21-101125-UP-Modification Request for SW Ret Walls-09-14-2021-V1
11235 s.e. 6th street, suite 150 t 425.453.9501
bellevue, wa 98004 f 425.453.8208
navixeng.com e info@navixeng.com
Memo
To: Stacey Welsh, AICP
From: Joe Taflin, P.E.
CC: Ben Childs, Graves Associates
Date: September 7, 2021
Subject: Modification Request for Landscaping at Base of SW Retaining Walls
The FNW Federal Way project is seeking a modification per FWRC 19.120.050 for relief from meeting the
requirement set forth in FWRC 19.120.120 (b) that states "there shall be a minimum three-foot landscaped setback
at the base of each rockery or retaining wall." This requirement cannot be feasibly or reasonably met in two
isolated areas at the SW corner of the site, as described below.
Background
The subject site includes over 45 feet of fall from the high point near the NW corner along S. 348th Street to the SW
corner along 1st Ave S., and there is over 41 feet of fall alone along 1st Ave S. Given the FWRC code requirements
for retaining walls to be six feet maximum height and five feet setbacks between terraced walls, site requirements
for ADA pathways, and emergency vehicle access through the site, a significant area of the site is dedicated to
tiered retaining walls and landscaping.
At the SW corner of the site, there are several constraints near the access driveway, which is provided for both fire
and resident use, that limit the ability to meet all retaining wall requirements, and in particular the three feet of
planting at the base of each wall. Constraints include:
6-foot maximum height retaining walls with 5-foot spacing between walls results in a 3-tiered wall layout to
cover the grade delta from 1st Ave S. to the on-site parking area
5' landscape buffer required along the south property line
Any retaining structure is required to be provided entirely on site, (no portion of the wall can extend past
the property line)
No work to build the retaining walls can occur past the property line
SW Area 1 (South Property Line)
At the south property line, the project has 7 feet of space at the west end and approximately 5 feet at the east end
of wall to provide for the retaining wall construction, the wall itself, and landscaping to the back of curb. Providing
3 feet of landscaping at the toe of the wall is not feasible in all areas due to the site constraints mentioned above.
The project evaluated multiple retaining wall types for each location on site. The wall types evaluated for use in this
area include Mechanically Stabilized Earth (MSE), Rockery, and Cast-in-Place (CIP) Concrete. Each has advantages
and disadvantages for use in this area, which are discussed below.
MSE Wall: The MSE wall has the advantage of being relatively narrow (12" to 18") in plan view. However, the wall
requires geotextile fabric to extend behind the wall face, typically 6 to 10 feet for a 6-foot wall, to retain soil. The
MSE wall is not feasible here since excavation and access would be required onto the property south of the subject
site and there is not adequate space for the geotextile fabric on site. Therefore, MSE wall is not recommended.
CIP Concrete Wall: The CIP concrete wall has the advantage of being narrow (8" to 12") in plan view above
grade. However, the wall requires a significant footing below grade, placed on a flat grade. Given the constraints of
the adjacent property line, most or all of the wall footing would be required north of the wall face, opposite of
where the retained earth is located. This design would require a very large footing with multiple steps as grade
climbs with the driveway to the east from 1st Ave S. Additionally, the footing would be about 12" below grade,
which would not support landscaping growth. Therefore, the CIP concrete wall is not recommended.
Rockery: A rockery wall is typically 3 to 4 feet thick in plan view, and placed with a 6H:1V batter, resulting in 4 to
5 feet in plan required when complete. Additional space is required to install drainage material behind the wall. The
rockery can be placed in cut or fill conditions, and in the case of a cut wall condition at the subject site, with stable
soils, the excavations is anticipated to be contained within the subject property. The rockery is the recommended
wall type in this location.
If a rockery is utilized at the SW corner access drive, approximately 6 to 8 feet of horizontal space is required for
construction of the wall and 4 to 5 feet is required in plan view for the wall itself. The site layout can support this
design if allowed, but 3 feet of landscaping at the base of wall is not feasible. The provided landscaping space is
anticipated to be 1 to 2 feet wide.
SW Area 2 (Base of Tiered Walls)
SW Area 2 is the tiered walls that allow grade to climb north of the proposed driveway up to the proposed parking
area. The walls are likely to be CIP concrete or MSE walls given the space constraints and total grade that must be
climbed.
At least 3 feet of landscape area will be provided along approximately 90 feet of wall, extending from the west wall
chamfer near 1st Ave S. From this point eastward, the drive aisle widens for access to and from the gate. The wider
access allows cars trying to enter the site to turn around at the gate, as well as for fire access from the site towards
1st Ave S.
At the narrowest point, there is approximately 13 feet of space for two wall tiers that provide 10 feet of grade
change. These walls are separated by 5 feet of landscaped area and are approximately 2 feet wide each.
Additionally, room along the parking lot is provided for fence and/or guardrail. This requires 11 feet of space,
leaving approximately two feet of room at the base of wall for planting area.
Summary
The proposed project at 1st Ave S. and S. 348th Street utilized creative site planning and grading to meet FWRC
code requirements for retaining wall spacing, height, and landscaping across the site. Given the significant grade
drop that occurs along the 1st Ave S. frontage, there are limited areas that cannot reasonably meet the
requirement for 3 feet of landscaping at the base of the wall.
The project seeks relief from this requirement pursuant to FWRC 19.120.50. Please refer to the attached site plan
exhibits, which have been prepared in accordance with FWRC 19.120.50(2).
1ST AVE S