Alberto Ledesma

Estimation of parameters in geotechnical backanalysis — I. Maximum likelihood approach

The estimation of soil and rock parameters based on field instrumentation data is a common procedure in geomechanics. The use of system ident

Estimation of parameters in geotechnical backanalysis — II. Application to a tunnel excavation problem

cation and optimization techniques allows the performance of this type of analyses in a more rational and objective manner. In this paper a probabilistic formulation for the backanalysis problem is presented. The procedure described involves the evaluation of the measurement covariance matrices, which are derived for some geotechnical instruments used in jield instrumentation. The algorithm used to solve the mathematical problem of optimization is also presented, as well as its coupling to aJinite element code. The algorithm requires the computation of the sensitivity matrix, which can be evaluated “exactly” in terms of thefinite element method. Finally, a synthetic example, based on the excavation of a tunnel, is presented in which the elastic modulus E and the Ko parameter of the material are identljiedfrom measured displacements. The eflect of the number of measurements and their error structure is also discussed.

Parameter and variance estimation in geotechnical backanalysis using prior information

A general statistical framework to perform backanalysis in geotechnical problems from

Characterisation of a volcanic residual soil and its implications for large landslide phenomena: application to Tenerife, Canary Islands

eld instrumentation has been presented in a companion paper. Here, an application to a real case involving the excavation of a tunnel in st@ overconsolidated clay is described. Both, extensometer and inclinometer measurements are used as input data and elastic moduli of the ground and the value of the Ko coeficient are estimated. The finite element method is used as the computational procedure to solve the direct problem, and has been coupled to the identtfication algorithm as described in the companion paper. In addition, a discussion on the reliability of the parameters identtjied is presented.

Backanalysis of thermohydraulic bentonite properties from laboratory tests

SUMMARY A probabilistic framework to perform inverse analysis of geotechnical problems is presented. The formulation allows the incorporation of existing prior information on the parameters in a consistent way. The method is based on the maximum likelihood approach that allows a straightforward introduction of the error structure of field measurements and prior information. The difficulty of ascribing definite values to the uncertainties associated with the various types of observations is overcome by including the corresponding variances in the set of parameters to be identified. The inverse analysis results in a minimization problem that is solved by coupling the optimization technique to the finite element method. Two examples are presented to illustrate the performance of the method. The first one corresponds to a synthetic case simulating the excavation of a tunnel. Young’s modulus, KO value and measurements variances are identified. The second case concerns the excavation of a large underground cavern in which again Young’s modulus and KO are identified. It is shown that introduction of prior information permits the estimation of parameters more consistent with all available informations that include not only monitored displacements but also results from in situ tests carried out during the site investigation stage.

Mechanical relationship between catastrophic volcanic landslides and caldera collapses

Large landslides are common processes during the evolution of volcanoes and individual events can exceed several cubic kilometres in volume. Volcanic slope failures are a significant risk for the neighbouring population due to their huge volumes and great runout distances. Around the Canary archipelago, a total of seventeen deposits of large landslides have been found, and on Tenerife, seven large landslides have affected the subaerial and submarine morphology during the last ∼6 Ma. However, the causes of such mass movements are still poorly understood. This work analyses the events around the Canary Islands and focuses on the ones that occurred on Tenerife in order to obtain new insights into the mechanisms of large volcanic landslides. The study is divided into a first part that includes site investigations examining the general features favouring large-scale failures at volcanoes. The second part describes the laboratory tests used to analyse a residual soil that may be the potential slip surface of the slides on Tenerife. The site investigation revealed that regional tectonics and the climate have a significant influence on the spatial distribution of the landslides. Moreover, morphological and geological features such as deep fluvial canyons, a high coastal cliff and persistent dike intrusion may favour the initiation of slope failure. A typical residual soil sample from the lateral scarp of the La Orotava amphitheatre on Tenerife was studied by carrying out standard laboratory tests. The microstructure was analysed using environmental scanning electron microscopy and a particular bonding was found. This bonding was also detected by the geotechnical tests. Consolidation tests and direct shear tests revealed that the mechanical behaviour of the residual soil changes greatly if the bonding of the soil is broken. The bonded structure generally fails when the effective normal stress surpasses the yield strength of the bonding. In the case of large volcanic landslides with thicknesses up to several hundred meters, the high overburden easily exceeds this yield strength and generates a broken bonding. Therefore, volcanic residual soils, such as the one analysed in this study, are perfect candidates for the potential failure surfaces of large volcanic landslides. Referring to the La Orotava events, we assume that residual soil layers and morphological, geological and climatic features reduced the slope stability to critical conditions, whereas a strong earthquake associated with a caldera collapse episode may have finally triggered the landslide. The results obtained indicate that the residual soils play an important role in affecting the stability of volcano slopes and their destabilising influence significantly favours large-scale sliding. We suggest that the results obtained from this study can be applied to other locations since volcanic residual soils are common in volcanic areas.

