Geological origin as an input variable in reliability-based designs: for an accurate exploration in geotechnical engineering

Soil is one of the most difficult materials to characterize realistically, which partly explains the uncertainty between the designs and the geostructures real behavior. Different recommendations have arisen with respect to carrying out field investigations in order to reduce the uncertainties inherent to the soil. However, the field exploration and the implementation of sophisticated geotechnical models have proven to be insufficient to mitigate the geotechnical uncertainty. Therefore, Reliability-Based Designs (RBD) emerge as a decision-making tool through the definition of the probability of failure in conjunction with the typical Factors of Safety. RBD requires a previous understanding of the most appropriate soil probabilistic models, such as the Shear Strength Varying with Depth (SSVD) analysis, traditional Monte Carlo simulations or random fields. Soil shear strength uncertainty is related to soil geological characteristics, however, geology has been commonly used in geotechnical engineering as a definition of the layers’ distribution on the soil mass, where the definition of the accurate RBD models according to the geological origin has been missing. Therefore, two geological formations were analyzed: residual soils (stationary origin) and mudflows (dynamic origin). The results show that random fields are more related to the mudflows due to the random nature of these soils, thus the exploration resources should be focused on the determination of the Probability Density Functions (PDF) and the spatial variability of the shear strength (SS) properties (laboratory tests have priority over the in situ tests). Residual soils present a higher SS space uniformity because these soils have not been previously mobilized, thus the exploration resources should be focused on the determination of the SSVD (field tests have priority over the laboratory tests). Therefore, defining the geological origin as an “input variable” will allow recognizing the most important variables and the definition of the best soil exploration for an accurate and cost-effective RBD in geotechnical engineering

Probabilistic Analysis of the Active Earth Pressure on Earth Retaining Walls for c‑φ Soils According to the Mazindrani and Ganjali Method

The determination of the earth pressure coefficients (K) in geotechnical engineering is one of the most critical procedures in designing earth retaining walls. However, most earth pressure theories are made for either clay or sands, where the c-ϕ soils are the least analysed. In this paper, an analysis of the earth pressure for drained mixed soils based in Mazindrani and Ganjali (J Geotech Geoenviron Eng 123:110–112, 1997) theory was carried out. Earth pressure coefficients are generally used in a deterministic way and can represent designs under an inadmissible risk. Therefore, Reliability-based design arises as an essential tool to deal with soil variability as one of the main aspects of the geotechnical uncertainties. The influence of the soil variability in the active earth pressure for a c-ϕ soil was performed through probabilistic analysis concerning the Ka coefficient of variation (Cv) of both shear strength parameters. The sensitivity analysis shows a Cv in which the cohesion begins to have a more significant correlation with Ka than the friction angle. The results show an increase of the statistical Ka concerning the deterministic value as the soil variability and the soil slope (β) increase. Although the statistical value does not increase significantly, a statistical analysis on gravity walls and sheet pile walls in c-ϕ soils shows a significant probability of failure (pf) increase. The pf obtained through the c-ϕ variability can be considered inadmissible even if the required FS are met

Características de la aceptación del riesgo sobre obras de infraestructura en laderas del Valle de Aburrá

RESUMEN: Los crecimientos urbanísticos alrededor del mundo afrontan grandes retos del orden social, ambiental y económico, donde la ingeniería juega un papel importante a través del diseño y construcción de distintas obras de infraestructura seguras, para el desarrollo de las distintas sociedades. Debido a las condiciones topográficas y alto crecimiento demográfico, las sociedades se vieron obligadas a crecer urbanísticamente sobre las laderas, donde poco a poco se asumieron mayores retos con el fin de atender los cada vez más exigentes proyectos de urbanísticos, o en su defecto asumir las condiciones de riesgo relacionado con los posibles problemas de inestabilidad

Influence of Limit Equilibrium Methods in the Design of Contiguous Cantilever Sheet Pile Walls Through Reliability-Based Analysis

