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Home My WebLink AboutSubsoil StudyG 0"oftv 1-i tech HEPWORTH-PAWLAK GEOTECHNICAL SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED ADDITIONS LOT 10, STIRLING RANCH 704 SKIPPER DRIVE GARFIELD COUNTY, COLORADO JOB NO. 115 125A MAY 29, 2015 PREPARED FOR: SCOTT POWELL AND LEAH O' DONNELL 704 SKIPPER DRIVE CARBONDALE, COLORADO 81623 (scott.yowell(gy,axis oillt.co TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY........................................:....................... - 1 - PROPOSED CONSTRUCTION...................................................................................- 1 - SITECONDITIONS....................................................................................................- 2 - -FIELFIELD D EXPLORATION..............................._..........................:...................................- 2 - -SUBSURFACE SUBSURFACE CONDITIONS..............................._....................................................- 2 - -DESIGN DESIGN RECOMMENDATIONS...............................................................................- FOUNDATIONS......................................................................................................- 3 - FOUNDATION AND RETAINING WALLS 3 - ..............................._...........................- FLOORSLABS 4 - .......................................................................................................- UNDERDRAINSYSTEM 5 - ........................................................................................- SURFACEDRAINAGE 5 - ...........................................................................................- 6 - LIMITATIONS............................................................................................................- 6 - -FIGURE FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURES 4 AND 5- SWELL -CONSOLIDATION TEST RESULTS TABLE I- SUMMARY OF LABORATORY TEST RESULTS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for proposed additions to be located at Lot 10, Stirling Ranch, 704 Skipper Drive, Garfield County, Colorado. The project site is shown on Figure 1. The purpose of the study was to develop recommendations for the foundation design. The study was conducted in accordance with our proposal for geotechnical engineering services to you dated March 19, 2015. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils obtained during the field exploration were tested in the laboratory to determine their classification, compressibility or swell and other engineering characteristics. The results of the field exploration and laboratory testing were analyzed to develop recommendations for foundation types, depths and allowable pressures for the proposed building foundation. This report summarizes the data obtained during this study and presents our conclusions, design recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION The proposed residence addition will be a two story wood frame structure above a crawlspace. The garage addition will be one story wood frame construction with slab -on - grade floor. Grading for the structures is assumed to be relatively minor with cut depths between about 3 to 4 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. The additions will be located on the east side of each existing structure. If building loadings, location or grading plans change significantly from those described above, we should be notified to re-evaluate the recommendations contained in this report. Job No. 115 125A GetPtech -2- SITE CONDITIONS The existing residence is one and two story wood frame construction. The existing detached garage is a one story wood frame structure with slab -on -grade floor. The area is landscaped with evergreen trees, brush and grass. The site is located on an upland rolling mesa and the addition building area is on a relatively flat topographic knoll with slight slopes. FIELD EXPLORATION The field exploration for the project was conducted on April 7, 2015. Two exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. The borings were advanced with 4 inch diameter continuous flight augers powered by a truck -mounted CME -45B drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with 1% inch and 2 inch I.D. spoon samplers. The samplers were driven into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This test is similar to the standard penetration test described by ASTM Method D-1586. The penetration resistance values are an indication of the relative density or consistency of the subsoils. Depths at which the samples were taken and the penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils were variable and consisted of gravelly sandy silt overlying basalt cobbles and boulders in a sandy silt matrix at Boring 1 (garage addition) and about 6 inches of Job No. 115 125A Ge Ptech -3- topsoil overlying sandy silty clay at Boring 2 (house addition). Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit at Boring 1. Laboratory testing performed on samples obtained from the borings included natural moisture content, density and percent finer than sand size gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4 and 5, indicate low to moderate compressibility under conditions of loading and wetting. The sample tested from Boring 2 at 5 feet showed a minor expansion potential when wetted. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist to moist. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, we recommend the building be founded with spread footings bearing on the natural soils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural granular soils should be designed for an allowable bearing pressure of 1,500 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. 2) The footings should have a minimum width of 16 inches for continuous walls and 2 feet for isolated pads. 3) Exterior footings and footings beneath unheated areas should be provided with adequate soil cover above their bearing elevation for frost protection. Job No. 115 125A G(eTPtech Placement of foundations at least 42 inches below exterior grade is typically used in this area. 4) 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 should also be designed to resist a lateral earth pressure corresponding to an equivalent fluid unit weight of at least 50 pcf. 5) All existing fill, topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively undisturbed natural soils. The exposed soils in footing area should then be moistened and compacted. If water seepage is encountered, the footing areas should be dewatered before concrete placement. