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Sawicki A.,Institute of Hydro Engineering
Archives of Mechanics | Year: 2014

THE PROBLEM OF PORE WATER PRESSURE changes in the seabed is considered. Two mechanisms of pore pressure changes are distinguished. The first is caused by external excitations, such as earthquakes, when pore pressure is gradually generated, leading to liquefaction. The second mechanism is caused by water waves, and it leads to cyclic changes in pore water pressure and the mean effective stress. Under certain conditions, when the effective stress path tends to exceed the failure condition, the regrouping of effective stresses takes place, as the soil should accommodate to new conditions. Then, the mechanism of resolidification of the seabed is described. It is concluded that after resolidification, the seabed is in a virgin state, as liquefaction erases the previous history of the seabed structure. A critical discussion of selected existing approaches to the problem of pore-pressure changes and the mechanism of liquefaction is presented in detail, in the form of extensive appendices. Some of these appendices deal with the crucial aspects of the mechanics of liquefaction such as, for example, the drained/undrained conditions. Copyright © 2014 by IPPT PAN

Sawicki A.,Institute of Hydro Engineering
Geotechnical Engineering | Year: 2011

Possible links between certain aspects of pre-failure instabilities of sand (instability line) and plastic dilation are studied. The starting point is experimental results obtained from triaxial investigations, which are approximated separately by analytical formulae for initially contractive and dilative sands. The irreversible strains are deduced from the condition that plastic work (dissipation) should be positive. Then, analytical formulae for plastic dilation are derived and presented in graphical form. In the case of initially contractive sand, a possible link between the instability line and maximum of the plastic dilatancy function is demonstrated. This condition is equivalent to the minimum dilation parameter or maximum of Rowe's function. In the case of initially dilative sand, it is shown that the negative work done by a mean stress on expanding (dilating) soil should not be treated as dissipative because of thermodynamical requirements. Consequently, the plastic dilation function is zero for shearing stress exceeding the instability line.

Sawicki A.,Institute of Hydro Engineering | Swidzinski W.,Institute of Hydro Engineering
Archives of Civil Engineering | Year: 2013

This paper presents numerical simulations of the behavior of a sandy layer subjected to a cyclic horizontal acceleration in shaking table tests, with a particular attention focused on the settlements of a dry sand layer, and on the liquefaction of saturated sand. A compaction/liquefaction model (C/L) is applied to these simulations. The infl uence of specifi c parameters of the model on the compaction and liquefaction of a sandy layer is shown and discussed. The results of simulations are compared with selected experimental data.

Sawicki A.,Institute of Hydro Engineering | Kulczykowski M.,Institute of Hydro Engineering
Journal of Earthquake Engineering | Year: 2014

Experimental results showing the frequency-dependent behavior of both dry and saturated sandy layers subjected to a horizontal excitation on a shaking table are presented. The largest settlements of a dry layer correspond to two specific frequencies. In the case of a saturated layer, there is a single peak frequency corresponding to the largest depth of sinking of a measuring plate in liquefied subsoil. The first peak of settlements coincides with the single peak of sinking in liquefied soil. The eigenfrequencies of the layer were estimated. A modification of the compaction law was proposed for low shaking frequencies. Copyright © A. S. Elnashai.

Sawicki A.,Institute of Hydro Engineering | Swidzinski Waldemar W.,Institute of Hydro Engineering
Computers and Geotechnics | Year: 2010

A simple incremental model describing the pre-failure behaviour of granular soils is presented. The model describes both the dry/fully drained and undrained response. It takes into account an initial anisotropy of soil and an initial state defined as either contractive or dilative. A physically sound definition of loading/unloading is assumed, which differs from elasto-plastic approaches. The model is based on extensive empirical data and gives predictions conformable with experimental results. It also describes pre-failure instabilities of granular soils, both dry/fully drained and undrained. The Hill's criterion was used to examine stability. It was shown that this condition can be formulated either in terms of the effective stresses or by the total stresses. In the extreme cases of either dry/fully drained or undrained conditions, these alternative formulations are equivalent. This is not so in the case of partial drainage of pore water and associated volumetric deformations as well as pore pressure changes. The model describes the pre-failure instabilities well, and additionally allows for analytical derivation of the instability line. It was shown that the second order work, appearing in the Hill's condition, is equivalent to the entropy source. © 2010 Elsevier Ltd.

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