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HomeMy WebLinkAboutSubsoil Study for Foundation Design 12.31.2013HEPW ORTH*AWL AK ("EOTEC 1 INI( 11 December 31, 2013 Jan Burke P.O. Box 1123 Glenwood Springs, Colorado 81602 (pi poittit kc 2rte allttil.coitt Job No. 113 452A Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 77, Springridge Reserve, 34 Hidden Valley Road, Garfield County, Colorado Dear Jan: 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 agreement for geotechnical engineering services to you dated December 3, 2013. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. We previously performed a preliminary geotechnical study for the subdivision development (known as Springridge Place, Phase I1 at that time) which included a geologic site assessment and presented our findings in a report dated June 22, 2004, Job No. 101 126. Proposed Construction: Based on information provided, the residence will be a two story, wood frame structure over a crawlspace with an attached garage located on the lot as shown on Figure 1. The garage floor will be slab -on -grade. Cut depths are expected to range between about 3 to 5 feet. Foundation loadings are assumed to be relatively light and typical of the assumed 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 was vacant at the time of our field exploration and is located north of Hidden Valley Drive and east of Elk Ridge Drive as shown on Figure 1. Two small abandoned irrigation ditches crossed the site west of the proposed building envelope. The overall topography of the lot slopes gently to moderately down to the southwest from a small hill located to the east of the property. The ground surface in the proposed building area slopes strongly down to the southwest at a grade of around 10%. There is about 6 feet of elevation difference across the proposed building footprint. Steeper slopes are located east of the lot. The site was covered with 1 to 117 feet of snow at the time of our field exploration. 2_ The hill to the east of the property rises approximately 50 feet along a gently slope. Rock outcrops capable of producing boulders large enough to impact the property were not observed and the gentle slope of the hillside does not appear adequate to produce a rockfall hazard. In our opinion, a rockfall hazard to the property is not present. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits just outside of the proposed building footprint at the approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 1 foot of topsoil (root zone), consist of loose to medium dense, silty to very silty and clayey sand and sandy clay with scattered gravel and gravel lenses down to the maximum explored depth of 10 feet. Results of swell - consolidation testing performed on relatively undisturbed samples of the clay and sand soils, presented on Figures 3, 4 and 5, typically indicate low compressibility under conditions of light loading and moderate to high compressibility after wetting. No free water was observed in the pits at the time of excavation and the soils were slightly moist. Foundation Bearing Conditions: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, a shallow spread footing foundation or reinforced mat foundation should be feasible for support of the proposed residence founded on the native soils with some potential settlement risk. Isolated pad footings should be avoided and the building foundation designed to be relatively rigid to help limit building distress due to differential foundation settlement potential. Particular care should be taken to avoid wetting of the foundation bearing soils. Recommendations for a shallow spread footing or reinforced mat foundation are presented in the section below. As an alternative with a lower risk of foundation settlement, a deep foundation system could be used to support the proposed residence. Typical deep foundation systems which may be feasible at the site include drilled piers and heavy-duty "screw" piles. In order to evaluate the feasibility of a deep foundation system, at least one exploratory boring should be drilled at the site to identify the depth of relatively incompressible bearing strata (bedrock) below the proposed building footprint. If a deep foundation system is proposed, we should be contacted to provide additional analysis and recommendations. A pit dug for the preliminary geotechnical study located east of the subject site roughly on Lot 75 encountered sandstone bedrock at a depth of about 4 feet. Over -excavating the natural soil in the foundation areas to a depth of about 2 feet and placing the footings on properly compacted structural fill would also help mitigate some of the settlement risk. Required structural fill can consist of the on-site sandy soils or imported well graded sand and gravel (such as road base) devoid of vegetation, topsoil and oversized rock. The fill should be moisture conditioned to near optimum moisture, placed in maximum 8 inch loose lifts and compacted to a minimum of 98% of a standard Proctor value. The structural fill should extend at least 2 feet beyond the perimeter of the footings. Job No. 113 452A • G•irlSGech -3 - Foundation Recommendations: Spread footings or mat foundations may be placed on the undisturbed natural soil and designed for a maximum allowable soil bearing pressure of 1,200 psf. The natural soils are compressible after wetting and there could be post - construction differential foundation settlement. We expect initial settlements to be up to about 1 inch, and additional differential settlements of h to 11/2 inches depending on the depth of wetting of the bearing soils up to about 10 feet. Precautions should be taken to prevent post -construction wetting of the bearing soils. Footings should be a minimum width of 20 inches for continuous walls. 