IMPROVED DRAINAGE BY DRILLED FILTER DRAINS, DAM SAFETY UPGRADE, CASE STUDY-Paper

Paper

Pdf Summary

Storfinnforsen Embankment Dam in Sweden is 23 m high and was completed in 1954. Its central impervious barrier includes untreated timber sheet piles, which were believed to have deteriorated over time. Monitoring detected a gradual rise in groundwater pressure in the downstream glacial-till support fill. Investigations found relatively high pore pressures near a coarse transition layer at the downstream toe, whose gradation could allow internal erosion to begin. Some turbidity in seepage water added to the concern.<br /><br />A downstream rockfill berm and a new drainage and leakage-measurement system were installed in 2013. These measures were intended to limit the consequences of a potential erosion failure, but would not prevent erosion from starting. The authors considered retaining the status quo, constructing a new impervious seal, or improving drainage. A new seal offered long-term safety benefits but was costly and presented construction challenges. Vertical sand drains were selected as a comparatively quick, lower-cost alternative that could reduce pore pressure while leaving open the option of a future seal.<br /><br />A trial section installed in 2015 used vertical drains with strainer pipes surrounded by filter sand. One year later, measured pore-water pressure had fallen by about 2.5–3.5 m with 2 m drain spacing and 3–4 m with 1 m spacing. Following this positive result, installation along the full 330 m dam was under way, with 1 m spacing and drain lengths of 15–25 m.<br /><br />The case demonstrates the importance of responding to small changes in monitoring data. The drains, combined with the berm and continued surveillance, were intended to reduce internal-erosion risk and provide long-term dam safety.

Keywords

Storfinnforsen

embankment dam

Sweden

internal erosion

pore pressure

groundwater monitoring

vertical sand drains

drainage berm

timber sheet piles

dam safety