rainfall events, while physically-based modelling was essential to verify the effects on the slope stability of determined rainfall depths. Empirical relationships between rainfall and landslide consist of the calculation of rainfall Depth-Duration-Frequency (DDF) curves, which allow one to determine rainfall depth (or intensity) as a function of duration for given return periods or probabilities of exceedance (frequencies). Physically-based modelling was performed through coupled seepage and slope stability analyses. Combining results from empirical and physically-based modelling, the minimum alert threshold for a reactivation of the phenomenon was found in rainfall cumulated up to 60 days with a return period of 2 yr. These results were used to set up a hydrological alert system based on the calibration of DDF curves which can be used as a sort of abacus to plot in real time rainfall depths and to set increasing levels of alert on the basis of the degree of exceptionality of rainfall. The alert system for Val di Maso was successfully tested by the rainfall events that produced displacements which have been recorded by extensometers placed in the crown area after the November 2010 landslide. However, further tests are recommendable to improve the process-based model that led to the implementation of the alert system. To this end, a monitoring system is currently being realized. In the near future, monitoring data will help in testing and improving landslide evolution and alert models. The proposed hydrological alert system proves to be effective mainly because it can be applied to different scales of investigation and geological and geomorphological contexts. In fact, it might also be applicable to territorial scale analyses, as showed by the brief example provided in this paper on how the alert system could be used for landslide early warning in the area surrounding Val di Maso. Furthermore, it is easy to set up. The needed components are a rain gauge station, a software that compares rainfall data to rainfall events with different return periods and degree of alert, and a transmission system of the warning levels to authorities.

The prehistoric Vajont rockslide: An updated geological model

This study presents the detailed reconstruction of the entire structure of the prehistoric Vajont rockslide (about 270–300millionm3 of rocks and debris) for the first time, describing the complex geometry and the characteristic superimposition of distinct rigid blocks on a very thick shear zone. The prehistoric Vajont rockslide was characterized by an enormous ‘en masse’ motion of a rigid overlying rock mass (100–130m thick) that moved downslope, sliding onto a very thick shear zone (40–50m thick, on average) made up of a chaotic assemblage of blocks, limestone angular gravel, and high plasticity clays (montmorillonitic clays).Coarse loose sediments, still exposed on the 9 October 1963 detachment surface, are always associated with large blocks made of strongly fractured rock masses (Fonzaso Formation: middle‐upper Jurassic) preserving the stratification. The blocks of stratified and folded limestone sequences appear to be ‘sheared off’ from the underlying bedrock and can be considered as displaced rock masses planed off by the motion of the overlying rigid rock mass (‘rock mass shavings’). The prehistoric Vajont rockslide was characterized by a multistage failure with a marked retrogressive evolution. The first rupture (Pian del Toc block) rapidly destabilized the upper slope, mobilizing a second rock mass block (Pian della Pozza block) that, in turn, determined the multiple rupture of the revealed shear zone material (Massalezza lobe). Even if the exact timing of the different phases is not known, the entire multistaged failure process was very rapid. At the end of the multistage retrogressive failure, the slope morphology of the northern toe of Mt. Toc was drastically changed and the large failed rock mass settled into the preexisting Vajont Valley assuming the unusual chair-like geometry.The Vajont rockslide represents a very significant example on how a complex geological situation, if not adequately analyzed and reconstructed, can lead to dangerous misinterpretations or even to erroneous engineering–geological and geotechnical models. Accurate fieldwork and modern technologies can be fundamental in solving such a very intriguing ‘geological puzzle.’

Control of landslide retrogression by discontinuities: evidence by the integration of airborne- and ground-based geophysical information

The objective of this work is to present a multitechnique approach to define the geometry, the kinematics, and the failure mechanism of a retrogressive large landslide (upper part of the La Valette landslide, South French Alps) by the combination of airborne and terrestrial laser scanning data and ground-based seismic tomography data. The advantage of combining different methods is to constrain the geometrical and failure mechanism models by integrating different sources of information. Because of an important point density at the ground surface (4. 1 points m−2), a small laser footprint (0.09 m) and an accurate three-dimensional positioning (0.07 m), airborne laser scanning data are adapted as a source of information to analyze morphological structures at the surface. Seismic tomography surveys (P-wave and S-wave velocities) may highlight the presence of low-seismic-velocity zones that characterize the presence of dense fracture networks at the subsurface. The surface displacements measured from the terrestrial laser scanning data over a period of 2 years (May 2008–May 2010) allow one to quantify the landslide activity at the direct vicinity of the identified discontinuities. An important subsidence of the crown area with an average subsidence rate of 3.07 m year–1 is determined. The displacement directions indicate that the retrogression is controlled structurally by the preexisting discontinuities. A conceptual structural model is proposed to explain the failure mechanism and the retrogressive evolution of the main scarp. Uphill, the crown area is affected by planar sliding included in a deeper wedge failure system constrained by two preexisting fractures. Downhill, the landslide body acts as a buttress for the upper part. Consequently, the progression of the landslide body downhill allows the development of dip-slope failures, and coherent blocks start sliding along planar discontinuities. The volume of the failed mass in the crown area is estimated at 500,000 m3 with the sloping local base level method.

