Pore pressures in embankments

Fills and compacted soils (such as those used in embankments) have an inherent suction when they are first compacted. The magnitude of this suction can be a few hundred kilopascals even under normal compaction conditions.

Infiltration into fills or compacted ground will increase the water content and reduce the suction. Frequently therefore the assumption of no suction is used for the assessment of slopes. Attempts to retain suction and hence stability by the introduction of vegetation onto slopes has had mixed success. Aside from providing natural drainage, vegetation introduces root reinforcement, it intercepts a significant amount of the infiltration that can lead to the loss of soil suction and it is often aesthetically pleasing.

However, vegetation also creates seasonal drying and wetting, which causes subsequent changes to the shallow in situ pore water pressures. Recent field observations suggest that old railway embankments constructed of dumped London clay fill deform as the shallow pore pressures vary in a seasonal manner. Finite element analyses indicate that seasonal cyclic stress changes cause a gradual outward movement, which induces strain softening and this can eventually lead to collapse through a mechanism of progressive failure (Kovacevic et al. 2001). These analyses suggest that the horizontal mid-slope movements and the number of cycles required to cause failure are linked to the amplitude of the pore pressure variation. Retaining a small suction at the boundary at the end of winter can significantly prolong the time to failure. Therefore if analyses such as these are to be used, in pro-active way, to assess the serviceability of embankments it is essential that they be fed with good data on the seasonal variation of the pore pressures, obtained from reliable field measurements.

Geotechnical Observations has extensive experience of measuring seasonal variations of pore water pressures in embankments using our flushable piezometers, modular piezometer system and suction probes.  The figure below shows data gathered from an old railway embankment in the south east of England. This was published in The Skempton Conference.

References

Kovacevic, K., Potts D.M. & Vaughan P.R. (2001) Progressive failure in clay embankments due to seasonal climate changes. Proc. 15 th Int. Conf. Soil Mech. Geotech. Engng , Istanbul , 3 , 2117-2130.

Ridley A.M., Vaughan P.R., McGinnity B. and Brady K. Pore pressure measurements in infrastructure embankments. In “Advances in Geotechnical Engineering: The Skempton Conference, 2004” , Thomas Telford, London . Vol. 2, pp 922-932.

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