HomeMy WebLinkAbout2016_0708_Geotech_Design_Recommend_Sunset Pk Ph IT E C H N I C A L M E M O R A N D U M
CH2M HILL, INC. 1
Geotechnical Design Recommendations for Sunset
Park Phase I
PREPARED FORPREPARED FORPREPARED FORPREPARED FOR:::: Dean Koonts, Hough, Beck & Baird
Karla Kasick, P.E.
COPY TO:COPY TO:COPY TO:COPY TO: Karen Dawson P.E.
PREPARED BY:PREPARED BY:PREPARED BY:PREPARED BY: Menzer Pehlivan
DATE:DATE:DATE:DATE: July 8, 2016
PROJECT NUMBER:PROJECT NUMBER:PROJECT NUMBER:PROJECT NUMBER: 677149
REVISION NO.:REVISION NO.:REVISION NO.:REVISION NO.: 0
This technical memorandum is to supplement the Geotechnical Data and Recommendation Report for
the Renton Sunset Stormwater Retrofit/LID Project (CH2M 2012), and provides geotechnical design
assumptions and recommendations for the proposed foundations for the lightweight structures that
includes bathroom facilities and pergola that are going to be constructed as part of the Phase I of the
Master Plan for the Sunset Terrace Neighborhood Park (the “Project”) in Renton, Washington. The
recommendations have been made to support 30 percent design of the project. This document is a
deliverable under subtask 03.35.02 of the contract between CH2M and HBB Landscape Architecture.
Project Summary
The primary objective of the project is to provide a mix of spaces for the adjacent and surrounding
community: open lawn areas for play and events, structured play and picnic areas, environmental
facilities, restrooms, and convenient access points all arranged in a curving, informal layout that
balances open vistas, focal points, and pastoral aesthetic.
The park site was formerly residential housing that was recently demolished or will be demolished under
the Sunset Lane construction contract. The demolition is intended to remove building foundations and
light pole bases, but abandon utilities in-place. Harrington Avenue and Glenwood Avenue right of way
will also be vacated (between NE 10th and Sunset Lane NE) for construction of the new park (Figure 1).
The project will also include an extension of NE 10th Street west to Glenwood Avenue NE. The right-of-
way and curb line for Sunset Lane NE will be shifted to the north to allow for development on the south
side of the street. This new curb line will be the southern boundary of the park.
Improvements to the park will be completed in two phases because of funding limitations. The
improvements for the Phase I are summarized below.
Phase 1:
• Rough grading and seeding on west side of park, minimal landscaping (street trees) on east side
of park
• Install water line and sewer line for bathrooms and a water feature
GEOTECHNICAL DESIGN RECOMMENDATIONS FOR SUNSET PARK PHASE I
CH2M HILL, INC. 2
• Demolish or abandon existing utilities (overhead and buried) and undergrounding the electrical
and communication lines.
• Construct bathroom facilities
• Install a pergola
• Construct curb and gutter including temporary curb inside the park for area west of the
bathrooms
• Install irrigation systems
• Install electrical the restroom, and lightings (conduit only for lighting).
Phase 2 could add a gazebo, walkways, park equipment, water feature, and additional landscaping.
Limitations
This preliminary Geotechnical Design Memorandum has been prepared for the exclusive use of City of
Renton and the CH2M-led internal design team for specific application to the Phase I of Master Plan for
the Sunset Terrace Neighborhood Park Project. The memorandum was prepared in accordance with
generally accepted geotechnical engineering practice. No other warranty, expressed or implied, is made.
The conclusions in this preliminary memorandum are based on our understanding of the project at the
conceptual design phase. Calculations may be modified as design is developed, final recommendations
may change, and this document should be verified or updated to reflect updates made during final
design.
The preliminary recommendations in this memorandum are based on the subsurface conditions
interpreted from explorations described in this document. If conditions differing from these are
observed during subsequent explorations or during construction, the recommendations provided in this
document must be verified or revised in writing.
Subsurface Conditions
The subsurface conditions described in this document are based on explorations performed for the City
of Renton Sunset Stormwater Retrofit / LID project and documented in the Geotechnical Data and
Recommendations Report prepared by CH2M for the City of Renton (CH2M 2012).
Area GeologyArea GeologyArea GeologyArea Geology
The geologic units in the project vicinity, based on a Geologic Map (Washington State Department of
Natural Resources, 2012) are Vashon Glacial Outwash (Qgo) and Vashon Glacial Till (Qgt).
Vashon Glacial Outwash chiefly consists of stratified sand, gravel, and cobbles with minor silt and clay
interbeds deposited in delta, ice-contact, beach, and meltwater environments (Dragovich, 2002) with
generally moderate to high saturated hydraulic conductivity. Vashon Glacial Till is a highly compacted
mixture of clay, silt, sand, gravel, and boulders deposited by glacial ice (Dragovich, 2002).