In geotechnical engineering the design of bored- pile walls is commonly performed in terms of the embedment length (D) defined according to the limit equilibrium methods (also known as balance methods) formulated by Krey, Blum, Rowe, Hansen and the empirical method of Look in sandy soils. The limit equilibrium methods are calculations associated with the shape of the earth pressures distributions in the shaft, earth pressures theories, geotechnical properties and safety factors, while the empirical criteria establish that the relationship between embedment and the free height (H) is of the order of D/H = 1.5. These design methods are complemented by reliability-based analyzes to evaluate the uncertainties associated with soil variability and the use of deterministic concepts. The Monte Carlo method is applied to evaluate the inherent variability of soil friction angle and the soil-wall interface angle in the calculation of the stability of adjacent bored-pile walls. The results show that Hansen’s method requires a lower D/H ratio to obtain the failure probabilities of the USACE (PF = 0.001) in comparison with the other methods. The above is because the method considers that the passive earth pressure increases linearly from the excavation level to the bottom of the embedment depth, however, the other methods considers an earth pressure balance around the pivot point in the shaft. The results show that for a probability of failure established at 0.001, the obtained D/H by the Krey, Blum, Rowe and Hansen methods are 2.13, 1.97, 1.97 and 1.93, respectively. These values represent differences between 28.7% and 42.0% with respect to the deterministic design criteria of Look. Thus, the Look criterion, from the probabilistic point of view, can be considered as an inadmissible risk method, especially since it does not consider surcharge and water table

Reliability-Based Designs Procedure of Earth Retaining Walls in Geotechnical Engineering

The design and construction of foundations, retaining structures and slopes are usually based on deterministic formulations that do not allow the distinction between the natural variability and the inherent dispersion in the geotechnical parameters. Due to the inherent variability of the soil properties, there is a growing trend to implement reliability-based designs in geotechnical engineering to reduce design uncertainties by probabilistic methods. The reliability designs require the definition of the probability density functions of the geotechnical properties, as well as knowledge of the spatial variability of soils. This paper identifies the procedures, type of soil investigations, simulations and the most commonly studied areas in geotechnical reliability-based designs. The importance of the correlation length in defining the reduction factors to determine the probabilities of occurrence of events, with Monte Carlo as the most used simulation method in this type of designs, is highlighted. The most commonly studied problems in this regard are related to foundation design and slope stability analyses whereas earth retaining walls and gabion walls are the less studied. Furthermore, no study was found in the determination of the variation of the soil-wall friction nor in the geological influence for this type of structures, which implies a great potential for future research in these areas

Geology Influence in the Slope Stability Analysis by Limit Equilibrium, Finite Elements and Random Finite Elements Methods

Slope stability analyzes correspond to the evaluation of the safety factors commonly performed through the limit equilibrium and finite element methods. Due to the uncertainty of the geotechnical parameters, different statistical methods have emerged such as the limit equilibrium combined with the Monte Carlo simulation method and the random finite elements, which have been widely accepted in recent years. Normally, these analyzes are not performed taking into account the geological influence on the geotechnical properties, so in this work we proceed with the comparison of several geometrical typologies of slopes for mudflows and a residual soils of the Stock San Diego. These geologies according to the standard penetration test presents similar geo-mechanical characteristics in addition to similar grain size distribution, but their statistical parameters obtained by several laboratory tests shows a noticeable difference between them. The results show that the long-term safety factors of the mudflows are more susceptible to geometric changes with respect to the residual soils. The residual soils present a better conditions of stability in the short term analysis. The statistical parameters obtain higher safety factors in the residual soils. The probabilities of failure by random finite elements are substantially greater on the residuals and, finally, that the greater deformations occur in the mudflows under 1:1 slopes and 1: 0 in the residual soils

Reliability Analysis of Active Earth Pressure for Cantilever Walls on Mudflows and Residual Soils of the Aburrá Valley

