Cálculo de columnas de grava: algunos puntos oscuros

RESUMEN: Se presenta una revisión de los métodos de cálculo de la capacidad de carga y asientos de columnas de grava, y sus puntos más inciertos o deficientes, así como recientes avances que permiten una solución más ajustada. El análisis se refiere al caso de columnas bajo terraplenes o áreas extensas cargadas. En primer lugar, se presenta el problema de la predicción de la estabilidad de las columnas en suelos blandos, para lo que la aplicación de los distintos métodos conduce a resultados dispares. El tema central de la conferencia es el análisis del reparto de cargas entre terreno y columnas. Este reparto, variable a lo largo del proceso de consolidación, está gobernado por la rigidez relativa de columnas y terreno y tiene múltiples consecuencias: la evaluación de la estabilidad de las columnas en suelos blandos, la magnitud de los asientos producidos, la velocidad de consolidación radial hacia las columnas como elementos drenantes y la capacidad portante media del terreno reforzado. En el texto se presentan avances recientes de carácter teórico, contrastados con experimentación en laboratorio y observación de casos reales, que suponen una importante mejora de los métodos de cálculo tradicionalesABSTRACT: The lecture presents a review of the available methods for calculation of the bearing capacity and settlements of stone columns under embankments or large areas with distributed loads. First, the prediction of column stability in soft toils is commented: the application of existing methods leads to very different results, often due to misinterpretation of the inherent assumptions associated to each of them. The central part of the text deals wíth the load transfer between soil and columns, governed by their relatíve stiffness and variable along the consolidation process, and its consequences on the magnitude on magnitude of the settlements, their evolution in time (radial consolidation towards the draining columns), and the resultíng bearing capacity of the treated ground. In these aspects, recent developments in the design methods are presented, . against laboratory experiments and observation of real cases. These advances mean a significant improvement of the quality of the predictionsEl trabajo descrito es parte de un proyecto de investigación sobre “Tratamiento de suelos blandos mediante columnas de grava para la cimentación de terraplenes de carretera. Análisis del proceso y criterios de dimensionamiento”, financiado por el Ministerio de Fomento para el periodo 2003-07 (Ref.: 03-A634), habiendo sido especialmente positiva la labor de la coordinadora del proyecto, Dra. M.D. Cancela. En el desarrollo han participado, además del autor del presente texto, los profesores del Grupo de Geotecnia J. Cañizal y A. da Costa, y los entonces becarios J. Castro y A. Cimentada. El primero de ellos ha recibido también una beca de FPU del Ministerio de Educación y ha realizado estancias en las universidades de Graz (Austria) y Strathclyde (Glasgow, U.K.), en relación con el presente trabajo. La instrumentación de la obra en la Variante de Sueca (Valencia) fue posible gracias al apoyo del Ministerio de Fomento, y en concreto de la Demarcación de Carreteras de Valencia. El contratista principal fue Dragados, S.A., la construcción de las columnas correspondió a la U.T.E. Geocisa-Kellerterra, y la instrumentación a la empresa Geocis

Three decades of evolution of numerical analysis of tunnels. Apersonal view to the light of a real case.

RESUMEN: Se presenta un esbozo de la evolución de los métodos de análisis numérico en problemas geotécnicos desde sus comienzos, haciendo hincapié en las tres últimas décadas. Para ello, se ha tomado un caso real de una obra bien documentada, que permita un reanálisis con las herramientas de cálculo disponibles actualmente. La obra elegida ha sido la estación de Baixa-Chiado, del Metro de Lisboa, construida en la década de los 90. El objetivo no es la evaluación de la obra en sí, sino de la evolución de las técnicas de modelización numérica reflejadas en su aplicación al caso, en respuesta a la invitación expresada por el Comité Editorial de la revista. La presentación del caso y de los métodos de cálculo tiene una componente subjetiva importante, por lo que las referencias de apoyo se han tomado en lo posible de la experiencia propia o del entorno próximo de los autores.ABSTARCT: An overview of the development of numerical methods for the analysis of Geotechnical problems is presented, with special emphasis on the last three decades. The presentation follows the development of a real, well documented engineering project: the Baixa-Chiado Metro station in Lisbon, built in the nineties. The goal is not to present the project or its analysis, but rather to show the advances in the numerical techniques, in response to the invitation of the Editorial Committee of the Journal. The presentation follows a personal point of view, and this leads to a list of references with a relative high number of sources from the personal experience of the Authors or of their close environmen

Pull-Out Capacity and Failure Mechanisms of Strip Anchors in Clay

ABSTRACT: Plate anchors are a well-established solution for supporting the efforts of floating platforms for wind and marine renewable energies. The behavior of ultrathin rigid plate anchors buried in purely cohesive soils under undrained and plane-strain conditions is analyzed. As already known, a dimensional analysis shows that the pull-out capacity of the anchor may be expressed using a weightless break-out factor (Nc0) that only depends on the ratio between the depth and the anchor width (H/B). Using finite element analyses, tabulated values of the weightless break-out factor are provided in this paper and three different failure mechanisms are identified, namely very shallow (quasi-vertical), shallow or intermediate (semi-vertical), and deep (rotational). For very shallow failure mechanisms, the studied problem is completely equivalent to the trapdoor problem because immediate breakaway at the bottom part of the anchor is considered (vented conditions). The existing analytical solutions for the very shallow (Nc0 = 1.956 H/B) and deep cases (Nc = 3? + 2) using the slip-line method are reviewed and an analytical limit is proposed for the first time for the very shallow mechanism (H/B = 1.314). For shallow (intermediate) cases, the failure mechanism is identified and the angle of the main slip lines is numerically evaluated

