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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