剪切带图案的复杂性及应力-应变曲线的离散性

在平面应变压缩条件下,通过在岩样左边界靠近下端面的位置及右边界的不同位置预制材料缺陷,采用拉格朗日元法研究了材料缺陷位置不同时的剪切带图案启动、演变及试样的宏观力学行为。在数值计算中,采用了莫尔-库仑与拉破坏复合的破坏准则,峰后岩石的本构关系为线性应变软化。当试样左、右边界上的缺陷距离较远时,剪切带仅出现1条,以通过两缺陷形式贯通试样两边界;当两缺陷距离较近时,启动于缺陷附近的剪切带按各自其固有方向发展,剪切带图案十分复杂。当右缺陷距离试样上端面较近时,由于上端面的约束,仅位于缺陷下方的剪切带能得到充分的发展。若剪切带数量较少且贯通试样两边界时,试样发生脆性剪切破坏,应力-变形曲线软化段比较陡峭;反之,发生韧性剪切破坏,软化段比较平缓。当剪切带的前端迎面遇到试样上、下端面附近的上、下三角形区域时,由于端面的约束,剪切带的方向将改变。当剪切带的前端与试样的纵向对称线距离较近时,剪切带发生折射现象;当距离较远时,剪切带将发生反射现象。     

瞬态与稳态瑞利波法频散曲线等价性研究

简述了稳态及瞬态瑞利波测试系统和瑞利波相速度频散曲线的求取原理,指明了瞬态SASW法所得的频散曲线与稳态法结果等价的理论前提,提出了一种通过控制测试过程来改善SASW法测试效果的优化相干函数方法。     

黄河大堤非饱和土土-水特性试验研究

模拟工程中非饱和土体的实际受力状态,利用改进的非饱和土三轴仪对黄河大堤非饱和土在不同围压条件下的土-水特性进行了试验研究。根据试验结果,分析了黄河大堤非饱和土体在外荷作用下土-水特征曲线及其变化规律。并对相应的土-水特征曲线模型进行了拟合,得出其拟合函数。根据土-水特征曲线可知,非饱和土的基质吸力随着土体所受周围压力的增大而减小,随着含水量的增大而减小;对于某一类土而言,在高含水量(含水量＞20 %) 时,土的基质吸力随含水量的变化较小,而在低含水量 (含水量≤20 %)时,土的基质吸力随含水量的变化较大。随着含水量的增加,非饱和土的工程性质向弱性变化。     

煤岩两体模型变形破坏数值模拟

采用拉格朗日元法,在弹性岩石与弹性-应变软化煤体所构成的平面应变两体模型的上、下端面上不存在水平方向摩擦力条件下,模拟了模型的破坏过程、岩石高度对模型及煤体全程应力-应变曲线、煤体变形速率、煤体破坏模式及剪切应变增量分布的影响。结果表明,当模型的全程应力-应变曲线达到峰值时煤体内部的剪切带图案已经十分明显,在模型的应变硬化阶段,煤体中的应变局部化可视为模型失稳破坏的前兆,随岩石高度的增加,模型应力-应变曲线的软化段变得陡峭,这与单轴压缩条件下的解析解在定性上是一致的;煤体应力-应变曲线的软化段变得平缓,煤体消耗能量的能力增强;弹性阶段煤体的变形速率降低;煤体内部的剪切应变增量增加。煤体应力-应变曲线的软化段的斜率、弹性阶段煤体的变形速率、煤体内部的剪切应变增量及塑性耗散能都受岩石高度的影响,说明了岩石几何尺寸对煤体的影响(煤岩相互作用)是不容忽视的。     

昆仑山隧道冻胀力现场测试及其数据分析

冻胀力是冻土隧道支护结构产生病害的主要原因之一,为掌握多年冻土区昆仑山隧道冻胀力的分布形态以及随温度变化的规律,进行了现场测试研究。研究结果表明,到目前为止昆仑山隧道整个支护结构是安全可靠的;冻胀力随着气温、季节的变化而变化,2004年后其变化基本趋于稳定,实测到的冻胀力最大值为0.3 MPa,分布于围岩与初期支护之间仰拱部位;初期支护与二次衬砌之间的冻胀力很小,二次衬砌支护强度可考虑适当减弱;运用影响线理论绘制出了冻胀力包络图,其为确定作用于支护结构上的最不利荷载提供了一种合理有效的方法。     

灰色理论在快速法载荷试验数据处理中的应用

探讨了灰色理论方法在快速法载荷试验数据处理中的应用,并运用此理论对快速载荷试验各级荷载下的t-s曲线及总体的P-S曲线进行预测,以得到荷载作用下与慢速法一致的稳定沉降量。用VB语言编制了实用计算程序,通过对实例计算,得到的预测值与稳定沉降进行对比,证明了该法对于两种曲线的预测是可行的。同时评价了GM(1,1)以及等维灰数递补GM(1,1)两种模型对于远距离预测的精度。算例表明,对于P-S曲线的外推预测,最好只进行2～3级,外推级别较多时,精度不高,若应用于远距离预测,必须对模型作进一步改进。     

