Frame transformation of spherical-harmonic coefficients of differential particle intensity     

C. K. Ng 《Astrophysics and Space Science》1986,126(2):313-327

We present a general second-order-correct frame transformation on spherical-harmonic coefficients of differential particle intensity. The transformation, valid for relativistic particles as well, provides a clear view of the Compton-Getting effect. It shows explicitly how each transformed harmonic coefficient depends on a subset of the original harmonic coefficients. The general expression for the first-order Compton-Getting vector anisotropy is derived and interpreted. In addition, we show how the new transformation allows one to simplify a current procedure for determining the directional intensity in a comoving frame. This involves the directional particle data measured on a spacecraft.  相似文献   

Natural concentrations of lead in ancient Arctic and Antarctic ice     

Amy Ng  Clair Patterson 《Geochimica et cosmochimica acta》1981,45(11):2109-2121

Concentrations of Pb and K were determined in a series of veneer layers chiseled in sequence from the outside toward the center of each of the five 1500–5500yr old ice core sections that had been drilled in Greenland and Antarctic ice. They were analogs of very old ice samples analyzed earlier by Herronet al. (1977) and Craginet al. (1975), who reported high concentrations of Pb in them. Lead contamination, existing at exterior concentrations of about 10⁶ ng/kg ice, had intruded to the centers of the cores, establishing interior values of at least 1.4 ng/kg ice in three electromechanically drilled Camp Century core sections taken from fluid filled drill holes. Corresponding Pb concentration changes were 3 × 10⁴ ng/kg ice to 1.2 ng/kg ice in two thermally drilled New Byrd Station core sections taken from non-fluid filled drill holes. Contamination made the lowest center concentrations serve only as upper limits to the original concentrations of Pb in the ice.Potassium concentrations decreased from exterior values of about 5 × 10⁵ ng/kg ice to an interior value of 2 × 10³ ng/kg ice in the Camp Century core sections and from 8 × 10⁴ ng/kg ice to 9 × 10² ng/kg ice in New Byrd Station core sections. Potassium contamination effects were not large within the central portions of the cores.These data verify earlier findings by Murozumiet al. (1969) and extend to a broader geographical significance the general validity of their observation of a ~ 300-fold increase of Pb concentrations in the Greenland ice sheet during the past 3000 yr. Our findings refute claims by Herronet al. (1977) and Craguinet al. (1975) that 100-fold excesses of natural Pb exist in 800 yr old Greenland ice above levels contributed by silicate dusts. Our new data also show that average Pb concentrations of 26 ng Pb/kg ice, claimed by Boutron and Lorius (1979) to be natural and present for 60 yr in snow strata in Antarctica, did not exist in old Antarctic ice, and that Pb concentrations have increased at least 10-fold in that ice during the past century.Virtually all of the present day ~300-fold excess of Pb above natural levels in Greenland ice can be shown to be caused by industrial Pb emissions to the atmosphere on the basis of the following factors: (1) the historic increase of Pb in snow strata coincides with the historic increase of industrial Pb production and atmospheric emissions (2) mass inventories of industrial emissions can account for the excess Pb in polar snow (3) new quantitative measurements of Pb emissions from volcanic plumes by Buat-Menard and Arnold (1978), Pattersonet al. (1981), and Buat-Menardet al. (1981), and from sea spray by Ng and Patterson (1981) and Settle and Patterson (1981). show that these natural sources cannot account for 99% of the excess Pb above contributions by silicate dusts observed today in the atmosphere; and (4) the historic increase of Pb in snow strata is paralleled by analogous increases of excess Pb shown by isotopic tracers to be industrial in water-laid sediments in a remote continental region (Shirahataet al., 1980). It is now known, however, that snows display about a 10-fold greater excess of industrial Pb above crustal silicate concentrations than exists in the air above the snows.  相似文献   

The dilatant behaviour of sand–pile interface subjected to loading and stress relief     

