Erosion of the intertidal chalk platform in the vicinity of groynes and seawalls is evident to the naked eye along many stretches of the engineered coastline of southeast England, leading to undermining and eventually failure of these structures. However, quantification of the magnitude and spatial extent of the erosion has been difficult to date because of a lack of data about the past elevation of the platform. The application of softcopy photogrammetry makes it possible to recreate past platform elevations from historic air photographs and to compare these with elevations from modern air photographs. Coastal sea defence structures have been installed along the chalk coast east of Brighton at various dates over the past 70 years. During this period, the construction methods have changed from predominantly manual labour to a reliance on heavy machines. The analysis of erosion patterns around structures built since the 1970s using heavy machinery show that surface lowering is 4 to 25 times greater in the vicinity of these structures than across the platform as a whole. In contrast, there is no similar pattern of increased erosion around structures built using predominantly manual labour in the 1930s. A four fold increase in average surface lowering is found also along a vehicle trackway that crosses the mid platform. Depressions developed by enhanced lowering in the front of seawalls generate their own dynamic of increased erosion by trapping pebbles and cobbles that enhance the abrasion of the chalk through bedload transport under standing waves in front of the walls. 相似文献
Structures in locations susceptible to severe seismic disturbances should be designed properly in order to resist lateral forces induced by earthquake motions. Steel offshore platforms are some of those structures which are built to withstand environmental and accidental loads during oil exploitation operation. Particular attention is being paid to earthquake loads in seismic active areas because it directly influences the capacity of the offshore installations.
In this paper, a small-scaled planar platform has been modelled analytically using nonlinear finite element program, based on an experimental test, conducted simultaneously in order to assess the local and global behavior of pile–leg interaction in Jacket Type Offshore Platforms (JTOPs). A combination of nonlinear beam column elements and fatigue affected elements are used to capture the inelastic cyclic behavior of planar frame as accurately as possible. Results of analytical tests are to be compared with experiments and it is concluded that an analytical approach can be best used for modelling JTOPs with reasonable accuracy regardless of the type and scale of the structure. Moreover, a special study on joints has been carried out and the best model has been selected to simulate brittle behavior of joints resulting from heat affected zone. 相似文献
Molar tooth structure (MTS) represented by complex ptygmatical shapes is widely distributed in the Proterozoic of the world. MTS filled by fine, equant sparry calcite (or dolomite) displays an abrupt contact with hosting rocks, which are mainly composed of carbonaceous micrites and fine-grained carbonates with local silts and stormdominated deposits with graded, cross or wave beddings, numerous erosional surfaces and truncated and fills or guttered bases. Occurrence of MTS suggests a result of the constraint of sedimentary facies, and the storm-base in ramp settings is the maximum depth for the formation of MTS. Vertical succession of MTS-bearing carbonates shows a deposition stacked by high-frequency shallow subtidal and peritidal cycles. An individual cyclic MTS-bearing sequence is characterized by thinning, shallowing and dynamic decreasing-upward, and peritidal caps of purple red iron and organic carbonaceous sediments with more complicated shapes of MTS are common on the top of individual MTS-bearing sequences. 相似文献
The primordially structural-lithofacial relationships in the Adriatic Carbonate Platform (AdCP) of Croatia were formed by a Late Cretaceous synsedimentary tectonics. During Cenomanian, an extensional tectonic regime differentiated AdCP into several kilometres large paleoenvironmental segments which behaved as individual depocenteres. The latest Cenomanian and earliest Turonian were tectonically relatively quiet periods during which sediments only recorded a relative sea-level rise. Compression commenced during the middle Santonian and formed first (NW-SE) gentle folds in the frontal part of the Split-Dubrovnik thrust. These folds had amplitudes of tens to hundreds of metres and are up to ten kilometres in strike. The apical parts of the anticlines were dominated by shallow-marine deposition with short emergences simultaneously, slope deposition of pelagic sediments took place in the synclines. By the end of the Campanian, compression weakened and younger sediments infilled former depressions while the deposition ended in the Adriatic hinterland of Croatia. During the Maastrichtian the compression recommenced and the index of older folds increased while new folds and reverse faults were formed. Such deformations created a differentiated morphology at the surface subsequently overlaid by Palaeogene sediments. Clastic sediments accumulated indeed in this paleodepression during the Palaeogene and Miocene–Quaternary, forming favourable structural conditions for hydrocarbon generation. 相似文献