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HomeMy WebLinkAboutSoils Report 12.06.2017ii-P--KIJ Geotechnlcal Engineering I Engineering Geology Materials Testing I Environmental Office Locations: Denver (HQ), Parker, Colorado 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Springs, Fort Collins, Glenwood Springs, Summit County, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT B6, ASPEN EQUESTRIAN ESTATES EQUESTRIAN WAY GARFIELD COUNTY, COLORADO PROJECT NO. 17-7-848 DECEMBER 6, 2017 PREPARED FOR: KRISTIN MELBYE P.O. BOX 1658 ASPEN, COLORADO 81612 Locibve@aol.com RECEIVED APR 02 2018 GARFIELD COUNTY COMMUNITY DEVELOPMENT TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS _ 1 - SUBSIDENCE POTENTIAL - 2 - FIELD EXPLORATION _ 2 - SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS _ 4 - FLOOR SLABS - 5 - UNDERDRAIN SYSTEM - 5 - SURFACE DRAINAGE - 6 - LIMITATIONS - 6 FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS 1-1-P4-.KUMAR 0 res...". AI.. 1 7 7 O A O PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot B6, Aspen Equestrian Estates, Equestrian Way, Garfield County, Colorado. The project site is shown on Figure 1. The purpose of the study was to assess the feasibility of construction for the purchase/sale of the property in which our client is the buyer and to develop recommendations for the foundation design. The study was conducted in accordance with our agreement for geotechnical engineering services to Kristin Melbye, dated December 1, 2017. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. The results of the field exploration 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 will be 2,500 to 2,800 square feet in area and will be a one to two story wood frame structure over a crawlspace or slab on grade ground floor. Grading for the structure is assumed to be relatively minor with cut depths between about 2 to 5 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. 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. SITE CONDITIONS The building area is relatively flat with a slight slope down to the southwest. There is a relatively large irrigation ditch located just south of the building envelope, the bottom of which is about 7 feet lower than the grade at the southern building envelope line. The irrigation ditch is H-PMKUMAR /.1.. 17 7 DA -2 - currently empty. The slope down to the ditch from the building envelope is about 25 to 50 percent down to the south. Vegetation in the building area is mostly grass and weeds. There is a large willow tree between the northern lot line and the pavement on Equestrian Way and a row of small fir trees near the eastern property line. There are fallen dead trees in or near the irrigation ditch and a cottonwood tree near the southwest building envelope corner. The lot to the east is developed with a single family residence. The lot to the west is vacant. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the lot. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous work in the area, several sinkholes were observed scattered throughout the lower Roaring Fork Valley from Basalt to south Glenwood Springs. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River Valley. Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory borings were relatively shallow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot B6 throughout the service life of the proposed residence, in our opinion, is low and similar to other lots in the lower Roaring Fork Valley; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. FIELD EXPLORATION The field exploration for the project was conducted on December 5, 2017. Two exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. H-PKUMAR Project No. 17-7-848 e --)- 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 H-P/Kumar. Samples of the subsoils were taken with a 1% 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. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils below about /2 foot of topsoil, consist of 11/2 to 21/2 feet of medium dense, silty sand overlying relatively dense, slightly silty, sandy gravel and cobbles down to the bottom of the borings at 6 to 61/2 feet. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and possible boulders and drilling refusal was encountered in the deposit. No free water was encountered in the borings at the time of drilling. The subsoils were slightly moist. FOUNDATION BEARING CONDITIONS Construction of a residence supported on a conventional spread footing foundation should be feasible at this site. The dense gravel soils encountered at 2 to 3 feet below the ground surface are suitable for support of the proposed residence. Houses in this area are typically built over slab -on -grade or shallow crawlspace due to the typically shallow groundwater during irrigation season. We expect that groundwater during irrigation season will be at 5 to 7 feet below the ground surface, or about the same level as the water level in the nearby ditch. H-P%-KUMAR Project No. 17-7-848 -4 - 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 dense granular 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 3,000 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. 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 10 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 45 pcf for the onsite sand and gravel soils as backfill. 5) All existing medium dense sand, topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively dense natural granular soils. The exposed soils in footing areas should then be moistened and compacted. If water seepage is encountered, the footing areas should be dewatered before concrete placement and the water condition evaluated for additional mitigation measures. H-PKUMAR Project No. 17-7-848 -5- E) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. 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 slabs. 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 or a suitable imported gravel 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 deep crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. A shallow crawlspace (less than 4 feet deep) or a slab -on -grade floor near exterior grade should not need an underdrain system unless groundwater is encountered during construction. If installed, 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 -PMKUMAR Project No. 17-7-848 -6 - sloped at a minimum 1% to a suitable gravity outlet or sump and pump. Free -draining granular material used in the underdrain system should contain less than 2% passing the No. 200 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 11/2 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. 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 6 inches in the first 10 feet in unpaved areas and a minimum slope of 21/ inches in the first 10 feet in paved areas. Free -draining wall backfill (if any) should be capped with about 2 feet of the on-site finer graded soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 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. 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 H-P�KUMAR Pr'ject No. 17-7-848 -7 - 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, H -P KUMAR • , : Daniel E. Hardin, P.E.;,,;° 24 43 Steven L. Pawlak, P.E. Reviewed by: DEH/kac H-Pk-KUMAR Project No. 17-7-848 s ?-4 way pro >`." . 7 Zoe B6 14.300 Sq Ft 0.33 Ac BORING 1 (V cont Lot) 4 BORING 2 0 8270 — 45.0' 0-ernbe r . Rear Setback LNo neihtoran (do'.cigyk s - to' curia & I� 0. 4:10. Ulf�,e 1 - sra 0 700 year Rood Nein nr Per Reception tun. 564578 - lf6tlptds CorineaNaa Per F_n`{renmT1 f & Ec+oioylcol Resources, 11C 947/99 Per Reception NA 564578 0 Nola' This tap Accuracy srca: 90X of paints and vaN dervrc Melt true poste spot domOVarts kr this spot 10 0 10 20 APPROXIMATE SCALE -FEET 17-7-848 H-PWUMAR LOCATION OF EXPLORATORY BORINGS Fig. 1 W w a 0 5 10 BORING 1 EL. 6271' 50/12 50/6 LEGEND BORING 2 EL. 6270.5' 50/3 50/2 1 TOPSOIL; ORGANIC SILTY SAND, FIRM, SLIGHTLY MOIST, DARK BROWN. SAND (SM); SILTY. SCATTERED GRAVEL, MEDIUM DENSE, SLIGHTLY MOIST, BROWN. GRAVEL (GM—GP); WITH COBBLES, POSSIBLE BOULDERS, SANDY, SLIGHTLY SILTY, DENSE, SLIGHTLY IJOIST, BROWN. DRIVE SAMPLE; STANDARD PENETRATION TEST (SPT), 1 3/8 INCH I.D. SPLIT SPOON SAMPLE, ASTM D-1586. DRIVE SAMPLE BLOW COUNT. INDICATES THAT 50 BLOWS OF A 140—POUND HAMMER FALLING 30 INCHFS WERE REQUIRED TO DRIVE THE SPT SAMPLER 12 INCHES. PRACTICAL AUGER REFUSAL. 50/12 t NOTES 1. THE EXPLORATORY BORINGS WERE DRILLED ON DECEMBER 5, 2017 WITH A 4—INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER. 2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. 2. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN PROVIDED. 4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD 8E 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 THE TRANSITIONS MAY BE GRADUAL. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING. 17-7-848 1 -1 -Pk --KU MAR LOGS OF EXPLORATORY BORINGS 0 5 --- Fig. 2