HomeMy WebLinkAboutRS_Geotech_Coal_Mine_Assessment_240418_v1January 23, 2023
Gilroy Rental, LLC
1316 South 10th Street
Renton, WA 98057
(253)310-1718
Geotechnical Report
Proposed Addition to Single-Family Residence
& Detached Accessory Dwelling Unit
1318 South 7th Street
Renton, Washington
PN: 722140-0287
Doc ID: Gilroy.S7thSt.RG
INTRODUCTION
This Geotechnical Report summarizes our assessment of coal mine hazards and provides
mitigation recommendations for a proposed addition to a single-family residence and detached
accessory dwelling unit at 1318 South 7th Street in Renton, Washington. The approximate site
location is shown on the Site Location Map, Figure 1.
Our understanding of the project is based on our discussions with you, our December 12,
2023 site visit, our understanding of Renton Municipal Code Title IV Development Regulations, and
our experience in King County and the Renton area with residential projects. The site is developed
as a single-family residence in an existing residential neighborhood. An 8-foot addition to the north
side of the existing residence and a future detached accessory dwelling unit (DADU) in the northwest
corner is proposed at the site. The site is located above the historic mine workings of the Renton
Coal Mine and is mapped by the City of Renton sensitive areas as a “high” coal mine hazard area.
The City of Renton coal mine hazard mapping overlayed on the proposed site plan for the addition is
shown on the attached Site Plan, Figure 2.
SCOPE
The purpose of our services is to evaluate the site conditions as a basis for assessing coal
mine hazards on and adjacent to the site. Specifically, the scope of services for this project included
the following:
1.Performing a data search for existing available geologic information (USGS and USDA);
2.Completing a search of the historic coal mine maps available from the Washington
Department of Natural Resources (WA DNR) and analyze the maps for mine workings
(counters, gangways, tunnels) within 300 feet (horizontal and vertical) from the addition to
the existing residence and future DADU;
3.Completing a site reconnaissance to search for indicators of mine hazards at the site; and,
4.Preparing this Geotechnical Report in accordance with the City of Renton Municipal Code,
summarizing our findings.
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Our scope of work was originally outlined in our Proposal for Services dated November 27,
2023. We received written authorization to proceed with our scope of work on November 28, 2023.
SITE CONDITIONS
Surface Conditions
As stated, the site is a single tax parcel located at 1318 South 7th Street in the City of Renton,
Washington. According to City of Renton COR MAPS, the site measures approximately 67 feet wide
(east to west) by approximately 121 feet long (north to south) and encompasses about 0.2 acres.
The site is bounded by existing residential development to the north and west, by South 7th Street to
the south and Grand Avenue South to the east.
Based on 2-foot elevation contours provided by the City of Renton COR MAPS and our field
observations, the topography of the parcel and surrounding area generally slopes down to the west.
The slope on site slopes down to the west at approximately 8 percent over a vertical relief of 48
vertical feet. The vertical relief across the site is on the order of about 55 feet.
Vegetation across the site is ornamental grass and landscaping shrubs and perennials. No
surface water or erosion features were observed at the time of our site visit. The site topography
and existing configuration are shown on the attached Figure 3.
Site Soils
The USDA Natural Resource Conservation Service (NRCS) Web Soil Survey for the King
County Area (WA633) maps the soil at the site as Arents, Everett material (An). An excerpt of the
NRCS mapping that covers the area of interest is attached as Figure 4.
•Arents, Everett material (An): This soil is derived from basal till, forms on slopes of 0 to 5
percent, has a “slight” erosion hazard when exposed, and is included in hydrologic soils
group A.
Site Geology
The Geologic Map of the Renton quadrangle, King County, Washington by Mullineaux (1965)
maps the site and site vicinity as being underlain by Renton formation (Tr), Recessional stratified
drift – kame deposits (Qik), Recessional stratified drift – kame terrace deposits (Qit), and ground
moraine deposits (Qgt). Detailed descriptions of the above listed geologic units are included below.
An excerpt of the geologic map for the area is included as Figure 5.
