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Home My WebLink AboutSWP272247 TERRA ASSOCIATES, Inc. Consultants in Geotechnical Engineering, Geol Geology .,W., and Environmental Earth Sciences February 20, 1996 Project No.T-2970-1 Mr. Bob Tozer RFC-RIVED P.O. Box 53312 199s Bellevue,Washington 98015-3312 DEVELOPMENT PLANNING Subject: Coal Mine Hazard Evaluation CITY OF RENTON Renton Place Renton, Washington Reference: Geotechnical Report, Project No. T-2970, by Terra Associates, Inc. dated September 16, 1995 Dear Mr. Tozer: As requested and in accordance with the recommendations outlined in the referenced report, we have completed an evaluation of the abandoned coal mine hazard on the subject property. Our field exploration identified evidence of possible mining activity in the southwest corner of the property. This evidence is a 2.5 foot high void encountered approximately 105 feet below the existing surface grade. Based on the conditions we encountered and the geologic structure at the site, it is our opinion that this condition poses a minimal geologic hazard and associated risk to public health and safety. The following discussion outlines the results of our study and summarizes our evaluation and conclusions. BACKGROUND INFORMATION The subject site is located northeast and adjacent the intersection of South 19th Street and Talbot Road in Renton, Washington. This general region of Renton is known to have been actively mined for coal in the late 1800s to early 1900s. This site is in the general vicinity of the Patton mine, which was opened in 1872 and abandoned some time prior to 1912. Historical records on the workings of the Patton mine are limited. Information indicates that the workings were confined to a single incline down the dip of one coal stratum, from which coal was mined along two levels. It is reported that the Patton mine was not extensively developed, possibly because of groundwater and roof support problems. The overburden above the coal seam that was mined was relatively thin, estimated to be generally less than 100 feet. 12525 Willows Road, Suite 101, Kirkland, Washington 98034 9 Phone (206) 821-7777 Mr. Bob Tozer February 20, 1996 In reviewing available records, there was one reported incidence of surface subsidence related to coal mining activities from the Patton mine. This was on property located at 320 South 19th Avenue, approximately 1/4 mile west of the subject site. In 1984, this site was extensively studied by the Morrison Knutson company under contract with the Office of Surface Mining. The reported problems and damage to the home included cracking of the interior and exterior fireplace chimney, a 1.5-inch sag in the kitchen floor, cracking of concrete walks and driveways, and ground subsidence below a rear deck and floor joist support. Extensive field exploration performed by Morrison Knutson revealed that the overburden above the coal seam mine in this area was relatively thin, less than 35 feet. Test borings performed above the mined tunnel noted that the overburden soils were disturbed, indicating slumping or subsidence of the soil zone. FIELD EXPLORATION RESULTS On January 31 through February 9, 1996, we observed the drilling of three soil test borings along the southern portion of the site. The approximate locations of the test borings are shown on the Exploration Location Plan, Figure 2. The borings were performed with a truck-mounted drill rig and advanced using mud rotary methods. During boring advancement, soil samples were obtained at regular intervals using a three-inch diameter split