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1.
Uncertainty in the predicted ultimate pullout strength of soil nails can be significant due to the complexity of nail–soil interactions, inherent variability in soil properties and the effects of nail installation. The paper first presents a statistical evaluation of the accuracy of ultimate bond strength of soil nails using the effective stress method (ESM) equation that has been adopted in Hong Kong. A total of 113 ultimate nail capacity data points from field pullout tests were collected from the literature and used to estimate the accuracy of the current ESM. Based on the available data, the current ESM default pullout model is found to be excessively conservative (on average) by at least a factor of three. The spread in prediction accuracy measured by the coefficient of variation (COV) of bias is in the range of 36–43 % after removing anomalous test data. Here, bias is the ratio of measured to predicted pullout load capacity. In addition, the accuracy of the current ESM equation for prediction of nail bond strength is shown to be dependent on the magnitude of predicted ultimate bond strength and magnitude of nominal vertical effective stress which is undesirable. The paper examines four candidate-modified bond strength equations with empirical coefficients that have been back-fitted to measured bond strengths to improve the overall accuracy of the equation and to reduce or remove the undesirable dependencies noted above. One equation with an empirically corrected stress-dependent term is judged to be the best candidate model based on the mean of bias values, spread (COV) of bias values, lack of dependencies and simplicity. Finally, the relative contributions of random variability in soil shear strength to measurement bias in bond strength (prediction accuracy) for each soil type are computed for the best bond strength model. Analysis of the contribution of soil shear strength to prediction accuracy showed that the combination of variability due to factors other than soil shear strength was greater than the variability in soil shear strength alone, where the latter is defined by the soil secant friction coefficient. 相似文献
2.
Acta Geotechnica - Statistical evaluation of the accuracy of effective stress method (ESM) equations for prediction of the ultimate bond strength of soil nails was performed using the maximum... 相似文献
3.
This paper presents reliability analyses of soil nail walls against two external ultimate limit states, global and sliding stabilities, which are related to the external stability failures of soil nail walls. Reliability analyses are conducted using Monte Carlo simulation technique. Soil nailing is a popular retaining system in highway construction and slope stabilization, and its current design practice is still based on the working stress design. There remains a need to establish a more rational design framework—load and resistance factor design—based on the concept of limit state design and reliability analysis for soil nail walls. The development of load and resistance factor design approach must consider multiple ultimate limit states, associated with external, internal, and facing failures. The analyses of resistance factors against two external failures are conducted in this study considering various influencing factors, including statistical parameters of soil friction angle, ultimate bond strength between soil and nails, soil type, wall geometry (wall height, back slope angle, and face batter angle), and nail configurations (nail inclination angle, drillhole diameter, and nail spacing). In the end, a series of resistance factors are proposed for potential application of load and resistance factor design approach against external failures for soil nail walls according to different design codes. 相似文献
4.
Spatial data that are incomplete because of observations arising below or above a detection limit occur in many settings, for example, in mining, hydrology, and pollution monitoring. These observations are referred to as censored observations. For example, in a life test, censoring may occur at random times because of accident or breakdown of equipment. Also, censoring may occur when failures are discovered only at periodic inspections. Because the informational content of censored observations is less than that of uncensored ones, censored data create difficulties in an analysis, particularly when such data are spatially dependent. Traditional methodology applicable for uncensored data needs to be adapted to deal with censorship. In this paper we propose an adaptation of the traditional methodology using the so-called Expectation-Maximization (EM) algorithm. This approach permits estimation of the drift coefficients of a spatial linear model when censoring is present. As a by-product, predictions of unobservable values of the response variable are possible. Some aspects of the spatial structure of the data related to the implicit correlation also are discussed. We illustrate the results with an example on uranium concentrations at various depths. 相似文献
5.
An empirical approach to estimating population parameters from censored fracture-length distributions 总被引：1，自引：0，他引：1
Pascal de Caprariis 《Mathematical Geology》1988,20(7):803-814
Most fracture data sets are length-censored because of incomplete exposure at the surface, so estimating values of parameters of the sampled populations is difficult. Unless the form of the distribution function of the population is known, or one which is analytically tractable is assumed, length-censoring presents formidable problems in determining population parameters. Tests conducted on experimental fracture patterns developed in clay samples subjected to simple shear loading are the basis of a distribution-free way to estimate population parameters. Comparison of random samples of censored and uncensored fracture lengths shows that a useful, homogeneous data set consists of those cracks which lie entirely within the sampling area (two-ended cracks). The properties of this data set can be used to estimate the mean and variance of uncensored data. Estimates of the maximum fracture length of uncensored data, using the variance and maximum length of these two-ended cracks, show good agreement with measured values. Knowledge of the mean, variance, and maximum value of fracture-length populations are of interest in engineering and hydraulic studies, as well as in remote sensing studies of the Earth and other planets. Application of these results to data on rock masses are subject to the caveat that different crack-growth mechanisms in clay and rock may affect the accuracy of the calculations. 相似文献
6.
