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Home My WebLink AboutSubsoils Report14" 1r,;n'rth I'.nvl,rl' #**'1 r,r h n i. tl. lrs, S*Jf Lorlnr; R$.i,J $$4 #l*rtrr nrrJ $pri*gr. {.^rltrgn,.!,; lil {* } lli*:**: S?f .$'$ i'?9.1$ {aEWW #RT*4*3 AW Le,W" ffi E*Tg**"{ h* $*A!-F,rr:9?*'945.*4i4 *t:t:li1; hygc*#hffL{}t{.{}-1.{{rit: April 18,2016 Brikor A$sociates Attn: Briston Peterson 20 Sunset Drive, Unit I Basalt, Colorado 81621 @ Job No. 116 l09A Subject:Observation of Exploratory Pit, Proposed Residence, Lot A8, Aspen Glen, 417 River's Bend, Garfield County, Colorado Dear Mr. Peterson: As requested, a representative of Hepworth-Pawlak Geotechnical,Inc. observed an exploratory pit at the subject site on April 12, 2016 to evaluate the soils exposed for foundation support. The findings of our cbservations and recommendations for the foundation design are presented in this report. The services were performed in accordance with our agreement for professional engineering services to Brikor Associates, dated April 12,2016. Chen-Northem,Inc. previously conducted geotechnical engineering studies for the Aspen Glen development and presented their findings in reports dated December 20, 1991 and May 28, 1993, Job No. 4 I I292. Building plans for the proposed residence are conceptual and this report was prepared for purchase of the property. A one and two story wood frame structure with an attached garage is anticipated. Main floor and garage floor will be slab-on-grade. The site is located on the north side of the Roaring Fork River and is vegetated with grass and weeds. The ground surface is relatively flat with a slight slope down to the south, Rock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen development. These rocks are a sequence of glpsiferous shale, fine-grained sandstone/siltstone and limestone with some massive beds of gypsum. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous studies in the area, several broad subsidence areas and smaller size sinkholes were observed scattered throughout the Aspen Glen devclopment (Chen-Northern,Inc. l99l and 1993). The nearest mapped broad depressions are Ijffi to 1,600 feet north and northwest and mapped sinkholes are I,?00 feet north and northwest of the subject site. These sinkholes appear similar to others sssociated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley, W P*rlq*r .3#i"#41"?I1S * Cr:lu:r;r#*Sprir"ags ?1S"S3l"5SS: * Silv*rtfu*ra"re 9T*-4SS"lSSf Brikor Associates April 18,2016 ?age2 The lot is not located within a broad subsidence area and existing sinkholes were not observed in the immediate area of the subject lot. The nearest mapped broad depression is 1300 feet north and the nearest mapped sinkhole is 1700 feet northwest of the site. No evidence of cavities was encountered in the subsurface materials: however, the exploratory borings were relatively shallow, for foundaiion design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot A8 throughout the service life of the proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. At rhe time of our visit to the site, an exploratory pit had been dug near the middle of the proposed building footprint down to about 7 feet below the adjacent ground surface. The soils exposed in the pit below about 6 inches of topsoil consisted of 4 feet of sandy silty clay overlying silty sandy gravel with cobbles and small boulders down to the bottorn of the pit at 7 feet. Results of swell-consolidation testing performed on a sample of sandy clay taken from the site, shown on Figure l, indicate the soils are moderately compressible under conditions of loading and wetting. The results of a gradation analysis performed on a sample of silty sandy gravel with cobbles (minus S-inch fraction) obtained from the pit are presented on Figure 2. Free water was encountered in the pit at 1t&feeland the upper soils were msist to very moist. lVe expect the groundwater