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 14 feet. The bearing soils should be moistened and compacted in footing areas. We should observe the building excavation prior to footing construction for exposed bearing conditions. Foundation and Retaining Walls: Foundation walls and retaining structures which are laterally supported and can be expected to undergo only a slight amount of deflection should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 50 pcf for backfill consisting of the on-site soils. Cantilevered retaining structures which are separate from the residence and can be expected to deflect sufficiently to mobilize the full active earth pressure condition should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of the on-site soils. 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. 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 over -compact 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.backf ll. The lateral resistance of foundation or retaining wall footings will be a combination of the sliding resistance of the footing on the foundation materials and passive earth pressure against the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated based on a coefficient of friction of 0.40. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 300 pcf. The coefficient of friction and passive pressure values recommended above assume ultimate soil strength. Suitable factors of safety should be included in the design to limit the strain which will occur at the ultimate strength, particularly in the case Job No. 113 452A GiMech -4 - of passive resistance. Fill placed against the sides of the footings to resist lateral loads should be compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. Floor Slabs: The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab -on -grade construction. The soils are typically compressible and there is a risk of slab settlement and distress, especially if the subgrade soils are wetted. Care should be taken to prevent post -construction wetting of the slab subgrade soils. 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. All fill materials for support of floor slabs should be compacted to at least 95% of maximum standard Proctor density at near optimum moisture content. Required fill can consist of the on-site soils or imported well graded sand and gravel (such as road base) devoid of vegetation, topsoil and oversized rock. Surface Drainage: Keeping the bearing soils dry will be critical to Iimit building settlement and distress. An underdrain should not be provided next to building foundations to help inhibit collecting and spreading of water to the bearing soils. 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. 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 of the structure to direct surface runoff around the residence and should be designed by a civil engineer. 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 the building. Consideration should be given to the use of xeriscape to limit potential wetting of soils below the foundation 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 Job No. 113 452A -5 - 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. 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 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, pleas: call our office. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECH AL, INC. 47281 James A. Parker, P.E., P.G. Reviewed by: 7-t Steven L. Pawlak, P.E. JAP/ksw attachments Figure 1 — Locations of Exploratory Pits Figure 2 -- Logs of Exploratory Pits Figures 3, 4 & 5 — Swell -Consolidation Test Results Table 1 — Summary of Laboratory Test Results cc: Girard Homes -- Roger Girard (giards(trof.net) Juga No. I i 3 452A G& 'tech LOT 78 / *. f / / LOT 77 / 1/ !/ /' / / /' TST IT 11 ,rr /' t �. t� / PROPO ED ,' `---".j_. �: `I;�� ES PIT 2 // r //i V \ O •• \ \ ` / / \ S. / l f /f / / r r ELK RIDGE DRIVE \ \ ,/ / TO HIDDEIf'\ • \N./ / VALLEY • ti\ DRIVE \ \ N \r / / r / /' LOT 76 / / / / rr APPROXIMATE SCALE: r r / / ' 1"=30' / 113 452A Gtech 1i1SPWURTH. AWIJ(KGE01ECIiNIC�I LOCATIONS OF EXPLORATORY PITS FIGURE 1 DEPTH - FEET 10 15 LEGEND: SI PIT 1 ELEV. =96' WC= 3.7 DD=99 -200=61 PIT 2 ELEV. =99' WC=4.5 DD=87 0 5 10 15__, TOPSOIL; silty sandy clay; with roots, moist, brown. CLAY AND SAND (CL -SC); silty, with gravel and gravel lenses, medium dense, slightly moist, light brown. SAND AND GRAVEL (SM -GM); silty and slightly clayey, gravel lenses, loose to medium dense, slightly moist, brown - red brown. Relatively undisturbed 2 -inch diameter liner sample. Disturbed bulk sample. NOTES: 1. Exploratory pits were excavated on December 11, 2013 with a backhoe. 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 water was encountered in the pits at the time of excavating. 7. Laboratory Testing Results: WC = Water Content (©/o ) DD — Dry Density ( pcf ) -200 = Percent passing No. 200 sieve 113 452A Gtech IEPWORTII-PAwIJFK(LO1LCIiNAl LOGS OF EXPLORATORY PITS FIGURE 2 COMPRESSION (% ) 0 1 2 3 4 5 6 7 8 9 Moisture Content = 33 percent Dry Density = 99 pot Sample of: Silty Sandy Clay From: Test Pit 1 at 1 1/2 Feet MIN MA. IN= EMICompression upon wetting 1111111 01 10 10 APPLIED PRESSURE (ksf ) 100 113 452A Glcrtech HEPWOI TII.PAW4.AKGEOTECIINICAL SWELL. -CONSOLIDATION TEST RESULTS FIGURE 3 COMPRESSION ( % 2 Moisture Content - 7.2 percent Dry Density = 100 pcf Sample of: Silty Clay and Sand From: Test Pit 1 at 7 Feet Compression upon wetting 0.1 1 0 10 APPLIED PRESSURE (ksf ) 100 113 452A Gtech k ILInNORTI I-PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS FIGURE 4 COMPRESSION (% ) 10 12 14 16 18 Moisture Content = 4.5 percent Dry Density = 87 pcf Sample of: Silty Clay and Sand From: Test Pit 2 at 4 Feet (Possible Disturbed Sample) Compression upon wetting b 01 113 452A 10 10 APPLIED PRESSURE ( ksf ) G1,-.1Stech I LPWCIliT11 PAW LAK GCOTEC1 I NICAL SWELL -CONSOLIDATION TEST RESULTS 100 FIGURE 5 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 113 452A SAMPLE LOCATION NATURAL MOISTURE CONTENT (961 NATURAL DRY DENSITY (Poi GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH (PSF) I SOIL OR BEDROCK TYPE TEST PIT DEPTH (tt) GRAVEL (%) SAND (%) r LIQUID LIMIT (%) P3.A5TlC INDEX , (%1 1 1 1/2 3.7 99 61 Silty Sandy Clay ! 3 1/2 4.3 87 Silty Clay and Sand 7 7.2 100 Silty Clay and Sand 2 4 4.5 87 Silty Clay and Sand