Vincenzo De Gennaro

Experimental Investigation on the Time Dependent Behaviour of a Multiphase Chalk

A series of high pressure oedometer compression tests were carried out on samples of Lixhe chalk (Belgium) containing various pore fluids: water, air (dry sample), oil (Soltrol) and oil/water under a controlled suction of 200 kPa. Strain curves with respect to time of tests carried out under a constant load were examined in a purpose of investigating the time dependent behaviour of chalk. A simple empirical exponential law able to account for the combined effects of stress and suction is proposed. Experiments show that it provides satisfactory results to predict a feature of multiphase chalk behaviour that is important in the study of the behaviour of oilfields.

Characterization of Soft Deepwater West Africa Clays: SHANSEP Testing is Not Recommended for Sensitive Structured Clays

With the development of deepwater fields offshore West Africa, in water depths currently ranging between about 600 and 1,500 metres, geotechnical characterization of the sediments has become an issue since the properties of the soft Gulf of Guinea clays differ from those of the equivalent Gulf of Mexico or North Sea clays. In particular, the highly plastic (plasticity index Ip over 80 %) West Africa deepwater clays have a very low unit weight, are highly compressible, and have a relatively strong natural structure. As part of a joint research effort carried out within the CLAROM framework in France, a high quality triaxial testing programme was carried out on clay specimens from an oilfield located in 1,300 metres of water. The paper describes the laboratory test procedure and the results obtained, comparing the results of SHANSEP testing with the results of other triaxial tests at low confining stresses representative of the in situ conditions. With the SHANSEP testing procedure, the micro-structure of the soft West Africa clay is altered by the reconsolidation stresses in the over-consolidated range through large consolidation deformations. The paper shows that the SHANSEP procedure provides undrained shear strength values that are similar to those of a disturbed or reconstituted sample. High quality triaxial testing under confining stresses representative of the in situ stress level appears as the only way to provide geotechnical parameters that are representative of the in situ clay strength. Therefore, the SHANSEP triaxial testing procedure is not recommended for the soft sensitive structured clays from deepwater West Africa sites.

Water Retention Properties of a Mine Chalk

The water retention properties of a chalk from an abandoned underground mine in Northern France were studied and related to changes in relative humidity (RH) in the mine in an attempt to better predict the long term stability of the mine. Chalk microstructure was investigated by scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). It has been shown that some experimental techniques commonly employed for controlling and measuring suction in unsaturated soils can be easily used to determine the retention properties of chalk. The comparisons between the results obtained using these techniques allowed to have an insight into the effect of mineralogy and pore morphology on the retention properties of the chalk.

Water-Rock Interaction Mechanisms and Ageing Processes in Chalk

The microstructures of geomaterials and their evolution under the effects of applied loading and/or environmental conditions can affect the integrity of the solid skeletons and eventually change the mechanical behaviours of the materials at the macroscopic scale. Analyses of geomaterial microstructure and its ‘ageing’ are therefore critical to the understanding of their mechanical behaviour and performance in engineering environments. This process of progressive ageing of the microstructure is mainly related to the interaction between the solid skeleton and the fluids that partially or completely saturate the porous network. Water/rock interaction mechanisms and ageing processes in geomaterials are often slow, leading to structural and textural changes that are generally imperceptible to the naked eye but which can significantly affect matrix integrity and cause sudden collapse. This is a particularly well-known phenomenon in shallow abandoned chalk mines, such as those found in France, where the chalk may remain stable over many tens of years but then suddenly break down, leading to collapse of the openings and the ground above them and subsidence at a macroscopic scale (Sorgi, C. & Watelet, J., 2007). In this chapter we present results from a research program (led by Institut National de l’Environnement Industriel et des Risques (INERIS), France, in collaboration with Ecole des Ponts ParisTech – CERMES, France) that was carried out to evaluate the mechanical behaviour of chalk in the shallow underground mine of Estreux, located in northern France. The study was conducted at three different scales: i. site scale (stability analysis) ii. laboratory scale (standard core testing) iii. microscopic scale (electron scanning environmental microscope (ESEM) observations and micro-testing) In situ characteristics of the Estreux mine are first described, including mine geometry, excavation method, overburden lithology, pillar monitoring, and in situ measurements. Then, petrophysical properties, microstructural characterisation, retention properties, and mechanical behaviour of the chalk investigated at laboratory scale (oedometric and triaxial tests) are presented. A microstructural analysis of the Estreux chalk is also presented. This part of the study was conducted using an ESEM, a recent technology that allows the observation of

Advanced multiphysical testing for soft rocks

ABSTRACT Coupled phenomena are of paramount importance for a correct understanding and a reliable description of the mechanical response of geomaterials to external actions. This response depends on the applied mechanical loads, the type of saturating fluids and the interaction between these fluids and the solid skeleton. Fluids and solid skeleton are in turn variably influenced by intensive state variables: temperature, pressure and chemical potential. Given the variability of the conditions influencing the response of geomaterials, laboratory investigations often require advanced multiphysical testing techniques capable to approach at best the real conditions in terms of intensive state variables. This paper presents some results obtained using advanced laboratory testing to characterise failure in chalks coping with their intrinsic multiphysical nature.

Elasto-plastic Modelling of the Behaviour of Granular Soil-structure Interfaces

It is well known that interfaces usually play a major role in the definition of the mechanical behaviour of engineering structures having interactions with the soil. In this paper the general framework of an elasto-plastic constitutive model developed on purpose for describing the interface behaviour is presented. The model is based on a Mohr-Coulomb failure criterion, including deviatoric hardening/softening, phase transformation state (compaction and dilatancy) and critical state. The choice of the constitutive parameters and their identification is first discussed. The predictions of the model are then presented and compared with available experimental data from various interface tests between sand and metal plates. The results of the numerical analyses emphasise the key role played by the volumetric behaviour of the interface (compaction and dilatancy), linked in some cases with the change in the normal stress acting on the structure surface and, consequently, controlling the shear resistance at the interface.