Orr Springs Road (CR 223) Slide at MP 39.20 – Permanent Repair

Mendocino County, California

The subject road failure has resulted in the complete loss of the outboard (west-bound) lane and over half of the inboard (east-bound) lane for a distance of approximately 55 feet and maximum depth of 16 feet.

CAInc prepared a Geotechnical Memo which described the existing site and soil conditions, and analyzed soil samples obtained from test borings. The road failure occurred primarily within residual soil and/or fill material, as well as the uppermost portion of the soft decomposed sandstone. The primary causes of slope failure were due to the inherent weakness of the fill and intensely weathered sandstone, the high degree of saturation from seasonal storm water infiltration during this past very wet winter, and possible internal seepage pressures – including seepage from within the underlying weathered rock.

Recommendations include a soldier pile tieback wall. This option will achieve secure support within the sedimentary rock and provide lateral resistance to active pressures. This option allows for Orr Springs Road (a Major Collector) to remain open during construction, as well as limiting the environmental impact downslope of the failure.

Old Cazadero Road @ PM 17.43 – Geotechnical Investigation

Sonoma County, California

Severe storms in February 2017 resulted in the failure of a 145’ section of the outboard lane. The original 12 ft road width is reduced to about 4’ at the narrowest point. CAInc prepared a Preliminary Site Assessment for Old Cazadero Road. Upon reviewing the site and geologic setting, a preliminary cause of failure was determined to be saturation of weak, unconsolidated fill/colluvium from seasonal storm water infiltration on an over-steepened slope. Runoff from the upslope watershed appeared to collect in the curved section of road at the failure and may have concentrated at this point. The failure appeared to have been a debris-flow type of failure involving mostly the outer fill section and depositing the slide debris about 65 feet below the road (slope distance). CAInc provided preliminary repair alternatives, concluding that a tie-back soldier pile wall to be the best feasible option for the site. Based on the Preliminary Site Assessment, CAInc will prepare a Draft Work Plan detailing the site investigation and a Final Work Plan incorporating any comments by the County and FEMA.

Oro-Quincy Highway 2017 Storm Damage Sites

Butte County, California

Site 1 @ PM 12.1 This road failure has involved both lanes of the highway and closed the road at this location. The failure has affected a 200+ft long section that has dropped about 15 ft vertically and displaced about 10 ft horizontally. CAInc performed a field investigation of three exploratory borings at locations near the slide and performed laboratory testing on the samples. Based on data obtained from the study of the subsurface conditions, CAInc determined material properties and limit-equilibrium analysis and concluded that the failure cause appears to be saturation of the fill, weak “dg” from springs and seasonal storm water infiltration, and discharge from storm drains. Recommendations include installation of subdrainage; excavation of slide debris; key reconstruction section into the “dg” material; excavation of slide material; providing subdrainage relief; replacement of excavated materials; supplement the excavated material with lightweight fill; construct the exterior finish slope at 2:1 (H:V) trimmed to match the adjacent embankment slopes and to buttress the private driveway on the west side; construct and maintain the finished roadway surface such that surface water run-off is diverted away from the embankment slope to prevent erosion; and construct a trenched underdrain along the inner road area to intercept shallow seepage.

Site 2 @ PM 12.6 A section of an existing MSE wall, about 200 feet in length, has settled up to 2-3 feet vertically and about 6” laterally along the highway. CAInc performed a field investigation of six exploratory borings and performed laboratory testing and corrosion tests on the samples. CAInc determined the most likely causes of distress are considered to be settlement of the native soil and “weak” decomposed rock at the base of the existing wall, and possible settlement of the “select granular backfill” within the wall. Recommendations include removal of the MSE wall, backfill, and drain strips within the limits of the roadway distress; drainage of any springs; replacement of excavated soil with new granular fill per Caltrans Standard Specifications; replacement of geocomposite sheet drains; construction of new MSE wall; and construction of new pavement sections.