The interpretation of conventional and non-conventional laboratory tests for challenging geotechnical problems

This overview paper discusses the use of laboratory testing in conjunction with in situ determination of reference parameters as a step forward to have consistent analyses of the behaviour of geotechnical constructions. In the first part of the paper, the key role of those reference properties, such as the elastic stiffness values obtained in field conditions at repose stress state, is highlighted. By comparing these values with those determined in laboratory reconditioned samples, their "undisturbed" conditions are evaluated and the reliability of advanced laboratory techniques can thereby be assured. The use of seismic wave velocity (V-s) measurements in the field and the laboratory is discussed, and examples are given. Particular emphasis is placed on using the ratio of V-s,V-field/V-s,V-lab for the correction of constitutive stress-strain laws -i.e. the stiffness " decay" (G-gamma) curves determined in laboratory tests. This is becoming a more feasible approach because of the increasing prevalence of laboratory Vs measurement using " bender elements" in laboratory tests at different stages of testing and under diverse stress paths, coupled with the increasing use of seismic cone penetration tests (SCPT) for determination of V-s,V-field. A second subject that is discussed in some detail is the advantage of looking at soil liquefaction as an elasto-plastic mechanical behaviour that is well modelled by critical states concepts, while recognising that it takes places in a wide range of materials and conditions. These issues are outlined in this section, as the critical state framework has now been extended to other materials apart from sands. This approach integrates the knowledge of influence of the micro-mechanics of particles and their contacts on the observed behaviour, and takes account of the effects of continued particle breakage and change in uniformity. The objectives of performance-based design are presented in the light of laboratory and field tests that may be able to identify the triggering risk of both cyclic and static liquefaction and how those tests can be performed and their results interpreted to predict these phenomena, under a global mechanical modelling approach. The third part of the paper describes some special geotechnical testing procedures (equipment and methods of interpretation of test results) that are commonly used in advanced laboratories working for the offshore industry. Being scarcely known and used in current laboratories, even with advanced equipment, these techniques have a great potential in answering to the challenging problems involved in this area of activity, but is it also demonstrated that they can give very useful information in other geotechnical problems. The final section describes a new dosage methodology, based on rational criteria, for cement-soil mixtures, where porosity/cement ratio plays a fundamental role in the assessment of the target mechanical properties of artificially cemented soils, which are used increasingly in many geotechnical engineering solutions.