Public Works Department
City of Kelso
906 Croy Street
Kelso, Washington 98626

This letter transmits four copies of our "Report, Geotechnical Engineering Services, Slope Stability Evaluation, Vista Neighborhood, Kelso, Washington." Our scope of services for this phase was presented in our proposal authorized on March 25, 1999. On June 11, 1999, we transmitted draft copies of this report for your review and comment. This final report incorporates some corrections and your comments and revisions. We anticipate that additional evaluations including drilling of borings and laboratory testing will be necessary to further define areas of slope instability and possible mitigation recommendations.

We appreciate the opportunity to provide continued service to the City of Kelso. Please do not hesitate to contact us if you have any questions regarding our findings and conclusions.

Yours very truly,

GeoEngineers, Inc.



Kenneth G. Buss, P.E.
Principal

 

cc: Washington Cities Insurance Authority (two copies) Hagens Berman (two copies)
c/o Law, Lyman, Daniel Kamerrer and Bogdanovich 1301 – 5th Ave., Ste 2900
910 Lakeridge Way SW Seattle, WA 98101
P.O. Box 11880 Attn: Carl Hagens
Olympia, WA 98508-1880
Attn: Donald L. Law

REPORT
GEOTECHNICAL ENGINEERING SERVICES
SLOPE STABILITY EVALUATION
VISTA NEIGHBORHOOD
KELSO, WASHINGTON
FOR
CITY OF KELSO

introduction

This report summarizes our preliminary evaluation of the stability of the slopes within the Vista neighborhood of Kelso, Washington. For purposes of this evaluation, the Vista neighborhood is defined as that served by Apple Lane, West Vista Way, North Vista Way, South Vista Way, Cris Road, and the south end of Haussler Road south of West Vista. The study area location is shown on the Vicinity Map, Figure 1. This project was initiated at the request of the Kelso City Council on March 9, 1999 and authorized by Doug Robinson, City Manager, on March 25, 1999. The study of slope stability within the Vista neighborhood was requested by the City Council to respond to concerns by residents that slope movements were causing damage to residences and property.

At the time of this report, some homes within the Vista neighborhood are currently experiencing damage because of apparent slope movements. Work is being completed on a geotechnical study of Haussler Road north of West Vista Way to Highland Park Drive. That study is in response to slope movements earlier this year that damaged Haussler Road and the utilities within the roadway. The damage to Haussler Road was such that access was threatened to all the residences south and east of the landslide limits. The Vista neighborhood study has been completed concurrently with the Haussler Road study as portions of the individual study areas overlap.

scope

The purpose of the Vista neighborhood study is to evaluate the stability of hillside slopes and the extent of existing and potential slope movements. In addition, this first phase of study is to identify areas within the neighborhood where more detailed explorations should be recommended, if any, to determine depth and rate of active slope movements. Specifically, our proposed scope of work included the following:

SITE DESCRIPTION

General

The Vista neighborhood is located on the west flank of a north-south trending ridge located south of the Coweeman River and east of Interstate Route 5 in Kelso, Washington. The study area includes developed and undeveloped residential properties, together with the infrastructure roadway and utility systems. Older homes in the neighborhood are generally older than 40 or 50 years. Other homes are from 30 to 40 years old, and some are as recent as 4 to 5 years old. A topographic map showing the areas of ancient landslides and also the areas of present slope movement within the Davis Terrace neighborhood, Aldercrest subdivision, Haussler Road neighborhood and Vista neighborhood is presented as the Study Area Slope Stability Map, Figure 2. The Vista neighborhood is shown with respect to these other features in Figure 2. The details of the Vista neighborhood study area are shown in Figure 3.

As part of our detailed surface reconnaissance, we completed two field developed cross sections (FDCS) that depict the relative location of features on the hillside from the top of the ancient landslide scarp down to Kelso Drive. The FDCS locations were selected to especially evaluate the areas of more active slope movement and to incorporate the information developed as part of the Haussler Road landslide study. The FDCS are presented respectively as Field Developed Cross Sections A-A’ and B-B’, Figures 4 and 5.

Regional Geology

Bedrock in the project area is composed of interbedded sedimentary and volcanic materials which comprise the Tertiary Cowlitz Formation overlain by partially consolidated alluvial sediments of the Troutdale Formation (Livingston, 1966). The Cowlitz Formation generally consists of sandstone, siltstone and shale deposited in a near-shore marine environment. These deposits contain coal seams and interbeds of basalt. The Troutdale Formation ranges from approximately 40 to 500 feet thick in the Kelso area and is made up of two distinct members. The lower member is a coarse conglomerate composed of well-rounded gravel and cobbles with occasional sandstone beds in a sandy silty clay matrix. In the upper portion of the Troutdale Formation, the sediments include layers of silty fine sand or silty clay. The Troutdale Formation is capped by several feet of sandy silt to silty clay.

