Compaction of alluvial sands by deep blasting

Abstract

The Jebba Hydroelectric Development, Nigeria, presently under construction, features a 42 m high zoned rockfill embankment founded on river alluvium up to 70 m in depth. The riverbed alluvium comprises fine- to medium- to coarse-grained, quartzitic sands with traces of gravel. The relative densities of the sands vary appreciably in both the vertical and horizontal directions. During the dam design it was found that it would be necessary to densify the loose to medium dense portion of the river sands to maintain settlements within acceptable limits and to withstand design earthquake forces. The top 25 m of the river alluvium is densified by the vibrocompaction method, and below this level down to depths of 40 m the alluvium is densified by blasting. There is no precedent found in the literature for densification by blasting at such depth. This paper presents details of a test program and the production work. Test results conclusively show increases in static cone penetration resistance in loose sand layers several months after blasting. Keywords: cohesionless soils, compaction, dam foundation, deep blasting, densification (soil), penetration tests, sands.