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HomeMy WebLinkAboutRS_13701SEMayValley Road-WoldeSiteMitigationLetter Civil & Geotechnical Engineering Consultants Elias Wolde June 27, 2023 13701 SE May Valley Road Renton, WA 98059 Residential Geotechnical Mitigation Report Site Address; 13701 SE May Valley RD Renton, WA Parcel No. 342405-9113 Site Visit 12/09/2022 Introduction This Report has been prepared by The Concept Group to respond to a request from the City of Renton to support a mitigation plan for the development adjacent to May Creek at 13701 SE May Valley Road in Renton, WA. The nearly 0.52-acre parcel (22,771 sf) is a developed residential parcel extending from SE May Valley Road south to the shoreline of May Creek. The City of Renton requires the removal of a patio, walls, stairs and vegetation (Ivy) along the north shoreline and slope of May Creek. The City has requested, A geotechnical report is needed to determine issues and mitigation associated with removing the unpermitted development. A geotechnical report is referenced in the critical area study, but it doesn’t appear to have been submitted. The city has requested a Base Flood Elevation Study, Critical Area Study, Mitigation Plan, Geotechnical, Title Report and Erosion and Sediment Control Plans. This report is intended for the exclusive use of Elias Wolde, his consultants and contractors for the intended purpose described. Site investigations and observations described in this evaluation represent the standard of practice for the industry. Sources of information cited are uniformly accepted resources when utilized in conjunction with field reconnaissance as confirmation. Opinions are based on using these standardized practices to adequately characterize the local surficial geology and general conditions at the site. Information Sources Soil classification and mapping for this assessment is supported by research and on-site soil evaluation performed on the above-mentioned dates. Geologic information for this assessment is supported by information from the United States Geological Survey (USGS) Geologic Map of the Black Diamond Quadrangle, Washington. Our understanding of site geology is supported by the review of geologic mapping, published topographic and relief map layers from the King County Geographical Information System (GIS), Soil exploration by IGS and site observations. Our opinions are based on our interpretation of the cumulative information and the contemporary conditions of the geologic setting. Published Information Accuracy It should be noted that the Survey, the USGS and/or DNR geologic maps, and the King County GIS define general areas of soil deposits, geology, and landforms. Given the large areas to identify and limited sample points, the authors of the above sources had to infer boundaries, contacts, and other representations in some areas. Only through on-site reconnaissance can we further detail and adjust information from the maps as they relate to a specific site. They are not (from our experience) accurate on a lot-by-lot basis in all cases. In this case, the USGS unit identification is generally in concurrence with observed field conditions, while the Survey identification is not wholly in concurrence with observed conditions. A Critical Area Study was prepared by Confluence Environmental Company dated June, 2022 and a Hydrologic and Hydraulic Evaluation was performed by Aspect Consulting dated July, 2022 to address Flood Hazard Data based on predevelopment conditions. Copies of each document were provided to Concept Engineering for our review. Concept Engineering prepared a geotechnical evaluation for the developed site which included a review of the predeveloped conditions, developed conditions, slope stability and soil characteristics. Site and Project Description The nearly 22,771 sf parcel (0.52-acre) is situated between SE May Valley Road on the north and May Creek on the south. The existing home is constructed on a terrace on the north valley wall of May Creek. The northern two thirds of the residential lot is nearly level with access from SE May Valley Road. The parcel is surrounded by similar residential development. Soil characteristics and conditions were observed in the face of the north and south valley walls of the May Creek drainage. Soil observed in the valley wall on the south side of May Creek consists of a very dense silty fine sand with some gravel and rocks. The south shoreline is the outside meander of the creek flow which is being undercut creating a nearly vertical slope. At the time of our site visit the south shoreline of the creek had experienced some sloughing and erosion. The main channel of May Creek flows along the toe of the steep southern shoreline. Wolde Property (nts) Topography and Aerial Photography obtained from King County GIS and Site plan from Terrane Predevelopment photos show the north shoreline is a wide, gently sloping meander with grade less than 5 percent extending north from the creek to the toe of the steep north valley wall. The north valley wall is approximately 23 ft high with a slope grade of nearly 35 percent. A small area near the center of the west side of the slope has grades of 40 to 50 percent. Predevelopment conditions are based on photos of the area showing the inside meander of the creek. Figure 3 below shows the predevelopment conditions consisted of a sparkly vegetated area with two large deciduous trees, the surface is covered with an unknown low growing vegetation. The east and west sides of the area are defined by green fencing and a wooden curb line along the north. The north shoreline of the creek appears to contain a wood log and possibly some rocks or other materials upland from the channel. The north valley wall is covered with a combination of Ivy and ferns. A dirt path with a few wooden steps descends the slope. Development of the area shown in Figure 4 below shows a masonry and flagstone walk, and stairway have been constructed down the moderate to steep slope facilitating the existing topography. A short block wall extends across the toe of the moderate to steep slope. This wall is 4 ft tall at the west property line and 5 ft tall at the eastern line. This masonry wall appears to have been founded at the toe of the original slope and backfilled to create a shallow grade to the east and a moderate slope to the west of the central stairway. The north shoreline of May creek has been