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FOUNDATION AND RETAINING WALLS All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The pressures recommended above assume drained conditions behind the walls and a horizontal backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls. Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at a moisture content near optimum. Backfill in pavement and walkway areas should be compacted to at least 95% of the maximum standard Proctor density. Care should be taken not to overcompact the backfill or use large equipment near the wall, since this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall backfill should be expected, even if the material is placed correctly, and could result in distress to facilities constructed on the backfill. Job No. 115 125A G( CTteCh -5 - 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 at least 50% retained on 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 granular 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 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 at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to a suitable gravity outlet. Free -draining granular material used in the underdrain system should contain less than 2% passing the No. 200 Job No. 115 125A r i GecPtech M 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 1 %2 feet deep. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the additions have 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. 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 paved areas. Free -draining wall backfill should be capped with about 2 feet of the on- site soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation should be located at least 10 feet from foundation walls. Consideration should be given to use of xeriscape to reduce the potential for wetting of soils below the building caused by irrigation. 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 borings drilled at the locations indicated on Figure 1, the proposed type of construction and our experience in the area. Job No. 115 125A Ge~cPitech -7 - 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 extrapolation of the subsurface conditions identified at the exploratory borings 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 so that 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. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Louis E. Eller Reviewed by: Steven L. Pawlak, P.E. LEE/ksw cc: Black Shack Architects - Glenn Rappaport ( Job No. 115 125A Gtech / 1 6 7158 JA55 / \--� /� 7159 7160 ,76 EXISTING EXISTING SEPTIC TANK RESIDENCE 611 j6' \ \ EXISTING RETAINING WALL 7159 / \ \ \ \ - ^ r EXISTING RESIDENCE \ _ EXISTING 1 � EXISTING GARAGE SEPTIC TANK \---�CONCRETE 11 DRIVE /BORING 1 BORING 2 0 �7��\ J I \ I GRAVEL \� DRIVE \ APPROXIMATE SCALE 1"= 30' 115 125A I I LOCATION OF EXPLORATORY BORINGS I Figure 1 I-Isanwnhh_Cn.uL.L ee.,a__a. _;__. BORING 1 ELEV.= 7158.3' I 15 20 BORING 2 ELEV.= 7158.7' 11/12 WC=18.6 DD=104 11/12 WC=9.6 DD=98 11/12 WC= 11.0 DD=98 -200=92 14/12 Note: Explanation of symbols is shown on Figure 3. U 5 10 u. 15 20 115 125A II LOGS OF EXPLORATORY BORINGS I Figure 2 Hepworth—Pawlak Geotechnirnl 0- n 18/12 WC=14.5 DD=84 5 11/12 WC=15.0 DD=70 va. 200=14 e,Do 50/0 10 LL 0 - CD 15 20 BORING 2 ELEV.= 7158.7' 11/12 WC=18.6 DD=104 11/12 WC=9.6 DD=98 11/12 WC= 11.0 DD=98 -200=92 14/12 Note: Explanation of symbols is shown on Figure 3. U 5 10 u. 15 20 115 125A II LOGS OF EXPLORATORY BORINGS I Figure 2 Hepworth—Pawlak Geotechnirnl 0- n LEGEND: ® TOPSOIL; organic sandy silt and clay, moist, brown. CLAY (CL); silty, slightly sandy to sandy, medium stiff, moist, brown. PSAND AND SILT (SM -ML); gravelly, medium dense, slightly moist, calcareous, white to light brown. F.M. H. - BASALT COBBLES AND BOULDERS (GM); in a sandy silt matrix, dense, slightly moist, light brown, calcareous. Relatively undisturbed drive sample; 2 -inch I. D. California liner sample. Drive sample; standard penetration test (SPT), 1 3/8 inch 1. D. split spoon sample, ASTM D-1586. 18/12 Drive sample blow count; indicates that 18 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. TPractical drilling refusal. NOTES: 1. Exploratory borings were drilled on April 7, 2015 with 4 -inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory borings were obtained by interpolation between contours shown on the site plan provided and checked by instrument level. 4. The exploratory boring 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 boring logs represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the borings at the time of drilling. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) -200 = Percent passing No. 200 sieve 115 125A I I LEGEND AND NOTES Hepworth—Pawlak Geotechnical Figure 3 0 1 2 0 3 c 0 C/) 4 N n E 0 U 5 6 7 0 01 � 1 0 0 n 2 E 0 U 3 4 Moisture Content = 14.5 Dry Density = 84 Sample of: Gravelly Sandy Silt From: Boring 1 at 2 % Feet Compression upon wetting 7rcent 0.1 1.0 10 100 APPLIED PRESSURE - ksf Moisture Content = 18.6 Dry Density = 104 Sample of: Sandy Silty Clay From: Boring 2 at 2 % Feet Compression upon wetting percent pcf � 0.1 1.0 10 100 APPLIED PRESSURE - ksf 115 125A ~ ��ri SWELL -CONSOLIDATION TEST RESULTS Figure 4 HeDworth—Pawlak Genterhnirnl Moisture Content = 9.6 percent Dry Density = 98 pcf Sample of: Sandy Silty Clay From: Boring 2 at 5 Feet 1 0 0 0 1 c c� s? x Lu 2 0 Expansion U, upon co 3 wetting a E 0 U 4 5 0.1 1.0 10 100 APPLIED PRESSURE - ksf 115 125A II SWELL -CONSOLIDATION TEST RESULTS I Figure 5 Hepworth—Pawlak GeoterhninM Q LO N T" LO r O z m O (j U) Z J J fn W U � Z F— 2 cn U W W ~ F- } W Cf - 0 aJ0 0 a�- _j a LL = O IX � o a 0 - LU X Cl) c� W 0 U CL O Y cC M wx U U 8 w V)-� >,�Cj 00 a� v� C7 rid v� v� W w>x W 0 CL a 0� j U U N old QZ Mi- CL cD w w LU J J