A study of the behaviour of and the forces in a bed protecting mattress: The falling apron

In many cases the bottom around a structure that is exposed to current has to be protected by a mattress. The purpose of the mattress is to protect the soil besides the structure from erosion and scour, thus preserving the strength of the foundation of the structure. Examples are the bottom protection around bridge piers, along guide bunds and behind discharge sluices. At the edge of the bottom protection parallel to the current (e.g. along guide bunds) or at the trailing edge (e.g. behind discharge sluice) scourholes will be formed in the unprotected bed. The slopes of these scourholes may be so steep that local soil failure follows. In that case the mattress has to span the local depression caused by the slope failure and maintain its protective function (see Figure 2-1). The mattress should therefore be able to withstand the tension forces that build up in the fabric. In this report a sequence of models is proposed and verified to estimate the maximum force in a mattress spanning a local soil failure. The study was commisioned by CUR in Gouda as a contribution to the design of the guide bunds of the Jamuna Bridge