降雨及地下水对边坡稳定性动态影响的初步研究

论述了降雨及地下水对边坡稳定性的动态影响及作用特征。这种影响特征具有典型的周期性和滞后性，通过对坡体地下水位的动态模拟，取得了边坡稳定性随时间而变化的规律认识。     

Time effects on dynamic soil properties under random excitation conditions

Two types of clayey soils, a kaolinite and a bentonite, were tested using a resonant column apparatus under random excitation conditions. The concept of root mean square (rms) strain was utilized for the purpose of strain calculations during random loading. The conventional estimator of the transfer function was used for random vibration analysis. The effect of confinement duration (at a constant pressure) on dynamic soil properties, namely damping and shear modulus, was evaluated. The results indicate that for both cohesive soils, the effect of time was less pronounced during random vibration than sinusoidal loading at the same rms strain. This effect is however more pronounced when the peak shearing strain of sinusoidal loading is considered. Furthermore, time effects were more pronounced at low strain levels than at high strain levels.     

River channel adjustments downstream from channelization works in England and Wales

Relatively little attention has been given to river channel adjustments that occur downstream from channelization works. This study is concerned with the nature of channel adjustments downstream from a total of 46 channelization works located in low and high energy environments in England and Wales. Channel changes are identified principally by the method of field survey and by reconstructing the original positions of eroded beds and banks. Use is also made of maps, aerial photographs, and engineering drawings of different dates and the technique of space-for-time substitution is applied. Enlargement of channel cross-sections through erosion had occurred downstream from a variety of types, sizes, and dates of channelization works. The maximum increase of channel size was 153 per cent. Out of a total of 14 sites with enlarged channel cross-sections, seven had undergone a change of width only, at a further three width increased rather than depth, and at the remaining four sites depth increases were dominant. These sites all have relatively high stream powers. Factors causing spatial variation of erosion included tree roots locally binding bank sediments and the occurrence of bends. Planform change had taken place at only one site. A further three high stream power sites had downstream reaches incised into bedrock and therefore did not exhibit adjustment. Channel enlargement is explained in terms of increased flood flows downstream from channelization works causing higher stream velocities, which in turn cause erosion, thereby increasing channel width and/or depth. Examination of flow records for 35 stations revealed flood events which would formerly have spread overbank but are now confined by the channelization works and are therefore likely to alter downstream flows. At sites with downstream change it is proposed that the energy of increased flows was sufficient to exceed a threshold required for erosion of perimeter sediments. By contrast the absence of change at a majority of sites in low energy lowland areas could be a reflection of both the incompetence of increased flows to erode and resistance provided by perimeter sediments. Sites with erosion features appear not to have yet attained new equilibrium conditions.     

The effect of the Mt St Helens eruption on tropospheric and stratospheric ions

Balloon-borne observations of electrical conductivity in the troposphere and stratosphere were performed using conductivity sondes at Garmisch-Partenkirchen, West Germany, from June to November, 1980, after the Mt St Helens eruption. A significant decrease of atmospheric ions in the altitudes from the troposphere to lower stratosphere has been detected until several months after the eruption in comparison with the observational results obtained before the eruption. Simulteneous ruby lidar observation a month after the eruption indicates an increased amount of aerosol at nearly the same altitude as that of conductivity decrease. Several months after the eruption it appears that aerosols detected by lidar and those effective in reducing ion concentration have different profiles.     

Direct and Indirect pCO2 Measurements in a Wide Range of pCO2 and Salinity Values (The Scheldt Estuary)

Recent improvements in both Infra-red spectroscopy and equilibrator techniqueshave allowed to determine, for the first time, pCO₂using simultaneously and continuously both the direct and indirect methods in an estuary where pCO₂ values range from 500 to 8500 atm and salinity from 0 to 30. Our results show that both methods are in excellent agreement in the wholeestuary (r² = 0.999, n = 1075, p < 0.0001). Thus, the NBS (US National Bureau of Standards) scale, although inadequate for seawater samples, is appropriate for estuarine waters and can be applied with confidence to calculate pCO₂.     

西安地区地下水位下降及其环境负效应

本文通过西安地区多年地下水位动态资料,分析区域潜水及城郊自备井集中开采区承压水水位持续下降状况,阐述了地下水位持续下降对水环境、农业环境及城市地质环境所产生的负效应。     

Relationships between land-use and forced-pool characteristics in the Colorado Front Range