•Recessional stratified drift – kame deposits (Qik): Kame deposits typically consist of poorly to
moderately well stratified mixtures of sand and gravel, pebbly sand, sand, and cobble gravel
deposited by fluvial processes in contact with the continental ice sheet. Kame deposits
generally slumped and collapsed as the ice receded causing mixing of the glacial sediment
and disturbance of the original fluvial stratification. As such, the kame deposits are
considered normally consolidated and generally have moderate strength and compressibility
characteristics when undisturbed. The ice contact deposits are mapped to underlie the level
to slightly sloping southeast portion of the site.
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•Recessional stratified drift – kame terrace deposits (Qit): Recessional stratified drift typically
consists of a well graded, lightly stratified mixture of sand and gravel that may locally contain
silt and clay. This soil was typically deposited by meltwater emanating from channels and
plains during the retreat (melting) of the ice sheet associated with the Vashon Stade of the
Fraser Glaciation. Kame terrace deposits are the result of scattered terraces that formed at
the contact of the ice sheet and valley walls, with the terraces collapsing during the retreat of
the ice sheet. The recessional stratified kame terrace deposits are typically encountered in a
loose to medium dense condition, considered normally consolidated, and generally exhibit
moderate strength and low compressibility characteristics when undisturbed.
•Ground moraine deposits (Qgt): The glacial till sediments were deposited during the Vashon
Stade of the Fraser Glaciation, some 12,000 to 15,000 years ago. Glacial till typically consists of
a heterogeneous mixture of clay, silt, sand, and gravel deposited at the base of the continental
ice mass and was subsequently over-ridden. Accordingly, these deposits are considered to be
over-consolidated and offer high strength and low compressibility characteristics, when
undisturbed. The infiltration potential of glacial till is generally limited given the density and
fines content of the deposit.
•Renton Formation (Tr): The Renton Formation is the upper member of the late Eocene age
Puget Group and consists of sandstone to siltstone with interbeds of coal and carbonaceous
shale.
Bedrock Geology of the Renton Area
The Renton and Tukwila Formations outcrop in the Renton area, with these formations part of
the stratigraphy of the Puget Group. The historic coal mining in Western Washington was completed
within the strata of the Puget Group. In general, the group consists of yellow, fine-grained sandstone
and very fine arenaceous shales interbedded with beds of carbonaceous shale and coal. The group
extends to occupy a large part of the Puget Sound basin and extends to the western flank of the
Cascade Mountain Range. The Puget Group overlies rocks from the Late Cretaceous age.
Structural Geology
The Renton formation is regionally folded in a two anticline and one syncline sequence. The
western limbs of the folds are slightly dipping while the eastern limbs dip at 55 degrees or greater.
There is a normal fault on the far eastern limb of the second anticline, with displacement measured
at about 20 feet.
Sequence Stratigraphy of the Coal Bearing Strata in Renton
According to The Coal Fields of King County (Evans, 1912), coal was discovered in the Renton
area in 1873. There were three economically feasible coal beds mined in the Renton area, herein
after referred to Bed #1, Bed #2, and Bed #3. Bed #1 was stratigraphically the highest of the three,
with Bed #3 the lowest. The overburden strata recorded on a Cedar River outcrop was 165 feet of
conglomerate and 335 feet of concealed beds of the Puget Group. Underneath the concealed beds
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is Bed #1, with a thickness of 16 feet and 11.5 inches. 110 feet of sandstone and shales separate
Bed #1 and Bed #2, with Bed #2 thickness recorded as 15 feet and 4 inches 72 feet of sandstone
and shales separated Bed #2 and Bed #3, with Bed #3 recorded as 11 feet and 5 inches thick.
Workings of the Renton Coal Mine
Based on data provided in The Coal Fields of King County (Evans, 1912), portions of the
abandoned workings of the Renton Coal Mine are located beneath the project site. The mine
opened in 1874, closed amid labor disputes in 1886, and re-opened by the Renton Cooperative Coal
Company in 1895. In 1901 the mine was purchased by the Seattle Electric Company and was
operated by Seattle Electric until its closure in 1918.
