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701.
Analytical equations for the moment–rotation response of a rigid foundation on a Winkler soil model are presented. An equation is derived for the uplift-yield condition and is combined with equations for uplift- and yield-only conditions to enable the definition of the entire static moment–rotation response. The results obtained from the developed model show that the inverse of the factor of safety, χ, has a significant effect on the moment–rotation curve. The value of χ=0.5 not only determines whether uplift or yield occurs first but also defines the condition of the maximum moment–rotation response of the footing. A Winkler model is developed based on the derived equations and is used to analyze the TRISEE experiments. The computed moment–rotation response agrees well with the experimental results when the subgrade modulus is estimated using the unload–reload stiffness from static plate load–deformation tests. A comparison with the recommended NEHRP guidelines based on the FEMA 273/274 documents shows that the choice of value of the effective shear modulus significantly affected the comparison. 相似文献
702.
Aseismic base isolation is an effective method used to protect structures and their contents against earthquakes. An isolated structure may be designed to remain elastic throughout major ground motions as a result of the efficiency of the isolation systems. In this paper, the equations of motion of two‐dimensional elastic structures supported on a new base isolation system called the Sliding Concave Foundation (SCF) are presented and a procedure for their solution is suggested. The responses of a number of structures subjected to different earthquake records are evaluated and the results are compared with those of the same structures supported on two other famous isolation systems and also a fixed base condition. The results indicate the effectiveness of the SCF in protecting the supported structures even during very strong and/or long period earthquakes. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
703.
Shoreline positions established from beach profile surveys combined with wave data are jointly analyzed, as a function of their contribution to coastal processes, to investigate the interaction between waves, shoreline orientation and coastal structures along the Nile delta promontories, Rosetta, Burullus and Damietta. Repeated beach profile surveys along the promontory sectors (64 km long in total) have been analyzed to determine rates of shoreline changes prior to construction (1971–1990) and after construction of protective structures (1990–2000). The behavior of coastline pre- and post-construction indicates that coastal erosion fronting protective structures has declined in the case of the seawalls at the tips of the Rosetta and Damietta promontories, or has been partially replaced by sand accumulation in the case of detached breakwaters at Baltim (east of Burullus promontory) and at Ras El Bar (west of the Damietta promontory). As a consequence, downdrift erosion has been initiated in local areas adjacent to these structures in the direction of longshore sediment transport. The 5-km-long seawall protecting the Rosetta promontory has stopped the dramatic erosion of this highly eroded area (formerly shoreward retreated 88 m/year), with adverse local erosion at its west and east ends, being 3 and 13 m/year, respectively. Similarly, the 6-km-long seawall built on the eastern tip of the Damietta Promontory, still under construction, has nearly stopped the severe erosion, which was formerly 10 m/year. The detached breakwaters at both Baltim and at Ras El Bar have accumulated sand at accretion rates of 37 and 14 m/year, respectively. This sand accumulation is associated with downdrift erosion of 25 and 13 m/year at Baltim and Ras El Bar, respectively. Results reaffirm that the original erosion/accretion patterns along the Nile delta promontories have been reshaped due to the massive protective structures built during the last decade. This reshaping along the examined promontories is generally controlled by the temporal variability in the intensity and reversibility of wave directions and associated longshore currents, coastline orientation and by the existing coastal protection structures. 相似文献
704.
: The knowledge of the volume and duration of low-flow events in river channels is essential for water management and the
design of hydraulics structures. In this study, both preceding characteristics, X
1 and X
2, are considered simultaneously via two types of bivariate distributions whose marginals are exponential. One of these bivariate
distributions has been presented by Nagao and Kadoya (1971) and the other has been used by Singh and Singh (1991) to the study
of rainfall intensity and rainfall depth. The results are applied to the low-flow series (“peaks-below-threshold”) of Lepreau
River (station 01AQ001) in New Brunswick, Canada. These results show that the model that was successfully employed by Singh
and Singh (1991) to study rainfall, presents certain difficulties when a very strong correlation, ρ, between the two random
variables X
1 and X
2, exists. The model by Nagao and Kadoya (1971) seems to be more satisfactory for such situations, although this model seems
also to be quite sensitive to variations in ρ. 相似文献
705.
The occurrence of γ-phase, a high-pressure polymorph of olivine (α-phase), in the shock veins of Sixiangkou chondrite was
due to a greater cooling rate (> 10 000°C·s-1) in the veins. Because γ-phase partially reverted to β-phase and no back-transformation from β-phase to α-phase took place,
the shock veins of Peace River chondrite with a cooling rate of 1 000–2 000δC·s-1 contain a great amount of β-phase. In the shock veins of Mbale chondrite with a cooling rate of <500°C·s-1, the majority of γ-phase reverted to α-phase. The heat dissipation in shock veins took place after a stage of shock compression
of chondrite parent body, and the parent body was broken into fragmental pieces. Cooling rate in the shock veins constrained
the back-transformations of (Mg, Fe)2SiO4 high-pressure polymorphs.
Project of Chen and Xie supported by the National Natural Science Foundation of China (Grant No. 496720981, Natural Science
Foundation of Guangdong Province (Grant No. 960500), and the Science Foundation of Academia Sinica for the returned scholars. 相似文献
706.
