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HomeMy WebLinkAboutSoils Report 02.05.2018H-PI(UMAR Geotechnical Engineering 1 Engineering Geology Materials Testing 1 Environmental 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 81, SPRING RIDGE RESERVE ELIC RIDGE DU.IVE GARFIELD COUNTY, COLORADO PROJECT NO. 18-7-102 FEBRUARY 5, 2018 PREPARED FOR: >i EITH WITTENBERG 470 HIDDEN VALLEY DRIVE GLENWOOD SPRINGS, COLORADO 81601 (,I<<iggs09@ yahoo.conI) TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 1 - ROCKFALL HAZARD - 2 - FIELD EXPLORATION - 3 - SUBSURFACE CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS - 4 FOUNDATION AND RETAINING WALLS - 5 - FLOOR SLABS - 6 - UNDERDRAIN SYSTEM - 6 - SURFACE DRAINAGE - 7 - LIMITATIONS - 7 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS H-P*KUMAR Project No 18-7-102 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at Lot 81, Spring Ridge 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 general accordance with our proposal for geotechnical engineering services to Keith Wittenberg dated December 19, 2017. Hepworth- Pawlak Geotechnical previously performed a preliminary geotechnical study for the subdivision development and reported their findings February 26, 2001, Job No. 101 126 and updated the study in a report dated June 22, 2004. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils and bedrock 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 one and two-story, wood frame construction with an attached garage at the upper, main level and a walkout lower level located as shown on Figure 1. Ground floors will slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 8 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 residence will be located in the lower, western part of the building envelope near Elk Ridge Drive as shown on Figure 1. Vegetation consists of grass and weeds with scattered stands of H-PvIWMAR Project No 18-7-102 _2 - scrub oak above the building site. The ground surface slope is down to the west at about 10% across the building area and steepens somewhat in the upper lot area. An abandoned irrigation ditch crosses through the lot just below the building area. ROCKFALL HAZARD The subject lot slopes up to the east-southeast at about 8 to 10 percent across the proposed building portion of the lot. An abandoned, north trending irrigation ditch with a small berm bordering the ditch, is located west of the building area in the northwest part of the lot. Above the building area, the terrain generally slopes up at about 20 to 25 percent with localized slopes of up to about 30 to 40 percent near the crest of the slope. The slope is about 300 feet in length from the building area to the crest. At the time of our site visit, this building areas was clear of snow and snow cover ranged up to about 8 inches in depth on the slope above the lot. Surficial soils on the hillside above the proposed residence area consist of relatively shallow colluvium, comprised of silty, clayey sand with gravel, cobbles and small boulders, overlying sandstone bedrock of the Maroon Formation. Scattered, mostly flat (tabular) shaped rock fragments, typically up to about 1 foot in size, are exposed on the hillside above the residence area. An area of cobbles, larger boulders and the remnants of a bedrock outcrop occurs near the crest of the ridge, with boulders ranging from 1 foot to several feet in size. The boulders are generally tabular in shape, and typically partially embedded in the overburden soil. Drainage on the slope above the proposed residence appears to be primarily by sheet flow. Based on our observations and professional experience, we conclude that the rockfall hazard to the proposed construction is low, and rockfall mitigation measures are not warranted. Although low in probability, down-slope rock movement, if it occurs, will likely originate from the steeper, rocky zone located near the crest of the slope. Cobbles and boulders in the rocky zone are typically tabular in shape and not prone to movement. Scattered cobbles and boulders with sub- angular to sub -rounded shapes were also observed and will be more prone to movement down- slope. Storm and seasonal events, resulting in erosion of surficial soils, may eventually cause instability of rock fragments on the hillside. If the owner is concerned with the potential for down-slope rock movement, we recommend periodic observation by the owner of the slope to identify potential rocks prone to movement, and we should be contacted if further investigation is desired at that time. If alterations to the slope above the residence are planned in the future, H-P-KUMAR Project No 18-7-102 -3 - we should be consulted to review the planned alterations for potential slope stability and rockfall hazards. FIELD EXPLORATION The field exploration for the project was conducted on January 10, 2018. 