barrel sampler driven into the ground using a 300 pound weight falling 30 inches. The number of blows required to drive the sampler a depth of 12 inches after an initial six-inch set depth was recorded. This sampling procedure is referred to as penetration testing. The number of blows required to drive the sampler is the N value. The N values obtained by this procedure were correlated to Standard Penetration Test values as described in ASTM Test Designation D-1586. This Standard Penetration Test uses a two-inch diameter sampler which is driven by a 140 pound weight falling 30 inches. The correlated N values are recorded on the Boring Logs, Figures 4 through 6. The test borings performed at the site encountered soils of glacial origin to depths of 95 to 105 feet. The upper 50 to 80 feet of soil is a glacial till comprised predominantly of a silty sand with gravel, cobble, and boulder inclusions. The thickness of this upper till is greatest along the western portion of the property and thins to the east. Underlying the till, the glacial soils are an advanced outwash unit composed predominantly of sand with silt and gravel. Penetration resistance values indicate that these glacial sediments are in a very dense field condition. Our visual examination of the soil samples did not find any evidence of disturbance. At Boring B-1, advanced near the southwest corner of the site, a void was encountered beneath the outwash soils. The presence of this void was detected by a complete loss of drilling fluid once the void was penetrated. This occurred at about 105.5 feet below the existing surface grade. Sounding indicated that the void had a height of about 2.5 feet. Beneath the void, we encountered a very dense fine to medium grained sand. This sand is likely a soft sandstone of the Renton formation. The void is indicative of mine workings, and it is likely that the sand formation encountered beneath the void is roof collapse following extraction of the coal. Project No. T-2970-1 Page No. 2 Mr. Bob Tozer February 20, 1996 Borings B-2 and B-3, advanced in the southeast corner and central portion of the site, did not encounter a void beneath the outwash deposits. Underlying the outwash at these locations, we encountered a 10 to 12-foot thick layer of soft sandstone of the Renton formation. Underlying the sandstone, interbedded layers of shale and coal were encountered to boring completion depths of 133 to 134 feet below existing surface grade. The preceding discussion is intended as a general review of subsurface conditions encountered at the test boring locations. For a more detailed description of the conditions encountered, please refer to the Boring Logs. DISCUSSION Available information indicates that this lower level working of the Patton mine drove from west to east towards the subject property and then veered a short distance to the southeast. Our field exploration appeared to confirm this recorded location of the mine workings, since the information obtained at Borings B-2 and B-3 suggest that mining activity did not occur eastward along the south portion of the site. Based on review of the available mine records and the results of our field exploration, it is our opinion that the only portion of the subject site which would be subject to the coal mine hazard includes planned Lots 11 and 12 and the existing residence at the southwest corner of the site. The hazard in this area involves the potential for surface subsidence to occur if the void or opening closes by natural processes. In our opinion, the only subsidence mechanism which could potentially