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HomeMy WebLinkAboutSoil's Report.pdfh HEPWORTH-PAWLAK GEOTECHNICAL. llui ,r,m1'1-1'.1tvli1,tic, Ircluucal, Inc. i0't'I 1 (;14.11%!, ..J 'l F1I . i i1,vn..l:, tti(A.) l CII;111: SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT E-15, ASPEN GLEN SUBDIVISION GARFIELD COUNTY, COLORADO JOB NO. 114 152A MAY 28, 2014 PREPARED FOR: ATTN: JOHN PLACEK P.O. BOX 4004 BASALT, COLORADO 81621 (crvstalconst(a-)sopris.nct) Parker 303-841-7119 • ColorAlo Springs 719-633-5562 • Silti•erthorne 970.468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION -1- SITE CONDITIONS - 2 - SUBSIDENCE POTENTIAL _ 2 - FIELD EXPLORATION - 3 - SUBSURFACE CONDITIONS - 3 - DEVELOPMENT IN SURFACE DEPRESSION AREAS - 4 - DESIGN RECOMMENDATIONS - 5 - FOUNDATIONS _ 5 - FLOOR SLABS - 6 - UNDERDRAIN SYSTEM - 6 - SURFACE DRAINAGE - 6 - LIMITATIONS - 7 - REFERENCES - 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 E-15, Aspen Glen Subdivision, 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 Crystal Constructors dated May 6, 2014. Chen - Northern, Inc. previously conducted a preliminary geotechnical engineering study for the development (Chen -Northern, 1991) and another geotechnical engineering study for preliminary plat design (Chen -Northern, 1993). 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 one story wood frame constriction above a crawlspace with an attached garage. Garage floor will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 5 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. Job No. 114 I52A C 6eP - ch -2 - SITE CONDITIONS The lot is vacant and vegetated with sparse grass and weeds. The ground surface is relatively flat with a slight slope down to the southeast. The site appears to have been graded during subdivision development with topsoil and some sandy clay removed. There is a small low lying area in the southeast corner of the lot with willows and cattails. An irrigation ditch parallels the eastern property line. SUBSIDENCE POTENTIAL SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies most of the lower Roaring Fork Valley and the Aspen Glen Subdivision. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive keds 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 studies in the area, several sinkholes were observed scattered throughout the Aspen Glen development (Chen -Northern, Inc., 1991). These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork Valley. The site is mapped as lying within a broad depression area and a sinkhole was mapped about 60 feet west of the building envelope. The lot is located in a wide surface depression area thought to be associated with long-term subsidence potential. 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 E-15 throughout the service life of the proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further Job No. 114 152A Gtech -3 - 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 May 12, 2014. 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- drill rig. The borings were logged by a representative of Hepworth-Fawlak Geotechnical, Inc. Samples of the subsoils were taken with 1% inch 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 2 to 4 feet of stiff sandy clay overlying slightly silty sandy gravel with cobbles and small boulders. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered at 11 to 14 feet deep in the deposit. Laboratory testing performed on samples obtained from the borings included natural moisture content and gradation analyses. Results of swell -consolidation testing performed on a relatively undisturbed drive sample, presented on Figure 4, indicate low to moderate compressibility under conditions of loading and wetting. Results of Job No. 114152A Gatech -4 - gradation analyses performed on small diameter drive samples (minus 1 %2 inch fraction) of the coarse granular 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 