Loading...
HomeMy WebLinkAboutMOD_justification_modification_request_20190116_v1Memo To: Jill Ding, Senior Planner From: Kristi Rettmann City of Renton, Community and Economic Development 1055 S Grady Way, 6th Floor Renton, WA 98057 Stantec 11130 NE 33rd Place Suite 200 Bellevue WA 98004 File: 2002003611 Date: January 16, 2019 Reference: Justification for the Modification Request to employ the use of some retaining walls exceeding the maximum permitted height of 6 feet for the Thunder Hills Sanitary Sewer Replacement Project The following are statements addressing and justifying how the modification request complies with each of the following issues as requested in the Submittal Requirements Modification Checklist. • Substantially implements the policy direction of the policies and objectives of the Comprehensive Plan Land Use Element and the Community Design Element and the proposed modification is the minimum adjustment necessary to implement these policies and objectives: Two of the 2016-2021 goals listed in the City of Renton’s 2015 Comprehensive Plan include “Promote a walkable, pedestrian and bicycle friendly city with complete streets, trails, and connections between neighborhoods and community focal points” and “Plan, develop, and maintain quality services, infrastructure, and amenities.” The modification request to have some retaining walls greater than 6 feet high in areas of steeper slopes, supports the implementation of these two goals by 1.) improving an access path used by pedestrians in the area that is appropriately reinforced to prevent slumping and erosion into the access path, and 2.) replacing an aging sanitary sewer line to provide quality infrastructure and amenities for the community. Walls greater than 6 feet high will only be utilized where necessary and will be adequately embedded into the unweathered sandstone to provide adequate protection to maintain the integrity and function of the maintenance access path and provide necessary environmental protection from slumping soils into the creek. Land Use Element policies within City of Renton’s 2015 Comprehensive Plan that this modification request substantially implements include but are not limited to the following: Policy L-29: Minimize erosion and sedimentation in and near sensitive areas by requiring appropriate construction techniques and resource practices, such as low impact development. Policy L-32: Protect buffers along wetlands and surface waters to facilitate infiltration and maintain stable water temperatures, provide for biological diversity, reduce amount and velocity of run-off, and provide for wildlife habitat. Policy L-37: Land uses in areas subject to flooding, seismic, geologic, and coal mine hazards should be designed to prevent property damage and environmental degradation before, during, and after construction. • Will meet the objectives and safety, function, appearance, environmental protection and maintainability intended by the Code requirements, based upon sound engineering judgment: Several areas along the project alignment show signs of soil movement or erosion. While much of this movement and erosion is along the west side of the ravine north of Station 5+50. Landslide activity and surface erosion has occurred in this area within the past 50 years (approximately). The movements include sliding of colluvium and highly weathered bedrock from the underlying hard bedrock. Slide thicknesses range from 1 to 3 feet in thickness, 10 to 30 feet in width, and up to 40 feet long. Additionally, there is ongoing erosion along the stream banks and within the channel. In order to support the adjacent hillside along the trail, prevent further erosion from undermining the hillsides where several apartment buildings are immediately above, and to prevent the undercutting and erosion of the new sewer line several areas along the existing maintenance path, a certified engineering geologist recommends installation of retaining walls in several areas along the project. Some of these walls will exceed 6 feet in height. However, several locations include very steep slopes (40% grade or higher) and are at a higher risk of hillside slumping, landslides and erosion. As a result, these areas require proportionately taller retaining walls to provide appropriate support that does not compromise the safety of people utilizing the path for walking and aid in preventing the hillside from slumping and eroding along steeper January 16, 2019 Jill Ding, Senior Planner Page 2 of 2 areas where apartment homes are located immediately up the hillside. Additionally, these higher walls will only be utilized where necessary and will be adequately embedded into the unweathered sandstone to provide adequate protection to maintain the integrity and function of the maintenance access path, provide necessary environmental protection from slumping soils into the creek, and will provide protection and maintainability of the new sewer line for the duration of its lifespan. • Will not be injurious to other property(ies) in the vicinity: Where installed, walls exceeding 6 feet high will be down below the apartment housing structures. These walls will provide appropriate support to prevent the erosion and landslide potential along these very steep slope areas, thereby providing a positive and structurally supportive impact for the properties along the creek. These supporting walls will be installed so that they are structurally sound for the area. No undercutting or intentional injurious action to the adjacent properties as a result of the installation of these walls (regardless of height). • Conforms to the intent and purpose of the Code: RMC 4-4-040 regulations are intended to regulate the height of fences, hedges, and retaining walls, particularly in front yards and in areas abutting public rights-of-way, so as to promote traffic and public safety and create aesthetically pleasing neighborhoods. The retaining walls for this project will assist in tree retention, preventing the hillside from slumping and eroding along steeper areas where apartment homes are located immediately up the hillside, and will reduce the amount of imported material needed for project-related activities. The retaining walls for this project will conform to this code as they will be out of site (down in a ravine) of adjoining property lines, will not infringe on any sight distance along public rights-of-way (they will be supporting the steep slopes and not blocking view for pedestrians on the path, and will promote public safety by providing erosion or landslide hazard in areas where they are installed. • Can be shown to be justified and required for the use and situation intended: The slopes extending downward into the Thunder Hills Creek valley between Stations 0+25 and 6+40 are very steep, with magnitudes of 100 to 150 percent. There are localized slope areas that are near vertical (200 percent magnitude) to overturned due to excavation, sloughing, and/or landslide activity. Generally, slope magnitudes south of this area range from 40 to 100 percent. These areas are at a higher risk of hillside slumping, landslides and erosion. As a result, these areas require proportionately taller retaining walls to provide appropriate support that does not compromise the safety of folks utilizing the path for walking, aids in preventing the hillside from slumping and eroding along steeper areas where apartment homes are located immediately up the hillside and provide protection and maintainability of the new sewer line for the duration of its lifespan. • Will not create adverse impacts to other property(ies) in the vicinity: Where installed, walls exceeding 6 feet high will be down below the apartment housing structures. These walls will provide appropriate support to prevent the erosion and landslide potential along these very steep slope areas, thereby providing a positive and structurally supportive impact on the properties along the creek. Best Regards, Kristi Rettmann Environmental Scientist Phone: (425) 289-7338 kristi.rettmann@stantec.com Attachment: None c. Brian Wolf Wayne Wright