Rotational Landslides Research Articles

Aspects of landslide activity in the Mercantour Massif and the French Riviera, southeastern France

Landslides pose serious hazards in the Mercantour Massif and the French Riviera in southeastern France. The context for landslide development is a particularly favourable one, both in terms of the geomorphic and structural setting of this Alpine region, and of the climatic, hydrologic and seismic factors that trigger such failures. High mountain relief and steep slopes constitute a very favourable setting for failures affecting massive basement rocks and a very heterogeneous sedimentary cover whose resistance has been weakened by weathering, tectonic stresses, and cambering due to gravity. Among trigger factors, the important appears to be the precipitation regime. Rainfalls are commonly concentrated into short high-intensity downpours or into bursts of sustained falls over periods of several days, leading to soil saturation and lubrication of potential failure planes. Snowmelt also contributes to these lubrication processes. Earthquakes affecting this region are also a potentially important landslide trigger. However, while a lot of work has been done on the relationship between extreme climatic events and landslide activity, much less is known of the trigger effects of earthquakes. Both the background factors that promote landslide development and the factors that trigger such failures are discussed within a time frame of landslide development. Progressive changes in soil strength due to weathering, rock cambering and shattering processes lead to long-term reduction in resistance. Superimposed on these progressive changes are episodic triggerings related to rainfall and snowmelt episodes or earthquakes. Landslides may occur as shallow, low-volume “one-time” events or may be part of a progressive long-term failure. The former generally affect unconsolidated or clay-rich sedimentary rocks, especially on the coastal hillslopes of the French Riviera. A notable exception of a major, voluminous “one-time” event was the submarine landslide of the Var Delta in 1979. This landslide, like numerous other smaller subaerial landslides onland, was largely a result of human activities. This landslide occurred following extensive modification of the Var Delta and, notably, reclamation of the steep, fine-grained delta front. Deforestation, quarrying, urbanisation and road network developments are various ways in which human activity has destabilized the coastal hillslopes, favouring the development of numerous shallow landslides following each episode of heavy rainfall. Progressive landslides on the upper hillslopes of the Mercantour Massif have developed over long time spans (order of 10 1 to 10 5 yrs) and have involved more complex interactions between lithological controls, slope characteristics and trigger factors. The Collelongue and Bois de Malbosc landslides have evolved into now stable failures buttressed by resistant migmatitic diorites or amphibolites. The more voluminous and well monitored Clapière landslide is a relatively simple rotational landslide of the toe-failure type. This active landslide poses a serious to inhabitants and infrastructure in the Tinée Valley. The importance of continued field monitoring, modelling and mapping of landslides and their hazards is emphasised.

An example of a low-temperature-triggered landslide

This paper describes the evolution of a rotational landslide triggered during a cold winter in 1991. The landslide involved the eastern slope of the Ru delle Roe basin located in the Alpine Dolomite area near Belluno (Italy). Because of the hazardous position of the village of Molin, located 500 m downhill from where the landslide occurred, it was necessary to systematically check the evolution of the mass movement. A warning system was installed and topographic surveys have been carried out since the beginning of the emergency to protect the village of Molin and to collect data to predict the landslide behaviour. Moreover studies of old cartographic maps and the aerial photographs led to the identification of morphological features that indicate the dynamic evolution of the area.

A reconstruction of Holocene geomorphology and climate, western Cypress Hills, Alberta and Saskatchewan

The Cypress Hills of southwestern Saskatchewan and southeastern Alberta are a key area for Quaternary research because the permanent lakes and coniferous forest are sources of proxy paleoenvironmental data generally not available on the surrounding northern Great Plains. In this paper, the Holocene paleoclimate of the western Cypress Hills is reconstructed from the pollen and organic matter contents of a 9.6 m sediment core from Harris Lake, Saskatchewan. This climatic history is considerably longer than others from the semiarid Interior Plains of Canada. Chronological control consists of a basal conventional radiocarbon date of 9120 ± 250 BP, Mazama tephra, and three accelerator mass spectrometry dates from plant macrofossils. The data from Harris Lake indicate significant climatic change from the warm, dry altithermal, 7700–5100 BP, to cooler and moister conditions of the late Holocene. Low lake sediment organic matter from 3000 to 2400 BP is attributed to lower temperatures and coincides with a well-documented global neoglacial advance. Correlations among the proposed climatic history, sedimentation rates in Harris Lake, and dated soils and sediments described in the literature are the basis for a reconstruction of the Holocene geomorphology. This information suggests a transition in dominant geomorphic processes from fluvial and aeolian erosion before 5100 BP to rotational landsliding after 4500 BP.

Landslides: The deduction of strength parameters of materials from equilibrium analysis

Summary Rotational landslides (slumps) varying in size can develop at different slope angles in the same type of cohesive regolith material. Therefore it is possible to set up independent equilibrium equations for these rotational slides of different size. In this way, a good estimate of the mean strength parameters of a certain type of material can be obtained on the basis of a number of these independent equilibrium equations. This can be done by means of regression analyses and the correlation coefficients and variances give a good indication of the reliability of the estimate of the strength parameters. In this paper the strength parameters of two regolith materials on schist and claystone are determined in this way and compared with the strength parameters measured in the laboratory.

