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HomeMy WebLinkAbout5744 Big Canyon Dr - Special Inspections/Engineering - 12/19/2012ADVISORY December 19, 2012 Ryland Homes 8100 East Maplewood Avenue Suite 100 Greenwood Village, Colorado 80111 Subject: Soils and Foundation Summary Letter 58 Lots at McClelland's Creek, 3rd Filing Lot 21, Block 3 Fort Collins, Colorado FC05543.003-120 Although some expansive soils may be present at this subdivision, which could be a geologic hazard, the soils found in our investigation were predominantly non -expansive. 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 important role you must accept to maintain the structure and mitigate risk of excessive wetting, we recommend you contact a competent geotechnical (soils) engineer for advice. CTL I Thompson, Inc. performed a Soils and Foundation Investigation for 58 Lots at McClelland's Creek, Filing No. 3 (Project No. FC05543.003-120; report dated December 11, 2012). This letter presents a summary of our findings and recommendations for the subject lot. The report referenced above should be reviewed for foundation design. Colorado is a challenging location to practice geotechnical (soils) engineering. The climate is relatively and and the near -surface soils are typically dry and relatively stiff. These soils and related sedimentary bedrock formations tend to react to changes in moisture conditions. Some of the soils and bedrock swell as they increase in moisture and are called expansive soils. Other soils can settle significantly upon wetting and are referred to as collapsing soils. Most of,the land available for development east of the Front Range is underlain by expansive clay 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, homeowners must assume responsibility for maintaining the structure and use appropriate practices regarding drainage and landscaping. In summary, the subsurface conditions encountered in the boring on this lot consisted of 4 feet of sandy clay fill over 5 feet of clayey sand over 9 feet of sandy clay over 4 feet of weathered claystone underlain by claystone to a depth of 30 feet. Groundwater was encountered at a depth of 21 feet during drilling and when measured several days later. The soils are predominantly non -expansive. Use of footing foundation system is recommended. Slab -on -grade construction can be used for the basement floor. Further details are described in following paragraphs. Based on our investigation, comparatively non -expansive soils are present at depths that will likely influence the foundation and slab performance at this site. We believe there is moderate risk of ground heave or settlement and associated damages of slabs -on -grade and foundations. The foundations and slabs may settle if loose or soft fill and soils are present under the footings and slabs. The risk of foundation and slab movements can be 351 Linden Street 1 Suite 1401 Fort Collins. Colorado 80524 Telephone:970-206-9455 Fax:970-205-9441 mitigated, but not eliminated by careful design, construction and maintenance procedures. We believe the recommendations in our report will help control risk of foundation and slab damage; they will not eliminate that risk. The builder and home buyers should understand that slabs -on -grade and, in some instances, foundations may be affected by the subsoils. Homeowner 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 431 was designed to provide this information. Laboratory tests were performed on samples from this lot and nearby lots. Exhibit A provides a discussion of slab performance risk evaluation, as well as slab installation and maintenance recommendations. We performed calculations of total potential ground heave as part of our study. The calculations indicate the ground surface and basement level slab heave potential is between 2.2 and 2.7 inches. It is not certain this movement will occur. 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. We recommend extending upper level footings to basement levels. As an alternative, the soils below upper level footings can be excavated to basement level and replaced with moisture -conditioned, compacted fill. Footings should be designed for a maximum allowable soil pressure of 3,000 pounds per square foot (psf) and a minimum dead load of 1,000 psf. Footings should be at least 16 inches in width. Column pads should be at least 20 inches square. 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 55 pounds per cubic foot (pcf). 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 homeowner. Proper design, construction and maintenance of surface drainage are critical to the satisfactory performance of foundations, slabs -on -grade, and other improvements. 1"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. RYLAND HOMES 2 58 LOTS AT MCCLELLANDS'S CREEK, FILING 3 LOT 21, BLOCK 3 CTL I T PROJECT NO. FC05543.003-120 ram) t. 1'� al c✓ 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, primarily because the methods used to develop geotechnical recommendations do not comprise an exact science. We never have complete knowledge of subsurface conditions. Our analysis must be tempered with engineering judgment and experience. Therefore, the recommendations presented in any geotechnical evaluation should not be considered risk - free. We cannot provide a guarantee that the interaction between the soils and a proposed structure will be as desired or intended. 