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HomeMy WebLinkAboutSubsoil Study for Foundation Design 02.15.16~ech Hep1vunh ,P;11vlal: G.:011.-chnic;il, In ... 5020 0111mr Ro~J 154 GlenwocJo.I Srrini:s. Colm:1d11 f.1601 Phone: 970-945·7988 HEPWORTH-PAWi.AK GEOTEG.INtCAL-F:ix: 970-94j·84H -r~1v1r~, 'J ~~· J: k . ··:...,. J "-' ~m.1il : hrl!~o1t\.p1?co11:ch .com JUL O 5 1016 ,.,,,!-< ... cL0vvU'"i1 ,,,..,., .. ... o· · t:' ;r,\M~(,1 '( Dt' rl ,r- .SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 30, SUN MEADOW ESTATES SOUTH MEADOW COURT SOUTH OF ANTONELLI LANE (COUNTY ROAD 216) GARFIELD COUNTY, COLORADO JOB NO. 116 019A FEBRUARY 15, 2016 PREPARED FOR: LUIS ARREOLA 698 BRISTLECONE WAY SILT, COLORADO 81652 arreola2006@gmail.com Parker 303-84l-7119 ° ColoradoSprings 719-633-5562 ° Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY .............................................................................. - I - PROPOSED CONSTRUCTION .......................................... -._. ............................... -I - SITE CONDITIONS······························································-··-··································-2 - FIELD EXPLORATION ................................................................................................. -2 - SUBSURFACE CONDITIONS ...................................................................................... -2 - FOUNDATION BEARrnG CONDITIONS .................................................................. - 3 - DESIGN RECO~NDATIONS ············!···································································-3. -FOUNDATIONS ........................................................................................................ -3 - FOUNDATION AND RETAmING WALLS ........................................................... -5 - FLOOR SLABS .......................................................................................................... - 6 - UNDERDRAIN" SYSTEM .......................................................................................... -7 - SU"RFACE DRAINAGE ............................................................................................. -8 ~ LJl\.fIT ATIONS ............................................................................................................... • 9 - AGURE 1 -LOCATION OF EXPLORATORY BORINGS AGURE 2 -LOGS OF EXPLORATORY BORINGS FIGURE 3 -LEGEND AND NOTES FIGURES 4 ANDS -SWELL-CONSOLIDATION TEST RESULTS TABLE l-SUMMARY OF LABORATORY TEST RESULTS Job No . 116 019A PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 30, Sun Meadow Estates,.South Meadow Court, Garfield County, Colorado. The project site is shown on Figure 1. The purpose of the study was to develop recommendations for foundation design. The study was conducted in accordance with our agreement for geotechnical engineering services to Luis Arreola, dated February 3, 2016. 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, recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION At the time of our study, design plans for the residence had not been developed. The building is proposed in the location shown on Figure I. The residence will be one story over a walkout basement or crawlspace with an attached garage. Basement and garage floors are proposed to be slab-on-grade. The house will be about 2,800 square feet in size. We assume excavation for the building will have a maximum cut depth of one level, about 10 feet below the existing ground surface. For the purpose of our analysis, foundation loadings for the structure were assumed to be relatively light and typical of the proposed type of construction. Job No . 116 Ol9A -2- If bm1ding loadings, location or gialfing plans are significantlylfiffi:rentfrom those describeil above, we should be notified lo re-evaluate the recommendations contained in this report. .sITE CONDITIONS The property was vacant and covered with 1 ~ feet of snow at the time of our site visit. The snow had been plowed by the client in the area of the borings to allow access to our drill rig. The site slopes gently to moderately down to the west. Vegetation consists of grass and weeds with scattered brush. There were some deciduous trees on the west side of the lot. FIELD EXPLORATION The field exploration for the project was conducted on February 5, 2016. Two exploratory borings were drilled at the locations shown on Figure I to evaluate the subsurface conditions. The borings were advanced with 4-inch diameter continuous flight auger powered by a truck-mounted CME-45B drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotecbnical, Inc. Samples of the subsoils were taken with a 2 inch I.D. spoon sampler. The sampler was 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 profiles encountered at the site are shown oa Figure 2. Below about 1/z foot of organic topsoil, the subsoils consist of 13~ to 15~ feet of very Job No. 116 Ol!