The Sensitivity Framework: Behaviour Of Richmond River Estuarine Clays
The stability and long-term settlement behaviour of NSW estuarine clays under load has become increasingly significant as a result of the large-scale infrastructure development currently occurring in coastal NSW.
The Structured Clay Framework (SCF) developed by a number of authors over the past 20 yrs (Burland et al., 1996; Chandler, 2000; Cotecchia and Chandler, 2000) and based on the work of Burland (1990), provides a general framework for understanding the behaviour of natural structured clays. This is analogous to the framework for remoulded soils provided by Critical State Soil Mechanics (CSSM).
The basis of the SCF is a normalised void ratio vs. effective stress relationship generated from oedometer tests on remoulded clay. The parameters used in the normalisation procedure are the intrinsic properties of e*o, e*100 and e*1000 the void ratios of the remoulded clay at the liquid limit, (eo), σ’vo=100kPa, (σ’100, e*100), and at σ’vo=1000kPa (σ’1000, e*1000). The “intrinsic” properties represent the values of clay with no microstructure i.e. its baseline properties.
Unlike standard empirical correlations between geotechnical properties and Atterberg limits there is a rigorous analytical basis (i.e. CSSM) for these correlations (Burland, 1990). The behaviour of high quality undisturbed tests can then be compared and classified according to this intrinsic baseline. The SCF provides a quantitative measure of the structure component of the clays consolidation and shear behaviour above that predicted based purely on void ratio stress relationships (Burland, 1990).
The work presented in this paper describes the implementation of the SCF on Holocene estuarine clays from the Richmond River in Northern NSW. These geologically normally consolidated clays form a significant component of the foundations of the proposed Ballina Bypass and it has been noted, anecdotally, that a number of these clay deposits suffer from large (3 m under a 7 m embankment) settlements and have relatively high sensitivities (4-10, shear vane). Both these factors suggest micro-structure may play a significant role in the overall behaviour.
The initial results indicate that the SCF method provides a useful tool for relating mechanical behaviour, in terms of oedometer and shear vane results, to periods in the geological evolution of the estuarine deposits. Significantly, zones of high sensitivity clays, the deposition of which relates to a period of rapid flooding within the estuary, have been identified. They have much higher intrinsic values that can be accounted for under normal sedimentation conditions, suggesting that they are highly structured. This fact is not apparent when using the standard Atterberg correlations.