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1.
Large rock slope failures from near‐vertical cliffs are an important geomorphic process driving the evolution of mountainous landscapes, particularly glacially steepened cliffs. The morphology and age of a 2·19 × 106 m3 rock avalanche deposit beneath El Capitan in Yosemite Valley indicates a massive prehistoric failure of a large expanse of the southeast face. Geologic mapping of the deposit and the cliff face constrains the rock avalanche source to an area near the summit of ～8·5 × 104 m2. The rock mass free fell ～650 m, reaching a maximum velocity of 100 m s?1, impacted the talus slope and spread across the valley floor, extending 670 m from the base of the cliff. Cosmogenic beryllium‐10 exposure ages from boulders in the deposit yield a mean age of 3·6 ± 0·2 ka. The ～13 kyr time lag between deglaciation and failure suggests that the rock avalanche did not occur as a direct result of glacial debuttressing. The ～3·6 ka age for the rock avalanche does coincide with estimated late Holocene rupture of the Owens Valley fault and/or White Mountain fault between 3·3 and 3·8 ka. The coincidence of ages, combined with the fact that the most recent (AD 1872) Owens Valley fault rupture triggered numerous large rock falls in Yosemite Valley, suggest that a large magnitude earthquake (≥M7.0) centered in the south‐eastern Sierra Nevada may have triggered the rock avalanche. If correct, the extreme hazard posed by rock avalanches in Yosemite Valley remains present and depends on local earthquake recurrence intervals. Published in 2010 by John Wiley & Sons, Ltd.  相似文献   

2.
Sea cliff morphology and erosion rates are modulated by several factors, including rock control that reflects both lithology and rock structure. Erosion is anticipated to preferentially exploit ‘fractures’, broadly meant as any discontinuity in an otherwise continuous medium, where the rock mass is weakest. Unpicking the direct control of such fractures on the spatial and temporal pattern of erosion remains, however, challenging. To analyse how such fractures control erosion, we monitored the evolution of a 400 m-long stretch of highly structured sedimentary cliffs in Socoa, Basque Country, France. The rock is known as the Socoa flysch formation. This formation combines decimetre-thick turbidites composed of repeat triplets of medium to strong calcareous sandstone, laminated siltstones and argillaceous marls. The sequence plunges at 45° into the sea with a shore-parallel strike. The cliffs are cross-cut by two normal and reverse fault families, with 10–100 m alongshore spacing, with primary and secondary strata-bound fractures perpendicular to the bedding, which combined delimit the cliff rock mass into discrete blocks that are exploited by the erosion process. Erosion, and sometimes plucking, of such beds and blocks on the cliff face was monitored using ground-based structure-from-motion (SfM) photogrammetry, over the course of 5.7 years between 2011 and 2017. To compare with longer time change, cliff-top retreat rate was assessed using SfM-orthorectified archive aerial photographs spanning 1954–2008. We show that the 13,250 m2 cliff face released 4500 blocks exceeding 1.45 × 10−3 m3, removing a total volume of 170 m3. This equates to an average cliff erosion rate of 3.4 mm/year, which is slightly slower than the 54-year-long local cliff-top retreat (10.8 ± 1.8 mm/year). The vertical distribution of erosion reflects the height of sea water inundation, where the maximum erosion intensity occurs ca. 2 m above high spring-tide water level. Alongshore, the distribution of rockfall scars is concentrated along bed edges bounding cross-cutting faults; the extent of block detachment is controlled by secondary tectonic joints, which may extend through several beds locally sharing similar mechanical strength; and rockfall depth is always a multiple of bed thickness. Over the longer term, we explain block detachment and resultant cliff collapse as a cycle. Erosion nucleates on readily exploitable fractures but elsewhere, the sea only meets defect-free medium-strong to strong rock slabs offering few morphological features for exploitation. Structurally delimited blocks are quarried, and with sufficient time, carve semi-elliptic scars reaching progressively deeper strata to be eroded. Lateral propagation of erosion is directed along mechanical weaknesses in the bedding, and large episodic collapses affect the overhanging slabs via sliding on the weak marl beds. Collapse geometry is confined to one or several triplets of turbidite beds, but never reaches deeper into the cliff than the eroded depth at the foot. We contend that this fracture-limited model of sea-cliff erosion, inferred from the Socoa site dynamics and its peculiar sets of fractures, applies more broadly to other fractured cliff contexts, albeit with site-specific geometries. The initiation of erosion, the propagation of incremental block release and the ultimate full failure of the cliff, have each been shown to be fundamentally directly controlled by structure, which remains a vital control in understanding how cliffed coasts have changed in the past and will change in the future.  相似文献   

