ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Mon, 17 Jun 2019 07:01:46 GMT2019-06-17T07:01:46Z502121- Stochastic analysis method of critical slip surfaces in soil slopes considering spatial variabilityhttps://scholarbank.nus.edu.sg/handle/10635/59219Title: Stochastic analysis method of critical slip surfaces in soil slopes considering spatial variability
Authors: Qi, X.-H.; Li, D.-Q.; Zhou, C.-B.; Phoon, K.-K.
Abstract: The effect of spatial variability in soil parameters on the critical slip surfaces has not been investigated substantially. Therefore, this paper aims to propose a stochastic method to determine the critical slip surfaces in soil slopes considering spatial variability of soil strength properties. First, the spectral representation method is adopted to simulate random field of spatially varying soils. Based on this random field model, the critical slip surface is determined by means of the auto-searching method with SIGMA/W and SLOPE/W. Second, the effect of spatial variability in soil parameters on the characteristics of the critical slip surfaces is investigated using a non-intrusive stochastic analysis method. Finally, an illustrative example is presented to demonstrate the validity of the proposed method. The results indicate that the proposed stochastic method can determine the characteristics of the critical slip surfaces effectively. The spatial variability has a significant influence on the position and scale of the critical slip surfaces. The longer the scale of fluctuation is, the wider the critical slip surface distribution is. The possibility of upper-located local sliding increases with the increasing ratio of horizontal fluctuation scale to vertical fluctuation scale. As the coefficients of variation of soil parameters increase, the range of critical slip surface will increase, and small-scale local sliding is more likely to happen.
Mon, 01 Apr 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/592192013-04-01T00:00:00Z
- Some observations on the performance of the signal matching technique in assessment of pile integrityhttps://scholarbank.nus.edu.sg/handle/10635/59213Title: Some observations on the performance of the signal matching technique in assessment of pile integrity
Authors: Chai, H.Y.; Wei, C.-F.; Phoon, K.-K.; Yang, Y.-M.
Abstract: The pulse-echo method is commonly used to assess pile integrity in a nondestructive way. One of the strategies for detecting relative variation in pile impedance is to analyze the wave reflections from the anomalies based on the 1-D stress wave theory. In current practice, however, several difficulties remain to be resolved in interpreting the wave patterns. Firstly, due to possible three-dimensional (3-D) behavior near the source and dispersion behavior far from the source, 1-D stress wave theory is not always applicable in analyzing the reflections from the anomalies. Secondly, reflections can be produced continuously along the shaft due to the pile-soil interaction, so that the reflection patterns are highly correlated to those from the pile body in complex layered soil profiles, and thus it is generally difficult to distinguish whether the reflections are produced by pile anomalies or by the changes in the soil profiles. In this paper, actual wave characteristics are analyzed based on numerical simulations and guided wave theory, the conditions for 1-D approximation are suggested, and the a method for uncoupling the soil resistance and the pile impedance effects is presented. The evaluation of pile integrity can be improved with help of the 1-D based signal matching technique. The technique is applied to experiments conducted on model piles, test piles for accreditation of pile inspectors, and routine in-situ piles. The results show that 1-D stress wave theory is approximately applicable in analyzing the reflections from deep anomalies under certain limited conditions, and the soil resistance and the pile impedance effects can be effectively uncoupled by relating the pile-soil interaction to the pile radius and the properties of the surrounding soils. © 2011 Springer Science+Business Media, LLC.
Thu, 01 Dec 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/592132011-12-01T00:00:00Z
- Slope stability analysis considering antecedent rainfall processhttps://scholarbank.nus.edu.sg/handle/10635/59212Title: Slope stability analysis considering antecedent rainfall process
Authors: Tang, D.; Li, D.-Q.; Zhou, C.-B.; Phoon, K.-K.
Abstract: Based on the measured rainfall data in the Three Gorges reservoir area, this paper investigates the effect of different initial conditions on different types of soil slope stability. A method to reflect the real initial conditions of the water content in the slope is recommended. Unsaturated seepage analysis is adopted to study the effect of antecedent rainfall on different types of soil slope stability. The typical sand and clay slopes are taken as examples to investigate the effect of antecedent rainfall on slope stability. The results indicate that the effect of the initial conditions on different types of soil slope stability can differ considerably. It is recommended that the steady-state seepage field under average annual rainfall can be used as the initial conditions for transient seepage analysis. The lower soil permeability coefficient, the greater impact of antecedent rainfall can on slope stability, and the longer time affected by antecedent rainfall. It is recommended that more than 15 days antecedent rainfall should be considered for sand and clay slopes stability analysis. Furthermore, more computational effort for sand slopes will depend on the 5 days rainfall before these 15 days rainfall. The short term and high density antecedent rainfall has a significant effect on sand slope stability, whereas the long term and low density antecedent rainfall has a significant effect on clay slope. The cumulative antecedent rainfall can be taken as a criterion to determine the minimum factor of safety. For the sand slopes, the time corresponding to the minimum factor of safety matches well with the time corresponding to the maximum 10-day cumulative antecedent rainfall, whereas, for the clay slopes, it matches well with the time corresponding to the maximum 15-day cumulative antecedent rainfall.
Fri, 01 Nov 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/592122013-11-01T00:00:00Z
- Probabilistic analysis of load-displacement hyperbolic curves of single pile using Copulahttps://scholarbank.nus.edu.sg/handle/10635/59173Title: Probabilistic analysis of load-displacement hyperbolic curves of single pile using Copula
Authors: Tang, X.-S.; Li, D.-Q.; Zhou, C.-B.; Phoon, K.-K.
Abstract: This paper aims to propose a method for probabilistic analysis of load-displacement hyperbolic curves of single pile using Copula. Firstly, the uncertainties of probabilistic load-displacement hyperbolic model are transformed to the uncertainties of two curve-fitting parameters. Then the Copula theory is adopted to model the joint probability distribution function of the two curve-fitting parameters. The direct integration method is used to carry out the reliability computations for serviceability limit state of single pile. Finally, an example of load-test dataset for Augered Cast-in-Place (ACIP) pile is illustrated to demonstrate the validity and capability of the proposed method. The results indicate that the proposed Copula-based method can provide a more general and flexible way of modeling and simulating the bivariate probability distribution of the two curve-fitting parameters in isolation from their marginal probability distributions, which facilitates the generation of normalized load-displacement hyperbolic curves and thus results in more accurate reliability results for serviceability limit state of single pile. A strong negative correlation between the two parameters underlying the normalized load-displacement hyperbolic model can be observed. Neglecting such negative correlation will overestimate the probability of failure. In addition, the Gaussian Copula, often adopted out of expedience without proper validation, is not the best Copula for modeling the dependence structure underlying the two curve-fitting parameters. If the Gaussian Copula is adopted, the probability of failure of ACIP pile will be underestimated significantly.