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. Samples of the subsoils were taken with a 2 -inch I.D. spoon sampler. The sampler was driven into the subsurface materials 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, 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, below about six inches of topsoil, consist of about 8 to 13 feet of stiff to hard, silty sandy clay to very sandy clay with depth, underlain by hard to very hard, siltstone/sandstone bedrock to the drilled depth of 16 to 21 feet. Laboratory testing performed on samples obtained from the borings included natural moisture content and density and percent finer than sand size gradation analyses. Results of swell - consolidation testing performed on relatively undisturbed drive sample of clay soils, presented on Figure 4, generally indicate low to moderate compressibility under conditions of loading and wetting. The sample from Boring 2 at 21/2 feet showed a low expansion potential when wetted under light loading. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils and bedrock were slightly moist. H-PkKUMAR Project No 18-7-102 soils should be further evaluated at the time of excavation i -4 - DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings, the nature of the proposed construction and our experience in this area, we recommend the building be founded with spread footings bearing on the natural soils. The expansion potential of the clay sample from Boring 2 at 21/2 feet appears to be an anomaly and the expansion potential of the exposed or possible mitigation measures. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural soils should be designed for an allowable hearing pressure of 2,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be less than 1 inch. Additional differential movement of about 1/2 to 1 inch could occur if the bearing soils are wetted and precautions should be taken to keep the bearing soils dry. 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 heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 14 feet. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressure as described below in the "Foundation and Retaining Walls" section. 5) All topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the undisturbed native soils. The exposed soils in footing area should then be moistened and compacted as needed. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. H-P-KUMAR Project No 18-7-102 -5 - 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 55 pcf for backfill consisting of the on-site 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 45 pcf for backfill consisting of the on-site 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. Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at near optimum moisture content. Backfill placed in pavement and walkway areas should be compacted to at least 95% 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 0.35. 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 at the ultimate strength, particularly in the case of passive resistance. Fill placed against H-P--14KUMAR Project No 18-7-102 -6 - the sides of the footings to resist lateral loads should be compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab -on -grade construction. The expansion potential and need for sub -excavation to remove expansive clay soils should be evaluated at the time of excavation. 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 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 soils or imported relatively well graded granular soils 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 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, 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 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to a suitable gravity outlet. 