Factors affecting the transition between the various types of mass movement in coastal cliffs consisting largely of overconsolidated clay with special reference to Southern England

Summary At present, the most widely accepted hypothesis for the transition from mudsliding to deep-seated rotational slipping as the dominant agent of mass transport in actively eroding clay cliffs is that the transition depends on the rate of marine erosion at the toe of the slope. This paper explores an alternative theory, that the transition from one type of behaviour to another depends largely on the nature of the materials at the crest of the slope and to a lesser, but still important, extent on the groundwater hydrology. The main argument behind this alternative theory is that the presence of stronger soil strata at the crest of the slope inhibits the mudslide behaviour that would otherwise occur and hence form a toe-protecting ‘mudslide barrier’. This allows marine action to erode the toe of the slope directly, causing it to be oversteepened, and ultimately precipitates a deep-seated rotational landslide. Where the coastal cliffs are capped with thick, hard and jointed caprock the slides tend to be of the multiple rotational kind.

Characteristics of natural slopes in the Athabasca Oil Sands

The Athabasca Oil Sands are largely within the McMurray Formation, which is a transgressive blanket quartz sand of Lower Cretaceous age. Millennia of erosion have resulted in extensive exposures of oil sand along river valleys in the vicinity of Fort McMurray. Study of these slopes has contributed to understanding the nature and behavior of oil sands. Oil sand slopes with active toe erosion are characteristically high and steep (up to 70 m at slopes over 50°), have an indurated outer face and display a stress–relief exfoliation joint system that controls slope recession phenomena. Bitumen does not contribute mechanically to slope stability. The major agents affecting slope morphology are the lithology, the aspect and the basal stratigraphy. Ravelling along exfoliation fractures is the major failure mode, block falls are a minor failure mode and rotational landslides have not been observed. Remolded oil sand may flow viscously, but intact oil sand displays an unusually high strength.

A Discussion on valley slopes and cliffs in southern England: morphology, mechanics and Quaternary history - The evolution of landslide slopes in Dorset

The form and evolution of Fairy Dell, an active landslide complex on the Dorset coast, is described using a combination of cartographic, air photograph and field survey techniques. Erosion rates for the main landslide scars, undercliffs and sea cliffs are calculated and the spatial patterns of landslide evolution demonstrated by the use of sequences of maps and geological cross sections. Two dominant mechanisms are identified and described: (1) rotational landsliding and (2) block disruption, the breakup of originally large landslide units as they move downslope and over time. The role of small-scale erosion, in combination with the infilling of depressions by scree, wash debris and mudslides so as to produce an increasingly subdued topography as the landslides degrade, is emphasized and simple evolutionary models are proposed. The active landslide complex is then compared with the now stable, degraded landslide slopes inland. It is shown how the spatial patterns of landforms recognized in these areas on morphological maps and the complex subsurface forms revealed in sections can be better understood by reference to the suggested evolutionary sequence developed for the active complex. The sections in the stable but degraded slopes clearly support the idea of retrogressive rotational landsliding followed by block disruption, infilling and the downslope reduction of topographic expression of landslide units. Finally, it is suggested that this evolutionary interpretation might assist in the understanding of similar areas elsewhere and, if used in conjunction with geomorphological surveys, could result in the planning of more efficient site investigations.

A Discussion on valley slopes and cliffs in southern England: morphology, mechanics and Quaternary history - The history and stability of two Lias clay slopes in the upper Gwash valley, Rutland

Two detailed valley side sections are described. Both are of landslipped slopes cut in Upper Lias at the point where the Inferior Oolite escarpment is breached by the east­ ward flowing river Gwash, a few kilometres east of Oakham, Rutland. Field mapping suggests that the slopes were cut in Ipswichian times either shortly before or during the deposition of the Second Terrace of the river Welland, of which the Gwash is a tribu- tary. One of the slopes (Barnsdale) was oversteepened by fluvial erosion which stripped the cambered Inferior Oolite and frost-disturbed Lias from the escarpment face, leaving, at the slope foot, a prominent erosion surface (the £Hambleton Surface’). After cessation of erosion Head accumulated on the Surface and the slope degraded, much of the degradation possibly occurring in the Middle Devensian, though land­ slide movement continued to the end of the Late Devensian. The other slope (Hambleton) which had formerly been cambered, suffered only slight steepening contemporaneously with the Barnsdale slope and the large, shallow rotational landslide that developed in the frost-disturbed Lias clay was subsequently partly obscured by Late Devensian solifluction. Stability analyses of these two landslides, together with three other previously pub­ lished case records, show that the field values of residual strength of Upper Lias clay are strongly stress dependent, with the magnitude of p'T (4 = 0)falling to 10° as the normal stress increases. Laboratory measurements of residual strength using the ring-shear apparatus are similarly stress dependent, but show considerable strength variations between different samples with similar index properties. The ring-shear strengths underestimate the field strengths by up to 11%.