Rock Face Research Articles

A rigorous multiscale random field approach to generate large scale rough rock surfaces

Estimating the shear strength of large in situ rock discontinuities is non-trivial because of the multiscale nature of roughness and the fact that only a very limited extent of discontinuity morphology is visible along traces. Recently, a novel stochastic method based on random field theory and Monte Carlo semi-analytical estimation of shear strength was proposed. The method was validated at laboratory scale and its application to one large natural surface showed that it has the potential to bypass the scale effect. However, a critical issue was reported by the authors of the study: it was found that the random field approach used could not generate the correct distribution of gradients on the simulated surfaces, which translates into an inaccurate prediction of shear strength. The authors had to manually adjust the input of the model to achieve a satisfactory prediction. This paper presents a new multiscale approach using random field theory, which now allows a rigorous generation of large synthetic 3D surfaces with controlled distribution of asperity heights and gradients from the profile of a 2D fracture trace (as might be visible in a rock face) referred to as a seed trace. Each seed trace is first decomposed into three daughter profiles corresponding to three levels of roughness. The statistics of each daughter profile form the input of the random field model at each scale level, allowing synthetic daughter surfaces to be created at each scale level. The synthetic daughter surfaces are then superimposed to obtain a composite rough surface comprising three levels of roughness. The approach was successfully validated with 25 input seed traces, coming from 5 different natural surfaces. This rigorous multiscale approach is essential to apply the stochastic method that was recently developed to predict the shear strength of large in situ discontinuities.

An Infrared Thermography Approach to Evaluate the Strength of a Rock Cliff

The mechanical strength is a fundamental characteristic of rock masses that can be empirically related to a number of properties and to the likelihood of instability phenomena. Direct field acquisition of mechanical information on tall cliffs, however, is challenging, particularly in coastal and alpine environments. Here, we propose a method to evaluate the compressive strength of rock blocks by monitoring their thermal behaviour over a 24-h period by infrared thermography. Using a drone-mounted thermal camera and a Schmidt (rebound) hammer, we surveyed granitoid and aphanitic blocks in a coastal cliff in south-east Sardinia, Italy. We observed a strong correlation between a simple cooling index, evaluated in the hours succeeding the temperature peak, and strength values estimated from rebound hammer test results. We also noticed different heating-cooling patterns in relation to the nature and structure of the rock blocks and to the size of the fractures. Although further validation is warranted in different morpho-lithological settings, we believe the proposed method may prove a valid tool for the characterisation of non-directly accessible rock faces, and may serve as a basis for the formulation, calibration, and validation of thermo-hydro-mechanical constitutive models.

A fully automatic-image-based approach to quantifying the geological strength index of underground rock mass

The conventional discontinuity survey process in the mining industry is known to be a time-consuming one and it is technically challenging due to the limited accessibility of fresh rock exposures. A rapid and robust rock mass property quantification system is desirable for rock structure design during mining operations. In this work we develop an image-based and fully automatic rock mass Geological Strength Index (GSI) rating system. The proposed method involves a series of novel algorithms to quantify the GSI rating based on data recovered from digital images. The proposed GSI system includes both Structure Rating (SR) and Joint Condition Digital Imaging (JCDI) to represent the bulk rock and discontinuity surface conditions of the rock mass. Local histogram equalization and self-adaptive gamma correction were introduced into the pre-processing of the rock face images. Compared to conventional histogram equalization, local histogram equalization can effectively restrain the uneven distribution of brightness often present. Self-adaptive gamma correction based on the image properties automatically enlarges the difference between discontinuity areas and intact rock. A novel algorithm combining region growing and the Hough transform is proposed for the automatic extraction of discontinuity areas. Laboratory and field tests demonstrated that the algorithm possesses an advantage over existing methods with regard to better noise suppression and that it can yield reasonable results for images taken in poor photography conditions. Discontinuities were characterized using a novel algorithm comprising area thinning, skeleton linking, spurs removal, and sampling. Using this algorithm, four parameters of the discontinuity can be quantified: length, orientation, separation width, and JRC value. The proposed approach was validated by two field-based case studies.