3.
The triggering factors of rock falls remain unknown due to a lack of exhaustive, regular and accurate surveys. Based on an inventory of 331 rock falls collected weekly between 2002 and 2009 from Veules‐les‐Roses to Le Treport (Upper Normandy), the relationships between coastal chalk cliff rock falls (dates and geomorphological features) and external factors commonly agreed as triggering (rainfall, temperature variations, tide and wind) are studied. The combination of multivariate statistical and empirical analyses indicates that (1) ‘cold and dry weather’ and ‘high rainfall and high wind’ are the conditions most likely to trigger rock falls, (2) the main triggering factors of rock falls are effective rainfall (for rock falls mostly between 200 and 1400 m3 or larger than 10 000 m3 and coming from the whole cliff face), freeze/thaw cycles (especially for rock falls smaller than 200 m3 and coming from the foot and top of the cliff face) and marine roughness (rock falls mainly smaller than 200 m3 and coming from the cliff foot). However, the contribution of each factor to triggering is difficult to determine because of combinations of factors (85% of 331 cases), relays of processes and hysteresis phenomena. In view of these first results, it is still presumptuous to predict the location and time of triggering of rock falls. However, the statistical and naturalistic approaches adopted and the observations made in this study are from an original database, and constitute a real starting point for the prediction and prevention of the hazard of coastal chalk cliff rock falls in Upper Normandy. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

4.
We analyzed a combination of airborne and terrestrial LiDAR, high‐resolution photography, seismic, and acoustic data in order to gain insights into the initiation, dynamics, and talus deposition of a complex rock fall. A large (46 700 m3) rock fall originated from near Ahwiyah Point in eastern Yosemite Valley and fell a total of 730 m to the valley floor on 28 March 2009. Analyses of remote sensing, seismic, and acoustic data were integrated to reconstruct the rock fall, which consisted of (1) the triggering of a 25 400 m3 rock block in an area of intersecting and sometimes highly weathered joint planes, (2) the sliding and subsequent ballistic trajectory of the block from a steeply dipping ledge, (3) dislodging of additional rock from the cliff surface from beneath the rock fall source area, (4) a mid‐cliff ledge impact that detached a volume of rock nearly equivalent in volume to the initial block, (5) sliding of the deteriorating rock mass down the remainder of the cliff, and (6) final impact at the base of the cliff that remobilized the existing talus downward and outward and produced an airblast that knocked down hundreds of trees. The depositional geomorphology indicates that the porosity of the fresh talus is significantly lower than that expected for typical blocky talus slopes, likely because the rock debris from this event was pulverized into smaller, more poorly sorted fragments and densified via dynamic compaction when compared to less energetic, fragmental‐type rock falls. These results suggest that accumulation of individual rock‐fall boulders tends to steepen talus slopes, whereas large, energetic rock falls tend to flatten them. Detachment and impact signals were recorded by seismic and acoustic instruments and highlight the potential use of this type of instrumentation for generalized rock fall monitoring, while LiDAR and photography data were able to quantify the cliff geometry, rock fall volume, source and impact locations, and geomorphological changes to the cliff and talus. Published in 2012. This article is a US Government work and is in the public domain in the USA.  相似文献   

5.
Cliff stability within the Pembrokeshire Coast National Park was evaluated using a numerical model applied at four sites representative of rock mass failure phenomena and major sedimentary geological sequences. The sites were: Mill Bay, (Old Red Sandstone); St Govan's Head, Carboniferous Limestone (Dinantian); Druidston, Millstone Grit (Namurian) and Lower Coal Measures (Westphalian); and Wiseman's Bridge, Lower Coal Measures (Westphalian). The study integrated a range of geotechnical parameters, measured in the field and laboratory, into a model to predict the likely failure mechanisms. The model is based on the existence of rock prisms delineated by structural parameters, i.e. joints, bedding planes and critical tension fractures behind the cliff face. An iterative approach is used to define the dip of the most probable, stepped failure surface at the base of any potentially unstable multiblock system and to calculate the sliding and toppling forces for each block in the cliff mass. Prediction compared favourably with field observations at three of the four selected sites, i.e. Druidston, St Govan's Head and Wiseman's Bridge. At Druidston sliding is predicted and dominates in the basal blocks, whilst toppling is confined to the upper cliff and is dependent on movement of the lower structural units. St Govan-s Head is shown to have a low risk of toppling and sliding and this was predicted except where basal undercutting reaches a depth of 1·0 m or lateral forces exceed 100 kN m−2 when failure could occur. Copyright © 1998 John Wiley & Sons, Ltd.  相似文献   

6.
An in-situ experimental program in a jointed rock mass was designed and implemented to investigate the rock joint effects on stress wave propagation. Accelerometers were placed on rock surface along three lines at 0, 45 and 90° to the predominant rock joint strike direction. Eight blasts were detonated in a charge hole drilled in the rock mass. The equivalent TNT blast weight ranged from 2.5 to 50.0 kg and the loading density (charge weight divided by the charge chamber volume) varied from 1 to 20.0 kg/m3. A fully coupled detonation with a charge weight of 25.0 kg was also exploded to study the coupling effect. The recorded data are presented and analyzed in this paper. The effects of rock joints on characteristics of stress wave propagation such as peak value attenuation, spectrum, and spatial variations are discussed.  相似文献   

7.
We have measured concentrations of cosmogenic 10Be and 26Al produced in situ at bare bedrock surfaces of successive sheets developing on a granite dome in Korea and calculated the exfoliation rate of sheeting joints. The exfoliation rate was obtained using a simple model in which the sheeting joints experience intermittent denudation, i.e. peeling off along the bedrock face. We find that the average exfoliation (erosion) rate of the episodic peeling‐off process is 5·6 cm/ka?1. The analysis is useful for understanding the evolution of granite sheeting structures on this dome in Korea. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

8.
Large cliff failures involving forward toppling over a basal hinge have occurred on more than half of the plateau edge of Ben Lomond, northeastern Tasmania. This mode of failure, which is readily identified from the columnar structure of the dolerite involved, has affected up to 107 m3 of rock at a time and a total of more than 50 × 106 m3 in all the cases which can still be identified. It represents perhaps the most important form of cliff retreat, amounting to a rate of 0.2 mm yr?1 over the last 100,000 years. Topographic evidence and joint surveys suggest that two different mechanisms have produced the topples on Ben Lomond. One has involved failure in the sediments underlying the dolerite with consequent foundering and cambering of large sections of the plateau edge. This mechanism accounts for relatively few of the Ben Lomond topples, though it includes the largest individual cases. The second mechanism, dominant in most of the topples, involved slab failure in the cliffs. Both modes of failure have been facilitated by vertical weaknesses within the bedrock and both require an initially steep cliff profile. Because of the latter requirement, which is not met on the other mountains of northeastern Tasmania, large-scale topples are found only on Ben Lomond, and only there where glacial steepening of the cliff has been possible. Following the initial failure, topples of both types have migrated downslope by block sliding for distances up to 2 km.  相似文献   

9.
In identifying controls on rock slope form a distinction is made between: (1) rock slopes with joints which dip steeply out of a cliff and hence are subject to mass failure of the rock mass above a critical joint; and (2) rock slopes with inclinations which are either in equilibrium with the mass strength of their rocks, or have profiles which will develop towards strength equilibrium as cross joints open. In the first class of slope, stability results not just from the basic frictional resistance of the rock but also from the frictional roughness along the critical joint and from the normal stress acting across that joint. Stability may be reduced by weathering and loss of strength of the joint wall rock. As a result of normal stress variations with depth, induced by overburdens, high cliffs which are not undercut have a concave profile. The second group of slopes includes those with inclinations controlled at the scale of individual joint blocks, buttressed slopes and those on unjointed rock masses. Buttressed and unjointed rock masses develop towards a condition of mass strength equilibrium as cross joints open. Strength equilibrium slopes may be recognized by application of a rock mass strength classification proposed for geomorphic purposes. Eleven propositions are formulated which identify controls on rock slope development and some consequences of these controls.  相似文献   

