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Home My WebLink AboutSubsoil Study for Foundation Design 04.13.16~ech HEPWORTH-PAWLAK GEOTECHNICAL April 13, 2016 Charlie Apple 491 Elk Springs Drive Glenwood Springs, Colorado 81601 (capple .. 9@ msn.com) Hepworth p,'" I 11.. Gwtc:chnic .11 In c 50ZO Count} ll.u:id 154 Gknwi",J Srnn,::,, Col.1r.1Jl, 81601 rhl•nc. 970 945 79 ~1) F:ix 970-945-845-1 cm.iii hN:cn'lVhrl!LtltLch.cnm Job No.116 091A Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 9 , Filing 6, Elk Springs, Garfield County, Colorado Dear Mr. Apple: As requested, Hepworth-Pawlak Geotechnical, Inc. performed n subsoil study for design of foundations at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated April 4, 2016. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Hepworth-Pawlak Geotechnical, Inc., previously performed a preliminary geotechnical study for Filings 6 through 9, Elk Springs (formerly Los Amigos Ranch PUD) and reported our findings on February 14, 1997, Job No. 197 617. Proposed Construction: The proposed residence will be one and two story wood frame construction above a crawlspace with an attached garage. Garage floor will be slab-on- grade. Cut depths are expected to range between about 3 to 5 feel. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. If building conditions or foundation loadings are significantly different from those described above, we should be notified to re.evaluate the recommendations presented in this report. Site Conditions: The lot is vacant and was free of snow at the time of our field exploration. The site is located on a rolling upland mesa. Vegetation consists of sage brush with grass and weeds. The ground surface slopes moderately down to the northwest. The site elevation is approximately 7,165 feet. Subsurface Conditions: The subsurface conditions al the site were evaluated by excavating two exploratory pits at the approximate locations shown on Figure I. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 1 Vz feet of topsoil, consist of up to 3 feet of sandy silty clay overlying dense basalt cobbles and boulders in a sandy gravel matrix . Refusal to backhoe digging was encountered in the Parker 303-841-7119 • Colorado Springs 719-633-5562 • Sikerrhomc 970-468-1989 -2- pits at about 4 feet deep . Results of swell-consolidation testing performed on a relatively undisturbed sample of sandy silty clay, presented on Figure 3, indicate low lo moderate compressibility under existing moisture conditions and light loading. Results of a gradation analysis performed on a sample of sandy gravel matrix soils (minus 3-inch fraction) obtained from the site are presented on Figure 4. No free water was observed in the pits nt the time of excavation nnd the soils were moist. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, we recommend spread footings placed on the undisturbed natural soil designed for an aJlowable soil bearing pressure of 2,000 psf for support of the proposed residence. Footings should be a minimum width of 16 inches for continuous walls nnd 2 feet for columns . Utility trenches and deeper cut areas below about 4 feet may require rock excavating techn iques such ns chipping or blasting. Loose and disturbed soils encountered at the foundation bear ing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural soils. Voids created from boulder removal at footing grade should be filled with a structural material such as concrete or road base compacted to 98 percent standard Proctor density at a moisture content near optimum. Exterior footings should be provided with adequate cover above their bearing elevations for frost protection. Placement of footings at least 36 inches below the exterior grade is typically used in this area. 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 ns retaining structures should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on·site soil ns backfill. Floor Slabs: The natural on -site soils, exclusive of topsoil, are suitable to support lightly loaded slab -on -grade construction . To reduce the effects of some differential movement, floor slabs should be separated from all bearing walls and columns with expansion joints which allow unrestrained vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage cracking. The requirements for joint spacing and slab reinforcement should be established by the designer based on experience and the intended slab use. A minimum 4-inch layer of free -draining gravel should be placed beneath slabs to facilitate drainage. This material should consist of minus 2 inch aggregate with less than 50% passing the No. 4 sieve and less than 2% passing the No. 200 sieve. Underdrain System: Although free water was not encountered during our exploration, it has been our experience in the area that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched condition. We recommend below -grade construction, such as retaining walls and crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above the invert level with free -draining granular material. The drain should be placed at each level of excavation and al least l foot below lowest adjacent finish grade and sloped at a minimum I% to a suitable gravity outlet. Free -draining granular J ob No.116 091A -3- malerial used in the underdrain system should contain less than 2% passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have n maximum size of 2 inches. The drain gravel backfill should be at least 1 Vi feet deep. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed : 1) Inundation of the foundacion excavations and underslab areas should be avoided during construction . 