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Rock Strength (rock + strength)

Distribution by Scientific Domains

Earth and Environmental Science	66%

Selected Abstracts

A minimum sample size required from Schmidt hammer measurements

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 13 2009
Tomasz Niedzielski
Abstract The Schmidt hammer is a useful tool applied by geomorphologists to measure rock strength in field conditions. The essence of field application is to obtain a sufficiently large dataset of individual rebound values, which yields a meaningful numerical value of mean strength. Although there is general agreement that a certain minimum sample size is required to proceed with the statistics, the choice of size (i.e. number of individual impacts) was usually intuitive and arbitrary. In this paper we show a simple statistical method, based on the two-sample Student's t -test, to objectively estimate the minimum number of rebound measurements. We present the results as (1) the ,mean' and ,median' solutions, each providing a single estimate value, and (2) the empirical probability distribution of such estimates based on many field samples. Schmidt hammer data for 14 lithologies, 13,81 samples for each, with each sample consisting of 40 individual readings, have been evaluated, assuming different significance levels. The principal recommendations are: (1) the recommended minimum sample size for weak and moderately strong rock is 25; (2) a sample size of 15 is sufficient for sandstones and shales; (3) strong and coarse rocks require 30 readings at a site; (4) the minimum sample size may be reduced by one-third if the context of research allows for higher significance level for test statistics. Interpretations based on less than 10 readings from a site should definitely be avoided. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Effects of earthquake and cyclone sequencing on landsliding and fluvial sediment transfer in a mountain catchment

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2008
Guan-Wei Lin
Abstract Patterns and rates of landsliding and fluvial sediment transfer in mountain catchments are determined by the strength and location of rain storms and earthquakes, and by the sequence in which they occur. To explore this notion, landslides caused by three tropical cyclones and a very large earthquake have been mapped in the Chenyoulan catchment in the Taiwan Central Range, where water and sediment discharges and rock strengths are well known. Prior to the MW 7·6 Chi-Chi earthquake in 1999, storm-driven landslide rates were modest. Landslides occurred primarily low within the landscape in shallow slopes, reworking older colluvial material. The Chi-Chi earthquake caused wide-spread landsliding in the steepest bedrock slopes high within the catchment due to topographic focusing of incoming seismic waves. After the earthquake landslide rates remained elevated, landslide patterns closely tracking the distribution of coseismic landslides. These patterns have not been strongly affected by rock strength. Sediment loads of the Chenyoulan River have been limited by supply from hillslopes. Prior to the Chi-Chi earthquake, the erosion budget was dominated by one exceptionally large flood, with anomalously high sediment concentrations, caused by typhoon Herb in 1996. Sediment concentrations were much higher than normal in intermediate size floods during the first 5 years after the earthquake, giving high sediment yields. In 2005, sediment concentrations had decreased to values prevalent before 1999. The hillslope response to the Chi-Chi earthquake has been much stronger than the five-fold increase of fluvial sediment loads and concentrations, but since the earthquake, hillslope sediment sources have become increasingly disconnected from the channel system, with 90 per cent of landslides not reaching into channels. Downslope advection of landslide debris associated with the Chi-Chi earthquake is driven by the impact of tropical cyclones, but occurs on a time-scale longer than this study. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Contribution of deep-seated bedrock landslides to erosion of a glaciated basin in southern Alaska