Sun, 01 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591732012-01-01T00:00:00Z
- Quantile value method versus design value method for calibration of reliability-based geotechnical codeshttps://scholarbank.nus.edu.sg/handle/10635/59176Title: Quantile value method versus design value method for calibration of reliability-based geotechnical codes
Authors: Ching, J.; Phoon, K.-K
Abstract: This paper compares two methods for geotechnical reliability code calibration, namely the well known design value method (DVM) based on first-order reliability method and a recently developed method based on quantile, called the quantile value method (QVM). The feasibility of calibrating a single partial factor to cover the wide range of coefficients of variation (COVs) commonly encountered in geotechnical designs is studied. For analytical tractability, a simple design example consisting of one resistance random variable and one load random variable is first examined. A resistance factor is first calibrated using a single calibration case associated with a typical COV. The objective is to evaluate the departure from the target reliability index analytically when this calibrated resistance factor is applied to validation cases associated with a range of COVs. The results show that QVM is more robust than DVM in terms of achieving a more uniform reliability level over a range of COVs. Two realistic geotechnical design examples are studied to demonstrate that the theoretical insights garnered in the simple analytical example are applicable. © 2013 Elsevier Ltd.
Sun, 01 Sep 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591762013-09-01T00:00:00Z
- Probability distribution for mobilised shear strengths of spatially variable soils under uniform stress stateshttps://scholarbank.nus.edu.sg/handle/10635/59174Title: Probability distribution for mobilised shear strengths of spatially variable soils under uniform stress states
Authors: Ching, J.; Phoon, K.-K.
Abstract: This paper studies the probability distribution for the mobilised shear strength of a spatially variable soil mass that is subjected to a uniform stress state. Based on the mechanisms identified in two previous studies conducted by the authors, this study further proposes a probability distribution model for the mobilised shear strength that is based on the extreme value of normal random variables. It is concluded that the probability distribution of the mobilised shear strength of a spatially variable soil mass is affected by the line averaging effect along the potential slip plane and the number of independent potential slip planes. These two factors depend on the stress state and the orientation of the potential slip planes. With this model, the mobilised shear strength of a spatially variable soil mass can be simulated without the need of conducting random-field finite-element analyses. In addition, the strength characteristic value that is the 5% quantile in the Eurocodes can be easily derived from this model. © 2013 Taylor & Francis.
Sun, 01 Sep 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591742013-09-01T00:00:00Z
- Second-moment characterization of undrained shear strengths from different test procedureshttps://scholarbank.nus.edu.sg/handle/10635/59200Title: Second-moment characterization of undrained shear strengths from different test procedures
Authors: Ching, J.; Phoon, K.-K.; Lee, W.-T.
Abstract: The undrained shear strength (su) of a clay is not a constant. In particular, su of a clay evaluated by different test procedures are different because these tests may have different stress states, stress histories, degrees of sampling disturbance, and strain rates. This study summarizes the mean value and variability of the normalized undrained shear strength (su/σ'v) from various test procedures based on a large clay database. These second-moment statistics can be seen as a probabilistic extension of Eq-4.55 developed in Kulhawy & Mayne (1990), which provides an estimation of the normalized undrained shear strength based on three factors accounting for test mode, overconsolidation ratio, and strain rate. The second-moment statistics are essential for reliability analysis and reliability-based design. © 2013 American Society of Civil Engineers.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/592002013-01-01T00:00:00Z
- Modified Metropolis-Hastings algorithm with reduced chain correlation for efficient subset simulationhttps://scholarbank.nus.edu.sg/handle/10635/59127Title: Modified Metropolis-Hastings algorithm with reduced chain correlation for efficient subset simulation
Authors: Santoso, A.M.; Phoon, K.K.; Quek, S.T.
Abstract: Simulation of Markov chain samples using the MetropolisHastings algorithm is useful for reliability estimation. Subset simulation is an example of the reliability estimation method utilizing this algorithm. The efficiency of the simulation is governed by the correlation between the simulated Markov chain samples. The objective of this study is to propose a modified MetropolisHastings algorithm with reduced chain correlation. The modified algorithm differs from the original in terms of the transition probability. It has been verified that the modified algorithm satisfies the reversibility condition and therefore the simulated samples follow the target distribution for the correct theoretical reasons. When applied to subset simulation, the modified algorithm produces a more accurate estimate of failure probability as indicated by a lower coefficient of variation and a lower mean square error. The advantage is more significant for small failure probability. Examples of soil slope with spatially variable properties were presented to demonstrate the applicability of the proposed modification to reliability estimation of engineering problems. It was found that the modified algorithm produces a more accurate estimator over the range of random dimensions studied. © 2010 Elsevier Ltd. All rights reserved.
Fri, 01 Apr 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591272011-04-01T00:00:00Z
- Modeling dependence between shear strength parameters using Copulas and its effect on slope reliabilityhttps://scholarbank.nus.edu.sg/handle/10635/59118Title: Modeling dependence between shear strength parameters using Copulas and its effect on slope reliability
Authors: Tang, X.-S.; Li, D.-Q.; Zhou, C.-B.; Phoon, K.-K.
Abstract: The influence of Copulas for modeling dependence between shear strength parameters on slope reliability is investigated. Firstly, four data sets of shear strength parameters in hydropower projects are collected. Then, the Akaike information criterion is adopted for identifying the best-fit Copula underlying the collected shear strength parameters. The formulae for probability of failure of an infinite slope using the direct integration are derived. Finally, an example of reliability analysis for the infinite slope is presented to demonstrate the influence of Copulas on the probability of slope failure. The results indicate that the Copulas have a significant influence on the probability of slope failure. The probabilities of slope failure produced by different Copulas may differ considerably. Such difference increases with increasing slope factor of safety (or decreasing probability of slope failure), increasing negative dependence between cohesion and internal friction angle and decreasing coefficient of variation of shear strength parameters. In addition, the Gaussian Copula is not the best-fit one for modeling the dependence structure underlying the shear strength parameters. If the Gaussian Copula is adopted, the probability of slope failure will be underestimated significantly. It is emphasized that the Gaussian Copula should not be adopted out of expedience without proper validation to model the dependence between the shear strength parameters.
Sat, 01 Dec 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591182012-12-01T00:00:00Z
- Modeling parameters of structured clays as a multivariate normal distributionhttps://scholarbank.nus.edu.sg/handle/10635/59120Title: Modeling parameters of structured clays as a multivariate normal distribution
Authors: Ching, J.; Phoon, K.-K.
Abstract: This study explores the possibility of modeling liquidity index, undrained shear strength, remolded undrained shear strength, preconsolidation stress, and vertical effective stress of structured clays (sensitive or quick clays) as a multivariate normal distribution. The literature is replete with correlation equations between two soil parameters. Consistent synthesis of more than two soil parameters through construction of a multivariate probability distribution function is rare, despite obvious practical usefulness of such an approach. This study compiles a large database of structured clays to construct the multivariate probability distribution among the aforementioned five soil parameters. This multivariate distribution is then used to simulate the correlations between soil parameters of interest and to derive useful equations for Bayesian inference. This constructed multivariate distribution and equations are further validated by another independent database of structured clays as well as by empirical equations proposed in the literature.
Tue, 01 May 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591202012-05-01T00:00:00Z
- Multivariate model for soil parameters based on Johnson distributionshttps://scholarbank.nus.edu.sg/handle/10635/59131Title: Multivariate model for soil parameters based on Johnson distributions
Authors: Phoon, K.-K.; Ching, J.
