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HomeMy WebLinkAboutRS_DD_Civil_Narrative_for_Onsite_Improvements_LPD_220111_v1.pdf 1932 First Avenue, Suite 201 Seattle, WA 98101 p. 206.725.1211 f. 206.973.5344 lpdengineering.com DESIGN DEVELOPMENT CIVIL NARRATIVE FOR ONSITE IMPROVEMENTS Lindbergh High School January 11, 2022 The following civil narrative outlines design development-level of onsite improvements for proposed building renovations at Lindbergh High School. Site Overview and Existing Conditions The school site consists of three contiguous parcels associated with the primary address of 16426 128th Avenue SE in Renton, Washington. The main high school facility spans the two largest adjacent parcels (parcel numbers: 2823059004 and 2823059042). The existing science wing resides on the easternmost parcel, and the administrative/health center sits on the westernmost parcel. There are regulated slopes designated throughout the school site including within the proposed building renovation areas. Topographically, the specific renovation site area is relatively flat, but generally slopes down from the east to the west from a high elevation of approximately 430-feet. Elevations descend to a low point approximately 400-feet at the surface parking area adjacent to the west of the major portions of the school facility. Per the USDA web soil survey, the soil consists of Alderwood gravelly sandy loam (AgC) which may have some infiltrative capabilities. However, based on the existing stormwater infrastructure on-site, there are no implications of infiltrative facilities being utilized for stormwater management. The site resides within the Soos Creek Water and Sewer District service area. Stormwater is under the jurisdiction of the City of Renton. As such, stormwater improvements will be evaluated and designed in accordance with the requirements in the 2017 City of Renton Surface Water Design Manual. A full drainage review that evaluates the Core Requirements (CR) #1-9 will be required since the project proposes more than 2,000 square feet (SF) of new plus replaced impervious surface. CR #3 Flow Control (detention) will likely not be required since it is anticipated that the target surfaces required to be mitigated only include the new impervious surfaces and will, therefore, not increase the peak flow rate of the 100-year storm event by 0.15 cubic feet per second (cfs). Additionally, it is anticipated that CR#8 Water Quality (treatment facility) will not be required since the project will propose less than 5,000 SF of new plus replaced pollution-generating impervious surface. CR#9 On-site Best Management Practices (BMPs) will likely be required to be evaluated. Target surfaces for the application of On-site BMPs will include new impervious surfaces that are not already mitigated by an approved on-site BMP or flow control facility. Based on the existing stormwater infrastructure and what is known about the site soils, infiltrative BMPs such as bioretention may not be feasible. Since there is limited vegetated surfaces directly downstream of the proposed improvements, dispersion devices are not likely feasible. There is significant amount of existing permeable pavement along the northeast side of the existing building. The location of the existing pavement is at the toe of a grassed slope and a portion is beneath tree coverage. As a result, the pavement is subject to sheet flow and underlying saturated soils which has caused moss and silt to clog the void space of the pavement. The pavement does not appear to function as originally intended; therefore, the additional permeable pavement is not anticipated to be January 11, 2022 Lindbergh High School Design Development Civil Narrative for Onsite Improvements Page 2 utilized. One possible BMP that could potentially be utilized is perforated pipe connections for the proposed roof downspouts. Proposed Site Improvements: Science Wing Renovation and Addition Temporary Erosion and Sediment Control (TESC) The TESC design incorporates various measures to mitigate impacts resulting from the proposed development activities on the site. TESC measures include inlet protection, tree protection fencing, compost socks, etc. Water Water improvements associated with the science wing is not anticipated. Sewer There does not appear to be any external sewer line on the east side of the building. It is assumed that since this wing is operating as the science wing currently that sewer is available internally. Dry Utilities The building addition footprint conflicts with the existing gas line. It is assumed this line will require rerouting per the direction of the mechanical engineer. There is also a power riser on the corner as well as pedestrian lights that will be impacted and existing connection points will require further confirmation and direction from the electrical engineer. Site Drainage The existing storm drainage system will require rerouting and new connections for downspouts, footing drains, and french drains. These improvements are not anticipated to have a significant impact on the capacity of the system. There is an existing grassed area designated as a stormwater dispersion path for the parking lot that will be slightly impacted by the proposed renovations. Measures will be taken to mitigate elsewhere on the site where feasible. Additional Site Infrastructure Improvements There will be a new walkway connection between the existing parking lot and science wing addition. Paving improvements will also be established in the center courtyard area adjacent to the science wing. January 11, 2022 Lindbergh High School Design Development Civil Narrative for Onsite Improvements Page 3 Proposed Site Improvements: Administration / Health Center Renovation The administration area renovations will include a remodel of the interior space, an infill of an existing exterior triangular space between building units, and a one-story addition at the south end of the building renovation area. Temporary Erosion and Sediment Control (TESC) The TESC design incorporates various measures to mitigate impacts resulting from the proposed development activities on the site. TESC measures include inlet protection, tree protection fencing, etc. Water Per the Soos Creek as-built drawings and the survey, there is an 8-inch water main underneath the proposed addition and is required to be rerouted. Approximately 130 feet of water main need to be removed and replaced. This will impact a portion of the asphalt drive at the entryway and the vertical curb. The new water main will conflict with the existing utility tunnel. Core drilling thru the tunnel for the new water main will likely be required. It is assumed the domestic water service for the renovation of the admin and health center will be from within the existing building. Sewer It is assumed that sewer service for the renovation of the admin and health center will be from within the existing building. Approximately 300 feet of 8-inch sanitary sewer line will be removed or abandoned in place where not below structure, and two sanitary sewer manholes (SSMH) will be removed. The sewer line will be routed around the new addition requiring approximately 200 feet of 8-inch pipe and five new SSMHs. There will be a portion of the system that extends from under the existing building that will need to be addressed in the plumbing design. Site Drainage Downspouts on the proposed renovated building area will connect to the existing roof overflow drain, which is internally routed. The existing storm system will be rerouted, and approximately 200 feet of storm drainage pipe will need to be removed or abandoned in place where not below structure. Approximately 200 feet of new storm drainage piping will be required for the reroute, in addition to, a new manhole and a catch basin. This will impact a portion of the asphalt drive at the entryway and the vertical curb. Site Paving Areas impacted by the rerouting utilities will be required to be replaced in kind. This includes new concrete paving, asphalt drive paving, and vertical curb. Paving improvements will be required to comply with current ADA standards. January 11, 2022 Lindbergh High School Design Development Civil Narrative for Onsite Improvements Page 4 Proposed Site Improvements: Parking Lot Paving Replacement The existing parking at the southwest corner of the site is in poor condition. This includes three (3) parking bays with approximately 162 stalls and two drive entrances. Temporary Erosion and Sediment Control (TESC) The TESC design incorporates various measures to mitigate impacts resulting from the proposed development activities on the site. TESC measures include inlet protection, tree protection fencing, etc. Site Paving Per the geotechnical recommendations, a full thickness repair is recommended for the longevity of the parking lot. Three parking bays and two drive entrances will be replaced and the subbase will be repaired as needed. The majoring of the existing curbing around the three parking bays in in poor condition and will not be able to be saved during the asphalt placement repair process. This will be replaced with vertical curb. The parking lot stalls and channelization will be re- striped. Water Water improvements within the parking lot is not anticipated. Sewer Sewer improvements within the parking lot is not anticipated. Site Drainage Existing catch basin structures within the replaced parking lot area will need to be adjusted to grade.