HomeMy WebLinkAboutRS_GEOTECH_ADDENDUM_REPORT_210318_V2
13705 Bel-Red Road, Bellevue, Washington 98005
Phone 425/649-8757 · Fax 425/649-8758
December 28, 2020 G-4661
Hari Ghadia
4th Creek Meadows LLC
12505 Bel-Red Rd, Suite 212
Bellevue, WA 98005-2510
Send via: ghadia_hari@hotmail.com
Subject: ADDENDUM LETTER – RESPONSE TO CITY COMMENTS
PROPOSED 4TH CREEK MEADOWS DEVELOPMENT
4502 NE 4TH ST
RENTON, WASHINGTON
Ref: See end of letter
Dear Hari Ghadia:
We have been provided with preliminary plans with “redlined” City comments and asked to
provide responses by Mr. Martin Reimers of Concept Architecture. The pdf document provided
is 599 pages and a link was emailed on October, 26, 2020. On November 17, 2020 we were
provided with a link to another set of plans, documents and review comments having a total of
455 pages. We have reviewed both pdf documents.
The following letter has been prepared in order to document our review of the plans and provide
recommendations with regard to the “redlined” City comments. This letter shall serve as an
addendum to the referenced geotechnical report. We have been asked to specifically address
comments directed to the geotechnical engineer on plan sheets: C5.2 and C4.0.
Site and Project Description
At the time of our site investigation the subject site consisted of a developed parcel containing
one building which is located at the south side of the lot. The existing building is a 1-story
building with daylight basement which daylights toward the north. The property has an
approximate area of 0.55 acres. There is an ecology block retaining wall which is located at the
west side of the lot or at the adjacent west right-of-way which has an estimated height of around
12-feet and retains Bremerton Ave NE. The site topography includes a relative depression at the
north-central portion of the lot with moderate to steep relatively minor slopes at the north and
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south sides of the lot. In addition, we understand that there is underground stormwater piping
which transits the central portion of the lot with water flowing from east to west.
Based upon review of the referenced plans we understand that the proposed development is to
consist of removing the existing building and associated development and the construction of a
new 4-story commercial mixed-use building having one level of underground parking. A
detention tank is proposed at the north side of the building. The building is to have a finish floor
elevation of 410.33, presumably at the main floor level. The plans also indicate street/sidewalk
improvements at the frontage within the adjacent right-of-ways for NE 4th St and Bremerton Ave
NE.
We understand that the project proposes to remove a section of the existing underground
stormwater piping which crosses the site and allow stormwater to flow and backwater below the
building within a compensatory storage area since the downstream pipe is undersized for the
design flood event. Concrete columns will be used the support the north half of the building
within the compensatory storage area. The current infrastructure induced floodplain is at an
elevation of 394.87-feet. The plans indicate that the 100-year flood elevation for the project is at
394.81-feet and the calculated 100-year backwater is 399.5-feet. Based upon the plans the
lowest parking garage level located directly above the compensatory storage is at an elevation of
400.17-feet. An eight-foot wide and one-foot deep concrete lined channel will be constructed at
the flow line between the pipe discharge at the east and the pipe inlet at the west. The concrete
channel is to be 6-inch thickness of Class 3000 concrete constructed on top of a 2-inch thickness
of crushed surface top course, presumably placed on top of the existing grades. The concrete
channel will be reinforced with #4 bars at 16-inches on-center, both directions. Two-feet wide
ballast rock surfacing areas are proposed at both the sides of the flow channel. Based upon the
Technical Information Report the anticipated flow rate during flooding is 16.7 cfs. We note that
the concrete channel has a cross-sectional area of 18 square-feet.
The project plans indicate that the new footings are to be a minimum of 18-inches below
adjacent finish grades, this includes footings within the compensatory storage area.
The referenced project site plans do not note the existence or proposed removal of the existing
ecology block retaining wall at the Bremerton Ave NE right-of-way where street improvements
are proposed. The wall is shown on the survey. At the time of geotechnical report preparation
the wall was proposed to be removed.
