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HomeMy WebLinkAboutSubsoil StudyHEPWORTH - PAWLAI< GEOTECHNICAL October 31, 2005 Jordan Architecture Attn: Darryl Anderson P.O. Box 1031 Glenwood Springs, Colorado 81602 Hc1�tr, rt1,-r'a�;^laL (3eoteQlsnical, l t. 502( Counry Road 154 Glenwood Spring:, Colorado 81601 PIir ne: 970-9'15-7(M Fa':: 970-945..S45N email: hpg lftilpgr<)tec1;.c,.nn Job No.105 855 Subject: Subsoil Study for Founc:ation Design, Proposed Burkholder Residence, Lot A-14, The Peninsula at Aspen Glen, Whispering Water and River's Bend, Garfield County, Colorado. Dear Mr. Anderson: As requested, Hepworth-Pawiak Geotechnical, Inc. performed a subsoil study for design of foundations at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to Jordan Architecture dated September 23, 2005. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: The proposed residence will be a two story wood frame structure with an attached garage. Ground floor will either be structural over a crawlspace or a basement level with slab -on -grade floor. Cut depths are expected to range between about 3 to 10 feet. 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 site was vacant at the time of our field exploration. There is sone fill on the lot from overlot grading as part of the subdivision development. The ground surface is relatively flat with a slight slope down to the south. There is about 2 to 3 feet of elevation difference across the lot. Vegetation consists of scattered grass and weeds. Cobbles are exposed on the ground surface. Subsidence Potential: Aspen Glen is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits. 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, sinkholes were observed in Aspen Glen development. Sinkholes were not observed in the immediate area of the subject lot. The nearest mapped sinkhole is approximately 1540 feet to the north- northwest of the lot. The exploratory pits were relatively shallow, for foundation design only. Based on our present knowledge of the site, it cannot be said for certain that sinkholes will not develop. In our opinion, the risk of ground subsidence at Lot A-14 Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 FILE copy -2 - throughout the service life of the residence is low and similar to other lots in the area but the owner should be aware of the potential for sinkhole development. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating three exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 1/2 to 3 feet of clay fill, generally consist of relatively dense, slightly silty sandy gravel with cobbles and scattered boulders. A 1 to 3 foot thick slightly silty to silty sand layer was encountered within the gravels in Pits 1 and 2. Results of a gradation analysis performed on a sample of the gravels (minus 5 inch fraction) obtained from the site are presented on Figure 3. The laboratory testing is summarized on Table 1. No free water was observed in the pits at the time of excavation and the soils were slightly 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 gravel soil designed for an allowable soil bearing pressure of 3,000 psf for support of the proposed residence. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Topsoil and loose disturbed soils encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural gravel soils. 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 10 feet. Foundation walls acting as retaining structures should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 45 pcf for the on-site soil as 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 flee -draining gravel should be placed beneath basement level slabs to facilitate drainage. This material should consist ofminus 2 inch aggregate with less than 50% passing the No. 4 sieve and less than 2% passing the No. 200 sieve. All fill materials for support of floor slabs should be compacted to at least 95% of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on-site soils devoid of vegetation, topsoil and oversized rock. 