Abstract Recognition of weathering as a primary process in the development of landforms dates back at least to the late 19th and early 20th century, as indicated by several contributions from various countries (e.g. Cortese 1895 ; Branner 1896 ; Falconer 1911 ; Jutson 1914 ; Walther 1915 ); nevertheless, it was only in the second half of the last century that systematic studies on weathering and related topics such as geomorphology, engineering geology and petrography were carried out. In a recent review paper Ehlen (2005) found through an on-line search more than 9000 hits where the term ‘weathering’ was used, dating from the mid-1950s to the beginning of the 21st century. Such a huge number of citations clearly demonstrates the attention that the scientific community has dedicated to the in-place breakdown of rocks by chemical, physical and biological processes. However, despite the high frequency of landslides and erosional phenomena in weathered materials, and the damage and casualties they repeatedly cause, not very much is known about the direct and indirect relationships between weathering and slope movements. The matter is further complicated by the high variability of landslide features ( Working Party on World Landslide Inventory 1993 ). According to the local conditions, a variety of slope movements may take place in the weathered rock masses. Shallow soil slips evolving to rapid and catastrophic debris flows are probably the most common type in steep residual soil slopes, whereas thicker failures, also including deep-seated complex

Abstract Some huge landslides have occurred in deeply weathered rock, but most geologists and engineers do not appreciate the great depths that weathering can attain. Rocks can be weathered to depths of hundreds of metres, often in a very irregular manner. Fresh rock is converted to weathered rock called saprolite by isometric chemical alteration. Some saprolite is later eroded, and fresh rock appears in the landscape in distinctive landforms. This erosion of an irregularly weathered landscape is called stripping (meaning the stripping of saprolite from fresh rock). The age of the weathering may be measured in geological periods, as can the age of stripping, and may be related to past climate different from that of today. Deep weathering occurs basically by hydrolysis, which requires groundwater. Slope failure in saprolite depends on the engineering property of the material, and its relation to bedrock and corestones, and relationship to degree of weathering of adjacent saprolite. This paper emphasizes the deep weathering phenomenon itself, but a few examples are given to illustrate the variety of landslides in deeply weathered material.

Abstract Soil deterioration is one of the main causes of slope failure in hard clays and clay shales, but so far a clear general framework of the causes of deterioration is still missing. Based on laboratory data, field observations and speculation, this paper examines some processes of deterioration of stiff overconsolidated clays and clay shales with particular regard to their influence on the stability of slopes. In particular, deterioration localized within soil bands or involving large soil masses is described with reference to its mechanical or physico-chemical causes. Coupling between mechanical and physico-chemical processes is considered.

Abstract The paper assesses the contribution of mass movement to the development of landforms in weathered granite and gneiss terrain, using examples from SE Brazil, southern Mexico, SE China, and other locations. The style of slope failures varies depending on local relief and the grade of weathering, which mutually influence each other. Shallow debris slides and boulder falls are common in less weathered rock and steep terrain, whereas rotational slides appear to dominate in highly weathered rock and in more subdued topography. In certain topographic circumstances, slides may turn into debris flows or earthflows. Extreme rainfall is a typical trigger of mass movement in weathered terrain. Mapping of past landslides may considerably assist in hazard assessment, as they usually leave a clear geomorphological record.

Abstract The strength degradation of two marl formations (Pamplona marls in Spain and Abadia marls in Portugal) is analysed in the paper. In the case of Pamplona marls, a number of large-scale direct shear tests on natural and saturated specimens of the undisturbed material were performed. Tests on marl–concrete interfaces were performed and are also presented. Both cases analysed show that the saturation of the marl leads to a strong reduction of friction and effective cohesion. Negative effects of suction changes on the mechanical behaviour of the marls were investigated in more detail in suction controlled tests on Abadia marls, a more plastic material. A numerical simulation of wetting effects on a marl fragment of finite size helps to identify the nature of degradation mechanisms, which are associated with the development of tensile surfaces inside the marl matrix. They, in turn, are due to the expansive behaviour of the clay fraction of the marl relative to suction changes.