Geologic maps of the area (Livingston, 1966) and other previous geotechnical studies suggest that the regional bedding attitude within the Cowlitz Formation is nearly horizontal or tilted down slightly to the east.

SURFACE CONDITIONS

Topography

The Vista neighborhood study area is roughly rectangular in shape and is centered along the western flank of a north-south trending ridge. The crest of the ridge forms the eastern border of the study area and occurs at approximately Elevation 425 feet Mean Sea Level (MSL). The top of the ridge is fairly level and is approximately 250 to 600 feet wide. Continuing west from the ridge top, the ground surface drops along a steep slope that has previously been identified as an ancient landslide scarp. The ancient scarp is inclined from approximately 25 to 35 degrees (2.1H:1V to 1.4H:1V) in the study area. The ground surface continues to slope downward to the west from the base of the ancient landslide scarp to Kelso Drive at an average slope of approximately 8 to 10 degrees (7.1H:1V to 5.7H:1V).

Two prominent east-west trending elongate hollows extend across the scarp face in the Vista neighborhood. The more prominent of these features is centered along West Vista Way. The second hollow is located southeast of the southern end of Cris Road.

Several lobes/fans extend west from the base of the scarp. During our reconnaissance we observed exposures of gravel and cobbles in these features, which we interpret as derived from the Troutdale Formation. The fans/lobes have an irregular surface with occasional visible blocky structures and intermittent water erosion grooves.

West of Haussler Road the overall ground surface inclination is more gradual. During the reconnaissance, extensive regrading was observed along FDCS A-A’ (Figure 4) and in the southern portion of the study area bordered by Cedar Falls Drive. The undisturbed ground surface north of this area generally consists of irregular, hummocky topography, which we interpret as ancient slide deposits.

Vegetation

Vegetation along the ancient landslide scarp consists of a mature second-growth forest of mixed deciduous and coniferous trees with an understory of dense shrubs and ground cover. Fir, maple, and alder trees with trunk diameters up to 2.5 feet were observed along the scarp face. The understory vegetation consists of blackberries, nettles, sword ferns, and other deciduous shrubs.

Seepage areas at the base of the scarp and in the gently sloping area west of Haussler Road are vegetated with hydric plants. These include, salmonberries, occasional skunk cabbage, horsetails and water tolerant grasses and reeds, which indicate saturated soil conditions. Maple, alder, and occasional cedar trees were also observed in these areas.

Surface Water Features

The primary surface water features observed during our reconnaissance were seepage zones along the base of the ancient landslide scarp and in the slide debris west of Haussler Road. A small drainage was also observed extending from a culvert under the southern end of Haussler Road, downslope to Kelso Drive.

Seepage zones were encountered along both of the FDCS (see Figures 4 and 5). At the base of the ancient landslide scarp extensive seepage results in surface flow, hydric vegetation, and occasional deposition of fine sediments. Standing water was observed at the base of the scarp along FDCS A-A’ (Figure 4). A prominent seep was observed behind the northeast corner of 506 Haussler Road with an approximate discharge of 2-3 gallons per minute (gpm) (as observed April 20, 1999).

West of Haussler Road wide-spread seepage zones, saturated soils, and deposition of silts and clays are visible. A sag pond identified in previous reconnaissance was observed west of 537 Haussler Road and is shown on FDCS A to A’ (Figure 4). This pond is approximately 35 to 45 feet wide and 100 feet long. The sag pond outflow is conveyed downstream in a channel approximately 1 to 2 feet wide, 1 foot deep, with a discharge rate of approximately 3-4 gpm (observed March 20, 1999).

A culvert extends under the southern section of Haussler Road just north of the driveway for 546 Haussler Road. An estimated discharge of 2-3 gpm was observed from this culvert on March 20, 1990. During our reconnaissance seepage and surface runoff appeared to be the source for the culvert inflow. Outflow from the culvert continues down slope, connecting with a channel identified in a previous study. Several small man-made detention ponds intercept the channel discharge north of Cedar Falls Drive.