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. Homeowners 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 Investigation from which this summary is taken. CTL I. THO Er A. 73 376 RYLAND HOMES 3 58 LOTS AT MCCLELLANDS'S CREEK, FILING 3 LOT 21, BLOCK 3 CTL I T PROJECT NO. FC05543.003-120 The rating of slab performance risk on a site as low or high is not absolute. Rather, this represents a judgment. Movement of slabs may occur v ,1ith time in low, moderate, high and very high risk areas as the expansive soils respond to increases in moisture content. Overall, the severity and frequency of slab damage usually is greater in high and very high rated ;:real. Heave of slabs -on -grade of 3 to 5 inches is not uncommon in areas rated as high or very high risk. On low and moderate risk sites, slab heave of 1 to 2 inches is considered normal and we believe in the majority of instances, movements of this magnitude constitute reasonable slab performance. Slabs can be affected on all sites. On lots rated as high or very high risk, there is more likelihood of need to repair, maintain or replace basement and garage floors and exterior flatwork. CTL J Thompson, Inc. recommends use of structurally supported basement floors, known as Astructural floors, @ for all lots rated as high and very high risk. We also generally recommend use of structural basement floors on walkout lots rated as moderate, high or very high risk. Home buyers should consider structural floors on any lot where they intend to finish the basement. Home buyers should be advised the floor slabs in the basement may move and crack due to heave or settlement, and there may be maintenance costs associated during and after the builder warranty period. We believe these movements are most likely in the first three to five years following construction as the soils and bedrock respond to changes in availability of moisture. If buyers choose to finish basement areas, they must accept risk of slab heave, cracking and consequential damages. Heave or settlement may require maintenance of finish details to control damage. We suggest delaying finish in basements until at least 3 years after construction or start of irrigation, whichever is later. For portions of houses where conventional slab -on -grade are used, we recommend the following precautions. These measures will not keep slabs -on -grade from heaving; they tend to mitigate damages due to slab heave. (Basemerti) 1. Slab -on -grade floor construction should be limited to areas such as garages and basements where slab movement and cracking are acceptable. 2. Slabs should be placed directly -on the exposed subsoils or properly moisture conditioned, compacted fill. The 2006 International Building Code (IBC) or 2006 Intemational Residential Code (IRC) require a vapor retarder be placed between base course or subgrade soils and the concrete slab -on -grade floor. The merits of installation of a vapor retarder below floor slabs depend on the sensitivity of floor coverings and building use to moisture. A properly installed vapor retarder (10 mil minimum) is more beneficial below concrete slab -on -grade floors where floor coverings, painted floor surfaces or products stored on the floor will be sensitive to moisture. The vapor retarder is most effective when concrete is placed directly on top of it, rather than placing a sand or gravel leveling course between the vapor retarder and the floor slab. The placement of concrete on the vapor retarder may increase the risk of shrinkage cracking and curling. Use of concrete with reduced shrinkage characteristics including minimized water content, maximized coarse aggregate content, and reasonably low slump will reduce the risk of shrinkage cracking and curling. Considerations and recommendations for the installation of vapor retarders below concrete slabs are outlined in Section 3.2.3 of the 2006 report of American Concrete Institute (ACI) Exhibit A-2 Committee 302, "Guide for Concrete Floor and Slab Construction (ACI 302. R-96)". S. Conventional slabs should be separated from exterior walls and interior bearing members with a slip joint, which allows free vertical movement of the slabs. These joints must be maintained by home buyers to avoid transfer of movement. 4. Underslab plumbing should be thoroughly pressure tested during construction for leaks and be provided with flexible couplings. Gas and eater lines leading to slab -supported appliances should be constructed with flexibility. Home buyers must maintain these connections. 5. Slab bearing partitions should be minimized. Where such partitions are necessary, a slip joint (or float) allowing at least 2 inches of free vertical slab movement should be used. Doorways should also be designed to allow vertical movement of slabs. To limit damage in the event of movement, sheetrock should not extend to the floor. The home buyer should monitor partition voids and other connections, and re-establish the voids before they close to less than 1/2 inch. 6. Plumbing and utilities which pass through slabs should be isolated from the slabs. Heating and air conditioning systems constructed on slabs should be provided with flexible connections capable of at least 2 inches of vertical movement so that slab movement is not transmitted to the duct work. These connections must be maintained by the home buyer. 