On the steep slopes of Mercer Island we investigated a landslide that reached just below the basement footings of a large two-story residential home. During our site subsurface investigation we provided our client with several potential remedial options along with the relative risks implied by each. The client, after weighing the costs/benefits selected a combination of options, then requested our design and overall project management oversight to install a permanent soil nailed wall with steel reinforced shotcrete facing that included an anchored HDPE Pipe drainage system. The challenges involved working on this steep slope and finding the right contractors that were flexible enough to help speed field adjustments of the design to accommodate the unusual slope shape, thus avoiding costly potential survey revisions and layout time.
This project on the east slopes of Mercer Island was completed successfully with the speed needed to avoid the heavy "La Niña" storms. Slope movement and erosion of this area has been significantly reduced with no measurable issues after several rainy seasons.
We continue a long-term monitoring program for keeping tabs on past landslide movement, excessive erosion from surface water and groundwater seepage at this project site where Mr. Adams led the design for three horizontal drain galleries, rockery wall/reinforced earth buttresses, an HDPE pipe collection system and a permanent soil nailed wall. The project began after the storm events in the winter of 1996-1997, impacting five separate properties adjacent to the Golden Gardens Park. Drainage and reinforcement efforts have successfully kept continued movement and erosion to extremely low levels since the completion of the project elements.
Several landslides were activated during the 2010 - 2011 winter. We are currently working on two neighborhood Debris Slides that threaten several properties near Leschi beach. Initial remedial measures have significantly slowed erosion through the winter on these steep slopes. We will have more about the challenges as the investigations unfold.
We completed a geotechnical engineering investigation of this slide to help the owner slope select remediation options adjacent to their new three-story condominium. The slide scarp reached right up to the house footings and groundwater was seeping into the lowest crawl space area.