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2005
Ann M. Arsenault
Abstract Landslides represent a key component of catchment-scale denudation, though their relative contribution to the erosion of glaciated basins is not well known. Bedrock landslide contribution was investigated on the surface of one of eleven glaciers on a glaciated ridge in the Chugach-St Elias Range of southern Alaska, where the debris from four major landslides is easily distinguished from moraines and other supraglacial material. A series of aerial and satellite photos from 1972 to 2000 and field observations in 2001 and 2002 indicate that three of four landslides have fallen onto the surface of the glacier since about 1978. The landslides, which originated from the steeply dipping (60,70°) bedrock walls, were deposited onto the glacier in the ablation zone and are currently being transported downstream supraglacially. Individual glacial valleys with topographic relief of ,400 m are cut into high-grade metamorphic rock characterized by a steep north-dipping foliation and fractured by numerous large joints. Measurements of landslide area and average thickness obtained from high-resolution survey data indicate a total landslide volume of ,2·3 × 105 m3. This volume suggests a basin-averaged erosion rate from landslides of 0·48 mm a,1. An overall basin-scale erosion rate of 0·7 to 1·7 mm a,1 can be inferred, but depends on the percentage of the total-basin sediment yield contributed by supraglacial sources. A mean rockwall retreat rate of 6·7 mm a,1 is calculated and is considerably higher than published rates, which range from 0·04 to 4·0 mm a,1. Controls on landslide generation include seismicity, freeze,thaw processes, topography, rock strength, and debuttressing. It is likely all of these factors contribute to failure, although the primary controls for the landslides in this study are thought to be rock strength and topography. The absence of landslides on ten of the eleven glaciers on this ridge is attributed to landslide magnitude,frequency relationships and short temporal scale of this study. Large-volume bedrock landslides (>100 000 m3) may have low frequency, occurring less than once in a 55-year time frame. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Analytical simulation of the dynamic compressive strength of a granite using the sliding crack model

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 9 2001
H. B. Li
Abstract A sliding crack model is employed to simulate rock strength under dynamic compression. It is assumed that the growth and nucleation of a sliding crack array presented results in the shear fault failure and dominate the mechanical properties of rock material. The pseudo-tractions method is used to calculate the stress intensity factor of the sliding crack array under compression. With the utilization of a dynamic crack growth criterion, the growth of the sliding crack array is studied and the simulated strengths of a granite under dynamic compression are correspondingly obtained. It is concluded that the simulated rock strengths increase with increasing strain rates at different confining pressures, and the rising rates have a trend to decrease with increasing confining pressures. It is also indicated that the simulated rock strengths increase with increment of confining pressure at different strain rates, and the rising rates are almost identical at different strain rates. The simulation results are validated by the experimental data for the granite. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Metamorphism and microstructures along a high-temperature metamorphic field gradient: the north,eastern boundary of the Královský hvozd unit (Bohemian Massif, Czech Republic)

JOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2002
D. Scheuvens
Abstract A metamorphic field gradient has been investigated in the Moldanubian zone of the central European Variscides encompassing, from base to the top, a staurolite,kyanite zone, a muscovite,sillimanite zone, a K-feldspar,sillimanite zone, and a K-feldspar,cordierite zone, respectively. The observed reaction textures in the anatectic metapsammopelites of the higher grade zones are fully compatible with experimental data and petrogenetic grids that are based on fluid-absent melting reactions. From structural and microstructural observations it can be concluded that the boundary between the kyanite,staurolite zone and the muscovite- and K-feldspar,sillimanite zones coincides with an important switch in deformation mechanism(s). Besides minor syn-anatectic shearing (melt-enhanced deformation), microstructural criteria point (a) to a switch in deformation mechanism from rotation recrystallization (climb-accommodated dislocation creep) to prism slip and high-temperature (fast) grain boundary migration in quartz (b) to the activity of diffusion creep in quartz,feldspar layers, and (c) to accommodation of strain by intense shearing in fibrolite,biotite layers. It is suggested that any combination of these deformation mechanisms will profoundly affect the rheological characteristics of high-grade metamorphic rocks and significantly lower rock strength. Hence, the boundary between these zones marks a major rheological barrier in the investigated cross section and probably also in other low- to medium-pressure/high-temperature areas. At still higher metamorphic grades (K-feldspar-cordierite zone), where the rheologically critical melt percentage is reached, rock rheology is mainly governed by the melt and other deformation mechanisms are of minor importance. In the study area, the switch in deformation mechanism(s) is responsible for large-scale strain partitioning and concentration of deformation within the higher-temperature hanging wall during top-to-the-S thrusting, thus preserving a more complete petrostructural record within the rocks of the footwall including indications for a ?Devonian high- to medium-pressure/medium-temperature metamorphic event. Thrusting is accompanied by diapiric ascent of diatexites of the K-feldspar-cordierite zone and infolding of the footwall, suggesting local crustal overturn in this part of the Moldanubian zone. [source]