Abstract: The objective of this paper is to demonstrate the practical construction of a multivariate probability distribution function using an actual soil database containing su(CIUC), OCR, and four piezocone parameters. Five hundred and thirty-five multivariate data points were compiled from 40 clay sites around the world (Brazil, Canada, Hong Kong, Italy, Malaysia, Norway, Singapore, Sweden, UK, USA, and Venezuela). It was found that a multivariate probability distribution can be constructed by transforming each component of a multivariate normal distribution to a Johnson distribution. Existing bivariate regression equations focus on strong correlations. Weak correlations are typically discarded. Site investigation is a costly exercise, and, ideally, one should exploit all measured geotechnical data for design. The multivariate distribution is a concise model to summarize all available information. Conditional distributions can be easily derived to update the marginal distribution of any one parameter or the multivariate distribution of any group of parameters given information from other parameters. One of the objectives of site investigation is to perform cost-effective field tests and to evaluate design parameters based on these field measurements. Clearly, conditioning involving updating one or more design parameters using one or more field measurements is a natural probabilistic generalization of the current soil property evaluation methodology. © 2013 American Society of Civil Engineers.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591312013-01-01T00:00:00Z
- Performance of reliability-based design code formats for foundations in layered soilshttps://scholarbank.nus.edu.sg/handle/10635/59157Title: Performance of reliability-based design code formats for foundations in layered soils
Authors: Phoon, K.-K.; Ching, J.; Chen, J.-R.
Abstract: Simplified reliability-based design (RBD) formats in the form of Load and Resistance Factor Design (LRFD) and Multiple Load and Resistance Factor Design (MRFD) are applied to 42 drilled shafts installed in layered soil profiles underlying the city of Taipei. Clay, sand, gravel and rock layers or some partial combination thereof are encountered within the depth of penetration. It is reasonable to require a foundation reliability-based design code to achieve a reasonably consistent target reliability index over this range of design scenarios, as they are representative of the ground conditions underlying Taipei. The simplified RBD formats are calibrated using two approaches. They are the design value method (DVM) and the more recently proposed quantile value method (QVM). In general, for layered soil profiles, conventional formats containing resistance and load factors are unable to achieve the prescribed target reliability index with the same consistency as that reported for homogeneous soil profiles, regardless of the reliability calibration approach. For the drilled shaft examples considered in this study, it was found that the direct application of quantiles in the simplified RBD format is more reasonable for layered soil profiles. © 2013 Elsevier Ltd.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591572013-01-01T00:00:00Z
- Performance of translation approach for modeling correlated non-normal variableshttps://scholarbank.nus.edu.sg/handle/10635/59158Title: Performance of translation approach for modeling correlated non-normal variables
Authors: Li, D.-Q.; Wu, S.-B.; Zhou, C.-B.; Phoon, K.K.
Abstract: It is common to construct a consistent multivariate distribution from non-normal marginals and Pearson product-moment correlations using the well known translation approach. A practical variant of this approach is to match the Spearman rank correlations of the measured data, rather than the Pearson correlations. In this paper, the performance of these translation methods is evaluated based on their abilities to match the following exact solutions from one benchmark bivariate example where the joint distribution is known: (1) high order joint moments, (2) joint probability density functions (PDFs), and (3) probabilities of failure. It is not surprising to find significant errors in the joint moments and PDFs. However, it is interesting to observe that the Pearson and Spearman methods produce very similar results and neither method is consistently more accurate or more conservative than the other in terms of probabilities of failure. In addition, the maximum error in the probability of failure may not be associated with a large correlation. It can happen at an intermediate correlation. © 2012 Elsevier Ltd.
Thu, 01 Nov 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591582012-11-01T00:00:00Z
- Reliability analysis of slopes considering spatial variability of soil parameters using non-intrusive stochastic finite element methodhttps://scholarbank.nus.edu.sg/handle/10635/59183Title: Reliability analysis of slopes considering spatial variability of soil parameters using non-intrusive stochastic finite element method
Authors: Li, D.-Q.; Jiang, S.-H.; Zhou, C.-B.; Phoon, K.K.
Abstract: A non-intrusive stochastic finite element method (NISFEM) is proposed for the reliability analysis of slope stability considering spatial variability of soil parameters. Firstly, the Karhunen-Loeve (K-L) expansion method is used to characterize the spatial variability of shear strength parameters, where the wavelet-Galerkin technique is employed to numerically solve the eigenvalue problem of the Fredholm integral equation. Thereafter, the finite element method is used for slope stability analysis, the factor of safety is explicitly expressed using the Hermite polynomial chaos expansion (PCE), and the flow chart of procedure is also presented. Finally, the proposed NISFEM is studied by application to the reliability analysis of a homogeneous slope. The results indicate that the proposed method can greatly improve the calculation efficiency for slope reliability analysis considering spatial variability of soil parameters, and that it provides on effective way for solving complex slope reliability problems. There exists a critical coefficient of variation for slope reliability analysis, which increases as the factor of safety increases. If the spatial variability of soil properties is ignored, it will lead to overestimating the probability of failure when the coefficient of variation of shear strength parameters is less than the critical value. The probability of failure does not always increase with the coefficient of variation when the factor of safety is less than 1. 0. In addition, the correlation between the random fields of effective cohesion and internal friction angle has a very significant effect on the probability of slope failure.
Thu, 01 Aug 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591832013-08-01T00:00:00Z
- Reliability analysis of slopes with incomplete probability informationhttps://scholarbank.nus.edu.sg/handle/10635/59184Title: Reliability analysis of slopes with incomplete probability information
Authors: Tang, X.-S.; Li, D.-Q.; Zhou, C.-B.; Phoon, K.-K.
Abstract: Reliability analysis of slopes with incomplete probability information is a challenging problem. This paper aims to propose Copula-based methods for the reliability analysis of slopes with incomplete probability information. First, the method for constructing the joint probability distribution of shear strength parameters using Copulas is briefly introduced. Thereafter, the concept of nominal factor of safety for slope stability analysis is defined. The dispersion of probability of slope failure is derived. Finally, an example of reliability analysis for an infinite slope is presented to demonstrate the capability and validity of the proposed methods. The results indicate that the probability of slope failure can not be determined uniquely based on the incomplete probability information of shear strength parameters. The probabilities of slope failure produced by different Copulas can differ considerably. Such differences directly lead to large dispersion of probability of slope failure. The dispersion of probability of slope failure increases with increasing nominal factor of slope safety, and it increases with decreasing probability of slope failure. The proposed three Copula-based methods can narrow the dispersion of probability of slope failure effectively, which can improve the reliability estimation of slopes efficiently. These methods provide an effective tool for the reliability analysis of slope failure with incomplete probability information.
Sat, 01 Jun 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591842013-06-01T00:00:00Z
- Reliability analysis with scarce information: Comparing alternative approaches in a geotechnical engineering contexthttps://scholarbank.nus.edu.sg/handle/10635/59185Title: Reliability analysis with scarce information: Comparing alternative approaches in a geotechnical engineering context
Authors: Beer, M.; Zhang, Y.; Quek, S.T.; Phoon, K.K.
Abstract: In this paper the problem of dealing with scarce information in a reliability analysis is investigated in a geotechnical engineering context. Common probabilistic methods are compared with interval analysis as an alternative non-probabilistic approach. The potential of imprecise probabilities is discussed as an option for combining probabilistic and non-probabilistic information. The selected methods are examined in view of (i) an appropriate modeling of the information actually available in practical cases, (ii) the transfer of the uncertainty to the computational results, and (iii) the interpretation of the results. Key issue of this investigation is the meaning of the results as the basis to derive engineering decisions if only scarce information is available. This involves particular attention to low-probability-but-high-consequence events, which are often essential for risk assessment. A retaining wall structure is adopted for this study, and the failure mechanisms are considered as known in a deterministic form. This describes the input-to-output transformation of the available information in all cases. Representative available information about the parameters of the retaining wall problem is taken from literature, and typical practical specifications for the parameters are considered. © 2012 Elsevier Ltd.
