PROJECT: HILLIS HILL LANDSLIDE, Clark County, Washington

Horizontal Drain Installation
Horizontal Drain Installation to Dewater the Landslide

The Hillis Hill Landslide area is located on a west-facing slope above the Lewis River, approximately 1 mile south of Woodland on the Pacific Highway (Clark County Road 1). The slide area is a locally active portion of a much larger ancient landslide that extends approximately 1,200 feet along the highway, and 2,000 feet upslope from the Lewis River. LT assisted Clark County with the stabilization of two active segments of the slide, which were termed the “Hillis Hill” and “Hillis Hill No. 2” Slides. The investigation and repair of the original Hillis Hill Slide occurred between 1997 and 2001. Following phases of investigation, monitoring, and design, the original slide was mitigated with a horizontal drain system installed in October 2001, and has remained stable since then.

The Hillis Hill No. 2 Slide occurred on January 18, 2006 during an intense winter rainstorm. The slide was located below the outboard (southbound) lane of the highway, and was moving downhill toward the floodplain of the Lewis River. The slide headscarp had extended to the edge of pavement, and the shoulder of the road embankment had dropped 5 to 7 feet. As a precaution, the Clark County Department of Public Works closed the southbound lane and restricted the road to one-way traffic through the slide area.

LT provided a phased approach to the landslide investigation. The preliminary phase consisted of a geologic reconnaissance to identify the scale of the problem area and further investigation needs. The second phase included subsurface explorations with installation of piezometers, monitoring of the instrumentation, slope stability analyses, and evaluation of mitigation options and conceptual costs estimates. Following the second phase, LT met with the County to present the slide stabilization options, and to discuss the advantages and disadvantages of each. The selected option consisted of a horizontal drain array near the downhill end of the slide, and a series of finger drains constructed across the uphill end. The final phase consisted of preparation of construction drawings; assistance in obtaining permits, and construction observation services. The drain systems were installed in the fall of 2006, and the roadway has since remained stable.