HomeMy WebLinkAboutRS_G-5204 GeoTechnical Evauation - 212 S. 20th Place, Renton - Karen Lang_V1
13705 Bel-Red Road · Bellevue, Washington 98005
Phone 425/649-8757 · Fax 425/649-8758
October 5, 2020 Project No. G-5204
City of Renton: Building Division
Attn: Alan D. Findlay, Building Plan Reviewer
1055 - S Grady Way
Renton, WA 98057
Subject: Geotechnical Evaluation
Backyard Deck
212 – S 20th Place
Renton, Washington
Reference: Building Permit Structural Plan Review – Prescreen Submittal
Addressed to Ms. Karen Lang, Dated August 14th, 2020.
Dear Mr. Findlay,
GEO Group Northwest, Inc., has reviewed the above-referenced letter addressed to Ms. Karen
Lang on August 14th, 2020. Our findings related to the deck foundation design and geotechnical
comments presented in the prescreen submittal can be found in the following sections of this
letter. The deck designer, Z Dawitt, will address the structural and architectural comments of the
above-referenced letter.
Deck Foundation
The foundation is comprised of 33, 10-inch diameter, cylindrical concrete footings extending
4½-feet into the ground. The footings are bedded on 6 inches of compacted 5/8-minus crushed
rock underlain by dense, native site soils. The concrete footings extend approximately 4 feet
below the ground surface. 6x6 wooden supports are attached to the footings and connected to
the girder beams via clip attachments and joints. The frost depth for the local area is 12 inches.
October 5, 2020 G-5204
Alan D. Findlay, Building Plan Reviewer | City of Renton: Building Division Page 2
GEO Group Northwest, Inc.
Settlement
During our site investigation on July 22nd, 2020, we took laser-level measurements of the deck to
check for signs of settlement. No measurable amount of settlement was observed. Future
settlement or differential settlement across the deck width is estimated to be equal to or less than
1/4-inch.
Slope Stability
It is our understanding that the soils on the site consist of dense soil derived from weathered
sandstone from the Renton Formation (Tr). We consider the presence of shallow bedrock a
mitigating factor in regards to stability of the slope at the northern extent of the site. It is our
interpretation that the slope is comprised of bedrock, which is evidenced by local geologic
mapping, and because the slope is currently at inclinations steeper than a soils’ natural angle of
repose. In our opinion, no additional mitigation is required for slope stability or deck support.
Soil Bearing Pressure
The dense soils on the site are underlain by shallow bedrock. With the footings at a depth of 4
feet, we have recommended an allowable bearing pressure of 5,000 psf (pounds per square foot).
The existing deck has 33 footings, and assuming a deck load of 60 psf, each deck footing has a
bearing pressure of approximately 1,600 psf.
CLOSING
In our opinion, the backyard deck at 212 – S 20th Place, in Renton, Washington, is well
supported from a geotechnical standpoint. It is our opinion that the stability of the slope at the
northern extent of the site is not adversely affected by construction of the backyard deck.
Please do not hesitate to contact us if you have any questions regarding this report.
October 5, 2020 G-5204
Alan D. Findlay, Building Plan Reviewer | City of Renton: Building Division Page 3
GEO Group Northwest, Inc.
Sincerely,
GEO Group Northwest, Inc.
Garrett Dean, G.I.T. William Chang, P.E.
Staff Engineering Geologist Principal Engineer
Attachments:
Plate 1 – Deck Foundation Plan