NearNearNearNear----Surface Soil ConditionsSurface Soil ConditionsSurface Soil ConditionsSurface Soil Conditions
Surficial soils for the project study area were mapped by the United States Department of Agriculture
(USDA) Soil Conservation Service (SCS), now referred to as the Natural Resources Conservation Service
(NRCS). Typically, the NRCS maps depict conditions within 5 feet of the ground surface, and usually do
not indicate representative conditions at greater depths. The surficial soils mapped by NRCS (2012) in
the project area consist of Arents-Alderwood material (AmC), Ragnar-Indianola association (RdC), and
Urban land (Ur). Arents-Alderwood material consists of moderately drained material from basal till. The
material is classified with a very low to moderately low capability to transmit water (Ksat) with values
ranging from less than 0.01 to 0.06 in/hr. The typical profile is gravelly sandy loam to very gravelly sandy
GEOTECHNICAL DESIGN RECOMMENDATIONS FOR SUNSET PARK PHASE I
CH2M HILL, INC. 3
loam to a depth of 60 inches. Ragnar-Indianola association material consists of a combination of
somewhat excessively drained to well drained material formed from glacial outwash and glacial drift.
The material is classified with a high capability to transmit water (Ksat) with values ranging from 2 to 6
in/hr. The typical profile is fine sandy loam and loamy fine sand to a depth of approximately 30 inches
underlain by sand and loamy sand to 60 inches. Urban land consists of fill material placed for
development and can vary widely in composition, depending on it origin.
The surficial soils mapped by the NRCS are fairly consistent with the soils encountered during the field
explorations. Most of the borings to the west side of the project area, which is mapped mostly as Amc,
encountered till between 1 and 4.5 feet below the ground surface (bgs). The majority of the surficial
soils encountered throughout the project site contained primarily fine sand with estimated 15 to 20
percent material passing the number 200 sieve. Groundwater found within the project site was below
the surficial soils described in the NRCS.
General Subsurface ConditionsGeneral Subsurface ConditionsGeneral Subsurface ConditionsGeneral Subsurface Conditions
The subsurface explorations performed in the vicinity of the project site as a part of the 2012 subsurface
exploration program are B-6-12, B-8-12, B-10-12, TP-1-12, and TP-2-12 (Figure 2). The soil boring and
test pit logs associated with these explorations are provided in Appendix A of the Geotechnical Data and
Recommendation Report (CH2M 2012). Based on the subsurface explorations, the subsurface profile at
the project site consists of following sequence starting at the ground surface:
Fill/Outwash: The soil layer nearest to the ground surface was typically medium dense to dense silty
sand (SM) and silty sand with gravel (SM), most likely originating as fill or recessional outwash. This
soil unit typically contained 15 to 25 percent non-plastic fines. The gravel content varied from 0 to
20 percent. In boring B-6-12 and B-8-12, the Fill/Outwash unit was encountered between the
surface and approximately 3 to 4.5 feet below ground surface (bgs) with Standard Penetration Test
(SPT) N values ranging from 27 to 51. In boring B-10-12 and test pit TP-2_12 the Fill/Outwash unit
was encountered up to a depth of approximately 20 feet bgs; in the upper 10 feet SPT N values
ranged from 3 to 14 and in the lower 10 feet SPT N values ranged from 27 to 51.
Glacially Overconsolidated Till: Glacially overconsolidated till was encountered below the Fill/Outwash
soils at depths ranging between 3 and 20 feet bgs in the locations explored within the project area.
This layer usually consisted of very dense silty sand (SM) or silty sand with gravel (SM) with SPT N
values greater than 50 and non-plastic or low-plasticity fines content between 15 and 30 percent.
Although no specific instances of cobbles larger than 5 inches or boulders were logged in the test
pits, cobbles and boulders are commonly found in this geologic deposit and should be anticipated in
excavations at the site.
Ground water table at the project site was encountered at 15.5 and 20 feet bgs at the time of drilling
(June 2012) in B-8-12 and B-10-12, respectively. Ground water was not encountered at the time of
drilling in B-6-12.
