Construction of a run-of-river hydroelectric power plant required an extensive cofferdam to be built within a steep river valley aligned with a major regional fault structure. Cofferdam construction was at risk of being undermined through porous rock formations during the summer runoff season when high water levels flow through the river valley.
The contractor used overburden drilling equipment to install casing and systematically drill open holes using down stage drilling techniques as deep as 25 m below surface. Adjacent drill holes were sequentially positioned following primary, secondary and tertiary spacing.
High-density cement-flyash grout was delivered to site from a central batch plant using ready-mix trucks and pumped through drill casing using small concrete pumps to achieve high rates of grouting performance while monitoring grout delivery volumes and pressures.
Drilling and grouting operations proceeded on a split spacing pattern to consolidate faulted and porous ground conditions and to strengthen foundation conditions beneath the cofferdam.
Cofferdam remained intact during summer runoff and provided suitable working conditions for ongoing construction work within the cofferdam area.
“Large Scale Cement Grouting for Forrest Kerr Cofferdam Construction” – by Peter White, P.Eng.