Development of a probabilistic method in seismic microzonation mapping accounting for uncertainties of the 3D geological model and geotechnical soil parameters

A key aspect of geotechnical earthquake engineering is related to the evaluation of the expected intensity and the dominant period of the seismic shaking at a given location. Knowledge of the geological (e.g., stratigraphy, basin topography, and thickness) and geotechnical (e.g., soil type, shear modulus, and damping ratio) properties of the surficial sediments is important in this respect since they tend to modify the amplitude and frequency content of the incoming seismic waves, a phenomenon known as seismic site effect. Site effect proxies, such as the time-averaged shear wave velocity of the top 30 m (Vs,30) and the fundamental site period (T0), are the main parameters commonly used for evaluating the potential amplification of seismic shaking and conducting seismic microzonation mapping. Seismic microzonation study is conducted to refine the seismic hazard model for the study area with complex surficial geology. The Quaternary geology underlying the lowlands of southern Quebec, including the Saguenay City territory, shows complex glacial and postglacial stratigraphy with a number of buried valleys filled with glaciofluvial and glaciomarine sediments. High seismic impedance contrast between rock formations and surficial sediments may cause seismic amplification. Considering the stratigraphic setting and soil type heterogeneity, a multistep stochastic methodology is developed for 3D geological modeling and quantification of the associated uncertainties. Empirical Bayesian kriging (EBK) is applied to generate a bedrock topography map and determine the thickness of the till sediments and their uncertainties. The locally varying mean and variance obtained by the EBK method enable accounting for data complexity and moderate nonstationarity. Sequential indicator simulation is then performed to determine the occurrence probability of the discontinuous postglacial sediments (e.g., clay, sand, and gravel) on top of the basal till layer. The correlation of shear wave velocity (Vs) with piezocone penetration test (CPTu) parameters is investigated in postglacial sediments in the lowlands of the St. Lawrence and Saguenay rivers. In establishing CPTu-Vs correlations, the sparsity of Vs measurements is remedied by using extensive geotechnical soil databases and the developed CPTu-Vs correlations. The compiled database includes 991 CPTu–Vs measurements at 40 sites. The objectives are to examine the applicability of different CPTu–Vs correlations, identify the leading CPTu parameters, and develop specific CPTu–Vs correlations considering the effects of soil type (e.g., sandy or clayey) and geological setting (Champlain or Laflamme sea sediments). Results reveal that the application of the existing correlations is biased in varying degrees, denoting a need for site-specific correlations for the study area. A multivariate statistical analysis allows the development of empirical correlations among Vs, geotechnical parameters, depth, and soil types, along with the evaluation of their uncertainties. Consideration of soil type and geological setting contributes to a reduction in uncertainties for the CPTu–Vs correlations for fine-grained soils. A combined treatment of various sources of uncertainties, from geological to geotechnical, is applied to develop a 3D shear wave velocity model and evaluate the associated uncertainty. A 3D Vs model is created using Vs correlations and the occurrence probability of postglacial soils. The propagated uncertainty is also quantified by considering the combined variance of the geological and geotechnical properties. The final step involves transforming the 3D Vs model into 2D maps representing the spatial distribution of Vs,30 and T0 together with related uncertainties. Results indicate that seismic maps and their uncertainty are influenced by soil thickness, soil geotechnical properties, and soil type probabilities. Among which soil thickness is one of the most critical; in shallow sediments, ??,30 and T0 maps represent rock or very stiff soil conditions with the seismic response in short vibration periods ≤ 0.2 s. By contrast, regions with thicker sediments present sites with a potential response that resembles medium to soft soil conditions with longer dominant vibration periods. The respective ???,30 and ??0 maps represent the inherent random and epistemic uncertainty in the models, which are associated with the spatial variability of the geological units and the statistical dispersion of the Vs data. Consequently, the combined uncertainty of the geological and geotechnical models is genuinely quantified as it decreases in the vicinity of the geological boreholes due to the higher certainty of the geological model, resulting in lower uncertainty of Vs,30 and T0. Un problème important en génie géotechnique appliquée à la séismicité est l'évaluation de l'intensité attendue des ondes et de