Edward N. Bromhead

Volcanic rock-mass properties from Snowdonia and Tenerife: implications for volcano edifice strength

Although volcano instability is increasingly recognized as a societal hazard, numerical data on the relevant mechanical properties remain sparse. We report new field data on the rock-mass properties of volcanic materials from Snowdonia (North Wales, UK) and Tenerife (Canary Islands). Using rock types that range in composition from phonolite to rhyolite, we summarize a method for estimating the overall strength of a volcanic edifice based on the rock-mass rating index (RMR) and the Hoek–Brown criterion. We show that the average rock-mass compressive (σcm) and cohesive (c) strengths decrease exponentially with RMR according to σcm=0.652(0.0559RMR) and c=0.035(0.0669RMR), respectively, and appear insensitive to both initial magma composition and relative age. This exponential relationship provides a new predictive tool for directly estimating rock-mass strength from the RMR. Our analysis further predicts a marked reduction of up to 96% in the rock-mass compressive strength relative to the intact rock value based on laboratory tests and that, overall, the combined results from both study areas yield cohesive strength values from 4.8 to 0.44 MPa. Estimated values of rock-mass angle of friction range from 28° to c. 38°. Recent modifications to the Hoek–Brown criterion, in particular the inclusion of the disturbance factor D, suggest that even these low values of rock-mass strength and cohesion may be optimistic, and the true values of rock parameters relevant for accurate predictions of volcano edifice strength may be up to 30% weaker still.

Landsliding in London Clay coastal cliffs

Factors controlling rotational landsliding in the unprotected London Clay coastal cliffs atWarden Point, Isle of Sheppey, Kent, aredescribed. Surveys of the landslides have been carried out at regular intervals since1971, and these are supported by an extensive subsurface investigation that included the installation of 56 piezometers. These studies have resulted in an understanding of the pore water pressures in the cliffs, including the role of undrained unloading resulting from slope formation, and pore pressures within the landslide mass ‘carried down’ the slope. The zone of depressed pore pressures is carried inland with the slope as the coastline retreats. Factors controlling the shear strength mobilized during first-time slides, and rates of movement, include geological control on the position of the slip surface and reduction in shear strength to close to the residual value along the basal section of the slip surface. This strength reduction results from stress relief controlled pre-failure deformations. A simple model is proposed to allow calculation of pore water pressures associated with the degrading slide mass. These have enabled back-analyses to be carried out which extend the residual shear strength envelope for London Clay to high effective normal stress levels, and explain the rate of re-activated slide movements.

Three-dimensional stability analysis of a coastal landslide at Hanover Point, Isle of Wight

The applicability of 3-D slope stability analysis is discussed in the first part of the paper. A conclusion is drawn that 2-D analysis is often deficient in one or more respects, and often fails to model properly the true mechanics of landslides. The paper describes the application of 3-D slope stability analysis using Hungrs Method of Columns to a small but active landslide in Wealden strata at Hanover Point on the SW coast of the Isle of Wight. The basal slide surface of this very small landslide follows a bedding plane. This bedding plane has components of dip both across the landslide and also into the slope. The shape of the slide mass is affected by the shape of the basal shear, but as the slide is also clearly prevented from moving in the direction of steepest dip (inland) by the confining effect of the embayment within which it is developing, the slide is a strong candidate for 3-D analysis. Systematic analyses of the slide have been made using the Method of Columns, which is essentially a 3-D variant of the 2-D Method of Slices. By changing the assumed movement direction by a small amount between analyses, and covering all possibilities, it is demonstrated that there is a vector direction of movement that exhibits the least Factor of Safety. This vector direction is comparable to the actual movement direction observed in the field. The paper draws conclusions as to the applicability of 3-D methods of analysis, and also concerning the evolution of bedding controlled landslides with dips into or out of the slope, when subject to strong marine attack and cliff retreat.

