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Home My WebLink AboutDrilled Pier Recommendations 03.16.2017H-PI<UMAR 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Parker, Glenwood Springs, and Silverthorne, Colorado March 16, 2017 Karon and Steve Smith 4pip 3909 South Eagle Street �� Aurora, Colorado 80014 �1 karon@karonsmith.com karonsmith.com Project No. 16-7-577 Subject: Drilled Pier Recommendations, Proposed Residence, Lot 53, Heron Crossing at Ironbridge, Blue Heron Drive, Garfield County, Colorado Dear Karon and Steve: As requested by RM Construction, we are providing drilled pier recommendations for foundation support at the subject site. We previously conducted a subsoil study for design of foundations at the site and presented our findings in a report dated December 2, 2016, Project No. 16-7-577. The residence is proposed to be single -story above crawlspace with slab -on -grade garage and located in the upper, back part of the lot. The recommended settlement precaution with spread footings was to place at least 3 feet of compacted soil below the footings. As an alternative to spread footings, a drilled pier foundation which extends down into the underlying dense, gravel and cobble soil could be used with low settlement potential. Drilled Pier Alternative: Considering the subsoil conditions encountered in the exploratory borings on the lot and the nature of the proposed construction, straight -shaft piers drilled into the underlying gravel and cobble soils can be used for building support. The design and construction criteria presented below should be observed for a straight -shaft drilled pier foundation system. 1) The piers should be designed for an allowable end bearing pressure of 10,000 psf and a skin friction of 1,000 psf for that portion of the pier embedded in gravel. Pier penetration through the upper silt and clay soils should be neglected in the skin friction calculations. 2) All piers should have a minimum total embedment length of 10 feet and a minimum penetration into the gravel of 1 foot. The gravel and cobble soils will tend to cave and penetration into the bearing soils should be limited to about 2 feet. 3) The pier holes should be properly cleaned prior to placement of concrete. The natural silt and clay soils are stiff which indicates that casing of the holes should not be required. Some caving and difficult drilling may be experienced in the bearing soils due to cobbles and possible boulders. Placing concrete in the pier hole the same day as drilling is recommended. Karon and Steve Smith March 16, 2017 Page 2 4) The pier drilling contractor should mobilize equipment of sufficient size to achieve the design pier sizes and depths. We recommend a minimum pier diameter of 12 inches. 5) Grade beams and pier caps should have a minimum depth of 3 feet for frost cover and void form below them is not needed. 6) Free water was not encountered in the borings made at the site and dewatering should not be needed. 7) A representative of the geotechnical engineer should observe pier drilling operations on a full -tune basis. If you have any questions or need further assistance, please call our office. Sincerely, H -P = KU MAR Steven L. Pawlak, SLP/ksw cc: RM Construction — (blake)buildwitlum.com) H -P ; KUMAR Project No. 16-7-570 H-PKUMAR Geotechnical Engineering l Engineering Geology Materials Testing j Environmental 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Parker, Glenwood Springs, and Silverthome, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 53, HERON CROSSING AT IRONBRIDGE BLUE HERON DRIVE GARFIELD COUNTY, COLORADO PROJECT NO. 16-7-577 DECEMBER 2, 2016 PREPARED FOR: KARON AND STEVE SMITH 3909 SOUTH EAGLE STREET AURORA, COLORADO 80014 karon@karonsmith.com TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 - SUBSIDENCE POTENTIAL - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS - 4 - FLOOR SLABS - 5 - UNDERDRAIN SYSTEM - 6 - SURFACE DRAINAGE - 6 - LIMITATIONS - 7 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURES 3 and 4 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS H-1 KUMAR Project No. 10-7-577 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 53, Heron Crossing at Ironbridge, Blue Heron 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 Karon and Steve Smith dated November 8, 2016. Hepworth-Pawlak Geotechnical (now H-P/Kumar) previously conducted a preliminary geotechnical study in the Heron Crossing at Ironbridge development and presented the findings in a report dated February 28, 2014, Job No. 113 471A. 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 will be a single -story wood frame structure with an attached garage located on the lot as shown on Figure 1. Ground floor will be structural above crawlspace in the living area and slab -on -grade in the garage. Grading for the structure is assumed to be relatively minor with cut depths of about 2 to 4 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. KUMAR Project No, 167-577 -2 - SITE CONDITIONS The lot was vacant and appeared to have been stripped of vegetation and topsoil at the time of our site visit. The ground surface is relatively flat with a gentle slope down to the north with roughly two feet of elevation difference across the building area. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge development. 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. A sinkhole opened in the cart storage parking lot located east of the Pro Shop and south of the Heron Crossing at Ironbridge development in January 2005. Irregular bedrock conditions have been identified in the affordable housing area located to the south of Heron Crossing at Ironbridge. Subsurface exploration performed in the area of the proposed residence on Lot 53 did not encounter voids which could indicate past ground subsidence, however, the exploratory borings were relatively shallow, for foundation design only. In our opinion, the risk of future ground subsidence on Lot 53 in Heron Crossing at Ironbridge throughout the service life of the proposed residence is low and similar to other areas of the Roaring Fork River valley where there have not been indications of ground subsidence, but 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 November 15, 2016. 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 H-P/Kumar. H -I: KUMAR Project No. 16.7-577 -3 - Samples of the subsoils were taken with 1% 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 consist of about 13 to 15 feet of stiff, sandy silt and clay overlying dense, silty sandy gravel and cobbles with boulders. Drilling in the coarse granular soils with auger equipment was difficult due to the cobbles and boulders and practical auger drilling refusal was encountered in the deposit. Laboratory testing performed on samples obtained from the borings included natural moisture content and density, and finer than sand size gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples of the upper fine-grained soils, presented on Figures 3 and 4, indicate low compressibility under light loading with a low collapse potential (settlement under constant load) when wetted and moderate compressibility under additional loading after wetting. The laboratory testing is summarized in Table 1. The soils were slightly moist and no free water was encountered in the borings at the time of drilling. FOUNDATION BEARING CONDITIONS The silt and clay soils encountered at expected foundation level tend to settle when they become wetted. A shallow foundation placed on the upper natural silt and clay soils will have a risk of excessive settlement and building distress. The amount of settlement will be mainly related to the depth and extent of subsurface wetting and it will be critical to the long term performance of H-� KUMAR Project No. 16-7-577 _4 - the structure that the recommendations for surface drainage contained in this report be followed. Extensive wetting of the natural soils could cause 3 inches of settlement and mitigation should be used to reduce the settlement potential. Recommended forms of settlement mitigation include: 1) deep compaction, 2) a deep foundation such as helical piers bearing on the underlying dense gravel and cobble soils, or 3) a heavily reinforced structural slab foundation. Presented below are recommendations for compaction below shallow footings with a settlement risk. If recommendations for a deep foundation or structural slab are desired, we should be contacted. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, the building can be founded with spread footings bearing on a minimum 3 feet of compacted structural fill soils with a risk of settlement mainly if the bearing soils become wetted and acceptable to the owner. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on at least 3 feet of compacted fill soils should be designed for an allowable bearing pressure of 1,200 psf. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. Additional differential settlements of about 1 to 11/2 inches could occur if the underlying natural silt and clay soils become wetted. 2) The footings should have a minimum width of 20 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 Ieast 36 inches below exterior grade is typically used in this area. H-> KUMAR Project No. I6-7-577 -5- 4) Continuous foundation walls should be heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 15 feet. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressures corresponding to an equivalent fluid unit weight of at least 50 pcf. 5) Any topsoil and loose disturbed soils should be removed from the building area and down to at least 3 feet below design bearing level. The exposed soils in footing area should then be moistened and compacted. Structural fill should consist of low permeable soil such as the onsite soils or CDOT Class 6 base course extended at Least 2 feet beyond footing edges and compacted to at least 98% of standard Proctor density at near optimum moisture content. 6) A representative of the geotechnical engineer should observe the building excavation for bearing conditions and evaluate compaction of the structural fill during its placement and prior to concrete placement. FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -on -grade construction with a risk of settlement if the bearing soils are wetted. We recommend at least 2 feet of compacted structural fill similar to that placed below footings be placed below the floor slab to help mitigate the settlement potential. 