A design approach to residual rockfall hazard of drapery systems: example from Clifton Hill, Sumner

Unsecured draperies are routinely used as a rockfall mitigation system. These systems do not completely eliminate the rockfall hazard and are typically classified as passive structures aiming primarily at controlling rockfall motions by constraining them towards the slope. Rockfall hazard is seldom completely eliminated, therefore requiring another line of defence such as a catch ditch or a berm. This paper presents a new approach that was implemented on Sumner Road at Clifton Hill in Christchurch to quantify the efficiency of the drapery to control the motions of potential rock falls, therefore optimising the design requirements for the catch ditch at the base of the slope. A discrete element model (DEM) was previously developed to specifically simulate the interaction between falling blocks and draperies. Results of 3D rockfall trajectory simulations were used to characterise the influence of block size, shape and fall height on the efficiency of a drapery to reduce the impact kinetic energy and constraint the bottom impact location. Simplified relationships were derived and used to estimate the rebounds at the base of the slope and subsequently design the catch ditch.