The Howard A. Hanson Dam (HHD) is a critical flood protection and water storage structure on the Green River in the Eagle Gorge in western Washington. Construction of the earth embankment structure was completed in 1962 and evidence of significant seepage was observed on the downstream slopes shortly after completion. Several different mitigation measures were attempted to address the seepage, such as filter blankets, one 640-foot long drainage tunnel, vertical and horizontal drains, and one 475-foot long grout curtain. Malcolm was contracted to install vertical and horizontal drains and additional instrumentation to permanently reduce the high flood risk.
Twelve dewatering wells, along the alignment of a future drainage tunnel were constructed in fully cased boreholes, utilizing 8-inch stainless steel well screen and silica filter sand. Twenty-six vertical drains were drilled directly into the existing concrete drainage tunnel (through the crown and floor), allowing seepage above and below the tunnel to drain.