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HomeMy WebLinkAboutRS_Geotechnical_Addendum_190405 Corporate Office 17522 Bothell Way Northeast Bothell, Washington 98011 Phone 425.415.0551 ♦ Fax 425.415.0311 www.riley-group.com June 13, 2018 Sukvinder Singh 9217 220th Lane South Kent, Washington 98031 Subject: Geotechnical Addendum Renton Short Plat 901 Renton Avenue South Renton, Washington RGI Project No. 2017-002 Reference: City of Renton, Community & Economic Development, “On Hold” Notice, S. Singh Short Plat, LUA17-000803, ECF, SHPL-A, dated January 11, 2018 Dear Mr. Singh: As requested, The Riley Group, Inc. (RGI) is providing this letter to address coal mine hazard issues, requested in the referenced notice from the City of Renton. We previously prepared a Geotechnical Engineering Report (GER) for the project, dated March 14, 2017. Coal Mine Hazard Areas The subject site is located on Renton Hill, an area extensively mined for coal in the late 1800’s and early 1900’s. Portions of the site are mapped as Medium and High Coal Mine Hazard Areas on the City of Renton online GIS. In assessing potential coal mine hazards at the site, we reviewed The Washington State Coal Mine Map Collection: A Catalog, Index, and User’s Guide by Henry W. Schasse, etc. (1994), and The Coal Fields of King County by George Watkins Evans (1912), and online mine maps from the Renton Mine on the Washington DNR Geologic Information Portal. Subsurface Geology of the Renton Mine The site is underlain by sandstone, mudstone, shale, and coal of the Renton Formation, deposited in the Upper Eocene. Portions of three coal beds were mined at the Renton Mine; Bed No. 1, No. 2, and No.3. The coal beds ranged in thickness from about 11.5 to 17 feet and were separated by large sandstone and shale units ranging in thicknesses from 85 feet between the lowest No.3 and the middle No. 2, and 110 feet between the No. 2 and the upper No. 1 coal bed. The main slope of the Renton Mine was originally sunk at an angle of about 10 to 11 degrees down the No. 3 coal bed from where it outcropped on the surface (approximately 130 feet west of the subject site). The attached Cross Section (Figure 2) was prepared along the alignment of the main slope using surface topography and approximate dip and location of the No. 3 slope. The relative locations of the No. 2 and No. 1 coal beds are also shown. Geotechnical Addendum Page 2 June 15, 2018 Renton Short Plat, Renton, Washington RGI Project No. 2017-002 Mining Methods The primary mining method on the No. 3 bed at the Renton Mine was room-and-pillar coal mining. Gangway shafts were driven every 250 to 300 feet both north and south off the main slope within the No. 3 bed, then areas between the gangways were mined by driving chutes from the lower gangway almost to the upper gangway at intervals, with crosscuts between for ventilation, forming a grid of mined out areas and coal pillars left to support the overlying rock. Once the room-and-pillar grid between gangways was complete, they mined the pillars as they worked back out toward the main slope, allowing the overlying rock to collapse. By 1912, north and south gangways 1 through 7 at the Renton Mine were almost completely mined out, with coal recovery by the room-and-pillar method reaching as high as 95 percent. Some apparently exploratory tunnels were driven into the No. 2 and No. 1 beds by means of a water-level early in the mines history, with a tunnel that stays just above the water level to prevent flooding. Mine Workings in the Site Vicinity Early in the mines history, a water-level tunnel was mined into the No. 2 coal bed from the north slope of Renton Hill, above the Cedar River that extended into the northern portion of the site. Some maps show it may have been mined by the Denny –Renton Clay and Coal Company. The Renton Mine main slope was sunk from an outcrop about 130 feet west of the subject site, descending at about 10 to 11 degrees down the dip of the No. 3 coal bed. A parallel airway shaft was sunk about 70 feet to the south. The 1st north gangway was extended to the north off the main slope through the western portion of the site, and 2nd north and 1st south gangways were extended off the main slope near the southeast site corner. Some maps show an airway tunnel extending off the main slope airway below the main slope and 2nd north gangway, possibly in the underlying sandstone, extending north, then east through the southeastern portion of the site. A rock tunnel was extended east from the 2nd north gangway, intersecting the No. 2 coal bed east of Grant Avenue (one block east of the site), with a tunnel extending in a north-south direction there. Two tunnels were mined west, up the slope of the No. 2 and a second north-south tunnel was mined to the east of Renton Avenue South. A rock chute was sunk from this No. 2 tunnel to the 1st south gangway on the No. 3 near the southeast site corner. To more efficiently move coal to the valley floor to the west of the site, a rock tunnel was sunk in the sandstone bedrock below the No. 3, starting between the 1st north and 2nd north gangways, and extending about 900 feet in length to near the valley floor. The No. 3 bed was mined by room-and-pillar method, with most of the area worked out and collapsed. Remnants of the gangways likely remained for some time, as the downslope east wall was often not mined or partially mined to control water, and some adjacent pillars were left to prevent a runaway collapse. Portions of the 1st north gangway just north of the main slope were