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95% of the maximum standard Proctor density in pavement and slab areas and to at least 90% of the maximum standard Proctor density in landscape areas. Free-draining wall backfill should be capped with about 2 feel of the on-site, finer graded soils to reduce surface water infiltration . 3) The ground surface surrounding the exterior of the building should be sloped to drain away from the foundation in all directions. We recommend a minimum slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in pavement and walkway areas . A swale may be needed uphill to direct surface runoff around the residence. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. Limitations: This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the exploratory pits excavated at the locations indicated on Figure 1 and to the depths shown on Figure 2, the proposed type of construction, and our experience in the area. Our services do not include determining the presence, prevention or possibility of mold or other biological contaminants (MOBC) developing in the future . H the client is concerned about MOBC, then a professional in this special field of practice should be consulted . Our findings include interpolation and extrapolation of the subsurface conditions identified at the exploratory pits and variations in the subsurface conditions may not become evident until excavation is performed. If conditions encountered during construction appear different from those described in this report, we should be notified at once so re-evaluation of the recommendations may be made. This report has been prepared for the exclusive use by our client for design purposes. We are not responsible for technical interpretations by others of our information. As the project evolves, we should provide continued consultation and field services during construction to review and monitor the implementation of our recommendations, and to verify that the recommendations have been appropriately interpreted. Significant design changes may require additional analysis or modifications to the recommendations presented herein . We recommend on -site observation of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnical engineer. Job No . 116 091A -4- If you have any questions or if we mny be of further assistance, please let us know. Respectfully Submitted, HEPWORTH -PAWLAK GEOTECHNICAL, INC. Louis E. Eller Reviewed by: LEE/ksw attachments Job No.116 091A -~---~--- APPROXIMATE SCALE 1· = 50 71so - -............ _____ __ -- 7155 - ------ ------\~':> \ --...... 1 \ ........ ........ ...... ........ _______ ..,.,. LOT9 7160 -- co § l'--~----- 1 -........ ---- 716s ___ j ------1---7160 --- --I - - - - - - - - -7165 I J 7110---_, __ l::..:: ' • PIT 1 ~BUILD/NG£ -~~~ -1110 116 091A LOCATION OF EXPLORATORY PITS Figure 1 Qj Q) u.. ' ~ Q) c 0 5 10 LEGEND : PIT 1 ELEV.= 7168' wc .. 22a DD ;c 95 PIT2 ELEV.= 7165' --+4 ... 79 I -..I -200 -3 ~ TOPSOIL; organic sandy silt an clay, firm, moist, dark brown . D CLAY (Cl); sandy, silty, gravelly, stiff, moist , brown, calcareous 0 5 10 BASALT COBBLES AND BOULDERS (GC); in a sandy clay matr:x, dense, moist, light brown, calcareous 2· Diameter hand dtiven liner sample. Disturbed bulk sample . Practical digging refusal refusal in basalt boulders . NOTES : 1. Exploratory pits were excavated on April 6, 2016 with a Bobcat min-excavator. 2. Locations of exploratory pits were measured approximately by pacing from features shown on the site plan provided. 3 . Elevations of exploratory pits were obtained by interpolation between contours shown on the site plan provided 4 . The exploratory pit locations and elevations should be considered accurate only to the degree implied by the method used . 5. The lines between materials shown on the exploratory pit logs represent the approximate boundaries between material types and transitions may be gradual . 6. No free waler was encountered in the pits al the time of excavating Fluctuation in water level may occur with time 7. Laboratory Testing Aesulls : WC = Water Content (%) OD = Ory Density (pcf} +4 =Percent retained on the No . 4 sieve -200 = Percent passing No. 200 sieve Ci) Q.J u.. • .c a Q) a 116 091A LOGS OF EXPLORATORY PITS Figure 2 Moisture Content ~ 22.3 percent Dry Densi ty = 95 pcl Sample ol: Sandy Silty Clay From: Pit 1 at 2 ~ Feet 0 ---~ i'"'-. r--.. ....... ~ i.- 1 --_. r--1'1 K I' No movement -upon ~ 2 wetting c 'I\. 0 ·c;; Cl) 3 Q) ... \ a. ~ (,.) 4 \ \ 5 I~ 0 1 1 0 10 100 APPLIED PRESSURE ksl 116 091A ~ Hepworth-Pawlak ~\11chnk:al SWELL-CONSOLIDATION TEST AESUL TS Figure 3 I HYDROMETEAANAl.YSIS I SEVEANALYSIS 7 HR TIME READINGS US STANDARD SER ES I O ~~ t\~ 15 MIN 60MIN19MIN.4 MIN 1 MIN 1200 #100 150 130 116 #8 14 Cl.EAR SOUARE OPEN NGS 3/B' 314· 1112· 3· s ·6· a-100 10 90 : 20 BO 30 70 0 60 (.!) w 40 z z u; C( (/] I-< w a. 0::: so 50 I-I-z z LI.I w u u Q: 0::: UJ w 4 0 a. a. 60 70 30 BO 20 90 10 100 0 001 .002 005 009 019 .037 .074 .150 300 600 118 236 475 95 190 375 762 152 203 125 127 DIAMETER OF PARTICLES IN M LLJMETCRS GRAVEL 79 % SAND 18 % SILT AND CLAY 3 % LIQUID LIMIT PLASTICITY INDEX % SAMPLE OF. Sandy Gravel Matrix Soil FROM Pit 2 al 3 lo 4 Feet 116 091A GRADATION TEST RESULTS Figure 4 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 Job No. 116 091A SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE PIT DEPTH CONTENT DENSITY NO. 200 LIMIT INDEX STRENGTH SOIL TYPE W•> (%) (ft) (%\ lDcn SIEVE '°"' l%) IPSF) I 2 1h 22 .3 95 Sandy Silty Clay 2 3 to 4 79 18 3 Sandy Gravel f'\41.r, ~ !n ~