Loading...
HomeMy WebLinkAboutSubsoils Reportrcn $jffi[',ffi:lrHit*&*"' An Employcc Orncd Compony 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 email : kaglenwood@,kumarusa.com www.kumarusa.com OffEce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED GARAGE ADDITION AND FOUNDATION REPAIR LOT 126, PHASE 1, TRONBRTDGE 196 SILVER MOUNTATIN DRIVE GARFIELD COUNTY, COLORADO PROJECT NO.23-7-658 DECEMBERt9,2023 PREPARED FOR: RICIIARD DOOLEY P.O. BOX 183 GLENWOOD SPRTNGS, COLORADO 81602 richard.doolev@ gmail.com TABLE OF CONTENTS PURPOSE AND SCOPE OT.' S'I'UDY PROPOSED CONSTRUCTION SITE CONDITIONS......... SUBSIDENCE POTENTIAL FIELD EXPLORATION SUBSURFACE CONDITIONS FOUNDATION BEARING CONDITIONS DESIGN RECOMMENDATIONS ................ FLOOR SLABS UNDERDRAIN SYSTEM 1 I I I 2- -2- -3- ..................- 3 - DEEP FOUNDATIONS FOUNDATION AND RETAINING WALLS -3- a-J- ....,.,.,.,.- 4 - ............- 5 - LIMITATIONS.... FIGURE 1 - LOCATIONS OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 . LEGEND AND NOTES TABLE I- SUMMARY OF LABORATORY TEST RESULTS ........- 5 - Kumar & Associates, lnc. @ Project No.23-7-658 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed garage addition and foundation repair for an existing residence located on Lot 126, Phase l, Ironbridge,196 Silver Mountain 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 Richard Dooley dated November 14,2023. 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 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 existing residence is a two-story wood-frame structure with affached garage. The existing residence foundation has settled since its construction resulting in distress. The existing residence is proposed to be underpinned with pilings down to dense gravel. The proposed addition plan is to extend the garage approximately 8 feet along the existing driveway alignment. Ground floor will be slab-on-grade. Grading for the addition is assumed to be relatively minor with cut depths between about 2to 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. SITE CONDITIONS The lot was developed with a two-story wood-frame residence with attached garage at the time of our field exploration. The ground surface was gently sloping down to the east at a grade of about 5 percent. Vegetation consists of landscaped grass, bushes and trees. Underpinning operations for pile installation was in progress at the time of our study. 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 Kumar & Associates, lnc. @ Project No.23-7-658 ", 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 Ironbridge development. These sinkholes appear similar to others assooiated with the Eagle Valley Evaporite in areas of the lower Roaring Fork 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 126, Phase 1 throughout the service life of the proposed residenceo in our opinion, is low and similar to other lots in the area; 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 6, 2023. 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 Kumar & Associates, Inc. Samples of the subsoils were taken with l%-inch and Z-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-I586. 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 eonsist of a-bout one foot of topsoil or 3 inehcs of aspha-lt overlying medium stiffto stifl sandy, silty clay to between 14% and2l feet deep where medium dense to dense, silty sandy gravel with cobbles was encountered to the maximum drilled depth of 26 feet. Drilling in the coarse granular soils with auger equipment was difficult due to the cobbles and possible boulders and drilling refusal was encountered in the deposit. Kumar & Associates, Inc. @ Project No. 23-7-658 -J- Laboratory testing performed on samples obtained from the borings included natural moisture content and density. The laboratory testing is summarized in Table l. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist to moist. FOUNDATION BEARING CONDITIONS The subsoils at the site are variable and include compressible natural soils. The natural soils possess low bearing capacity and moderate to high sefflement potential. These soils are hydro- compressive and could seffle under load especially when wetted. In residential areas there are several sources of subsurface wetting, such as irrigation, surface water runoff and utility line leaks. Following the recommendations in the "surface Drainage" section of this report will be critical to the long-term performance of the proposed residence. Spread footings placed on the natural sand in distress to the the piles have the advantage of providing moderate load capacity with a relatively small settlement potential if properly designed and constructed. DESIGN RECOMMENDATIONS DEEP FOLINDATIONS Considering the subsoil conditions encountered in the and the nature of the area. type is recommendations. 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 60 pcf for backfill consisting of the on-site fine-grained 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 50 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 Kumar & Associates, lnc. @ Project No.23-7-658 -4- 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 90% of the maximum standard Proctor density at a moisture content slightly above optimum. Backfill placed in pavement and walkway areas should be compacted to at least95%o 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 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 0.30. 