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 Job No, l 05 855 satech _3 - walls, crawlspace and basement 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 at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to a suitable gravity outlet dry well or sump and pump, Free -draining granular material 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 a maximum size of 2 inches. The drain gravel backfill should be at Least 11/2 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 foundation 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 feet of the an -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 6 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. 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 xnold or other biological contaminants (MOBC) developing in the future. If 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 Job No.105 855 -4 - 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. If you have any questions or if we may be of further assistance, please let us know. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Jordy Z. Adamson, Jr., P.E. Reviewed by: Daniel E. Hardin, P.E. JZA/ksw attachments Figure 1-- Location of Exploratory Pits Figure 2— Logs of Exploratory Pits Figure 3 -- Gradation Test Results Table 1-- Summary of Laboratory Test Results Job No.105 B55 1 LOT Aly APPROXIMATE SCALE 1"=30' wRf `SPtR/!vC 1yq TER 7 r 1 \ ,.' 1 i 1 PIT 1 1 1 1 LOT A14 I 1 1 PIT 2 L1 1 ■ ` // 1 / W......_„ / \\ PIT ■ ` // r r r r LOT A13 1 � ` ~~ '� - BENCH MARK; GROUND SURFACE AT PROPERTY CORNER; ELEV. = 100.0; ASSUMED. RIVER'S BEND 105 855 Hepworth -Pawl* Geot- foal LOCATION OF EXPLORATORY PITS Figure 1 5 10 LEGEND: PIT 1 ELEV.= 102.6' WC=1.9 DD=97 -200=5 PMT 2 ELEV.= 101.0' a y i + 1 +4=62 j -200=6 WC=2.7 DD=101 -200=4 PIT 3 ELEV.= 101,2' FILL; sandy silty clay, organics in Pit 3, firm, slightly moist, brown to reddish brown. SAND (SP -SM); slightly silty to silty, medium dense, slightly molst, light brown. GRAVEL (GP -GM); sandy, slightly silty with cobbles and scattered boulders, dense, slightly moist, light brown, subrounded to rounded rock. 2' Diameter hand driven liner sample. Disturbed bulk sample. 0 5 10 NOTES: 1. Exploratory pits were excavated on October 11, 2005 with a Caterpillar 420D backhoe. 2. Locations of exploratory plts 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 and checked by instrument level. 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 water was encountered In the pits at the time of excavating. Fluctuation In water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) +4 = Percent retained on the No. 4 sieve -200 = Percent passing No. 200 sieve 105 855 M Hap worth—PawIat atatttnical LOGS OF EXPLORATORY PITS Figure 2 • .MMM.••.. Ha, HYDROMETER ANALYSIS j SIEVE ANALYSIS TIME 1 p 24 MIN. 15 MIN. 6QM N19M44 MIN, READINGS, 1 RAIN. #2100 #100 U # 0 #30S. 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MM i I.Ir.I.i rr►�=NEM MEN MMr•laMi AMi IUP•_ lr Ml•i M—••I MAMA WMMMMM.a•1..I11I111•M !—.M•i MM•01 =M.MI lMMMii iMIr�"Ii i011 `�MO .MMMT ..Mi =U MMI !li IMIM Mr•r iMa.l i•MIMM 0=ZMI M• MIMIi l aMMM i =• M•••• mi !MIEN I MiI••••••=1M NII^ Mt Ili.Ia CLAY TOSILT DIAMETER OF PARTICLES IN MILLIMETERS SAND dRAVEi FINE 1 IJEDIUM }COARSE fl i mum GOBBLES GRAVEL 62 % LIQUID LIMIT q SAMPLE OF: Slightly Silty Sandy Gravel with Cobbles SAND 32 % SILT AND CLAY 6 % PLASTICITY INDEX % FROM: Pit 2 at 5 to 7 Feet 90 80 70 60 50 40 30 20 10 0 T PAS ,r 105 855 • H GRADATION TEST RESULTS Hepworth—Powlak Geotec1 nTeal Figure 3 0 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 Job No. 105 855 SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION PIT DEPTH (ft) NATURAL MOISTURE CONTENT (%) NATURAL DRY DENSITY (p) GRADATION GRAVEL ((XI) SAND (%) PERMIT PASSING NO. 200 SIEVE ATTERBERG LIMITS LIQUID PLASTIC LIMIT INDEX (°) (%) UNCONFINED COMPRESSIVE STRENGTH (PSF) SOIL OR BEDROCK TYPE 1 6 1.9 97 5 Slightly silty sand 2 5-7 62 32 6 1 Slightly silty sandy gravel with cobbles i 7 2.7 101 4 sand