Fri, 01 Mar 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591852013-03-01T00:00:00Z
- Robustness of constant load and resistance factor design factors for drilled shafts in multiple stratahttps://scholarbank.nus.edu.sg/handle/10635/59195Title: Robustness of constant load and resistance factor design factors for drilled shafts in multiple strata
Authors: Ching, J.; Phoon, K.-K.; Chen, J.-R.; Park, J.H.
Abstract: This study addresses the issue of adopting constant resistance factors for the LRFD approach to the axial compression of drilled shafts in multiple strata of clay, sand, gravel, and rock. The focus is on how to achieve a uniform reliability level for a wide range of stratum scenarios with constant or nonconstant LRFD factors. The main finding is that a uniform reliability level cannot be achieved if the LRFD factors are constant and are independent of the thicknesses or relative contributions of the strata. This conclusion is confirmed in three separate ways from examination exercises: (1) by using LRFD factors recommended in a recent Federal Highway Administration manual, (2) by using tailormade LRFD factors derived from optimization, and (3) by using LRFD factors calibrated from the first-order reliability method. A possible strategy of calibrating nonconstant LRFD factors without resulting in a proliferation of factors is proposed in this study. The use of this approach is demonstrated by a design example. © 2013 American Society of Civil Engineers.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/591952013-01-01T00:00:00Z
- Physical modeling of soil uncertaintyhttps://scholarbank.nus.edu.sg/handle/10635/125088Title: Physical modeling of soil uncertainty
Authors: Garzón, L.X.; Caicedo, B.; Sánchez-Silva, M.; Phoon, K.K.
Abstract: This paper presents a new technique to prepare reduced scale soils models with controlled spatial variability. Soil spatial variability is obtained analytically by using a discrete random field generation method. Then, heterogeneous models are constructed by using nine homogeneous soils, which are created by mixing different percentages of kaolin and bentonite at water content of 1.5 times the liquid limit. Next, the heterogeneous models are subjected to a consolidation process that is developed in twostages: oedometric path and geotechnical centrifuge. Finally, the mechanical soil properties of the heterogeneous models were estimated by measurement of the unit weight and the water content of the random field elements. Results clearly show that the mechanical properties do not only depend on its intrinsic behavior, but also on its space locations. The method proposed in this paper offers new possibilities for the study of the effect of soil variability on the behavior of geotechnical structures. © 2014 Taylor & Francis Group.
Wed, 01 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1250882014-01-01T00:00:00Z
- Updating uncertainties in undrained shear strengths by multivariate correlationshttps://scholarbank.nus.edu.sg/handle/10635/84745Title: Updating uncertainties in undrained shear strengths by multivariate correlations
Authors: Ching, J.; Phoon, K.-K.
Abstract: Undrained shear strengths (su) play important roles in geotechnical designs. In the context of geotechnical reliability-based design, reducing uncertainties in su can be an important research topic. There are at least two ways of reducing uncertainties in su: conduct (1) laboratory or (2) in-situ tests to obtain indices or parameters to correlate su indirectly. The way of reducing uncertainties in su can be challenging. The challenge lies in the fact that the so-obtained indices and parameters, e.g. CPT tip resistance, cannot be directly used to estimate s u but can estimate su only through correlations. Above all, there is a challenge in combining information: how to reduce uncertainties in su when there are multiple sources of information? In this paper we will address the aforementioned challenges and propose a probabilistic framework to handle these difficulties. Sets of simplified equations will be obtained through the probabilistic analysis for the purpose of reducing uncertainties: the inputs to the equations are the results of in-situ or laboratory tests and the outputs are the updated mean values and coefficients of variation (c.o.v.) of the desirable undrained shear strengths. The uncertainties in su will decrease when the number of inputs increase, i.e. more information is available. The results of this research may be beneficial to geotechnical reliability-based design. © 2010 ASCE.
Fri, 01 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/847452010-01-01T00:00:00Z
- Uncertainty quantification using multi-dimensional hermite polynomialshttps://scholarbank.nus.edu.sg/handle/10635/84744Title: Uncertainty quantification using multi-dimensional hermite polynomials
Authors: Phoon, K.K.; Huang, S.P.
Abstract: The general stochastic problem involves the propagation of input uncertainties through a computation model to arrive at a random output vector. This paper presents the application of the multi-dimensional Hermite polynomials to reduce an unknown random output vector into a significantly simpler unknown vector of numbers. The unknown numbers are evaluated using a collocation method because it has the important practical advantage of allowing existing deterministic numerical codes to be used as "black boxes". A simple laterally loaded pile example involving two input random variables demonstrated that a third- or fourth-order Hermite expansion is adequate to reproduce probabilities of failure between 10 -3 and 10 -4. A simple and efficient 2-term recurrence method for obtaining Hermite polynomials of any order in the case of two random dimensions is proposed. To our knowledge, this proposal appears to be original. Copyright ASCE 2007.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/847442007-01-01T00:00:00Z
- Random field characterisation of stress-normalised cone penetration testing parametershttps://scholarbank.nus.edu.sg/handle/10635/84733Title: Random field characterisation of stress-normalised cone penetration testing parameters
Authors: Uzielli, M.; Vannucchi, G.; Phoon, K.K.
Abstract: Random field modelling of soil variability allows significant statistical results to be inferred from field data; moreover, it provides a consistent framework for incorporating such variability in reliability-based design. Cone penetration testing (CPT) is increasingly appreciated because of its near-continuity and repeatability. Stress-normalised CPT parameters are Included in widely used engineering procedures. Nonetheless, the results of variability analyses for these parameters are surprisingly limited. This paper attempts to characterise normalised cone tip resistance (qc1N) and friction ratio (F R) rigorously using a finite-scale weakly stationary random field model. It must be emphasised that inherent soil variability so determined strictly refers to the variability of the mechanical response of soils to cone penetration. The variability of soil response potentially depends on the failure mode (shear for sleeve friction or bearing for tip resistance) and most probably on the volume of soil influenced (averaging effect). To investigate spatial variability, 70 physically homogeneous CPT profiles were first identified from 304 soundings (subdivided into five regional sites) and subsequently assessed for weak stationarity using the modified Bartlett test. Only 40 qC1N profiles and 25 FR profiles were deemed sufficiently homogeneous from both physical and statistical considerations for the scales of fluctuations to be valid and for estimation of the coefficient of variation of inherent soil variability. The majority of the acceptable profiles were found in sandy soils. The remaining profiles are in fine-grained soils, with a few in intermediate soils. Trends in the estimated random field parameters indicate that qc1N is more strongly autocorrelated than FR, probably because qc1N is influenced by a larger volume of soil around the cone tip, and that the mechanical response of cohesionless soils to cone penetration is significantly more variable and erratic than that of cohesive soils. Comparison with literature data indicates that normalisation leads to a decrease in the scale of fluctuation for cone tip resistance and a reduction in the coefficient of variation. A tentative explanation is that normalisation tends to minimise systematic in situ effects that are explainable by physical causes.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/847332006-01-01T00:00:00Z
- Probabilistic analysis of soil water characteristic curves from sandy clay loamhttps://scholarbank.nus.edu.sg/handle/10635/84731Title: Probabilistic analysis of soil water characteristic curves from sandy clay loam
Authors: Phoon, K.-K.; Santoso, A.; Cheng, Y.
