The outer breakwaters of the new Barcelona harbour are currently under construction. Two types of breakwater are used: the classical trapezoidal rockfill structures and vertical caissons. The foundation soils are relatively uniform: soft normally consolidated silty clays with thin interlayers of silts and fine sands. These deltaic deposits reach a thickness in excess of 50m along the breakwaters. Below, a stronger level of gravels in a clay matrix is found. The paper describes the properties of the deltaic foundation soils as determined in the field and in the laboratory. Particular attention is given to the cyclic instability of foundation soils. A stiffness model for the deltaic deposit was also developed on the basis of the tests performed. An instrumented preload test provided also a field estimation of soil stiffness and consolidation properties which is compared with parameters determined in the laboratory.