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HomeMy WebLinkAboutSubsurface Study for Foundation Design 03.25.16[{ep!1'orth-Parvlak (ieotechnical, Inc. 5020 CoLrnty Roasl 154 Glenrvtx,¡d Springs, Colorad,r B l 60l Phonc 970"945-7988 H EPWORTH*PAWLAK € EOTECHN ¡CAL Fax: 970"941-8454 ernail: h¡rgco-@ìl4rgcutr:ch.cotrr SUBSURFACE STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 7&,SPRING RIDGE RESERVE ELK RIDGE DRIVE GARFTELD COUNTY, COLORADO JOB NO. 116 0614 MARCH 25,2016 PREPARED FOR: KRISTEN KELLOGG AND RANDY HILL 214 PINE STREET GLENWOOD SPRINGS, COLORADO 81601 kikelloee6 @ comcâst.net rladder49@aol.com H Parker 3A3-B4l-7119 . CokrradoSprings 719-633-5562. r Silverthorne 970-4ó8-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY PROPOSED CONSTRUCTION SITE CONDITIONS FIELD EXPLORATION.... SUB SURFACE CONDITIONS .. DES IGN RECOMMENDATIONS FOUNDATIONS FOUNDATION AND RETAINING V/ALLS FLOOR SLABS ... UNDERDRAIN SYSTEM..... SURFACE DRAINAGE BEDROCK EXCAVATION.. LMITATIONS. FIGURE 1 - LOCATIONS OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES I 1 I 2- 2- 3 J 4 5 5 6 7 7- JobNo. ll606lA cå&ecrr PURPOSE AND SCOPE OF STUDY This report prcsents the results of a subsurface study for a proposed residence to be located on Lot 78, Springridge Reserve, Elk Ridge 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 agreement for geotechnical engineering services to you dated March 1O,2016. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. The results of the field exploration were analyzedto 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 recofitmendations, and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION Plans for the proposed residence had not been developed at the time of our study. In general, we understand that the building will be located in the front (southern) half of the building envelope and will likely be cut into the hillside with a walkout lower level. V/e assume relatively light foundation loadings typical of the general building construction. When specific building location, grading and loading information have been developed, we should be notified to re-evaluate the recoÍrmendations presented in this repolt. SITE CONDITIONS The site was vacant at the time of our field exploration. It is bounded by vacant residential lots to the east and west, open space land to the north, and Elk RdgE Drive to the south. The lot slopes moderately steeply to steeply down to the southwest. Vegetation in the proposed building area consisted of grass and weeds. An irrigation JobNo. ll606lA cå5tecn -2- ditch runs along the hillside above the proposed building area. There were scattered cobbles on the ground surface. FIELD EXPLORATION The field exploration for the project was conducted on March 17,20L6. 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-458 drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsurface materials 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 and hardness of the bedrock. Depths at which the samples were taken and the penetration resistance values are shown on the Logs of Exploratory Borings, Figuie 2. The samples were returned to our laboratory for review by the project engineer. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils generally consisted of about Vzto t foot of topsoil underlain by hard to very hard sandstone/siltstone bedrock of the Maroon Formation. A l-foot thick layer of clay and silt was encountered in Boring 2 between the topsoil and bedrock. Drilling in the hard bedrock was difficult and drilling refusal was encountered in Boring 2 at a depth of 7 feet. Sampling in the hard bedrock was difficult and sample recovery was limited. The Maroon Formation bedrock generally dips steeply down to the west-southwest in this area. No free water was encountered in the borings at the time of drilling and the subsoils and bedrock were slightly moist. JobNo. l1606lA cåFtecrr -3 - 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 sandstone/siltstone bedrock. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed entirely on the undisturbed bedrock should be designed for an allowable bearing pressure of 4,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be minor. 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. Placement of foundations at least 36 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 l0 feet. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressure as presented in the "Foundation and Retaining Wall" section of this report. 5) The topsoil, silt and clay soils and any loose or disturbed soils should be removed and the footing bearing level extended down to undisturbed bedrock. 6) A representative ofthe geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. Job No. 116 06lA cåStecn -4- 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 45 pcf for backfill consisting of imported granular soils or well-broken bedrock. 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 40 pcf for backfill consisting of imported granular soils or well-broken bedrock. Backfill should not contain organics or rock larger than about 6 inches. 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 90Vo of the maximum standard Proctor density at near optimum moisture content. Backfill placed in pavement and walkway areas should be compacted to at least 95Vo 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. 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 JobNo.1l6061A cåStecrr 5 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 0.50. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 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 compacted to at least 95Vo of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural bedrock, exclusive of topsoil, is 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 gravel material should consist of minus 2-inch aggregate with at least 50Vo retained on the No. 4 sieve and less than 27o passing the No. 200 sieve. All fill materials for support of floor slabs should be compacted to at least 95Vo of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of imported granular soils or well-broken bedrock 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 and where bedrock is shallow that local perched groundwater can JobNo.1l6061A cstecn -6- develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also 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 at least I foot below lowest adjacent finish grade and sloped at a minimum l7o to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2Vo passing the No. 200 sieve, less than 507o passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least lYzfeetdeep. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Inundation ofthe foundation excavations and underslab areas should be avoided during construction. 2) Exterior backfîll should be adjusted to near optimum moisture and compacted to at least 957o of the maximum standard Proctor density in pavement and slab areas and to at least 9OVo of the mærimum 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 recoÍrmend 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 at least2 feet of the on- site soils to reduce surface water infiltration. A swale may be needed on the uphill side of the house to direct surface runoff around the house. Job No. 116 06lA c$ecr¡ -7- 4)Roof downspouts and drains should discharge well beyond the limits of all backfill. BEDROCK EXCAVATION Based on our experience in this area, we expect that the Maroon Formation bedrock can be excavated with typical large track excavators used for residential excavation for a limited depth of 5 to 8 feet. Excavation of cemented sandstone layers, narrow utility trenches and in comers of the building excavation may be more difficult and require other rock excavation techniques such as chipping or blasting. Stepping of the foundation down the hillside could be done to avoid deeper cut depths. The typical bedding of the Maroon formation in this area is down to the south-southwest at about 50o from horizontal. This bedding dip will be adverse to side slope stability in southwest and west facing excavation cut faces. Cut slopes in these areas should be laid back to at least the bedding angle. LIVIITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the exploratory borings drilled at the locations indicated on Figure l, the proposed type of construction and our experience in the area. Our services do not include determining the presence, prevention or possibility of mold or other biological contaminants (MOBC) developing in the future. If the client is concerned about MOBC, then a professional in this special field of practice should be consulted. Our findings include interpolation and extrapolation of the subsurface conditions identified at the exploratory borings and variations in the subsurface conditions may not become evident until excavation is performed. If conditions JobNo.11606lA c$tecfr -8- 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. 'W'e 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, MPWORTH - PAV/LAK GEOTECHNICAL, INC. âW'rBrrn¡rr Robyn C. Brown, P.E. Reviewed by: Daniel E. Hardin P.E. RCB/ksw ¡¿r pq4d3 zì 0 t\) Ë() l;¡3 vl Ç q LÀlA JobNo,1160614 cåBtecrt I I I LOT79 \_-__ | -- \ -, - .-=Q*ote€ooe\ _, _ \ \_. _______ \\\\\r\ j I / I IIIIII I I I II .t¡ BORING 1 fI I / I I I LOT78 I IIII I \,\\. IIII I I I o BORING 2 IIIIg(K,?/ÒG€o,ga|-v I LOT77 I APPROXIMATE SCALT 1":50' 1 16 0614 & HEPIIVORTH.PAWLAK GEOTECHNICAL LOCATIONS OF EXPLORATORY BORINGS Figure 1 BORING 1 BORING 2 0 0 50/1 66/6 5 50/2 5 5013 10 30/6,50/5 '10 o) 0)LL I -c. o_ o)o o o) LL I -c o- c)o '15 50/1 15 20 50/2 20 25 25 Note: Explanation of symbols is shown on Figure 3 1 16 0614 & HEPI,VoRTH.PAVI¿LAK GEOTECHNICAL LOGS OF EXPLORATORY BORINGS Figure 2 LEGEND: ã TOPSOIL; silty claywith gravel, roots and organics, loose, moist, red-brown CLAY AND SILT (CL-ML); sandy, with gravel, stiff, slightly moist, red-brown W SANDSTONE/SILTSTONE, hard to very hard, slightly moist, red. Maroon Formation. Relatively undisturbed drive sample; 2-inch l.D. California liner sample. I I Drive sample; standard penetration test (SPT), 1 3/8 inch l.D. split spoon sample, ASTM D-1586.r Drive sample blow count; indicates that 39 blows of a '140 pound hammer falling 30 inches were 39112 required tó drive the California or SPT sampler 12 inches. T Refusal to auger drilling. I NOTES 1. Exploratory borings were drilled on March 17,2016 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 not measured and the logs of exploratory borings are drawn to depth. 4. The exploratory boring locations should be considered accurate only to the degree implied by the method used. b. The lines between materials shown on the exploratory boring logs represent the approximate boundaries between materialtypes 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. 1160614 & EP\,VORTH.PAWLAK GEOTECHNTCALH LEGEND AND NOTES Figure 3