>A -3- stiff to hard, sandy silty clay overlying up to 27 feet of sandy silt and clay. At a depth of about 41 feet in Boring 1, dense silty sandy gravel with cobbles was encountered down to the bottom of the boring, 45 feet. Laboratory testing performed on samples obtained during the field exploration included natural moisture content and density, percent finer than sand size gradation analyses and liquid and plastic limits. Swell-consolidation testing was performed on relatively undisturbed drive samples of the clay soils. The swell-consolidation test results, presented on Figures 4 and S, indicate low compressibility under relatively light surcharge loading and a low to moderate expansion potential when wetted under a constant light surcharge. Expansion pressures ranged from 1,500 to 7,000 psf. Liquid and plastic limits testing indicates the clay soils have low plasticity. The laboratory testing is summarized in Table I. Free water was encountered in Boring 1 at a depth of 33 feet at time of drilling. When checked 6 days later, Boring 1 had caved at 35% feet and Boring 2 had caved at 27 feet below the ground surface and no water was observed. The subsoils above the groundwater were slightly moist to moist with depth. FOUNDATION BEARING CONDITIONS The upper clay soils encountered at the site possess low to moderate expansion potential when wetted. The expansion potential can probably be partly mitigated by placing at least 3 feet of compacted structural fiJJ below spread footings to reduce swelling in the event of wetting below the foundation bearing level. Surface runoff, landscape irrigation, and utility leakage are possible sources of water which could cause wetting. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of tlie proposed construction, the residence can be founded with spread footings placed on at least 3 feet of compacted structural fill with some movement risk. Job No, 116 019A The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the at least 3 feet of structural fill .on undisturbed natural soils can be designed for an allowable be~ pressure of 3,000 psf. The footings should also be designed for a minimum dead load pressure of 800 psf. In order to satisfy the minimum dead load pressure under lightly loaded areas, it may be necessary to concentrate loads by using a grade beam and pad system. Wall-on-grade construction is not recommended at this site to achieve the minimum dead load. 2) Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be up to about 1 inch . There could be some additional movement if the bearing soils were to become wet. The amount of movement would be related to the depth and extent of wetting and could be 1 to 2 inches. 3) The footings should have a minimum width of 16 inches for continuous footings and 24 inches for isolated pads. 4) Continuous foundation walls should be reinforced top and bottom to span local anomalies and limit the risk of differential movement. One method of analysis is to design the foundation wall to span an unsupported length of at least 14 feet. Foundation walls acting as retaining structures should aJso be designed to resist a lateral earth pressure as discussed in the "Foundation and Retaining Walls" section of this report. 5) Exterior footings and footings beneath unheated areas should be provided with ade·quate soil cover above their bearing elevation for frost protection. Placement of foundations at least 36 inches below the exterior grade is typically used in this area. 6) Prior to the structural fill placement, topsoil and loose or disturbed soils should be removed and the footing bearing level extended down to competent bearing soils at least 3 feet below design footing grade. Structural fill should be compacted in thin lifts to at least 98% of the Job No. 116019A -5- maximum standard Proctor density at a moisture content near optimum. The structural fill should extend out on either side of the footing a distance of at least half the total fill depth below the footing. Structural fill should consist of an imported silty sandy gravel of restricted penneability, such as Class 6 road base. 7) A representative of the geotechnical engineer should observe aJI footing excavations prior to concrete placement to evaluate bearing conditions. FOUNDATION AND RETAINING WALLS Found~tion walls and retainir~g structures which are lateraUy supported and can be expected to undergo only a slight amount of detlection should be designed for a lateral earth pressure computed on the basis of an equivalent fluid. unit weight of at least 55 pcf for backfill consisting of the on~site fine-grained soils and at least 45 pcf for backfill consisting of imported granular material such as Class 6 road base. 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 fine-grained 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. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls. Wall backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at a moisture content slightly above optimum. Back.fill in pavement areas should be compacted to at least 95% of the maximum standard Proctor Job No . 