armored with a short masonry wall situated upgrade from the waterline. The two deciduous trees have been circled with short masonry walls. Site Development Photos Photos obtained from Confluence Environmental Company / Critical Area Study, June 2022 Mitigation Recommendations The recommendations outlined in this report are based on the statement from the city insisting the site must be restored to a “Natural State” not a predevelopment condition. This assumption is based on the City Comment: Please be aware that if we do not receive all the requested documents, and they do not address complete removal of the unpermitted improvements and full restoration, in the next document submittal I will be handing this code case back to code enforcement which could mean additional fines and/or other penalties. English Ivy is an invasive species and needs to be removed. The geotechnical report will need to address removal of the ivy. Prior to any surface work or site disturbance the shoreline of May Creek should be protected by installing silt fencing along the outside of the small masonry wall. Erosion Control: Before new construction begins, an appropriate erosion control system should be installed. This system should collect and filter all surface run off through either silt fencing or a series of properly placed and secured straw bales. We anticipate a system of berms and drainage ditches around construction areas will provide an adequate collection system. If silt fencing is selected as a filter, this fencing fabric should meet the requirements of WSDOT Standard Specification 9-33.2 Table 3. In addition, silt fencing should embed a minimum of 6 inches below existing grade. If straw baling is used as a filter, bales should be secured to the ground so that they will not shift under the weight of retained water. Regardless of the silt filter selected, an erosion control system requires occasional observation and maintenance. Specifically, holes in the filter and areas where the filter has shifted above ground surface should be replaced or repaired as soon as they are identified. The city as stated the English Ivy which dominates the north valley wall slope must be removed. Removal of the Ivy should be accomplished in stages removing a portion of the plant and replacing it with a native deep rooting vine or ground cover to protect the slope. If all of the ivy is removed across the slope, the face of the slope should be protected with a “jute matt” or other erosion blanket material which will allow planting on or through the material. The blanket material should be allowed to deteriorate as the plants take over the slope. Plant selection should include low maintenance, deep rooting, quick growing species. May Creek to Toe of Slope (Inside Meander) The limited access to the site will require dismantling and hand removal of the masonry walls along the shoreline and encircling the two deciduous trees along with the flagstone pavers and fire pit. Any imported foundation rock or footings will have to be broken up and removed. The concrete pad located at the toe of the slope, in the northwest corner of the meander, will have to be broken up and removed from the toe of the slope. The short masonry wall north of the creek shoreline appears to have been backfilled to level the area north of the wall. Backfill soils will have to be removed, exposing the natural surface along the north side of May Creek. This surface may consist of glacial till, gravel or a combination of these native materials. The crushed rock surface placed across and around the flagstone sitting area will have to be removed to expose the native glacial subgrade, gravel or combination. The fencing material along the east and west sides of the lot should be removed. The restored surface of the meander should be vegetated with native species of shrubs and plants. Surface armoring may also include placement of logs, rocks or other natural materials. North Valley Wall The masonry block wall constructed across the toe of the moderate to steep north valley wall appears to be founded at the toe of the slope and backfilled. The stairway near the center of the slope appears to be approximately the grade of the original slope. The masonry wall should be dismantled and the backfill removed to expose the original slope. If the slope has been cut to install the wall, the backfill material behind the wall should be utilized to backslope the toe of the slope. The stairway near the middle of the slope, landing and series of stairs and landings extending upslope to the west must be removed to the top of the slope. Portions of the stairways and landings appear to have been cut into the upslope portion of the grade. To restore the slope grade, the backfill material used behind the toe wall should be placed onto the cut and graded to match the existing slope. Backfill material should be keyed into the slope and compacted to a non-yielding condition. Surface vegetation should be restored using native, low maintenance deep rooting species. Grades greater than 3H:1V (30%) should be covered with “Jute Matt” or other surface erosion blanket which will allow planting through the material. Conclusions The recommendations presented are based on our evaluation of the site’s soil and geologic conditions. We caution against the mass removal of the English Ivy exposing the underlying soil and proximity to the creek. Removal of the Ivy should be done in stages and each stage restored with native plant species. Erosion control should remain in place until the slope vegetation is restored. The slope stability was evaluated and was found to be stable in its current condition. Removal of the walls, stairways, landings and vegetation will increase the potential for surface erosion and/or shallow translational slides. No surface water runoff should be directed towards or down the face of the slope. Closure The observations presented in this letter are based, in part, on our interpretations and assumptions regarding subsurface conditions; therefore, if variations in the site conditions are observed later, we may need to modify this letter to reflect those changes. We appreciate the opportunity to be of service on this project. If you have any questions regarding this letter or any aspects of the project, please feel free to contact our office. Sincerely, Anna-Trang Nguyen, P.E. Project Engineer Rex Humphrey, L.E.G Engineering Geologist 06/27/2023 06/27/2023