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 relatively well graded sand and gravel such as road base should be placed beneath interior slabs for support. This material should consist of minus 2 -inch aggregate with at least 50% retained on the No. 4 sieve and less than 12% passing the No. 200 sieve. M -I KUMAR Project No. 16-7-577 -6 - 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 soils devoid of vegetation and topsoil or a suitable imported material such as road base. UNDERDRAIN SYSTEM It is our understanding the ground finished floor elevation of the residence is at or above the surrounding grade. Therefore, a foundation drain system is not required. 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 basement areas, if provided, be protected from wetting and hydrostatic pressure buildup by an underdrain and wall drain system. An underdrain is not recommended around the crawlspace area to help limit the potential for wetting below the shallow footings. If the finished floor elevation of the proposed structure has a floor level below the surrounding grade, we should be contacted to provide recommendations for an underdrain system. AlI earth retaining structures should be properly drained. SURFACE DRAINAGE It will be critical to the building performance to keep the bearing soils dry. 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 H-� KUMAR Project No, 16-7-577 -7 - 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. Graded swales should have a minimum slope of 3%. 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 Ieast 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 the time of this study. 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 practice should be consulted. Our findings include interpolation 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 H -I KUMAR Project No. 16-7.577 -8 - of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnical engineer. Respectfully Submitted, H -P `KUMAR Steven L. Pawlak, P Reviewed by: Daniel E. Hardin, P.E. SLPtksw Q! `4,11 !x el'J'i.m,E'h�'.��j�� H-� KUMAR Pmjcct No. 16-7.577 LOT 52 B' UE HERRON DRIVE LOT 53 LOT 54 10 0 s0 20 APPROXIMATE SCALE—FEET 16-7-577 H-P44KUMAR LOCATION OF EXPLORATORY BORINGS Fig. 1 5 d - 0 --5 — 10 •--- 15 BORING 1 BORING 2 19/12 WC=5.4 DD=99 -200=83 17/12 WC=5.9 DD=95 17/12 14/12 13/12 WC=5.3 DD=101 -200=68 18/12 WC=6.3 DD=93 52/6 10 15 --- 20 20 16-7-577 H-Pk-KUMAR LOGS OF EXPLORATORY BORINGS DEPTH -FEET Fig. 2 LEGEND 7 li 19/12 f NOTES SILT AND CLAY (ML—CL); SANDY, STIFF, SLIGHTLY MOIST, LIGHT BROWN, SLIGHTLY POROUS. GRAVEL AND COBBLES (GM—GP); SLIGHTLY SILTY, SANDY, PROBABLE BOULDERS, DENSE, SLIGHTLY MOIST, BROWN. ROUNDED ROCK. RELATIVELY UNDISTURBED DRIVE SAMPLE; 2—INCH I.D. CALIFORNIA LINER SAMPLE. DRIVE SAMPLE; STANDARD PENETRATION TEST (SPT), 1 3/8 INCH I.D. SPLIT SPOON SAMPLE, ASTM D-1586. DRIVE SAMPLE BLOW COUNT. INDICATES THAT 19 BLOWS OF A 140—POUND HAMMER FALLING 30 INCHES WERE REQUIRED TO DRIVE THE CALIFORNIA OR SPT SAMPLER 12 INCHES. PRACTICAL AUGER REFUSAL. 1. THE EXPLORATORY BORINGS WERE DRILLED ON NOVEMBER 15, 2016 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. 3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE NOT MEASURED AND THE LOGS OF THE EXPLORATORY BORINGS ARE PLOTTED TO DEPTH. 4. THE EXPLORATORY BORING LOCATIONS 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 THE TRANSITIONS MAY BE GRADUAL. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (%) (ASTM D 2216); DD = DRY DENSITY (pef) (ASTM 0 2216); —200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM 0 1140). 16-7-577 H-P�KUMAR LEGEND AND NOTES Fig. 3 CONSOLIDATION - SWELL 2 0 — 2 _4 — 6 — 8 — 10 — 12 .1 16-7-577 1.0 APPi1E0 PRESS4. K5F H -P- KUMAR I0 SWELL -CONSOLIDATION TEST RESULT 100 Fig. 4 SAMPLE OF: Sandy Slit and Clay FROM: Boring 1 0 5' WC = 5.9 X, DD = 95 pcf I 1 ADDITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING Thu, SMI .RK/ Awl/ M4' W ti ,rryl,. 14•L- Rr WIYf, Iprl MIM Lf b IM ivork.+d. l./post AAL mtleul Ifth MAIM qp.M I INf AMNIA* f,Y .. NI 111 Ail G -76r6. .1 16-7-577 1.0 APPi1E0 PRESS4. K5F H -P- KUMAR I0 SWELL -CONSOLIDATION TEST RESULT 100 Fig. 4 n...• rr ln.A.Vr p 1N rw .. I.. . r'46914. M u.s 1 - — .11.1 Y. nprNro.L ..aol In Mom" 4,'4 r �w�sww, i SAMPLE OF: Sandy Silf and Clay FROM: Baring 2 0 10' WC = 6.3 X. DD = 93 pcf ADDITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING .1 16-7-577 1.0 APPLIED PRESSURE - I Si ID I00 H -PKU MAR SWELL -CONSOLIDATION TEST RESULT Fig. 5 5 3 1 1 1 H-PKUMAR TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 16-7-577 SAMPLE LOCATION NATURAL MOISTURE CONTENT (%) NATURAL DRY DENSITY (pct) GRADATION PERCENTCOMPRESSIVE PASSING NO. 20(1 SIEVE ATTERBERG LIMITS UNCONFINED STRENGTH (PSF) SOIL OR BEDROCK TYPE BORING _ DEPTH (ft) GRAVEL (%) SAND (%) LIQUID LIMIT (°lo! PLASTIC INDEX (%) 1 2% 5.4 99 83 Sandy Silt and Clay 5 5.9 95 Sandy Silt and Clay 2 5 5.3 101 68 Sandy Silt and Clay 10 6.3 93 Sandy Silt and Clay