la période de résonance dominante des terrains à un endroit donné. La connaissance des caractéristiques des sédiments de surface est importante à cet égard. En effet, les facteurs géologiques (par exemple la stratigraphie des dépôts, leur épaisseur, la topographie du bassin) et géotechniques (par exemple les types de sol, leur module de cisaillement, taux d’amortissement) des dépôts de surfaceront tendance à modifier l'amplitude et le contenu fréquentiel des ondes sismiques qui les traversent, un phénomène connu sous le nom d'effet de site. Des indicateurs d’effet de site tels que la vitesse moyenne des ondes de cisaillement des 30 premiers mètres (Vs30) et la période fondamentale de résonance du site (T0) sont les principaux paramètres utilisés pour évaluer l'amplification potentielle de la secousse sismique et effectuer une cartographie de microzonation sismique. Cette étude de microzonation sismique est menée pour affiner le modèle d'aléa sismique pour un territoire ayant une géologie de surface complexe. La géologie quaternaire sous-jacente aux basses-terres du sud du Québec, incluant le territoire de la Ville de Saguenay présente une stratigraphie glaciaire et postglaciaire complexe avec un certain nombre de vallées enfouies remplies de sédiments fluvioglaciaire et glaciomarine. Le contraste élevé d'impédance entre les formations rocheuses et les sédiments superficiels peut causer une amplification sismique. Compte tenu du cadre stratigraphique et de la variabilité des dépôts, une méthodologie stochastique à plusieurs étapes est développée pour la modélisation géologique 3D et la quantification des incertitudes associées. Le krigeage bayésien empirique (EBK) est appliqué pour générer la carte de la topographie du substratum rocheux et déterminer l'épaisseur des sédiments de till et leurs incertitudes. La moyenne et la variance localement variables estimées par la méthode EBK permettent de tenir compte de la complexité des données et de la non-stationnarité modérée. Une simulation séquentielle d'indicateurs est ensuite effectuée pour déterminer la probabilité d'occurrence des sédiments postglaciaires discontinus (argile, sable et gravier) au-dessus de la couche basale de till. La corrélation de la vitesse des ondes de cisaillement (Vs) avec les paramètres mesurés par des essais de pénétration de piézocône (CPTu) est étudiée dans les sédiments postglaciaires des basses-terres du Saint-Laurent et du Saguenay. Les valeurs supplémentaires de Vs, obtenues grâce à ces corrélations empiriques permettent de palier la faible quantité de mesures directes de Vs et d’augmenter les bases de données géotechniques appliquées à la séismicité. La base de données ainsi compilée comprend 991 mesures CPTu-Vs sur 40 sites. Les objectifs sont d'examiner l'applicabilité de différentes corrélations CPTu-Vs, d'identifier les principaux paramètres CPTu et de développer des corrélations CPTu-Vs spécifiques en tenant compte des effets du type de sol (par exemple sableux ou argileux) et du contexte géologique (sédiments marins de la mer de Champlain ou du golfe de Laflamme). Les résultats révèlent que l'application des corrélations déjà utilisées est biaisée à des degrés divers, dénotant un besoin de corrélations spécifiques aux sites localisés dans la zone d'étude. L'analyse statistique multivariée permet de développer des corrélations empiriques entre les valeurs de Vs, les paramètres géotechniques, la profondeur et les types de sol, ainsi que l'évaluation de leurs incertitudes. La prise en compte du type de sol et du cadre géologique contribuent à une réduction des incertitudes dans les corrélations CPTu-Vs développée pour les sols à grain fin. Un traitement combiné de diverses sources d'incertitudes est appliqué, considérant les facteurs géologiques et géotechniques, pour développer un modèle 3D de vitesse d'onde de cisaillement et évaluer l'incertitude associée Le modèle 3D de Vs est créé en utilisant des corrélations Vs et la probabilité d'occurrence des sols postglaciaires. L'incertitude propagée est également quantifiée par la prise en compte de la variance combinée et elle dépend à la fois de la variabilité géologique et géotechnique. La dernière étape consiste à transformer le modèle 3D de Vs en cartes 2D représentant la distribution spatiale de Vs,30 et T0 avec les incertitudes associées. Les résultats indiquent que les cartes sismiques et leur incertitude sont influencées par l'épaisseur du sol, les probabilités de type de sol et les propriétés géotechniques du sol. L'épaisseur du sol ressort comme un des facteurs les plus critiques. Dans les sédiments peu profonds, les cartes de Vs,30 et de T0 représentent des conditions de roche ou de sol très rigide avec une réponse sismique dans de courtes périodes de vibration ≤ 0,2 s. En revanche, les régions avec des sédiments plus épais présentent des sites avec une réponse potentielle qui ressemble à des conditions de sol moyennes à molles, avec des périodes de vibration dominantes plus longues. Les cartes de variance respectives, soi

