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The stress-strain behaviour of unsaturated soils containing discrete bubbles of gas has been studied in a programme of experimental and theoretical research. The research is of particular relevance to the offshore environment, where bubbles of methane, nitrogen and carbon dioxide are formed within the seabed by the decomposition of organic matter. The presence of these gas bubbles can have a major influence on the engineering properties of the soil. As gas bubbles in marine sediments are typically much larger than the normal void spaces, the bubbles cannot be considered as simply changing the compressibility of the pore fluid. Chapters 2 to 4 describe a series of undrained triaxial tests on reconstituted samples of clayey silt from an estuarine site. Bubbles of methane were formed within the soil by using a chemical technique. The test results provide evidence of the effect of gas bubbles on the undrained shear strength, together with useful information on the stress-strain behaviour prior to failure, the generation of pore pressures during shearing, the elastic moduli and the isotropic consolidation behaviour. A theoretical model for soils containing large gas bubbles is developed in Chapter 5. The model consists of spherical bubble cavities surrounded by a matrix of saturated soil. In the following three chapters various aspects of the soil behaviour are examined by analysing the theoretical model. Chapter 6 covers the elastic moduli, Chapter 7 the consolidation behaviour and Chapter 8 the undrained shear strength. In Chapter 9 the predictions of the theoretical model are compared with the experimental results. The agreement between theory and experiment is excellent, suggesting that the theoretical approach developed and analysed in Chapters 5 to 8 is a reasonable model for the behaviour of soils containing large gas bubbles.

An improved relationship for the variation of degree of saturation in an unsaturated soil is presented, incorporating the influence of changes of void ratio. When combined with an elasto-plastic stress–strain model, this is able to represent irreversible changes of degree of saturation and changes of degree of saturation caused by shearing. Experimental data from tests on compacted Speswhite kaolin are used to demonstrate the success of the proposed new expression for degree of saturation. The experimental data involve a wide variety of stress paths, including wetting, isotropic loading and unloading under constant suction, constant suction shearing, and constant water content shearing. Improved representation of the variation of degree of saturation has important consequences for numerical modelling of coupled flow-deformation problems, where the expression used for the degree of saturation can influence significantly the suction generated within the soil and hence the predicted stress–strain behaviour.

The ATLAS Trigger requires high efficiency and selectivity in order to keep the full physics potential of the experiment and to reject uninteresting processes from the 40 MHz event production rate of the LHC. These goals are achieved with a trigger composed of three sequential levels of increasing accuracy that have to reduce the output event rate down to ~100 Hz. This work focuses on muon reconstruction and identification for the third level (Event Filter), for which specific algorithms from the off-line environment have been adapted to work in the trigger framework. Two different strategies for accessing data (wrapped and seeded modes) are described and their reconstruction potential is then shown in terms of efficiencies, resolutions and fake muon rejection power.

The ATLAS combined test beam in the second half of 2004 saw the first deployment of the ATLAS High-Level Triggers (HLT). The next steps are deployment on the pre-series farms in the experimental area during 2005, commissioning and cosmics tests in 2006 and collisions in 2007. This paper reviews the experience gained in the test beam, describes the current status and discusses the further enhancements to be made. We address issues related to the dataflow, selection algorithms, testing, software distribution, installation and improvements.

The Large Hadron Collider at CERN offers unprecedented challenges to the design and construction of detectors and trigger/data acquisition systems. For ATLAS, a three level trigger system has been developed to extract interesting physics signatures with a 10^6 rate reduction. To accomplish this, components of physics analysis traditionally deferred to offline physics analysis must be embedded within the online trigger system. For the Muon trigger, the specific off-line algorithms MOORE (Muon Object Oriented REconstruction) and MuId (Muon Identification) have been adopted so far for the on-line use, imposing an operation in a Bayesian-like environment where only specific hypotheses must be validated. After a short review of the ATLAS trigger, the paper shows the general strategy of the Muon Identification and Selection accessing the full event data, or being seeded from results derived at a previous stage of the trigger chain.