Abstract: It is costly to measure the soil water characteristic curve. A first-order estimate from statistical generalization of experimental data belonging to soils with similar textural and structural properties is useful. A simple approach is to fit the data with a nonlinear function and model the curve-fitting parameters as a random vector. This approach is demonstrated using sandy clay loam data in UNSODA. It was found that the classical lognormal distribution is unable to reproduce the skewness and kurtosis of the curve-fitting parameters accurately. A simple four-term Hermite expansion was found to be more suitable. The Hermite coefficients are determined by minimizing lack-of-fit errors in the 25th, 50th, and 75th quantiles. Copyright ASCE 2008.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/847312008-01-01T00:00:00Z
- Reliability analysis of infinite slope using subset simulationhttps://scholarbank.nus.edu.sg/handle/10635/74322Title: Reliability analysis of infinite slope using subset simulation
Authors: Santoso, A.; Phoon, K-K.; Quek, S-T.
Abstract: It is important to incorporate the spatial variability of soil properties in slope stability analysis. Most studies use a random field to model the variability of soil properties and estimate the reliability using direct Monte Carlo simulation (MCS), which is not efficient for problems with small failure probabilities. The slope reliability can be more efficiently estimated using the subset simulation method, as it computes a small failure probability as a product of larger conditional probabilities. The approach is demonstrated using an infinite slope with undrained shear strength varying with depth. It will be shown that subset simulation requires much less samples than MCS to achieve a given accuracy. Subset simulation results are within the first order probability bounds derived from classical system reliability theory. It is found that the failure surface can be developed in any soil layer, not necessarily atthe soil-rock interface. Copyright ASCE 2009.
Thu, 01 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743222009-01-01T00:00:00Z
- Soil variability analysis for geotechnical practicehttps://scholarbank.nus.edu.sg/handle/10635/74357Title: Soil variability analysis for geotechnical practice
Authors: Uzielli, M.; Lacasse, S.; Nadim, F.; Phoon, K.K.
Abstract: The heterogeneity of natural soils is well known in geotechnical practice. However, the importance of quantifying the resulting variability in geotechnical characterisation and design parameters is not adequately recognised. Variability should not be approached suspiciously. Rather, it should be accepted as a positive contribution to geotechnical design as its consistent modelling and utilisation lead, with limited additional computations and conceptual effort on the part of the engineer, to more rational and economic design. The paper presents a structured - though necessarily partial - review of approaches and methodologies for the quantification of soil variability, as well as selected examples of its utilisation in reliability-based geotechnical design. © 2007 Taylor & Francis Group, London.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743572007-01-01T00:00:00Z
- Some numerical considerations in unsaturated slope stability analysis due to rainfall infiltrationhttps://scholarbank.nus.edu.sg/handle/10635/74359Title: Some numerical considerations in unsaturated slope stability analysis due to rainfall infiltration
Authors: Phoon, K.K.; Cheng, Y.
Abstract: Accurate prediction of the propagating wetting front arising from rainfall infiltration into an unsaturated soil slope is of considerable importance to stability analysis of the slope. However, difficulties exist in numerical simulations of unsaturated flow analyses because of the highly nonlinear soil hydraulic properties. This paper emphasizes the slow convergence of calculated pressure heads with the refinement of mesh sizes and time steps, and discusses possible solutions. © 2008 Science Press Beijing and Springer-Verlag GmbH Berlin Heidelberg Geotechnical Engineering for Disaster Mitigation and Rehabilitation.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743592008-01-01T00:00:00Z
- Some critical issues in geo-RBD calibrations for foundationshttps://scholarbank.nus.edu.sg/handle/10635/74358Title: Some critical issues in geo-RBD calibrations for foundations
Authors: Kulhawy, F.H.; Phoon, K.K.
Abstract: Calibrations for reliability-based design (RBD) of foundations are critical but are largely "invisible". Some key methods of calibration are discussed, including issues ranging from the basic assumptions involved, distributions of key variables, uncertainties in both load and resistance, target reliability, etc. The goal is to assist engineers in understanding the level of sophistication, or lack thereof, in different approaches to calibrations, which then translates into the degree of confidence one can have in a particular calibration or code. Copyright ASCE 2006.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743582006-01-01T00:00:00Z
- Probabilistic hyperbolic models for foundation uplift movementshttps://scholarbank.nus.edu.sg/handle/10635/74312Title: Probabilistic hyperbolic models for foundation uplift movements
Authors: Phoon, K.K.; Chen, J.-R.; Kulhawy, F.H.
Abstract: There is a need to consider both ultimate limit state and serviceability limit state uniformly within the reliability-based design framework. However, it is more difficult to characterize uncertainties associated with the latter, because load-displacement curves are nonlinear functions that cannot be reduced to a single random variable. This paper demonstrates that a random vector containing two curve-fitting hyperbolic parameters is an adequate probabilistic model for uplift load-displacement curves associated with spread foundations, drilled shafts, and pressure-injected footings. A simple but general computational approach, based on Hermite polynomials, can be used to simulate these hyperbolic parameters, which typically are correlated and may not follow closed-form classical distributions. Copyright ASCE 2007.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743122007-01-01T00:00:00Z
- Simulation of multi-dimensional non-Gaussian fields using Karhunen-Loève expansionhttps://scholarbank.nus.edu.sg/handle/10635/74349Title: Simulation of multi-dimensional non-Gaussian fields using Karhunen-Loève expansion
Authors: Phoon, K.K.; Quek, S.T.; Li, L.B.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743492007-01-01T00:00:00Z
- Some observations on assessment of gaussianity for correlated profileshttps://scholarbank.nus.edu.sg/handle/10635/74360Title: Some observations on assessment of gaussianity for correlated profiles
Authors: Uzielli, M.; Phoon, K.-K.
Abstract: The classical Anderson-Darling test for Gaussianity cannot be applied on correlated data. It is possible to circumvent correlation by resampling the data at wider spacing. A simple resampling criterion is developed using digitally simulated standard Gaussian realizations following a single exponential correlation structure. The effects of resampling on Gaussianity assessment are assessed on actual normalized cone tip resistance profiles from highly homogeneous soil units. Copyright ASCE 2006.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743602006-01-01T00:00:00Z
- Prediction of changes in pore-water pressure response due to rainfall eventshttps://scholarbank.nus.edu.sg/handle/10635/74309Title: Prediction of changes in pore-water pressure response due to rainfall events
Authors: Karthikeyan, M.; Toll, D.G.; Phoon, K.K.
