County targets active Kingston landslide

Kitsap County Public Works will pursue horizontal drains on unstable slope

KINGSTON — The northern end of Kingston’s Washington Boulevard is a dead end, but a crucial one for some residents.

The road is the only connection from the town of Kingston to the properties of a number of local residents. But the looming presence of an active landslide along Washington Boulevard constantly threatens to cut these residents off from the town and emergency responders, should the slope finally give way.

Following a significant slide at the site in 2006, Kitsap County Public Works began monitoring the Washington Boulevard landslide. Public Works held a meeting April 24 to let Kingston residents know what they intend to do about the street’s slowly sloughing soil. The meeting seemed to mark a departure from the county’s established habit of monitoring and studying movement along the slope, without employing any preventative measures.

Tina Nelson, a senior manager for Kitsap County Public Works, said at the meeting, “In November 2016 we said, ‘We will do something different than continue to maintain and monitor. We are going to come up with some solution that is going to provide some more predictability.’ ”

The county, Nelson said, has decided to pursue a full-slope dewatering system to rein in the active slide along Washington Boulevard.

Andy Holmson, associate geotechnical engineer for Aspect Consulting — the environmental engineering group contracted by the county for the past 12 years to gather data on the slide — provided some insight into the solution.

“In essence, there’s only a certain number of ways to stabilize a landslide and they all come down to basically two things: can you reduce what we call the driving forces on the landslide, those are the forces pushing soil down the hill; or can you increase the resisting forces, which are the forces holding the soil up the hill,” Holmson said.

“Dewatering is a concept that tries to take groundwater levels within the landslide and lower them.”

The engineer explained that by removing groundwater, a driving force for the slide would be decreased.

“[Groundwater] adds weight to the soil, it decreases what we call the shear strength of the soil,” Holmson said.

“During the wintertime, we see evidence of that at the landslide. Groundwater levels come up in response to precipitation, the landslide moves. Through our monitoring and efforts like that, we know that is the primary cause of movement at Washington Boulevard.”

He said that by installing horizontal, passive groundwater drains, the extra weight added to the soil from groundwater could be mitigated. The drains would be installed near the shoreline on an uphill gradient and allow gravity and the permeability of the soil to do the work. Groundwater permeates slotted drain piping and then travels down the pipe to a dewatering pit located downhill.

One of the proposed alternatives included the installation of seven dewatering pits about halfway down the slope from Washington Boulevard.

A pair of passive groundwater drains would connect to each pit. A header system would then collect the water from these mid-slope dewatering pits and pipe it down to a shoreline outfall. The report noted, however, that this half-slope dewatering system would not affect the landslide’s stability. The initial cost of this alternative was estimated at $550,000.

The full-slope plan being pursued by Public Works will have longer extensions of the passive groundwater drains and will locate the dewatering pits farther downhill with a shorter header system running to a shoreline outfall. While similar to the half-slope design, this full-slope dewatering system has the added bonus of providing increased stabilization to the slide area. The initial cost of this alternative was estimated at $620,000.

While horizontal drains seemed to strike a balance between feasibility and providing a solution to the slide, Holmson did say that there would be some maintenance costs associated with the horizontal drains: occasionally, the headers will require pressurized water jetting to dislodge blockages that form over time.

County officials said they intend to seek a design contract for the project in August.

Other alternatives and their associated costs were:

  • Long-term refined monitoring and continued roadway maintenance. Referred to as the status-quo alternative, this would not require any additional funding;
  • Removing and replacing the roadway embankment with lightweight fill — $950,000;
  • Constructing an anchored pile wall to retain the roadway and underlying soils — $1,850,000, and;
  • Constructing a bridge upon large-diameter foundations to effectively bypass the active landslide — $8,000,000.

— Nick Twietmeyer is a reporter with Kitsap News Group. Nick can be reached at ntwietmeyer@soundpublishing.com.

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