affect this area of the site is trough subsidence. Trough subsidence is time-dependent and occurs as a result of gradual bending of the soil strata overlying the opening, causing the roof to sag and eventual closure of the opening. This type of subsidence can result in both vertical and lateral displacements, the surface extent of which could extend beyond the horizontal dimension of the mine opening. The potential for development of a subsidence trough is dependent on many factors, including the dimensions of the mine workings and the strength and deformation characteristics of the overlying strata. The dimensions of the mine workings in this area are undocumented. However, in their previous study of the mine, Morrison and Knutson speculated that the mine width along this lower level was likely five to eight feet. The sandstone which overlies the coal seam is a weak formation and, as reported, caused considerable difficulty during the mining operation with respect to roof collapse. The sand encountered beneath the void at Boring B-1 is likely roof collapse material which occurred during or shortly following extraction of the coal. The overlying glacial sediments exhibit high strength characteristics and based on visual examination of the soil samples and the penetration test results, do not appear to have been affected or disturbed by this roof collapse. There are no surface features present that would suggest trough subsidence has occurred. Given the conditions observed, it is more probable that the mine workings are relatively narrow and that the glacial sediments are arching or bridging the void which developed following collapse of the sandstone formation. Arching is a natural characteristic of the granular formation and is often relied upon in tunnel and shoring design. Project No. T-2970-1 Page No. 3 Mr. Bob Tozer February 20, 1996 CONCLUSIONS Based on our study, it is our opinion that the hazard associated with the presence of the coal mine workings beneath the southwest corner of the site is minimal. Currently, there are no surface features or reported difficulties with the existing structures that would suggest surface subsidence has occurred or is occurring. The results of our exploration indicate that the glacial sediments are currently bridging or arching above a void which likely resulted following roof collapse upon coal extraction. Given the likely narrow width of the mine workings, it is our opinion that this arching effect will continue. In the event of an earthquake, additional stress generated by ground shaking could affect the balance of this current equilibrium. Historically, since closing of the mine, the Puget Sound region has been subjected to two major earthquakes which occurred in 1949 and 1965. There are no surface expressions that would suggest the ground motions during these earthquakes had any effect on the current mine stability. However, a strong motion earthquake could result in additional collapse of soil into the boring, the results of which may be reflected at the ground surface. While it is our opinion that the hazard and risk associated with the coal mine workings on the subject property are minimal, the hazard and risk do exist. Mitigating measures to eliminate or further reduce the risk include filling the void using grouting techniques or designing foundation systems to structurally span areas of potential subsidence. We trust the information presented is sufficient to meet your current needs. If you have any questions or need additional information, please call. Sincerely yours, TERRA ASSOCIATES, INC. �2 TheodoreJ. Sche.pper, P.E. Principal En* „o-- TJS:lh,'-j RFq f,,,_ Eric]: Figure 1 - Vicinity Map Figure 2-Exploration Location Plan Figure 3 - Soil Classification System Figures 4 through 6 -Boring Logs cc: Mr. Ken Williams, Group Four, Inc. 