Abstract: Global climate change is expected to result in worldwide increases in temperature and alteration of rainfall patterns. Such changes have the potential to activate rainfall triggered landslides and a study is underway in Singapore to investigate these possible effects. The main objective of this work is to calibrate a numerical model for future prediction by making use of cases where in-situ measurements of pore-water pressures/suctions have been made in Singapore. The results presented in this study show that the prediction of changes in the pore-water pressure profile is highly sensitive to the soil-hydraulic properties used in the analysis. It was found that a SEEP/W flow model is able to capture the general trend of field behaviour of the changes in the pore-water pressure profile response due to rainfall events. However, the results from the numerical models indicate that further research is warranted to improve the accuracy of the numerical analysis by better definition of critical input parameters. © 2008 Taylor & Francis Group, London.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743092008-01-01T00:00:00Z
- Serviceability considerations in reliability-based foundation designhttps://scholarbank.nus.edu.sg/handle/10635/74344Title: Serviceability considerations in reliability-based foundation design
Authors: Zhang, L.M.; Phoon, K.K.
Abstract: Serviceability limit state (SLS) considerations often govern the design of foundations. The objective of this paper is to review the limited literature on reliability-based design for SLS and to suggest useful directions for future work. A review of several new design codes indicates that ultimate and serviceability limit states are not verified in the same way. The former check conforms to reliability principles, but the latter is essentially verified in a deterministic way. To address SLS within the reliability framework, it is necessary to characterize the uncertainties associated with foundation displacements and tolerable displacements. The former is discussed with particular attention to spatial correlation, model uncertainties, and load-displacement nonlinearities. Uncertainties in tolerable displacements are discussed using two case histories. Copyright ASCE 2006.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743442006-01-01T00:00:00Z
- Geotechnical reliability analyses: Towards development of some user-friendly toolshttps://scholarbank.nus.edu.sg/handle/10635/74188Title: Geotechnical reliability analyses: Towards development of some user-friendly tools
Authors: Phoon, K.K.; Honjo, Y.
Abstract: This paper discusses two powerful and yet user-friendly reliability techniques that could be implemented on the PC easily by practitioners. The techniques are representation of non-Gaussian random variables using Hermite polynomials and subset Markov chain Monte Carlo simulation. It is believed that ease of implementation would popularize use of reliability-based design in practice.
Sat, 01 Jan 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/741882005-01-01T00:00:00Z
- Geotechnical probabilistic analysis using collocation-based stochastic response surface methodhttps://scholarbank.nus.edu.sg/handle/10635/74187Title: Geotechnical probabilistic analysis using collocation-based stochastic response surface method
Authors: Phoon, K.K.; Huang, S.P.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/741872007-01-01T00:00:00Z
- A new 2D failure mechanism for face stability analysis of a pressurized tunnel in spatially variable sandshttps://scholarbank.nus.edu.sg/handle/10635/74035Title: A new 2D failure mechanism for face stability analysis of a pressurized tunnel in spatially variable sands
Authors: Mollon, G.; Phoon, K.K.; Dias, D.; Soubra, A.-H.
Abstract: This paper presents a method to consider the spatial variability of the soil shear strength parameters for determining the critical collapse pressure of a pressurized tunnel face. Only the case of a cohesionless soil is considered in the analysis. The present method is based on the kinematic theorem of limit analysis. A new 2D kinematically admissible collapse mechanism whose shape depends on the spatial distribution of the soil friction angle (φ) is proposed. In this mechanism, the normality condition imposed by limit analysis is enforced everywhere along the slip surfaces of the failure mechanism. The results obtained using the present approach are presented and compared to those based on common numerical methods such as the Finite Element Method (FEM) or the Finite Difference Method (FDM). The proposed method is computationally more efficient and has significant potential for simulation studies involving random fields. © 2010 ASCE.
Fri, 01 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/740352010-01-01T00:00:00Z
- Application of the probabilistic SBRA method in the steel frame designhttps://scholarbank.nus.edu.sg/handle/10635/74069Title: Application of the probabilistic SBRA method in the steel frame design
Authors: Krivy, V.; Marek, P.; Phoon, K.K.
Abstract: This paper is concerned with the alternative reliability assessment of steel frame structures using the probabilistic SBRA method (Simulation-Based Reliability Assessment). The attention is focused on the selection of appropriate reference values, which serve to define the resistance (in case of safety assessment) and appropriate limitations (in case of serviceability assessment) applied in the calculation of the probability of failure Pf of the structure and its members. Next it is necessary to evaluate the suitability of particular transformation models serving as tools for transforming the loading into the response of the structure to the load with regard to the substance of the stochastic method. In this connection attention is paid to the transition from the traditional assessment of buckling strength in compliance with contemporary standards (buckling lengths, buckling factor or stress) to the "strength stability concept" with respect to the effects corresponding to the second order theory. Possible solution based on the probabilistic SBRA method and increasing capacity of computer techniques is outlined and demonstrated using an example of reliability assessment of a multistory frame structure.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/740692007-01-01T00:00:00Z
- Bootstrap estimation of sample autocorrelation functionshttps://scholarbank.nus.edu.sg/handle/10635/74086Title: Bootstrap estimation of sample autocorrelation functions
Authors: Phoon, K.-K.
Abstract: It is rarely appreciated that the sample autocorrelation function is a non-stationary stochastic process with mean and variance that are functions of the lag distance. For Gaussian processes, some complex formulae for the mean and covariance of the sample autocorrelation function are available. Analytical solutions for non-Gaussian processes are probably intractable and none seems to be available. A more hopeful strategy is to exploit growing desktop computational power and increasingly powerful simulation techniques for stochastic processes to perform bootstrapping. Numerical results presented in this study show that fairly accurate mean estimates of the sample autocorrelation function can be obtained for both Gaussian and non-Gaussian processes using bootstrapping. Variance estimates are less accurate, but even crude estimates are useful in identifying the level of noise at large lags and reducing misinterpretation of how the actual autocorrelation function decays with lag distance. Copyright ASCE 2006.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/740862006-01-01T00:00:00Z
- Characterisation of model uncertainties for laterally loaded rigid drilled shaftshttps://scholarbank.nus.edu.sg/handle/10635/74097Title: Characterisation of model uncertainties for laterally loaded rigid drilled shafts
Authors: Phoon, K.-K.; Kulhawy, F.H.
Abstract: This paper presents a critical evaluation of model factors for laterally loaded rigid drilled shafts (bored piles). Both the lateral or moment limit and hyperbolic capacity are considered to make explicit the dependence of model factors on the criterion for interpreting 'capacity' from load test data. Although the hyperbolic capacity may be closest to the theoretical ultimate state or upper bound, results indicate that it generally does not produce a mean model factor of 1. When the measured capacity is interpreted consistently from load test data, the coefficient of variation (COV) appears to remain relatively constant between 30% and 40%. The range of the mean bias for the lateral or moment limit is 0·67 to 1·49, whereas that of the hyperbolic capacity is 0·98 to 2·28. Based on available data, a log-normal probability model appears adequate for reliability analysis. Laboratory-scale load tests conducted in uniform soil deposits prepared under controlled laboratory conditions are ideal for establishing benchmarks on the probable magnitude of uncertainty arising from model idealisations alone. However, the limited range of geometric and geotechnical parameters in a laboratory load test database may not produce a representative mean model factor. A field load test database typically contains more diverse geometric and geotechnical parameters, but it entails an unknown degree of extraneous uncertainties. A comparative study indicates that model statistics are surprisingly robust and appear not to be seriously affected by the above concerns (possibly because of normalisation). Model factors from drained analysis seem to be more variable than those from undrained analysis. A more detailed examination indicates that the higher COV of about 40% for these drained model factors arises because they are not completely random. There are reasons to believe that applying a more complete force system for drained analysis could minimise some of the undesired correlations and reduce the COV to a level comparable to undrained analysis.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/740972006-01-01T00:00:00Z
- Characterization of model uncertainties for for Augered Cast-In-Place (ACIP) piles under axial compressionhttps://scholarbank.nus.edu.sg/handle/10635/74101Title: Characterization of model uncertainties for for Augered Cast-In-Place (ACIP) piles under axial compression
Authors: Phoon, K.K.; Chen, J.R.; Kulhawy, F.H.
