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481.
In this study the spatial patterns and dynamics of vegetation patches along a grazing gradient in the steppe ofLarrea divaricataandStipaspp. in NE Patagonia (Argentina) are described. A general effect of grazing is the reduction of total plant cover resulting from the decrease in cover of perennial grasses (Stipa speciosa, Poa ligularis, Stipa tenuis) and some tall shrubs (Chuquiraga hystrix, Bougainvillea spinosa, Lycium chilense). Dwarf shrubs (Nassauvia fueguianaandJunellia seriphioides) increase their cover under medium and/or high grazing pressures. Plant species are spatially grouped into patches which alternate with areas of bare soil. Eleven types of vegetation patch differing in the dominant plant functional type or species, floristic richness and size were identified with different relative frequency along the grazing gradient. Based on these results, it is postulated that grazing forces the replacement of large patches dominated by tall shrubs with high species richness, byLarrea divaricatapatches or small dwarf shrub patches with low species richness and the extinction of grass patches. This results from: (1) disruption of local balances of species deletions and additions; (2) fragmentation of large patches; and (3) formation of new vegetation patches. These changes lead to differing plant spatial organization and heterogeneity along the grazing gradient which may be described by characteristic arrays of vegetation patches. 相似文献
482.
ANDREW J. NEWELL DAVID J. GOWER† MICHAEL J. BENTON‡ VALENTIN P. TVERDOKHLEBOV§ 《Sedimentology》2007,54(4):835-845
The aim of this study was to determine how Unio bivalve shells fragment within the channel of the Sakmara River (southern Urals, Russia). The Sakmara River has an abundant bivalve population and a highly variable flow regime which, at low flow, allowed much of the channel bed to be examined. A large data set of 1013 shells (Unio sp.) was examined and these were shown to have consistent patterns of orientation, aspect, shell abrasion, perforation and fracture. The close spatial relationship between areas of shell abrasion, shell perforation and shell fracture showed that they form part of a continuum whereby areas of abrasion evolve into perforations and perforations coalesce and enlarge into fractures. The mechanism of shell damage proposed is one of abrasion in place, whereby the shell remains stationary on the surface of the point bar and is impacted by bedload. Underpinning this process are the hydrodynamic properties of the bivalve shell, with consistency in the orientation and aspect of the valve in a flowing current producing consistency in the distribution of damage on the shell surface. Valves preferentially lie in a convex‐up position and orientate in the flow such that the umbo faces upstream. The elevated, upstream‐facing umbo region is exposed to particle impact and is the first to be abraded and perforated. The vulnerability of the umbo to perforation is greatly increased by the thinness of the shell at the umbo cavity. The in situ abrasion process is enhanced by the development of an armoured gravel bed which restricts valve mobility and maintains shells within the abrasion zone at the sediment–water interface. The in situ abrasion process shows that broken shells are not a reliable indicator of long distance transport. The study also raises the issue that tumbling barrel experiments, which are generally used to simulate shell abrasion, will not replicate the type of directionally focused sand‐blasting which appears to be the principal cause of shell fragmentation in the Sakmara River. 相似文献
483.
高密度电法探测基岩起伏和隐伏断裂中的应用 总被引:3,自引:0,他引:3
蓟县山前地带兴建高档住宅小区,勘察过程中发现基岩埋深变化较大,采用高密度电法进行探测,根据视电阻率等值线的变化规律较好地解决了这一问题,同时对测区内可能存在的隐伏断裂进行了分析判断。 相似文献
484.
485.
高密度电法具有点距密、数据采集密度大、施工效率高和分辨率高的特点,工程地质勘察、管线探测、物探找水、岩溶及地质灾害调查等工程物探中已逐渐成为常用的方法。本文首先介绍了高密度电法的基本原理,并以常用的、比较稳健的温纳装置方式进行剖析,然后从4个测区高密度电法工作结果与钻孔验证的实例来说明高密度电法在工程上的应用效果。 相似文献
486.
487.
488.
Empirical approaches for predicting fragmentation from blasting continue to play a significant role in the mining industry in spite of a number of inherent limitations associated with such methods. These methods can be successfully applied provided the users understand or recognize their limitations. Arguably, the most successful empirical based fragmentation models have been those applicable to surface blasting (e.g., Kuz-Ram/Kuznetsov based models). With widespread adoption of fragmentation assessment technologies in underground operations, an opportunity has arisen to extend and further develop these type approaches to underground production blasting.
This paper discusses the development of a new fragmentation modelling framework for underground ring blasting applications. The approach is based on the back-analysis of geotechnical, blasting and fragmentation data gathered at the Ridgeway sub level caving (SLC) operation in conjunction with experiences from a number of surface blasting operations.
The basis of the model are, relating a peak particle velocity (PPV) breakage threshold to a breakage uniformity index; modelling of the coarse end of the size distribution with the Rosin-Rammler distribution; and modelling the generation of fines with a newly developed approach that allows the prediction of the volume of crushing around blastholes.
Preliminary validations of the proposed model have shown encouraging results. Further testing and validation of the proposed model framework continues and the approach is currently being incorporated into an underground blast design and analysis software to facilitate its application. 相似文献
This paper discusses the development of a new fragmentation modelling framework for underground ring blasting applications. The approach is based on the back-analysis of geotechnical, blasting and fragmentation data gathered at the Ridgeway sub level caving (SLC) operation in conjunction with experiences from a number of surface blasting operations.
The basis of the model are, relating a peak particle velocity (PPV) breakage threshold to a breakage uniformity index; modelling of the coarse end of the size distribution with the Rosin-Rammler distribution; and modelling the generation of fines with a newly developed approach that allows the prediction of the volume of crushing around blastholes.
Preliminary validations of the proposed model have shown encouraging results. Further testing and validation of the proposed model framework continues and the approach is currently being incorporated into an underground blast design and analysis software to facilitate its application. 相似文献
489.
根据大孔径梯段爆破开挖对底部基岩扰动影响的声波检测情况,对检测资料进行了分析研究,给出了大孔径开挖底部基岩预留保护层厚度,供工程参考。 相似文献
490.
Lindsey J. McEwen & John A. Matthews 《Geografiska Annaler: Series A, Physical Geography》1998,80(1):17-36
A geomorphological and sedimentological case study is made of the Sprongdøla, a medium-sized river flowing in a predominantly bedrock channel in the mountains of southern Norway. Particular attention is given to bed material characteristics and potential sediment sources. Downstream patterns in clast size and shape, including sphericity and roundness, are poorly developed in the Sprongdøla. There is an apparent absence of down-stream fining and a large proportion of bedload in the lower channel remains in very angular and angular roundness categories. Covariance analysis of clast shape and roundness allows this to be attributed to the widespread input of coarse, angular rock particles derived from snow-avalanche activity on the valley sides and especially from frost-shattering of bedrock in the channel, which produces characteristically slabby clasts. The latter process is important in channel widening and in steepening the channel walls. Strictly fluvial processes, avalanche activity and frost-shattering are controlled in different ways by related annual environmental cycles. Their interaction in the context of the periglacial river channel produces a unique process-sediment-landform association that distinguishes alpine periglacio-fluvial systems from those associated with glacio-fluvial rivers and fluvial landscapes in the temperate zone. 相似文献