S. Y. Peng  C. W. W. Ng  G. Zheng 《Acta Geotechnica》2014,9(3):425-437

Property and behaviour of sand–pile interface are crucial to shaft resistance of piles. Dilation or contraction of the interface soil induces change in normal stress, which in turn influences the shear stress mobilised at the interface. Although previous studies have demonstrated this mechanism by laboratory tests and numerical simulations, the interface responses are not analysed systematically in terms of soil state (i.e. density and stress level). The objective of this study is to understand and quantify any increase in normal stress of different pile–soil interfaces when they are subjected to loading and stress relief. Distinct element modelling was carried out. Input parameters and modelling procedure were verified by experimental data from laboratory element tests. Parametric simulations of shearbox tests were conducted under the constant normal stiffness, constant normal load and constant volume boundary conditions. Key parameters including initial normal stress ( $ \sigma_{{{\text{n}}0}}^{\prime } $ ), initial void ratio (e ₀), normal stiffness constraining the interface and loading–unloading stress history were investigated. It is shown that mobilised stress ratio ( $ \tau /\sigma_{\text{n}}^{\prime } $ ) and normal stress increment ( $ \Updelta \sigma_{\text{n}}^{\prime } $ ) on a given interface are governed by $ \sigma_{{{\text{n}}0}}^{\prime } $ and e ₀. An increase in $ \sigma_{{{\text{n}}0}}^{\prime } $ from 100 to 400 kPa leads to a 30 % reduction in $ \Updelta \sigma_{\text{n}}^{\prime } $ . An increase in e ₀ from 0.18 to 0.30 reduces $ \Updelta \sigma_{\text{n}}^{\prime } $ by more than 90 %, and therefore, shaft resistance is much lower for piles in loose sands. A unique relationship between $ \Updelta \sigma_{\text{n}}^{\prime } $ and normal stiffness is established for different soil states. It can be applied to assess the shaft resistance of piles in soils with different densities and subjected to loading and stress relief. Fairly good agreement is obtained between the calculated shaft resistance based on the proposed relationship and the measured results in centrifuge model tests.  相似文献   

Dynamic response of flexible rockfall barriers under different loading geometries   总被引：1，自引：1，他引：0  

Raymond C. H. Koo  Julian S. H. Kwan  Carlos Lam  Charles W. W. Ng  Jack Yiu  Clarence E. Choi  Axel K. L. Ng  Ken K. S. Ho  W. K. Pun 《Landslides》2017,14(3):905-916

Flexible steel barriers are commonly constructed on steep hillsides to mitigate rockfall. The evaluation of the dynamic response of proprietary flexible barriers is conventionally performed using full-scale field tests by dropping a weight onto the barriers in accordance with the European test standard ETAG 27. The weight typically has a spherical or polyhedral shape and cannot reproduce more complex rockfall scenarios encountered in the field. A rigid slab may load a barrier over a larger area and its effect has not been investigated. In this study, a calibrated three-dimensional finite-element model was developed to study the performance of vertically and horizontally orientated rockfall barriers under concentrated areal impact loads. A new bilinear force-displacement model was incorporated into the model to simulate the behavior of the energy-dissipating devices on the barriers. The effect of different weight geometries was studied by considering impacts by a rigid single spherical boulder and a rigid slab. Results reveal that areal loading induced by a rigid slab increases the loading on the barrier foundation by up to 40 % in both horizontally and vertically positioned barriers when compared to a concentrated load scenario with a single boulder. This indicates that barriers tested under the current test standard does not give the worst-case scenario in terms of foundation loads, and barrier designers should take into account the possible effect of increased foundation loads by reinforcing the barrier posts and/or increasing their spacing.  相似文献   

Velocity attenuation of debris flows and a new momentum-based load model for rigid barriers   总被引：2，自引：1，他引：1  

