Field measurements were taken during construction of a double track tunnel for the northern extension of Santiago Metro Line 2 in Chile, between 2004 and 2005. The tunnel was driven through a thick deposit of a fine fluvial-lacustrine sediment, consisted mainly by silts and clays with low deformability, with consistency moderate to high and with presence of isolated thin lenses of silty fine sand, gravel and volcanic ash. The monitoring data made possible Bayesian updating of the coefficient of earth pressure at rest (K0) and of the distribution of soil deformability with depth, represented by a hyperbolic model that requires knowing the initial tangent modulus (Ei). The back-analysed values for K0 and modulus distribution with depth were different from what was used for design of Line 2 and were incorporated into the design of the new Line 3 under construction for Santiago Metro. © 2014 Korean Geotechnical Society.