共查询到20条相似文献，搜索用时 24 毫秒
1.
The dynamic soil-tunnel interaction is studied by the model of a rigid tunnel embedded in layered half-space,which is simplified as a single soil layer on elastic bedrock to the excitation of P- and SV-waves.The indirect boundary element method is used,combined with the Green's function of distributed loads acting on inclined lines.It is shown that the dynamic characteristics of soil-tunnel interaction in layered half-space are different much from that in homogeneous half-space,and that the mechanism of soil-tunnel interaction is also different much from that of soil-foundation-superstructure interaction.For oblique incidence,the tunnel response for in-plane incident SV-waves is completely different from that for incident SH-waves,while the tunnel response for vertically incident SV-wave is very similar to that of vertically incident SH-wave. 相似文献
2.
Isis Brook 《地球表面变化过程与地形》2006,31(13):1600-1605
Philosophical concerns about restoring landscapes often revolve around two, connected, issues. First is the idea that a restored landscape, even if it is a perfect replica, has lost some of its value. The claim might appeal to a break in the continuity of the landscape and that continuity is part of what is valuable. Alternatively, often in the case of natural landscapes, the appeal is that any human manipulation is inauthentic; here the analogy is sometimes made with the art world and the restoration is deemed a fake. The second problem highlighted in philosophical debates is that the greater the success of restoration projects, the more threatened natural landscapes become: any claim that something must be preserved in its pristine or historically layered state is undermined by the claim that it could be put back again. Initially I discuss two opposing potential responses to these claims: (1) that humans are part of nature and thus cannot be an alien dominating force outside of nature; and (2) that nature is itself a social construct. Neither of these positions is entirely satisfactory, but what they jointly reveal is the reality of our fluid and multifaceted relationship with the world. I then show that Elliot's claim of the additional value of pristine nature is actually not based on an inherent value, but is dependent on the human valuation of it. I propose an alternative that places the source of value in the thing itself and thus arrive at a positive role for restoration as the setting in train and guiding of positive relationships above and beyond their social or public amenity value to us. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
3.
用丰富的高质量数字强震记录对台湾集集大地震及余震的长周期地震动特性进行了研究. 本文主要研究了长周期地震动的强度随震级、距离及场地条件的变化情况;同时对近断层的长周期地震动特性进行了分析. 结果表明,集集大地震及余震的长周期地震动反应谱的形状主要受场地条件和震级控制（D+E类场地的反应谱要比B+C类的宽,震级大的反应谱要比震级小的宽）,而受距离影响并不显著;近断层长周期地震动明显受断层活动特性影响:上盘的长周期地震动比下盘的强,北部的比南部的强. 相似文献
4.
《Water Policy》1998,1(4):383-395
Clean water is an important natural resource. In recent times, there has been a radical change in the institutional and regulatory mechanism for providing clean water to the rural communities of Ghana. The object of this paper is to examine the strengths and weaknesses of the two regimes for providing water to rural communities in Ghana. These are the traditional regime and the Community Water and Sanitation Program (CWSP). We shall argue that the strength of the traditional regime is that almost everybody knows the laws and institutions that regulate the use of water. Its major weakness is that water is used in an untreated stage and is not available all year round. The strength of the CWSP is that it treats water as an economic resource and it is managed in a manner that makes it sustainable. Its major weakness is that, for its execution, it depends on a number of state institutions which can be notoriously weak. Very little has also been accomplished on the sanitation aspect of the program. 相似文献
5.
6.
A discrete point sampler has been developed that overcomes disadvantages inherent in several current small-volume samplers. It is designed to obtain ground water samples after a well has been purged with a pump. It consists of a sample chamber, two ports, and a stopcock for withdrawing sample aliquots. After lowering the sampler into a well, sampling is initiated by pulling on a line that sequentially removes the plugs in the lower and the upper level ports. The sample chamber fills from the bottom port and vents air from the top port. The device is suitable for sampling for volatile organic compounds in ground waters that are not subject to spontaneous bubble degassing. The upper port is sufficiently far above the lower port that none of the water that is sampled is exposed to the vented air. The sample chamber fills in such a way that the water that is taken from the chamber for analysis is not exposed to the headspace in the chamber. 相似文献
7.
