Loading...
HomeMy WebLinkAboutSoils Report.pdf4290 HEPWORTH-PAWLAK GEOTECHNICAL March 7, 2011 Brent Lough 311 Crystal Canyon Drive Carbondale, Colorado 81623 l fCp A•c)rd,-Paa•Iak Gcotecl,nical, Inc. 5020 County Roast 754 ClcnaooJ Springs, Colorado 81601 970-945-7988 l=ax: 970-945-5454 J mail: 1,pgcu I,pgcotccl,.com Job No.100 879-1 Subject: Addendum to Subsoil Study for Foundation Design, Proposed New Residence, Lot A-1, Lough Subdivision, 1655 County Road 109, Garfield County, Colorado Dear Brent: As requested by Dale Kaup, structural engineer, we have re-evaluated the potential downdrag on the proposed micropiles from settlement of the subsoils due to future wetting. We recently provided a subsoil study for the new residence dated February 23, 2011 which included an estimate of 10 kips per pile allowance for downdrag. We had assumed 6 -inch diameter, 50 -kip piles. We understand that 25 -kip piles that are 4 inches in diameter are currently proposed. Based on the subsoil profile of 40 to 60 feet of settlement prone soils and the assumption that about half of the depth will become wetted, we estimate the downdrag force will be about 4 to 6 kips per pile. We recommend that the piles be designed for an average downdrag force of 5 kips. If conditions encountered during construction appear different from those previously described, we should be notified at once so re-evaluation of the recommendations may be made. If you have any questions or if we may be of further assistance, please let us know. Respectfully Submitted, HEPWORTH - PAWLAK Ctig;EC a 1\0 Rfujt QgC®CC3ecn. E. Daniel E. Hardin, P.4 24443 rCr r °°assoa ioNAL lrrglrent�fs,:utPi0 Rev. by: SLP DEH/ksw cc: Kaup Engineering — Dale Kaup (kaupeng irof.net) Parker 303-841-7119 • Colorado Springs 719-633-5562 • Siiverthorne 970-468-1989 HEPWORTH -PAWLAK GEOTECHNICAL February 23, 2011 Brent Lough 311 Crystal Canyon Drive Carbondale, Colorado 81623 Job No.100 879-1 Subject: Subsoil Study for Foundation Design, Proposed New Residence, Lot A-1, Lough Subdivision, 1655 County Road 109, Garfield County, Colorado Dear Brent As requested, Hepworth-Pawlak Geotechnical, Inc. performed additional drilling for a subsoil study for foundation design of a new house proposed at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated February I, 2011. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Evaluation of potential geologic hazard impacts on the site are beyond the scope of this study. We previously performed a subsoil study for underpinning the existing residence dated December 29, 2005, Job No. 100 879-1. Proposed Construction: The proposed residence will be a one story log structure over a crawlspace located in the same area as the existing house on the site as shown on Figure I. Ground floors are proposed to be structural over crawlspace. Cut depths are expected to range between about 5 to 10 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. Due to the settlement prone soils, the house is proposed to be constructed on a deep foundation consisting of micro -piles. The existing house and former garage will be razed. The existing barn will remain. If building conditions or foundation loadings are significantly different from those described above, we should be notified to re-evaluate the recommendations presented in this report Existing Building and Site Conditions: The existing house was built in 1997. The house started settling noticeably after a crawlspace flood event in August, 2000. The house was subsequently underpinned with pin piles_ The garage experienced movement and a new slab was placed in 2004. The garage continued to settle and the garage doors 71))6:), 2 o cil tith,"n ne `) '7‘. -2 - were subsequently removed and replaced with siding, making the garage into a storage shed. The garage continues to settle to this day. The mud room/laundry room between the house and the garage has been removed. The grading and landscaping around the house is mostly similar to that observed during our 2005 visit. The leach field area downhill of the house has settled 2 to 3 feet. Subsurface Conditions: The subsurface conditions at the site were evaluated by drilling an exploratory boring near the northwest (downhill) corner of the existing house at the approximate location shown on Figure 1. The logs of the previous Borings 1 