City of Renton Geotechnically Related Comments
Below we list the City of Renton plan review comments which we have addressed, where
appropriate, in the Conclusions and Recommendations section of this letter.
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1. At several locations on the referenced plans, our reviewed geotechnical report (pdf
document) as well as the Oct. 6, 2020 letter from the City of Renton there is a
requirement that GEO Group Northwest review the plans and produce a plan review
letter approving of the plans, per the IBC requirement. We have performed a plan review
for this letter. However, we have provided additional recommendations herein, which we
recommend are implemented in the final plan set, prior to issuance of our plan review
letter.
2. The following comment by the City of Renton was placed on the referenced geotechnical
report and also on sheet C4.0 of the “redlined” plans by Harmsen (6-22-20):
The geotechnical and structural design of the 4th Dimension Building and the portion of
the building providing compensatory storage must be prepared in a way that will prevent
damage to the subject building and surrounding properties from seepage of water into the
subbase material and potential settlement and erosion that might result. Possible means
of accomplishing this include providing a watertight concrete base slab and/or a
waterproof liner below the open channel and compensatory storage area to prevent
infiltration of water into the subbase material. The project’s geotechnical and structural
engineers must provide the City with a correspondence bearing a professional
engineering stamp stating that in their opinion the project as designed will not result in
property damage caused by settlement or erosion of the subbase material.
3. The “redlined” (City Comments) plans by Harmsen Engineers C5.2 (6-22-20) has the
following City comment related to the road widening at project site frontage (NE 4th St
and Bremerton Ave NE:
Add Pavement Section: Provide pavement design per RMC 4-6-060.F.5 for arterials and
collectors. Include pavement design in geotechnical report.
Conclusions and Recommendations
General
The subject site is mapped as being overlain with glacially consolidated soils. However,
thicknesses of loose and unsuitable fills were encountered at some of the test pit locations.
Please see the referenced geotechnical report for the locations and depths of encountered
overlying apparent loose soils and fills. We note that these unsuitable soil conditions were found
to a depth of 9-feet below ground surface at the test pit TP-3. We recommend that the loose and
unsuitable fills are removed from all foundation subgrade areas and areas where the structural
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support is required for slabs, including at the compensatory storage concrete channel. Where the
excavations extend below the proposed bottom of footing or slabs these excavations are termed
over-excavated. In our geotechnical report over-excavated areas at footing and slab subgrade
locations are recommended to be filled with lean mix concrete or compacted structural fills
placed and compacted in accordance with the geotechnical report. Due to the proposed drainage
conditions, the creation of compensatory storage below the building, we revise our
recommendation for fill placement below footings and slab-on-grade floors within the building
footprint to consist of either clean crushed ballast rock (no fines, poorly graded), controlled
density fills (CDF) or lean-mix concrete. Over-excavation areas should be completely de-
watered prior to the placement of these fill materials. We also recommend that GEO Group
Northwest is retained to observe the condition of prepared foundation subgrade areas prior to the
placement of fills and then also monitor the placement of fills to verify compliance with our
recommendations.
Footings at Compensatory Storage Area
We have reviewed the referenced project plans. General specifications on the structural plans
indicate that the footings shall be founded at 18-inches below adjacent/finish grade. The footing
schedules indicate that the footing “depth” dimension (no relation to depth below grade) will
range from 18 to 24-inches. Accordingly, the current plans would allow for a portion of the
footings to presumably project above the adjacent grade elevation at some locations. In addition,
it appears that portions of the footings proposed at gridline F will be located at the compensatory
storage channel location.
The creation of a compensatory storage area for drainage below the building does present risks to
the building from erosion and footing subgrade damage. For these reasons we recommend risk
mitigation include the following and that these items are illustrated on the plans:
1. The over-excavated slab and footing areas as noted above are recommended to be filled
with clean crushed ballast rock (no fines, poorly graded), controlled density fills (CDF)
or lean-mix concrete. It is recommended that over-excavated trench areas which are to
be filled with lean-mix concrete or CDF have dimensions which are equal to or wider
than the proposed footings or slab areas which they support. If clean crushed rock is used
for backfill at over-excavated footing subgrades then the bottom of the over-excavated
trench should be widened as necessary to insure that an approved fill prism extends
downward from the outside/bottom edge of the footings at a 1H:1V imaginary plane.