Automatic and continuously recording samplers are deployed in a Hertfordshire gravel-bed stream to show that bedload transport is related to stream power. The pattern is similar to that already established for North American channels but, because the record is so detailed, it is possible to identify the cause of the considerable scatter that is normal in such relationships. A major factor is the occurrence of rhythmic pulses in bedload discharge that are not matched by similar fluctuations in hydraulic variables. It is suggested that these pulses reflect downstream differences in the concentration of mobile particles in a slow-moving traction carpet, and that they may be likened to kinematic waves. The record also reveals that the threshold of sediment transport—always presumed hithero to be associated with incipient motion—is related to the cessation of bedload transport in a river flood. Indeed, the mean value of stream power at the finish of bedload transport is only 20 percent of that prevailing at the moment of incipient sediment motion. Because of this, there is an inevitably poor correlation between actual bedload transport rates and those predicted by bedload equations which rely upon a single traction threshold. These new data show that the general inverse relationship between bedload discharge and water-depth : grain-size ratio proposed by Bagnold (1977, 1980) is not universal. Transport efficiency for this gravel-bed stream is typically 0.05 per cent of available stream power, which compares with 1.6 per cent for a river moving both gravel and sand, and 5 per cent for another channel where bedload is composed predominantly of sand-sized particles. It is argued that coarse and fine-grained alluvial channels may need to be considered separately. By allowing for differences in traction threshold at the beginning and end of bedload events, and by averaging bedload discharge flood by flood in order to smooth out the effect of pulses, it is possible to achieve a reasonably good prediction of average bedload transport rate in terms of stream power. 相似文献
8.
杨贵 《地震地磁观测与研究》2011,32(6):17-21
对比福建数字地震台网测定的中国台湾中深源地震目录与中国台湾中央气象局公布的相应中深源地震目录,分析震源位置和发震时刻的差别情况,基本认为:福建台网测定中国台湾中深源地震的结果,在震中位置上,纬度偏大,即偏北;经度偏小,即偏向台网方向;采用单纯型方法定位的结果,发震时刻略偏小,震源深度偏小较多;用Locsat方法定位的结... 相似文献
9.
Willem V. R. Malkus 《地球物理与天体物理流体动力学》2013,107(1):267-278
AbstractHiggins and Kennedy (1971) concluded that the Earth's fluid core has a stable stratification if it is at its melting point. Busse (1972) and Elsasser suggested as an alternative that a hydrostatic-isentropic distribution of particulate solid can produce neutral stability in a partially molten core. Here this suggestion is quantified and a determination is made of the efficiency of the production of fluid motion from the heat flux. This is used to establish that macroscopic convection can exist only if the particulate solid is of sufficiently small size. A thermal history of the core compatible with upper mantle heat flux is advanced in which it is suggested that the inner core is a fairly recent feature. The implication of these results for convection-driven and precession-driven dynamos is that both can function for a small enough suspended particulate, that the convection-dynamo will fail for particles greater than one micron in diameter, and that the precession-driven dynamo probably cannot survive particles greater than ten microns in diameter. 相似文献
10.
通过跟踪分析湖州地震台数字化水管倾斜仪和垂直摆倾斜仪连续2年的资料质量,发现水管倾斜仪观测精度高于垂直摆倾斜仪,并且资料稳定性逐年提高。从映震效能看,水管倾斜仪同震响应幅度远大于垂直摆,水管倾斜仪的自振周期远大于垂直摆倾斜仪而接近远震面波的周期,形成谐振,但两者的同震响应时间相当。 相似文献
11.
Introduction The study of attenuation relations for long-period ground motion is to meet the requirement of economy development. During the earthquakes such as the June 16, 1964 Niigata, Japan, M=7.5 earthquake, the March 24, 1977 Romania M=7.3 earthquake, the March 29, 1970 Gediz, Turkey, M=7.1 earthquake, the May 26, 1983 M=7.7 earthquake in the middle Japanese Sea, the September 19, 1985 Mexico M=8.1 earthquake, the September 16, 1994 M=7.3 earthquake in Tai- wan Strait, and the No… 相似文献
12.