Abstract: The uncertainties in normalized load-displacement curves for ACIP piles are quantified using a pair of correlated lognormal curve-fitting parameters. To correct for simplifications in the capacity model, it is preferable to use linear regression rather than the standard multiplicative form. The former definition prevents correlation by removing variations that are explainable by the computed capacity. Copyright ASCE 2006.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/741012006-01-01T00:00:00Z
- Characterization of model uncertainties for drilled shafts under undrained axial loadinghttps://scholarbank.nus.edu.sg/handle/10635/74100Title: Characterization of model uncertainties for drilled shafts under undrained axial loading
Authors: Phoon, K.K.; Kulhawy, F.H.
Abstract: This invited paper presents a rigorous examination of model statistics for drilled shafts under undrained axial loading. To apply a model factor as an independent random variable on the predictive capacity equation, it is desirable to verify that it is not varying systematically with some underlying factors. The measured and computed capacities appear as the numerator and denominator, respectively, in the standard definition of the model factor and are the most obvious factors to check for such statistical dependencies. The Spearman rank correlation is a simple and useful method to perform this check. Even though the adhesion factor typically is expressed as a function of the undrained shear strength, this approach only is marginally acceptable from a rank correlation perspective. For undrained compression tip resistance, a generalized model factor is recommended because the existing definition produces strong statistical dependencies with the measured and computed alues.
Sat, 01 Jan 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/741002005-01-01T00:00:00Z
- Comparison of MSSOR versus ILU(0) preconditioners for Biot's FEM consolidation equationshttps://scholarbank.nus.edu.sg/handle/10635/74107Title: Comparison of MSSOR versus ILU(0) preconditioners for Biot's FEM consolidation equations
Authors: Phoon, K.K.; Chaudhary, K.B.; Toh, K.C.
Abstract: Numerical performance of two different preconditioning approaches, modified SSOR (MSSOR) preconditioner and incomplete factorization with zero fill-in (ILU0) preconditioner, is compared for the iterative solution of symmetric indefinite linear systems arising from finite element discretization of the Biot's consolidation equations. Numerical results show that the nodal ordering affect the performance of ILU0 whereas MSSOR is less affected. The statistics proposed by Chow and Saad (1997) is demonstrated to be useful in diagnosing the failure of ILU factorization. The stabilized ILU0 coupled with symmetric quasi-minimal residual (SQMR) method is about 10-50% efficient time-wise (especially in the heterogeneous soil condition) than MSSOR-preconditioned system. However, the determination of a proper stabilization parameter (thresh value) for the replacement of smaller pivots is largely problem dependant. The optimal balance can only be identified through numerical experiments - a luxury that practitioners can ill-afford and completely self-defeating if the goal is to solve a problem in a shortest time.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/741072008-01-01T00:00:00Z
- Complexity of limit equilibrium based slope reliability problemshttps://scholarbank.nus.edu.sg/handle/10635/74110Title: Complexity of limit equilibrium based slope reliability problems
Authors: Ching, J.; Phoon, K.-K.; Hu, Y.-G.
Abstract: This study addresses the complexity of slope reliability problems based on limit equilibrium methods (LEM). The main focus is on the existence of multiple failure modes that poses difficulty to many LEM-based slope reliability methods. In particular, when weak seams are present, the failure modes associated with those seams may be difficult to detect. A systematic way of searching the failure modes is proposed, and its robustness over slopes with weak seams is demonstrated. It is found that in the presence of weak seams, assuming circular slip surfaces may cause underestimation of slope failure probability. © 2010 ASCE.
Fri, 01 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/741102010-01-01T00:00:00Z
- Pile group settlement. A probabilistic approachhttps://scholarbank.nus.edu.sg/handle/10635/84652Title: Pile group settlement. A probabilistic approach
Authors: Quek, S.T.; Phoon, K.K.; Chow, Y.K.
Abstract: The uncertain settlement response of pile groups is determined using a 'hybrid' formulation and a first-order perturbation technique. The spatially varying soil modulus, which gives rise to the uncertainties in the pile group settlement, is modeled as a homogeneous random field. The random field is assumed to be one-dimensional since the 'hybrid' formulation does not account for horizontal variation in the soil properties. Using the proposed method, the coefficient of variation of the pile group settlement is computed. The single-pile solution obtained compare favorably with the solutions from a conventional stochastic finite element analysis. Pile groups of sizes ranging from two to twenty-five piles are studied. It is observed that the coefficient of variation is not significantly affected by the pile spacing as well as the group size. By defining an appropriate performance function, the reliability index of a pile group system is also found to be approximately the same as that of a single-pile system. These observations suggest that the solutions for a single pile may be used to estimate the uncertainties in the settlement response of pile groups.
Fri, 01 Nov 1991 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/846521991-11-01T00:00:00Z
- Closure to "Low strain integrity testing of piles: Three-dimensional effects" by Y. K. Chow, K. K. Phoon, W. F. Chow, and K. Y. Wonghttps://scholarbank.nus.edu.sg/handle/10635/67859Title: Closure to "Low strain integrity testing of piles: Three-dimensional effects" by Y. K. Chow, K. K. Phoon, W. F. Chow, and K. Y. Wong
Authors: Chow, Y.K.; Phoon, K.K.; Chow, W.F.; Wong, K.Y.
Tue, 01 Feb 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/678592005-02-01T00:00:00Z
- Reliability-based design of foundations for transmission line structureshttps://scholarbank.nus.edu.sg/handle/10635/74325Title: Reliability-based design of foundations for transmission line structures
Authors: Kulhawy, F.H.; Phoon, K.K.; Prakoso, W.A.; Hirany, A.
Abstract: An overview is presented of research at Cornell University on reliability-based design (RBD) of foundations for transmission line structures (TLS). Under sponsorship of the Electric Power Research Institute (EPRI), Empire State Electric Energy Research Corporation (ESEERCO), and others, a long-term research effort was conducted to develop a comprehensive RBD methodology for TLS foundations in both soil and rock under uplift, compression, and lateral/moment loading. The results of these studies are described herein in conceptual form, illustrating the basic concepts involved and key issues of geotechnical variability and uncertainty. Many are described in more detail in the geotechnical literature, so references are provided to the pertinent works. These very useful tools are readily available to the TLS design community. Copyright ASCE 2007.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743252006-01-01T00:00:00Z
- Reliability-based design of drilled shafts under undrained lateral-moment loadinghttps://scholarbank.nus.edu.sg/handle/10635/74324Title: Reliability-based design of drilled shafts under undrained lateral-moment loading
Authors: Kulhawy, F.H.; Phoon, K.-K.