In the early years, Bed #2 was opened to the east of the project site on the Cedar River valley
side. For the workings underneath the project site, the Renton Bed #3 was the principal bed mined,
with rock tunnels connecting the workings of Bed #2 to the workings of Bed #3. The main slope on
Bed #3 was driven at a roughly 11-to-12-degree downdip from the toe of the valley slope located
about 1,400 horizontal feet slightly southwest of the project site. From the main slope, 11 levels
with gangways were driven roughly horizontal from the slope to the north and south directions on
Bed #3. The gangways acted as haulage tunnels to the main slope and the coal ultimately pulled up
on the main slope to the mine entrance using both mechanical and animal drawn methods. The
northern workings of the Renton Coal Mine as recorded upon its closure are attached as the Worked
Out North Section of Renton Coal Mine, Figure 6.
For removing the coal, breast and pillar mining techniques were used (Evans, 1912). Breast
and pillar mining is a variant of room and pillar mining. As defined, “room and pillar mining is a
system in which mining material is extracted across a horizontal plane to create horizontally arrayed
chambers and columns.” Most of the coal was “worked out” on the gangways, with a 95 percent
extraction success recorded using the breast and pillar mining techniques in the Renton Coal Mine
(Evans, 1912).
The project site is located directly above the 2nd level gangway and breast and pillar workings
of the Renton Coal Mine. By referencing the horizontal length measurements for the main slope
and the recorded average dip of the slope, we estimated that the 2nd level gangway was driven
horizontally from the main slope with an estimated vertical depth of 219 feet from the project site to
the 2nd level gangway. Cross Section A-A’ shows the estimated vertical depth of the workings of Bed
#3 to the existing surface grade as is shown on the attached Figure 6. The cross section line A-A’
that crosses the mine workings is shown on the attached Figure 7.
CONCLUSIONS AND RECOMMENDATIONS
The site is located per the City of Renton Code in a “high” mine hazard area because of the
estimated vertical depth to the mine workings from the project site. The greatest risk from the mine
workings underneath the project site is from mine subsidence. Mine subsidence is defined as
“lateral or vertical ground movement caused by a failure initiated at the mine level.” Based on the
results of our data review, site reconnaissance, and our experience in the area, it is our opinion that
the proposed addition to the residence and DADU is at no greater risk to subsidence related to the
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mine workings than the surrounding residential development. Below we also recommend practical
mitigation measures for construction of the addition and future DADU.
Coal Mine Hazards
The City of Renton Title IV Chapter 3 Section 4-3-050G for Development Standards in a
Critical Area defines a coal mine hazard based on the following criteria.
i.Low Coal Mine Hazards (CL): Areas with no known min workings and no predicted
subsidence. While no mines are known in these areas, undocumented mining is
known to have occurred.
ii.Medium Coal Mine Hazards (CM): Areas where mine workings are deeper than two
hundred feet (200’) for steeply dipping seams, or deeper than fifteen (15) times the
thickness of the seam or workings for gently dipping seams. These areas may be
affected by subsidence.
iii.High Coal Mine Hazard (CH): Areas with abandoned and improperly sealed mine
openings and areas underlain by mine workings shallower than two hundred feet
(200’) in depth for steeply dipping seams, or shallower than fifteen (15) times the
thickness of the seam or workings for gently dipping seams. These areas may be
affected by collapse or other subsidence.
The site meets the definition of a high coal mine hazard (CH), as we agree with the City of
Renton mapping for coal mine hazards that covers the site location. Residential use within the High
Coal Mine Hazard Area should be allowed provided that the following mitigations for design and
construction are implemented:
1.Use of rigid foundations (conventional reinforced concrete spread footings) supporting a
flexible superstructure (wood‐frame).
2.Small, square, or nearly square‐shaped building pads should be favored over large,
irregularly shaped building pads.
3.Crawl‐space construction rather than slab‐on‐grade. However, slab‐on‐grade may be used in
garage and driveway areas.
4.Buildings should be constructed such that they are easily releveled.
5.The edges of foundations should be backfilled with loose soil or other compressible material
to allow for potential ground compression.
6.Underground utilities should be designed with flexible and/or telescopic couplings or fittings.
7.Utilities that depend on gravity for flow (sewers and storm drain) should be designed to
compensate for the potential for ground subsidence.
