Cable-tied blocks as an alternative for protecting bridge piers against scour under mobile-bed conditions

When a bridge pier requires augmented protection against scour, riprap is typically added to the base of the pier. Over the years, however, a number of alternatives to riprap have been tested in the laboratory and proposed for use in the field. These include cable-tied blocks, grout-filled bags and mattresses, gabions, tetrapods and related units, pier- or bed-attached vanes, collars or other flow deflectors and sacrificial piles. Nearly all laboratory tests of these countermeasures have, however, been performed under so-called `clear-water' conditions, for which the ambient bed is not mobile. The mobile-bed conditions associated with floods in rivers, and in particular the passage of bedforms, can cause riprap layers to sink and disperse even though the stones are never directly entrained by the flow. With this in mind, a comprehensive set of experiments were performed using a variety of alternatives to riprap under mobile-bed conditions that can model floods in sand-bed rivers. The experiments were conducted at the St. Anthony Falls Laboratory, University of Minnesota, USA. Cable-tied blocks countermeasure are reported herein. Of the various alternatives, a mat of cable-tied blocks underlain by a geotextile of an aerial extent that is less than the mat itself was found to provide excellent protection against scour. Sealing of the junction between the mat and the pier itself, however, is necessary to ensure good performance.