10.
Three numerical models of cliff failure have been developed, based on analyses of rockfalls along the 22 km Liassic coastline of the Glamorgan Heritage Coast, U.K. Detailed field investigation of the bucklandi (limestone dominated) and angulata (mudstone dominated) Lias series at four locations confirm the veracity of the models. Translation failure was correctly predicted at the mainly limestone buttressed cliff sites in the bucklandi and at locations where the angulata series formed a high proportion of the cliff mass. Toppling was predicted for vertical and overhanging cliffs with basal undercutting. The factor of safety reduces as the ratio of undercutting depth (d) to tension fracture distance (D) from the cliff face increases. Instability can be triggered by thrust forces generated by wave/tide impact, freeze/thaw and expansion/contraction associated with clay infill. Thrust forces can be modest, ranging from 1·7 to 2·7 MN m2, but can exceed the cross-joint strength as weathering proceeds.  相似文献   

11.
Moraines that dam proglacial lakes pose an increasing hazard to communities in the Andes and other mountain ranges. The moraines are prone to failure through collapse, overtopping by lake waters or the effect of displacement waves resulting from ice and rock avalanches. Resulting floods have led to the loss of thousands of lives in the Cordillera Blanca mountains of Peru alone in the last 100 years. On 22 April 2002 a rock avalanche occurred immediately to the south‐west of Laguna Safuna Alta, in the Cordillera Blanca. The geomorphic evidence for the nature, magnitude and consequences of this event was investigated in August 2002. Field mapping indicated that the avalanche deposited 8–20 × 106 m3 of rock into the lake and onto the surface of the frontal region of Glaciar Pucajirca, which flows into the lake. Repeated bathymetric surveying indicated that ～5 × 106 m3 of this material was deposited directly into the lake. The immediate effect of this event was to create a displacement wave that gained in height as it travelled along the lake basin, overtopping the impounding moraine at the lake's northern end. To achieve overtopping, the maximum wave height must have been greater than 100 m. This, and subsequent seiche waves, caused extensive erosion of both the proximal and distal faces of the impounding terminal moraine. Further deep gullying of the distal face of this moraine resulted from the supply of pressurized water to the face via a relief overflow tunnel constructed in 1978. Two‐dimensional, steady‐state analysis of the stability of the post‐avalanche moraine rampart indicates that its proximal face remains susceptible to major large‐scale rotational failure. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

12.
At Writing-On-Stone Provincial Park in southern Alberta, Canada, weathering is causing deterioration and loss of archaeologically important Indian rock art. A procedure devised for the use of park personnel identified four classes of weathering ranging from largely unweathered rock to severely weathered. The technique employed simple visual, qualitative assessment and photo interpretation of 50 sample sections of sandstone cliff face covering a total area of 354 m2. Schmidt hammer tests indicated large variations in rock strength and provided a numerical basis for the visual assessment. About 43 per cent of the cliffs are severely to completely weathered, 41 per cent show moderate weathering.  相似文献   

