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HomeMy WebLinkAboutSoils Report 05.30.2014HEPWORTH—PAWLAK GEOTECHNICAL Idepv+urtlri :;tak Gcntechnte:ti, 1t . 5020 Cioutity Rt tad I54 Glenwood Sprmes. Ci I r JAs 81601 Phone 970.945 79hL F,tx 970 945.8454 cnnul 111)gerghpe01(erlt Cum SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 45, OAK MEADOWS FILING 4B, PHASE II 639 OLD MIDLAND SPUR GARFIELD COUNTY, COLORADO JOB NO. 114 160A MAY 30, 2014 PREPARED FOR: KEN GARRETT 3900 SOUTH WADSWORTH BOULEVARD SUITE 465 LAKEWOOD, COLORADO 80235 (kenjarn'ri nisn.eom) Parker 303-841.7119 • Colorado Springs 719-633 5562 • Stiverncorne 970-4681989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - I - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 GEOLOGIC CONDITIONS - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 3 - 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 - g - LIMITATIONS - g - FIGURE 1 - LOCATION OF EXPLORATORY BORING FIGURE 2 - LOG OF EXPLORATORY BORING FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS TABLE I- 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 on Lot 45, Oak Meadows Filing 4B, Phase II, 639 Old Midland Spur, 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 Ken Garrett dated May 7, 2014. A field exploration program consisting of an exploratory boring 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 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 suaunarizes 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 design was conceptual at the time of our study. In general, the residence will be multi-level and located in the middle of the lot and with a walkout lower level Ground floors could be slab -on -grade and structural above crawlspace. Grading for the structure will be relatively extensive due to the steep slope of the lot with assumed cut and fill depths up to about 10 to 12 feet. We assume relatively light foundation loadings, typical of the assumed type of construction. When building location, grading and Ioading information have been developed, we should be notified to re-evaluate the recommendations presented in this report. Job No. 114 160A -2 - SITE CONDITIONS The lot was vacant and covered with grass and weeds with sage brush in the middle to lower part of the lot. The ground surface slope is steep off of Old Midland Spur consisting of an earth embankment for the mad construction on the order of 15 feet high. The natural ground surface in the middle part is moderately steep, on the order of 15% then about 25 to 30% in the lower part. Basalt boulders were observed in the area which may be left from the mad and utility infrastructure construction. GEOLOGIC CONDITIONS Lot 45 is located near the lower limit of a mapped, very large, dormant landslide complex. Hepworth-Pawlak Geotechnical evaluated the overall stability ofthe landslide as part of the subdivision approval by Garfield County in 1999. The evaluation included depth to bedrock and depth to groundwater level, both being relatively deep. The conclusion was that the landslide complex was not near critical stability condition and moderate cut and fill depths made for the subdivision infrastructure and individual lot developments should not affect the overall stability of the landslide. FIELD EXPLORATION The field exploration for the project was conducted on May 8, 2014. One exploratory boring was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The boring was advanced with 4 inch diameter continuous flight augers powered by a truck -mounted CME -45B drill rig. The borings were logged by a representative ofHepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with a 2 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 Jab No. 114160A -3 - consistency of the subsoils. Depths at which the samples were taken and the penetration resistance values are shown on the Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The subsoils consist of about one foot of topsoil overlying stiff to very stiff sandy clay becoming hard and calcareous at about 13 feet below ground surface to the drilled depth of 21 feet. The boring was located just below the toe of the roadway fill slope in the natural terrain. The fill depth could be up to about 15 feet. Laboratory testing performed on samples obtained from the boring included natural moisture content and density and unconfined compressive strength. Results of swell - consolidation testing performed on relatively undisturbed drive samples of the clay soils, presented on Figure 4, indicate low to moderate compressibility under loading and minor expansion potential when wetted. Results of the unconfined compressive strength test indicted very stiff consistency. The laboratory testing is summarized in Table 1. No free water was encountered in the boring at the time of drilling and the subsoils were moist to slightly moist with depth. FOUNDATION BEARING CONDITIONS The natural clay soils have moderate bearing capacity and low settlement potential under light load such as from a residence typical of this area. Shalbw spread footings placed on the natural soils should have relatively low settlement potential The expansion potential of the clay appears minor and should be further evaluated at the time of construction. The existing fill (and topsoil) are not suitable for building foundation support. The fill may be suitable for the driveway embankment support and should be further evaluated at the time of construction. Job No. 114 160A -4 - DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, we recommend the building be founded with spread footings bearing on the natural soils below topsoil and existing fill. 