J. S. Griffiths

Anatomy of a ‘fossil’ landslide from the Pleistocene of SE Spain

Abstract Pleistocene landslide failures are rarely recorded in the literature due to difficulties in recognition in the field. An adaptation of standard landslide classification based on activity is used to identify and examine the significance of a ‘fossil’ Pleistocene landslide from the Mocatan catchment of the Sorbas Basin, SE Spain. Recognition of the landslide is based on identifying key, but subtle anomalies in the geology and drainage. The fossil landslide at Mocatan developed principally in response to a rapid increase in valley incision rates associated with capture-induced base-level changes in the master drainage. This was facilitated by the weak sand and silt lithology of the study area. The landslide was a significant morphological modifier of the environment. Holocene drainage follows the original landslide morphology, whilst more recent gullying and piping exploits planes of weakness developed within the internal body of the landslide feature. In addition, the landslide removed much of the Pleistocene divide, facilitating future river capture.

Landslide susceptibility in the Río Aguas catchment, SE Spain

The definitions of hazard and risk in natural hazard studies are well established in the scientific literature. However, many examples of ‘landslide hazard assessment’ only identify the susceptibility of slopes to failure and make no statement on the frequency of occurrence that would be necessary for a complete hazard evaluation. In a research programme undertaken in SE Spain the issue of landslide susceptibility in a semi-arid, neotectonic environment was examined, with some attempt to evaluate the hazard. This work involved establishing the occurrence of landsliding within the 550 km2 Río Aguas catchment through remote sensing interpretation and field mapping. These data were compiled in an inventory containing the records of nearly 250 landslides that was analysed to establish the nature and extent of landslide susceptible situations. Within the catchment anticipated combinations of geological materials proved to be susceptible to failure, and relationships between landslide volume and travel angle were examined in relation to standard models. The highest incidence of contemporary landsliding appeared to be related to the proximity of a major river capture site, a geomorphological event that had been dated at 100 000 BP. This produced localized rapid incision, a ten fold increase in sediment removal and the creation of oversteepened slopes that were only recently degrading to their long-term angle of stability. In addition to contemporary landslides, field mapping identified anomalous geological structures that proved to be degraded erosional remnants of ancient landslides. Relating these remnants to the river terrace sequence in the region provided some control on the relative ages of these ancient or ‘fossil’ landslides. It was concluded that any assessment of landslide risk in the study area would need to take into account not only geological materials and the contemporary geomorphological environment but also the geomorphological history of the region.

Geomorphological Investigations for the Channel Tunnel Terminal and Portal

Extensive geomorphological investigations of the Channel Tunnel UK portal and terminal site were commissioned by TML during 1986/87, prior to the commencement of the detailed ground investigations. These studies were primarily concerned with identifying and delimiting the spatial extent of landforms, superficial materials and in particular the evidence for contemporary or relict slope instability. The investigations took the form of large-scale (1:500) geomorphological mapping of the proposed development area. This paper describes the results of the mapping and how the information was incorporated into the site investigation and subsequent construction programme. In addition, some conclusions are drawn about the geomorphological evolution of the landslides that had developed off the chalk escarpment in the study area.

Engineering geomorphological input to ground models: an approach based on Earth systems

The engineering geomorphological input to ground (‘geo’) models as part of standard site investigations tends to lack a systematic approach and is often restricted only to morphological mapping, the naming of landform features and usage of outdated morphoclimatic concepts. Ground models require more sophisticated geomorphological approaches and this is particularly apparent in regions such as Europe and North America where ground conditions can be strongly influenced by geomorphological processes linked to Quaternary or older environmental changes. Within this paper it is proposed that an ‘Earth system science’ approach underpinned by the ‘rock cycle’ can provide a framework for a systematic and more detailed geomorphological input into ground models. Such a framework lends itself to recognizing the fact that landscapes comprise multiple ‘fragments’ of process–response landforms, which, under contemporary conditions, may be: relict and inactive; relict but reactivated; subject to change once certain threshold events occur; or fully active. Using the rock cycle, a simple basis for landform categorization is outlined starting with a four-fold subdivision into structural, weathering, erosional and depositional landforms that allows the user to focus initially on the historical development of the landscape. Each category comprises a system of landforms that occur over standard geomorphological scales (pico-, nano-, micro-, meso-, macro- and mega-scale) as part of a process–response system. Macro-scale landforms are readily identifiable within a landscape, typically occurring as six principal terrain types: hills, ridges, mountains, plains, valleys and basins. These in turn comprise a series of meso-scale features that, once identified, can inform about the geomorphological processes that have resulted in their formation. The geomorphological input into ground models benefits primarily from recognition of these meso-scale landforms and the processes responsible for their formation, although smaller-scale landforms may have importance for specific engineering structures.