13.
Development of a notch at the base of a cliff reduces cliff stability and often induces a collapse. Pleistocene limestone coastal cliffs of elevation 5?m in Kuro‐shima, Ryukyu Islands, have a prominent notch with a depth of 3–4?m at their bases. Around these coastal cliffs, collapses different from previous studies of cliff collapses in the Ryukyu Islands were found; collapses in Kuro‐shima have a horizontal failure surface. The horizontal failure surface, situated at the height of the failure surface corresponding to the retreat point of the notch, is bounded by vertical joints cutting the whole cliff and the reef flat in front of the cliff. Two types of horizontal failure surface were found, triangular and quadrangular; the distinction appears to depend on the angle between the vertical joints and the front face of the cliff. Prior to collapse, these cliffs appear to have been separated from the adjacent cliffs by the development of vertical joints. Consequently, a cliff that will collapse can be identified in advance; cliff instability is strongly dependent on the development of a notch. To study the effect of notch development on cliff collapse, the notch depth at which collapse occurs was calculated using stability analysis. Instability of a cliff increases with notch depth; collapse occurs at the horizontal failure surface when the ratio of the notch depth to the seaward length of the cliff is approximately 0·5–0·7 for a triangular failure surface, and 0·7–0·9 for a quadrangular failure surface. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

14.
The propagation characteristics of blast-induced shock waves in a jointed rock mass have been monitored and studied. Accelerometers were set up on a rock surface along three lines, at 0°, 45° and 90° with respect to the orientation of the predominant joint strikes. Cylindrical charges were detonated in a charge hole, and ground accelerations in both vertical and radial directions at various points on the rock surface were recorded. Results show that rock joints have significant effects on the propagation characteristics of blast-induced shock waves. The amplitude and principal frequency of shock waves attenuate with the increase of distance from the charge centre, and the increase of incident angle between the joint strike and the wave propagation path. The measured data were compared with the empirical equations of shock wave attenuation proposed by other authors. The mechanism of rock joint effect, the attenuation of shock waves in relation to the propagation distance, the charge weight and the incident angle, are discussed in this paper.  相似文献   

15.
为研究地震产生的应力波在断续节理岩体中的传播规律和应力分布趋势,首先,采用数值模拟方法分析应力波通过贯通节理的传播规律,并与已有理论研究结果进行对比,验证数值分析的准确性和适用性;然后,对应力波在断续节理岩体中的传播进行数值模拟,分析透射系数在水平方向的分布趋势以及不同节理连续性对波传播的影响,并结合波的衍射原理,给出定性的理论解释。结果表明:应力波通过断续节理时,节理的透射作用会使应力波振幅减小,引起波的衰减,岩桥的衍射作用则会使波阵面由平面变为曲面,波的传播方向发生改变,从而导致应力波振幅在水平方向的分布发生变化;应力波通过断续节理的透射系数与岩桥尺寸Lr和衍射角μ相关,当衍射角比较小时,透射系数主要受岩桥尺寸Lr的影响,当衍射角较大时,岩桥尺寸Lr和衍射角μ共同影响应力波在岩体中的传播。   相似文献   

16.
Influence of rock mass strength on the erosion rate of alpine cliffs   总被引：1，自引：0，他引：1  
Collapse of cliff faces by rockfall is a primary mode of bedrock erosion in alpine environments and exerts a first‐order control on the morphologic development of these landscapes. In this work we investigate the influence of rock mass strength on the retreat rate of alpine cliffs. To quantify rockwall competence we employed the Slope Mass Rating (SMR) geomechanical strength index, a metric that combines numerous factors contributing to the strength of a rock mass. The magnitude of cliff retreat was calculated by estimating the volume of talus at the toe of each rockwall and projecting that material back on to the cliff face, while accounting for the loss of production area as talus buries the base of the wall. Selecting sites within basins swept clean by advancing Last Glacial Maximum (LGM) glaciers allowed us to estimate the time period over which talus accumulation occurred (i.e. the production time). Dividing the magnitude of normal cliff retreat by the production time, we calculated recession rates for each site. Our study area included a portion of the Sierra Nevada between Yosemite National Park and Lake Tahoe. Rockwall recession rates determined for 40 alpine cliffs in this region range from 0·02 to 1·22 mm/year, with an average value of 0·28 mm/year. We found good correlation between rockwall recession rate and SMR which is best characterized by an exponential decrease in erosion rate with increasing rock mass strength. Analysis of the individual components of the SMR reveals that joint orientation (with respect to the cliff face) is the most important parameter affecting the rockwall erosion rate. The complete SMR score, however, best synthesizes the lithologic variables that contribute to the strength and erodibility of these rock slopes. Our data reveal no strong independent correlations between rockwall retreat rate and topographic attributes such as elevation, aspect, or slope angle. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