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 H-PvKU MAR Project No 18-7-102 -7 - maximum size of 2 inches. The drain gravel backfill should be at least 11/ feet deep. An impervious membrane, such as 20 mil PVC should be placed below the drain gravel in a trough shape and attached to the foundation wallwith mastic to prevent wetting of the bearing soils. 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 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 covered with filter fabric and 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. 5) Landscaping which requires regular heavy irrigation should be located at least 10 feet from foundation walls. 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 Project No 18-7-102 -8 - practice should be consulted. Our findings include interpolation and extrapolation of the subsurface conditions identified al 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 KU MAR Steven L. Pawlak, P. Reviewed by: „. e Daniel E. Hardin, P.E. SLP/kac 1 Cc: Don Pettygrove (clgpengineeringllc@gmail.com) H-P�KUMAR Project No 18-7-102 W C7 —J 41 OPEN SPACE 25 0 25 5C' APPROXIMATE SCALE—FEET LOT 80 NOTE: CONTOURS LINES IN THE BUILDING AREA SHOWN ARE MODIFIED FROM THE EXISTING MOSTLY NATURAL CONTOUR LINES. 18-7-102 H -P-` KUMAR LOCATION OF EXPLORATORY BORINGS Fig. 1 18-7-102 UJ w w I z 0 1- > w J w BORING 1 EL.6338' BORING 2 EL.6330' 6340 6340 - 6335 6330 6325 6320 6315 44/12 6335 / / f] 9/12 WC=10.3 / DD=117 // f 16/12 / WC=8.7 DD=121 -200=58 50/4 50/3 / 30/12 WC=5.3 DD=117 / /] 20/12 / WC=7.4 DD=117 / -200=57 / /-; 50/5 WC=8.2 DD=124 / 150/4 6330 6325 6320 6315- 6310 315- 6310 6310 - H-P-� KUMAR ELEVATION -FEET LOGS OF EXPLORATORY BORINGS Fig. 2 LEGEND • • / 7 TOPSOIL; ORGANIC SANDY SILTY CLAY, FIRM, DARK, BROWN. CLAY (CL); SILTY, SANDY TO VERY SANDY AND SCATTERED GRAVEL WITH DEPTH, STIFF TO HARD, SLIGHTLY MOIST, RED, LOW PLASTICITY. pSILTSTONE/SANDSTONE BEDROCK, HARD TO VERY HARD WITH DEPTH, SLIGHTLY MOIST, RED, STEEP BEDDING DIP. MAROON FORMATION. RELATIVELY UNDISTURBED DRIVE SAMPLE; 2—INCH I.D. CALIFORNIA LINER SAMPLE. 44/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 44 BLOWS OF A 140—POUND HAMMER FALLING 30 INCHES WERE REQUIRED TO DRIVE THE CALIFORNIA SAMPLER 12 INCHES. NOTES 1. THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 10, 2018 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 OBTAINED BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN PROVIDED. 4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS 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 (pcf) (ASTM D 2216); —200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140). 18-7-102 H -P- KUMAR LEGEND AND NOTES Fig. 3 CONSOLIDATION SWELL CONSOLIDATION - SWELL 1 0 1 2 2 1 0 1 2 .1 10 APPLIED PRESSURE - KSF 10 100 SAMPLE OF: Sandy Silty Clay FROM: Boring 1 @ 5' WC = 10.3 %, DO = 117 pcf I EXPANSION UNDER CONSTANT PRESSURE UPON WETTING EXPANSION UNDER CONSTANT PRESSURE UPON WETTING •-,.< n Tal These lest results apply only to the samples tested. T e testing apart shall not be reproduced. enc pt in full, without the w itten appr vel of Kumar and Associates, Inc. Swell Consolidation tasting performed in accordance wilh ASTM 0-4546 .1 10 APPLIED PRESSURE - KSF 10 100 1 0 APPLIED PRESSURE - KSF 10 100 18-7-102 H -P- KUMAR SWELL -CONSOLIDATION TEST RESULTS Fig. 4 SAMPLE OF: Sandy Silty Clay FROM: Boring 2 @ 2.5' WC = 5.3 %, DD = 117 pcf I EXPANSION UNDER CONSTANT PRESSURE UPON WETTING n �11 These lest results apply only to the samples tested. T e testing apart shall not be reproduced. enc pt in full, without the w itten appr vel of Kumar and Associates, Inc. Swell Consolidation tasting performed in accordance wilh ASTM 0-4546 1 0 APPLIED PRESSURE - KSF 10 100 18-7-102 H -P- KUMAR SWELL -CONSOLIDATION TEST RESULTS Fig. 4 PKU MAR TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 18-7-102 SAMPLE LOCATION I NATURAL MOISTURE CONTENT (%) I GRADATION ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH (PSF) SOIL TYPE BORING DEPTH (ft) NATURAL DRY DENSITY(%) (pcf) GRAVEL SAND (%) PERCENT PASSING ', NO. 200 SIEVE LIQUID LIMIT (%) PLASTIC INDEX (%) 1 1 5 10.3 117 Sandy Silty Clay 10 8.7 121 58 Very Sandy Silty Clay 2 21/2 5.3 117 Sandy Silty Clay 5 7.4 117 57 Very Sandy Silty Clay 10 8.2 124 Weathered Siltstone/Sandstone