HomeMy WebLinkAboutAttach C.1 - 10 Pct Civil Basis of Design
MEMO
Boeing South Bridge Seismic Retrofit – Design Criteria
Date: June 9, 2025
To: Jared McDonald, P.E. (City of Renton)
From: Eric Mendel, P.E.; Christine Hawatmeh, P.E.
Subject: Panther Creek 10% Civil Design – Basis of Design
Existing Site
Panther Creek crosses from east to west under Talbot Rd S, which is a two-lane road aligned north
and south. The paved width of the road is approximately 40 feet with 11-foot lanes and 4-foot
shoulders. There is an existing 5-foot sidewalk west of the roadway and a guardrail with no sidewalk
on the eastern side. The roadway is crowned with a cross slope of approximately 4%. Average fill
slopes on the east and west of the roadway are 2:1. The existing Talbot Rd S pavement surface is
Hot-Mix Asphalt (HMA).
Existing drainage/utilities include:
• Two storm drains that discharge to the area upstream of and near to the project crossing.
• Drinking and wastewater main lines run underground north to south over the crossing
parallel to Talbot Rd S. The water main is a 12-inch steel pipe. There is also an abandoned
20-inch steel water main pipe adjacent to the active main. The wastewater main is an 18-
inch concrete pipe that runs adjacent to the roadway centerline.
• A 2-inch and 16-inch gas line have been identified along the project limits. Both lines run
adjacent to the roadway centerline.
• Multiple overhead power and cable lines and poles have been located along both sides of
Talbot Rd S at the project location.
The following sections of this Basis of Design Memo discuss Roadway, Utility, Stormwater, and
Maintenance of Traffic considerations for this project.
Roadway Considerations
The proposed roadway design will maintain the existing horizontal and vertical alignment for Talbot
Rd S, which meets current sight distance requirements. The roadway over the bridge structure will
include two,11-foot traveled way and sidewalks on both sides of the roadway. The proposed roadway
will add a 12-foot wide two-way turn lane to meet current roadway standards and tie into the two-
way turn lane south of the bridge. In addition, the design proposes adding a sidewalk along the east
side of the roadway as well as designated bike lanes along both sides of the roadway to meet current
City of Renton standards and increase pedestrian/bicyclist access.
The project proposes 5-foot sidewalk widths to maintain consistency within the corridor and meet
minimum sidewalk width requirements. Guardrail will not be required on this project as the proposed
roadway section satisfies clear zone requirements on both sides of the road. Traffic barrier and fall
protection will be included along the bridge and extended north and south of the bridge as needed
ATTACHMENT C.1
MEMO
to meet vertical grade difference requirements. The proposed roadway section can be seen in Figure
1, below.
Utility Considerations
The project proposes temporary relocation of existing buried and overhead utilities during
construction. Some overhead utilities and poles will likely require permanent relocation due to the
widened roadway section and new bridge.
Buried utilities listed below are to be supported by the new bridge in the permanent configuration:
• Buried Telecom Line
• 2” Gas Line
• 16” Gas Line
• 21” Wastewater Line
• 12” Water Line
Franchise utility companies will need to be contacted to verify the locations of existing facilities, and
depth of underground utilities, within the project limits. Coordination with all utilities will occur during
the design and construction process to find mutually beneficial solutions.
Stormwater Considerations
The project will add 1,966 square feet of new impervious area, due to the widened roadway section,
and replace 9,075 square feet of existing impervious surface. Since the project has over 2,000
square feet of new plus replaced impervious surface a full drainage review is required per the City
of Renton Surface Water Design Manual (RSWDM) (2022). The project will be required to meet Core
Requirements (CR) 1 through 9 in addition to Special Requirements (SR) 1 through 6 as identified
in Table 1.1.2.A of the RSWDM.
Figure 1: Typical Roadway Section
MEMO
This project is exempt from flow control and water quality treatment requirements per the City of
Renton Surface Water Design Manual (2022). The project will not increase the 100-year peak flow
from the new impervious surfaces by more than 0.15-cfs and will meet this flow control exemption
as listed in Section 1.2.3.1.B, exemption number 2. The project is water quality treatment exempt
due to the new impervious surfaces being less than 50% of the existing impervious surfaces; less
than 5,000 square feet of new pollution generating impervious surfaces are added; and less than
0.75 acres of new pollution generating pervious surfaces will be added to this project. This exemption
is listed in Section 1.2.8 as Exemption #2. Existing roadway drainage patterns will be largely
maintained. The existing catch basin along the southeast edge of the roadway reconstruction limits
will be adjusted to grade. The corresponding outfall pipe falls within the structure excavation limits
and is anticipated to be replaced.
Maintenance of Traffic Considerations
The project will require a short-term full roadway closure and detour that will affect all roadway users
equally. A key criterion of the MOT analysis was to ensure access to the hospital would be
maintained during the construction period. In August 2023, the City met with representatives of the
Valley Medical Center Hospital (a large regional hospital located just south of the project site) to
discuss access needs during construction of the project. The hospital stated a full road closure, for
approximately 3 to 4 months, would be acceptable with the proposed detours allowing access from
the north. The south access to the hospital will remain unaffected by this project. No residential
relocations are required for the project. The project is located within a bus route that includes nearby
bus stops. Coordination with King County Metro will occur during the design and construction
process to find mutually beneficial solutions to any impacts resulting from the full road closure.
Codes/References
The civil design criteria for this project are based upon the following design standards:
• City of Renton Surface Water Design Manual (2022)
• AASHTO Policy on Geometric Design of Highways and Streets, 7th edition (2018)
• WSDOT Local Agency Guidelines (LAG) Manual, Dec. 2024