R. C. H. Koo  J. S. H. Kwan  C. W. W. Ng  C. Lam  C. E. Choi  D. Song  W. K. Pun 《Landslides》2017,14(2):617-629

Effective design of mitigation measures against debris flow hazards remains a challenging geotechnical problem. At present, a pseudo-static approach is commonly used for the calculation of impact load acting on a rigid debris-resisting barrier. The impact load is normally calculated based on the maximum velocity observed in the transportation zone under free-field conditions without considering debris-barrier interaction. In reality, the impact load acting on a barrier varies with the change of debris momentum flux but this is seldom considered in barrier design. To provide a scientific basis for assessing debris momentum flux during impact, this paper presents results from a study of debris-barrier interaction using physical flume modelling. This study showed that, following the first stage of impact, the accumulated debris behind a barrier formed a stationary zone and caused the remaining debris to slow down in a run-up process. In the experiments, the peak debris momentum was 30 % lower compared to that observed under free-field conditions. A new momentum-based model was developed to take into account attenuation of momentum flux for predicting debris impact load on rigid barriers. The new rationalised model was assessed using data from the notable Yu Tung Road debris flow in Hong Kong. The assessment showed that the design bending moment at the base of the barrier wall could be reduced more than 30 % using the proposed model, compared with the current design approach. The adoption of the proposed model could offer a new opportunity for practitioners to optimise the design of rigid barriers.  相似文献   

香港岩石的硬度与点荷载指标和强度的关系   总被引：5，自引：0，他引：5  

谭国焕  李启光  徐钺 《岩土力学》1999,20(2):52-56

无损硬度测试——施密特锤及肖氏硬度实验和点荷载试验是三种经常用来间接测定岩石强度指标的方法。目前很多学者已提出了不同的经验关系,这些关系主要是把硬度指标、点荷载指标和单轴抗压强度、单轴抗拉强度与间接拉伸强度联系起来。但是在应用这些关系时,必须首先用当地的岩石特性数据加以验证,查看地质变化的影响,才可确保使用无误。本文的主要目的是根据大量香港岩石（包括花岗岩、石灰岩、大理岩和凝灰岩）的实验数据,提出一套适用于香港地区岩石的经验关系,另外也对其他经验关系作了考证。  相似文献   

香港花岗岩风化分级化学指标体系与风化壳分带   总被引：10，自引：3，他引：7  

吴宏伟  尚彦军 《工程地质学报》1999,7(2):125-134

火成岩的化学风化过程实质上就是碱、碱土金属组分水解淋失与脱硅富铁铝化的地球化学过程。不同风化程度的岩石处于整个化学风化过程的不同阶段,因而具有不同的化学成分和风化度指标。笔者在对香港九龙两个地方5个钻孔共计150件不同深度、不同风化程度的样品化学全分析结果与岩土风化程度宏观判别结果的相互对比、选择和确认的基础上,建立了香港地区花岗岩风化岩土风化程度工程分级的化学指标体系。这一研究成果的提出不仅有利于提高花岗岩风化岩土工程分类的科。学性、可靠性和实用性,而且可以为风化岩土工程特性与风化作用关系研究提供理论依据。  相似文献   

Use of coupled finite element analysis in unsaturated soil problems     

A. K. L. Ng  J. C. Small 《国际地质力学数值与分析法杂志》2000,24(1):73-94

This paper presents a variation of Biot's consolidation theory for analysing problems involving unsaturated soils, and implemented using the finite element method. The numerical method is applied to a few geotechnical problems as examples and the results obtained are compared to some published data. The illustrative examples show how the numerical method can be used to analyse seepage and consolidation problems associated with unsaturated soils and demonstrate the flexibility and applicability of the presented method. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

Experimental and numerical investigation of Weibullian behavior of grain crushing strength     

Quanshui Huang  Wei Zhou  Gang Ma  Tang-Tat Ng  Kun Xu 《地学前缘(英文版)》2020,(2):401-411

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Analysis of footing resting on non-homogeneous soil by double spline element     

L. G. Tham 《Computers and Geotechnics》1988,5(4):249-268

A numerical method is proposed for the analysis of rectangular footing resting on an elastic soil layer. The footing is represented by double spline elements and the elastic soil medium by finite layers. The effect of the rigidity of footing and the non-homogeneity of the soil on the behaviour of such foundation system is investigated, and the results are presented in form of design charts such that they may be used for hand calculation for the estimation of the settlement of footings for a wide range of practical cases.  相似文献