Preloading  is  commonly  used  to  accelerate  primary  consolidation  and  increase  strength  under  embankments founded on highly compressible clay. By installing prefabricated vertical drains, this process can be even further accelerated. Because of significant geotechnical uncertainties, primarily regarding the vertical and horizontal coefficients of consolidation, prediction of settlement development with time is difficult. This paper shows how Bayesian statistics can be used to update a prior prediction of  the  final  settlement  from  the design  stage, with observations of  settlement made during  the preloading. With  this  approach,  the  observations  during  construction  are  used  to  significantly  reduce  the  considerable  epistemic uncertainty present at the design stage. This allows the surcharge design to be adjusted to suit the actual ground conditions, which is done by increasing the surcharge height if the settlement develops too slowly.