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SE RID ST A. 5[ 77rro St 3� -i ^•SE 177N0 sr •• l N r -:Sf i- 17 51l�`38TH Sr ST r 01 u 113m_si- ST "7.9 S st I ST ..J ) T $�I7eTN St a�I'I7.lx ST f - «a Q`..� ; '.• 173"D' SN 39 ST J IE $� L 5C I75TN '^� IIaTN `,K«Y SE ' �!• e id rJJr` r `\ SE 176TH ST IT -F LU 1 a "ST 0.0 d . �; " SM uT /1ST ST -�T_�k�� !4 �1 �� _ _ s i Y1� = Cp I - „ R d �+ REFERENCE: THE THOMAS GUIDE, KING COUNTY, WASHINGTON, PAGES 655 and 656, 1995 EDITION. VICINITY MAP MINING EVALUATION TERRA iNGeotechnical RENTON PLACE ASSOCIATES RENTON, WASHINGTON Consultants Proj. No. 2970-1 Date 2/96 Figure 1 CO 1. 1 ----- ---- _ N88'30'S8"VP- � / Y v s '? � a 27.75 * - _ �w' `. ?� 7 .64�.. .,,.t j399+90 l ' t�91'3�';� m. E�R) / 1 1 ,Y WAtER EAw 10'C(WSTRUcTIt I , 8 ` EAMMENT 1 IX L,jc A t' ` ,• 1 I ' 1 SIDEE JJ '•/_^ ,ti /'boo +b, \ \ ` 9t!' /Y TIP ti } t `' TP-4 =� \ 1` j'�`a;� 11 i. Q , 1 •. 2 .�� of b �// _ /Q�� .•�. . TOP-1%10 1 t: .1 Z.#s� ..f / it t.' r oNc t t2'�.t0 �. 1, _ TP-2� 1 ,�� ` 1 APPROXIMATE SCALE �j 8' — ii # I • � � 50 0 50 100 ft B-1 TP-7 at, a (~ uR r+[t .sE VI i r HATE° uETER`T i4 v / - 85.93' •c,y _ _:�'+ `.12 I r�gf y \. 1 28.70 -• �i' / / / }f N88=30'S8'w W B-3 ,y g } � 1 N88'30 58 -�1 � sc� \ TIP B,x T .� o . it A'SCR .� _ — /•c:T• l.�ti. / �,' ♦' �. T-'4 \i `a Lb I 1 396+75. la c. ..x rc Y• N$8'30'4R"W 35.60• 6 299. 7''v x 14 4 �- ---- n .14� - + a c _ S• 19ih �7 T. �0.vro �E 9. .�• ✓ _ V.— • t; '—^ l ta. -MATEF 10�l 9 - y `°�1 r •,P� .I. / J�/. 2S( 1+(. I SIax'. :]/. tNC �rro f•r rr FrNTO ' '. . CB If {pr: f .AT as .,A.57 Er cnrF_iv« I I c nc 1 LEGEND: REFERENCE: APPROXIMATE BORING LOCATION UNTITLED AND UNDATED SITE PLAN PROVIDED BY CLIENT. EXPLORATION LOCATION PLAN APPROXIMATE LOCATION OF TEST PIT TERRA MINING EVALUATION DONE BY TERRA ASSOCIATES, PROJECT RENTON PLACE NUMBER 2970, DATED SEPTEMBER, 1995. ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No,2970-1 1 Date 2/96 Figure 2 MAJOR DIVISIONS LETTER GRAPH TYPICAL DESCRIPTION SYMBOL SYMBOL GRAVELS Clean GW P; O.•d Well-graded gravels, gravel-sand mixtures, little L Gravels •.Q•q• or no fines. m U) to (less than ... . .. Poorly-graded gravels, gravel-sand mixtures, little m 6 More than o GP ••• • •• _ ,� 5/o fines) or no fines. O _•�, 50% of coarse ••' • •• .F fraction is GM • Silty gravels, gravel-sand-silt mixtures, non- _ a) > larger than No. Gravels plastic fines. w 4 sieve. with flees GC Clayey gravels, gravel-sand-clay mixtures, plastic Z • fines. Ur O N SANDS Cleani Well-graded sands, gravelly sands, little or Sands SW no fines. w o fQ z More than (less than r r{;s{>"' Poorly-graded sands or gravelly sands, little Q C 0 5% fines S P ::::.;r:•:• .>{.;'' l :<>:>:<::: 50% of coarse ) :;;::::::::<.; r:;� x or no fines. O fraction is U +� smaller than SM Silty sands, sand-silt mixtures, non-plastic fines. No. 4 sieve. Sands with fines SC Clayey sands, sand-clay mixtures, plastic fines. SILTS AND CLAYS M� Inorganic silts and very fine sands, rock flour, silty or (/) clayey fine sands or clayey silts with slight plasticity. J LO m O 8 � CL Inorganic clays of low to medium plasticity, gravelly m (n Liquid limit is less than 50% clays, sandy clays, silty clays, lean clays. E oN Q L I I I I I I I I" Organic silts and organic clays of low plasticity. Z a < z Inorganic silts, micaceous or diatomaceous fine L SILTS AND CLAYS MH sandy or silty soils, elastic. C� :5 _ w z 0 8 Liquid limit is greater than 50% CH Inorganic clays of high plasticity, fat Clays. u- m - N OH Organic clays of medium to high plasticity, organic silts. HIGHLY ORGANIC SOILS I PT - Peat and other highly organic soils. 