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Full Program » In the geotechnical engineering practice complete water saturation below the phreatic surface are generally accepted. Case studies however can be reproduced only under the assignment of a much larger storage capacity than values expected for saturated sols. Unsaturated conditions due to the presence of even very small volumes of gas entrapped in the voids, for example as a result of natural water level fluctuations and/or biogenic gas generation is responsible for this performance. The impact of entrapped gas on pore pressure propagation and settlement response following mechanic and/or hydraulic loading changes is investigated by means of FE calculations based on Biot’s theory of poroelasticity. The analyses revealed that the impact of unsaturated conditions depends on the particular type of boundary condition. After a change in hydraulic head pore pressure propagation is retarded enhancing the occurrence of hydraulic gradients and seepage forces which eventually may affect soil stability. In case of a change of the mechanic loading the buildup of excess pore pressure is diminished facilitating however instant deformation of the soil compared to the fully saturated case. In order to asses the complex interactions between the degree of saturation and the response of the soil matrix, the gaseous phase and the pore pressure induced by water level variations a dimensionless diagram was developed. Due to the non-dimensionality these findings can be applied to other geotechnical engineering problems like rapid draw-down in submerged slopes and embankments or the estimation of the local stability of bank protection layers in waterways.

Author(s):

Héctor Montenegro
BAW Federal Waterways Engineering and Research Institute
Germany

Oliver Stelzer
BAW Federal Waterways Engineering and Research Institute
Germany

Bernhard Odenwald
BAW Federal Waterways Engineering and Research Institute
Germany