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 a.t the ultimate strength, pa-rticularly in the case of passive resistance. Fill placed against the sides of the footings to resist lateral loads should be compacted to at least 95Yo of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS There is a risk of floor slab settlement due to the existing fill and hydro-compressive soils. Structurally supported floors over crawlspace are recommended for the areas due to the section structural reinforcement can also be used to reduce the effects To reduce the effects of some differential movement, non-structural 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 forjoint spacing and slab reinforcement should be established This rnaterial should consist of minus 2-inclt aggregate with sieve and less than l2%o passing the No. 200 sieve. Kumar & Associates, hrc. o ProJect No. 23-7-658 5 All fill materials for support of floor slabs should be compacted to at least95%o of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the onsite material or an imported granular material such as CDOT Class 6 aggregate base course. TINDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in the area and where clay soils are present that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched condition. The proposed shallow foundations of the garage addition and existing crawlspace should not need a perimeter foundation drain, provided that the exterior foundation wall backfill is well-compacted and good surface drainage, as described below, is maintained around the house. We recommend below-grade construction (if any), such as retaining walls, deep crawlspace (greater than about 4 feet) and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. If installed, the drains should consist of rigid perforated PVC 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%%o to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2%o passing the No. 200 sieve, less than 50olo passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least lYzfeet deep and covered with filter fabric such as Mirafi 140N or 160N. An impervious membrane such as 20 mil PVC should be placed beneath the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. SURFACE DRAINAGE Providing and maintaining proper surface drainage will be critical to the long-term performance of the proposed construction. The following drainage precautions should be observed during construction and maintained at all times after the construction has been completed: l) Inundation ofthe 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 95Yo 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 paved areas. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. Drainage swales should have a minimum slope of 3%o. Kumar & Associates, lnc. @ Project No.23-7-658 -6- I-andscaping which requires regular heavy irrigation should be located at least l0 feet from foundation walls. 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 this report are based upon the data obtained fiom the borings drilled at the locations indicated on Figure 1, the proposed type of constmction and our experie,nce in the area. Our services do not include determining the pr€s€noe, prevention orpossibility of mold or other biological contaminants (MOBC) developing in the future. Ifthe client is concemed about MOBC, then a professional in this special field of pmactice shouldbe consulted. Our findings include interpolation and exhapolation of the subsurface conditions identified at the exploratory borings and variations in the subsurface mnditions may not become evide,nt until excavation is performed. If conditions encountered during constnrction appear different from those described in this report, we should be notified so that re.evaluation of the recommendations may be made. This report ha^s been prepared for the exclusive use by our client for design purposes. We are not responsible for technical intertrnetations by others of our information. As the project evolves, we should pmvide continued consultation and field services during construction to review and monitor the implementation of our recommendations, and to veriry that the recommendations have ben appropriately intupreted. Significant design changes may require additional analysis or modifications to the recommendations presented herein. We reiommend on-site observation of excavations and foundationbearing strata and testing of structural fill by a representative of the geotechnical engneer. Respoctftlly Submitted, Kumar & Associates, James H. Parsons, P Reviewedby: 5) *-/- Steveir L. Pawlalq P.E. JHPftac Kumar & Associates, lnc. e Project No,23-7-658 25 o APPROXIMATE SCALE-FEET 23-7-658 Kumar & Associates LOCATIONS OF EXPLORATORY BORINGS Fig. 1 E !l I BORING 1 BORING 2 0 (5)0 13/12 18/12 5 10/12 6/ 12 5 10 5/ 12 WC=16.1 DD=95 6/12 WC=17.7 DD= 1 06 10 FLILI [L I-F(L t!o 15 2s/12 5/ 12 WC=20.4 DD= 1 06 15 F UJ UJt! I-F(L LJo 20 45/7 20 25 1s/12 25 50 30 23-7-658 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2 ! g I LEGEND TOPSOIL; CLAY, SANDY, ORGANIC, FIRM' MOIST, MEDIUM BROWN (3)ASPHALT, THICKNESS IN INCHES SHOWN IN PARENTHESES TO LEFT OF THE LOG. CLAY (CL); SANDY, MEDIUM STIFF TO STIFF, MOIST, MEDIUM BROWN. GRAVEL (GU-Op); SANDY, SILTY, MEDIUM DENSE TO DENSE. F I DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE. DRTVE SAMPLE, 1 5/8-INCH l.D. SPLIT SPOON STANDARD PENETRATION TEST IZTTC DRIVE SAMPLE BLOW COUNT' INDICATES THAT 13 BLOWS OF A 140-POUND HAMMER'"/ '' FALL|NG s0 lNcHEs WERE REeU|RED To DRIVE THE SAMPLER 12 lNcHEs. f enlcrrclL AUGER REFUsAL. NOTES 1 THE EXPLORATORY BORINGS WERE DRILLED ON DECEMBER 6, 2023 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. 5. 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. I.ABORATORY TEST RESULTS: wc = WATER OONTENT (%) (ASTM D2216): DD = DRY DENSITY (pcf) (ASTM D2216). Fig. 3Kumar & Associates LEGEND AND NOTES23-7-658 ! I(+A :' lonE & Associsb,lnc." Geotechnical and Materials Engineers and Environmental Scientists TABLE 1 SUMMARY OF LABORATORY TEST RESULTS sailPl I OCAT|OI|GR toil ATI LIHITS BORING rft) DEPTH r%I IIATURAL TOISTURE CONTEIIT IIATURAL DRY DEt{SITY tbc0 GRAVEL (%) SAND (%) PERCENT PASSTNG l{O. 200 stEvE t%t LIQUID TIHIT r%l PTASTIC INDEX rhrfi UNCOT{FINEO cotPREsstvE STRENGTH SOIL WPE I 9 l6.l 95 Sandy Silty Clay 2 9 17.7 106 Sandy Silty Clay t4 20.4 106 Sandy Silty Clay