11 h DEFINITION OF TERMS AND SYMBOLS J Standard Penetration T 2" OUTSIDE DIAMETER SPLIT > Density Resistance in Blows/Foot 1 SPOON SAMPLER Q Of Very loose 0 4 2.4" INSIDE DIAMETER RING SAMPLER Loose 4-10 OR SHELBY TUBE SAMPLER ° Medium dense 10-30 P SAMPLER PUSHED 0 Dense 30-50 Very dense * SAMPLE NOT RECOVERED >50 WATER LEVEL (DATE) p WATER OBSERVATION STANDPIPE Standard Penetration C TORVANE READINGS, tsf Consistency Resistance in Blows/Foot g qu PENETROMETER READING, tsf 0 Very soft 0-2W MOISTURE, percent of dry weight o Soft 2-4 pcf DRY DENSITY, pounds per cubic foot J Medium stiff 4-8 ILL LIQUID LIMIT, percent 8-16 Stiff Very stiff 16 2 PI PLASTIC INDEX Hard >32 N STANDARD PENETRATION, blows per foot SOIL CLASSIFICATION SYSTEM TERRA MINING EVALUATION • RENTON PLACE • "' • ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No. T-2970-1 Date 2/96 Figure 3 Boring No. B-1 Logged by: MFS Date: 1/31/96 - 2/5/96 Approximate Elev. 189 a) Graph/ Relative Depth 0 (N) Water USCS Soil Description Density (ft.) Blows/ Content� foot M Topsoil Red-brown to gray-brown silty = 58 SM SAND, moist, mottled. Dense Weathered Till Gray silty SAND, moist, trace Z 50/6" SM of gravel. (TILL) Very Dense TF 10 ' • Gray-brown silty sandy GRAVEL, = 50/4" GM moist. Very Dense Dark gray silty SAND with = 50/4" gravel/cobble, moist. Very Dense 20 Very Dense Z 60/6" Becomes gray medium SAND, y moist,with trace of silt. Z 50/6" SM �30 Dark gray silty SAND with Very Dense = 50/0" gravel/cobble, moist. 40 80/6" Gray sandy SILT with gravel, moist. ML Very Hard 50 Z 60/6" U1 1 11 - Gray silty SAND with gravel/ SM cobble, moist. Very Dense = 50/0" * No recovery Page 1 of 2 BORING LOG MINING EVALUATION TERRA RENTON PLACE . ; ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No. T-2970-1 Date 2/96 Figure 4 Boring No. B-1 (Continued) Logged by: MFS Date: 1/31/96 - 2/5/96 Approximate Elev. 189 _a) Graph/ Relative Depth 0- (N) Water USCS Soil Description DensityE Blows/ Content (ft.) 11 foot (%) (As above) SM Note: Drilling through cobble/ = 50/3" gravel 58.5 to 63.5, 66 to 70 68.5, 3 to 78 feet. Gray medium SAND with 80 Z 93/6" Very Dense silt and gravel/cobble. 90 ZL 71/6" SM` I 100 { Void 105.5 to 108 feet. Light GRAY fine to medium sand, Very Dense 70 10" moist,with some organic SP laminations. (Renton Formation) Page 2 of 2 Boring terminated at 108.8 feet. Boring drilled with hollow stem auger to 13.5 feet, mud rotory 13.5 to 108.8 feet. BORING LOG TERRA MINING EVALUATION RENTON PLACE ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No. T-2970-1 Date 2/96 Figure 4 Boring No. B-2 Logged by: M FS Date: 2/5/96 - 2/7/96 Approximate Elev. 178 _a) Graph/ Relative Depth a (N) Water USCS Soil Description Density Depth E Blows/ Content CO foot (/o) FILL: Dark brown silty SAND, moist with construction debris Loose (" concrete brick tile). Brown silty SAND with gravel, Dense moist. (Glacial Till) to 5 Very Dense 10 40 Medium brown silty SAND with 15 gravel, moist, some orange- Very Dense gray mottling. 36/2 20 SM 25 36/6" 30 35 Dark gray silty SAND, moist, trace of fine gravel, well Very Dense a 36/2" cemented. 