F. Ashkar N. El Jabi M. Issa 《Stochastic Environmental Research and Risk Assessment (SERRA)》1998,12(2):97-116
: The knowledge of the volume and duration of low-flow events in river channels is essential for water management and the
design of hydraulics structures. In this study, both preceding characteristics, X
1 and X
2, are considered simultaneously via two types of bivariate distributions whose marginals are exponential. One of these bivariate
distributions has been presented by Nagao and Kadoya (1971) and the other has been used by Singh and Singh (1991) to the study
of rainfall intensity and rainfall depth. The results are applied to the low-flow series (“peaks-below-threshold”) of Lepreau
River (station 01AQ001) in New Brunswick, Canada. These results show that the model that was successfully employed by Singh
and Singh (1991) to study rainfall, presents certain difficulties when a very strong correlation, ρ, between the two random
variables X
1 and X
2, exists. The model by Nagao and Kadoya (1971) seems to be more satisfactory for such situations, although this model seems
also to be quite sensitive to variations in ρ. 相似文献
707.
Diagenetic origin of Basal Anhydrite in the Cretaceous Maha Sarakham salt: Khorat Plateau, NE Thailand 总被引:1,自引:0,他引:1
Mohamed El Tabakh B. Charlotte Schreiber Cherdsak Utha-Aroon Lee Coshell & John K. Warren 《Sedimentology》1998,45(3):579-594
Development of a diagenetic anhydrite bed at the base of the Cretaceous Maha Sarakham Saline Formation (the `Basal Anhydrite' member) of the Khorat Plateau in north-eastern Thailand took place due to leaching and/or pressure dissolution of salt at the contact between an underlying active sandstone aquifer system and an overlying massive halite-dominated evaporite sequence. Basal evaporites composed of halite with intercalated anhydrite of the latter sequence are undergoing dissolution as a result of subsurface flushing, with anhydrite produced as the insoluble residue. The result is a 1·1 m thick interval of nodular anhydrite displaying unique, basin-wide continuity. Observed textures, petrographic features and chemical data from the anhydrite and associated authigenic minerals support the origin of the Basal Anhydrite Member as an accumulation residue from the dissolution of the Maha Sarakham salts. Petrographically, the anhydrite in this unit is made up of crystals that are blocky and recrystallized, sheared, generally elongated and broken, and is bounded at the bottom by organic-rich stylolite surfaces. Authigenic and euhedral dolomite and calcite crystals are associated with the anhydrite. Traces of pyrite, galena and chalcopyrite are present along the stylolite surfaces suggesting supply of fresh water from the underlying sandstone at highly reducing conditions of burial. The δ34S of sulphate in the Basal Anhydrite averages 15 ‰ (CDT) and falls within the isotopic composition of the anhydrite in the Cretaceous Maha Sarakham Formation proper and the Cretaceous values of marine evaporites. Measured δ18O in dolomite range from ?4·37 to ?14·26‰ (PDB) suggesting a re-equilibration of dolomite with basinal water depleted in 18O and possible recrystallization of dolomite under relatively elevated temperatures. The δ13C, however, varies from +1·57 to ?2·53‰ (PDB) suggesting a contribution of carbon from oxidation of organic matter. This basal anhydrite bed, similar to basinwide beds found at the bottom of many giant evaporite sequences, has always been considered to be depositional. Here, at the base of the Maha Sarakham Formation, we demonstrate that the anhydrite is diagenetic in origin and was formed by accumulation of original anhydrite by dissolution of interbedded halite from waters circulating though the underlying aquifer: it represents an `upside-down' caprock. 相似文献
708.
Abderrazak El Harti Abderrazak Bannari El Mostafa Bachaoui El Mostafa Aarab Guillaume Girouard Abderrahmen El Ghmari 《Comptes Rendus Geoscience》2004,336(14):1311-1320
The geodynamic model of the Palaeozoic basin opening of central Jebilet has been unknown before this study in spite of the abundance in the geological studies carried out in the studied sector using conventional methods. This is due to the scarcity of the key beds and synsedimentary structures. Using the synergy between the image data of the Landsat satellite TM sensor and the ground data, we have highlighted, herein, new structural data allowing the design of a new model of the Palaeozoic basin opening of central Jebilet. This opening could have been made according to dextral submeridian transverse faults with the individualization of subequatorial normal faults. To cite this article: A. El Harti et al., C. R. Geoscience 336 (2004). 相似文献
709.
Jean-Gabriel Bréhéret Micheline Hanzo Abderrazzak El Albani Agnès Iatzoura 《Comptes Rendus Geoscience》2004,336(15):1355-1362
The study of calcareous nodules from four organic-rich black shale series allows us to suggest a prominent part for benthic organisms in creating heterogeneities in the sediment, thus favouring microbially mediated early diagenesis based on organic matter consumption. To cite this article: J.-G. Bréhéret et al., C. R. Geoscience 336 (2004). 相似文献
710.
The late Barremian succession in the Agadir Basin of the Moroccan Western High Atlas represents wave-dominated deltaic deposits. The succession is represented by stacked thickening and coarsening upwards parasequences 5–15 m thick formed during fifth- or fourth-order regression and building a third-order highstand systems tract. Vertical facies transitions in parasequences reflect flooding followed by shoaling of diverse shelf environments ranging from offshore transition interbedded mudstones, siltstones and thin sandstones, lower shoreface/lower delta front hummocky bedforms to upper shoreface/upper delta front cross-bedded sandstones. The regional configuration reflects the progradation of wave-dominated deltas over an offshore setting. The maximum sea-level fall led to the development of a sequence boundary that is an unconformity. The subsequent early Aptian relative sea-level rise contributes to the development of an extensive conglomerate lagged transgressive surface of erosion. The latter and the sequence boundary are amalgamated forming a composite surface. 相似文献