116 019A -6- 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. We recommend imported granular soils for back.filling foundation walls and retaining structures because their use results in lower lateral earth pressures. Imported granular wall backfill should contain Jess than 15% passing the No. 200 sieve and have a maximum size of 2 inches. Granular materials should be placed within 2 feet of the ground surface and to a minimum of 3 feet beyond the walls. The granular back.fill behind foundation and retain.ing walls should extend to an envelope defined as a line sloped up from the base of the wall at an angle of at least 30 degrees from the vertical. The upper 2 feet of the wall backfill should be a relatively impervious on-site soil or a pavement structure should be provided to prevent surface water infiltration into the backfill. 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 0.45. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 325 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 of passive resistance. Fill placed against the sides of the footings to resist lateral loads should be a nonexpansive material compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The on-site soils possess an expansion potential and slab heave could occur if the subgrade soils were to become wet. Slab-on-grade construction may be used in the Job No. I 16 019A -7- garage area provided precautions are taken to limit potential movement and the risk of distress to the building is accepted by the owner. The lower floor of the house should be a structurally supported floor over a crawlspace. The expansion potential of the soils at cut depth should be further evaluated at the time of excavation. To reduce the effects of some differential movement, structural fill at least 3 feet deep should be placed under the garage slab grade. The structural fill should be placed similarly to the structural fill placed under footings. Nonstructural floor slabs should be separated from all bearing walls and columns with expansion joints which allow unrestrained vertical movement. Interior non-bearing partitions resting on floor slabs should be provided with a slip joint at the bottom of the wall so that, if the slab moves, the movement cannot be transmitted to the upper structure. This detail is also important for wallboards, stairways and door frames. Slip joints which will allow at )east 1 ~ inches of vertical movement are recommended. Floor slab control joints should be used to reduce damage due to shrinkage cracking. Slab reinforcement and control joints should be established by the designer based on experience and the intended slab use. Required fill beneath slabs can consist of imported silty sandy grave such as Class 6 road base. The fill should be spread in thin horizontal lifts, adjusted to at or above optimum moisture content, and compacted to at least 95% of the maximum standard Proctor density. All vegetation, topsoil and loose or disturbed soil should be removed prior to fill placement. The above reconunendations will not prevent slab heave if the expansive soils underlying slabs-on-grade become wet. However, the recommendations will reduce the effects if slab heave occurs. All plumbing lines should be pressure tested before backfilling to help reduce the potential for wetting. UNDERDRAIN SYSTEM Although groundwater was not encountered within expected excavation depth during our exploration, it has been our experience in this area and where clay soils arc present, that JobNo. ll6019A .g. local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also create a perched condition. Therefore, we recommend below·grade construction, such as crawlspace and basement areas, be protected from wetting by an underdrain system. The drain should also act to prevent buildup of hydrostatic pressures behind foundation walls. The underdrain system should consist of a drainpipe surrounded by free·draining granular material placed at the bottom of the wall backfill. The drain lines should be placed at each level of excavation and at least 1 foot below lowest adjacent finish grade, and sloped at a minimum 1 % grade to a suitable gravity outlet. Free·draining granular material used in the drain system should consist of minus 2-inch aggregate with Jess than 50% passing the No. 4 sieve and less than 2% passing the No. 200 sieve. The drain gravel should be at least I V:t feet deep. Void fonn below the foundation can act as a conduit for water flow. An impervious liner such as 20 to 30 mil PVC should be placed below the drain gravel in a trough shape and attached to the foundation wall above the void form with mastic to keep drain water from flowing beneath the wall and to other areas of the building. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Excessive wetting or drying of the foundation excavations and underslab areas should be avoided during construction. Drying could increase the expansion potential of the soils. 