In practice, field pullout testing of soil nails is a widely accepted method for the construction quality control and the performance assessment of soil nail walls. At the design stage, field pullout tests provide useful information about the design bond strength of the nail–soil interface, which is critical in the determination of minimum soil nail length. To obtain a reliable and representative estimate of the design bond strength, it is necessary to conduct a sufficient number of field pullout tests. This study demonstrates the use of the concepts of simple random sampling and stratified random sampling to assess the adequacy of the ultimate bond strength adopted for design based on the minimum recommended number of field pullout tests. 相似文献
7.
Kok-Kwang Phoon 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2019,13(2):101-130
The calculated response from a numerical model will deviate from the measured one given the presence of modelling idealizations and real world construction effects. This deviation can be directly captured by a ratio between the measured and the calculated quantity. The ratio is also called a model factor in many design guides. The probabilistic distribution of the model factor is arguably the most common and simplest complete representation of model uncertainty. The characterisation of model uncertainty is identified as one of the critical elements in a geotechnical reliability-based design process in Annex D of ISO 2394:2015 “General Principles on Reliability of Structures”. This Spotlight paper reviews the databases for various geo-structures and determines their associated model statistics. Foundation load test databases are the most prevalent. A recent effort to compile a large generic database (PILE/2739) that contains 2739 field load tests conducted on various piles and installed in different soils and countries, is highlighted. This systematic compilation of load test data is part of a broader research agenda to digitalise foundation design for “precision construction”, which is targeted at characterising “site-specific” model factors and soil parameters based on both site-specific and generic data for further customisation of design to a particular site. The mean and COV of the model factor for a range of geo-structures, geomaterials, and limit states (both ultimate and serviceability) are summarized in a form suitable for adoption in design and codes of practice. Based on this summary, it is proposed that a model factor for a design model can be classified as: (1) moderately conservative (1?≤?mean?相似文献
8.
预应力锚索复合土钉支护的现场测试研究 总被引：2，自引：0，他引：2
结合北京熊猫环岛地铁车站深基坑支护工程,通过对预应力锚索与土钉联合使用条件下土钉轴力随时间变化、土钉轴力沿长度的分布、土钉轴力最大值的分布、锚索预应力的变化等现场测试及分析,从力学机制上研究了预应力锚索复合土钉的工作机理,研究表明:(1)土钉受力具有时间效应及开挖效应;(2)土钉轴力形成和空间分布与基坑潜在滑动趋势有关;(3)预应力锚索对作用范围内土钉轴力空间分布有一定影响;(4)预应力锚索可减小坡体位移,使得各土钉轴力相对减小,改善坡体应力状态;(5)工作状态下,土钉和预应力锚索的受力状态变化相互影响较小,各自发挥作用。 相似文献
9.
Eulogio Pardo-Igúzquiza 《Mathematical Geology》1998,30(1):95-108
In this paper, the maximum likelihood method for inferring the parameters of spatial covariances is examined. The advantages of the maximum likelihood estimation are discussed and it is shown that this method, derived assuming a multivariate Gaussian distribution for the data, gives a sound criterion of fitting covariance models irrespective of the multivariate distribution of the data. However, this distribution is impossible to verify in practice when only one realization of the random function is available. Then, the maximum entropy method is the only sound criterion of assigning probabilities in absence of information. Because the multivariate Gaussian distribution has the maximum entropy property for a fixed vector of means and covariance matrix, the multinormal distribution is the most logical choice as a default distribution for the experimental data. Nevertheless, it should be clear that the assumption of a multivariate Gaussian distribution is maintained only for the inference of spatial covariance parameters and not necessarily for other operations such as spatial interpolation, simulation or estimation of spatial distributions. Various results from simulations are presented to support the claim that the simultaneous use of maximum likelihood method and the classical nonparametric method of moments can considerably improve results in the estimation of geostatistical parameters. 相似文献
10.
