Modelling of viscous effects in natural clays
A new approach to allow the modelling of the viscous behaviour of natural clay soils, including creep, stress relaxation and the effects of applied strain rate on soil stiffness, has been developed based on the BRICK constitutive model (published by Simpson in 1992). The new model, "strain rate-dependent" (SRD) BRICK, was used in a series of simulations to demonstrate its capabilities in predicting realistic behaviour during one-dimensional compression and undrained triaxial tests in which applied strain rates were varied. Triaxial stress path tests conducted by Gasparre et al. in 2007 to assess the influence of creep, resulting from recent stress history, on soil stiffness were also simulated. The observed trends concerning the stiffness at small strains were successfully modelled.
INFLUENCES OF DIATOM MICROFOSSILS ON ENGINEERING PROPERTIES OF SOILS
This paper examines how geotechnical characteristics of soils can be influenced by the presence of diatom microfossils, using an artificially prepared mixture of soil and diatomite. Test results indicate that the presence of diatom microfossils substantially alters the index properties as well as other fundamental engineering behaviours of a soil, owing to the predominantly hollow structures of microfossil skeletons having rough and interlocking surfaces. For example, with the increase in diatomite content, compressibility and the internal friction angle of a soil increase. This study has also made an attempt to quantify the diatom microfossils present in natural soils.
Correlation of the Upper Cretaceous White Chalk (Turonian to Campanian) in the Anglo-Paris Basin
Using key lithological, biostratigraphic and geophysical borehole-log markers a correlation of the Upper Cretaceous Chalk stratigraphies across the whole Anglo-Paris Basin has been achieved. A common lithological marker bed stratigraphy applies and key fossil markers (inoceramids, Micraster and Echinocorys ) are consistently related to the same lithological markers throughout the basin. The major lithological changes consistently occur at the same levels but are not necessarily equally strongly developed. The magnificent dip section along the French Channel coast between the Somme and the Seine, and the inland sections at Precy-sur-Oise and Verlin (Loiret) illustrate the influence of long-lived tectonic lines on Chalk sedimentation history recognised by: (i) rapid thickness variations; (ii) channel flooring hardgrounds and associated anomalous bedding dips: (iii) stratigraphically anomalous infills of channels which may be metres to kilometres across; (iv) redeposited chalks containing abundant Bathichnus paramoudrae ; (v) tectonically induced slumps and slides with flint and chalk shatter beds (palaeoseismic horizons). These features are found along may axes including the Anticlinal du Bray at Precy and Dieppe, the Anticlinal de Margny-les-Compiegne at Nourard le-Franc and Le Treport (the Bresle Valley Axis) and the Fecamp and Verlin fault systems. The ‘events’ recognised along these axes link with similar events recognised in Sussex (e.g. Kingston Anticline), on Portsdown, at Culver Cliff, Isle of Wight (Sandown Pericline) and on the Weymouth Anticline (White Nothe). Such events are common in the high Turonian, low Coniacian, basal Santonian, and the low and mid Campanian. The recurrence of key marker beds dominated by a particular trace fossil association or sequence, Bathichnus-Thalassinoides suevicus-Zoophycos associated with major changes in lithology, micro and macro-fauna indicates more widespread ‘oceanographic’ cycles of transgression and regression related to Atlantic opening.