and 2 and the recent Boring 3 are presented on Figure 2. The subsoils encountered in the recent boring, below about 7 feet of fill, consist of clayey sand and sandy silty clay soils down to about 60 feet where firm bedrock of the Eagle Valley Evaporite Formation was encountered. The soil profile is similar to that encountered in the previous borings. Results of swell -consolidation testing performed on relatively undisturbed samples of sandy silty clay soils, presented on Figure 4, indicate low compressibility under existing moisture conditions and light loading and a low collapse potential (settlement under constant load) when wetted. The deeper sample showed a minor expansion potential when wetted. The laboratory test results are summarized in Table 1. No free water was observed in the boring at the time of drilling and the soils were slightly moist to moist. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory borings and the nature of the proposed construction, we recommend a deep foundation consisting of micropiles or screwpiles bearing in the underlying bedrock for support of the proposed residence. Micropiles should be drilled at least 10 feet into the hard bedrock. Screwpiles should be drilled to torque refusal in the bedrock. We expect pile Lengths on the order of 60 to 70 feet. Capacities of 50 to 80 kips should be feasible. An allowance of about 10 kips per pile should be included for downdrag on the piles caused by future settlement of the soil around the piles. The soils tend to compress after wetting and there could be some post -construction settlement of developed areas not supported on piles. We recommend that any decks or porches attached to the house be structurally supported on piles. Placement of foundation walls (grade beams) at least 36 inches below the exterior grade is typically used in this area for frost protection. Foundation walls acting as retaining structures should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site soil as backfill. Exterior Patios: We expect that some settlement of the soils around the house will continue. We understand that the owner plans to construct exterior patios with sand set Job No.100 879-1 -3 - flagstone or pavers which can more easily be periodically releveled if significant settlement occurs. The new fill placed around the new house could be strengthened with biaxial geogrid or by adding cement to reduce the effects of differential settlement on the patios. All fill materials for support of patios 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 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 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, be protected from wetting and hydrostatic pressure buildup by an underdrain system Underdrain for crawlspace areas are not suggested because they tend to collect surface water and increase wetting to the soils. The drains should consist of drainpipe placed near 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 1 %2 feet deep. An impervious membrane such as 20 mil PVC should be placed beneath the drain gravel in a trough shape and attached to the retaining wall with mastic to prevent wetting of the underlying soils. Surface Drainage: Proper surface drainage is critical to keeping the soils dry below the building. The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: I) Inundation of the building excavations and exterior patio 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 patio areas and to at least 90% of the maximum standard Proctor density in landscape areas. Free -draining wall backfill should be capped with at least 2 feet of the on-site, finer graded soils to reduce surface water infiltration. Job No. t 00 879-I G tech -4- 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 pavement and walkway areas. A swale will be needed uphill to direct surface runoff around the residence. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) We understand that the owner will typically use xeriscape to help limit potential wetting of soils below the building caused by irrigation. Planters should be lined and drain lines provided to route excess irrigation water well away from the house. Subsidence Potential: Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the site. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds 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 work in the area, several sinkholes were observed scattered throughout the Roaring Fork River valley. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in this area Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory borings were 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 due to subsurface voids on this site 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 investigation of possible cavities in the bedrock below the site is desired, we should be contacted. 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 expressed 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 Job No.100 879-1 -5 - is concerned about MOBC, then a professional in this special field ofpractice should be consulted. Our findings include interpolation and extrapolation of the subsurface conditions identified at the exploratory pits 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 at once so 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. If you have any questions or if we may be of fiarther assistance, please let us know. Respectfully Submitted, HEPWORTH - PAWLAK GEV CHN1CAL, INC. too eri " lJttfr ,•iz� ®nasoee S%•j/ty. a4b N, E. P, e � O y o Daniel E_ Hardin, P.> , t 24 43 4 Reviewed by: %mss %/� //1 we <4.% .. _` \ 4' /1//��1gf lI t}iftlil° /�; 17/Q NAL t" NIP�`. Steven L. Pawlak, P.E. DEH/ksw attachments cc: Figure I — Locations 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 Kaup Engineering — Dale Kaup (kaup ganCLof.net) Job No.100 879-1 C c tech TO COUNTY ROAD 109 BORING 3 5' 15' BORING 2 22 BENCH MARK: FINISH FLOOR AT DOORWAY; ELEV. = 100.0', ASSUMED. EXISTING DRIVEWAY EXISTING GARAGE PATIO 8' BORING 1 NOT TO SCALE 100 879-1 G l' c 1 HEa'+YORTH-DAWAK GEOTECHNICAL LOCATIONS OF EXPLORATORY BORINGS FIGURE 1 DEPTH - FEET 0 10 20 30 40 50 60 70 BORING 1 ELEV.= 97.9' 5/12 8/12 WC=103 DID= 114 -200=43 12/12 WC=9.9 00=116 10/12 WC= 27.1 DD=96 21/12 14/12 WC=10.3 DD=126 -200=46 14/12 47/12 50/12 WC=33 DID= 119 75/6 WC=2.3 D0=135 BORING 2 ELEV.= 98.2' 4/12 10/12 9/12 WC=14.2 DD=116 -200=63 38/12 15;12 WC=8.5 DO= 124 11/12 WC=9.7 00=118 39/12 WC=3.5 DP =118 50/4 50/3 BORING 3 ELEV.= 89.6' 0 12/12 13/12 10 WC= 7.1 D0=108 -200=56 1a/12 WC=7.3 D0=108 -200=61 30(12 WC=5.9 00=115 -200=60 20 30 40 60/12 50 WC =63 LW =124 -290=70 60 70 80 80 NOTE: Explanation of symbols is shown on FIGURE 3. DEPTH - FEET 100 879-1 ±ect 1 HE'WD 01—PAWL&4 9EWTECF99C4. LOGS OF EXPLORATORY BORINGS FIGURE 2 LEGEND: x x 0 FILL; on-site sand and clay with shale fragments, soft to medium stiff, very moist, brown. SAND AND CLAY (SC -CL); clayey sand to sandy clay with shale fragments, silty, medium dense to stiff to hard, very moist to slightly moist with depth, gray brown to brown. SILTSTONE BEDROCK; hard, slightly moist, light brown and gray brown. Eagle Valley Evaporite 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. 5/12 Drive sample blow count; indicates that 5 blows of 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. NOTES: 1. Exploratory Borings 1 and 2 were drilled on April 27, 2005 and Boring 3 was drilled on February 14, 2011 with 4 -inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pacing from the existing building shown on the site plan provided. 3. Elevations of exploratory borings were measured by instrument level and refer to the Bench Mark shown on Figure 1. Logs 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 occur with time. 7. Laboratory Testing Results: WC = Water Content (46 ) DD = Dry Density (pcf ) -200 = Percent passing No. 200 sieve 100 879-1 HErfORRTh PAwux GEOTECHNICAL LEGEND AND NOTES FIGURE 3 COMPRESSION (%) COMPRESSION - EXPANSION (% ) 0 1 2 3 4 5 6 7 8 1 0 1 2 0.1 1 0 10 APPLIED PRESSURE ( ksf ) 100 O Moisture Content = 7.3 percent Dry Density = 108 pcf 200 = 61 percent Sample of: Gravelly Sandy Silty Clay From: Boring 3 at 14 Feet • Compression upon wetting • C Expansion upon wetting • 0.1 1 0 10 APPLIED PRESSURE ( ksf ) 100 01 1.0 10 APPLIED PRESSURE ( ksf ) 100 100 879-1 HER/3 ThOAW:X GED'ECHWCA- SWELL-CONSOLIDATION TEST RESULTS FIGURE 4 Moisture Content = 5.9 percent Dry Density = 115 pcf 200 = 60 percent Sample of: Gravelly Sandy Silty Clay From: Boring 3 at 24 Feet C Expansion upon wetting 01 1.0 10 APPLIED PRESSURE ( ksf ) 100 100 879-1 HER/3 ThOAW:X GED'ECHWCA- SWELL-CONSOLIDATION TEST RESULTS FIGURE 4