Abstract This paper reviews the nature and mechanics of landslides in the weathered terrain of Hong Kong. The vast majority of landslides are very shallow (a few metres depth) and occur during intense rainstorms. Deeper-seated landslides, in contrast, may occur days or weeks after intense rainstorms. The time of occurrence of landslides can be linked to hydrological and hydrogeological factors, and a hydrogeological grouping of landslide mechanisms is introduced related to timing in a storm. A relationship is presented that links intensity of landsliding to 24 h rainfall. The gradual deterioration and internal erosion of slopes prior to detachment is discussed and allows some realistic opportunity for identifying progressive major landslides. In particular, the growth of natural piping systems and infilling of dilated fracture networks are recommended as important indicators of landslide development. The conclusions are supported by case examples of slope failures, the study of some of which has been taken to a forensic level.

Abstract This paper reviews approximately 40 years of studies focused on landslides in weathered crystalline rocks of Calabria, southern Italy. In these studies geological, geomorphological, mineralogical, petrographical, engineering-geological and geotechnical aspects of the Palaeozoic granitoids and the high-grade metamorphic rocks cropping out in the Calabrian massifs have been considered. This in turn has allowed depiction of the regional landsliding scenario, where almost all the main typologies of mass movements have been recognized, ranging from shallow soil slips and extremely rapid debris flows to slow-moving deep-seated gravitational slope deformations. From north to south, the Sila Massif, Coastal Chain, and Poro, Serre and Aspromonte Massifs are described, with reference to the studies dealing with weathering and slope movements in the different geological and morphological settings. The regional extent of the weathering processes gave researchers the opportunity to adopt and calibrate mapping methods, specifically devoted to analysis and mitigation of the landslide hazard in weathered materials, which have also been applied to important civil engineering works. A further distinctive character of the Calabrian weathering is its age, which has been ascribed to Tertiary time. Such an ancient initiation could be regarded as one of the main reasons for the depth of the weathered mantle, which, despite the rugged topography, in some districts exceeds 150–200 m.

Abstract Lalibela is located in the northern–central part of Ethiopia, c . 600 km north of Addis Ababa in Northern Wollo (Ahmara Region). The town, which has about 12 000 inhabitants, is situated at an altitude of 2500 m. In its centre a unique complex of 11 rock-hewn Christian Orthodox churches is located, cut out of the rock some 800 years ago. Their construction is attributed to King Lalibela (1167–1207). All the churches exhibit widespread evidence of structural damage and weathering processes, which are posing a serious problem for the preservation of the monuments. Preliminary geological field surveys, in situ geotechnical analyses and laboratory tests have been carried out in the area to understand the cause of weathering and the likely consequences on the structural stability of the churches. Laboratory tests on volcanic materials detected a deep alteration of rocks as a result of the widespread presence of montmorillonite. This is the primary cause of the progressive deterioration of the physical and mechanical characteristics of the slope-forming rocks in Lalibela. Major consequences are the alveolar weathering of churches' façade, the degradation of the roofs and a reduction of rock strength, which are causing a progressive structural instability in some rock-hewn monuments. Also, sliding of façades along bedding joints was detected. Understanding the weathering processes affecting the Lalibela churches allows the detection of problems and the timely implementation of medium- to long-term protection strategies for conservation of the monuments.

Abstract The Hunters Crossing landslide is a slow-moving, weathered rock slide affecting a small community of condominiums in the town of Waynesville in the Blue Ridge Mountains of Haywood County, North Carolina. In November 2005, studies were begun to assess the characteristics of this landslide and the potential for further movement and damage to structures. Work included drilling several boreholes, performing seismic velocity surveys, and surveying benchmarks among other investigations. Data indicate that the potential failure surface is located no more than 11 m below the ground surface, possibly at the contact between saprolite and partially weathered rock. However, inclinometers installed at two locations on the slope have not detected enough movement to corroborate that assessment. Studies continue at this site to determine the location of the failure surface, to identify the mechanisms that accelerate movement, and to relate these findings to a broader understanding of weathered rock slides elsewhere in the southeastern USA.

Abstract Research on the properties of residual soils has been continuing in Brazil since the pioneering work by Vargas in 1953. A great number of earth dams built since then and the initiation of graduate courses since 1960 have increased the rate of in-depth studies on this theme. This contribution considers the more recent research on the properties of these soils, mainly shear strength parameters.

