SLADUS: AN INTRA-EUROPEAN FELLOWSHIP IN COLLABORATION BETWEEN THE UNIVERSITY OF GLASGOW AND THE UNIVERSITY OF NAPLES FEDERICO II Francesca D Onza, Università degli Studi di Napoli Federico II Domenico Gallipoli, Simon Wheeler University of Glasgow, Glasgow, UK Claudio Mancuso Università degli Studi di Napoli Federico II Summary Unsaturated soils occur naturally (as superficial ground above the water table) or in manmade structures (as compacted earth in infrastructure embankments, underground nuclear waste repositories and flood defences). Unsaturated soil mechanics has experienced significant advances during recent years, instigated by the use of compacted earth as sustainable building material and by the need of improving techniques for management and appraisal of earth structures. Seminal contributions to unsaturated soil mechanics have been made over the past two decades but the current state-of-the-art is still unable to provide an accurate understanding of pre-failure behaviour in compacted soils, which is crucial to ensure long-term serviceability and cost-effective maintenance of earth structures. Mechanical non-linearity and dependency of small strain stiffness, as well as damping, on stress history are important properties not properly described by existing constitutive models. Understanding the response of compacted soils at small strains is not only central to applications in engineering dynamics, such as predicting ground motion during earthquakes or in the proximity of high-speed railways, but also to the analysis of earth structures under static loads of service. The current project will contribute to fill such gaps of knowledge by pursuing two intertwined lines of investigations. Firstly, it will undertake a wide-ranging programme of triaxial and resonant column tests on unsaturated clayey silt samples compacted both in the laboratory and in-situ following standard procedures to ensure comparable material fabrics. Secondly, it will formulate a constitutive model capable of describing mechanical behaviour from small to large strains and will highlight advantages and limitations of such model when reproducing the behaviour of soils compacted both in the laboratory and in-situ. Introduction Recent challenges in geotechnical engineering have emphasized the need for an interdisciplinary approach drawing upon knowledge at the interface between soil mechanics, material science and geophysics. They have also demonstrated the inadequacy of classical saturated soil mechanics as a scientific framework for a comprehensive understanding of soil behaviour (Jennings & Burland, 1962). In geotechnical practice soils are very often unsaturated and the inter-granular pores are filled partly by a liquid (usually water) and partly by a gas (usually air). Unsaturated soils are therefore a tri-phase material with distinct engineering properties not accounted for in constitutive models developed for bi-phase saturated soils. Nevertheless, virtually all design methods currently used in geotechnical engineering still rely on classical saturated soil mechanics, mainly due to the failure in understanding the nature and engineering relevance of the complex interactions between multiple pore fluids and the solid skeleton. Saturated models are unable to reproduce the full range of mechanical and hydraulic soil properties with consequent negative repercussions on safety and cost-effectiveness of current geotechnical design.
Another very distinctive feature of soil behaviour is the strong non-linearity of the stressstrain response and the dependency of stiffness on the stress path followed during loading. Recent research has documented such non-linear behaviour starting from levels of strain as low as 0.0001% for most soils. Advances in the techniques for in-situ testing, field monitoring and numerical modelling have also led to an improved understanding of ground response during construction and service life of geotechnical structures. Especially in hard soils, the construction of earthworks such as embankments, foundations and tunnels is likely to generate large variations of stiffness across the ground due to the heterogeneity of the induced strain field and the non-linearity of the mechanical response at small strains. Accurate modelling of soil stiffness over the full range of deformations is therefore very important not only for applications in earthquake engineering or underground propagation of mechanical waves, but also for the prediction of ground settlements under static loads of service in most geotechnical applications, when strains remain relatively small over the majority of the soil domain (Atkinson, 2000). Research methodology The project will take place during an overall period of 24 months. The general research aims outlined in the previous section will be achieved through four interrelated tasks (Table 1), which are listed below together with a description of the relevant research methodologies. Table 1. Work plan of the project.