Reflections on the residual strength of clay soils, with special reference to bedding-controlled landslides

This Glossop Lecture is about landslides and their slip surfaces at residual strength in clays. Particularly in southern England, but also elsewhere, landslides in infrastructure cuttings and many natural slopes are commonly found to be slowly moving compound landslides with a component of their basal shear surfaces following a particular bed (or ‘slide-prone horizon’). A selection of both historical and modern case records of this type of landslide are presented briefly. The geotechnical conditions that give rise to this occurrence are discussed, and the dominant factor relates to the dip of the strata, which must be of low inclination for the landslide mass to remain in place over the critical clay bed in the geological sequence after sliding has been initiated. Observations of the slip surfaces in the field lead to the conclusion that the bedding-controlled elements of this type of landslide develop along thin, slide-prone or slide-susceptible, horizons in the bedding. The question of what caused the formation of those horizons in the first place is answered by putting forward two hypotheses to explain why bedding-controlled slip surfaces form where they do, and considering the evidence for or against each of them. The conclusion is reached that despite the attractiveness of the concept that these slip surfaces form by a progressive failure mechanism at the junction of two materials with dissimilar properties, the alternative concept that they occur where there is a bed of slightly enhanced smectite content better fits the observations. The mechanisms for such local changes in clay mineralogy are linked to inputs of volcanic ash at the time of deposition. Definitive proof of concept is, however, lacking, but taking into account how clay sediments are deposited in sedimentary basins, this paper makes suggestions for future lines of enquiry. Even now, nearly a half-century after Skempton’s seminal Rankine Lecture that introduced the concept of residual strength of clays to the wider geotechnical profession, the corpus of data is rather limited. Some of the datasets are shown to exhibit remarkable similarities, and the implications of this tend to support the preferred explanation of the origin of slide-prone horizons.

Cultural Heritage and Landslides: Research for Risk Prevention and Conservation

The impact of natural hazards on our cultural heritage represents an important theme, involving a multi-disciplinary approach. In case of landsliding, engineering geologists can play a key role, through the identification of relationships between soil and structures. This chapter starts from the large perspective of UNESCO Convention (1972), as a bird’s-eye view of the general problem of heritage conservation, arriving at the presentation of a series of case histories from different countries. This varied approaches to the problem of landslides and cultural heritage reflects the multitudes of interests associated with this topics.

An Overview of Landslide Problems in the British Isles, with Reference to Geology, Geography and Conservation

An overview is presented of the landslide problems experienced in Britain, and their primary causes. Principally, landslides occur in Britain where the strata are argillaceous and there is sufficient topographic relief. This combination occurs in swathes through the southern and central parts of Britain and along the lengthy coastline. In these parts of Britain, the main rock types are sedimentary, and they often exhibit low angles of dip. This gives rise to the occurrence of compound landslides, often with a bedding-controlled flat basal shear surface. Issues relating to this type of landslide are discussed in the article, which concludes with a discussion of the conflicts between interests of land users and the need for conservation, especially in some areas that are of significant interest in the history of the geological and other sciences, and to research in the present day.

The Vaiont landslide : re-assessment of the evidence leads to rejection of the consensus

There appears to be a clear general consensus in the literature regarding four critical issues that define the problem of the October 1963 Vaiont landslide and its behaviour that are central to the disaster: (1) the 1963 failure was a reactivation of an ancient landslide; (2) failure took place along thin clay seams (already at residual strength); (3) the sliding surface had a ‘chair’ shape with a (sub)horizontal base; and (4) failure was triggered by inundation of the toe of the slide mass by rising reservoir levels. The key to understanding the Vaiont landslide is the failure surface geometry, which was controlled by the structural geology. It now appears that the so-called chair structure (that was assumed to define the shape of the failure surface) does not exist, and without it, the first consensual point is untenable, and the fourth may not contain the whole truth. We have systematically re-examined the published evidence and undertaken our own new research in order to test the logical and geotechnical validity of the four elements of the consensus. Glacial processes can account for the pre-failure morphology of the landslide site; the clay seams must therefore have been at peak shear strength as there was no ancient landslide. Tectonic processes can account for the failure surface geometry, which does not have a ‘chair’ shape, as well as small-scale structures; and rainfall appears to have been an essential element in the initiation and development of the landslide. Our findings largely contradict the consensus position and thus form the basis of a new overarching hypothesis for the landslide that should account for all of the observed and known features, events and data.