2. It is recommended that the bottom of all footings within or adjacent to the compensatory
storage area have a depth of at least 24-inches below the finish adjacent grade in order to
mitigate erosion risks. In addition, we recommend that at all areas within the
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compensatory storage area except for the concrete channel the finish grade should be
overlain with a minimum 8-inch thickness of clean crushed ballast rock (2-inch clean). If
vehicular traffic (for maintenance) is ever anticipated at the grades within the
compensatory storage area than it is recommended that the ballast rock is placed on top
of a layer of filter fabric.
3. Where the plans indicate that new building footings intersect the compensatory storage
concrete channel we recommend that the footing is deepened as necessary to maintain 1-
foot clearance from the bottom of the concrete channel to the top of the footing. Fills
placed below the channel are recommended to be primarily clean crushed ballast rock
although the designer may use filter fabric separation to place a top-course (crushed rock
minus) as currently specified.
Response to City Comment #2
It is the opinion of GEO Group Northwest that provided the project foundations are bearing on
the underlying competent site soils or fills, as specified herein, which are placed on top of the
competent and approved subgrade soils then the risk of soil related settlements at the building is
minimized and mitigated per our geotechnical report. The risk of erosion at foundation elements
can be mitigated at the compensatory storage area by properly implementing the
recommendations presented herein. Additional mitigation regarding these risks can be
implemented by constructing a waterproof slab or installing a waterproof liner as suggested by
the reviewer. If a waterproof liner is designed to be installed we recommend that it is properly
protected from damage due to ballast rock fill placement.
Pavement Recommendations and Design – Site Frontage
The following recommendations and the pavement design attached as Appendix A – Pavement
Design have been prepared to address the City of Renton comment #3 noted above.
We understand that the project proposes to construct some new pavement at the adjacent NE 4th
Street and Bremerton Ave NE right-of-ways where the street widening fronting the subject site is
proposed. We recommend that new asphalt pavements are constructed on firm, unyielding
medium dense to dense site soils or compacted structural fills meeting the following compaction
(relative density) standards per ASTM-D-1557 (modified Proctor):
1. 90% or higher relative density for all depths of greater than 12-inches from the pavement
surface;
2. 95% or higher relative density for all depths of less than 12-inches from the pavement
surface.
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We recommend that GEO Group Northwest is retained to observe the condition of excavated
pavement subgrade areas and the placement of compacted structural fills at such areas.
Based upon the traffic counts noted in the referenced Gibson Traffic Consultants study (July
2019) GEO Group Northwest has performed calculations in support of pavement section design
for the street widening areas. Our calculations and supporting documentation are attached as
Appendix A – Pavement Design. Accordingly, we recommend the following minimum
pavement sections:
Site Frontage at South (NE 4th St) Site Frontage at West (Bremerton)
Asphalt (in) 5 4
Crushed Surfacing (in) 8 6
Existing Ecology Block Retaining Wall
We note that the existing 12-foot tall ecology block retaining wall at or adjacent to the west
property line is not labeled on the current site plan or grading plans. At the time of our
geotechnical report preparation the plans indicated the wall would be removed and presumably
the area would be filled as necessary to match the proposed finish grades at the building and at
the right-of-way. The referenced current plan set suggests that the Bremerton Ave NE roadway
widening will occur at the location of the existing retaining wall. So, if the wall is left in place
the roadway will bear on top of the existing wall. We do not recommend constructing a roadway
on top of the existing retaining wall. We did not find details or specifications in the referenced
plans clarifying the removal process for the existing block wall. We recommend that the
designer clarify the plans to indicate the existing wall removal, temporary excavation
requirements and structural fill placement requirements at the existing wall location.
We recommend that GEO Group Northwest be retained to perform a final review of the project
plans prior to permit issuance in order to verify that our recommendations have been properly
incorporated into the project construction drawings. We also recommend that we are retained to
provide geotechnical construction monitoring services for the project in order to verify that our
recommendations are properly impacted and to insure that appropriate revisions can be made if
site conditions are found to vary from our subsurface investigation.