An objective replacement method for censored geochemical data 总被引：1，自引：0，他引：1
Geochemical data are commonly censored, that is, concentrations for some samples are reported as less than or greater than some value. Censored data hampers statistical analysis because certain computational techniques used in statistical analysis require a complete set of uncensored data. We show that the simple substitution method for creating an uncensored dataset, e.g., replacement by3/4 times the detection limit, has serious flaws, and we present an objective method to determine the replacement value. Our basic premise is that the replacement value should equal the mean of the actual values represented by the qualified data. We adapt the maximum likelihood approach (Cohen, 1961) to estimate this mean. This method reproduces the mean and skewness as well or better than a simple substitution method using3/4 of the lower detection limit or3/4 of the upper detection limit. For a small proportion of less than substitutions, a simple-substitution replacement factor of 0.55 is preferable to3/4; for a small proportion of greater than substitutions, a simple-substitution replacement factor of 1.7 is preferable to4/3, provided the resulting replacement value does not exceed 100%. For more than 10% replacement, a mean empirical factor may be used. However, empirically determined simple-substitution replacement factors usually vary among different data sets and are less reliable with more replacements. Therefore, a maximum likelihood method is superior in general. Theoretical and empirical analyses show that true replacement factors for less thans decrease in magnitude with more replacements and larger standard deviation; those for greater thans increase in magnitude with more replacements and larger standard deviation. In contrast to any simple substitution method, the maximum likelihood method reproduces these variations. Using the maximum likelihood method for replacing less thans in our sample data set, correlation coefficients were reasonably accurately estimated in 90% of the cases for as much as 40% replacement and in 60% of the cases for 80% replacement. These results suggest that censored data can be utilized more than is commonly realized. 相似文献
11.
12.
Acta Geotechnica - Helical soil nails are passive elements installed in the soil which attains its bond strength through skin friction and bearing from helices. The present study examines the... 相似文献
13.
W. D. Guo 《国际地质力学数值与分析法杂志》2014,38(18):1969-1989
Recent study indicates that the response of rigid passive piles is dominated by elastic pile–soil interaction and may be estimated using theory for lateral piles. The difference lies in that passive piles normally are associated with a large scatter of the ratio of maximum bending moment over maximum shear force and induce a limiting pressure that is ~1/3 that on laterally loaded piles. This disparity prompts this study. This paper proposes pressure‐based pile–soil models and develops their associated solutions to capture response of rigid piles subjected to soil movement. The impact of soil movement was encapsulated into a power‐law distributed loading over a sliding depth, and load transfer model was adopted to mimic the pile–soil interaction. The solutions are presented in explicit expressions and can be readily obtained. They are capable of capturing responses of model piles in a sliding soil owing to the impact of sliding depth and relative strength between sliding and stable layer on limiting force prior to ultimate state. In comparison with available solutions for ultimate state, this study reveals the 1/3 limiting pressure (of the active piles) on passive piles was induced by elastic interaction. The current models employing distributed pressure for moving soil are more pertinent to passive piles (rather than plastic soil flow). An example calculation against instrumented model piles is provided, which demonstrates the accuracy of the current solutions for design slope stabilising piles. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
14.
结合基桩承载过程中的桩周土体应力状态分析和桩土界面摩擦特性分析,推导出基于土体应力状态的沿桩土界面的土体抗剪强度计算模型和基于界面摩擦特性的界面抗剪强度计算模型。通过对比土体抗剪强度和界面抗剪强度,推导出受该两种强度耦合作用影响的极限侧阻力计算模型,并用上海某工程算例验证该模型的合理性和可行性,同时利用该模型分析不同类型的土的极限侧阻力随埋深的分布规律。结果显示：因内摩擦角和侧压力系数之间的不同关系,极限侧阻力随埋深表现出不同的分布特点,在特定关系下,存在一个临界深度,超过该深度极限侧阻力维持在一个稳定值甚至不断减小直至为零。 相似文献
15.
深基坑土钉支护现场测试分析研究 总被引：19，自引：3，他引：16
土钉支护技术在我国深基坑开挖和支护中己得到了广泛的应用,但对其工作机理和计算方法的研究尚不完善。以一个基坑土钉支护工程为实例,对基坑水平位移、土钉拉力进行现场测试,得出了土钉水平位移和拉力的分布规律:(1)基坑最大位移发生在基坑顶部;(2)沿基坑深度范围受力最大的土钉在中部;(3)单根土钉最大拉力作用点在其长度的中部,沿基坑深度方向土钉最大拉力作用点的连线形成的曲线是潜在最危险滑动面的位置。 相似文献
16.
We develop an improved methodology for modelling the relationship between mean track length, temperature, and time in fission track annealing experiments. We consider “fanning Arrhenius” models, in which contours of constant mean length on an Arrhenius plot are straight lines meeting at a common point. Features of our approach are explicit use of subject matter knowledge, treating mean length as the response variable, modelling of the mean-variance relationship with two components of variance, improved modelling of the control sample, and using information from experiments in which no tracks are seen.
This approach overcomes several weaknesses in previous models and provides a robust six parameter model that is widely applicable. Estimation is via direct maximum likelihood which can be implemented using a standard numerical optimisation package. Because the model is highly nonlinear, some reparameterisations are needed to achieve stable estimation and calculation of precisions. Experience suggests that precisions are more convincingly estimated from profile log-likelihood functions than from the information matrix.
