HomeMy WebLinkAboutSpringbrook Boardwalk Geotech Design Memo
1101 Fawcett Avenue, Suite 200
Tacoma, Washington 98402
253.383.4940
July 31, 2024
Moffat & Nichol
600 University Street, Suite 610
Seattle, Washington 98101
Attention: Matt Perry
Subject: Geotechnical Evaluation Services
Springbrook Trail Boardwalk
Renton, Washington
File No. 0693-094-00
Introduction and Project Understanding
This letter report provides our geotechnical engineering and design recommendations for the repair of the
Springbrook Trail Boardwalk in Renton, Washington. The boardwalk is located at 800 SW 27th Street
immediately east of Springbrook Creek and provides pedestrian access through the Springbrook Creek
Wetland and Habitat Mitigation Bank. The site is shown in the attached Vicinity Map, Figure 1. We
understand a fire occurred on Saturday, April 15, 2023 that resulted in the total loss of the northern-most
100 linear feet of the boardwalk structure to the south of where the trail intersects SW 27th Street. We
understand that the City of Renton (City) desires to replace the destroyed boardwalk with a similar structure.
The remaining (and destroyed) boardwalk is supported on a Diamond Pier® foundation system.
The purpose of our geotechnical engineering services is to provide input on foundation types for the
replacement structure based on existing and regional information and a site visit to observe current site
conditions. Site specific geotechnical subsurface explorations were not completed as part of the project.
Our services are being completed in accordance with our project proposal dated January 5, 2024 and
contract with Moffat & Nichol authorized June 10, 2024. This report includes our opinion on the suitability
of Diamond Pier or similar small diameter driven pile system and recommended soil parameters for use in
design of the foundation systems.
Moffat & Nichol | July 31, 2024 Page 2
File No. 0693-094-00
Site Conditions
GEOLOGY
Geologic conditions at the site were evaluated in part by reviewing the “Geologic Map of the Renton
Quadrangle, King County, Washington” (Mullineaux, D. R., 1965). Geological materials mapped at and
surrounding the site consisted of late Pleistocene to Holocene era lacustrine peat deposits (map unit Qlp).
Peat can include large amounts of organic materials as well as minor amounts of inorganic clay and silt.
The water contents may be as much as 10 times the dry weight. Peat deposits are described as being “very
poor” for foundation stability with low shear strength and bearing capacity and will settle excessively under
load. The areas in which peat is mapped has been found to have peat deposits consistently 3 or more feet
thick, though the maximum known thickness is approximately 23 feet. The next closest mapped unit is
Alluvium (Qaw), which is mapped west of the site near Oakesdale Avenue SW. Alluvium is chiefly mapped
as sand, silt and clay with channel gravels and peat lenses. The upper part is mostly clayey silt and fine
sand and locally peaty.
REVIEW OF EXISITING SITE GEOTECHNICAL INFORMATION
We reviewed “Geotechnical Evaluation and Design Recommendations; Diamond Pier Foundations
Springbrook Creek Boardwalk, Renton, Washington” prepared by Creative Engineering Options, Inc (CEO)
dated April 2, 2007. Shallow hand probes were performed in the project area as part of CEO’s services.
Conditions were noted to be a 6-inch-thick layer of “dense sod” predominately underlain by a shallow
stratum of very soft to soft, organic and plastic silt, possibly containing some peat and/or peat inclusions.
The depths of these soil deposits extended between 7.5 and 10 feet below ground surface. We interpret
the “dense sod” layer to be the desiccated upper layer of alluvium soil. CEO’s report recommended
Diamond Pier foundations be installed to 6.5 to 8.5 feet, with 6.5-foot-long pins proposed in the northern
segment of the boardwalk (current project site).
We also reviewed the “Geotechnical Baseline Report: I-405 Springbrook Creek Wetlands and Habitat
Mitigation Bank Project, Renton, Washington” prepared by Hart Crowser dated October 4, 2005. This report
encompasses three sites that are located to the southwest of the project site. Site “C” is closest to the
project site. Explorations found sandy gravel and sand to silt sand (fill) underlain by interbedded silt and
sand (alluvium deposits) in a loose to medium dense condition. The interbedded layers were found to be
approximately 11 feet thick.
CURRENT SITE CONDITIONS
GeoEngineers visited the site on June 20, 2024. We observed that the northern 100 feet of the destroyed
boardwalk alignment had been removed. The northern concrete abutment of the boardwalk remained in
place. Some, but not all, Diamond Pier heads remained, however, most of the remaining Diamond Pier
heads appeared badly damaged, either by the fire or from subsequent demolition work.
Subgrade conditions were evaluated visually and by using a ½-inch diameter steel probe rod. In the
northern 45 feet of the former boardwalk alignment, subgrade soils consisted of silty sand with gravel.
Probe depths in this area were on the order of 0 to 2 inches. In the southern 55 feet of the alignment, soils
appeared to consist of brown silty sand to sandy silt with noticeably increased moisture content. Probe
depths in this area ranged between 18 and 30 inches. We interpret site soils to be either fill or alluvial soils.
The surficial soil in the northern 45 feet of the alignment is interpreted to be in a desiccated condition, and
we anticipate the soil profile grades softer with depth within the upper 5 to 10 feet below grade. We did not
observe peat-like soils in the project alignment.