Delayed failure of quarry slopes in stiff clays: the case of the Lucera landslide

This paper concerns the landslide process occurring on a cut slope in stiff clays located at the north of the town of Lucera in southern Italy. This unstable slope lies between a hospital at the top and an abandoned quarry at the toe: the quarry was active until the end of the 1970s. A first landslide occurred around 1980, and was characterised by a subsequent retrogressive evolution. This paper presents an interpretation of the slope failure mechanism based on the results of geomorphological studies, field monitoring, laboratory testing, and both limit equilibrium and coupled numerical analyses, these latter carried out with FLAC2D. In particular, the numerical analyses have been performed in order to interpret the evolution of the slope movements with time, and to assess the influence of the quarrying-induced excess pore pressures on the development of the failure process. The numerical results show that during the excavation stages the negative excess pore water pressures due to undrained unloading allowed for a temporary stability of the slope. The analyses also indicate that the process of pore pressure equalisation triggered the 1980 failure, and initiated the retrogressive evolution of the landslide.

Modeling the evolution of natural cliffs subject to weathering: 1. Limit analysis approach

[1] Retrogressive landsliding evolution of natural slopes subjected to weathering has been modeled by assuming Mohr-Coulomb material behavior and by using an analytical method. The case of weathering-limited slope conditions, with complete erosion of the accumulated debris, has been modeled. The limit analysis upper-bound method is used to study slope instability induced by a homogeneous decrease of material strength in space and time. The only assumption required in the model concerns the degree of weathering within the slope, and for this we assumed and tested different weathering laws. By means of this method, the evolution of cliffs subject to strong weathering conditions (weathering-limited conditions) was predicted. The discrete succession of failures taking place was modeled taking into account the geometry assumed by slopes as a consequence of previous mass movements. The results have been compared with published data from long-term slope monitoring and show a good match between experimental observations and analytical predictions. The retrogressive evolution of the slope occurs with decreasing size of the unstable blocks, following a logarithmic volume-frequency relationship. A nonlinear relationship is found between mass flux and average slope gradient. A set of normalized solutions is presented both by nomograms and tables for different values of slope angle, cohesion, and internal friction angle.

A counter‐clockwise P–T path for the Voltri Massif eclogites (Ligurian Alps, Italy)

AbstractIntegrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P–T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim‐ward resorption, likely related to re‐equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca‐, Na/Ca‐amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist‐to‐amphibolite facies metamorphic stage (M1 stage) at c. 450–500 °C and 5–8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 °C and 20 kbar, conditions which were considered as minimum P–T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote–blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid‐assisted Na‐amphibole‐bearing symplectites) produced at the expense of the primary M2 garnet‐clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non‐coaxial fabric, with dominant top‐to‐the‐N sense of shear during rock exhumation. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P–T deformation data are consistent with an overall counter‐clockwise path, from the greenschist/amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high‐pressure roots.

Analysis of the flexural failure of an overhanging rock slab

A reservoir-induced landslide is a common geomorphological problem in the TGR (Three Gorges Reservoir) region, China. This study considers the Quchi landslide as an example of a large-scale partially reactivated ancient landslide in the TGR region; the possible kinematic responses and corresponding triggering mechanisms are investigated by analyzing eight-year monitoring data sets, field surveys, observations, and laboratory tests. The reactivated zone is primarily located in the south-western part of the ancient landslide, and it mainly moves along the deep sliding surface. The long-term monitoring data sets show that the presence of faster movements from June to September (rainy season), followed by slower deformation in the remaining nine months of the hydrological year. Both strong rainfall and drawdown during the low period of RWL (reservoir water level) are important hydrological factors responsible for the occurrence of faster movements. An RWL of 160 m a.s.l. (above sea level) during drawdown is considered the key water level for the initiation of faster movements. The motion pattern is primarily retrogressive, and the displacement velocity increases towards the toe. At present, the stability of the reactivated zone is indeterminate; a multistage failure with significant retrogressive evolution can possibly occur in the future. The findings in this study can help in managing reservoir landsides and designing landslide reinforcements in the regions of the word with geological settings similar to that of the TGR region.