We appreciate the opportunity to provide geotechnical consulting regarding the proposed
development. Please contact us if there are any questions or concerns.
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Sincerely,
GEO GROUP NORTHWEST, INC.
Adam Gaston
Project Engineer
William Chang, P.E.
Principal
Attachment: Appendix A – Pavement Design
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REFERENCES
Plans:“4th Dimension Mixed Use”, Concept Architecture, 6-1-2020, Sheets: A0.1, A0.2, A0.2.1,
A0.2.2, A0.2.3, A0.2.4, A1.0, A2.1, A2.2, A2.3, A2.4, A2.5, A2.6, A2.7, A2.8, A2.9,
A2.10, A2.11, A3.1, A3.2, A3.3, A3.3B, A3.3C, A3.5, A3.6, A3.7, A4.1, A4.2, A4.3,
A4.4, A5.1, A5.2, A5.3, A6.1, A7.1, A7.2, A8.1, A8.2, A8.3, A9.1, A9.2, A9.3, A9.4,
A9.5, A9.6.
“4th Dimension Mixed Use, Renton, WA”, Harmsen Engineers Surveyors, 3-20-20,
Sheets C1.0, C2.0, C3.0, C3.1, C4.0, C4.1, C4.2, C4.3, C4.4, C4.5, C5.0, C5.1, C5.2,
C6.0.
“4th Dimension Mixed Use”, Coffman Engineers, 3-27-20, Sheets S1.1, S1.2, S1.3, S2.1,
S2.2, S2.3, S2.3a, S2.4, S2.5, S2.6, S5.1, S5.2, S5.3, S5.4, S5.5, S5.6, S5.7, S5.8, S5.11,
S5.12, S5.13, S5.14, S5.15, SA1.
“4th Dimension Mixed Use”, Origin Design Group, 3-4-20, Sheets L-1, L-2, L-3, L-4, L-
5.
“4th Dimension Mixed Use”, Berona Engineers, 6-04-20, Sheets: M0.0, M0.1, M2.1,
M2.2, M2.3, M2.4, M2.5, M2.6, M3.0, P0.0, P0.1, P2.0, P2.1, P2.2, P2.3, P2.4, P2.5,
P2.6, P3.0, P4.0.
“4th Dimension Mixed Use”, Cross Engineers, 8-9-2018, Sheets E0.0, E1.0, E2.0, E2.1,
E2.2, E2.3, E2.4, E2.5, E3.0, E4.0, E4.1, E5.0, E5.1, E6.0,
“Geotechnical Report with Infiltration Evaluation, 4502 NE 4th St, Renton, Washington”, GEO
Group Northwest, May 14, 2014.
“Building Permit Structural Plan Review – First Submittal, 4502 NE 4th Street (B20003319)”,
City of Renton, Oct. 6, 2020.
“4th Dimension Building Traffic Impact Analysis”, Gibson Traffic Consultants, GTC #18-102,
July 2019.
“Technical Memorandum, 4th Dimension Renton, Stream and Wetland Reconnaissance”,
Raedeke Associates, Project No: 2019-052-001, June 25, 2019.
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“Technical Information Report for the 4th Dimension Mixed Use Building, Renton,
Washington”, Harmsen, March 13, 2020.
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APPENDIX A
PAVEMENT DESIGN
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Pavement Design Calculations G-4661
Project: 4th Creek Meadows Page 1 of 4
Address: 4502 NE 4th St, Renton, WA
Traffic Counts (Daily):
NE 4th St @ E side of Int.Eastbound:13909
Westbound: 14822
Bremerton Ave NE @ N side of int.Northbound:727
Southbound:681.0
Per Gibson Traffic Consultants anticipated daily trips from completed project:277
*Assume average daily traffic in one direction adjacent to site in 2020 (2.5% growth rate) with project added trips:
NE 4th St westbound:15665
One Lane (1/2):7833 Site Frontage at South
Bremerton Ave NE northbound:1042 Site Frontage at West
7833 x 0.045 x 0.4=140.994 ESAL/Day 1042 x 0.045 x 0.4=18.756 ESAL/Day
7833 x 0.005 x 1.0=5.21 ESAL/Day 1042 x 0.005 x 1.0=5.21 ESAL/Day
146 ESAL/Day 24 ESAL/Day
53364 ESAL/Year 8748 ESAL/Year
Growth (%): 2.5
Design Life (yr): 20
Growth Factor: 25.544658
1363177 ESAL/design life 223454 ESAL/design life
12/22/20
Site Frontage at South Site Frontage at West
GEO GROUP NORTHWEST, INC.