Preliminary flood frequency estimates for Lebanon

Abstract

This paper describes a first attempt at developing a regional flood estimation methodology for Lebanon. The analyses are based on instantaneous flood peak data for the whole country, and cover the period from the start of observations in the 1930s to the start of the civil war in the mid-1970s. Three main flood-generating zones are identified, and regional flood growth curves are derived for each zone using the Generalized Extreme Value distribution fitted by probability-weighted moments. Typical parameter values are presented, together with regression coefficients for estimating the mean annual flood. Based on this work, several recommendations are made on the future data collection and analysis requirements to develop a national flood estimation methodology for Lebanon.     

Variation in curve numbers derived from plot runoff data for New South Wales (Australia)

The curve number method is a simple one parameter (the curve number) rainfall runoff model. While its theoretical underpinning has been questioned it remains a powerful hydrological tool in the absence of detailed data and is therefore used extensively in hydrological models. This study aims to characterize the variation in maximum retention values (S), which underlie curve numbers, for a range of agricultural treatments across a large spatial area in New South Wales (NSW), Australia. The data used for the analysis spans several decades of rainfall runoff observations. A range of different derivation methods result in variation in mean and variance of S. In particular, methods that emphasize the larger storms result in greater S and thus lower runoff. For larger spatial scales, emphasis on larger storms gives more reliable estimates of S. Systematic variation in S arises from variations in treatment, pre‐runoff soil moisture, rainfall depth, and variations in cover. On the basis of the analysis, a table of curve number values for different land uses found in NSW is presented. The resulting distributions of S and curve numbers provide guidance for rainfall runoff modelling studies in the agricultural important areas of NSW. Copyright © 2011 John Wiley & Sons, Ltd.     

Rating curve estimation under epistemic uncertainty

River discharge values, estimated using a rating curve, are subject to both random and epistemic errors. We present a new likelihood function, the ‘Voting Point’ likelihood that accounts for both error types and enables generation of multiple possible multisegment power‐law rating curve samples that aim to represent the total uncertainty. The rating curve samples can be used for subsequent discharge analysis that needs total uncertainty estimation, e.g. regionalisation studies or calculation of hydrological signatures. We demonstrate the method using four catchments with diverse rating curve error characteristics, where epistemic uncertainty sources include weed growth, scour and redeposition of the bed gravels in a braided river, and unconfined high flows. The results show that typically, the posterior rating curve distributions include all of the gauging points and succeed in representing the spread of discharge values caused by epistemic rating errors. We aim to provide a useful method for hydrology practitioners to assess rating curve, and hence discharge, uncertainty that is easily applicable to a wide range of catchments and does not require prior specification of the particular types and causes of epistemic error at the gauged location. Copyright © 2014 John Wiley & Sons, Ltd.     

SWAT application to estimate design runoff curve number for South Korean conditions

Runoff estimations based on the standard USDA–NRCS curve number (CN) table without calibration have a tendency to give inaccurate results when the CN values are applied in South Korea which has many high slope watersheds and that has a continental monsoon climate. Particularly for the design flood estimation, accurately calibrated CN values are required because the estimated peak flow is very sensitive to the selection of CN. However, the lack of flood data makes it difficult to calibrate and assign runoff CNs to Korean watersheds. Even if sufficient data are available to estimate CN values, it is also difficult to obtain the direct flows by separating base flows from total runoff hydrographs due to the temporal irregularity of rainfall events and the resulting complex pattern of runoff. Therefore, an alternative method for estimating CNs needs to be developed to overcome these issues. The purpose of this study is to present a method for estimating runoff CNs using the soil and water assessment tool (SWAT) model which can take into account watershed heterogeneities such as climate conditions, land use and soil types. The proposed CN estimation method uses the simulated flow data by SWAT instead of using measured flow data. This method has advantages in estimating CN values spatially for each subbasin division considering watershed characteristics. The use of daily data can reduce the sensitivity to the abnormality that is commonly involved in flow data with a small time scale. The SWAT‐based CN estimation method, combined with the asymptotic CN method, was applied to the Chungju dam watershed in South Korea. A regression equation was then developed from this approach, which was used to estimate CN values that decrease exponentially as rainfall amounts increase and that converge to 60·6 and 79·4 without and with considering subsurface lateral flow, respectively. Furthermore, the CN values for the antecedent moisture conditions were determined using the probabilistic approach. The CN associated with the 50% probability for the Chungju dam watershed is 87·8 which can be taken to be representative of antecedent moisture condition (AMC) II. The CN_I and CN_III associated with 90% and 10% probabilities are 78·9 and 94·1, respectively. The estimated CN_II = 87·8 differs markedly from the geographic information system (GIS)‐based CN 65·0, which implies that the standard USDA–NRCS CN method should be calibrated to the studied area of interest. Copyright © 2010 John Wiley & Sons, Ltd.