Slope Movement Observations

Within the Vista study area there are several locations exhibiting evidence of ongoing, active movement. Along the uphill side of Apple Lane some homes are experiencing structural distortion, apparently because of slope movement. We have seen the distortion in some of these homes. We have not visited other homes but have been told of the distortion and damage by residents and neighbors. This distortion is evidenced by damage such as cracks above corners of door and window frames, nails popping out of sheetrock, cracks in ceiling, tilting of floors, sticking doors, and support beams moved on their supports. At the time of our field visits, there was no visible evidence such as pavement cracks or distortion in the street (Apple Lane). The concrete curbs and gutters on both sides of the street show no apparent distortion or displacement. Water drains continuously from drainpipes, mostly 4-inch-diameter PVC, that extend through the curb and discharge into the gutter on the uphill side of the street. We understand that in May, the water line servicing a house near the south end of Apple Lane pulled apart near the house and had to be repaired.

Slope movement is apparent at the Cozad residence above Haussler Road. A two-course high ecology block wall was used to support the driveway just downslope of the residence. Slope movement has been sufficient to topple the blocks and move several down slope. A scarp with a vertical offset of about 2 to 3 feet and a horizontal offset of 1 to 2 feet is evident passing through the ecology block wall location and extending into the neighboring property to the south. The general location of this scarp is shown on the Site Plan, Figure 3.

On the upslope (east) side of Haussler Road just south of West Vista Way, the hillside slope appears to have moved out (to the west) 2 to 3 feet onto Haussler Road. At this location there is no apparent cracking or distortion in the pavement on Haussler Road that would be caused by slope movement. A slope indicator installed at the intersection of West Vista Way and Haussler Road has shown no movement through May 28, 1999.

On South Vista Way there are several residences with structural damage that appears to be the result of slope movement. This visible damage includes cracks in door and window frames, nails popping out of sheetrock, sticking doors and cracks in ceilings. Cracks are also apparent in foundation walls, driveways, and exterior chimneys. Our interpretation of the general area of active slope movement is shown on the Site Plan, Figure 3.

Slope inclinometers were also installed as part of the Haussler Road landslide study at the intersection of Apple Lane and Haussler Road and in the street in front of the residence at 125 Apple Lane. As of May 28, 1999, we observed no indication of slope movement in these slope inclinometer installations. The approximate locations of these slope indicators are also shown on the Site Plan, Figure 3.

DISCUSSION

General

The Vista neighborhood is located on an ancient landslide. The entire area from the base of the ancient landslide scarp down to Kelso Drive, appears to consist of ancient slide deposits overlying very stiff or hard silt and clay. Presently, areas within the Vista neighborhood are exhibiting active movement with displacement and bulging of slopes, failure of retaining walls, tension cracks, and continuing damage of residences. At this time the rate of movement appears to be relatively slow. However, it is likely that the rate will vary with changes in rainfall and ground water levels. It is also likely that amounts and rates of movement will vary greatly with variations in local soil, topography, geology, and water conditions, both surface and subsurface.

conclusions and recommendations

In the areas of more active recent movement, outlined in Figure 3, it is likely that the existing soil structure has been disturbed thereby reducing the internal strength of the soil mass. Because of this, these areas will be more susceptible to future movement, especially in the presence of increased rainfall and elevated ground water, and continued movement must be expected.

At this time, we do not have sufficient subsurface information to determine if mitigation measures are feasible. Such measures might include drainage, dewatering, structural support, regrading, buttressing, or in-situ soil strength improvements. The feasibility of such measures is dependent on cost, access, depth to water, soil permeability, depth to competent soils, and topography. We recommend that borings be drilled at selected locations in the Vista neighborhood to provide information regarding subsurface soil and water conditions and to provide soil samples for laboratory testing and analysis. We also recommend that slope inclinometer casing be installed in the borings to monitor the depth to any failure surfaces and the amount of movement of the slope. The data obtained from the borings, laboratory testing, and slope inclinometers would be used to evaluate the effectiveness and feasibility of potential mitigation measures. The data would also be used to provide design and construction recommendations for mitigation measures determined to be feasible.

We recommend that five borings be drilled to provide the above described information. The recommended approximate locations of the five borings are shown on the Site Plan, Figure 3. We also recommend that slope inclinometers and piezometers be installed in each boring to develop data regarding depth and rate of any slope movements and ground water levels. We can provide a more detailed outline of work scope and schedule for this recommended additional study, if requested.

We trust this provides the information you require to assist you in making decisions concerning the Vista neighborhood. We will be pleased to discuss our conclusions and recommendations with you in greater detail should you so request. Please contact us if you have any questions concerning our findings and recommendations.