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95% of the maximum standard Proctor density in pavement 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 to 3 feel of the on-site soils to reduce surface water infiltration. Job No. 116 019A -9- 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 I 0 feet in paved areas . 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 l, 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 borings and variations in the subsurface conditions may not become evident until excavation is performed. If conditions encountered during construction appear to be 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 Job No. 116 019A -10- verify that the recommendations have been appropriately interpreted. SW1ificant design changes may require additional analysis or modifications of the recommendations presenced herein. We recommend on-site observation of excavations and foundation bearing strata and testing of structural fill b_y a representative of the geotechnical engineer. Respectfully Submittedt HEPWORTH -PAWLAK OEOTECHNICAL. INC. Reviewed by: Steven L. Pawlak, P.E. DEH/ksw Job No. 116 019A 116019A BORING9 HPGEOTECH JOB NO. 100 169 0 LOT30 LOT31 \ \ \ APPROXIMATE SCALE 1 c:: 60' SOUTH MEADOW DRIVE LOT29 ~ LOCATION OF EXPLORATORY BORINGS Figure 1 He worth-Pcnwlak Geoledmlc:ol 0 5 10 15 20 ii if I .c a Ql 0 25 30 35 40 45 116 019A BORING 1 28/12 WC•S.1 00•119 30/12 WCaS.4 00=111 -2001:174 LL•29 Pla14 34/12 WC=4.2 tJ0=128 34/12 WC•4.6 00c127 19/12 WC=4.B 0Da12S ·200=57 13112 13112 50/6 BORING2 29/12 wc-s .o 0Da11B 25/12 WC•S.3 00=115 2&'12 WC=5.B 00=117 ·200=63 33/12 wc .. 30 00-121 33112 WC=5.3 00•126 29/12 17/12 Note: Explanation of symbols Is shown on Figure 3. LOGS OF EXPLORATORY BORINGS 0 5 10 15 20 ~ I .i: g. D 25 30 35 40 45 Figure 2 LEGEND: ~ TOPSOIL; sandy clay, organic with roots, soft, moist, dark±rown. D CLAY (Cl); silly, sandy, very stiff to hard, slightly moist, tlgltbrown. ~ SILT AND CLAY (Ml-CL); silty, hard to stiff with depth, sJ!gh!Jy moist to wet with depth, browrt I GRAVEL (GM); sandy, silty, with cobbles, dense, wet, bro~ p Relatively undisturbed drive sample; 2-inch l.D . California liner sample. 26/12 Drive sample blow count; Indicates that 28 blows of a 140 pound hammer falling 30 inches were required to drive the California sampler 12 Inches. 0 Free water level in boring and number of days following drilllng measurement was taken. Depth at which boring had caved when cliecked on February 11, 2016. NOTES: 1. Exploratory borings were drilled on February 5, 2016 with 4-lnch 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 not measured and the logs of exploratory borings are drawn to depth. Boring 1 is 5 feet lower than Boring 2. 4. The exploratory boring locations and elevations sholJd 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 transitlons may be gradual. 6. Water level readings shown on the logs were made at the lime and under the conditions indicated. Fluctuations In water level may occur with time. 7. Laboratory Testing Results: WC • Water Content (%) DD = Ory Density (pcQ -200 = Percent passing No. 200 sieve LL = Liquid Limit (%) Pl = Plasticity Index (%) oG'1ech Heaworth-Pawlllk GeatedinlClll 115 019A LEGEND AND NOTES Figure 3 Moisture Content = 5.1 percent Dry Densily .. 119 pcf Sample of: Sandy Silty Clay From: Boring 1 at 2 Xi Feet 3 2 !C~ ... ~ ~ \ c: ·~ 1 a Expansion ~ Q. upon \ ~ 0 wetting • ·~ ' ,, \ (/) ~ Cl) 1 ._ ti 1 E 0 (.) 2 0.1 1.0 10 100 APPLIED PRESSURE -ksf Moisture Content ... 4.2 percent Ory Density = 128 pcf Sample of: Sandy Silty Clay From: Boring 1 at 9 Feet ~ c: 1 0 4~ c;; c: co Q. 0 in M)~ ~ I r---.... \ c ~ .. ·fjl 1 en Expansion I'\> ~ upon 0 2 wetting u 0.1 1.0 10 100 APPLIED PRESSURE • ksf 1'16 019A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 4 Heoworih-Powtalc Geolechnlcal Moisture Content = 5.3 percent Dry Density= 115 pcf 3 Sample of: Sandy Siity Clay From: Boring 2 at 4 Feet 2 c,.__ ' "'t fl. c: 1 0 ·u; Expansion \ c: IO upon Q. [\ .n 0 wetting I c: ' \ 0 ·c;; U) ! a. 1 \ ' E ' 0 (,) 2 \ ) 3 0.1 1.0 10 100 APPLIED PRESSURE -ksf Moisture Content = 3.8 percent Dry Density = 121 pcf Sample of: Sandy Silty Clay ~ From: Boring 2 at 14 Feet c: 1 0 u; c: <U a. 0 ari :~ I c 0 r---.... ·u; 1 U) Q) ~ ''-.. a. ~) E 0 (,) 2 Expansion upon wetting 0.1 1.0 10 100 APPLIED PRESSURE -ksf 116 019A ~ SWELL-CONSOLIDATION TEST RESULTS Figure 5 Hmworth-PQwlak GecllethnlcCll HEPWORTH-PAWLAK GEOTECHNICAL. INC. TABLE1 Job No. 116 019A SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBeRG LIMITS PERCENT SWELL % MOISTURE OR\' GRAVEL SAND PASSING LIQUID PLASTIC PRESSURE SWELL SOILOR BORING DEPTH CONTENT DENSITY (%) (%) N0.2DO LIMIT INDEX BEDROCK TYPE uu ,.") focn SIEVE Co/el l%l IPSF\ 1 2'h 5.1 119 7,000 3.1 Sandy Silty Clay 4 5.4 111 74 29 14 Sandy Silty Cla)I 9 4.2 128 3,500 1.3 Sandy Silty Clay 14 4.6 127 Sandy Silt and Clay 19 4.8 125 57 Sandy Silty and Clay 2 2 1h 5.0 118 Sandy Silty Clay 4 S.3 115 5,000 2.5 Sandy Silty Clay 9 5.8 117 63 Sandy Silty Clay 14 3.7 121 1,500 0.3 Sandy Silty Clay 19 5.3 126 Sandy Silt and Clay