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HomeMy WebLinkAbout452 Zeppelin Way - Special Inspections/Engineering - 12/16/2016December 16, 2016 Calatlantic Group, Inc. 6161 South Syracuse Way Suite 200 Greenwood Village, Colorado 80111 Subject: Soils and Foundation Summary Letter East Ridge Subdivision Phase 1 Lot 5/6, Block 4 Fort Collins, Colorado CTLIT Project No. FC06953.004-120 B1607734 La HOME BUYER ADVISORY Expansive soils are present at this subdivision; this results in a geologic hazard. This letter describes the soil conditions on this lot more specifically. Prospective home buyers are strongly advised to read this letter and the referenced documents. If you do not understand the risk(s) associated with the hazard and the important role you must accept to manage and mitigate the risk(s), we recommend you contact a competent geotech- nical (soils) engineer for advice. CTL I Thompson, Inc. is performing a Soils and Foundation Investigation for 46 lots within the East Ridge Subdivision. This letter presents a summary of our findings and recommendations for the subject lot. The final report should be reviewed for founda- tion design. Colorado is a challenging location to practice geotechnical engineering. The cli- mate is relatively dry and the near -surface soils are typically dry and comparatively stiff. These soils and related sedimentary bedrock formations tend to react to changes in moisture conditions. Some of the soils swell as they increase in moisture and are referred to as expansive soils. Other soils,can compress significantly upon wetting and are identified as collapsing soils. Most of the land available for development east of the Front Range is underlain by expansive clay and/or claystone bedrock near the surface. The soils that exhibit collapse are more likely west of the Continental Divide; however, both types of soils occur throughout the state. Covering the ground with houses, streets, driveways, patios, etc., coupled with landscape irrigation and changing drainage patterns, leads to an increase in subsurface moisture conditions. As a result, some soil movement is inevitable. It is critical that all recommendations in the referenced report are followed to increase the chances that foundations and concrete slabs -on -grade will perform satisfactorily. After construction, home owners must assume responsibility for maintaining the structure and use appro- priate practices regarding drainage and landscaping. In summary, the strata encountered in the boring on this lot consisted of 2 feet of fill and 15 feet of clayey sand underlain by sandy gravel to the maximum depth explored of 30 feet. Groundwater was encountered at a depth of 17 feet. Use of footings with minimum deadload is recommended. A slab -on -grade concrete basement floor can -be used provided risk of movement and cracking is acceptable. Further details are de- scribed in following paragraphs. Expansive soils and bedrock are present at this site. The presence of expansive soils and bedrock, collectively referred to as expansive or swelling soils, constitutes a geologic hazard. There is risk that ground heave or settlement will damage slabs -on - grade and foundations. The risks associated with swelling soils can be mitigated but not 400 North Link Lane I Fort Collins, Colorado 80521 Telephone: (970) 206-9455 Fax: (970) 206-9441 eliminated by careful design, construction and maintenance procedures. We believe the recommendations in the referenced report will help control risk of foundation and/or slab damage; they will not eliminate that risk. The builder and home buyers should under- stand that slabs -on -grade and, in some instances, foundations may be affected by swelling soils. Home owner maintenance will be required to minimize this risk. We recommend the builder provide a booklet to the home buyer that describes swelling soils and includes recommendations for care and maintenance of homes constructed on expansive soils. Colorado Geological Survey Special Publication 43, was designed to provide this information. Laboratory tests were.performed on samples from this lot and nearby lots. . Samples from this lot swelled 0.8 to 5.2 percent when wetted. Based upon results of laboratory tests and other factors, we judge basement slab performance risk for this lot to be moderate. Exhibit A provides a discussion of slab performance risk evaluation, as well as slab installation and maintenance recommendations. If home buyers cannot tolerate movement of a slab -on -grade basement floor, they should select a lot where a structurally supported floor will be constructed or request that a structurally supported floor be installed. Considering the subsurface conditions at this lot, we recommend construction of the proposed residence on a footing foundation with minimum deadload. Footings should be designed for a maximum allowable soil pressure of 2,000 psf with a minimum deadload pressure of 700 psf. Because of expansive clay in upper 5 feet, all footings should extend to basement levels. Footings should be at least 16 inches in width. Column, pads should be at least 20 inches square. There should be a 4-inch continuous void between the footings to concentrate the deadload, if interrupted footings are necessary. Exterior footings should be protected from frost action with at least 30 inches of cover. It is sometimes necessary to alter the foundation design based on conditions exposed during construction. The buyer can discuss the changes, if any; with the builder. Basement and/or foundation walls and grade beams that extend below grade should be designed for lateral earth pressures where backfill is not present to about the same extent on both sides of the wall. Our experience suggests basement walls can deflect or rotate slightly under normal design loads and that this deflection typically does not affect the structural integrity of the walls. We recommend design of the basement walls on this lot using an equivalent fluid density of at least 50 pounds per cubic foot. This value assumes slight deflection of the wall can occur, generally less than 0.5 to 1 percent of the wall height. Some minor cracking of the walls may occur. A subsurface drain is recommended around the entire perimeter of the lowest excavation area for this residence. The drain should lead to a positive gravity outlet or to a sump where water can be removed with a pump. The provision of the drain will not eliminate slab movement or prevent moist conditions in crawl spaces. The pump must be maintained by the home owner. "A Guide to Swelling Soils for Colorado Homebuyers and Homeowners," Second Edition Revised and Updated by David C. Noe, Colorado Geological Survey, Department of Natural Resources, Denver, Colorado, 2007. 2 CALATtANTIC GROUP, INC EAST RIDGE SUBDIVISION PHASE 1 LOT 516. BLOCK 4 CTL I T PROJECT NO. FC06953.004-120 Proper design, construction and maintenance of surface drainage are critical to the satisfactory performance of foundations, slabs -on -grade, and other improvements. Landscaping and irrigation practices will also affect performance. Exhibit B contains our recommendations for surface drainage, irrigation, and maintenance. The concept of risk is an important aspect with any geotechnical evaluation, pri- manly because the methods used to develop geotechnical recommendations do not comprise an exact science. We never have complete knowledge of subsurface condi- tions. Our analysis must be tempered with engineering judgment and experience. Therefore, the recommendations presented in any geotechnical evaluation should not be considered risk -free. Our recommendations represent.our judgment of'those measures that are necessary to, increase the chances that the structure will perform satisfactorily. It is critical that all recommendations in the referenced report are followed. Home owners must assume responsibility for maintaining the structure and use appropriate practices regarding drainage and landscaping. As this letter is meant only as a; summary of our findings and recommendations for the subject lot, we recommend home buyers review the Soils and Foundation Inves- tigation from which this summary is taken. CTL Spencer SchraMIN.,,,,,,,./ Geotechnical De qer 3 CALATLANTIC GROUP, INC EAST RIDGE SUBDIVISION PHASE 1 LOT 5/6. BLOCK 4 CTL I T PROJECT NO. FC06853.004-120