7. Exterior patio and porch slabs with overhanging roofs should be structurally supported on the same foundations as the residence. B. Patio slabs (no roofs) and other exterior flatwork should be isolated from the foundation or be designed so that slab movement does not affect the foundation. 9. Frequent control joints should be provided In conventional slabs -on -grade to reduce problems associated with shrinkage and curling. Panels which are approximately square generally perform better than rectangular areas. We advocate an additional joint about 3 feet from and parallel to foundation walls. (Basement) Exhibit A-3 EXHIBIT B SURFACE DRAINAGE, IRRIGATION AND MAINTENANCE Performance of foundations and concrete flatwork is influenced by the moisture conditions existing within the foundation soils. Surface drainage should be designed to provide rapid runoff of surface water away from proposed residences. Good surface drainage and irrigation practices can help control the amount of surface water which penetrates to foundation levels and contributes to settlement or heave of soils and bedrock which support foundations and slabs -on -grade. Good drainage away from the foundation and avoidance of irrigation near foundations also help to avoid excessive wetting of backfill soils, which can lead to increased backfill settlement and possibly to higher lateral earth pressures due to increased weight and reduced strength of the backfill. CTL I Thompson, Inc. recommends the following precautions. Home buyers should maintain surface drain2*e nc install irrigation systerns which suiostaniialh; conform to these recommendations. Wetting or drying of the open foundation excavations should be avoided. 2. The ground surface surrounding the exterior of each residence should be sloped to drain away from the building in all directions. We recommend a minimum constructed slope of at least 12 inches in the first 10 feet (10%) in landscaped areas around each residence, where practical. We do not view the recommendation to provide a 10% slope away from the foundation as an absolute. It is desirable to create this slope where practical, because we know that backfill will likely settle to some degree. By starting with good slope, positive drainage can be maintained for most settlement conditions. There are many situations around a residence where a 10% slope cannot be achieved practically, such as around patios, at inside foundation comers, and between a house and nearby sidewalk. In these areas, we believe it is desirable to establish as much slope as practical, and to avoid irrigation in the area. We believe it is acceptable to use a slope on the order of 5% perpendicular to the foundations in these limited areas. For lots graded to direct drainage from the rear yard to the front, it is difficult to achieve 10% slope at the high point behind the house. We believe it is acceptable to use a slope of about 6 inches in the first 10 feet (5%) at this location. Between houses which are separated by a distance less than 20 feet, the constructed slope should generally be at least 10 percent to the swale used to convey water out of this area. For lots which are graded to drain to the front and back, we believe it is acceptable to install a slope of 5 to 8 percent at the high point (aka "break point") between houses. 3. Swales used to convey water across yards and between houses should be sloped so that water moves quicidy and does not pond for extended periods of time. We recommend a minimum slope of 2 percent in grassed areas, and believe 2.5 percent Exhibit B-1 I II - -�� I is preferable; this cannot always be achieved without using steep driveways or steps from the house to the garage. The slope should also be at least 2 percent where landscaping Bock or other materials are present. If slopes less than about 2 percent are necessary, concrete -lined channels or plastic pipe should be used. 4. Backfill around the foundation walls should be moistened and compacted. 5. Roof downspouts and drains should discharge well beyond the limits of all backfill. Splash blocks and/or extensions should be provided at all downspouts so that water discharges onto the ground beyond the backfill. We generally recommend against burial of downspout discharge. Where it is necessary to bury downspout discharge, solid pipe should be used and it should slope to an open gravity outlet. All downspout extensions, splash block and buried outlets must be maintained by the home buyer. 6. The imrjortance of 000d hoi neovitner ininationi wactices cannot be oy r-rmohasized Irrioation should be IirrxitEdto the minimum amount sufficient to maintainyecleiation-. ao lication of morc- v.,eiet viili increase likelihood of slab and found ationtion movements. Landscaping should be carefully designed and maintained to minimize irrigation. Plants used close to foundation walls should be limited to those with low moisture requirements. Irrigated grass should not be located within 5 feet of the foundation. Sprinklers should not discharge within 5 feet of foundations. Exhibit B-2