New, simplified and improved interpretation of the Vaiont landslide mechanics

Both the occurrence and behaviour of the Vaiont landslide have not been satisfactorily explained previously because of difficulties arising from the assumption that the failure surface was ‘chair’ shaped. It is now known that there was no ‘chair’, which means that the 1963 landslide could not have been a reactivated ancient landslide because the residual strength of the clay interbeds would have been insufficient for stability prior to 1963. Furthermore, the moderately translational geometry reduces the influence of reservoir-induced groundwater and hence of submergence. Standard stability analyses now show that prior to 1960, the average shear strength must have significantly exceeded the peak shear strength of the clay interbeds known to have formed the majority of the failure surface. Three-dimensional stability analyses confirm these results and show that at the time of the first significant movements in 1960, the rising reservoir level had a negligible effect on the Factor of Safety. According to these results, the Vaiont landslide was most likely initiated by pore water pressures associated with transient rainfall-induced ‘perched’ groundwater above the clay layers, in combination with a smaller than hitherto assumed effect of reservoir impounding, then developed by brittle crack propagation within the clay beds, thus displaying progressive failure. Further, very heavy rainfall accelerated the process, possibly due to reservoir-induced groundwater impeding drainage of the rainwater, until the limestone beds at the northeast margin failed. With the shear strength suddenly reduced to residual throughout, the entire mass was released and was able to accelerate as observed.

The ruined town of Campomaggiore Vecchio, Basilicata, Italy

Abstract This paper describes the geological and geomorphological setting of the ruined town of Campomaggiore Vecchio, abandoned by its inhabitants following a large displacement landslide a little over a hundred years ago. The reasons for the original landslide, and future prospects for the historical ruins, are discussed in the following text, and the photographs show clearly the effects both of the original slide and the depredations of over a century of abandonment.

Introduction to the Stone Cycle and the Conservation of Historic Buildings

This Thematic Set of papers relating to the life cycle of building stone was initiated by a call for papers in order to better recognize the contribution that the disciplines, and practitioners, of engineering geology and hydrogeology make to the conservation of historical buildings, which is intrinsically multidisciplinary. The call for papers particularly focused upon the issues of different stone types used in historical buildings, as well as the performance, durability and conservation of stone in historical settings. The response was overwhelming, with many more abstracts submitted than could possibly be published in the Quarterly Journal of Engineering Geology and Hydrogeology ( QJEGH ). Accordingly, the papers were divided into two sets, with one set destined to appear in QJEGH as described herein and the second set to appear in a Geological Society Special Publication (Cassar et al. 2014). The presence of a particular paper in one set or the other is not a reflection on quality, but merely a reflection of the need to divide the papers into two sets each of which reflects subtly differing themes. History has been written in stone, from prehistoric monuments to modern-day buildings, and all types of stone, limestones and sandstones, granites and marbles, have been utilized to build, to clad, and to decorate. The buildings that are symbols of a city, a region, or a country are mostly built of stone. We immediately think of England when we see an image of Stonehenge; the Acropolis symbolizes Athens; the Coliseum Rome; Machu Picchu Peru; and the Taj Mahal India. The immense varieties and diverse properties of building stone, its workability and its (im)permanence have been observed and studied since time immemorial. Vitruvius, in the first century BC, writing in the Ten Books on Architecture , Chapter VII (Stone), says the following: ‘The stone in quarries …