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Potential for Subsidence
Where coal is removed by the room and pillar extraction method, as was done in the Renton
Coal Mine using a variant of this method, the ability to predict subsidence is difficult. This is
particularly true where there is irregular room development, nonuniform barrier pillars, and poor
definition of panels (Lee and Abel, 1983). Under these circumstances, as is the case in the Renton
Coal Mine, it is usually impossible to predict the time, magnitude, or occurrence of subsidence.
Subsidence for room and pillar mining may be erratic, intermittent, and long delayed (F. T. Lee and J.
F. Abel, Jr., 1983). This is especially true for deeper coal extraction relative to the surface elevation.
Subsidence is better understood and therefore easier to predict in longwall mining.
Where bedrock overburden is less than 164 feet (50 meters) thick, smaller sinkholes features
tend to predominate. If greater than this depth, subsidence tends to be more trough like (F. T. Lee
and J. F. Abel, Jr., 1983). Based on our interpretation of the mapping for the Renton Coal Mine, we
would anticipate any potential subsidence would be trough like.
During our site visit on December 13, 2023, we observed no evidence of past or developing
subsidence related to a deeper mine collapse. No depressions or evidence of subsidence related to
settlement of the existing residence was observed at the time of our visit. Nor were indications of
deeper mine failure related subsidence observed in the available LiDAR or topographic data for the
site and surrounding area. An excerpt of a Digital Terrain Model Hillshade from the most recent
Lidar that covers the site is attached as Figure 8.
Therefore, it is our opinion that the risk of subsidence at or near to the proposed addition is
no greater than the surrounding residential development. Additionally, the construction of the
proposed addition is unlikely to alter the potential for subsidence on the subject or adjacent
properties. Subsidence related to historic (greater than 100 years ago) room and pillar mining
typically require reactive mitigation measures in lieu of proactive mitigation measures without
investigative drilling into the mine workings.
LIMITATIONS
We have prepared this report for use by Gilroy Rentals, LLC and other members of the design
team for use in the design of a portion of this project. The data used in preparing this report and this
report should be provided to prospective contractors for their bidding or estimating purposes only.
Our report, conclusions and interpretations are based on our surface observations, data from others
and limited site reconnaissance, and should not be construed as a warranty of the subsurface
conditions.
Variations in subsurface conditions are possible and may also occur with time. A contingency
for unanticipated conditions should be included in the budget and schedule. Sufficient monitoring,
testing and consultation should be provided by our firm during construction to confirm that the
conditions encountered are consistent with those indicated in our report and to provide
recommendations for design changes should the conditions revealed during the work differ from
those anticipated.
The scope of our services does not include services related to environmental remediation and
construction safety precautions. Our recommendations are not intended to direct the contractor’s
(
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January 23, 2024
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methods, techniques, sequences, or procedures, except as specifically described in our report for
consideration in design.
If there are any changes in the loads, grades, locations, configurations or type of structures to
be constructed, the conclusions and recommendations presented in this report may not be fully
applicable. If such changes are made, we should be given the opportunity to review our
recommendations and provide written modifications or verifications, as appropriate.
♦ ♦ ♦
We have appreciated the opportunity to be of service to you on this project. If you have any
questions or comments, please do not hesitate to call at your earliest convenience.
Respectfully submitted,
GeoResources, LLC
ERIK FINA
Erikj. Fina, LG
Project Geologist
EJF:STM/ejf
DoclD: Gilroy.S7thSt.RG
Attachments: Figure 1: Site Location Map
Figure 2: Site Plan
Figure 3: Site Vicinity Map
Figure 4: NRCS Soils Map
Figure 5: Geologic Map
Eric W. Heller, PE, LG
Senior Geotechnical Engineer
Figure 6: Worked Out North Section of Renton Coal Mine
Figure 7: Cross Section A-A'
Figure 8: DTM Hillshade 61 -King County West
GEORESOURCES earth science & geotechnical engineering
Gilroy.S7thSt.RG
January 23, 2024
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REFERENCES
•George Watkin Evans, 1912, The Coal Fields of King County: Washington Geological Survey.
Bulletin No.3.
