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HomeMy WebLinkAboutSubsoil Study for Foundation Designr f I f 1 ! . i l ~ ' ~ ; 1! • f I , I p * •' : ~I • • .. 1 l t' • ( ' • ~· •• '. SUBSOIL STUDY fr • ' t: - . . . ' FOR FOUNDATION DESIGN PROPOSED DUPLEX LOT D-19 ASPEN GLEN SUBDIVISION GARFIELD COUNTY, COLORADO JOB NO. 115 021A FEBRUARY 9, 2015 PREPARED FOR: ., . WOODBRIDGE MORTGAGE INVESTMENT FUND 2, LLC ATTN: RICK SALVATO 22 CENTER STREET, FRONT SUITE FREEHOLD, NEW JERSEY 07728 (rh:i h <!1 ;ml.mm) TABLE OF CONTENTS PlJRPOSE AND SCOPE OF STUDY ............................................................................ - 1 - PROPOSED CONSTRUCTION ....... _ ........................................................................... -1 - SITE CONDITIONS ....................................................................................................... -2 - SUBSIDENCE POTEN"l'IAL ......................................................................................... -2 - FIELD EXPLORATION ................................................................................................. -3 - SUBSURFACE CONDITIONS ...................................................................................... -3 - DESIGN RECOMMENDATIONS ................................................................................ -4 - FOUNDATIONS ........................................................................................................ - 4 - nooR SLABS ........................................ , ........................................................ , ........ -... _.-5 - UNl)ERDRA.IN SYSTEM .................................. .,. ...................................................... -6 - SURFACE DRAIN'AGE ............................................................................................. -6 - Lll\1.IT A TIO NS ................................................. -.............................. +.+•· .. •••••·•••••·••••• ................... -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 TABLE 1-SUMMARY OF LABORATORY TEST RESULTS PURPOSEANDSCOPEOFSTUDY This report presents the results of a subsoil study for a proposed duplex to be located at Lot D-19, Aspen Glen, Garfield County, Colorado. The project site is shown on Figure l. The purpose of the study was to develop recommendations for the foundation design. The study was conducted in accordance with our proposal for geotechnical engineering services to Woodbridge Mortgage Investment Fund 2, LLC dated January 20, 2015. Chen-Northern, Inc. (1991and1993) previously conducted preliminary gcotechnical engineering studies for the development and preliminary plat design. A field exploration program consisting of exploratory borings was conducted to obtain infonnation on the subsurface conditions. Samples of the subsoils obtained during the field exploration were tested in the laboratory to detennine 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 b~ed on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION The proposed duplex will be one and two story wood frame construction above a crawlspace and with attached ~arages. Garage floors will be slab-on-grade .. Grading for the structure is assumed to be relatively minor with cut depths between abqut 3 to 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 . Job No. 115 02JA -2- SITE CONDITIONS The vacant Jot was covered with about 6 to 12 inches of snow at the time of our exploration. Vegetation consists of grass and weeds. The area was graded during subdivision development with probable shallow cuts at the site. The ground surface is relatively flat with a slight slope down to the northwest. An irrigation ditch is located along the rear property line . SUBSIDENCE POTE.NTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen development These rocks are a sequence of gypsiferous shale, fine-grained sandstone/siltstone and limestone with some massive beds of gypsum. 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 broad subsidence areas and smaller size sinkhole areas were observed scattered throughout the Aspen Glen development, predominantly on the east side of the Roaring Fork River (Chen-Northern, Inc., 1993). These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley. Lot D-19 is located outside and about 450 feet east of one of the broad subsjdence areas mapped by Cbcn-Nonhem. Signs of active ground movements have not been observed in the subsidence area The nearest sinkhole was mapped about 1000 feet to the northwest of Lot D-19. Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encoumcred in the subsurface materials; however, the exploratory borings were relatively shaUow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be s~d for certain that sinkholes wiJl not develop. The risk of future ground subsidence on Lot D-19 throughout the service life of the proposed duplex . i n our opinion, is low but the site Job No. I IS 021A -3- should not be considered totally risk free. 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 January 21, 2015. Two exploratory b~rings were drilled at the locations shown on Figure l 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 Hepworth-Pawlak Geotcchnical, 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 al the site are shown on Figure 2. The subsoils consist of about 10 feet of sandy silty clay overlying silty sandy gravel with cobbles and boulders. Drilling in the dense granular soils with auger equipment was difficuJt due to the cobbles and boulders and drilling refusal was encountered in the deposit. Laboratory testing performed on samples obtained from the borings included natural moisture content and percent finer then sand gradation analyses. Results of swell- consolidation testing perfonned on relatively undisturbed drive samples, presented on JobNo. llS021A -4- Figure 4, indicate low to moderate compressibility under conditions of loading and wetting. The sampJes showed a low swell potentiaJ when wetted. 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. 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 sandy silty cJay soils . 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 bearing pressure of 2,000 psf. Based on experience, we expect settlement/heave 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 18 inches for continuous walls and 2 feet for isolated pads . 