or when checked 15 days later and the subsoils were slightly moist to moist. DEVELOPMENT TN SURFACE DEPRESSION AREAS As described in Chen-Northem's geotechnical report (Chen -Northern, 1993), "development within surface depression areas should be feasible provided appropriate mitigative designs are implemented for residential buildings". These design considerations included use of a rigid foundation, such as a stiffened slab or raft, and a simply shaped building footprint to reduce potential damage in the event of differential movement. The following architectural measures were also recommended: Relatively flexible structural systems such as wood frame construction, flexible exterior siding and dry wall interior partitions are preferable to less flexible masonry structural systems and exterior sidings. Interior non-bearing partitions resting on floor slab should be provided with slip joints so that slab movement is not transmitted to the upper structure. Limit building height to one or two stories. Building should have relatively small plan dimensions of 60 feet or Less. Building configuration should be a simple rectangular configuration with straight foundation walls and minimal side projections. Ground level should be a single level rather than using split level design. Basements are particularly susceptible to subsidence damage and are not recommended unless the entire foundation is at basement level and designed for lateral earth loading. To our knowledge, these mitigation measures recommended by Chen-Northem were not implemented by the Aspen Glen HOA in their design requirements. Over the last 20 Job No. 114 152A Gtech -5 - years, it has been our experience that no movements have been observed in the homes built within the surface depression. It is our opinion that the mitigation measures recommended by Chen -Northern, while they could decrease the risk of building distress in the event of ground movement, are not warranted. The risk of movement on this lot is similar to the surrounding lots in this area. 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 on the natural granular soils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural granular soils should be designed for an allowable bearing pressure of2,500 psf. Based on experience, we expect settlement Of footings designed and constructed as discussed in this section will be about I inch or less. 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 10 feet. Foundation walls acting as retaining structures should also be designed to resist a lateral earth pressure corresponding to an equivalent fluid unit weight of at least 45 pd. Job No. 114 152A Gg tech -6- 5) All existing fill, topsoil, sandy clay and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively dense natural granular soils. The exposed soils in footing area should then be moistened and compacted. If water seepage is encountered, the footing areas should be dewatered before concrete placement. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab - on -grade construction. 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 granular soils devoid of vegetation, topsoil and oversized rock. UNDERDRAIN SYSTEM An underdrain system is not needed for the proposed shallow crawlspace provided that good surface drainage as described below is maintained around the residence. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: Job No. 114 152A Gaech -7- I) 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% ofthe 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. Free -draining wall backfill (if any) should be capped with about 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. 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 practice should be consulted. Our findings include interpolation and extrapolation ofthe 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 Job No. 114 152A Gtech -8 - 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 - P WLAK GEOTECHNICAL, INC. sex Louis E. Eller Reviewed by: Daniel LEE/ljg 'T., ardin, P.E. t© ��43 < r � 5i.)51/44)„4- -‘1%..