Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Two aspects of seismically-induced liquefaction are discussed which are of vital engineering significance: the triggering condition and the consequences of liquefaction. The triggering condition is examined with respect to liquefaction analysis, note being taken of the onset condition which is governed by cyclic strength. Consequences of liquefaction are discussed with respect to post-seismic stability analysis, in which the residual strength plays a major role. Procedures used for liquefaction analysis based on the results of in situ sounding tests are introduced, and the applicability of this method for estimating associated ground settlements is discussed. The evaluation of residual strength requires a better understanding of undrained sand behaviour. Results of extensive laboratory rests on Japanese standard sand are examined and new index parameters are proposed to quantify undrained sand behaviour better. The results of laboratory tests on silty sands are examined in the same way. AH the results are...

Liquefaction and flow failure during earthquakes.

This paper examines the relationship between the microstructure and engineering properties of cement-treated marine clay. The microstructure was investigated using x-ray diffraction, scanning electron microscopy, pH measurement, mercury intrusion porosimetry, and laser diffractometric measurement of the particle size distribution. The engineering properties that were measured include the water content, void ratio, Atterberg limit, permeability, and unconfined compressive strength. The results indicate that the multitude of changes in the properties and behavior of cement-treated marine clay can be explained by interaction of four underlying microstructural mechanisms. These mechanisms are the production of hydrated lime by the hydration reaction which causes flocculation of the illite clay particles, preferential attack of the calcium ions on kaolinite rather than on illite in the pozzolanic reaction, surface deposition and shallow infilling by cementitious products on clay clusters, as well as the presence of water trapped within the clay clusters.

Physicochemical and engineering behavior of cement treated clays

The relationship between suction and swelling properties in a heavily compacted unsaturated clay

Mudstone is the most abundant sedimentary rock and variously acts as sources, seals, and shale gas reservoirs in petroleum systems. Many important physicochemical properties of mudstones are strongly influenced by the mineralogy and size of deposited grains, and by diagenetic changes (precompaction and postcompaction); these are commonly predictable. The diverse composition of mudstones reflects input and hydrodynamic segregation of detrital materials to basins, primary production within basins, and diagenetic processes (both precipitation and dissolution) in the sediment. High-magnification observations both in modern and ancient sediments demonstrate that mudstones are texturally and mineralogically heterogeneous; this variability is not always readily apparent. Although some mud is indeed deposited by suspension settling out of low-energy buoyant plumes, textural analyses reveal that it is commonly dispersed by a combination of waves, gravity-driven processes, and unidirectional currents driven variously by storms and tides. Such dispersal mechanisms mean that muddy successions are typically organized into packages that can be interpreted using sequence stratigraphy. Early bioturbation homogenizes mud, whereas early chemical diagenesis can result in highly cemented zones developing, especially at stratal surfaces. The nature of deeper burial diagenesis, which involves compaction, mineral dissolution, recrystallization, mineral reorientation and lithification, and petroleum generation, is preconditioned by depositional and early diagenetic characteristics of the mud. Although the petrophysical properties of homogeneous mudstones are reasonably well known, the quantitative implications of heterogeneity for petroleum expulsion, retention, petroleum migration, seal capacity, acoustic anisotropy, and identification of shale gas reservoir sweet spots are essentially unexplored. Future work should seek to redress this position.

Mudstone diversity: Origin and implications for source, seal, and reservoir properties in petroleum systems

The compressibility and strength characteristics of reconstituted clays are used as a basic frame of reference for interpreting the corresponding characteristics of natural sedimentary clays. The properties of reconstituted clays are termed ‘intrinsic’ properties since they are inherent to the soil and independent of the natural state. The properties of a natural clay differ from its intrinsic properties due to the influence of soil structure (fabric and bonding). Thus the intrinsic properties provide a frame of reference for assessing the in situ state of a natural clay and the influence of structure on its in situ properties. A new normalizing parameter called the void index is introduced to aid in correlating the compression characteristics of various clays. The sedimentation compression curves for most, but not all, natural clays lie well above the corresponding intrinsic compression curves. A consequence of this is that such clays are more sensitive and brittle than the reconstituted material and the...

On the compressibility and shear strength of natural clays

Current methods of determining soil moisture suction are outlined and their shortcomings identified. A new instrument is then described which will overcome a number of these problems by the use of a miniature pressure transducer, and minimum volume of water between the active face of the transducer and the porous stone. The instrument uses an Entran EPX-500 stainless steel miniature pore water pressure transducer, with a working range up to 3500 kPa, shrouded by a stainless steel sheath fitted with a porous stone. The mechanics of the instrument and its evaluation against known values of suction are outlined. Reliable measurements of suction were achieved up to 1500 kPa.

A new instrument for the measurement of soil moisture suction

Prediction of ground movements and assessment of risk of building damage due to bored tunnelling

Soil matrix suction is assuming an increasing importance in geotechnical engineering. It is now recognised that soil suction is not confined to arid or semi-arid regions of the world. It is frequently encountered in a great many geotechnical problems in temperate climates. For example, all fills and compacted soils have an inherent suction when they are formed; many slopes, both natural and man-made, have in-situ suctions; and all samples that are removed from the ground during a site investigation possess a soil suction in their unconfined state. Measurements of soil matrix suction can be used to: (a) identify disturbance and/or desiccation in samples removed from the ground; (b) estimate the average in-situ effective stress once the effect of disturbance and/or desiccation has been quantified; (c) apply more rigorous controls to the compaction of fill materials where swelling is likely to cause problems; and (d) assess the stability in natural and engineered slopes. This paper describes and compares thr...

Soil matrix suction: some examples of its measurement and application in geotechnical engineering

This paper describes a novel method of controlling suction by means of air circulation in combination with independent suction measurement. The technique has been incorporated in a standard triaxial stress path cell, and could easily be incorporated in other standard apparatus such as the direct shear box and oedometer. The application of the technique is demonstrated by means of an exploratory testing programme in which samples of a reconstituted saturated clayey silt were progressively dried to various predetermined suctions, which were then maintained constant during isotropic compression followed by shearing to failure. In this way the drying and stress history of the soil was carefully controlled in a way that could be made representative of the genesis of natural unsaturated soils. Though exploratory, the experimental investigation described in this paper has provided some interesting and thought-provoking results. Prior to desaturation the clayey silt behaves in accordance with the classical critic...

John Burland

Papers

On the compressibility and shear strength of natural clays

A new instrument for the measurement of soil moisture suction

Prediction of ground movements and assessment of risk of building damage due to bored tunnelling

Soil matrix suction: some examples of its measurement and application in geotechnical engineering

The mechanical behaviour of a reconstituted unsaturated silty clay