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File No. 0693-094-00
The foundations for the intact portions of the boardwalk south of the site were observed to be comprised
by Diamond Piers installed on 10-foot spacing, longitudinally in the direction of the boardwalk. Each
foundation “bent” are supported by a series of three piers spaced 3-foot on center, perpendicular to the
boardwalk. Each Diamond Pier is constructed with a 1 foot by 1 foot pier head supported by 4 pins driven
in at various batters. The remaining Diamond Pier heads in the area of the destroyed boardwalk were
observed to be in a similar configuration as that of the intact boardwalk.
Conclusions and Recommendations
SOIL PARAMETERS
Based on the conditions observed in the field and the reports reviewed above, we recommend the following
soil properties for design of the replacement foundation system. These parameters are based on the
existing information provided and may need to be revised during construction if there is a deviation in soil
conditions from those described above.
TABLE 1. SOIL DESIGN PARAMETERS
SOIL UNIT MOIST UNIT WEIGHT (PCF) SATURATED UNIT WEIGHT (PCF) FRICTION ANGLE (DEGREES)
Alluvium 95 33 25
FOUNDATION RECOMMENDATIONS
Recommendations for potential new foundation systems are presented in this section. Any new foundations
will need to consider the existing Diamond Peir heads and pins that currently remain. We recommend the
existing Diamond Pier heads and pins be removed if feasible. However, if the existing pins remain, new pins
or piles will need to be designed and constructed to avoid existing pins.
Diamond Pier Foundation
Based on our observations onsite and project understanding, the current Diamond Pier foundation system
is functioning as intended on the intact portions of the existing boardwalk. We further understand there
were no foundation issues with the destroyed boardwalk prior to the fire. In our opinion, a Diamond Pier
foundation system mimicking the previous design, and under similar design loads, will provide adequate
support for the boardwalk.
Alternative Foundations
The boardwalk repair may also be supported with an alternative small diameter pile foundation, such as
steel “pin” pipe piles. The piles would be driven close-ended into the denser layers within or underlaying
the alluvium. Typically, pin piles are installed to a specified driving resistance based on the size of pile and
the installation equipment utilized. We would anticipate 2- to 4-inch diameter pin piles can achieve an
allowable axial capacity between 4 and 20 kips per pile.
- Figure 1 - Vicinity Map
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Vicinity Map
Figure 1
Springbrook Trail Boardwalk
Renton, Washington
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Source(s):
• ESRI
Coordinate System: NAD 1983 UTM Zone 10N
Disclaimer: This figure was created for a specific purpose and project. Any use of this figure
for any other project or purpose shall be at the user's sole risk and without liability to GeoEngineers.
The locations of features shown may be approximate. GeoEngineers makes no warranty or
representation as to the accuracy, completeness, or suitability of the figure, or data contained
therein. The file containing this figure is a copy of a master document, the original of which is
retained by GeoEngineers and is the official document of record.
Appendix A
Report Limitations and Guidelines for Use
Moffat & Nichol| July 31, 2024 Page A1
Appendix A
Report Limitations and Guidelines for Use1
This appendix provides information to help you manage your risks with respect to the use of this report.
Report Use and Reliance
This report has been prepared for Moffat & Nichol. GeoEngineers structures its services to meet the specific
needs of its clients. No party other than Moffat & Nichol may rely on the product of our services unless we
agree to such reliance in advance and in writing. Within the limitations of the agreed scope of services for
the Project, and its schedule and budget, our services have been executed in accordance with our
Agreement with the Client dated June 10, 2024 and generally accepted geotechnical practices in this area
at the time this report was prepared. We do not authorize, and will not be responsible for, the use of this
report for any purposes or Projects other than those identified in this report.
If changes to the Project or property occur after the date of this report, GeoEngineers cannot be responsible
for any consequences of such changes in relation to this report unless we have been given the opportunity
to review our interpretations and recommendations in the context of such changes. Based on that review,
we can provide written modifications or confirmation, as appropriate.
Information Provided by Others
GeoEngineers has relied upon certain data or information provided or compiled by others in the
performance of our services. Although we use sources that we reasonably believe to be trustworthy,
GeoEngineers cannot warrant or guarantee the accuracy or completeness of information provided or
compiled by others.
Conditions Can Change
This report is based on conditions that existed at the time the study was performed. The findings and
conclusions of this report may be affected by the passage of time, by events such as construction on or
adjacent to the site, new information or technology that becomes available subsequent to the report date,
or by natural events such as floods, earthquakes, slope instability or groundwater fluctuations. If more
than a few months have passed since issuance of our report or work product, or if any of the described
events may have occurred, please contact GeoEngineers before applying this report for its intended
purpose so that we may evaluate whether changed conditions affect the continued reliability or applicability
of our conclusions and recommendations.
Professional Judgment
It is important to recognize that the geoscience practices (geotechnical engineering, geology and
environmental science) rely on professional judgment and opinion to a greater extent than other
engineering and natural science disciplines, where more precise and/or readily observable data may exist.
To help clients better understand how this difference pertains to its services, GeoEngineers includes these
explanatory “limitations” provisions in its reports. Please confer with GeoEngineers if you need to know
how these “Report Limitations and Guidelines for Use” apply to your Project or site.
1 Developed based on material provided by GBA, GeoProfessional Business Association; www.geoprofessional.org.