Wet winter pore pressures in railway embankments

Abstract

This paper demonstrates the influence of extreme wet winter weather on pore water pressures within clay fill railway embankments, using field-monitoring data and numerical modelling. Piezometer readings taken across the London Underground Ltd network following the wet winter of 2000-2001 were examined, and showed occurrences of hydrostatic pore water pressure within embankments, but also many readings below this. A correlation was found between the maximum pore water pressures and the permeability of the embankment foundation soil, with highpermeability foundation soils (of chalk or river terrace deposits) providing underdrainage and maintaining low pore water pressures within the overlying clay embankment fill. Numerical modelling of transient water flow in response to a climate boundary condition supports this conclusion, and has been used to demonstrate the influence of clay fill and underlying foundation permeability on transient pore water pressures during extreme (c. 1 in 100 years) and intermediate (c. 1 in 10 years) wet winter rainfall. For clay-founded embankments, extreme wet winter conditions increased pore water pressures significantly compared with an intermediate winter, whereas for embankments underlain by a permeable stratum pore water pressures were less sensitive to the extreme winter rainfall. Notation a inflection point on SWRC; slightly greater than airentry value k sat saturated hydraulic conductivity m, n constants Ł r residual water content Ł s saturated water content Geotechnical Engineering Volume 166 Issue GE5

Failures in transport infrastructure embankments

“…Anderson et al, 1982;Anderson & Kneale, 1980a;Garrett & Wale, 1985;Birch & Dewar, 2002;Ridley et al, 2004;Briggs et al, 2013a).…”

The influence of tree root water uptake on the long term hydrology of a clay fill railway embankment

“…Vadose/w has previously been used to investigate the sensitivity of pore water pressures in railway earthwork slopes to seasonal climate and soil permeability (Loveridge et al, 2010;Briggs et al, 2013a;Briggs et al, 2014).…”

“…Therefore the SWRC for clay fill was estimated from that for London Clay in drying and compared with in situ measurements of soil suction and volumetric water content ( Figure 5). The clay fill was assigned a lower air entry value and a curve of shallower gradient than the in situ London Clay, reflecting its greater specific volume and wider range of pore sizes, following Loveridge et al (2010) and Briggs et al (2013a). A SWRC for fouled ballast (given by Duong et al, 2013) was used to model the ash / ballast layer at the site.…”

“…Leaving trees on the lower third of an embankment slope makes it more likely that residual suctions will remain within the soil at this location during periods of wet weather. In extremely wet winters, such as 2000/2001 and 2013/2014, the likelihood of soil saturation, raised pore water pressures and hence embankment distress will inevitably increase (Briggs et al, 2013a). Nonetheless, in what is essentially a probabilistic design (i.e.…”

Editorial

“…A third paper, by Briggs et al (2013), demonstrates the influence of extreme wet winter weather on pore water pressures within clay fill railway embankments in temperate climates. The paper uses both field measurements and numerical modelling in the investigation, with access to considerable piezometer data from London Underground.…”