mined in a different manner, leaving elongated pillars, likely to keep the area stable as the bed approached the surface and to prevent collapse of the main slope and rock tunnel below. At some point before the mine closed, a gangway was extended south from the main slope, about 80 feet west of 1st south gangway on the No. 3, near the southwest corner of the development area, and areas west of this gangway were mined to near the ground surface. Geotechnical Addendum Page 3 June 15, 2018 Renton Short Plat, Renton, Washington RGI Project No. 2017-002 Recommendations The site is underlain by extensive coal mine workings at various depths. Due to the close proximity of the main slope and airway entrances on the No. 3, and the method of mining to the west of the 1st north gangway, and the relatively shallow depth of the main slope and rock tunnel below the surface of the western portion of the site, RGI recommends avoiding development of the western half of the site. The entrances to the west of the property were observed to either have been collapsed or filled in. In the eastern portion of the site, the No. 3 bed has mostly been worked out and was collapsed over 100 years ago, and the remnant mine workings of the 2nd north and 1st south gangways are over 200 feet in depth. Due to the mining method used and the elapsed time since they were abandoned, on our professional opinion, development can be safely completed over worked out areas and gangways of the No. 3 coal bed in this area without affecting the site or neighboring properties. The main slope and rock tunnel in the southern portion of the site remained open and were in use until the mine closed in this area. It is unclear how the Renton Mine was abandoned, whether they pulled support timbers and allowed it to collapse on its own or induced it to collapse. The intersection of the main slope and rock tunnel is in the general vicinity of the steep slope area, and could be indicative of past subsidence or collapse, and resulting in increased erosion. The status of the main slope east of this area is unknown, and may be less than 200 feet below the surface as it approaches proposed Lot 5 due to elevations in the ravine. We expect this mine was collapsed and the overlying materials will support the proposed development. RGI recommends further research or the potential that drilling in Lot 5 may be necessary prior to developing the lot with a residence. The area will need to be cleared to allow access for the drilling. A tunnel appears to extend into the northern portion of the site on the No. 2 bed. This extends through the 30 foot wide utility easement and into proposed Lot 1. The status of the tunnel is unknown, although it was excavated prior to the collapse of the No. 3 bed below, and may have already collapsed. This tunnel may be less than 100 feet below grade and development could affect Lot 1, and utilities and the residence to the north. RGI recommends drilling to confirm status of this tunnel. The area will need to be cleared to allow access for the drilling. Public Comment Letters RGI reviewed the relevant portions of the attached comment letters regarding the coal mines. As described above, this addendum addresses concerns over the development and potential effects on neighboring properties. RGI is not aware of an entrance/air way/shaft at 910 Renton Avenue South on maps of the area. If one is present at that property, it seems it would have to be associated with the No. 1 bed, which is not likely to be present west of Renton Avenue South. Limitations This letter is the property of RGI, Sukvinder Singh, and his designated agents. Within the limits of the scope and budget, this letter was prepared in accordance with generally accepted geotechnical engineering practices in the area at the time this letter was issued. This letter is intended for specific application to Parcel no. 0007200196 in Renton, Washington, and for the exclusive use of Sukvinder Singh and his authorized representatives. Airw a y N o . 3 Mai n S l o p e N o . 3 First South Gangway No. 3Rock Chute Between No. 3 and No. 2 Rock Tunnel Between No. 3 and No. 2 Worked Out No. 3 Worked Out No. 3 Worked Out No. 3 Worked Out No. 3 Renton Mine No. 22nd North Gangway No. 31st North Gangway No. 3Denny-Renton Clay and Coal No. 2Ro c k T u n n e l B e l o w N o . 3 N 06/2018 Corporate Office 17522 Bothell Way Northeast Bothell, Washington 98011 Phone: 425.415.0551 Fax: 425.415.0311 Renton Short Plat RGI Project Number 2017-002 Date Drawn: Address: 901 Renton Avenue South, Renton, Washington 98057 Site Map Figure 1 Approximate Scale: 1"=60' 0 30 60 120 *Mine workings based on Map Showing the Upper Coal Mining Operations of the Renton Mine Under the City of Renton (1955) 1st NorthGangwayRenton Coal Bed No. 32nd NorthGangwaySandstoneRock Tunnel to No. 2 BedDenny-Renton Clayand Coal No. 2 BedSandstone and Shale BedsRenton Coal Bed No. 2Sandstoneand ShaleProposedDevelopmentPropertyRenton MineNo. 2 BedRenton Coal Bed No. 1Renton Ave. SouthGrant Ave.280220160Mine Entrance1st SouthGangwayRock Tunnel Below No. 3Sandstone1004006/2018Corporate Office17522 Bothell Way NortheastBothell, Washington 98011Phone: 425.415.0551Fax: 425.415.0311Renton Short PlatRGI Project Number2017-002Date Drawn:Address: 901 Renton Avenue South, Renton, Washington 98057Cross SectionFigure 2Approximate Scale: 1"=60'03060120NNotes:1.Geologic beds approximated based on descriptions in TheCoal Fields of King County by George Watkins Evans (1912)2.Profile along main slope alignment from mine entrance to theeast