Seismic Active Earth Pressure on Narrow Backfill Retaining Walls Considering Strain Localization

The evaluation of seismic active earth pressure acting on narrow backfilled retaining walls is more essential during earthquake loading to ascertain safety and economical design. This paper proposes an analytical procedure to evaluate seismic active earth pressure for narrow backfill width retaining walls using the limit equilibrium method under earthquake loading. The existence of rock behind the retaining wall that affects the size and shape of the failure wedge in narrow backfill soil is considered. The study also accounts for the strain softening behavior under earthquake loading by considering the change in shear strength due to the reduction in friction angle from peak to residual along the bilinear failure slip surfaces in backfill soil. The formation of multiple failure surfaces due to multiple failure wedges in the narrow backfill soil is also considered. The proposed values of seismic active earth pressure are validated by comparing the obtained results with the earlier studies. The influence of backfill soil friction angle, dilatancy angle, interface friction angle between the retaining wall and backfill soil, interface friction angle between the rock face and backfill soil, backfill slope angle, rock-face slope angle, and the distance of constraint rock face boundary from retaining wall on the magnitude of seismic active earth pressure and its point of application are presented.

Automated structural discontinuity mapping in a rock face occluded by vegetation using mobile laser scanning

Abstract Mapping of structures, such as discontinuities, in an exposed rock mass is fundamental for slope stability analysis. This study investigates mobile laser scanning technology to identify structural discontinuity in a complex environment where the exposed rock face is partially covered with vegetation. The conventional terrestrial laser scanning and photogrammetry based approaches for structure identification rely solely on coplanarity criteria or point normal vectors and often miss several prominent discontinuity planes in a complex environment with inherent noise. To enhance structural mapping in such environments, this study tests a mobile scanner which has multi-view data collection to reduce blind spots. A new automated and robust algorithm termed clustering on local point descriptors (CLPD) is developed for more accurate discontinuity identification. The algorithm involves a rigorous pre-processing step to remove erroneous and irrelevant points followed by computation of local point descriptors. Five descriptors (i.e. eigenvalue descriptor (EVD), radial surface descriptor (RSD), fast point feature histogram (FPFH), normal and curvature) were generated for each point to capture spatial distribution, geometrical relationships and local surface variations in the point cloud. Finally, a K-Medoids clustering was performed on the computed descriptors using a histogram of normals to identify discontinuity planes. Results indicate that the proposed CLPD algorithm outperforms existing approaches in terms of accuracy in discontinuity orientation estimate (3.50° in dip angle and 4.32° in dip direction) for the study site. The stereonet comparison also validated that the poles distribution from CLPD is on par with the ground truth as well as results from commercially available software. The study presents a comparative evaluation of various approaches for kinematic feasibility of planar, wedge and sliding failures.

Geological evolution process, mechanism, and application of protective layer in semi-coal and rock face

Under the influence of geological structure, the surrounding rock of semi-coal seam is easy to be loose and fractured, and the repairing rate is high. Therefore, it is of great theoretical and practical significance to study semi-coal seam mining's mechanism and control technology. In this paper, the geological evolution principle of the semi-coal seam is analyzed theoretically. The mechanic's relationship between the mining height of the semi-coal face and the lower coal seam's pressure relief is interpreted. The mechanism of eliminating bad blind areas in non-pillar mining of upper protective seam is analyzed. Through numerical simulation analysis, it is concluded that the expansion deformation rate of a protective layer increases linearly with the increase of mining thickness of the protective layer, and the width of undesirable blind zone of coal seam increases linearly with the rise of protective layer thickness. This paper puts forward the technology of non-pillar mining in the protective seam's semi-coal and rock face. Field industrial test results show that the technology achieves the overall pressure relief and reflection reduction of low permeability and high gas coal seams. The underlying 3 + 4 protective layer achieves the general pressure relief and reflection reduction of low permeability and high gas coal seams. Coalbed methane can be pre-drained up to 18 m3/min with a concentration of 90%.