Landslide hazard mapping and risk assessment

Definitions Before landslide hazard mapping and risk assessment are reviewed, it is important to define terms and concepts closely in order to avoid the confusion and misuse that has occurred in some previously reported case histories. The most widely accepted and basic definitions in landslide studies are those provided by Varnes (1984). Natural Hazard: the probability of occurrence within a specified period of time and within a given area of a potentially damaging phenomenon. Vulnerability: the degree of loss to a given element or set of elements resulting from the occurrence of a natural phenomenon of a given magnitude. It is expressed on a scale from 0 (no damage) to 1 (total loss). Specific Risk: the expected degree of loss due to a particular natural phenomenon. It may be expressed by the product of Hazard and Vulnerability. Elements at Risk: the population, properties, economic activities, including public services, etc., at risk in a given area. Total Risk: the expected number of lives lost, persons injured, damage to property or disruption of economic activity due to a particular natural phenomenon. It is therefore the product of Specific Risk and Elements at Risk. These definitions have been expanded by the International Union of Geological Sciences (IUGS) Working Party on Landslides through its committee on Risk Assessment (IUGS 1997) but the main elements are essentially the same. Hazard, therefore, defines the potential to cause damage. With respect to landslides it is necessary to: identify the existence of a landslide or a potential slope failure; establish its size, depth, speed and travel distance; and estimate or calculate its frequency of movement or its probability of occurrence.

Feet on the ground: engineering geology past, present and future

Engineering geology has a long and rich heritage and the UK has been in the vanguard of the development of the subject as a distinct discipline, with the first book on the subject being published in London in 1880. Since then, engineering geology has been applied to projects around the world and engineering geologists have become core members of planning, investigation, design and construction teams in the civil engineering and mining industries. However, in the past few decades we have seen numerical analyses increasingly being accepted as the answer to all geotechnical design questions, although as engineering geologists we are used to dealing with natural materials and processes and recognize that their inherent variability cannot always be reduced to a simple numerical value. Consequently, how do we ensure that any proposed construction works in civil engineering or mining take full account of this variability and the uncertainties that result? To enable engineering geologists to understand and describe these uncertainties are there fundamental skills that define an engineering geologist and, if so, how can these skills be taught or acquired? Also, in a world dominated by readily accessible data that can be downloaded and analysed for so many planned development sites, how important are the field techniques of observation and mapping that an older generation of engineering geologists, including the author, considered their defining skill? Concentrating on the role of engineering geology in relation to civil engineering, these are amongst the questions explored in this paper, leading to observations as to how the profession might develop in the future in order to meet the needs of society.

The reactivation of a landslide during the construction of the Ok Ma tailings dam, Papua New Guinea

The Ok Ma dam (Ok is a local word meaning river) was to form part of the permanent tailings disposal system designed for the Ok Tedi gold and copper opencast mine located at Mount Fubilan in the Star Mountains of Papua New Guinea. During the construction works a landslide involving approximately 35 million cubic metres of soil and rock moved downslope to partially fill the foundation excavations of the dam. The failure of this landslide was the start of an insurance litigation that finally reached the Supreme Court of Papua New Guinea in November 1989. The geotechnical, geomorphological and engineering geological data that were available at the time of the failure are presented together with an evaluation of the key deductions from the case study.

Engineering geological significance of relict periglacial activity in South and East Devon

Quaternary glacial ice barely impinged on SW England with the maximum limits of the various ice advances now being reasonably well established. In SW England a probable Anglian ice front was located in Barnstaple Bay that is likely to have occupied some of the existing land surface in the area. Elsewhere the ice front does not appear to have got beyond the north coast of Devon and Cornwall, although a till has been identified on the Isles of Scilly and a possible glacial corrie has been described on Exmoor. Beyond these limits, however, the SW region will have been subject to repeated extensive and intensive periglacial activity. The effects of periglaciation will not have been removed by any subsequent ice advances, thus the region provides an excellent area to evaluate the importance of periglacial activity to engineering geology. Information on the nature and extent of periglaciation is mainly limited to detailed studies carried out in specific areas. However, the Quaternary history of SW England has recently been summarized and these summaries refer to the extent of periglacial processes. In this paper, the nature and engineering geological implications of periglacial processes and deposits from four contrasting areas of south and east Devon are discussed.

