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Home My WebLink About7439 Stonington Ct - Special Inspections/Engineering - 08/13/2003August 13, 2003 Melody Homes 11031 Sheridan Blvd. Westminster, Colorado 80020 Attn: Mr. John Johnson 80307708 Re: Geotechnical Engineering Report Ridgewood Hills 3rd Filing — Phase 8, Lot 291 7439 Stonington Court Fort Collins, Colorado Terracon Project No. 20005162 Irerracon 301 N. Howes P.O. Box 503 Fort Collins, Colorado 80521-0503 (970) 484-0359 Fax: (970) 484-0454 Terracon has completed a geotechnical engineering exploration for a proposed single family residence to be located on the above referenced lot within the Ridgewood Hills 3rd Filing — Phase 8 residential development project located in south Fort Collins, Colorado. The purpose of these services is to provide geotechnical- engineering recommendations for design of foundations, slab on grade, and perimeter drainage systems. Terracon performed a geotechnical engineering exploration for the 3rd Filing of Ridgewood Hills in October of 1997, which encompassed the above -referenced lot. For further information and additional geotechnical engineering recommendations please refer to our "Geotechnical Engineering Report' for the Ridgewood Hills 3rd Filing Residential Development revised January 16, 2001, Project No. 20975149. Terracon also performed a project specific geotechnical engineering subsurface exploration for the lots in which blasting operations were performed to accommodate the ease for basement excavations for areas of the site where very well cemented sandstone lenses were encountered. For further information and findings thereof, please refer to our "Geotechnical Engineering Report — Ridgewood Hills 3rd Filing — Phases 5 through 8, Blasted Lots to Accommodate Basement Construction" dated June 5, 2003, Project No. 20005162. We understand a blasting operation was conducted on this particular lot due to the relatively shallow depth to the very well cemented sandstone bedrock formation. The blasting program was completed by Yenter of Denver, Colorado to facilitate the ease for basement foundation excavation. We understand the blasted zone extended to approximate depths of 8-feet below finished site grades. The blasted materials were then over -excavated to the bottom of the blasted zone. The blasted template footprint was then backfifled with the over -excavated materials. FIELD EXPLORATIONS The additional site exploration for this lot was conducted on July 10, 2003, which consisted of drilling one test boring drilled near the central portion of the lot as requested. The purpose of the drilling activities was to evaluate the subsurface soils and/or blasted zone of replaced sandstone bedrock materials and to provide appropriate foundation design criteria for the lot. A Log of Boring describing the soil, bedrock and groundwater profile conditions, along with the laboratory test results, are attached with this letter. The materials analyzed for the above mentioned lot, based on the laboratory field and test results, are generally classified as being low swelling according to the Colorado Association of Geotechnical Engineers (CAGE) criteria. Arizona ■ Arkansas ■ Colorado ■ Georgia ■ Idaho ■ Illinois ■ Iowa ■ Kansas ■ Kentucky ■ Minnesota ■ Missouri Montana ■ Nebraska ■ Nevada ■ New Mexico ■ Oklahoma ■ Tennessee ■ Texas ■ Utah ■ Wisconsin ■ Wyoming Quality Engineering Since 1965 Geotechnical Engineering Report — Melody Homes Ridgewood Hills 3`d Filing Fort Collins, Colorado Page 2 Project No. 20005162 Terracon The subsurface soils encountered on this lot generally consisted of approximately 13-feet of fill material which was derived from the blasting operations, underlain by undisturbed sandstone bedrock at approximate depths of 5-feet below site grades and extending to the depths explored, approximately 19- feet. It is our understanding the proposed residence will be constructed having a full depth basement. Therefore, based on the properties of the soils/fill materials and cemented characteristics of bedrock present on this lot, Terracon recommends the proposed structure be supported by conventional type spread footings, (Type B) designed in accordance with the recommendation contained in the "project -specific' blasted lot geotechnical engineering report. Foundation Systems — Conventional Type Spread Footings To reduce the potential for differential movement and to increase the structural integrity of the bearing stratum within the blasted zone, we recommend overexcavating within the blasted template a minimum of 2- feet beneath all foundations and slabs, and replace the materials as engineered controlled fill within this zone. The controlled engineered fill material should extend laterally beyond the edges of all exterior footings an additional distance of 12-inches for every 12-inches of depth. If engineered fill is placed beneath the entire footprint, it should extend horizontally a minimum distance of 5-feet beyond the outside edge of perimeter footings. Terracon recommends the 2-foot of over -excavated blasted sandstone materials beneath all footings be stockpiled for re -use. The underlying subgrade layer beneath the removed section should be scarified a minimum of 12-inches, moisture conditioned and recompacted to a minimum of 98-percent of Standard Proctor Density ASTM D698. The over -excavated material should then be moisture conditioned to plus or minus 2 percent of optimum moisture content, placed in uniform 8 to 12-inch lifts and mechanically compacted to a minimum of 98-percent of Standard Proctor Density ASTM D698. A general schematic of the over -excavated and replaced foundation -bearing stratum for use of conventional -type spread footings is enclosed for your review. Spread footings bearing upon a minimum of 2-feet of controlled engineered fill may be designed