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HomeMy WebLinkAboutSubsoil Study for Foundation Design 10.30.15~ech HEPWORTH-PAWLAK GEOTECHNICAL October 30, 2015 Dan and Ayla Cacho danandayla@hotmail .com H1.p" 11rt h • P011\ l.11.: Gcn11:ch nu::il, Inc.. 5020 Coumy Ro.1J 154 Glcnwo .. .J Sprini:\, C.1luradt> 81601 Phone. 970-941-7988 F.r1: 9 iCl.9-l 5 -!H 5-f cm.nl hri:co@hpi:c•ncch.wm Job No.115 461A Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 25, Lookout Mountain Ranches, Lookout Mountain Road, Garfield County, Colorado. Dear Mr. nnd Mrs. Cacho: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design of foundations at the subject site. The study was conducted in accordance with our proposal for geotecbnical engineering services to you dated September 14, 2015, Proposal No. 165-15. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: At the time of our study, design plans for the residence had not been developed. The proposed residence is assumed to be a I to 2 story structure located in the area of Pits I and 2 on the site as shown on Figure 1. Ground floor is assumed be slab-on-gl".ide or structural above crawlspace. Cut depths are assumed to range between about 3 to 6 feel. Foundation loadings for this type of construction are assumed to be relutively light and typical of the proposed type of construction. When building location, grading and loading information have been developed, we should be notified to re-evaluate the recommendations presented in this report. Site Conditions: The site was vacant at the time of the field explor.ition. The proposed location for the residence is situated on a south facing hillside. The ground surface within the proposed building area is gently sloping down to the south and east with about 3 feet of elevation difference . The ground falls away more steeply below the proposed building area. Vegetation on the site consisted of scrub oak trees with limited grasses and weeds. Subsurface Conditions: The subsurface conditions at 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 Y.z to 1 foot of topsoil, consist of fractured and broken basalt rock in a silt matrix. Results of a gradation analysis performed on a sample of the fractured basalt obtained from the site are presented on Figure 3. No free water was observed in the pits at the time of excavation and the soils were slightly moist to moist. Parker 303-841-7119 • Culor,1Jo Sprmgs 719-633·5 562 • Silverthorne 970-468-l 989 -2 - 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 rocky soil designed for an allowable soil bearing pressure of 2,500 psf for support of the proposed residence. The soil matrix could tend to compress after wetting and result in minor post-construction foundation settlement of less than 1 inch. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. The 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 soils. Voids created by large rock removal should be backfilled with compacted road base or with concrete. 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 lo span local anomalies such as by assuming an unsupported length of at least 12 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 excluding rock larger than about 6 inches. 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 basement level 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. 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. Underdraln System: Although free water was not encountered during our exploration. it has been our experience in mountainous areas thnt 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, 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 al least I foot below lowest adjacent finish grade and sloped at a minimum I% to a suitable gravity outlet. Free-draining granular Job No llS461A material used in lhe undcrdrain syslem should contain Jess than 2% passing the No. 200 sieve, less 1han 50% passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least 1 ~ 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 on-site, finer graded soils to reduce surf ace 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 I 0 feel in unpaved areas and a minimum slope of 3 inches in the first l 0 feet in pavement and walkway areas. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. Percolation Testing: Percolation tests were conducted on October 8, 2015 to evaluate the feasibility of an infiltration septic disposal system at the site. One profile pit and three percolation holes were dug at the locations shown on Figure 1. The test holes (nominal 12 inch diameter by 12 inch deep) were hand dug at the bottom of shallow backhoe pits and were soaked with water one day prior to testing. The soils exposed in the percolation holes are similar to those exposed in the Profile Pit shown on Figure 2 and consist of extremely gravelly sand (USDA Classification). The percolation test results are presented in Table 2. Based on the subsurface conditions encountered and the percolation test results, the site should be suitable for an infiltration system but could require a sand filter system. A professional engineer should design the infiltration septic disposal system. 