3) Exterior footings and footings beneath unheated areac; 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 retain ing structures should also be Job No. I IS 021A -5- designed to resist a lateral earth pressure corresponding to an equivalent fluid unit weight of at least SO pcf. 5) AH topsoil and any loose or disturbed soiJs should be removed and the footing bearing level extended down to the relatively undisturbed 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 imported granular soils such as % inch road base devoid of oversized rock, vegetation and topsoil. Job No. 115 021A -6 - UNDERDRAINSYSTEM Although free water was not encountered during our exploration , it has been our experience in the area that local perched groundwater can develop during ti mes 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 and crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an underdrnin system. The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above the invert level with f ree-draining granular material. The drain should be placed at each level of excavation and at least 1 foot below lowest adjacent fi ni sh grade and sloped at a minimum 1% to a suitable gravity outlet, drywell based in the underlying gravel soils or sump and pump. 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 1 ~ feet deep . SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the duplex has been completed: 1) Inundation of the foundation excavations and unders lab 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 den sity i n pavement and sl ab areas and to at least 90% of the maximum stand ard 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 sl ope of 12 inches iu the first lO feet in unpaved Job No. l!S 021A -7- areas and a minimum slope of 3 inches in the first 10 feet in paved areas. Free-draining wall backfill should be capped with about 2 feet of the on- sitc soils to reduce surface water inftltration. 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 IO feet from foundation walls. Consideration should be given to use of xeriscape to reduce the potential for wetting of soils below the building caused by irrigation. LIMITATIONS This study has been conducted in accordance with generally accepted gcotechnical 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 arc based upon the data obtained from the exploratory borings drilled at the locations indicated on Figure I. 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 e;icploratory borings and variations in the subsurface condiLions may not become evident until excavation is perfonned. 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 rccommendaLions have been appropriately interpreted. Significant design Job No. llS 021A changes may require additional analysis or modilications to the recommendations presented herein. We recommend on-site observation of excavations and foundation bearing strata and teMing of structural Ii II by a representative of the geotechnical engineer. Respectfully Submillcd , l-IEPWORTll ·PAWLAK GEOTECHNICAL, INC. Louis E. Eller Reviewed by: LEE/ljf REFERENCES Chen-Northern, Inc., 1991, Preliminary Geotedmical £11gi11eeri11g Srm(r. Proposed A.\1Jl'll Glen De1 ·e/opme111. Gmjidcl Cmmty. Colomdo. prepared for Aspen Glen Company, dated December 20, 1991, Job No. 4 112 92. Chen -Northern, Inc., 1993, G,·oredmicnl Engi11eeri11g S111clyfor Pn:liminm:r Plat Design , Aspe11 Gh•n Dt•1 ·e/opm e111 . Gmfieltl Cou111y. Colorado. prepared for Aspen Glen Company. da ted May 28, 1993 . Job No. 4 112 92. ~ech \ APPROXIMATE SCALE 1· = 30' / / I\ I \ I \ I \ // \ / \ / \ / \ // \ \ / \ \ // \ / \ // '\ \ / \ // / \ // ' // ~ \ ---/ \ /,,-\ I / LOTD-19 \ ( ( . ~ \ \ BORING2 \ LOT D-18 \ \ ~ \ \ \ PROPOSED \ \ \ WPLEX \ \ I BORING 1 ·~ \ t I_ ' I ~G SETBACK LINE \ I \ r \ L_______ \ ------------------------~ IRRIGATION DITCH LOTC-1 115 021A G5'~ LOCATION OF EXPLORATORY BORINGS Figure 1 He warttt-Pawlak Geotedvllcol BORING 1 BORING2 0 0 9/1 2 12/12 WC=S.4 5 0Du98 5 12/12 -200=83 WC=11 .3 00=99 9/12 WC=B .7 ii) 00=93 '1il if tf I 10 3orl 10 • .c £ a. Cl. Q) Q) 0 0 3516,10/0 15 15 20 20 Note: Exp lanation of symbols is shown on Figure 3. 115 021A LOGS OF EXPLORATORY BORINGS Figure 2 LEGEND: CLAY (CL); sandy, silty to very silty, stirf to very stiff with depth, red, low plasticity, upper 6 Inches organic, upper portion porous. GRAVEL (GM); with cobbles and boulders, sandy, silly, dense, slightly moist, brown, subrounded rock. 9/12 T NOTES: Relatively undisturbed drive sample; 2-inch l.D . California liner sample. Drive sample; standard penetration tesl (SPn. 1 3/8 inch 1.0. split spoon sample, ASTM D-1586. Drive sample blow count; indicates thal 9 blows of a 140 pound hammer falling 30 inches were required lo drive the California or SPT sampler 12 inches . Practical drilling refusal. 1. Exploratory borings were drH!ed on January 21, 2015 with 4-inch diameter continuous night 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. Fluctuation In water level may 0CC1Jr with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Densily (pcf) -200 = Percent passing No. 200 sieve ~ H91:1warth-Pawlak Gaiatechnlcal 115 021A LEGEND AND NOTES Figure 3 Moisture Content = 11.3 percent Dry Density = 99 pcf Sample of: Sandy Silty Clay From: Boring 1 at 5 Feet 1 (fl. c~~ c 0 u; 0 a ~ "t a. " a'.i • 1 ' c .Q \ ) (/) (/) QI a. 2 E 0 Expansion 0 upon wetting 0.1 1.0 10 100 APPLIED PRESSURE -ksf Moisture Content = 8 .7 percent 2 Dry Density = 93 pcf Sample of: Sandy Silty Clay From: Boring 2 at 7 Feet 1 t I ~ ~ c: .Q 0 (/) c: ~ \ ro a. in • 1 c: Expansion I c \ u; rn upon !!? 2 wetting a. E 0 \ 0 3 ~ 4 0.1 1.0 10 100 APPLIED PRESSURE -ksf 115 021A c.s''1ech H111>warth-Pawlak Geatechnlcal SWELL-CONSOLIDATION TEST RESULTS Figure 4 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 Job No. 115 021A SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION PERCENT AmRBERG LIMITS UNCONFINED MOISTURE DRY GRAVEL SAND PASSING UQUID PU\STIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY NO. 200 UMIT INDEX STRENGTH BEDROO< TYPE (%) (%) SIEVE lit\ t%\ (od\ 1%) {%\ (PSf\ 1 5 11.3 99 Sandy Silty Clay 2 3 5.4 98 83 Sandy Silty Clay 7 8.7 93 Sandy Silty Clay