According to the variability of the local geology of the Aburrá Valley, two geological strata with similar grain size distribution were analyzed. The geologies are composed of mudflows and residual soils from the San Diego stock in the Poblado neighborhood. These soils show a similar mechanical behavior in relation to the number of blows of the standard penetration test SPT. The analysis was carried out by reliability methods for the determination of the active earth pressure for the geotechnical design of a cantilever walls. The analysis consists in the variation of the height of the retaining wall between 1 and 10 meters and the inclinations of the filling between 0 and 25 degrees, using design parameters obtained from laboratory tests (effective cohesion and friction angle) and the results obtained from correlations of the SPT test (equivalent friction angle). These input variables were represented by their probability distribution functions, based on the results of the field tests and the direct shear tests. The results indicate that, although these geologies show similar mechanical behavior, there is a geological influence from the probabilistic point of view, since these soils have different probability functions for the design parameters, which directly affect the probability function of the active earth pressure and therefore the probability of failure for the cantilever retaining walls analyzed

Procesos para los diseños por confiabilidad de muros de contención en ingeniería geotécnica

RESUMEN: El diseño y construcción de cimentaciones, estructuras de contención y taludes se basan generalmente en formulaciones deterministas que no permiten la distinción entre la variabilidad natural y la dispersión inherente de los parámetros geotécnicos. Debido a la variabilidad inherente de las propiedades del suelo, existe una tendencia cada vez mayor en la implementación de diseños basados en la confiabilidad en geotecnia, con el fin de reducir las incertidumbres con métodos probabilísticos. Los diseños por confiabilidad requieren la definición de las funciones de densidad de probabilidad de las propiedades geotécnicas, además de tener el conocimiento de la variabilidad espacial correspondiente a cada tipo de suelo. Este artículo identifica los procedimientos, tipo de investigación del subsuelo, simulaciones y las áreas de estudio más comunes en los diseños geotécnicos basados en la confiabilidad. Se resalta la importancia de la longitud de correlación en la definición de los factores de reducción para determinar las probabilidades de falla y que el método de simulación más utilizado en estos diseños es Monte Carlo. Las áreas de la geotecnia más estudiadas en confiablidad son las de diseño de cimentaciones y análisis de estabilidad de las laderas, mientras que los análisis de muros de contención y muros de gaviones son los menos estudiados. Por otra parte, no se encontraron estudios en la determinación de la variación de la fricción suelo-muro ni en la influencia geológica para este tipo de estructuras, lo que implica un gran potencial para futuras investigaciones.ABSTRACT: The design and construction of foundations, retaining structures and slopes are usually based on deterministic formulations that do not allow the distinction between the natural variability and the inherent dispersion in the geotechnical parameters. Due to the inherent variability of the soil properties, there is a growing trend to implement reliability-based designs in geotechnical engineering to reduce design uncertainties by probabilistic methods. The reliability designs require the definition of the probability density functions of the geotechnical properties, as well as knowledge of the spatial variability of soils. This paper identifies the procedures, type of soil investigations, simulations and the most commonly studied areas in geotechnical reliability-based designs. The importance of the correlation length in defining the reduction factors to determine the probabilities of occurrence of events, with Monte Carlo as the most used simulation method in this type of designs, is highlighted. The most commonly studied problems in this regard are related to foundation design and slope stability analyses whereas earth retaining walls and gabion walls are the less studied. Furthermore, no study was found in the determination of the variation of the soil-wall friction nor in the geological influence for this type of structures, which implies a great potential for future research in these areas

Caracterización de las Propiedades Indices de los Flujos de Lodos en 3 km2 de la Ladera Oriental de la Ciudad de Medellín

La caracterización de las propiedades índices de los geomateriales es una de las labores más importantes en la ingeniería geotécnica, al ser estos un insumo fundamental para la identificación y determinación de propiedades geomecánicas de los suelos. Las principales propiedades índices estudiadas en la geotecnia son el contenido de agua, los límites de Atterberg, la granulometría, el peso unitario y la gravedad específica, las cuales son la base para clasificación de suelos. En este artículo se presenta el análisis estadístico de las propiedades índices de suelos tipo flujo de lodos de la ladera oriental de Medellín, a partir del análisis de 10 proyectos de acceso público y con información suficiente para los análisis, los cuales cubren un área aproximada de 3 km2. Los resultados evidencian bajos coeficientes de variación de las propiedades índices de los flujos de lodos, especialmente para la densidad húmeda y granulometría, porcentajes de saturación altos y, según los límites de Atterberg, estos suelos se encuentran primordialmente en estado plástico