Mud volcanoes are structures that are formed through 'cold volcanism' and indicate soil liquefaction. Their evolution depends on the structure, state and excitation of fine-grained feeding sediments. The disturbance of the framework of a loose, fine-grained, saturated sediment causes shear deformation leading to a pore fluid pressure increase. Effective stresses are thereby reduced and can vanish; the soil is then totally liquefied. Small amounts of enclosed gas bubbles render the soil compressible and enhance local shearing, pore pressure build-up and structural damage. Liquefied, overpressurized sediments form mud chambers, whose excess pressure is released through cracks and other inherent weak channels caused, for example, by density variations in the overlying strata.Experiments were conducted on small-scale soil columns and on model slopes, using quartz powder as model material. Soil column tests were aimed at determining the influence of gas bubbles with regard to the liquefaction process, whereas model slope tests were targeted at the collapsability of gentle slopes, subject to atmospheric pressure changes. Different initial porosities were achieved through ion content variation. Liquefaction of fine-grained sediments and subsequent volcano evolution could be produced.
A much-discussed topic in seismology deals with how and under which loading conditions soil shows nonlinear behavior and how this can be verified from seismograms. Seismologists have been seriously searching for signatures of nonlinear soil response to earthquakes for about two decades. A mechanism explaining the dispersion in the P-wave spectra due to the interaction between compressional (P) and shear (S) waves is presented. Shear waves in granular materials induce longitudinal dilatancy waves (so-called D waves) with approximately double frequency. This can be explained with dilatancy and contractancy, which is characteristic of granulates under shear deformations. The predicted dispersion is observed in laboratory experiments and verified by comparing accelerograms from hard-rock and soil stations from the Vrancea region, Romania. The arrival-time difference between D waves and S waves may theoretically be indicative of the thickness of nonsaturated granular layers. These results, modeled with nonlinear constitutive relations of the rate type, show a specific type of nonlinearity in granular sediments also for earthquakes of moderate magnitudes.
Spontaneous flows of very loose sand deposits pose a serious environmental problem in abandoned lignite mines. Large areas in eastern Germany are not safe for public access. Slight disturbances have triggered large settlement flows, causing loss of lives and structures. To better understand the mechanisms of fast settlement flows, undisturbed samples from depths between 10 and 15 m below ground level were retrieved using the soil-freezing method. Conventional laboratory and computer tomographic soil analyses were performed on these samples. The results reveal a very low degree of saturation and large gas-filled macropores responsible for considerable density fluctuations within the phreatic zone. These spatial porosity variations are thought to strongly contribute to the collapsibility of loose sands.Key words: spontaneous liquefaction, undisturbed sampling, soil freezing, computer tomography, density fluctuations, gas inclusions.
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