Task A: In-situ measurements of shear stiffness and soil sampling The initial part of the project will be undertaken at a site near Mantova in northern Italy, where an experimental embankment has been erected for research purposes next to the main flood defences of the Po River (Figure 1). The construction and instrumentation of this embankment has been funded by a consortium of four Italian universities with the objective of enhancing current design of flood defence embankments and identifying the main factors that cause slope instabilities, breaches and overtopping in such structures. The embankment is 4 metres high, 194 metres long with a crest width of 4 metres and maximum slope 2:3. It is made of the same soil as the main flood defence embankment of the Po River (i.e. clayey silt); it is compacted in-situ at the optimum water content and dry density as determined by standard Proctor tests. Samples will be initially collected from the embankment fill for the determination of basic soil properties such as Attenberg limits, specific gravity, organic content and grain size distribution. Subsequently, a number of undisturbed samples will be retrieved by core drilling inside uncased boreholes at different locations across the full depth of the embankment. These samples will be used during an extensive laboratory programme of mechanical testing and suction measurements as described below (task B). Profiles of small-strain stiffness with depth will be measured at different locations over the embankment by using advanced field instrumentation for dynamic testing. (SASW, Down-Hole or Cross-Hole). The profiles of small strain stiffness as determined in-situ will be interpreted in conjunction with similar measurements taken in the laboratory, as described below, on undisturbed samples retrieved at various depths across the embankment. Figure 1.Experimental embankment in Viadana (MN): a) plan; b) section. Task B: Laboratory tests on undisturbed field-compacted samples from the embankment An extensive programme of laboratory tests will be carried out, on undisturbed samples retrieved from the embankment, at University of Naples Federico II. One set of undisturbed samples will be used for the determination of suction, water content and porosity profiles across the embankment depth. Porosity and water content will be measured in the laboratory using conventional methods. Given the larger uncertainties associated to suction measurements, suction will be determined by three alternative techniques, namely by axis translation, filter paper and suction probes. The comparison between such alternative methods will provide insight into the physical process of suction measurement and will help to identify advantages and limitations of each technique. Three different classes of laboratory tests will be performed on a second set of undisturbed samples to provide a comprehensive engineering characterization of material behaviour and to investigate the influence of the current soil state (in terms of suction, degree of saturation, and mean net stress) and loading history on the stiffness at small strains. The first
class of tests will use a Resonant Column Torsional Shear device (RCTS_ns) for testing unsaturated soils under suction control. A series of resonant column and cyclic torsional shear tests will be carried out on specimens subjected to isotropic loading/unloading paths at constant levels of suction. These tests are mainly designed to explore behaviour at small strains (i.e. over the range from 0.0001% to 1%) and, in particular, to investigate the dependency of shear stiffness and dissipative properties (such as damping) on the confining pressure at different levels of suction and degree of saturation. At the same time, these tests will make possible to investigate the dependency of volumetric compressibility on both suction and degree of saturation. This is particularly relevant to earth structures in the light of inconclusive evidence in the literature suggesting contradictory variations of compressibility with suction depending on the level of confining stress. The second class of tests will use the same device employed in the first one (RCTS_ns) but, this time, to explore the influence of water retention on mechanical response. Similarly to the first class of tests, measurements of small strain shear stiffness and damping will be taken at various points during cycles of wetting (suction decrease) and drying (suction increase) performed at different constant values of confining pressure. Due to the occurrence of hydraulic hysteresis, it is expected that different degrees of saturation will be achieved at the same suction depending on whether the soil follows a drying or wetting path, which will make possible to isolate the effects associated to changes of suction and degree of saturation. As previously mentioned, these results will be interpreted in the light of recently published theories assuming the existence of a dual link between mechanical and water retention behaviours in soils. The third and final class of tests will use a suction-controlled Triaxial Cell to investigate soil behaviour under the same isotropic compression stress paths and wetting-drying cycles as in the first two classes of tests respectively. In addition, at the end of each isotropic compression and wetting-drying cycle, soil samples will be sheared to critical state in order to investigate the pre-failure behaviour in the deviatoric plane, including the effects of history of suction, degree of saturation and confining stress on the mechanical behaviour. These tests will allow a useful comparison with the soil response observed in the RCTS_ns during similar stress paths and might highlight experimental difficulties associated to the use of a particular piece of equipment. The shearing stages performed in the Triaxial Cell will also offer the opportunity to assess the independent effects of suction and degree of saturation on critical state strength. Task C: Laboratory tests on laboratory-compacted samples A number of laboratory test programmes have been undertaken over recent years, at the University of Naples Federico II, to characterize the engineering properties of the experimental embankment considered in this research. In particular, tests have been performed on artificial samples compacted in the laboratory at values of water content and dry density similar to those existing in the embankment at the time of construction. Such data have been obtained during suction-controlled tests covering similar stress paths as those outlined in the previous task; however they include limited evidence on the soil response during wetting-drying cycles at constant confining pressure. A number of additional tests on similar artificial samples compacted in the laboratory will therefore be performed during this project to explore specifically such aspect of behaviour. The analysis of the existing data, together with additional experimental evidence generated during this project, will enable a comparison between the mechanical response of undisturbed field-compacted