17.
Results are presented of distinct element computer modelling used to examine rates and mechanisms of change in rock slopes and cliffs, where material intact properties determine process and form but the most significant controls are the joint pattern and cross-joint properties. The modelling approach does not appear to have been used before in a geomorphological context and provides an alternative approach for examining cliff development. Field and laboratory data have been collected for the Portland Limestone outcrop of the Isle of Purbeck, central southern England. The Portland Limestome is a hard, shelly, crystalline sediment of the Upper Jurassic. It has a regular discontinuity pattern throughout the outcrop in Purbeck. While joint orientation remains relatively constant, bedding changes from horizontal to vertical, a consequence of the Purbeck Monocline. There are resulting implications for spatial variations in rock slope evolution. The modelling exercise enhances previous knowledge on rock failure mechanisms and slope development along the Purbeck coast and demonstrates its potential in research where landforms are developed in lithified, jointed rock masses. © 1998 John Wiley & Sons, Ltd.  相似文献   

18.
The present study essentially employs a thin-layer interface model for filled rock joints to analyze wave propagation across the jointed rock masses. The thin-layer interface model treats the rough-surfaced joint and the filling material as a continuum medium with a finite thickness. The filling medium is sandwiched between the adjacent rock materials. By back analysis, the relation between the normal stress and the closure of the filled joint are derived, where the effect of joint deformation process on the wave propagation through the joint is analyzed. Analytical solutions and laboratory tests are compared to evaluate the validity of the thin-layer interface model for filled rock joints with linear and nonlinear mechanical properties. The advantages and the disadvantages of the present approach are also discussed.  相似文献   

19.
A traversing micro‐erosion meter was used to measure rock surface micro‐topography over 40 cm2 on a supra‐tidal cliff face from early morning to late evening in late spring. From 06:00 hours to 22:00 hours the relative heights of 188 coordinates were obtained using the meter at 2‐hour intervals, resulting in a data set of 1607 readings. Monitoring shows that rock surfaces are dynamic entities, with significant rise and fall relative to the first measurement at shorter timescales than previously reported. The maximum positive rise between readings was 0·261 mm and lowering was 0·126 mm. The pattern of change did not relate as expected to environmental variables such as temperature or insolation. Rather, the surface showed greater surface change in the early morning and late afternoon. It is hypothesized that this pattern relates to the expansion and contraction of lichen thalli as moisture is absorbed during higher humidity in the morning and late afternoon. The implications of these results for weathering studies are considered. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

20.
Research indicates that the aeolianite (Kurkar) cliffs along the Israeli Mediterranean coastline have continuously retreated eastward during the last few decades. There seems to be no dispute among Earth scientists regarding the general trend of cliff retreat. However the majority of papers displaying cliff retreat rates are based upon comparison of aerial photographs. Their lack of advanced geometric measurement methods causes a high margin of error. Public attention is focused upon the Beit‐Yannay coastal cliff since private homes are located along the southern section of the cliff crest. The current research compares the historic location of the cliff crest edge at Beit‐Yannay as observed in a series of aerial photographs taken during the period 1918–2000. Quantitative measurement methods included applications of satellite geodesy and digital photogrammetry and mapping. Research results offer quantitative, consecutive and highly accurate data regarding retreat rates over a relatively long period of 82 years. It is concluded that: 1. Annual average cliff retreat rates of the cliff crest is 20 cm/year. 2. Categorization of the study time span reveals periods displaying varying retreat rates such as 27 cm/year during 1918–1946, 21 cm/year during 1946–1973 and 10 cm/year during 1973–2000. 3. Maximum retreat distances of the cliff crest, over the study period were found to be approximately 25 m along the northern, lowest section of the cliff. Minimum distances of 11 m were identi?ed at the highest, southern section of the cliff. 4. The eolianite (Kurkar) cliffs along the Israeli Mediterranean coast throughout the 20th century have been an important source of sediment, contributing approximately 24 × 106 m3 of sediments to the sediment balance of Israeli beaches. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

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