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 I 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. If the client is concerned about MOBC, then a professional in this special field of practice should be consulted. Our findings include interpolation and Job Nol l!i 461A -4 - 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. If you have any questions or if we may be of funher assistance, please let us know . Respectfully Submitted, HEPWORTH -PAWLAK GEOTECHNICAL, INC . e-r;:: c ~u..-- Tom C. Brunner -Staff Engineer Reviewed by: TCB/ksw Attachments: Job No I IS 46lA APPROXIMATE SCALE 1· -200' RANCH22 I I I I I I I I I I I I I I I I I I I I I / I .::~\ 4";;::>-- ~ \. ·~'(s(~~,,. ,,. ~, ,,.~,,. RANCH24 '\ \ ,,. ,,. ,,. ,,. ,,. ,,. \.,,. ,,. / ~ y,,. I I I / ./ / ___________ .,,..,,. / / / RANCH25 I -----~--­r---- \ --------~ / ~-----EASEMENT_____ ~ \ ~ -~----~-~~~ ' -------~ , '~ ~~----~~-'' /,,. ..... ----\ \ / ,,. \ I / \ \ \ I I \ l \ I I \ \ I P 1 • PIT 1 • PIT2 \ ~ ,' P3 A A p 2 A • PROFILE I / I I / I I NOTE: PIT LOCATIONS ARE RELATIVE TO EACH OTHER AND NOT TO SCALE. PIT RANCH29 I I ....... ..... ....... ....... ....... ....... ....... ....... ..... ............ ........ ....... ...... 115 461A LOCATION OF EXPLORATORY PITS Figure 1 'a> Q) U-• ..c a. ~ 0 5 10 LEGEND: PIT 1 PIT 2 --I -i +4 -70 -.J ·200 " 8 I -.J ~ TOPSOIL; organic sandy silt and clay, sUghlly moist, black. PROFILE PIT 0 --; GRAVEL 54 • SAND •35 ! SILT -9 5 I CLAY -2 -.J LL -53 Pl •NP 10 ~ BASALT ROCK (GM): fractured and broken in a sill matrix, medium dense to dense, slightly moist , mixed ~ brown and gray. ~ 2· Diameter hand driven liner sample. ~ --: Disturbed bulk sample. ~ _.J NOTES: 1. Exploratory pits were excavated on October 7, 2015 with a mini-excavator. 2 . The exploratory pits were excavated at the house and septic disposal areas designated by the ciient. 3 . Elevations of exploratory pits were not measured and the logs ol exploratory pits are drawn to depth. 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 lree water was encountered In the pits at the time of excavating. Fluctuation in water level may occur with time 7 . Laboratory Testing Results: +4 = Percent retained on the No. 4 sieve -200 = Percent passing No. 200 sieve LL = Liquid Limit (%) Pl = Plasticity 1ndex (%) NP = Non-plastic Gravel = Percent retained on No. 10 Sieve Sand = Percent passing No. 1 o sieve and retained on No. 325 sieve Silt = Percent passing No. 325 sieve to particie size .002mm Clay= Percent smaller than particle size .002mm 1 .... I J:. a ~ 115 461A LOGS OF EXPLORATORY PITS Figure 2 a w z ~ "" a: ... z l&J u ffi D. I HYDROMETER ANALYSIS I SIEVE ANALYSIS CLEAR SQUARE OPEN NGS R 7 HR Tt.1E READINGS U S STANDARD SERIES j O ~~ ~N 15 MIN 60MIN19MIN 4 MIN 1 MIN 1200 1100 150 130 I 16 18 114 318" 3/4 11/'Z 3' 5'6" 8' 100 10 20 30 4() 50 60 70 80 90 100 .001 .002 005 009 019 037 074 150 300 600 118 236 4 75 95 190 37 5 12 5 76 2 152 203 12 7 DIAMETER OF PARTICLES IN MILLIMETERS CUT10SU GRAVEL 70 % SAND 22 O ' .o SILT AND CLAY B o • 10 UOUIOLIMIT % PLASTICITY INDEX o • .o SAMPLE OF: S~ghtly Silty Sandy Gravel FROM: Pil 2 al 3 lo 4 Feel 90 80 7l) 60 50 40 30 :L'O 10 0 115 461A ~ GRADATION TEST RESULTS Figure Ht0wurth-Pawlak Geotlldlnlcal u z Vi (IJ 4( a. r-z bJ u ffi EL 3 0 UI z ~ UJ a:: ~ z UJ 0 a:: It I HYDROMETER NW. VSIS I SIEVE ANALYSIS 24 R 7 HR TIME READINGS 1 MIN O 45 ~IN 15 MIN 60MIN19MIN 4 MIN #325 ,140 us STANDARD SERIES I #60 #JS #18 #10 114 Cl.EAR SQUARE OPENINGS 318 ' 314· 1 1/2" 3' 5' 6 8' 100 10 90 20 80 30 70 40 60 so 50 60 40 70 30 eo 20 90 10 100 0 001 .002 .oos .009 019 045 106 025 soo 1 00 2 .00 4 75 9 5 19 .0 37.5 762 152 203 DIAMETER OF PARTICLES IN MILLIMETERS Sll I v UE I lff. I .. s:. !CO"RSE ,, _, SMoU. I .. DU.I ,'ofl twi I CXlllllLfS GRAVEL 54 % SAND 35 % SILT 9 % CLAY 2 % USDA SOIL TYPE : Extremely Gravelly Sand FROM: Profile Pit at 3 to 6 Feet 115461A ~ USDA GRADATION TEST RESULTS Figure 4 Hepworth-Pawlak Geotedvllcal HEPWORTH·PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 115 461A SAMPLE LOCATION NATURAL NATURAL GRADATION PERCENT USDA SOIL TEXTURE MOISTURE ORV PASSING SOIL TYPE PIT DEPTH CONTENT DENSITY GRAVEL SAND NO. 200 GRAVEL SAND SILT CLAY SIEVE (fl) (•Ao) (pcf) (%) (%) (%) (%) (%) (%) 2 3-4 5.4 70 22 8 Slightly Silty Sandy Gravel Profile I 3-6 14.8 54 35 9 2 Extremely Gravelly Sand . . . HOLE NO . HOLE DEPTH (lNCHESt P·l 32 P-2 40 P-3 36 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 2 PERCOLATION TEST RESULTS LENGTH OF WATER DEPTH WATER DEPT H INTERVAL AT START OF AT ENO OF (MIN) INTERVAL INTERVAL (INCHES) {INCHES) 10 4 3~ 3% 3Y. Water Added SY. 4~ 4Ya 3~ 3y, 2Y. 2Y. rn Water Added 4il 3y, 3Ya 2Y, 10 4 2 Water Added SY, 3% 3Y. 1 Water Added SYi 3y, 3:.S 1% Water Add ed s 3 3 1 Water Added SY, 3y, 10 4y, 3Y. 3Y. 2 Water Added 4y, 3y, 3y, 2 Water Added 4 2Y. 2Y. Yi Water Added 4~ 2Y, 2Yi % JOB NO. 115 461A DROP IN AVERAGE WATER LEVEL PERCOLATION (INCHES) RATE (MIN./INCH) % ~ % 1 iy. y. 1 1 11 2 2Y. 2Y. 2 1% 2 2 2 s lY. rn 1 rn rn 1% 2 1% 5.5 Note: Percolation test holes were hand dug in the bottom of backhoe pits and soaked on October 7, 2015 . Percolation tests were conducted on October 8, 2015. The average percolation rates were based on the last three readings of each test.