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Home My WebLink AboutSoils Report 04.29.2016G&ech HEPWORTH - PAWLAK GEOTECHNICAL Herm orth•1'uw1.1 Ina. 5020 County Ro:id 151 Glenwood Springs, Cilnrid.z 81601 Mone: 970.945.7985 Fax_ 970.945.8454 cuta�l hpgco@hpgcure h.com SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 75, SPRINGRIDGE RESERVE 75 HIDDEN VALLEY DRIVE GARFIELD COUNTY, COLORADO JOB NO. 116 082A APRIL 29, 2016 PREPARED FOR: WHITNEY AND MICHAEL SCURLOCK (whitney360@gmai1.com) (Michael.scurlock @ riverrestoration.org) RECEIVED JAN 19 2017 GARFIELD COUNTY COMMUNITY DEVELOPMENT Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 2 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS - 4 - FOUNDATION AND RETAINING WALLS - 5 - FLOOR SLABS - 6 - UNDERDRAIN SYSTEM - 7 - SITE GRADING - 7 - SURFACE DRAINAGE - 8 - LIMITATIONS - 8 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS FIGURE 5 - GRADATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at Lot 75, Springridge Reserve, 75 Hidden Valley 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 agreement for geotechnical engineering services to Whitney and Michael Scurlock dated March 24, 2016. Hepworth-Pawlak Geotechnical previously conducted a preliminary geotechnical study for the Springridge Reserve Subdivision and presented our findings in a report dated June 22, 2004, Job No. 101 126. 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 Building plans for the residence were conceptual and we understand the findings of our study will be considered in the purchase of the property. The residence will likely be one to two story wood frame structures over a walkout basement level with an attached garage at the walkout level. Ground floors will probably be slab -on -grade. Grading for this type of structure is assumed to be relatively minor with cut depths between about 3 to 9 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. Job No. 116 082A Glitiamch -2 - When building location, grading and loading information have been developed, we should be notified to re-evaluate the recommendations presented in this report. SITE CONDITIONS The lot was vacant and the ground surface appeared mostly natural. The terrain is westerly sloping hillside above Hidden Valley Drive. The slope grades range from about 10 to 20%. Elevation difference across the assumed building area is about 8 to 12 feet. There are two abandoned ditches that trend through the site, see Figure 1. Vegetation consists of moderately tall grass and weeds. The adjacent lots are vacant. FIELD EXPLORATION The field exploration for the project was conducted on April 8, 2016. Three exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. A third boring was added during the field exploration due to the shallow bedrock being encountered. 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 Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils and bedrock were taken with 1'/s inch and 2 inch I.D. spoon samplers. The samplers were driven into the subsoils and bedrock 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 Togs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils encountered, below about 1/2 to 1 foot of organic topsoil, consisted Job No. 116 082A Ggrytech -3 - intermixed sandy clay and gravel with cobbles underlain at depths from about 31 to 5 feet by hard to very hard sandstone bedrock that extended down to the maximum depth drilled of 11 feet. Drilling in the sandstone with auger equipment was difficult due to its hardness and cemented condition, and drilling refusal was encountered in all three borings in the formation. The bedrock is of the Maroon Formation. Laboratory testing performed on samples obtained from the borings included natural moisture content and density, and gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples of the sandstone and sandy clay soils are presented on Figure 4. The swell -consolidation testing indicated indicate low to moderate compressibility under conditions of loading and wetting. The sandstone sample showed a low hydro -compression potential and the clay sample a minor swell potential. The sandstone sample was probably partly disturbed due to its fractured condition from the sample driving. Results of gradation analyses performed on a small diameter drive sample (minus 11/2 inch fraction) of the sandy clay with gravel subsoils are shown on Figure 5. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist to moist, and the bedrock was slightly moist. FOUNDATION BEARING CONDITIONS The soils possess low to moderate bearing capacity and the bedrock moderately high bearing capacity. Spread footings bearing on these materials appear feasible for foundation support of the building. To reduce the risk of differential movement, the footings should bear entirely on the sandstone bedrock, but can be further evaluated at the time of construction. Difficult excavation of the sandstone bedrock should be expected especially in confined utility trenches. Deeper foundation excavations or excavation in the utility trenches may require chipping. Job No. 116 052A Gmech allowable bearing pressure of 3,000 psf. 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 entirely on the sandstone bedrock. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed bedrock should be designed for an Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be less than 1 inch. 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 I2 feet. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) All topsoil, soil and any loose disturbed materials should be removed and the footing bearing level extended down to the undisturbed bedrock. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. Job N;371-16 082A - GBCed'I -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 50 pcf for backfill consisting of the on-site predominantly granular soils and 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 45 pcf for backfill consisting of the on-site predominantly granular soils and well broken bedrock. The backfill should not contain topsoil or oversized (plus 6 inch) rocks. 