Case Studies

BAINBRIDGE ISLAND, KITSAP COUNTY

Site Conditions:

Clay soils, daylighting groundwater 20 feet below the top of the slope. Slope is about 25 feet high but house is located only 5 to 10 feet back from the crest of the slope.

Site Drainage Features:

Large driveway area draining to a catch basin near the top of the slope. However, little water reaches the catch basin. Observation of driveway sheet flow shows water disappears into the joints between the driveway concrete slabs. Wood originally between the slabs had rotted away. Downspout discharges and other site drainage are routed to a tightline system that discharges below the slope. However, a leak is present within the tightline system.

Resulting Problem:

Recent landslide just below existing residence. Landslide on slope was caused by groundwater and uncontrolled surface water flows (tightline leak).

Solution:

Repair of the slide area using a retaining wall with good backfill drainage, repair of tightline leak, perform regular monitoring of the tightline system for possible future leaks, and place a flexible joint sealant between concrete driveway slabs.

HANSVILLE, KITSAP COUNTY

Site Conditions:

Sand at top of slope underlain by clay, groundwater daylighting at the sand/clay boundary located about 10 feet below the top of the slope. Slope is greater than 50 feet in height and house is over 80 feet back from the top of the slope.

Site Drainage Features:

All site runoff from roof, driveway, and foundation drains routed to a tightline system that appears to be functioning without leaking. Septic field located between the residence and the crest of slope, about 30 to 60 feet from the crest.

Resulting Problem:

Slope erosion and recent landslide. Landslide originated at the top of the slope near the sand/clay boundary.

Solution:

Construct an interceptor trench to collect the groundwater before it reaches the face of the slope and route the water collected in the trench via a tightline system down to the bottom of the slope. Location of the cutoff trench would be best between the slope and the residence but must be set well back from the crest of the slope. However, the septic system location creates a planning conflict. Even if there was sufficient room for the trench setback from the top of the slope, a cutoff trench too close to the septic system could pick up untreated or inadequately treated wastewater.

Two alternatives remain:

  1. Relocate the septic system to an area behind the house (away from the slope) and construct the cutoff trench in the area previously occupied by the septic system; or
  2. Locate the cutoff trench behind the house. Alternative 1 is more expensive but would provide a greater level of drainage control for the slope by intercepting groundwater closer to the problem area. Also, by locating the septic system behind the residence the septic system wastewater flows are further from the crest of the slope.

GRAPEVIEW, MASON COUNTY

Site Conditions:

High bluff, glacial till soils. The owner cut a sloping path from the residence to the stair system at the top of the bluff. At the bluff crest the path was notched into the bluff about 10 feet.

Site drainage Features:

No control of site surface water. All surface water allowed to flow towards the top of the slope. Most of the site surface water drained down the path.

Resulting Problem:

Serious erosion from the concentrated surface water flow undermined the stair foundations resulting in stair damage and loss of vegetation from the slope.

Solution:

Reroute site surface water flows where they originate at the house downspouts (tightline) and lawn (surface swale). Capture and route the water down the bluff in a tightline system. Capture as much of the site runoff before it gets near the path. As a backup system construct a swale and catch basin that captures and routes any water flow on the path before it reaches the bluff crest. Tightline the captured flow down the slope. Note that it would have been best not to have "notched" the bluff in the first place.

KINGSTON, KITSAP COUNTY

Site Conditions:

Construction of a new residence in a previously undeveloped area. Area is mapped in Coastal Zone Atlas as Historic Old Slide Area. Soils are sand over clay. There is evidence of groundwater seepage with water loving vegetation on a portion of the slope. The site slope shows indications of past sliding: leaning and rounded trees and hummocky ground.

Site Drainage Features:

None constructed prior to site development. Natural groundwater daylighting on the slope.

Resulting Problem:

Development triggers slide activity.

Solution:

Do not develop the site or do so with extreme caution. Develop only with the input and engineered designs by professional geotechnical and civil/drainage engineers experienced with development in slide areas. Recognize that development on known slide areas contains inherent risk, even with the best of engineering design.

FOX ISLAND, PIERCE COUNTY

Site Conditions:

Development of a 10 acre subdivision in a previously undeveloped and heavily vegetated area. Extensive slope clearing, grading, cuts, and fills performed.

Site Drainage Features:

None prior to development. Few erosion control measures incorporated into the construction or final site configuration. No engineering performed to evaluate fill placement on steep slope. Drainage system poorly laid out and not effective in intercepting and routing surface water flows.

Resulting Problem:

Site development without adequate construction or permanent stormwater, leads to heavy site erosion and landslides.

Solution:

Install drainage control measures including swales, detention, and tightlines down to the base of the slope. Provide erosion control and revegetation of the site. Reengineer the fills by recontouring, removing, reinforcing, or constructing a retaining wall.

Given the level of damage to the site and cost for repair, the developer would have been better off spending the money up front for adequate engineering and site construction erosion control measures.

LUMMI ISLAND, WHATCOM COUNTY

Site Conditions:

A shallow undeveloped waterfront lot has known drainage and slide problems. The only site vegetation is a narrow strip along the top of the bluff. Owner contacts someone for input on site development.

Site Drainage Features:

None on undeveloped lot. Recommended input on site develop- ment includes a deep interceptor trench within several feet of the bluff edge.

Resulting Problem:

Proposed trench construction so close to the bluff crest would remove almost all vegetation from the top of the bluff. Also, excavating a trench so close to the edge could actually cause rather than prevent a failure at the top of the slope.

Solution:

Do not construct a trench close to the top of the bluff crest.

When retaining someone to provide you with technical input for drainage design and slope stability improvement be sure they are State of Washington licensed civil engineers with education and experience as a geotechnical engineer and/or drainage engineer. Don't be afraid to ask questions and clarify recommendations that don't make sense to you. When in doubt get a second opinion.

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