level to be higher during flood events and spring runoff and to be close to the river level. Considering the conditions exposed in the exploratory pit and the nature of the proposed construction, spread footings placed on the undisturbed clay soil designed for an allowable soil bearing pressure of 2,000 psf should be adequate for support of the proposed residence. Footings placed of the deeper gravel soils can be designed for an allowable soil bearing pressure of 3,000 psf. Footings should be a minimum width of l8 inches for continuous walls and 2 feet for columns. Loose and disturbed soils in footing areas should be removed and the bearing level extended down to the undisturbed natural soils. Exterior footings should be provided with adequate soil cover above their bearing elevations for frost protection. Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 12 feet. Foundation walls acting as retaining structures (if any) should also be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 55 pcf for on*site clay soil as backfill. The proposed main floor level should be above the sunounding grade and the 100-year flood level and a perimeter foundation drain should not be required. Basement and crawlspaces ,u6 not recommended. Suuctural fill placed within floor slab areas can consist of the on*site soils compacted to at least 95Vo af standard Proctor density at a moisture content near optimum. Backfill placed around the structure should be compacted and the surface graded to prevent ponding within at least l0 fe.et of the building. JobNo. I 16 l09A e&Siecrr Brikor Associates April I8,2016 Page 3 The recommendations submitted in this letter are based on osr observation of the soils exposed within the exploratory pit and do not include deep subsurface exploration to evaluate the subsurface conditions within the loaded depth of foundation influence. This study is based on the assumption that soils beneath the footings have equal or better support than those exposed. The risk of foundation movement may be greater than indicated in this report because of possible variations in the subsurface conditions. In order to reveal the nature and extent of variations in the subsurface conditions below the excavation, drilling would be required. It is possible the data obtained by subsurface exploration could change the recommendations contained in this letter. Our services do not include determining the presence, prevention or possibility of mold or other biological contarninants (MOBC) developing in the future. If the client is concerned about MOBC, then a professional in this special field of praetice should be consulted. If you have any questions or need further assistance, please call our office. Sincerely, HEPWORTH - PAWLAK GEOTECHNICAL, INC Eller Reviewed by: Daniel E. Hardin, P.E. LEE/ksw attachments Figure I -S Test Results Figure 2 * Gradation Test Results JobNE. 116 l09A c&ftecrr Moisture Conlent * 29.6 Dry DensitY = 91 Sample of: Sandy Silty Clay Frsm: Pit 1 at 3ft Feet percent pc.f 7 (No movement upon wetting \\ \t 0 sOo\ L ,ooa dJ a. Eo CJ 1 2 3 4 0"1 1,0 l0 100 APPLIED PFESSUFE - Ksf <;e5hgcrr Hcpworlh-Powf ok Gcotachnlcd 1 16 1094 SWELL-CON SOLIDATION TEST RESU LTS Figure 1 TIMEREADINGS U.S. TANDARD SEFIES #50 #30 #16 #8 6TEAF SOUAFE OPENiNGS oltilfr.'3[,3u ,60MtNr9MtN.4 MtN. 1 MtN, #200 #100 #4 ale 3/4- I1lX 3', s'6" tr 100 10 20 30 40 gl 8{t 70 C)LJz{Ft!u F trJ()Eulo- 60 a'antn o- Fz Lil(,) E,lrl o- 50 60 70 80 s0 lfi) 40 30 m 10 0 ,001 ,00?005.00S ,01S "O37 ,o74 .1s0 ,300 .600 l"t8 2-36 OAqMETEH OF FARNCLES ; N M&I,#METERS 4.75 9.5 19 0 27.5 762 152 20312.5 127 cr,,AYfi3 5r. r ts6tf;tusrt I Fof I lrggrw lCOrFif I FUG I Co AsE I coo6r.6 GHAVEL 76 %SAND 14 %SILTANDCI.AY 10 6A LIOUID LIMIT OA PLASTICITY INDEX % SAMPLE OF: Slightly Silty $andy Graveiwith Cobbles FROM:Pit at 6 to F[zFeet J ++-E-ts-- E_ ---- --- :- 1 16 10gA ,.*ffi*l GRADATION TEST HESULTS Figure 2