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 desiane i for an allowable bearing pressure of 2,500 ps£ Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. There could be additional settlement/heave of about'' to 1 inch if the bearing soils are wetted. 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 12 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) The existing fill, topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the very stiff clay soils. The exposed soils in footing area should then be moistened and compacted. If water seepage is encountered, the footing areas should be dewatered and we should be contacted for additional recommendations. Job No. 114 160A -5- 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. 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. Backfill in pavement and walkway areas should be compacted to at Ieast 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 settlement potential can be limited by use of a relatively well graded granular soil, such as road base, and compaction to at least 98% of standard Proctor density. Job No. 114 160A -6 - 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 pct 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 compacted to at least 95% of the maximum standard Proctor density at near optimum moisture content. FLOOR SLABS The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab - on -grade construction. The clay soils may exhibit expansion potential and heave the floor slab if they become wetted. The expansion potential should be further evaluated at the time of construction for possible mitigation methods such as non -expansive structural fill below the slab and slip joints at the bottom of interior partition walls. 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 should consist of imported granular soils devoid of vegetation, topsoil and oversized rock. Job No. 114160A Gtech Although free water was not encountered during our exploration, it has been our experience in mountainous areas 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 maximum size of 2 inches. The drain gravel backfill should be at least 11/2 feet deep. SITE GRADING The risk of construction -induced slope instability at the site appears low provided the building is located in the less steep slope area ofthe lot as planned and cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about 10 to 12 feet. Fills should be limited to about 8 to 10 feet deep, especially at the downhill side of the residence where the slope steepens. We should evaluate the suitability ofthe imported fill material prior to its placement on-site. Embankment fills should 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% ofthe maximum standard Proctor density. The fill should be benched into slopes that exceed 20% grade. 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. The risk of slope Job No. 119 160A G ch -7- UNDERDRAIN SYSTEM -8 - instability will be increased if seepage is encountered in cuts and flatter slopes may be necessary. If seepage is encountered in permanent cuts, an investigation should be conducted to determine if the seepage will adversely affect the cut stability. This office should review site grading plans for the project prior to construction. 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 capped with at ]east 2 feet of the on- site 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 5 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 boring drilled at the location indicated on Figure 1, the proposed type of construction and our experience in the area. Our Job No. 114 160A -9 - 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 boring 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, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Steven L. Pawlak, P.E. Reviewed by: P±1 Daniel E. Hardin, P.E. SLP/Ijg Job No 114 160A APPROXIMATE SCALE 1s = 30' LOT 46 EXISTING RESIDENCE 114160A OLD MIDLAND SPUR 1 1 1 APPROXIMATE TOE 1 1/ OF EXISTING ROAD 1 FILL SLOPE 1 BORING —1 11 1 • 1 1 1 % 1 1 1 1 ! 1 LOT45 1 1 r 1 1 1 1 1 L—� 1 1- 1 1 H Hepworth —Po wlok Gov tnchn/col LOT 44 VACANT LOCATION OF EXPLORATORY BORING Figure 1 m 4 O 0 5 10 — 15 — 20 BORING 1 14/12 WC=18.8 DD= 107 UC -5,500 15/12 WC=19.9 DD=107 19/12 WC= 10.5 DD=98 32!12 40/12 0 5 10 15 20 _ . 25 25 114 160A NOTE: Explanation of symbols Is shown on Figure 3. H riteCh H worth-Ponlok GeatecinfenJ LOG OF EXPLORATORY BORING 1 m Figure 2 LEGEND: ® TOPSOIL; organic sandy clay, dark brown. CLAY (CL); sandy, stiff to very stiff, moist, brown, low to medium plasticity. CLAY (CL); silty, sandy, scattered gravel, very stiff to hard, slightly moist, light brown, calcareous, low plasticity. -7 e L riRelatively undisturbed drive sample; 2 -inch I.D. California liner sample. 14/12 Drive sample blow count; indicates that 14 blows of a 140 pound hemmer failing 30 inches were required to drive the California sampler 12 inches. NOTES: 1. The exploratory boring was drilled on May 8, 2014 with a 4 -inch diameter continuous flight power auger. 2. Location of the exploratory boring was measured approximately by pacing from features shown on the site plan provided. 3. The exploratory boring elevation was not measured and the log of exploratory boring is drawn to depth. 4. The exploratory boring location and elevation should be considered accurate only to the degree implied by the method used. 5, The lines between materials shown on the exploratory boring log represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the boring at the time of drilling. Fluctuation in water level may occur with time, 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) UC = Unconfined Compressive Strength (psi) 114 160A Herm orth—Pu*Iok Goolochrgco! LEGEND AND NOTES I Figure 3