In a paper by Koefoed and Kegge (1968), which was based on previous work of Wesley (1958), the electrical current pattern has been derived that is induced by an oscillating magnetic dipole in a semi-infinite thin plate of infinitesimal resistivity. In the present paper, the range of validity of the assumptions, on which the work of Wesley is based, is subjected to a theoretical analysis. It is found that the decisive factor for the validity of Wesley's derivation is the quotient of the square of the penetration depth of the electrical current over the product of the thickness of the plate and a distance that is indicative of the size of the current loops in the plate. Wesley's derivation is shown to be valid only when this factor is negligible. It is also shown that in this condition the imaginary component of the anomaly must be negligible. Model experiments are described in which the electrical current pattern is studied also in the range in which the derivation of Wesley is not valid. The procedure used in these model experiments was to measure the tangential component of the magnetic field strength very close to the metal plate that simulated the conductive dyke. In order to express the results of the measurements in terms of the imaginary to real ratio, these results are compared with an interpretation graph for field measurements that was published by Hedström and Parasnis (1958). It is found that the current pattern in the plate is essentially the same as that which follows from Wesley's derivation, provided that the imaginary to real ratio is less than one third. The measurements do not permit to draw conclusions regarding the current pattern in the plate in conditions that correspond to larger values of the imaginary to real ratio. 相似文献
13.
利用伪谱和有限差分混合方法对兰州盆地进行二维强地面运动模拟.假设3个不同的震源深度5、10、20 km的同一地震下,以一个5层的二维剖面作为模型,以此来研究地震波传播过程并分析复杂的二维非均质路径效应,以及局部地下速度构造对强地面运动振幅加乘的影响,从而了解路径效应对强地面运动最大峰值位移的影响程度.研究结果显示:伪谱和有限差分混合方法模拟地震波场结合了有限差分法和伪谱法的优点,弥补了二者的不足,能较好地处理介质不连续面的计算,同时保证了和伪谱法相当的计算精度.通过地震波场模拟可知沉积盆地的强地面运动较基岩相比具有放大效应,震源深度的不同对兰州盆地垂直分量地面最大峰值位移影响较大,在震源深度为5 km时最小,为0.06 cm;震源深度10 km时在水平剖面30～55 km范围内峰值位移最大,达到0.14 cm;但对水平分量的峰值位移影响较小. 相似文献
14.
In this paper two causal models that approximate the nearly frequency‐independent cyclic behaviour of soils are analysed in detail. The study was motivated by the need to conduct time‐domain viscoelastic analysis on soil structures without adopting the ad hoc assumption of Rayleigh damping. First, the causal hysteretic model is introduced in which its imaginary part is frequency independent the same way that is the imaginary part of the popular non‐causal constant hysteretic model. The adoption of an imaginary part that is frequency independent even at the zero‐frequency limit, in conjunction with the condition that the proposed model should be causal, yields a real part that is frequency dependent and singular at zero frequency. The paper shows that the causal hysteretic model, although pathological at the static limit, is the mathematical connection between the non‐causal constant hysteretic model and the physically realizable Biot model. The mathematical structure of the two causal models is examined and it is shown that the causal hysteretic model is precisely the high‐frequency limit of the Biot model. Although both models have a closed‐form time‐domain representation, only the Biot model is suitable for a time‐domain viscoelastic analysis with commercially available computer software. The paper demonstrates that the simplest, causal and physically realizable linear hysteretic model that can approximate the cyclic behaviour of soil is the Biot model. The proposed study elucidates how the dynamic analysis of soil structures can be conducted rigorously in terms of the viscoelastic properties of the soil material and not with the ad hoc Rayleigh damping approach which occasionally has been criticized that tends to overdamp the higher vibration modes. The study concludes that under pulse‐type motions the Rayleigh damping approximation tends to overestimate displacements because of the inappropriate viscous type of dissipation that is imposed. Under longer motions that induce several cycles, the concept of equivalent viscous damping is more appropriate and the Rayleigh damping approximation results to a response that is comparable to the response computed with a rigorous time‐domain viscoelastic finite element analysis. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
15.