•F. T. Lee and J. F. Abel, Jr., 1983, Subsidence from Underground Mining: Environmental Analysis
and Planning Considerations: U.S. Geological Survey Circular 876.
Approximate Site Location
Map created from City of Renton COR MAPS (https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps)
Not to Scale
Site Location Map
Addition to Single-Family Residence & DADU
1318 South 7th Street
Renton, WA
PN: 722140-0287
DocID: Gilroy.S7thSt.F Jan 2024 Figure 1
EagleView Technologies, Inc.Ü Addition to Single-Family Residence & DADU1318 South 7th StreetRenton, WAPN: 722140-0287 Figure 2DocID: Gilroy.S7thSt January 2024
CAD drawings of the proposed addition to the residence from the architectural drawingsby Amy DedominicisAerial from the King County 2021 Aerial Imagery
0 30 60 90 12015Feet
Site PlanLegend
Coalmine Severity
High Moderate
Unclassified
Approximate Site Location
Map created from City Of Rentron COR MAPS (https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps)
Not to Scale
Site Vicinity Map
Addition to Single-Family Residence & DADU
1318 South 7th Street
Renton, WA
PN: 722140-0287
DocID: Gilroy.S7thSt.F Jan 2024 Figure 3
Approximate Site Location
Map created from Web Soil Survey (http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx)
Soil
Type Soil Name Parent Material Slopes Erosion Hazard Hydrologic
Soils Group
An Arents, Everett material Basal till 0 to 5 Slight A
BeD Beausite gravelly sandy loam Till over residuum from
sandstone 15 to 30 Moderate C
Not to Scale
NRCS Soils Map
Addition to Single-Family Residence & DADU
1318 South 7th Street
Renton, WA
PN: 722140-0287
DocID: Gilroy.S7thSt.F Jan 2024 Figure 4
Approximate Site Location
An excerpt from the Geologic Map of the Renton Quadrangle, King County, Washington by Mullineaux, D.R. (1965)
Tr Renton Formation
Qik Recessional stratified drift – kame
deposits
Qit Recessional stratified drift – kame
terrace deposits
Qgt Ground moraine deposits
Not to Scale
Geologic Map
Addition to Single-Family Residence & DADU
1318 South 7th Street
Renton, WA
PN: 722140-0287
DocID: Gilroy.S7thSt.F Jan 2024 Figure 5
Qgt
^_
A
A'
Ü Addition to Single-Family Residence1318 South 7th StreetRenton, WAPN: 722140-0287
Worked Out North Section Renton Coal Mine
Figure 6DocID: Gilroy.S7thSt January 2024
North Section of the Seattle Electric Co Renton Coal Mine Date: April 12, 1912, Surveyed: 1918Washington DNR Coal Map K31_Ahttps://fortress.wa.gov/dnr/geologydata/coal_maps/pdfs/K31_A.pdf
0 200 400 600 800100Feet
Legend
^_Location of Proposed Addition to Residence A-A' Cross Section
179ft219ft277ft330ft380ft420ft492ft565ft647ft724ft842ftLocation of Proposed Addition to Existing ResidenceWorked Out Third Level NorthPN: 7221400287ColorMaterial NameGlacial DepositsPuget Group Sandstone, Shale, Siltstones, and ConglomeratesCoal Seam8006004002000-200-400-60002004006008001000120014001600180020002200240026002800ScenarioMaster ScenarioGroupCross Section A-A'CompanyGeoResources, LLCDrawn ByEJFFile NameFigure 7Date12/13/2023, 10:54:21 AMProjectCross Section A-A' for North Section Renton Coal Mine as Recorded in 1918
Ü Addition to Single-Family Residence & DADU1318 South 7th StreetRenton, WAPN: 722140-0287 Figure 8DocID: Gilroy.S7thSt January 2024015030045060075Feet
DTM Hillshade 61 - King County West
North Section of the Seattle Electric Co Renton Coal Mine Date: April 12, 1912, Surveyed: 1918Washington DNR Coal Map K31_Ahttps://fortress.wa.gov/dnr/geologydata/coal_maps/pdfs/K31_A.pdf
Legend
PN: 722140-0287