Evaluating rock mass disturbance within open-pit excavations using seismic methods: A case study from the Hinkley Point C nuclear power station

Understanding rock strength is essential when undertaking major excavation projects, as accurate assessments ensure both safe and cost-effective engineered slopes. Balancing the cost-safety trade-off becomes more imperative during the construction of critical infrastructure such as nuclear power stations, where key components are built within relatively deep excavations. Designing these engineered slopes is reliant on rock strength models, which are generally parameterised using estimates of rock properties (e.g. unconfined compressive strength, rock disturbance) measured prior to the commencement of works. However, the physical process of excavation weakens the remaining rock mass. Therefore, the model also requires an adjustment for the anticipated rock disturbance. In practice, this parameter is difficult to quantify and as a result it is often poorly constrained. This can have a significant impact on the final design and cost of excavation. We present results from passive and active seismic surveys, which image the extent and degree of disturbance within recently excavated slopes at the construction site of Hinkley Point C nuclear power station. Results from active seismic surveys indicate that the disturbance is primarily confined to 0.5 m from the excavated face. In conjunction, passive monitoring is used to detected seismic events corresponding to fracturing on the cm-scale and event locations are in agreement with 0.5 m of disturbance into the rock face. This suggests rock disturbance at this site is relatively low and occurred during and immediately after the excavation. A ratio of seismic velocities recorded before and after excavations are used to determine the disturbance parameter required for the Hoek–Brown rock failure criterion, and we assess that rock disturbance is low with the magnitude of the disturbance diminishing more quickly than expected into the excavated slope. Seismic methods provide a low-cost and quick method to assess excavation related rock mass disturbance, which can lead to cost reductions in large excavation projects.

Geomechanical Characterization of a Rock Cliff Hosting a Cultural Heritage through Ground and UAV Rock Mass Surveys for Its Sustainable Fruition

In mountainous areas around the world, dealing with rockfalls means facing some technical survey difficulties due to the low accessibility of areas and the height of slopes. If a cultural heritage is also threatened by such mass movement, the need of specific survey solutions, even in a combined asset, is required. This paper deals with the integration of ground and UAV rock mass surveys aimed at defining the rockfall attitude of an unstable rock cliff sector hosting an example of cultural heritage in tourist area of southern Italy, whose fruition has already been threatened by the occurrence of rockfalls. As an example of the defensive architecture of XII and XIII centuries, the Saracen Castle in Taormina is reached by hundreds of visitors each year, but its access path and the surrounding area are threatened by the unstable condition of the cliff, hosting unstable rock volumes on kinematically critical planes. In order to achieve a reliable geostructural setting of the cliff, aiming at its possible securing through proper mitigation works, ground rock mass surveys could not provide enough information due to the bad accessibility of the rock faces. Therefore, a survey by an Unmanned Aerial Vehicle, with a reliable verified accuracy, was carried out to map the discontinuity planes especially occurring at the highest portions of the cliff, achieving geostructural data of different fronts of the cliff. Ground and aerial data were combined and statistically analyzed to define the main kinematic failure patterns. In this perspective, a critical comparison between the two employed surveying methodologies is proposed herein, highlighting that both approaches are affected by potential and limitations and that the integration of the mutual dataset represents a suitable solution for a complete rock mass characterization in this type of areas. Furthermore, rockfall simulations allowed ascertaining that potential falling blocks would cross the access path to the castle, thus representing a natural threat to the fruition of cultural heritage, thus proving the need of mitigation measures to ensure the safe fruition of the cultural heritage.