Image Analysis for the Quantification of a Developing Crack Network on a Drying Soil

Abstract The paper presents a general methodology to perform backanalysis of laboratory tests involving thermohydraulic behaviour of bentonite in a systematic manner. The procedure is based on a maximum likelihood approach that defines a probabilistic framework in which error measurements and the reliability of the parameters identified can be estimated. The method is applied to the identification of some thermal and hydraulic properties of a bentonite specimen, using temperature and water content measurements as input data. Three basic cases have been considered: (a) thermal case, identifying the global thermal conductivity, λ e , and the global specific heat, c e ; (b) hydraulic case, identifying the tortuosity factor, τ , and the exponent of the unsaturated permeability law, n ; and (c) thermohydraulic case, in which τ , n , and the saturated thermal conductivity, λ sat , have been estimated. A numerical code that solves the coupled thermohydraulic equations has been used as main computational tool. A detailed description of the experimental device and a discussion on the tests results and on the parameters identified are also included. The values obtained are within the normal range of these parameters, but the method provides a systematic and consistent procedure to find the best parameters that reproduce the measurements for the selected models. Also the method gives an insight into the model structure, and allows detecting dependence and coupling between parameters.

Size Effect in the Cracking of Drying Soil

Large-scale sector collapses of volcanic edifices and collapse calderas represent some of the most catastrophic geological events taking place at the earths surface. Examples of these two processes occurring simultaneously suggest that a mechanical relationship between landslides and collapse calderas may exist. We demonstrate that a caldera collapse can trigger large-scale landslides in volcanic terrains. Moderate seismic shocks caused by seismogenic slip on the ring fault on which the caldera subsidence takes place act as the driving force necessary to destabilize the volcano flank. This process is favored on steep volcanoes and where flank strength is reduced by agents such as hydrothermal alteration, pore fluid pressure increase or the presence of weak soils.

Origin and Mechanism of Cracks Seen at the Bottom of a Desiccating Soil Specimen

The paper presents a methodology for quantifying surface crack patterns that appear in cohesive soils under drying conditions due to environmental changes, using image analysis techniques. This has practical applications in the study of many geotechnical problems related to soil cracking such as the impact of permeability changes due to cracking in clay barriers, development of preferential flow paths for contaminant transport along cracks, decreasing bearing capacity, and others. The study of soil cracking may become even more relevant with the current climate change that may induce more frequent and severe droughts in many parts of the world, increasing the areas at risk of cracking. Qualitative and quantitative characterization of the crack patterns is needed to study the mechanical behavior of a cracking soil, how cracks generate and propagate. For this purpose a simple laboratory set-up has been developed for continuous monitoring of the processes of formation and propagation of cracks due to desiccation, and to study the final crack pattern. The paper describes a simple technique to process sequences of images obtained during the laboratory tests, and how image analysis can be used to quantify the parameters that characterize the evolving and final crack patterns.

Experimental analysis of 3D cracking in drying soils using ground-penetrating radar

Cracking in soils due to water loss is a problem not much studied from a mechanical point of view, despite its environmental implications. For instance, if a clayey soil is used as an impervious barrier in open waste sites, an intense drought may origin cracks and therefore preferential flow paths for polluted water. Cracks produced by environmental agents also reduce the bearing capacity of the soil and increases its propensity to erosion. Previous works have studied the problem either from a Fracture Mechanics perspective (Vallejo [1], Prat et al. [2], Avila [3], Harison et al. [4], [5], Hallet and Newson [6]), analysing the conditions for crack propagation, or from a classical Soil Mechanics approach (Kodikara et al. [7], Abu-Hejleh and Znidarcic [8], Konrad and Ayad [9], Morris et al. [10], Lloret et al. [11], using the effective stress principle. In this case it has been observed that cracks initiate when soil is still close to saturation.