Seismic ConditionsSeismic ConditionsSeismic ConditionsSeismic Conditions
The site is classified as Site Class D based on International Building Code (IBC) 2012 guidelines. The short
period and one-second spectral accelerations mapped at the project site from 2008 Unites States
Geologic Survey (USGS) seismic design. The seismic design parameters estimated for site class D
conditions and Risk category I/II/III at the project site are as follows:
Design, 5 % damped, spectral response acceleration parameter at short-period, SDS = 0.953 g
Design, 5 % damped, spectral response acceleration parameter at a period of 1 second, SD1 = 0.537 g
Design, 5% damped, peak ground acceleration, PGA = 0.38 g
Seismic Design Category, SDS = D
GEOTECHNICAL DESIGN RECOMMENDATIONS FOR SUNSET PARK PHASE I
CH2M HILL, INC. 4
Engineering Properties of SoilsEngineering Properties of SoilsEngineering Properties of SoilsEngineering Properties of Soils
Recommended design engineering properties of the soils anticipated to be encountered in the project
area are provided in Table 1. These properties were estimated from the Standard Penetration Test (SPT)
N-values and empirical correlations provided by WSDOT Geotechnical Design Manual, available
laboratory test results, previous practice, and engineering judgement.
Table Table Table Table 1111
Engineering Properties of Soils
Soil
Unit Layer Description Parameters (unit) Recommended Value
1 Fill/Outwash
Unit Weight, γ (pcf)
Cohesion, c’ (psf)
Friction Angle, φ’ (degree)
125
0
34
2 Glacially Overconsolidated Till
Unit Weight, γ (pcf)
Cohesion, c’ (psf)
Friction Angle, φ’ (degree)
135
200
40
Lateral earth pressure coefficients for use in designing pole foundations and walls less than 5 feet high
are provided in Table 2. A factor of safety of 1.5 has been applied to the passive resistance to limit soil
deformation needed to mobilize passive resistance. Active earth pressures should only be used if the
wall or pole is free to rotate at least 0.01 times H, where H is the depth of pole or wall embedment
below the ground surface.
The lateral pressure on walls or poles is a triangular distribution, calculated as:
σh = K * γ * z, where;
K is the active, at-rest, or passive earth pressure coefficient from Table 2
γ is the unit weight for Fill/Outwash soil from Table 1
z is the depth below finish grade or the top of the wall
For poles, the lateral earth pressure loading is applied over one pole diameter and the resisting pressure
is applied over three pole diameters. Passive resistance within 2 feet of the finished ground surface
should be ignored (i.e., the resisting lateral earth pressure diagram is a truncated triangle). Walls or
structures in occupied spaces (International Building Code controlled design) are not planned as a part
of the project, therefore lateral earth pressures under seismic conditions have not been provided. If the
project plan changes or exterior walls greater than 5 feet high are proposed, the geotechnical engineer
of record should be consulted for specific recommendations.
Table 2Table 2Table 2Table 2
Lateral Earth Pressure Coefficients for Pole and Wall Design
Parameter Value
Active earth pressure coefficient, Ka 0.28
At-rest earth pressure coefficient, Ko 0.44
Passive earth pressure coefficient, Kp 2.4
GEOTECHNICAL DESIGN RECOMMENDATIONS FOR SUNSET PARK PHASE I
CH2M HILL, INC. 5
Foundation Design Recommendations
Strip footings or rectangular pads with minimum width and length of 1.5 feet founded a minimum of 2
feet below finish grade and slabs on grade will be suitable foundation types for the lightweight
structures in the Sunset Terrace Neighborhood Park considering the subsurface conditions,
constructability, cost, and environmental impacts. The allowable bearing capacity for strip footings
founded on medium dense Fill/Outwash is estimated as 3000 psf. A base friction factor of 0.5 should be
used between the foundation and footing.
Construction Considerations
The recommended allowable bearing capacity provided above is based on the construction
recommendations provided in the subsections below. All earth material names can be referenced to
WSDOT Standard Specifications for Highways and Bridges (hereafter referred to as “Section” only),
Section 9-03. All geotextile material names can be referenced to Section 9-33. Maximum density is
defined by ASTM D1557.
SiteSiteSiteSite and Subgradeand Subgradeand Subgradeand Subgrade PreparationPreparationPreparationPreparation for for for for Strip FootingStrip FootingStrip FootingStrip Footingssss
The subgrade should be cleared of trees, shrubs, and any other vegetation; grubbed of stumps and large
roots; and stripped of topsoil and underlying soils that contain significant amounts of roots or other
organic matter. Rubble or debris larger than 3 inches in maximum dimension should be removed. The
top 6 inches should be scarified and recompacted to at least 95 percent of maximum density. A
geotechnical representative should observe the subgrade prior to recompaction to verify acceptable
conditions.
A geotechnical representative was not on site during demolition of the existing structures which
included removal of footings and backfill with compacted granular material. It may be necessary to
overexcavate and recompact some of these areas if compaction was not well controlled. The
geotechnical representative should carefully probe these areas and may request proof rolling with a
loaded truck or other piece of heavy equipment to verify that the areas will provide an appropriate firm,
unyielding surface appropriate for the use.