for a net allowable bearing pressure of 2,500 psf. For this alternative, the footings should be sized to maintain a minimum dead load pressure of 750 psf. The minimum of 2-feet of structural fill material placed beneath the entire building footprint should create a more uniform bearing stratum and minimize the potential for differential movement. Footings should be proportioned to reduce differential foundation movement. Proportioning on the basis of equal total movement is recommended; however, proportioning to relative constant dead -load pressure will also reduce differential movement between adjacent footings. Total settlement resulting from the assumed structural loads is estimated to be on the order of 1-inch or less assuming that the "blasted -zone" beneath the 2-foot replacement sections to the undisturbed sandstone is relatively consistent. Movement could exceed 1- inch if water from any source infiltrates the foundation soils; therefore, proper drainage should be provided in the final design and during construction. Geotechnical Engineering Report — Melody Homes Ridgewood Hills Td Filing Fort Collins, Colorado Page 3 Project No. 20005162 Terracon Exterior footings should be placed a minimum of 30 inches below finished grade for frost protection and to provide confinement for the bearing soils. Finished grade is the lowest adjacent grade for perimeter footings. Basement Construction Groundwater was not encountered on the lot to the maximum depths of exploration, 15-feet, during the initial site exploration. Therefore full -depth basement construction is considered acceptable. To reduce the potential for groundwater and/or surface water infiltration to enter the basement of the residence, installation of a dewatering system is recommended. The drainage system should be constructed around the exterior perimeter of the basement foundation, and sloped at a minimum 1/8 inch per foot to a suitable outlet, such as a sump and pump system. The drainage system should consist of a 4-inch diameter perforated pipe, embedded in free -draining gravel, placed in a trench at least 12-inches in width. Gravel should extend a minimum of 3-inches beneath the bottom of the pipe, and at least 1-foot above the bottom of the foundation wall. The gravel should be covered with drainage fabric prior to placement of foundation backfill. Backfill placed around the outside perimeter of the structure should be placed in uniform lifts; moisture conditioned to optimum moisture content and mechanically compacted to 95% of Standard Proctor Density ASTM 698, after the walls have been laterally braced. Floor Slab Design and Construction The variability of the existing soils at approximate slab subgrade elevation could result in differential movement of floor slab -on -grade should the clay soils increase in moisture content. Based on the CAGE criteria, the subsoils at the site have a slab performance risk category of 'low". Movement not likely to exceed up to 2-inches or more is estimated if the subsoils below slabs become wetted for a zone of approximately 10-feet below the basement. Slab -on -grade construction for the basement of this residence should perform in accordance with the above criteria. The homeowner should be aware of the risk of this slab movement and should take this into account prior to purchasing the residence. To reduce potential slab movements, slabs should be constructed in accordance with the recommendations of the Geotechnical Engineering Report. Surface Drainage Positive drainage should be provided during construction and maintained throughout the life of the proposed residence. In areas where sidewalks or paving do not immediately adjoin the structure, we recommend that protective slopes be provided with a minimum grade of approximately 10 percent for at least 10-feet from perimeter walls. Backfill against foundations, exterior walls and in utility and sprinkler line trenches should be well compacted and free of all construction debris to reduce the possibility of moisture infiltration. Downspouts, roof drains or scuppers should discharge into splash blocks or Geotechnical Engineering Report — Melody Homes Ridgewood Hills 3`d Filing Fort Collins, Colorado Page 4 Project No. 20005162 Terracon extensions when the ground surface beneath such features is not protected by exterior slabs or paving. Sprinkler systems should not be installed within 5-feet of foundation walls. Construction Observation The following observations are recommended during construction: • Foundation Excavation; • Footing sizes and void form placement; • Foundation Wall Steel/Reinforcement; • Perimeter Drainage System; and • Floor Slab for Basement and Garage We appreciate the opportunity to be of service to you on this phase of your project. If you have any questions, or if we may be of further service to you, please feel free to contact us. Sincerely,tP�p TERRACON off.;..... 277' David A. Richer ss••••••°•°°G�'` Geotechnical Engin t Manager Copies to: Addressee (6) Enclosures: Log of Boring (1) LOG OF BORING NO. 291 Page 1 of 1 CLIENT ARCHITECT/ENGINEER Melody Homes SITE Ridgewood Hills-3rd Filing PROJECT Fort Collins, Colorado Ridgewood Hills-3rd Filing SAMPLES TESTS o W n DESCRIPTION O F r 1: x w 1 Ztn KW Z LL ZZ CL to m W O K 0 p 7 Z F K ym Qp �0 � Co. Z� Ito FILL MATERIAL Silty Clayey Sand with Sandstone Bedrock Fragments Tan, brown, moist, loose to medium dense SS 12 4 5 SS 12 30 10 13 SILTSTONE/SANDSTONE :: 14.2 Tan, gray, moist, cemented to well cemented BOTTOM OF BORING s 's s i The stratification lines represent the approximate boundary lines between soil and rock types: in -situ, the transition may be gradual. WATER LEVEL OBSERVATIONS, ft BORING STARTED 7-10-03 DRY WD COMPLETED 7 -10-03 WL T Y Ir �rr acon - - R GRING CME 75FFOREMAN CMG s WL I INITIAL WATER LEVEL READING APPROVED DAR I JOB # 20005162