We apply our method to the B-5 and Sr fluorapatite data of Crowley et al. (1991) and obtain well-fitting models in both cases. For the B-5 fluorapatite, our model exhibits less fanning than that of Crowley et al. (1991), although fitted mean values above 12 μm are fairly similar. However, predictions can be different, particularly for heavy annealing at geological time scales, where our model is less retentive. In addition, the refined error structure of our model results in tighter prediction errors, and has components of error that are easier to verify or modify. For the Sr fluorapatite, our fitted model for mean lengths does not differ greatly from that of Crowley et al. (1991), but our error structure is quite different. 相似文献
17.
Empirical Maximum Likelihood Kriging: The General Case 总被引：4，自引：0，他引：4
Although linear kriging is a distribution-free spatial interpolator, its efficiency is maximal only when the experimental data follow a Gaussian distribution. Transformation of the data to normality has thus always been appealing. The idea is to transform the experimental data to normal scores, krige values in the “Gaussian domain” and then back-transform the estimates and uncertainty measures to the “original domain.” An additional advantage of the Gaussian transform is that spatial variability is easier to model from the normal scores because the transformation reduces effects of extreme values. There are, however, difficulties with this methodology, particularly, choosing the transformation to be used and back-transforming the estimates in such a way as to ensure that the estimation is conditionally unbiased. The problem has been solved for cases in which the experimental data follow some particular type of distribution. In general, however, it is not possible to verify distributional assumptions on the basis of experimental histograms calculated from relatively few data and where the uncertainty is such that several distributional models could fit equally well. For the general case, we propose an empirical maximum likelihood method in which transformation to normality is via the empirical probability distribution function. Although the Gaussian domain simple kriging estimate is identical to the maximum likelihood estimate, we propose use of the latter, in the form of a likelihood profile, to solve the problem of conditional unbiasedness in the back-transformed estimates. Conditional unbiasedness is achieved by adopting a Bayesian procedure in which the likelihood profile is the posterior distribution of the unknown value to be estimated and the mean of the posterior distribution is the conditionally unbiased estimate. The likelihood profile also provides several ways of assessing the uncertainty of the estimation. Point estimates, interval estimates, and uncertainty measures can be calculated from the posterior distribution. 相似文献
18.
Castorina Silva Vieira Maria de Lurdes Lopes Laura M. Caldeira 《Geotechnical and Geological Engineering》2013,31(4):1123-1135
The paper presents results from a computer code, based on limit equilibrium analyses, able to quantify earth pressure coefficients for the internal design of geosynthetic reinforced soil structures and identify the potential failure surfaces. Failure mechanisms assuming bilinear or logarithmic spiral failure surfaces are considered. The influence of the potential failure surface and geosynthetic strength distribution on the earth pressure coefficient is analysed. Required reinforcement tensile strengths calculated by the developed program are compared with values published in the literature. To further evaluate the capabilities of limit equilibrium analyses, the numerical modelling of a geosynthetic reinforced steep slope, designed at ultimate limit state conditions (FS = 1), is also presented. Good agreement was achieved between the potential failure surfaces predicted by limit equilibrium analyses and those obtained with numerical modelling. 相似文献
19.
西北地区深大基础工程日益增多,兼顾基础抗浮和耐久性问题的研究空白,借助西宁火车站综合改造工程,引入大直径布袋桩技术,有效解决了基础抗浮和耐久性问题;选择6根试桩进行了现场单桩抗拔载荷试验,最大加载量为9 060 kN;运用MATLAB软件分别拟合出3种抗拔极限承载力预测函数模型的曲线,同时运用PLAXIS软件对不同等级荷载桩-土位移进行模拟,并与实测的荷载-位移曲线对比分析。研究发现：双曲线和幂函数模型较适合此类抗拔桩极限承载力预测;本地区类似地基预测大直径缓变形抗拔桩极限荷载所需的极限位移标准应由0.030D减小为0.025D;仅根据土层的物理力学特征确定抗拔桩桩周土的极限摩阻力不够完善,至少还要考虑埋深不同对具有相似物理力学特征土层性质的影响。 相似文献
20.
The present paper addresses statistical analysis and estimation of fracture-length distributions at scales influenced by the truncation and censoring effects. The computational method employed here uses fracture-length distributions of a given set of measurements and information about observational constraints (i.e., window of observation) to estimate the probability density of the truncated and censored parts of fracture data sets. The results are benchmarked against power-law based maximal likelihood estimations commonly used for the same purpose. The relationship between the accuracy of estimates and size of the window of observation is studied. The utility of employing statistical models with arbitrary probability distributions of fracture lengths in order to provide a valid statistical model approximation is also considered. A verification of the suggested approximation using the Kolmogorov–Smirnov test applied to truncated and censored data is proposed. Numerical computations show that the proposed method can represent an essential improvement compared to other commonly employed techniques. 相似文献