Undocumented slope failures since the initial construction of the road have prompted previous road realignments and placement of non-engineered road fills that most likely date to early in the last century. Several failures have occurred along this important local roadway which provides the link between the City of Placerville and the community of Swansboro. In 1997 the south bank of the roadway, failed slide requiring slope stabilization, Again in 2006 another failure occurred on the north side of the canyon.

In 1995 a slope repair was initiated to stabilize a rock slope failure which jeopardized the Mosquito Road Bridge.  Youngdahl conducted a detailed geologic evaluation and made remediation recommendations for the stabilization of the steep granitic hillside. Stabilization of the slope was achieved using a combination of rock bolts, rock netting and fencing.

Again in 2006 a slope failure on the north side of the river occurred along Mosquito Road occurred. Heavy rains caused fill soils and Mesozoic granites, with well-developed adverse joint sets, to become saturated and subsequently release down-slope as a semi-liquefied flowing mass, taking with it unstable, adversely jointed bedrock blocks. Youngdahl conducted a geologic evaluation and made remediation recommendations for a steep hillside granitic terrain landslide affecting the rural mountain road and adjacent community.

Youngdahl was involved in developing recommendations for roadway repair which consisted of a fast-track installation of a FEMA-funded soldier pile wall featuring 26 piles along a narrow switch-back with rock anchor tie-backs to resist block-topple failure potentially overturning the wall. In addition to geologic consultation, Youngdahl’s involvement with the design of the soldier piles included numerous global/slope stability evaluations to determine the appropriate lateral pressures the solider piles would have to resists.