Abstract Soft ground improvement techniques have evolved substantially in Brazil in recent years. However, their application in soft and very soft clays requires a good understanding of the fundamentals of ground improvement techniques suited to the problem as well as the actual field behavior when implemented on a real scale. This paper describes some of the most widely used ground improvement techniques in the context of very soft clays in Brazil. The techniques described in the paper use prefabricated vertical drains (PVD) such as vacuum preloading; or combine PVD and rigid inclusion, such as CPR grouting; or are purely column-like elements such as piled embankments (including those executed with the deep mixing technique, DSM); or combine column-like elements with the drainage function, such as stone columns and geosynthetic encased columns; or use cementitious binders such as shallow soil mixing. The paper reference condition is a soft clay foundation in which no strengthening is implemented, such as, an embankment with basal reinforcement or soft clay with vertical prefabricated drains, or the use of vacuum preloading to speed up the consolidation rate. The applications of the ground improvement techniques are illustrated by case histories, numerical analyses, or physical models. Different types of measurements are used to evaluate the performance of each technique, including settlements, horizontal displacements, excess pore pressures, embankment applied stresses, stress concentration factors, and clay strength following the ground treatment. The settlement improvement factor β, the ratio between the settlements for untreated and treated conditions, is shown to be a suitable parameter to assess the degree of improvement imposed in the soft foundation by ranking the various methods in increased order of strengthening effect.

Abstract Understanding the dynamic behavior of soils is essential to the study of the influence of seismic loads on the instability of submarine slopes, an important issue in Brazil and other countries. The shear modulus and the damping ratio are two fundamental parameters for the study of this behavior. Determining these parameters for Speswhite kaolin clay is the object of the present study using resonant column tests and dynamic centrifuge tests with accelerometers and pairs of bender elements. The curves obtained in the laboratory are compared with empirical curves and comparable data in the literature. Good agreement was observed between experimental data and the empirical prediction for the degradation curve of the normalized shear modulus. The damping curve for very low strains, obtained with resonant column tests, was consistent with the empirical curve. However, consistent with a trend observed in the literature, the centrifuge test results presented considerable scatter (dispersion), attributable to the difficulty in modelling damping dissipation mechanisms in the centrifuge.

Abstract Dynamic soil parameters such as the shear modulus suffer degradation while damping in soil increases under dynamic loading. These can be determined from various tests such as element tests, field tests and centrifuge experiments. Most of the studies about dynamic soil characterization have focused on evaluating these parameters assuming horizontal ground conditions without considering the effect of static shear stresses induced by ground inclination. This paper presents a dynamic centrifuge test conducted on a layered gentle slope comprising clay and sand, to obtain experimental data in terms of shear modulus and damping for various shear strains. Particle Image Velocimetry (PIV) was employed to measure the displacements and to calculate accelerations due to seismic loading at various depths throughout the slope model. The results suggest that the static shear stress caused by the profile inclination causes a more pronounced degradation of the shear modulus when compared to flat ground conditions. Moreover, the damping during the centrifuge test exhibited larger values than expected, following a similar behavior observed in other experimental programs. The strain rate analysis revealed the mobilization of shear stresses higher than the monotonic shear strength for the clay layers during the seismic shaking. experiments sand strains PIV (PIV model Moreover expected programs shaking

Abstract The test sites analyzed here consist of clay deposits located in the Jacarepaguá Lowlands in Rio de Janeiro, characterized by high plasticity, high compressibility and low undrained shear strength. The deposits are made up of lightly overconsolidated aged clays, montmorillonite being the predominant clay mineral. Soft clay deposits are usually superficial, with thicknesses generally varying between 6 m and 17 m and geologically recent and originated from marine regressions and transgressions, that occurred between 6000 and 3500 years ago. The objective of this study is to analyze a large database of undrained shear strength measurements obtained by 461 vane tests performed at 15 different sites. In general, most of the data correspond to very soft clays, with undrained shear strength values lower than 25 kPa. The undrained shear strength measurements are correlated with plasticity index and with maximum excess pore pressure, measured with piezocone tests. The method for estimating the undrained shear strength su(DT) of soil from the excess pore pressure generated during piezocone dissipation tests proposed by Mantaras et al. (2015) was validated against the vane test database.