Abstract: This invited paper presents a complete reliability calibration for drilled shafts subjected to undrained lateral-moment loading. The main purpose is to demonstrate that key RBD components are in place and it is practical to achieve close integration with structural RBD, without minimizing important geotechnical considerations such as soil variabilities.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/743242004-01-01T00:00:00Z
- Large scale three-dimensional finite element analysis of underground constructionhttps://scholarbank.nus.edu.sg/handle/10635/74230Title: Large scale three-dimensional finite element analysis of underground construction
Authors: Lee, F.H.; Phoon, K.K.; Lim, K.C.
Abstract: This paper discusses the possibility of using three-dimensional finite element analyses to analyse and design underground construction works. Two common problems associated with three-dimensional analyses mean very large memory requirement and long computing times. To overcome these problems, the Authors outline some recent developments in iterative solution algorithm which can lead to large savings in memory and time. It is shown that these iterative methods offer viable and faster alternatives to Gaussian elimination approach for solving very large finite element equations in geotechnical engineering, such as those arising from three-dimensional analyses, with significantly reduced memory requirement. The convergence characteristics of ill-conditioned coupled-flow problems can be improved by using a generalized Jacobi preconditioner. Drained problems can be efficiently solved using the preconditioned conjugate gradient method with the standard Jacobi preconditioner. On the other hand, undrained problems are better solved as "nearly-impermeable" consolidation problems, using quasi-minimal residual method with the generalized Jacobi preconditioner. The advantages of iterative methods increase with the size of the problem. For very large problems, the speed-up can be very significant. The applicability of iterative methods is illustrated by the study of a tunneling project involving earth pressure balance machine. The details of face pressure application, tunnel convergence around shield and lining installation were modeled. The results show that large-scale three-dimensional analyses with iterative solution cannot only be implemented on relatively modest computation platforms with reasonable turnaround times, but they are also able to illuminate ground mechanisms which cannot be reflected by two-dimensional analyses. © 2006 Taylor & Francis Group.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/742302006-01-01T00:00:00Z
- Iterative solution of intersecting tunnels using the generalised Jacobi preconditionerhttps://scholarbank.nus.edu.sg/handle/10635/74224Title: Iterative solution of intersecting tunnels using the generalised Jacobi preconditioner
Authors: Phoon, K.K.; Lee, F.H.; Chan, S.H.
Abstract: This study presents settlement analyses of intersecting tunnels using the 3-D finite element method. This study demonstrates that the reinforcing or strengthening effects due to the presence of existing tunnel(s) are significant on the problem being investigated. Any existing tunnel prior to the new tunnelling has to be incorporated into analyses if more realistic assessments on the tunnelling impacts are required. This study also shows that when multiple closely spaced tunnels are constructed, the assumption that each tunnel behaves independently and the movements that would have occurred for each tunnel acting independently can be superimposed, is unlikely to provide good estimations. Note that the linear superposition technique has been commonly used in practice to reduce the complexity of very large problems that involve construction of multiple tunnels. Consequently, the entire problem has to be modelled and large-scale computing is unavoidable. The symmetric quasi-minimal residual method (SQMR) incorporating the generalised Jacobi (GJ) preconditioner is shown to be very economical in solving such large-scale finite element problems on personal computers. © 2006 Taylor & Francis Group.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/742242006-01-01T00:00:00Z
- Iterative solution of intersecting tunnels using the generalised Jacobi preconditionerhttps://scholarbank.nus.edu.sg/handle/10635/74223Title: Iterative solution of intersecting tunnels using the generalised Jacobi preconditioner
Authors: Phoon, K.K.; Lee, F.H.; Chan, S.H.
Abstract: This study presents settlement analyses of intersecting tunnels using the 3-D finite element method. This study demonstrates that the reinforcing or strengthening effects due to the presence of existing tunnel(s) are significant on the problem being investigated. Any existing tunnel prior to the new tunnelling has to be incorporated into analyses if more realistic assessments on the tunnelling impacts are required. This study also shows that when multiple closely spaced tunnels are constructed, the assumption that each tunnel behaves independently and the movements that would have occurred for each tunnel acting independently can be superimposed, is unlikely to provide good estimations. Note that the linear superposition technique has been commonly used in practice to reduce the complexity of very large problems that involve construction of multiple tunnels. Consequently, the entire problem has to be modelled and large-scale computing is unavoidable. The symmetric quasi-minimal residual method (SQMR) incorporating the generalised Jacobi (GJ) preconditioner is shown to be very economical in solving such large-scale finite element problems on personal computers. © 2006 Taylor & Francis Group.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/742232006-01-01T00:00:00Z
- Large scale three-dimensional finite element analysis of underground constructionhttps://scholarbank.nus.edu.sg/handle/10635/74229Title: Large scale three-dimensional finite element analysis of underground construction
Authors: Lee, F.H.; Phoon, K.K.; Lim, K.C.
Abstract: This paper discusses the possibility of using three-dimensional finite element analyses to analyse and design underground construction works. Two common problems associated with three-dimensional analyses mean very large memory requirement and long computing times. To overcome these problems, the Authors outline some recent developments in iterative solution algorithm which can lead to large savings in memory and time. It is shown that these iterative methods offer viable and faster alternatives to Gaussian elimination approach for solving very large finite element equations in geotechnical engineering, such as those arising from three-dimensional analyses, with significantly reduced memory requirement. The convergence characteristics of ill-conditioned coupled-flow problems can be improved by using a generalized Jacobi preconditioner. Drained problems can be efficiently solved using the preconditioned conjugate gradient method with the standard Jacobi preconditioner. On the other hand, undrained problems are better solved as "nearly-impermeable" consolidation problems, using quasi-minimal residual method with the generalized Jacobi preconditioner. The advantages of iterative methods increase with the size of the problem. For very large problems, the speed-up can be very significant. The applicability of iterative methods is illustrated by the study of a tunneling project involving earth pressure balance machine. The details of face pressure application, tunnel convergence around shield and lining installation were modeled. The results show that large-scale three-dimensional analyses with iterative solution cannot only be implemented on relatively modest computation platforms with reasonable turnaround times, but they are also able to illuminate ground mechanisms which cannot be reflected by two-dimensional analyses. © 2006 Taylor & Francis Group.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/742292006-01-01T00:00:00Z
- Modeling and simulation of stochastic datahttps://scholarbank.nus.edu.sg/handle/10635/74247Title: Modeling and simulation of stochastic data
Authors: Phoon, K.K.
Abstract: Stochastic data appear as basic components in reliability analysis and geostatistics. It is rarely emphasized that the multivariate probability distributions underlying random vectors (or processes, fields) are very difficult to construct theoretically, to estimate empirically, and to simulate numerically. This paper discusses the modeling and simulation of non-Gaussian random vectors, highlighting some useful geotechnical applications, important limitations, and outstanding challenges. Discussion is restricted to translation vectors, although emerging techniques that can potentially simulate a wider class of non-Gaussian random vectors would be briefly introduced for completeness. The translation approach is quite natural and takes advantage of the practicality, theoretical generality, and simulation speed associated with the multivariate Gaussian distribution. Nonetheless, there are fundamental limitations that must be recognized. The present focus on probabilistic analysis should be balanced by more research on statistical inference. It is acknowledged that statistical inference for correlated data is a challenging problem, but its practical significance is obvious. Copyright ASCE 2006.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/742472006-01-01T00:00:00Z