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 a moisture content near optimum. Backfill 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 Job No, 116 082A -6 - 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 375 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 a suitable granular material such as road base compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural on-site soils and bedrock, exclusive of topsoil, are suitable to support lightly Ioaded slab -on -grade construction. There could be some differential movement for slabs that transition from soil to bedrock bearing areas, especially if the soil subgrade were to become wetted. We should evaluate the exposed slab subgrade conditions at the time of construction for need of possible subexcavation of the soils and replacement with structural fill. ,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 immediately 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 Job No. 116082A Gtech -7 - consist of the on-site granular soils and well broken bedrock, devoid of topsoil and oversized (plus 6 inch) rocks, or a suitable granular imported material such as road base. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in the area where clayey soils are present and bedrock is shallow that local perched groundwater can 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 welting 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, Tess than 50% passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least 11 feet deep and be covered by filter fabric such as Mirafi 140N or 160N. SITE GRADING The risk of construction -induced slope instability at the site appears low provided the cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about 10 feet. Embankment fills should be limited to about 10 feet deep be compacted to at least 95% of the maximum standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be carefully prepared by removing all vegetation and topsoil and compacting to at least 95% of the maximum standard Proctor density. The fill should be benched into the portions of the hillside exceeding 20% grade. Job No, 116 082A Gated" Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or flatter and protected against erosion by revegetation or other means. Steeper cuts into bedrock should be feasible and can be evaluated if desired. 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 3 inches in the first 10 feet in paved areas. This may require a swale at the uphill side. 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 5 feet from foundation walls. 6) The existing ditched on the lot should be backfilled. 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 Job No, 116 082A Getritech -9 - 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 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, HEPWO `, H - PAWLAK GEOTECHNICAL, INC. i� Louis E. Eller Reviewed by: %Ili Millie ..4 • 4 ir�•• David A. Young, P.E. LEE/ksw 32.216 LfJ r CID Job No. 1 i 6 082A - - Ggstech MI APPROXIMATE SCALE 1"=60' HIDDEN VALLEY DRIVE LOT 74 NOTE LOT BOUNDARIES, BUILDING ENVELOPE AND CONTOURS TAKEN FROM SUBDIVISION PLAT ai IL CL m 0 0 5 10 15 BORING 1 ELEV.= 6435' AY 7 1 50/1 i7r 5012 BORING 2 ELEV,= 6444 50/2 22/6,5015 WC=34 DD -125 BORING 3 ELEV.= 6444` Note: Explanation of symbols is shown on Figure 3. 10/6 27/6 WC -14 2 DD=111 66/12 WC -4.5 +4-12 -200=69 50/2 0 5 10 15 1v v LL a v 0 116 082A H Hepworth -Powick Geatechnicoi LOGS OF EXPLORATORY BORINGS Figure 2 LEGEND: MTOPSOIL; organic sandy clay and silt, firm, slightly moist, dark reddish brown. GRAVEL AND CLAY (GC -CL); sandy, with cobbles, medium dense/very stiff, slightly moist, reddish brown SANDSTONE; hard to very hard, slightly moist, reddish brown. Maroon Formation. 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. 37/12 Drive sample blow count; indicates that 37 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. Practical drilling refusal. NOTES: 1. Exploratory borings were drilled on April 8, 2016 with 4 -inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pacing from features shown on the Figure 1 plat plan of the lot. 3. Elevations of exploratory borings were obtained by interpolation between contours shown on the Figure 1 plat plan. 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 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. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) +4 = Percent retained on the No. 4 sieve -200 = Percent passing No. 200 sieve 116 082A ' I-1 Hepworth—PawIok Gaotechnlco! LEGEND AND NOTES Figure 3 Compression % Compression - Expansion % 0 1 2 3 4 0 1 2 Moisture Content = 3.4 percent Dry Density - 125 pcf Sample of: Sandstone From: Boring 2 at 5 Feet �'` Compression upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content = 14.2 Dry Density - 111 Sample of: Sandy Clay From: Boring 3 at 2 y2 Feet percent pcf Expansion upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 HYDROMETER ANALYSIS SIEVE ANALYSIS 24 HR. 7 Hp TIME READINGS U S STANDARD SERIES 1 CLEAR SQUARE OPENINGS 0 45 MIN 15 MIN 60MIN19MIN 4 MIN 1 MIN #200 #100 #50 #30 1116 #8 #4 3/W 3/4' 1 1I2' 3 5'6' B' 100 10 20 30 40 50 60 70 80 90 100 r 1 r T 1 001 002 005 .009 019 037 074 150 300 600 1 18 2 36 4 75 9 5 19 0 37 5 76 2 152 203 17 5 127 DIAMETER OF PARTICLES IN MILLIMETERS GRAVEL 12 % LIQUID LIMIT % SAMPLE OF; Sandy Clay with Gravel SAND 19 % SILT AND CLAY 69 % PLASTICITY INDEX ;o FROM: Boring 3 at 4 Feet 80 60 50 40 0 ��►I�F��LC� HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 116 082A SAMPLE LOCATION NATURAL MOISTURE CONTENT (%) NATURAL DRY DENSITY (pc� GRADATION PERCENT NAO.200 SIEVE ATTERBERG LIMITS UNCONFINED STRENGTH (PSF) SOIL OR BEDROCK TYPE BORING 1 DEPTH (ft) GRAVEL (%) SANDCOMPRESSIVE ("/aj LIQUIDLIMIT (%1 PLASTICINDEX (%) 2 5 3.4 125 Sandstone 3 2 1 14.2 111 Sandy Clay 4 4.5 12 19 69 Sandy Clay with Gravel