Use of underground storage tanks for heating oil is sometimes a concern during demolition of housing.
Though not included in this scope of work, we urge a records search to determine if oil was ever used to
heat the buildings. If the records indicate the potential for buried tanks that have not been removed, an
inadvertent discovery plan should be developed.
Backfill Backfill Backfill Backfill Under and Around FoundationsUnder and Around FoundationsUnder and Around FoundationsUnder and Around Foundations
A leveling course of crushed surfacing base course or top course at least 4 inches thick should be
compacted to at least 95 percent of maximum density beneath spread footings and slabs. Excavations
around footings should be backfilled with gravel borrow compacted in 6 inch maximum lifts to at least
95 of maximum density beneath paved areas and to at least 90 percent of maximum density in other
locations.
Unsuitable SoilsUnsuitable SoilsUnsuitable SoilsUnsuitable Soils
If loose material below the depth of scarification, fine-grained, organic, or oversized materials are
encountered, the geotechnical representative may direct overexcavation and replacement with gravel
borrow compacted to at least 95 percent maximum density. Wet, large, or deep areas of unsuitable
foundation materials may require replacement with a separation or reinforcement geotextile and ballast
at the direction of the geotechnical representative.
GEOTECHNICAL DESIGN RECOMMENDATIONS FOR SUNSET PARK PHASE I
CH2M HILL, INC. 6
Trench Trench Trench Trench Preparation and Preparation and Preparation and Preparation and BackfillBackfillBackfillBackfill
Remove loose and disturbed material and trim off high areas and ridges left by excavating bucket teeth
in the trench bottom. Remove debris and particles more than 4 inches in maximum dimension and
compact to provide a firm, non-yielding surface. The trench width should provide at least 12 inches of
clear space between the pipe barrel and trench walls or slope and at least 12 inches between pipes in
the same trench to allow for compaction.
Pipes and conduits should be bedded on a minimum 4 inches of bedding material. Bedding may be
imported or select on-site material from excavations with the following characteristics:
• Unfrozen, friable, and no clay balls, roots, or other organic material
• Clean sand (100 percent passing the No. 4 sieve) with less than 10 percent passing the No. 200
sieve, as determined in accordance with ASTM D1140
• Individual particles free of sharp edges (i.e., no recycled glass)
• Non-plastic fines as determined by ASTM D4318.
The bedding should be placed across the full width of the trench and graded compacted smooth. The
upper 1 to 2 inches should be loosened to provide a cushion before laying each section of pipe or
conduit.
After the pipe or conduit is uniformly supported along its length, backfill the pipe zone with bedding
material by placing material simultaneously in lifts on both sides of the pipe and between pipes and
conduit installed in the same trench. The first lift should be less than or equal to half of the pipe
diameter and subsequent lifts should be limited to 6 inches. Thoroughly tamp each lift, including area
under haunches, with handheld tamping bars supplemented by “walking in” and slicing material under
haunches with a shovel to ensure voids are completely filled before placing each succeeding lift. Do not
use power-driven impact compactors to compact pipe zone material. The pipe zone should extend at
least 6 inches above the crown of the pipe. Material between the pipe barrel and the trench should be
compacted to at least 90 percent of maximum density. Backfill above a horizontal projection of the pipe
should not be compacted until there is at least 3 feet of backfill above the pipe crown.
Trench backfill above the pipe zone should be well graded sand and gravel with a 3 inch maximum
particle size compacted per the requirements of the overlying land use, but to a minimum of 90 percent
of maximum density.
References
ASTM. Annual Book of ASTM Standards. American Society for Testing and Materials. Section Four –
Construction. Volume 04.08 – Soil and Rock (I): D 420- D 5779. Revisions issued annually.
CH2M (2012). Renton Sunset Stormwater Retrofit/ LID Project. Geotechnical Data and
Recommendations Report, prepared for City of Renton, August.
Dragovich, J.D, et al. 2002. Geologic Map of Washington – Northwest Quadrant. Washington State
Department of Natural Resources, Geologic Map GM-50.
International Building Code 2012.
NRCS 2007. Web Soil Survey. Natural Resources Conservation Service. Web Soil Survey Release 1.1,
accessed June 2016. http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx
Washington State Department of Transportation (2016). Standard Specifications for Road, Bridge, and
Municipal Construction. M41-10
United States Geologic Survey, 2008. U.S. Seismic Design Maps.
GEOTECHNICAL DESIGN RECOMMENDATIONS FOR SUNSET PARK PHASE I
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Figure 1. Sunset Terrace Neighborhood Park Phase I Plan
Figure 2. Boring Location Plan