Intro/Assumptions/References:
Require pavement thickness design for road widening fronting the subject site at the north side (northern-most westbound)
lane/shoulder of NE 4th St and at the east side (northbound) lane/shoulder at Bremerton Ave NE. Per plan new flexible
pavement width at NE 4th St is 5-feet. At Bremerton Ave NE the new pavement width is 11-feet.
Assume pavement design lifetime of 20 years and a 2.5% annual growth rate in traffic counts.
"4th Dimension Building Traffic Analysis", Gibson Traffic Consultants, July 2019 cites traffic count data from IDAX July 18,
2018 which is used as basis for the design.
Soils are assumed based upon "Geotechnical Report with Infiltration Evaluation, Proposed Development, 4502 NE 4th St,
Heavy Traffic Count Review:
From Gibson cited data. Percentage "heavy" units is 1% of total traffic at NE 4th St and 2% of total traffic at Bremerton Ave NE.
FHWA Classification System:
Assume 95% of vehicles are cars and other negligable traffic loads. 5% of vehicles are heavy with 4.5% single units and 0.5%
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Pavement Design Calculations G-4661
Project: 4th Creek Meadows Page 2 of 4
Address: 4502 NE 4th St, Renton, WA 12/22/20
1. W18 [Accumulated ESALs]1,363,177
Zr -1.28 ZR
Std Dev 0.45 S
ΔPSI 2.20 DPSI
2. Subgrade M[r]9600 psi
Surface mix Base mix P.A.B. subbase
a[i]0.44 0.34 0.14 0.11
D[i], inches 5.00 0.00 8.00 0.00 inches
m[i]1.00 1.00 1.00
3. Reliability, %90 R
4. Initial and terminal serviceability Po Pt
ΔPSI 4.20 2.00
Provided SN 3.32
Required SN (Solver will fill in)3.19 Adequate
log10(W18) =6.13 left side
6.13 right side
target cell 0.00
Site Frontage At South
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Pavement Design Calculations G-4661
Project: 4th Creek Meadows Page 3 of 4
Address: 4502 NE 4th St, Renton, WA 12/22/20
1. W18 [Accumulated ESALs]223,454
Zr -1.28 ZR
Std Dev 0.45 S
ΔPSI 2.20 DPSI
2. Subgrade M[r]9600 psi
Surface mix Base mix P.A.B. subbase
a[i]0.44 0.34 0.14 0.11
D[i], inches 4.00 0.00 6.00 0.00 inches
m[i]1.00 1.00 1.00
3. Reliability, %90 R
4. Initial and terminal serviceability Po Pt
ΔPSI 4.20 2.00
Provided SN 2.60
Required SN (Solver will fill in)2.42 Adequate
log10(W18) =5.35 left side
5.35 right side
target cell 0.00
Site Frontage At West
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Pavement Design Calculations G-4661
Project: 4th Creek Meadows Page 4 of 4
Address: 4502 NE 4th St, Renton, WA
AC - Asphaltic Concrete (Class B) (in):
Crushed Surfacing (in):
12/22/20
Site Frontage at South (NE 4th)
5
8
Site Frontage at West (Bremerton)
4
6
Summary:
For the proposed street widening, pavement design per AASHTO 1993 Empirical Method indicates that for the anticipated
design lifetime minimum flexible pavement thicknesses are as follows:
Checked By:
William Chang, P.E.
Principal
GEO GROUP NORTHWEST, INC.
Prepared By:
Adam Gaston
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