Alberto Ledesma

Design of deep excavations involves the definition of the retaining walls and struts to support earth pressures. Despite the experience and the procedures available to estimate the forces acting on the struts, their prediction is still difficult and comparison between field measurements and numerical simulations suggest that there are many factors not taken into account in the routine calculations. This paper presents a software (DAARWIN) that can be used to manage field data and to perform backanalysis in an automatic manner to predict soil parameters. It is a cloud based platform combined with the Plaxis Finite Element code, that uses genetic algorithms to solve the backanalysis. This software has been applied to the identification of soil parameters and strut loads in a real case involving a deep excavation from Toulouse Metro in France.

Shear strength is a classical topic in Soil Mechanics and generally there is little concern about the inconsistencies behind the theories used to predict its value. In fact the debate on this issue is rather limited as the geotechnical community considers this a well-established concept. This note intends to highlight the difficulties that arise when teaching that concept in an undergraduate Soil Mechanics course. Those difficulties are related to the drained/ undrained behavior of soils, but also to the fact that cohesion is a tricky parameter, with a misleading physical meaning, depending not only on the properties of the contacts between particles, but also on external conditions (i.e., saturation or unsaturation). All these aspects are not analyzed in detail in many textbooks, but they should be considered in a modern Soil Mechanics course.

High capacity tensiometers (HCT) are the only type of sensors in existence that can measure soil matric suction directly and are regarded to be the most accurate and reliable technique for the study of suction evolution in unsaturated soils. The measurement with HCTs is possible due to their design, composed of ceramic filter with a specific air entry value (AEV), small water reservoir, and pressure transducer. Where, the AEV of the ceramic filter, more precisely the largest pore size within the ceramic, plays the most important role in controlling the measuring range of HCTs. Specifically, decreasing the largest pore size within the ceramic results in an increase in the measuring range. In this work, a new development in HCTs is presented whereby the ceramic filter in HCTs was replaced from the typically used 1.5 MPa AEV ceramic filter with a new alumina ceramic filter with an estimated AEV of 3.5 MPa. To assess the performance of the new ceramic filter, similar designed HCTs were assembled with the Soilmoisture Equipment Corp. 1.5 MPa AEV and the new alumina ceramic filters. Early results show that the maximum measuring matric suction range is beyond 3 MPa for HCTs assembled with the new alumina ceramic filter.

This paper reviews the state of the art of soil behavior in the range of small strains and its constitutive modeling, which is an important issue when predicting displacements under serviceability conditions. The factors that control nonlinear, hysteretic and dependent on recent history soil behavior are described. Likewise, concepts of soil constitutive modeling are explored in detail and two criteria are explained and used to classify the analyzed models: (1) a first criterion based on the concept of tensorial zones; and (2) a second criterion based on the elements that defines the hysteretic soil behavior, including reversal criteria, memory rules and the effects of reversals on soil degradation and on soil stiffness recovery. The fundamentals of the formulation of the analyzed models are provided, as well as their scope of application, advantages and disadvantages.

The Elastoplastic Hysteretic Small Strain (EPHYSS) model is an advanced elastoplastic model as a result from the combination of the Hysteretic Quasi‐Hypoelastic (HQH) model that considers strain‐induced anisotropy and can reproduce the nonlinear reversible, hysteretic and dependent on recent history soil behavior, and the Cap‐Cone Hardening Soil Modified (HS_MOD) model that can reproduce soil plastic behavior. EPHYSS model uses state variables that define different short and long‐term memory levels which provide it with robustness for the reproduction of soil hysteretic behavior and confers it a great versatility and adaptability to experimental results. It also corrects some inconsistencies of the Hardening Soil with Small Strain Stiffness (HS‐SS) model of Plaxis whose effects can have a considerable influence on the numerical simulations of boundary problems. The performance of EPHYSS and a comparison with the HS‐SS model is presented in some experimental and numerical tests, and in a boundary value problem of a large excavation in Barcelona.

This paper presents the results and analysis of two cracking tests carried on specimens of silty clay. One specimen was prepared in slurry conditions without applying energy and the other specimen was compacted. They were dried in an environmental chamber at a constant temperature and relative humidity to study the effect of the initial consistency on the cracking behaviour. Weight measurements and photographic images taken at regular intervals documented the evolution of the specimens. THM models were then carried to capture the unsaturated flow in the porous medium due to evaporation, and its resulting shrinkage. All the numerical analyses were coupled, incorporating the effect of porosity change on the balance equations and the constitutive model. The transfer coefficients in the imposed drying boundary condition were based on calculations of aerodynamic surface resistances, taking into consideration the new fronts for evaporation created by the cracks. The constructed numerical model results capture the gravimetric water content loss and the occurring shrinkage for both specimen conditions.