Reclassification of Microseismic Events through Hypocenter Location: Case Study on an Unstable Rock Face in Northern Italy

Passive seismic methods are increasingly used in monitoring unstable rock slopes that are likely to cause rockfalls. Event classification is a basic step in microseismic monitoring. However, the classification of events generated by the propagation of fractures and rockfalls is still uncertain due to their similar features in the time and frequency domains. Hypocenter localization might be a powerful tool to distinguish events generated by fracture propagation from those caused by rockfalls. In this study, a classification procedure based on hypocenter location was validated using a selected subset of high-quality data recorded by a five-geophone network installed on a steep rock slope in Northern Italy. Considering the complexity and heterogeneity of the rock mass, a 3D velocity model that was derived from a tomographic experiment was used. We performed the localization using the equal differential time method. The location results fairly fit our expectations on suspected rockfall events because most signals were located near the rock face. However, only 4 out of 20 suspected fracture events were unquestionably confirmed as fractures being located inside the rock mass and far enough from the rock face. Further improvements in location accuracy are still necessary to distinguish suspected fracture events located close to the rock face from rockfalls. This study demonstrates that hypocenter location is a promising method to improve the final classification of microseismic events.

Corrected Rock Fracture Parameters and Other Empirical Considerations for the Rock Mechanics of Rock Masses of Doha, Qatar

The objective of this paper is to provide insights into the intact rock and rock mass properties of the rock formations under the city of Doha, State of Qatar. We also intend to scientifically clarify these properties by presenting and statistically characterizing the ranges of the parameters, and by discussing the correlations between the parameters with respect to their usage and research potential. The rock quality designation corrected (RQDC) parameter is validated and a new parameter, the fracture index corrected (FIC), is proposed. The significantly improved correlation between RQDC and FIC is demonstrated and their derivation is explained. The paper demonstrates the correlation between the rock mass estimation parameters obtained through rock face mapping and discusses the applicability of the Hoek–Brown criterion to the studied rock masses, which is found to be relevant. A discussion about how properly performed triaxial tests can directly provide the rock constant mi values for all geological members is presented. Other estimation approaches for mi are also validated and compared with the existing knowledge base. Data for laboratory and field intact rock and rock mass parameters are combined using equations from various authors to obtain narrow ranges for rock mass strength and rock mass elasticity modulus values. Finally, within the framework of previous studies by other authors on the low-end transition range of rocks toward soils, it is shown that only the Rus formation member is sufficiently soft and can be included in the range.

Use of the single rope access technique reveals habitat use patterns for the elusive Erebia christi (Lepidoptera: Nymphalidae)

In this study we apply a new monitoring technique for butterflies in order to get more insights into the habitat use patterns of Erebia christi Ratzer 1890 (Nymphalidae: Satyrinae). This European endemic is a rare and very localized butterfly species, found alongside steep slopes in a restricted area in the Alps, at the border between Italy and Switzerland. Even though it was discovered almost 140 years ago, captivating the interest of many lepidopterists, there are still several aspects about the biology of E. christi that are not fully known. We selected two monitoring sites in Veglia–Devero Natural Park, Italy and tested whether E. christi was more likely to use rocky, almost vertical slopes than other types of habitat. Since this habitat was almost inaccessible to humans, we used the single rope access technique, in which field operators rappelled down the slopes to explore the rock faces. We demonstrate that the main characteristics of the typical habitat of E. christi are indeed very steep rock faces with grassy ledges where Festuca sp. plants are present. We also show that, E. christi is more abundant than it was previously thought, with populations that could probably be structured in a metapopulation system. Long-term monitoring with the rope access technique could represent an optimal method to provide key insights into the biology and ecology not only of E. christi but of other butterflies that use similar habitats. Finally, we propose the upgrading of its IUCN category from Vulnerable to Endangered at European level. Our findings demonstrate how appropriate, long-term monitoring can contribute to improve the scarce knowledge of the ecology of an elusive species, and to devise informed proposals regarding the management and protection of populations, especially for species of conservation concern.

