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Europe 2009 Geotechnical Modelling Workshop

GEO-SLOPE International is pleased to announce that our next Geotechnical Modelling workshop will be held 1-3 June 2009 in Haarlem, The Netherlands. Our annual public workshops are well attended events where GEO-SLOPE clients from around the world gather to learn, enhance and fine-tune their numerical modelling skills under the guidance of GEO-SLOPE personnel.

See the Europe 2009 training page for a registration form and additional details on the workshop.

Volumetric Expansion due to Excavation

SIGMA/W is commonly used to simulate the deformation and pore water pressure response of foundation soils beneath the load of a stage-constructed embankment. A less common use of SIGMA/W is the simulation these phenomenon during unloading by means of an excavation. Rapid unloading can lead to the development of negative pore water pressures around the face of the excavation.

Embankment construction problems usually involve the generation of excess pore water pressures in the foundation soils if the stratigraphic units have a low hydraulic conductivity. The rapid placement of fill creates a tendency for volumetric compression; that is to say the soil void space wants to compress, but the pore water must be expulsed from the pore space in order for this to occur, leading to excess pore water pressures. The foundation soils consolidate over time as the excess pore water pressures dissipate at a rate that is dependent on the hydraulic conductivity of the soils, and the proximity to the drainage boundaries.

Keeping this in mind, consider what happens when a low hydraulic conductivity (e.g. clay) soil is unloaded by means of an excavation. The rapid removal of the soils in the excavation creates a tendency for volumetric expansion. The soil (void space) wants to expand because of the stress relief; however, this requires that the soil imbibe, as opposed to expulse, water. In other words, water must flow from the surrounding area to the unsaturated zone around the excavation. In extreme cases—such as rapid unloading (excavation) of very low hydraulic conductivity units—it may even be possible to depress the phreatic surface and require decades for the pore water pressures to recover to positive values.

Using the View Movie command in Contour, we can show the pore water pressure response during a SIGMA/W simulation of an excavation in a very low hydraulic conductivity soil. The excavation occurs rapidly and is completed in seven lifts. The unloading causes the phreatic surface to be depressed to an elevation of about 1 metre below the base of the excavation. The subsequent recovery of the negative pore water pressures occurs over a thirty year period.

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