Comparison of code-oriented site classification in the Saguenay region, Québec

Local geological and geotechnical conditions can have significant effects on the incoming seismic waves and their spatial variation. Commonly referred to as site-effects, certain conditions can significantly amplify the intensity of the seismic shaking and make it more destructive. Site effects are accounted for in national building codes by the selection of amplification factors as functions of soil category, seismic intensity and frequency. The soil type is categorized by considering representative shearwave velocity, the number of blows in the standard penetration test, and/or undrained shear strength in the top 30 meters. Although soil categorizations based on these geotechnical parameters are more or less similar across earthquake-prone countries, there are some differences, which may result in over- or underestimation of the potential site effects. The SaguenayLac-Saint-Jean region (SLSJ) is located in a moderate seismicity zone in Eastern Canada; the last major earthquake in this region occurred in 1988 with a moment magnitude of 5.9. The presence of important deposits of sensitive clays in SLSJ combined with the strong impedance contrast between the unconsolidated deposits and the underlying crystalline bedrock can play an important role in the soil dynamic response. Different soil classification criteria and respective amplification factors are compared: the 2015 National Earthquake Hazards Reduction Program (NEHRP), Eurocode 8, and the 2015 National Building Code of Canada (NBCC). The advantages and disadvantages of each classification approach are discussed

Probabilistic 3D modeling of layered soil deposits: Application in seismic risk assessment

Estimating the soil properties and the associated heterogeneity is critical in geotechnical risk assessment, particularly in a large urban area and for infrastructural development projects. Seismic hazard studies acknowledge the considerable impact of local soil conditions on the amplitude and frequency of incoming seismic waves. This is particularly challenging in areas with highly variable soil properties and limited soil sampling. A multi-step probabilistic approach is proposed to model the soil-types at a regional scale, involving a large database that incorporates surface and subsurface data with clustered sampling pattern, and highly skewed statistical parameter distributions. First, the Empirical Bayesian Kriging (EBK) method is applied for the interpolation of the total subsoil and till thickness. The results from the EBK method appear more accurate compared with the estimates from the triangulated irregular network. The soil-types and associated probability of occurrence are determined from the continuous till deposit at the bottom and the ground surface topography, using sequential indicator simulation. This simulation method allows predicting the probability of occurrence of discontinuous soil layers in the full 3D model, a real aspect of the soil variability. The predicted soil-types and their probabilities allow better consideration of key geological uncertainties in risk evaluation. L’estimation des propriétés des sols et de leur hétérogénéité est essentielles pour l'évaluation des risques géotechniques, en particulier dans une grande zone urbaine et pour les projets de développement d’infrastructures. Les études sur les risques sismiques reconnaissent l'impact considérable des conditions locales du sol sur l'amplitude et la fréquence des ondes sismiques. Cela est particulièrement difficile dans les zones où les propriétés du sol sont très variables et où l'échantillonnage du sol est limité. Une approche probabiliste en plusieurs étapes est proposée pour modéliser les types de sols à l'échelle régionale, impliquant une grande base de données qui incorpore des données de surface et souterraines avec un modèle d'échantillonnage en grappes et des distributions statistiques très asymétriques des paramètres. La méthode de krigeage bayésien empirique (EBK) a été appliquée pour l'interpolation de l'épaisseur totale du sous-sol et du till. Les résultats de la méthode EBK sont plus exacts que les estimations obtenues par réseau triangulé irrégulier. Les types de sol et leur probabilité d’occurrence sont déterminés depuis le dépôt de till continu jusqu’à topographie de la surface du sol, à l'aide d'une simulation d'indicateur séquentiel. Cette méthode de simulation permet de prédire la probabilité d'occurrence de couches de sol non continues dans le modèle 3D complet, un aspect réel de la variabilité spatiale du sol. Les types de sols prévus et leurs probabilités permettent une meilleure prise en compte des facteurs géologiques clés dans l'évaluation probabiliste des risques géotechniques

Probabilistic approach for seismic microzonation integrating 3D geological and geotechnical uncertainty

A novel probabilistic methodology for regional seismic site characterization is proposed and applied to a region with highly heterogeneous surficial geology and varying soil sediment thickness and stiffness. The method combines various sources of geological and geotechnical uncertainties to develop a three-dimensional (3D) shear-wave velocity (Vs) model and evaluate the associated uncertainties. A 3D geological model of the unconsolidated deposits was developed using geostatistical interpolation and simulation methods. Sequential indicator simulations produced a quantitative geologic model that explicitly quantified geological uncertainties based on the likelihood of specific soil types occurring. In situ measurements and multivariate statistical analysis allowed the development of empirical correlations between Vs, geotechnical parameters, depth, and soil types. The resulting 3D Vs values were estimated on the basis of Vs-depth correlations and the probability of occurrence of each soil type. In this approach, the propagated uncertainty was also quantified by considering the combined variance. Seismic microzonation mapping was then conducted by transforming the 3D Vs model into two-dimensional (2D) maps that represent the spatial distributions of the time-averaged shear-wave velocity of the top 30 m (Vs,30) and the fundamental site period (T0), along with their respective uncertainties using Monte Carlo simulations. The results indicate that microzonation maps and their uncertainties are influenced by the thickness, occurrence probability, and geotechnical properties of soils. The proposed method can be used to assess the probabilistic seismic risk at local and regional scales in areas with geologically and geotechnically complex soil properties