Influence of geotextile encasement on the behaviour of stone columns: Laboratory study

This paper presents a study of the influence of the geotextile encasement on the behaviour of soft soils improved with fully penetrating encased columns. This influence is analysed by means of measuring soil-column stress distribution, pore pressures and soil deformation during the consolidation process. For this purpose, a horizontal slice of a representative “unit cell” has been analysed by means of small-scale laboratory tests. The tests were carried out in a large instrumented Rowe-Barden oedometric cell. Results showed that the vertical stress supported by encased columns is about 1.7 times that sustained by the non-encased ones. The stress concentration factor for encased columns is between 11 and 25, which is clearly higher than that obtained in tests with non-encased columns, which are between 3 and 6. Finally, the improvement in relation to settlements is presented by the ratio of settlement in soils reinforced with ordinary or encased columns and the settlement of non-treated soft soil. This settlement reduction factor is around 0.6 when the soil is treated with encased columns and 0.8 for soil with non-encased columns

Deformation and consolidation around encased stone columns

A new analytical solution is presented to study soft soil improvement, both reduction of settlement and consolidation time, by means of encased stone columns. The proposed solution pretends to be a simple and useful tool for design. Therefore, only a unit cell, i.e. an end-bearing column and its surrounding soil, is modelled in axial symmetry under a rigid and constant load. Soil is assumed as elastic but plastic strains are considered in the column using the Mohr-Coulomb yield criterion and a non-associated flow rule, with a constant dilatancy angle. An elasto-plastic behaviour is also considered for the encasement by means of a limit tensile strength. The solution is presented in a closed form and is directly usable in a spreadsheet. Parametric studies of the settlement reduction, stress concentration and consolidation time show the efficiency of encasing the columns, which is mainly ruled by the encasement stiffness compared to that of the soil. Column encasement is equally useful for common area replacement ratios but columns of smaller diameters are better confined. Besides, the applied load should be limited to prevent the encasement from reaching its tensile strength. A simplified formulation of the solution is developed assuming drained condition. The results agree well with numerical analyses

A Criterion for Brittle Failure of Rocks Using the Theory of Critical Distances

This paper presents a new analytical criterion for brittle failure of rocks and heavily overconsolidated soils. Griffith’s model of a randomly oriented defect under a biaxial stress state is used to keep the criterion simple. The Griffith’s criterion is improved because the maximum tensile strength is not evaluated at the boundary of the defect but at a certain distance from the boundary, known as the critical distance. This fracture criterion is known as the Point Method, and is part of the Theory of Critical Distances, which is utilized in fracture mechanics. The proposed failure criterion has two parameters: the inherent tensile strength, ó0, and the ratio of the half-length of the initial crack/flaw to the critical distance, a/L. These parameters are difficult to measure but they may be correlated with the uniaxial compressive and tensile strengths, óc and ót. The proposed criterion is able to reproduce the common range of strength ratios for rocks and heavily overconsolidated soils (óc/ót=3-50) and the influence of several microstructural rock properties, such as texture and porosity. Good agreement with laboratory tests reported in the literature is found for tensile and low confining stresses.The work presented was initiated during a research project on “Structural integrity assessments of notch-type defects", for the Spanish Ministry of Science and Innovation (Ref.: MAT2010-15721)

Influence of gravel density in the behaviour of soft soils improved with stone columns

Stone columns are frequently employed to improve the bearing capacity of soft soils, to reduce settlements, and to increase the speed of consolidation. Their behaviour depends on several factors, such as the density of the aggregate that forms the column and the area replacement ratio. This paper presents a study of the influence of the density of the gravel forming the columns on the deformation and stresses around end-bearing stone columns installed in soft soils. For this purpose, the behaviour of a horizontal slice of a unit cell has been analyzed by small-scale laboratory tests performed in a Rowe–Barden cell. Tests have been performed with a gravel relative density of Dr = 30% and with two area replacement ratios. Their results have been analyzed along with those from similar tests performed with a gravel density of Dr = 100%. The study is focused on the soil–column stress concentration ratio and the reduction of settlements. Finally, the experimental results are compared with numerical simulations. The results show that a reduction of settlements around 10% occurs when the relative density of the gravel increases from Dr = 30% to 100%. Numerical analyses reproduce well the behaviour of stone columns and are in good agreement with the experimental results

The energy threshold in dynamic probing

ABSTRACT: In dynamic probing tests, penetration is closely related to the potential energy of the hammer (nominal energy). This energy stems from the mass and free fall of the hammer after being released from a certain height. Penetration depends on energy, although only on a portion of that nominal energy that is effectively transferred to the rods (ENTHRU) and, more precisely, the energy that reaches the cone (ENTHRUcone). ENTHRU can be measured by monitoring the upper part of the drive rods.To calculate ENTHRUcone, ENTHRU needs to be corrected in three ways. Firstly, the energy loss in the energy transmission through the rods has to be subtracted, as well as the energy loss due to the skin friction of the rods along the soil around them. It is also necessary to add the energy due to the rod weight penetrating the soil. The main hypothesis assumed and later experimentally proved in this paper is based on the fact that ENTHRUcone has to be greater than a certain value or minimum energy (energy threshold: Th) in order to be able to cause penetration. After analyzing more than one hundred blows with different hammer mass and drop height, a small but consistent Th and a linear relationship between energy and penetration beyond it were found. The energy that really produces penetration (ENPEN) will be ENTHRUcone, minus Th. This allows for improved energy corrections and correlations between results from various kinds of penetration tests