多层框架结构弹性和弹塑性动力响应比较分析 总被引：2，自引：0，他引：2
结构在静荷载作用下,考虑材料塑性变形所得到的结构某点的位移响应大于不考虑塑性变形(即认为结构为完全弹性)所得到的位移响应。通过基于LSDYNA程序的计算分析发现,相同的结论并不适用于结构在动力荷载作用下的响应。进入弹塑性阶段后,可以认为结构的动力响应受两个因素影响:(1)刚度降低引起结构振动特性的改变;(2)塑性变形引起的位移增长。这两个因素的影响程度决定了结构在动力荷载作用下由弹性阶段过渡到塑性阶段后的各种响应的改变。 相似文献
16.
17.
18.
John T. Donald & Paul H. Roberts 《地球物理与天体物理流体动力学》2013,107(5):367-384
Intermediate dynamos are axisymmetric, spherical models that evade Cowling's theorem by invoking an α-effect to create the meridional magnetic field from the zonal. Usually the energy source maintaining the motions is a specified thermal wind, but here the dynamo is driven by the buoyancy created by a uniform distribution of heat sources. It has been argued by Braginsky and Meytlis (this journal, vol. 55, 1990) that, in a rapidly rotating, strongly magnetic system such as the Earth's core, heat is transported principally by a small-scale turbulence that is highly anisotropic. They conclude that the diffusion of heat parallel to the rotation axis is then significantly greater than it is in directions away from that axis. A preliminary study of the consequences of this idea is reported here. Solutions are derived numerically using both isotropic and non-isotropic thermal diffusivity tensors, and the results are compared. It is shown that even a small degree of anisotropy can materially alter the character of the dynamo. 相似文献
19.
The eastern Himalayan syntaxis is located on the leading edge of Indian-Eurasian plate collision, and the uplift rate of Namche Barwa area is higher than that of the peripheral zones, which is considered as the core position of the eastern Himalayan syntaxis(Uplift Center).It is indicated according to the recent regional earthquake observation results that, the seismic activity is poor in the area of Namche Barwa, but with strong seismic activity in its southeast region. In order to study the current geodynamical characteristics of the eastern Himalayan syntaxis, the elevation frequency distribution and hypsometry curve of Namche Barwa area, its northwest and southeast as well as the northeast Assam area is analyzed using DEM data. It is shown according to the result that, the Namche Barwa area is in the mature stage of erosion and the regional tectonic uplift and denudation are in the highly balanced status. Influenced by plateau-climate weather effect, the denudation of this area is relatively poor, which indicates that the uplift of the Namche Barwa area is relatively slow at present. The geomorphology in the northwest and southeast as well as in northeast Assam is in young evolutionary phase, belonging to erosive infancy, and the geomorphology of northeast Assam is closer to the early stage of infancy. The geomorphic evolution stage on northwest side reflects that the regional erosion is poor and it still belongs to plateau-climate area; Influenced by south subtropical monsoons, there is rich rainfall in the area from southeast Namche Barwa to Assam area, and this area still belongs to erosive infancy, even the geomorphic development degree of northeast Assam is lower as it suffers from strong erosion effect, which means that the tectonic uplift in east Namche Barwa is very intensive, and the northeast Assam has the highest uplift rate. It is considered according to the research that, under the mode that India Plate moves towards the north at present, the core position of the eastern Himalayan syntaxis(Uplift Center)moves towards the southeast, and the new core position may be located in northeast Assam, where there is intensive regional tectonic uplift with high potential of great earthquake. 相似文献
20.
An analytical solution for the space-time variation of contaminant concentration in one-dimensional transient groundwater flow in a homogenous semi-infinite aquifer, subjected to time-dependent source contamination, is derived. The uniform and time varying dispersion along transient groundwater flow is investigated under two conditions. First, the flow velocity distribution in the aquifer is considered as a sinusoidally varying function, and second, the flow velocity distribution is treated as an exponentially increasing function of time. It is assumed that initially the aquifer is not solute free, so the initial background concentration is considered as an exponentially decreasing function of the space variable which is tending to zero at infinity. It is assumed that dispersion is directly proportional to the square of the velocity, noting that experimental observations indicate that dispersion is directly proportional to the velocity with a power ranging from 1 to 2. The analytical solution is illustrated using an example and may help benchmark numerical codes and solutions. 相似文献