Micro-Geomechanics Workshop, 2005


Workshop Overview A number of technical and professional factors motivated the organizers to hosting this workshop. To provide a framework for aspects of the event, these are summarized below. It is now possible to envisage the future clarification of micro-mechanisms such as creep, cyclic degradation (phase transformation, liquefaction), and collapse through de-structuring which cause immense uncertainty in geotechnical practice. There is a widespread acknowledgement that micro-mechanics has made a substantial contribution to other branches of materials engineering, including high-strength composites, ceramics and asphalt road pavements. The aim should be to identify fundamental mechanisms and thereby clarify the definition of useful tests and parameters. This is especially significant where macroscopic failures involve material behavior that is poorly understood. Geomechanics is certainly a field in which “cook-book” approaches still proliferate. However, fundamental understanding is improving rapidly. Anisotropy of stiffness, strength and hydraulic conductivity are understood to be due to the fabric of granular materials, even though the inspection of fabric is far from routine. The recent introduction of micro-mechanisms for grain crushing are also proving helpful in shedding light on volume compressibility even though grain size analysis in Civil Engineering remains rooted in 19^th century laboratory practice. It is the lack of a proper understanding of parameters and behavior that obstructs the geotechnical engineer, not the inability to make calculations based on those parameters once they have been selected. The ongoing specialization of geotechnology within sub-disciplines such as tunneling, pile driving, geosynthetics and ground improvement has tended to proliferate empirical approaches and draw attention away from the unifying theme of the mechanics of soils. Yet there is much to be gained from a holistic approach. There is certainly little confidence at present amongst designers and decision-makers in the selection of geotechnical parameters within advanced mathematical models. Yet it is increasingly apparent that a realistic treatment of soil grains - including the effects of elastic contacts, sliding, crushing, and rearrangement - offers a fruitful avenue to discuss the whole spectrum of soil behavior. For example, it has recently been suggested that soil crushability is a key to behavior under all but the smallest stress levels. The behavior of crushing sands in complex stress-path tests can be normalized using similar precepts to those of critical state soil mechanics that have previously been applied to clays. Plasticity in clays is then seen to be analogous with grain crushing observed in sands. This seems to offer the prospect of a new synthesis over-arching “cohesive” and “granular” materials. In this way, the plethora of parameters required to describe soil deformations might be reduced to a fundamental few which have proven physical significance at the micro-level. Further, having micro-structural evidence to accompany conventional element test data will augment the confidence of designers. The techniques and tools for investigating micro-mechanics are now expanding. Phenomena such as evolution of local void ratio distribution and particle reorientation can be quantified before and after testing in the laboratory using new technologies which have superseded techniques such as sieving, sedimentation and specimen dissection in other branches of materials science. There is an increasing use of microscopes and video cameras to image the deformations of soil fabric at the scale of particles. For example, the real-time crushing and rearrangement of sands during penetrometer investigations is under review, offering a more fundamental understanding of the credibility of correlations with engineering design parameters. Other studies have focused on localization and other phenomena within soils. A range of geophysical methods have become available for gaining information on soil fabric, such as electrical conductivity and dielectric properties, acoustic sensing and seismic velocity probing, nuclear magnetic resonance imaging, and computerized X-ray tomography. Furthermore, there are continuing advances in Discrete Element Models (DEM), which can represent an aggregate of grains in a numerical simulation. The matching of DEM simulations with the actual response of real soil fabric in element tests can lead to improved understanding of, and definitions for, all aspects of soil behavior. Soil constitutive models are now being derived directly from simple micro-mechanical principles. It is timely to hold a high-level Workshop to review Research Priorities in Micro-Geomechanics following the creation of ISSMGE TC35. The International Society of Soil Mechanics and Geotechnical Engineering acceded in January 2000 to form a new Technical Committee, TC 35 on the subject of Micro-Geomechanics. The first meeting of TC35 took place in Istanbul in August 2001, immediately prior to the 15^th International Conference. This meeting was preceded by a Workshop on the Granular Perspective in Geomechanics to bring recent work to the attention of TC members and other interested ISSMGE delegates. A debate and discussion on the physical significance (or otherwise) of soil parameters followed.