°..., O . � �P,��~ °rl'ONAL �'\ i410110Ek0 REFERENCES Chen -Northern, Inc., 1991, Preliminary Geotechnical Engineering Study, Proposed Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen Company, dated December 20, 1991, Job No. 4 112 92. Chen -Northern, Inc., 1993, Geotechnical Engineering Study for Preliminary Plat Design, Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen Company, dated May 28, 1993, Job No. 4 112 92. Jon No. 114 I52A GecPtech 1 0 LL 1 0] PREVIOUSLY MAPPED SINKHOLE DIAMOND A RANCH ROAD i • BORING 2 I L WWI BORING 1 • LOT E-15 LOT E-16 APPROXIMATE SCALE 1"=30' 114 152A Gtech HEPWORTtPAWLAK GEOTECHNICAL LOCATION OF EXPLORATORY BORINGS Figure 1 0 5 10 15 114152A I BORING 1 6/12 96/10 WC -1.3 +4=51 -200=14 7/12 91/7 BORING 2 0 17/12 WC -11.2 00=101 71/12 5 WC=1.0 +4=53 -200=9 L 39/12 — p 10 _.. Note: Explanation of symbols is shown on Figure 3. Gafech 15 Hepworth—Pawlak Geotechnled ILOGS OF EXPLORATORY BORINGS I Figure 2 LEGEND: -7 b 1 6/12 1 CLAY (CL); sandy, silty, upper 6 inches slightly organic, very stiff, moist, .red -brown. GRAVEL, COBBLES AND BOULDERS (GM -GP); slightly silty to silty, sandy, dense, slightly moist, light brown, subrounded rock. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. Drive sample; standard penetration test (SP1), 1 3/8 inch I.D. split spoon sample, ASTM D-1586. Drive sample blow count; indicates that 6 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 May 12, 2014 with 4 -inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory borings were not measured and the logs of exploratory borings are drawn to depth. 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 or when checked 15 days later. 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 114152A HaPworthh—•P'a vio Oeotechn Ic I LEGEND AND NOTES Figure 3 Compression % 0 1 2 4 Moisture Content = 11.2 percent Dry Density = 101 pcf Sample of: Sandy Silty Clay From: Boring 2 at 2 Feet 0.1 114 152A I 1.0 APPLIED PRESSURE - kef Gtecl Hepworth,--Piawlak Geotac tMail 1 No movement pon wetting 10 100 SWELL -CONSOLIDATION TEST RESULTS I Figure 4 'CENT RETAIN i HYDROMETER ANALYSIS TIME READINGS 24Mt 11n Q5 MIN. 15 MIN. 60MIN79MIN.4 MIN. 1 MIN 10 20 30 40 50 60 70 60 90 100 .001 200 SIEVE ANALYSIS U.S. STANDARD SERIES 1 CLEAR SQUARE OPENINGS #100 #50 #30 #16 #8 #4 3/8' 3/4' 11/21 3° 5'6' 8' r 1 CO2 .SLB 409 CLAY 10 ELT .019 .037 .014 .150 Sm 1.10 2.89 DIAMETER OF PARTICLES IN MILLIMETERS GRAVEL 51 % LIQUID LIMIT % SAMPLE OF: Silty Sandy Gravel — 1 4.15 9512.5 19.0 375 192 152 203 127 sANO FINE 1 MEDAM ICOARSE 1 EINE 1 COARSE COBBLES SAND 35 % SILT AND CLAY 14 % PLASTICITY INDEX % FROM: Boring 1 at 5 Feet 100 90 90 Z >a Z 2 m a 50 40 IA 0- 30 20 10 0 HYDROMETER ANALYSIS J SIEVE ANALYSIS 0qq H TIME READINGS l U.S. STANDARD SERIES 1 CLEAR SQUARE OPENINGS 5 ..15 MIN. 60MIN19MIN.4 MIN. 1 MIN. #200 #100 #50 #30 #16 #8 #4 3/8' 3/4' 1 1/2' 31 5'8' 8' 100 10 I , 90 20 I 80 30 1 70 Z 0) 40 0. co 80 60 50 40 w 30 80 ■■II■■■■.,.■■f■hi'. 4 90 • ufl .auI.I!..III 10 100 .001 .002 .005 .009 .019 .037 .074 .150 .300 .600 1.18 2.36 4.75 9.512.519.0 37,5 76.2 121752 203 DIAMETER OF PARTICLES IN MILLIMETERS 0 CLAY TOOLT FlNE GRAVEL 53 % SAND LIQUID LIMIT % SAMPLE OF: :SlightlySilty Sandy Gravel 114 152A Hepworth-Pawlak Geoteahntaal 3ANO GRAM_ I COBBLES 9E0414 ICOA119E FIE I COARSE 38 % SILT AND CLAY 9 % PLASTICITY INDEX % FROM: Boring 2 at 5 Feet GRADATION TEST RESULTS Figure 5 Job No. 114 152A ci k § / ry w 2 6 § \ I- §2 ƒ/\ ± § \ r a. I t 3 ATTERBERG LIMITS UNCONFINED LIQUID PLASTIC COMPRESSIVE SOIL OR LIMIT INDEX STRENGTH BEDROCK TYPE (%) (%) (PSFI Silty Sandy Gravel Sandy Silty Clay Slightly Silty Sandy Gravel % "; ta a. 0. 14 0\ §2/ \ ` m m (9 G G \k§ t § Hh zu 1.3 ti _ 3 LOCATION DEPTH (R) w in 0 ( N