Comparison between the performance of four metaheuristic algorithms in training a multilayer perceptron machine for gold grade estimation

Reserve evaluation is a very difficult and complex process. The most important and yet most challenging part of this process is grade estimation. Its difficulty derived from challenges in obtaining required data from the deposit by drilling boreholes, which is a very time consuming and costly act itself. Classic methods which are used to model the deposit are based on some preliminary assumptions about reserve continuity and grade spatial distribution which are not true about all kind of reserves. In this paper, a multilayer perceptron (MLP) artificial neural network (ANN) is applied to solve the problem of ore grade estimation of highly sparse data from zarshouran gold deposit in Iran. The network is trained using four metaheuristic algorithms in separate stages for each algorithm. These algorithms are artificial bee colony (ABC), genetic algorithm (GA), imperialist competitive algorithm (ICA) and particle swarm optimization (PSO). The accuracy of predictions obtained from each algorithm in each stage of experiments were compared with real gold grade values. We used unskillful value to check the accuracy and stability of each network. Results showed that the network trained with ABC algorithm outperforms other networks that trained with other algorithms in all stages having least unskillful value of 13.91 for validation data. Therefore, it can be more suitable for solving the problem of predicting ore grade values using highly sparse data

Development of empirical CPTu-Vs correlations for post-glacial sediments in Southern Quebec, Canada, in consideration of soil type and geological setting

The correlation of shear wave velocity (Vs) with piezocone penetration test (CPTu) parameters is investigated in post-glacial sediments along the St. Lawrence and Saguenay rivers, Southern Quebec, Canada. The compiled database includes 991 CPTu-Vs measurements at 40 sites. The objectives are to examine the applicability of existing CPTu-Vs correlations, identify the main CPTu parameters and develop specific CPTu-Vs correlations that account for the effects of soil type (sandy or clayey) and geological setting (Champlain or Laflamme sea sediments). Results reveal that the application of existing correlations is biased in varying degrees, denoting a need for site-specific correlations for the study area. Multivariate regression analyses confirm the importance of cone tip resistance and depth as Vs predictors assisted by normalised pore pressure and soil behaviour type index. Consideration of soil type and geological setting helps reduce uncertainties in CPTu-Vs correlations for fine-grained soils

3D probabilistic modelling and uncertainty analysis of glacial and post-glacial deposits of the City of Saguenay, Canada

Knowledge of the stratigraphic architecture and geotechnical properties of surficial soil sediments is essential for geotechnical risk assessment. In the Saguenay study area, the Quaternary deposits consist of a basal till layer and heterogeneous post-glacial deposits. Considering the stratigraphic setting and soil type heterogeneity, a multistep stochastic methodology is developed for 3D geological modelling and quantification of the associated uncertainties. This methodology is adopted for regional studies and involves geostatistical interpolation and simulation methods. Empirical Bayesian kriging (EBK) is applied to generate the bedrock topography map and determine the thickness of the till sediments and their uncertainties. The locally varying mean and variance of the EBK method enable accounting for data complexity and moderate nonstationarity. Sequential indicator simulation is then performed to determine the occurrence probability of the discontinuous post-glacial sediments (clay, sand and gravel) on top of the basal till layer. The individual thickness maps of the discontinuous soil layers and uncertainties are generated in a probabilistic manner. The proposed stochastic framework is suitable for heterogeneous soil deposits characterised with complex surface and subsurface datasets

Seismic microzonation of a region with complex surficial geology based on different site classification approaches

A seismic microzonation study was conducted to refine the seismic hazard model for the city of Saguenay, Canada. The Quaternary geology underlying Saguenay shows complex glacial and post-glacial stratigraphy with a number of buried valleys filled with fluvioglacial and glaciomarine sediments. High impedance contrast between rock formations and surficial sediments is prone to seismic amplification. To evaluate their applicability, advantages and limitations in capturing the geological specificity of the study area, four site classification methods were applied: the current National Building Code of Canada (NBCC) and Eurocode 8, both mainly based on the average shear-wave velocity for the surficial sediments (VS,avg) and for the top 30 m (VS,30); a method based on the fundamental site period (T0); and a hybrid method based on the combination of VS,30, T0 and VS,avg. The study specifically aimed to evaluate the importance of the site classification parameters on the resulting microzonation maps. VS,30 is capable to present the geological and geotechnical site conditions, however, the results may be further improved by considering Vs,avg in shallow and T0 in thick layers of soil sediments as secondary parameters. The T0 method gives also satisfactory results with T0 showing a better correlation to Vs,30 than to Vs,avg. The versatile hybrid method may be challenging to apply in certain cases with its nine different site categories and parameters