The three-stage rock failure dynamics of the Drus (Mont Blanc massif, France) since the June 2005 large event

Since the end of the Little Ice Age, the west face of the Drus (Mont Blanc massif, France) has been affected by a retrogressive erosion dynamic marked by large rockfall events. From the 1950s onwards, the rock failure frequency gradually increased until the large rockfall event (292,680 m3) of June 2005, which made the Bonatti Pillar disappear. Aiming to characterize the rock failure activity following this major event, which may be related to permafrost warming, the granitic rock face was scanned each autumn between October 2005 and September 2016 using medium- and long-range terrestrial laser scanners. All the point clouds were successively compared to establish a rockfall source inventory and determine a volume-frequency relationship. Eleven years of monitoring revealed a phase of rock failure activity decay until September 2008, a destabilization phase between September 2008 and November 2011, and a new phase of rock failure activity decay from November 2011 to September 2016. The destabilization phase was marked by three major rockfall events covering a total volume of 61,494 m3, resulting in the progressive collapse of a new pillar located in the northern part of the June 2005 rockfall scar. In the same way as for the Bonatti Pillar, rock failure instability propagated upward with increasing volumes. In addition to these major events, 304 rockfall sources ranging from 0.002 to 476 m3 were detected between 2005 and 2016. The temporal evolution of rock failure activity reveals that after a major event, the number of rockfall sources and the eroded volume both follow a rapid decrease. The rock failure activity is characterized by an exponential decay during the period following the major event and by a power-law decay for the eroded volume. The power law describing the distribution of the source volumes detected between 2005 and 2016 indicates an exponent of 0.48 and an average rock failure activity larger of more than six events larger than 1 m3 per year. Over the 1905–2016 period, a total of 426,611 m3 of rock collapsed from the Drus west face, indicating a very high rock wall retreat rate of 14.4 mm year−1 over a surface of 266,700 m2. Averaged over a time window of 1000 years, the long-term retreat rate derived from the frequency density integration of rock failure volumes is 2.9 mm year−1. Despite difficulty in accessing and monitoring the site, our study demonstrates that long-term surveys of high-elevation rock faces are possible and provide valuable information that helps improve our understanding of landscape evolution in mountainous settings subject to permafrost warming.

External Stability Analysis of Narrow Backfilled Gravity Retaining Walls

A three wedge failure mechanism is proposed to determine active earth pressure exerted by narrow backfills on gravity retaining walls and its point of application. Gravity retaining wall proportions are optimized against external failure modes using proposed active earth pressure and its line of action of resultant thrust. Overturning failure, eccentricity failure of the resultant force, sliding failure on its base, and bearing capacity failure are given due consideration for the determination of optimum proportions of gravity wall. Optimum dimensions of the gravity wall are evaluated such that factors of safety against overturning, eccentricity, sliding, and bearing capacity should be greater than 2.0, 1.0, 1.5, and 2.5 respectively. Significant reduction in active earth pressure in narrow backfill width gravity retaining walls is noticed due to the inadequate development of shear bands because of the constraint of rock face boundary. The study recommends employing an appropriate failure mechanism for the development of shear bands behind retaining wall based on depth and width of narrow backfill. The proposed study indicates that significant savings in the cross sectional area may be accomplished as compared to a classical design by Coulomb’s theory.

A leap of faith.

I spent a lot of Free Solo peeking through my fingers, waiting for disaster to strike. I was on a flight to visit family and had decided to watch the documentary about climber Alex Honnold's extraordinary, rope-free ascent of the 900-meter sheer cliff face of El Capitan. I have no head for heights, and maneuvering up a rock face is the last thing you'd find me doing. Yet one moment gave me an odd jolt of recognition. In it, Honnold has to let go of one set of handholds, step out over the void, and land on the next hold—just knowing that it will work out. It's a true leap of faith that made me think of the step into the unknown I had just made in my own career.

