非饱和土力学中几个基本问题的探讨

近些年非饱和土力学的研究非常活跃,但对一些基本问题的认识并不一致,有时甚至概念混淆。针对非饱和土力学的几个基本问题：非饱和土状态变量的选择、非饱和土有效应力变量的选择、吸力概念的界定和轴平移技术的局限性、非饱和土的结构的表征方法等问题进行了分析和探讨。其中非饱和土状态变量和有效应力的选择对于非饱和土力学的理论和相应本构模型的建立具有重要影响,因此,首先深入讨论了这一问题,概括论述了非饱和土有效应力的演变并深入探讨了目前各种形式有效应力的优缺点。其次,指出由于受负压孔隙水气化（液-气相变化）的影响,在实际场地中大于某一界限值的基质吸力是不存在的;目前被广泛使用的轴平移试验技术却掩盖了这一情况,而基于此所建立的非饱和土强度和变形理论的适用性需要进一步的研究和论证。再次,指出非饱和土的结构除了包括组构和颗粒之间作用力的综合效应外,还建议增加孔隙水和孔隙气的分布以及各相之间的相互作用和物理-化学作用。最后对一些容易混淆的概念进行了梳理。其目的是希望国内同行在今后的研究中对这些问题加以关注,并建立正确的认识,促进非饱和土力学沿着正确的方向发展。     

Site class mapping based on earthquake ground motion data recorded by regional seismographic network

This paper presents a methodology for site class mapping in regions without adequate geotechnical, geologic and geomorphologic data, which is prevailing in many less developed regions globally. The proposed methodology is based primarily on analysis of earthquake ground motion data recorded by regional seismographic network. Three analysis methods independently developed have been adopted with appropriate weightings, from which a continuous value of site class index ranging from 1 to 4 could be assigned to each station. Finally, a regional site class map could be developed by applying an interpolation procedure across all stations of the seismographic network of which the site classes were estimated.     

Method for estimating available groundwater volume of small coral islands

We present a simple modelling method to estimate the volume of available groundwater in the freshwater lens of atoll islands under steady-state conditions. Model inputs include annual rainfall depth, island width for cross-sections along the length of the island, aquifer hydraulic conductivity, and depth to the contact between the upper sand aquifer and the lower limestone aquifer. The methodology is tested for nine islands of varying size in the Maldives and Micronesia. Sensitivity analysis indicates that lens volume on large islands typically is governed by the depth to the discontinuity, whereas lens volume for smaller islands is governed by rainfall rate and hydraulic conductivity. Volume curves, which relate lens volume to lens thickness, are developed for each of the nine islands and for three generic island shapes to allow rapid estimation of lens volume given field-estimated lens thickness. The methods presented in this study can be used for any small atoll island.     

Characteristics of near-source ground motions from the 2012 Varzaghan–Ahar double earthquakes, Northwest of Iran

Wavelet-based analyses were used in this study to interpret the near-source ground-motion characteristics of records obtained during the 2012 Varzaghan–Ahar double earthquakes in the East-Azerbaijan province of Iran. For this purpose, the large pulse of ground motions was extracted by applying continuous wavelet transforms, and then, the residual motions were achieved by subtraction of extracted pulse from the original motions. Analysis of ground motions illustrated that the records obtained at Varzaghan station could be classified as pulse-like motions with pulse index around 0.9. As well, the acceleration response spectra for horizontal directions were compared with the design response spectrum of Iranian seismic design code (Standard No. 2800). The comparisons showed a relatively large amplification in spectral values due to near-source pulse effects.     

Near-field probabilistic seismic hazard analysis with characteristic earthquake effects

In conventional seismic hazard analysis, uniform distribution over area and magnitude range is assumed for the evaluation of source seismicity which is not able to capture peculiar characteristic of near-fault ground motion well. For near-field hazard analysis, two important factors need to be considered: (1) rupture directivity effects and (2) occurrence of scenario characteristic ruptures in the nearby sources. This study proposed a simple framework to consider these two effects by modifying the predictions from the conventional ground motion model based on pulse occurrence probability and adjustment of the magnitude frequency distribution to account for the rupture characteristic of the fault. The results of proposed approach are compared with those of deterministic and probabilistic seismic hazard analyses. The results indicate that characteristic earthquake and directivity consideration both have significant effects on seismic hazard analysis estimates. The implemented approach leads to results close to deterministic seismic hazard analysis in the short period ranges (T < 1.0 s) and follows probabilistic seismic hazard analysis results in the long period ranges (T > 1.0 s). Finally, seismic hazard maps based on the proposed method could be developed and compared with other methods.     

Modification of the GALDIT framework using statistical and entropy models to assess coastal aquifer vulnerability

Recently, groundwater vulnerability assessment of coastal aquifers using the GALDIT framework has been widely used to investigate the process of groundwater contamination. This study proposes multi-attribute decision-making (MADM) entropy and Wilcoxon non-parametric statistical test methods to improve the vulnerability index of coastal aquifers. The rates and weights of this framework were modified using Wilcoxon non-parametric and entropy methods, respectively, and a combined framework of GALDIT-entropy, Wilcoxon-GALDIT, and Wilcoxon-entropy was obtained. Pearson correlation coefficients between the mentioned vulnerability indices and total-dissolved solids (TDS) of 0.51, 0.66 and 0.75, respectively, were obtained. According to the results, the Wilcoxon-entropy index had the highest correlation with TDS. Generally, it can be concluded that the proposed frameworks provide a more accurate estimation of vulnerability distribution in coastal aquifers.     

Laboratory Study of Topographic Effects on the Near-surface Tornado Flow Field

To study topographic effects on the near-surface tornado flow field, the Iowa State University tornado simulator was used to simulate a translating tornado passing over three different two-dimensional topographies: a ridge, an escarpment and a valley. The effect of the translation speed on maximum horizontal wind speeds is observed for translation speeds of 0.15 and 0.50 \(\hbox {m}\,\hbox {s}^{-1}\), with the lower value resulting in a larger maximum horizontal wind speed. The tornado translation over the three topographies with respect to flat terrain is assessed for changes in: (a) the maximum horizontal wind speeds in terms of the flow-amplification factor; (b) the maximum aerodynamic drag in terms of the tornado speed-up ratio; (c) the maximum duration of exposure at any location to high wind speeds of a specific range in terms of the exposure amplification factor. Results show that both the maximum wind amplification factor of 14%, as well as the maximum speed-up ratio of 14%, occur on the ridge. For all topographies, the increase in aerodynamic drag is observed to be maximized for low-rise buildings, which illustrates the importance of the vertical profiles of the horizontal wind speed near the ground. The maximum exposure amplification factors, estimated for the range of wind speeds corresponding to the EF2 (50–60 \(\hbox {m}\,\hbox {s}^{-1}\)) and EF3 (61–75 \(\hbox {m}\,\hbox {s}^{-1})\) scales, are 86 and 110% for the ridge, 4 and 60% for the escarpment and ? 6 and 47% for the valley, respectively.