Effective stress

The effective stress can be defined as the stress, depending on the applied tension \boldsymbol{\sigma}_{ij} and pore pressure p, which controls the strain or strength behaviour of soil and rock (or a generic porous body) for whatever pore pressure value or, in other terms, the stress which applied over a dry porous body (i.e. at p = 0) provides the same strain or strength behaviour which is observed at p ≠ 0. In the case of granular media it can be viewed as a force that keeps a collection of particles rigid. Usually this applies to sand, soil, or gravel, as well as every kind of rock and several other porous materials such as concrete, metal powders, biological tissues etc. The usefulness of an appropriate ESP formulation consists in allowing to assess the behaviour of a porous body for whatever pore pressure value on the basis of experiments involving dry samples (i.e. carried out at zero pore pressure).

Long-term triggered seismicity on the Mosha fault by Damavand volcano, Iran: Implications on the seismic hazard of Tehran metropolis

Seyyedmaalek Momeni

The seismic history of the Mosha fault, the largest and most active fault of Eastern Tehran metropolis, and its relation to the Damavand active volcano, the highest mountain in the Middle East, is investigated. We deduce that the central Mosha, near the Damavand, has a higher seismicity than either its western or eastern segments. On 7 May 2020, an Mw 5.1 earthquake occurred on the central Mosha, about 40 km east of Tehran and 10 km southwest of the Damavand crest, and it was felt intensely in Tehran. Its rupture is imaged and located in a region that presented a relative seismic quiescence compared to its eastern and western parts, during the last 14 years, suggesting its partial locking and heterogeneous distribution of fault frictional strength on this segment of Mosha. Its significant directivity to the west is confirmed by the mainshock rupture model, its PGA distribution, and distribution of early aftershocks. The rupture model suggests a relatively small stress drop of 2.6 bar, which is consistent with the comparatively high rupture dimension of 9 km for a Mw 5.1 earthquake, and indicates the easy rupture expansion on the central Mosha near the Damavand Volcano. The central Mosha experienced earthquakes in 1930, 1955, and 1983, as well as high microseismic activity and the 2020 seismic sequence, all of which strongly point to a possible influence of the Damavand Volcano on the seismicity of the central Mosha. This is corroborated by the observation of hydrothermal zones on the Mosha fault and the extension of a sill-like Damavand young magma chamber until central Mosha in tomography studies. We propose that the existing heat may increase the pore pressure on the fault, which lowers the effective normal stress, facilitates the nucleation-expansion of the rupture, and unclamps the fault. Damavand could act as a fuse and nucleate earthquakes, and if the rupture extends toward the west, it could have a significant directivity effect on low-frequency seismic waves that reach Tehran without attenuation and affect tall structures. In addition, high site amplification for frequencies up to 16 Hz due to the deep sedimentary basin, mainly in the mid-city of Tehran, will be remarkable for short buildings.

2022

Comportement mécanique d'un limon non saturé

Although the majority of soils are unsaturated, their mechanical behaviour is still not well defined. This dissertation presents first an experimental study of an unsaturated silt and then proposes an elasto-plastic constitutive law to model the behaviour of unsaturated soils. The first part of this work deals with the characterisation of the mechanical behaviour of unsaturated soils by means of laboratory experiments on a remoulded silt. Suction is imposed by controlling the excess air pressure in the sample. Several techniques for the measurement of volume changes of samples are developed and compared. The saturated behaviour is studied by means of imposed suction and controlled tests under triaxial and oedometric (uniaxial strain) conditions. Other test performed in Richards cells are used for defining the purely so-called "hydric" behaviour of the soil. The analysis of test results were done in the effective stress plane (σ'=σ-uw,) as well as in the net stress plane (σ*=σ-ua). The hydric tests confirmed the hysteretic nature of the wetting-drying cycles. The hydric and mechanical response changes when the suction raises above the air entry value. Isotropic tests show that the preconsolidation pressure increases with the suction in the log(p')-e plane. However, this increase is rather limited in the net stress diagrams. Deviatoric tests show an increase of stiffness and peak strength with suction at constant mean effective or net pressure. Suction appears to have an effect similar to a mechanical overconsolidation when representing the result in the effective stress plane. Softening appears post peak for the cases of strong suction or weak net mean pressure. The evolution of the critical state is interpreted in terms of effective and net stresses. In the p'-q plane, the critical state line is shown to be independent of the suction (in the studied range of stress and suction); this is not true in the p*-q plane. The yield surfaces F derived from the test results grow with suction and both p'-q and p*-q planes, and the behaviour corresponds to non-associated plasticity. An existing elasto-plastic model originally developed for saturated soils (HISS-δ1) was improved and extended to integrate suction effects. The new proposed model, called δ1-unsat, is described by means of two independent stress state variables: the effective stress σ' and the suction s. Two yield surfaces, coupled through the hardening internal variable, are used in the model; the first one is linked to the mechanical behaviour, while the second one is used for hydric loading. This new model requires five new material parameters: three of them are used for the purely hydric behaviour, one describes the increase of peak strength with the suction and the last one describes the growth of yield surface with suction. The δ1-unsat model is shown to be able to reproduce the principal features of the unsaturated soils: (a) existence on a drying path of a saturated domain with non-zero suction where plastic (irrecoverable) strains appear, (b) elastic behaviour on drying path when the suction is larger then the air-entry value, (c) wetting collapse, (d) influence of mechanical stress state on the hydric behaviour, (e) increase of preconsolidation pressure with suction, (f) increase of peak strength with suction. The quantitative reliability of the δ1-unsat model is good quantitatively. In particular the prediction of the behaviour in the domain of low strains (pre-peak) is well described. The introduction of a damage concept (DSC - disturbed state concept) in the δ1-unsat model allowed the modelling of the post-peak behaviour as well. In conclusion, the mechanical behaviour of an unsaturated silt was characterised. A constitutive elasto-plastic model integrating suction and damage effects was developed and was proved to represent the principal constitutive features of unsaturated soils.

EPFL1999

Long-term triggered seismicity on the Mosha fault by Damavand volcano, Iran: Implications on the seismic hazard of Tehran metropolis

Seyyedmaalek Momeni

2022

Comportement mécanique d'un limon non saturé

EPFL1999

Applicability of a general effective stress concept to unsaturated soils

Long-term triggered seismicity on the Mosha fault by Damavand volcano, Iran: Implications on the seismic hazard of Tehran metropolis

Comportement mécanique d'un limon non saturé

Long-term triggered seismicity on the Mosha fault by Damavand volcano, Iran: Implications on the seismic hazard of Tehran metropolis

Applicability of a general effective stress concept to unsaturated soils

Comportement mécanique d'un limon non saturé