40 Page 1 of 3 BORING LOG MINING EVALUATION TERRA RENTON PLACE ' ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No. T-2970-1 Date 2/96 Figure 5 Boring No. B-2 (Continued) Logged by: MFS Date: 2/5/96 - 2/7/96 Approximate Elev. 178 a (N) Water GraphUSCS/ Soil Description Relative Gepth E Blows/ Content Density (ft.) afoot N `x•. ^%' Gray medium SAND, moist, 36/4" Very Dense 3 <? `> trace of gravel, no cementation. —50 55 36/4" Gray-brown SILT, wet. Hard 60 ML 65 Gray sandy SILT with gravel, Hard wet. 70 42/6" 7 5 Gray SAND with silt and gravel/ Very Dense cobble, moist, no cementation. a 42/6" SM SID 80 ::::•: - 85•K•kv;T:'riii'r: 50/2" Page 2 of 3 BORING LOG RONTERRA MINING RENTON PLACE EVALUATION ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No. T-2970-1 Date 2/96 Figure 5 Boring No. B-2 (Continued) Logged by: MFS Date: 2/5/96 - 2/7/96 Approximate Elev. 178 a) Graph/ Relative Depth 0- (N) Water USCS Soil Description Density E Blows/ Content U) C, foot N .............. Gray silty SAND with gravel/ Very Dense cobble, moist, no cementation. X Sm W....... —95 Light gray fine to medium Very Dense - — 100 SAND, moist, some dark brown (organic) laminations, slight - organic odor, light cementation. . ... 65/4 (Renton Formation 105 . .......... 110 Dark brown clay shale with black coal interbeds. Hard = 50/4" — 115 —120 50/4" Z Black coal with dark brown Hard —125 clay shale interbeds. - 130 50/6" Test boring terminated at 133.5 feet. Page 3 of 3 BORING LOG 477777 MINING EVALUATION TERRA RENTON PLACE............... .............. A-A ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No. T-2970-1 I Date 2/96 1 Figure 5 Boring No. B-3 Logged by: MFS Date: 2/8/96 - 2/9/96 Approximate Elev. 185 a� Graph/ Relative Dept (N) Water h USCS Soil Description Blows/ Content Density (ft.) 53) foot N I TOPSOIL Weathered Till: Brown silty SM SAND, moist, some orange Dense mottlin . L 5 Gray silty SAND, moist, trace of gravel. (Glacial Till) Very Dense 10 L 15 Gray silty SAND, moist with 60/10" some gravel/cobble. 20 SM E: 25 30 Note: Relatively smooth drilling 38.5 to 52 feet. 35 Gray silty gravelly SAND, moist. Very Dense 36/6" 40 Page 1 of 3 BORING LOG MINING EVALUATION TERRA RENTON PLACE ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No. T-2970-1 Date 2/96 Figure 6 Boring No. B-3 (Continued) Logged by: MFS Date: 2/8/96 - 2/9/96 Approximate Elev. 185 Graph/ Relative Depth C1 (N) Water USCS Soil Description Density E Blows/ Content foot N Gray silty gravelly SAND, moist. ............... -50 ....X-X .......... .... Note: Gravels/cobbles 54 to X X. 57 feet. - 55 ....... .. .... Gray medium SAND,wet, no Very Dense cementation,trace of silt 60 36/4" — Note: Gravels/cobble 63.5 to 93.5 feet. 65 70 SH SM Note: Sampler at 75 feet -75 a 42/0' bouncing on cobble. x !i;ix -80 ...... .... -85 .............. "T No recovery Page 2 of 3 BORING LOG MINING EVALUATION ....... TERRA RENTON PLACE ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No. T-2970-1 � Date 2/96 Figure 6 Boring No. B-3 (Continued) Logged by: MFS Date: 2/8/96 - 2/9/96 Approximate Elev. 185 a) Graph/ Relative Depth 0- (N) Water USCS Soil Description Density (ft ) E Blows/ Content COfoot N Note: Smoother drilling at 95 feet. Very Dense L 95 >' Light gray medium SAND, r moist. (Renton Formation) Very Dense a 43/6" 100 �rSP •: • — 105 f,..Sr{f: Brown clay shale with coal = interbeds. Hard 110 68/4 :»i> ::{:::;{•; Gray medium SAND, moist Very with some organic laminations. y Dense 115 Dark brown clay shale with Hard 120 coal interbeds. 60/5" 125 _= 130 — T 50/3" Test boring terminated at 134.3 feet. Page 3 of 3 BORING LOG MINING EVALUATION TERRA RENTON PLACE ASSOCIATES RENTON, WASHINGTON Geotechnical Consultants Proj. No. T-2970-1 Date 2/96 Figure 6