Seeking the research frontiers for UK engineering geology

Anumber of weaknesses have previously been identified that could curtail future development of engineering geology as an independent subject. In the UK one way to guarantee the continued vigour of the subject and provide a continual stream of new recruits into the field will be to ensure that engineering geology has a thriving research culture. At present, the UK research councils provide little support for research in engineering geology, partly because the potential areas of research either are ill-defined or have now been taken over by other disciplines, such as soil and/or rock mechanics. However, many areas where research is needed in the subject can be identified. The concern is that unless engineering geology is willing to push forward its own research frontiers, the very real possibility exists that the discipline will be subsumed as a minor branch of geotechnical engineering.

Engineering geomorphology: A UKuk perspective

Engineering geomorphology has developed at a rather disappointingly slow rate in the UK since publication of the earlier applications in the 1970’s. Among the more notable contributions of geomorphological techniques and expertise to civil engineering have been the various landslide mapping surveys carried out for road construction projects overseas and in the UK. However, while geomorphological mapping has proved to be a useful tool for landslide assessment and the planning of geotechnical ground investigations there are four main constraints that currently limit the professional application of engineering geomorphology to civil engineering practice. First, the subject has not received universal acceptance by civil engineers who often see it as too academic and not directly applicable to engineering design. Second, when geomorphological mapping has been used it has often been applied at the beginning of a project and frequently continual geomorphological interpretation is not allowed to take place as the geotechnical ground investigation and design take place. Third the technique of geomorphological mapping is the most familiar geomorphological tool known to the civil engineer and the valid and often cost-effective contributions that geomorphologists can make to other civil engineering studies using different techniques are frequently not recognised. Finally, so-called engineering geomorphologists trained in the UK often do not have sufficient knowledge of engineering design criteria to be gainfully employed on civil engineering projects.Some of these problems may be resolved by a reconsideration of the training given to potential engineering geomorphologists in the UK, possibly by the British Geomorphological Research Group and the Engineering Group of the Geological Society, and a greater awareness among civil engineers of the full potential that engineering geomorphology can offer.RésuméIl est assez décevant de constater l’assez faible développement des utilisations de la géomorphologie en génie civil depuis ses premières applications au début des années 1970. Parmi les contributions les plus notables des techniques géomorphologiques au Génie Civil figurent les cartographies de glissements de terrain réalisées au Royaume Uni et Outre-Mer pour des projets routiers. Cependant bien que la cartographic géomorphologique ait fait la preuve qu’elle était un outil utile pour l’étude des glissements de terrain et la planification des études géotechniques, son développement est limité par quatre contraintes principales. En premier lieu, tous les ingénieurs civils ne sont pas convaincus de son intérêt, le jugeant souvent trop théorique et non applicable directement à la conception d’ouvrages. En second lieu, lorsque la cartographie géomorphologique a été utilisée, c’est souvent au début des études et en général le suivi des interprétation géomorphologiques n’a pas été rendu possible au stade des études géotechniques et de la conception même du projet. En troisième lieu, la technique de la cartographie géomorphologique est l’outil géomorphologique le mieux connu de l’ingénieur civil et les contributions valables et souvent rentables que les géomorpholoques pourraient apporter à d’autres études de génie civil en utilisant d’autres techniques que la cartographie ne sont généralement pas reconnues. Enfin, les géomorphologues dits «apliqués» formés au Royaume Uni ont souvent une connaissance trop insuffisante des critères techniques propres aux projets du Génie Civil pour être utilisés avec profit dans le cadre des études de projets.Certains de ces problèmes peuvent être résolus en réexaminant la formation donnée à ces géomorphologue au Royaume Uni, peu être en liaison avec le groupe Britannique de Recherche Géomorphologique et le Groupe de Géologie de l’Ingénieur de la Société Géologique de Londres, et en faisant mieux connaître aux ingénieurs civils l’ensemble des apports que peut offrir la géomorphologie appliquée.