Onset dynamics of air-water menisci on rock fracture surfaces

ABSTRACT Information on menisci dynamics and equilibrium contact angles, θe, is needed for modeling multiphase flow of fluids in geologic formations. The wettability of fracture faces is relevant to applications such as waterflooding in enhanced oil recovery and evaluation of caprock integrity for geologic carbon sequestration. We investigated the vertical capillary rise of air-water menisci on exposed fracture faces for a sandstone, a shale, and three granites. The experimental design approximated that employed in the Wilhelmy plate method. Replicate fracture faces were prepared by fracturing cylindrical rock cores using the Brazilian method and splitting the fractured cores apart along their fracture planes. Meniscus onset was imaged from a cross-sectional perspective using dynamic neutron radiography, and quantified with change point analysis. An existing model for meniscus onset fitted the experimental data very well (median R2 = 0.96). Capillary rise was typified by t 2 3 scaling (where t is time), followed by a constant equilibrium height, ze. The average time taken for the menisci to reach ze was 0.39 s, with no differences between rock types. The menisci achieved a greater vertical extent on the fracture faces of the sedimentary rocks as compared to those of the granites. Apparent θe values were calculated from the best fit estimates of ze using the Wilhelmy plate equation for a smooth and planar vertical surface. Despite the fractal roughness of some of the fracture faces, there was a significant linear relationship (R2 = 0.80) between the apparent θe values and intrinsic θe values measured on polished surfaces of the same rocks using the sessile drop method. The regression equation indicated the apparent θe values were consistently greater than the intrinsic values. This overestimation may be due to averaging of pixels at the air-water-solid interface, a required step in the image analysis. Further exploration of the range of applicability and measurement accuracy of this proposed new technique would be valuable.

Tomographic Experiments for Defining the 3D Velocity Model of an Unstable Rock Slope to Support Microseismic Event Interpretation

To monitor the stability of a mountain slope in northern Italy, microseismic monitoring technique has been used since 2013. Locating microseismic events is a basic step of this technique. We performed a seismic tomographic survey on the mountain surface above the rock face to obtain a reliable velocity distribution in the rock mass for the localization procedure. Seismic travel-time inversion showed high heterogeneity of the rock mass with strong contrast in velocity distribution. Low velocities were found at shallow depth on the top of the rock cliff and intermediate velocities were observed in the most critical area of the rock face corresponding to a partially detached pillar. Using the 3D velocity model obtained from inversion, localization tests were performed based on the Equal Differential Time (EDT) localization method. The results showed hypocenter misfits to be around 15 m for the five geophones of the microseismic network and the error was significantly decreased compared to the results produced by a constant velocity model. Although the localization errors are relatively large, the accuracy is sufficient to distinguish microseismic events occurring in the most critical zone of the monitored rock mass from microseismic events generated far away. Thus, the 3D velocity model will be used in future studies to improve the classification of the recorded events.

SEMANTIC CLASSIFICATION OF SANDSTONE LANDSCAPE POINT CLOUD BASED ON NEIGHBOURHOOD FEATURES

Abstract. The technology of airborne laser scanning enables fast and accurate gathering spatial data containing also echoes from the terrain below the vegetation canopy that is beneficial for topographic mapping of wooded sandstone landscapes in Czechia, Poland, and Germany. The challengeable task is to determine the ground points in the point cloud because commonly used filtration methods do not successfully distinguish between vegetation and rock pillars and faces. In this paper, we replace filtration with classification approach using the features derived from characteristics of points within a neighbourhood of optimized sizes, such as eigenvalue-based features and echo ratio. Random Forest classifier is trained and tested on the manually labelled dataset with a density of almost 650 points/m2 from the Adršpach-Teplice Rocks. The overall accuracy reaches 87% but recall and precision of non-ground points are unsatisfactory. Misclassified non-ground points are located also within trees, thus we do not consider the result as suitable for DTM processing without further processing.

