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.
Recent stress history effects in clays and associated improvements to the BRICK model
It is known that the stiffness of many soils is higher at small strains than at the large 
strains used in typical triaxial testing. Understanding this initial high stiffness and 
the factors affecting it is key to accurate displacement predictions. 
The recent stress history (RSH) effect describes how the stiffness degradation curve 
at small strains is affected by the immediately preceding stress path. The effect is 
usually described in terms of stress path rotation angle between an approach path 
and the shear probe upon which stiffness degradation is measured. 
The principle aim of the work was to investigate the relationship between the RSH 
effect, approach path length, and time dependent effects in more detail. A combination 
of physical testing and simulations with the BRICK constitutive model were 
used to achieve this. This has shown that in multistage testing the residual RSH 
effect from the first test increased the stiffness exhibited in the second, thus masking 
the natural behaviour. In addition the stress path rotation from consolidation to 
testing caused a stiffness reduction in the first test. This effect, newly termed recent 
consolidation history (RCH), was found to be degraded by the first test and so did 
not impact on the second. When allowance was made for these effects trends seen 
in the literature were supported. 
Due to the complex stress paths used in the physical testing, a number of improvements 
were required to the strain rate dependent (SRD) BRICK model to allow accurate 
simulations to be run. These included implementation of a bisecting iteration 
within the strain calculations, and a coordinate decent routine to allow creep and 
creep rupture to be modelled. The model was then used to simulate the physical 
tests to allow more insight into the interplay between the effects identified in the 
physical testing, ultimately supporting the conclusions drawn.
Effect of pH on the measurement of biogenic silica.
For investigations of the biogenic silica content of plankton, sediment trap, and sediment samples, alkaline extraction is the most common procedure. Different extraction techniques, with respect to the pH of the leaching agent and the method applied to correct for the release of SiO2 by non-biogenic silica phases as clays or feldspars, are applied. By sequential extraction of marine sediment samples and clay standards with solutions of pH 8 to 14, we investigated the sensitivity of these leaching techniques to the pH of the leaching solution. In addition to the dissolved silica concentration, the temporal increase of the dissolved aluminum concentration of the leaching agent was measured. For sediments of low biogenic silica content, the results are considerably affected by the pH of the leaching agent and relative deviations of more than 300% were observed. Correction by the calculated SiO2/Al2O3 ratios of non-biogenic silica phases had no significant effect on the observed pH dependence. Measurements of samples which exceed ∼10% biogenic silica are only negligibly affected by the pH of the leaching solution. Based on the pH dependence and investigations of a pure Miocene diatom sample, sequential alkaline extraction with a pH of 12.5 is recommended for the determination of biogenic silica.