samples and that of artificial samples compacted in the laboratory at similar dry densities and water contents as detailed below. Task D: Interpretation of test results and constitutive modelling The experimental data obtained from suction-controlled tests performed at the University of Naples Federico II during the first part of the project (Task B and C), as well as existing tests described under Task C, will be subsequently employed at the University of Glasgow to develop a novel constitutive model able to describe the hydro-mechanical behaviour of the soil from very small to large strains. Unsaturated soil mechanics has made significant progress over the past twenty years and a number of features of soil behaviour are now well captured by existing constitutive models (Alonso et al., 1990; Wheeler, 2003; Gallipoli, 2008). These include the effect of suction on the mechanical response, the occurrence of plastic compression upon wetting, the increase of shear strength with suction and the incorporation of full saturation as a particular case of unsaturated soil behaviour. The model that will be developed during this project, will retain all these features while incorporating additional aspects of soil behaviour such as the dual dependency between mechanical behaviour and water retention, the description of stiffness starting from strains as small as 0.0001% and the occurrence of hydraulic hysteresis during wetting-drying cycles. The development of the model will originate from the large database of
experimental evidence relevant to the clayey silt used in the construction of the experimental embankment (available at the University of Naples Federico II) and will cover the response of undisturbed field-compacted samples as well as samples compacted in the laboratory. The use of such a varied database including results relevant to two different classes of compacted soils will ensure the generality of the proposed constitutive framework. It is also expected that the general material trends emerging from the tests, together with the proposed constitutive framework, will be equally applicable to other types of clays or silts (Wheeler & Sivakumar 2000). Two sets of parameter values, relative to the formulated model, will be determined, based on data relevant to field-compacted samples and laboratory-compacted samples respectively. The comparison of the two sets of parameter values obtained during these calibrations will help to identify potential inconsistencies between the two classes of material as well as emphasizing common trends of behaviour. This is particularly important because current geotechnical design often assumes that engineering properties of earth fills are identical to those of samples compacted in the laboratory at similar values of dry density and water content. Tests on samples compacted in the laboratory are therefore conventionally used to characterize the mechanical and water retention properties of earth fills. This practice, however, seems to overlook the fact that construction procedures in the field might differ significantly from the compaction techniques used in the laboratory and this can induce considerable differences in material fabric. The investigation planned in this project will provide further insight into this aspect and will help to endorse or refute the validity of such practice. Conclusions The present project has been instigated by the fact that unsaturated soils are widely encountered across manmade structures as compacted earth in infrastructure embankments, underground nuclear waste repositories and flood defences. This issue is even more crucial due to the increasing use of compacted earth as sustainable building material and growing pressures in the construction industry to improve techniques for management and appraisal of earth structures. Although unsaturated soil mechanics has experienced significant advances during recent years, current state-of-the-art is still unable to provide an accurate understanding of the hydromechanical behaviour of compacted soils and this research will try to address some of the current gaps of knowledge. In particular, the project will generate a considerable set of experimental data about the mechanical behaviour of unsaturated soils under controlled suction, which will cover the entire range of deformation from very small strains up to failure. These data will be complemented by further laboratory investigation into the soil-water retention and its dependency on deformation. The evidence gathered from such experimental campaigns will be used to explore the existence of a dual link between deformation and water retention in unsaturated soils as recently proposed in the literature. Subsequently, a novel coupled constitutive model will be formulated on the basis of the data generated in this project as well as additional experimental evidence from literature. Such model will describe a range of features of unsaturated soils behaviour including the description of small to large deformations, the evolution of strength with hydro-mechanical state and the soil-water retention within a unique constitutive framework taking into account the coupled nature of these different aspects. Because of its versatility, the proposed model will have the potential of being implemented in finite element codes for the analysis of a variety of engineering problems. In addition, the project will provide valuable insight into laboratory compaction as a suitable technique to produce soil samples representative of earth fills compacted in-situ by using conventional heavy machinery. The proposed research draws upon multi-disciplinary knowledge at the interface between soil mechanics, material science and geophysics while relying on different areas of expertise in continuum mechanics, constitutive modelling, laboratory testing and site investigation. Applications of potential findings from this project span a number of sectors in engineering from the analyses of traditional earth structures to the design of engineered clay barriers for underground nuclear waste repositories and the mitigation of hazards associated to earthquakes or landslides. References Alonso, E. E., Gens, A. and Josa, A. (1990). A constitutive model for partially saturated soils. Géootechnique, 40(3), 405-430. Atkinson, J.H. (2000). Non-linear soil stiffness in routine design. Géotechnique, 50(5):487-508. D. Gallipoli, A. Gens, G. Chen, F. D Onza (2008) - Modelling unsaturated soil behaviour during
normal consolidation and at critical state Computers and Geotechnics, 35(6): 825-834. Jennings, J.E.B. and Burland J.B. (1962). Limitations to the use of effective stress in unsaturated soils - Géotechnique, 12(2):125-144. Wheeler, S.J., Sharma, R.S. and Buisson, M.S.R. (2003). Coupling of hydraulic hysteresis and stress-strain behaviour in unsaturated soils. Géotechnique, 53(1): 41-54. Wheeler, S. J. and Sivakumar, V. (2000). Influence of compaction procedure on the mechanical behaviour of an unsaturated compacted clay. Part 2: shearing and constitutive modelling. Géotechnique, 50(4): 369-376.