Abstract The interrelationship between slope deformation and fault-induced weathering as a predisposing factor for the development of sliding is analysed through several case studies from the Western Carpathians in the Czech Republic. The study area comprises flysch nappes with alternating sandstone and shale of different permeability. These lithological structures are affected by systems of faults. Recurring slope instability is found associated with zones of deep weathering in tectonically weakened areas. Climatic variability of landslide activity can be identified during the Holocene by means of radiocarbon dating and pollen analysis. Areas affected by recurring landsliding suggest gradual and cyclic landslide frequency.

Abstract To clarify the erosion processes on a marly bare slope in the southern Alps, the erosion processes on a steep and erodible slope composed of Black Marls formations were observed using a time-lapse video camera. The observations revealed that miniature debris flows (MDFs) occurred at the time of the rainfall–runoff event during which the most severe erosion took place over the 3 month observation period. Analysing the camera images, we show some characteristics of the MDFs and discuss them in the context of real rainfall–runoff phenomena observed at the outlet of the small experimental basin that contained the visually observed slope. The following results were obtained. (1) It is roughly estimated that the total amount of sediment discharged by the MDFs was not quantitatively negligible in comparison with the total sediment discharge from the entire experimental basin. (2) The MDFs occurred only during the rising limb of the hydrograph, which lasted 6 min. The visual observation also provided information on triggering conditions. Compared with the precipitation data, it was found that a rainfall intensity of 1 mm min −1 triggered MDFs and an antecedent cumulative rainfall of 10.4 mm was also enough to trigger an MDF. This is consistent with a previous study in Draix. (3) Although the Darcy–Weisbach friction factor and Manning's n of the MDFs observed differ considerably, they are consistent with results in the literature reporting erosion phenomena on very steep slopes up to 36°. (4) Based on this observation and a review of the literature, on very steep and highly erodible slopes, MDFs or similar phenomena might play an important role in erosion and transport processes. It is important to consider this possibility when examining erosion models for catchments composed of steep and erodible slopes.

Abstract A new correlation is proposed between the residual shear strength and the methylene blue value (‘value of blue’; VB) for weathered clay soils on argillaceous bedrock and on alluvial soils in the Oltrepo Pavese area (Northern Italy), which can be considered as geologically representative of a large part of the Italian Apennines. Shallow landslides occur periodically in these soils as a result of high-intensity rainfall events. A number of soils were tested. Trench pits were used for sampling and for the soil profile description (lithology, structure, grade of weathering, thickness). Field surveys were integrated with some standard geotechnical laboratory tests. The methylene blue dye adsorption (VB) was determined in accordance with the French AFNOR standards. The residual strength friction angle was measured with direct shear tests; the procedure employed for the measure involved inserting the soil at the liquid limit in the direct shear ring, applying consolidation in stages and then shearing (Kanji method). The applicability of some existing correlations between the residual friction angle and index properties were tested. The correlations are characterized by a large amount of scatter in the data and tend to overestimate the residual friction angle. The VB test is easy and rapid to perform and appears to be a good indicator of the residual strength friction angle for the tested soils.

Abstract This volume is intended to provide an up-to-date overview of the approaches, methodologies and techniques devoted to better understanding of the weathering conditions of rock masses on slopes. According to the local conditions, a variety of slope movements may take place and involve weathered rock masses. Shallow and rapid soil slips evolving to debris flows are probably the most common type of slope movement. At the same time, deep-seated, intermittent landslides can also affect large volumes of weathered rocks and soils. Despite the high frequency of landslides in weathered materials, and the damage and casualties they repeatedly cause, little is known about the relationship between weathering and slope movements. This book presents worldwide case studies, where a variety of geological and geomorphological settings are discussed. The content is divided into three sections: the first is devoted to broad aspects of the weathering/landslide processes; the second and third sections include papers dealing with igneous/metamorphic and sedimentary weathered rocks, respectively.

Abstract This paper reports the research results over several years on Piperno, the most important ornamental architectural stone of Naples. Particular attention is paid to the rediscovery of the old exploitation sites of this rock and to the survey of the last underground quarry site, still accessible, at the base of the Camaldoli Hill (western Naples) at Pianura. The conservation state was assessed by means of specific surveys in view of possible future utilization. At present, the re-opening of abandoned quarries is not possible owing to unsafe site conditions. The cultural relevance of the Pianura quarry site could suggest its possible restoration as a museum of mining and a centre for teaching the working of ornamental stone within the Campania Region.