![]() ![]() ![]() 2 (digitized from 2) shows the behaviour of Fuji river sand subjected to large preshearing with subsequent cyclic undrained loading. Accordingly, loading histories reaching stress ratios smaller than PTL were termed small preshearing whereas those going beyond the PTL were called large preshearing. In 2 as well as widely in the geotechnical literature the stress ratio at which the soil behaviour changes from contraction to dilation is denoted as the phase transformation line (PTL). Under drained or undrained conditions dilation results in a decrease of volumetric strain or excess pore water pressure (increase of mean effective stress), respectively. Further increase of the stress ratio led, in contrast, to dilation and to much larger shear strains. Under drained conditions contraction results in an increase of volumetric strain, while under undrained shearing it renders an increase of excess pore water pressure. By increasing the stress ratio from the isotropic stress axis during preshearing, they observed a tendency of the sample to contract while relatively small shear strains developed. ![]() ![]() In the event of preshearing the soil experienced a given deviatoric stress before the subsequent loading. In case of precompression the soil experienced an isotropic compression stress greater than that at the beginning of the subsequent shearing. They interpreted the preloading as either precompression or preshearing. Using undrained triaxial tests, Ishihara and Okada 2 studied the influence of the loading history (preloading) on the liquefaction resistance of Fuji river sand. However, this is not true when the directions of preloading and subsequent loading differ. In general, undrained triaxial tests on samples consolidated under isotropic direction show that dense samples require more cycles of the applied load to reach liquefaction than loose ones. With regard to liquefaction, the influence of density, consolidation stress as well as the amplitude of cyclic loading have been widely studied over the last decades. Adequate predictions of the liquefaction and the stress and strain accumulation upon cyclic loading, including the influence of density and deposition method on the soil behaviour are therefore essential. Of note, an earthquake (undrained shearing) can present the preshearing of the next seismic loading. But even if the liquefaction does not take place, excessive accumulation of settlements due to cyclic and/or quasi-static loading during and after the groundwater flooding can endanger the re-cultivation processes. To prevent catastrophic events, the design of dump slopes requires information about the soil resistance to liquefaction during possible earthquakes. This holds especially for deep mines, like Hambach (Germany), where the loosely dumped granular layers may reach a depth of 400 m and the re-cultivation of the area after the lignite extraction is intended, see Fig. Slope stability and long-term settlements assessments belong to the most challenging aspects of the design of dump slopes for opencast lignite mines. This deficiency of the constitutive models can lead to unsafe designs due to the overestimation of the cyclic resistance to liquefaction and to the underestimation of long term settlements. Simulations of the experiments with four advanced constitutive models reveal that neither the long-lasting effect of preshearing nor the preparation method can adequately be captured by all of the models. It is shown that the preshearing as well as the preparation method have a major influence on the strain accumulation upon cyclic loading. To explore the influence of preshearing on the subsequent soil behaviour, a set of triaxial tests with a combination of undrained preshearing and drained stress cycles using two sample preparation methods is presented. This resistance depends not only on the initial stress, the initial density, and the amplitude of the cyclic loading, but also on the preshearing, that is, the deviatoric stress path applied to the soil before the cyclic loading. The design of deep dump slopes for opencast mines usually requires information about the soil resistance to liquefaction during earthquakes. ![]()
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