Temporal-Spatial Frequency Rockfall Data from Open-Pit Highwalls Using a Low-Cost Monitoring System

In surface mining, rockfall can seriously threaten the safety of personnel located at the base of highwalls and cause serious damage to equipment and machinery. Close-range photogrammetry for the continuous monitoring of rock surfaces represents a valid tool to efficiently assess the potential rockfall hazard and estimate the risk in the affected areas. This work presents an autonomous terrestrial stereo-pair photogrammetric monitoring system developed to observe volumes falling from sub-vertical rock faces located in surface mining environments. The system has the versatility for rapid installation and quick relocation in areas often constrained by accessibility and safety issues and it has the robustness to tolerate the rough environmental conditions typical of mining operations. It allows the collection of synchronised images at different periods with high-sensitivity digital single-lens reflex cameras, producing accurate digital surface models (DSM) of the rock face. Comparisons between successive DSMs can detect detachments and surface movements during defined observation periods. Detailed analysis of the changes in the rock surface, volumes and frequency of the rocks dislodging from the sub-vertical rock surfaces can provide accurate information on event magnitude and return period at very reasonable cost and, therefore, can generate the necessary data for a detailed inventory of the rockfall spatial-temporal occurrence and magnitude. The system was first validated in a trial site, and then applied on a mine site located in NSW (Australia). Results were analysed in terms of multi-temporal data acquired over a period of seven weeks. The excellent detail of the data allowed trends in rockfall event to be correlated to lithology and rainfall events, demonstrating the capability of the system to generate useful data that would otherwise require extended periods of direct observation.

The role of frost cracking in local denudation of steep Alpine rockwalls over millennia (Eiger, Switzerland)

Abstract. Denudation of steep rockwalls is driven by rock fall processes of various sizes and magnitudes. Rockwalls are sensitive to temperature changes mainly because thermo-cryogenic processes weaken bedrock through fracturing, which can precondition the occurrence of rock fall. However, it is still unclear how the fracturing of rock together with cryogenic processes impacts the denudation processes operating on steep rockwalls. In this study, we link data on long-term rockwall denudation rates at the Eiger (Central Swiss Alps) with the local bedrock fabric and the reconstructed temperature conditions at these sites, which depend on the insolation pattern. We then estimate the probability of bedrock for failure through the employment of a theoretical frost cracking model. The results show that the denudation rates are low in the upper part of the NW rockwall, but they are high both in the lower part of the NW rockwall and on the SE face, despite similar bedrock fabric conditions. The frost cracking model predicts a large difference in cracking intensity from ice segregation where the inferred efficiency is low in the upper part of the NW rockwall but relatively large on the lower section of the NW wall and on the SE rock face of the Eiger. We explain this pattern by the differences in insolation and temperature conditions at these sites. Throughout the last millennium, temperatures in bedrock have been very similar to the present. These data thus suggest the occurrence of large contrasts in microclimate between the NW and SE walls of the Eiger, conditioned by differences in insolation. We use these contrasts to explain the relatively low denudation rates in the upper part of the NW rockwall and the rapid denudation in the SW face and in the lower part of the NW rock face where frost cracking is more efficient.

Bulbine keiskammaensis, a new cliff-dwelling Bulbine species (Asphodelaceae) from the Eastern Cape

Bulbine keiskammaensis (Asphodelaceae) which is endemic to sheer rock faces of the lower Keiskamma River, Eastern Cape Province, South Africa, is described (Figure 1). Its pendent stems up to 250mm long with a persistent network of purplish leaf bases and linear lanceolate, acuminate leaves of up to 450mm long (of which the margin is smooth) are faintly striate and green. The description of this taxon forms part of an extensive survey of cliff-dwelling succulent and bulbous-succulent taxa in South Africa and Namibia. A table is provided with a selection of characters of similar broad-leaved species of Bulbine in the Eastern Cape. A key is also provided to the cliff-dwelling Bulbine species of the summer rainfall parts of South Africa.