Land use practices in Colorado during the last two centuries altered the supply of sediment and water to many channels in the upper South Platte Basin. As a result of increased supply of sediment and mobility and reduced peak flows, the characteristics of pools associated with channel constrictions, referred to as forced pools, may have been altered. Increased supply of sediment and reduced transport capacity of high flows could lead to aggradation in forced pools. Channel confined by road corridors could lead to high velocities at normal flows, increased energy dissipation from riprap, or even increased pool frequency resulting from failed riprap. To assess potential alterations, four hypotheses were tested: (1) impacted streams will show significantly different mean volume of pools than a control stream; (2) mean volume of pools on streams where land-use activities increased the supply of sediment will be significantly different from streams solely affected by flow regulation; (3) the strongest change in characteristics of pools of impacted streams will be a reduced volume of pools; (4) streams affected by road corridors will show statistically lower spacing of pools than streams unaffected by roads. The downstream spacing and residual volume of twenty consecutive forced pools were surveyed on five streams in the Colorado Front Range that varied from no contemporary impact to multiple historical and contemporary impacts. ANCOVA with stepwise model selection indicated that degree of land-use (categorical), bankfull spacing of pools, upstream riffle slope and expansion ratio were all significant (α = 0.1) predictors of volume of pools (R² = 0.73). Simple linear regression of mean volume of pools and stream specific variables (gradient, drainage area and discharge) and least square means comparison of mean volume of pools indicated a need to standardize volumes of pools by slope and discharge so that the volumes of pools could be compared among different levels of land-use. Significant correlations between drainage area and volume of pools allowed volume of pools to be standardized by drainage area and thereby discharge. This dimensionless variable was also significantly correlated with channel slope, which permitted the construction of a new variable, PV_QS (volume of pools standardized by discharge and slope). Least square means comparison of mean PV_QS revealed that the control reach was significantly different from road-impacted reaches. Mean volume of pools was significantly larger in the control reach compared to all but one road-impacted stream. This was likely a function of higher wood loading in the control reach and the competence of high flows in the road-impacted reach. Streams affected by road corridors did not have significantly different bankfull spacing of pools from streams not impacted by roads. The multiple interactions among control and response variables explored in this study indicate the need to identify the most constrained and sensitive response variables when attempting to assess channel response to land use.     

Structural and fabric analyses of basal slip zone of the Jin’nosuke-dani landslide, northern central Japan: its application to the slip mechanism of decollement

We describe in detail the deformation structures and textures of a large-scale landslide body that developed in the Betto-dani Valley in northern central Japan. We studied the shape-preferred orientation of clasts and clay flakes and the development of internal shear planes within the slip zone of the landslide. The slip has an average rate of 5–10 cm/year under the overburden pressure of approximately 1.6 MPa; these values are similar to those of the proto-decollement zone of the Nankai accretionary prism in SW Japan. The anisotropy of magnetic susceptibility of samples obtained from the slip zone reveals that the long axes of clay flakes define an imbricate structure. The slip was due to a long-term periodical creep, which occurs during the thaw seasons with an average slip rate of 0.16–0.32 μm/min. During the creep, the long axes of grains including clay flakes in the slip zone are developed from parallel to perpendicular to the slip direction. The observed textures provide a clue to elucidate the deformation textures and process in the decollement zone of the Nankai prism.     

Seismicity, seismic input and site effects in the Sahel—Algiers region (North Algeria)

Algiers city is located in a seismogenic zone. To reduce the impact of seismic risk in this Capital city, a realistic modelling of the seismic ground motion (SGM) is conducted by using the hybrid method that combines the finite differences method and the modal summation. For this purpose, a complete database of geological, geophysical and earthquake data is constructed. A critical re-appraisal of the seismicity of the zone [2.25°E–3.50°E, 36.50°N–37.00°N] is performed and an earthquake list, for the period 1359–2002, is compiled. The analysis of existing and newly retrieved macroseismic information allowed the definition of earthquake parameters of macroseismic events for which a degree of reliability is assigned. Geological cross sections have been built up to model the SGM in the city, caused by the 1989 Mont-Chenoua and the 1924 Douéra earthquakes. Synthetic seismograms and response spectral ratio is produced for Algiers, and they show that the soft sediments in Algiers centre are responsible of the noticed amplification of the SGM.     

A contribution to seismic shear design of R/C walls in dual structures

The paper focuses on the seismic response of walls in dual (frame + wall) structures, with particular emphasis on shear behaviour. Although dual structures are widely used in earthquake-resistant medium-rise and high-rise buildings, the provisions of modern seismic codes regarding design of walls for shear are not fully satisfactory, particularly in the (common) case that walls of substantially different length form part of the same structure. Relevant provisions of the leading seismic codes are first summarised and their limitations discussed. Then an extensive parametric study is presented, involving two multistorey dual systems, one with identical walls, and one with walls with unequal length, designed to the provisions of Eurocode 8 for two different ductility classes (H and M). The walls of the same structures are also designed to other methods such as those used in New Zealand and Greece. The resulting different designs are then assessed by subjecting the structures to a suite of strong ground motions, carrying out inelastic time history analysis, and comparing the results against design action effects. It is found that although modern code procedures generally lead to satisfactory performance (differences among them do exist), the design of walls seems to be less appropriate in the case of unequal length walls. For this case a modified procedure is proposed, consisting of an additional factor to account for the relative contribution of walls of the same length to the total base and an improved envelope of wall shears along the height; this improved method seems to work better than the other procedures evaluated herein, but further calibration is clearly required.