A Combination of Jet Grouting and MPSP Grouting in Iksar River Cascade Hydropower Plant in Bulgaria-Paper

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The Iskar River hydropower cascade in Bulgaria comprises nine planned plants, five of which are operating. Each plant is founded on fractured diorite or diabase bedrock covered by coarse alluvial deposits. To limit seepage beneath the structures and earth dams, the project combined jet grouting in the soil with MPSP (multiple packer sleeve pipe) cement grouting in the underlying rock.<br /><br />Two lines of double-fluid jet-grout columns formed the curtain through the alluvium. MPSP pipes, fitted with grout-injected obturator bags, divided boreholes into sealed sections for controlled treatment of the fractured rock. The method was designed to address vulnerable transition zones between soil and rock, excavation-disturbed bedrock, and contacts between the rock and concrete foundations. Three-dimensional seepage analyses informed the curtain design, and post-treatment Lefranc and Lugeon tests checked permeability.<br /><br />Temporary embankments protected construction sites from river flows. Jet-grout columns reinforced with steel pipes waterproofed and stabilized these embankments, enabling work to continue through most seasonal river-flow changes. This reduced construction schedules by an estimated four to six months per plant, although one flood overtopped an embankment and caused a temporary interruption.<br /><br />Final permeability tests confirmed that the treatments met their design targets. The authors conclude that combining jet grouting and MPSP rock grouting produced an effective, uniform watertight curtain, particularly at critical interfaces, while the reinforced temporary works improved construction efficiency and flood protection.

Keywords

Iskar River

hydropower cascade

Bulgaria

jet grouting

MPSP grouting

seepage control

alluvial deposits

fractured rock

permeability tests

watertight curtain