Here is a UMAT using an  elastic viscoplastic model for soil

Modifed Cam-clay model coupled with viscosity.

Detailed information can be found in the link

http://www.eng.uwo.ca/research/grc/software.htm

ABAQUS - UMAT (Free Download for Research Purposes)

A user-defined subroutine in ABAQUS for viscous soils.

Introduction: