Loading...
HomeMy WebLinkAbout3526 Pratolina Ct - Special Inspections/Engineering - 05/13/2014CTL I THOMPSON Ap­ May 13, 2014 Richmond American Homes 4350 South Monaco Street Denver, Colorado Subject: Soils and Foundation Summary Letter Bella Vira Subdivision Lot 6, Block 2 Fort Collins, Colorado FC06224.001-120 ADVISORY Expansive soils are present at this subdivision; this is 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 geotechnical (soils) engineer for advice. CTL I Thompson, Inc. performed a Soils and Foundation Investigation for lots within Bella Vira Subdivision (Project No. FC06224.001-120). 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 arid 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 lawn irrigation and changing drainage patterns, leads to an increase in subsurface moisture conditions. As a result, some soil movement is inevitable. To mitigate the impacts of expansive soil and bedrock, sub -excavation was performed at this site to treat the expansive soil and bedrock with moisture conditioning, prior to recompacting these materials as a layer of structural fill. Our investigation indicates sub -excavation was effective in reducing swell. The methods used to mitigate expansive soil and bedrock through sub -excavation are described in Exhibit C. Sub -excavation will not eliminate potential movements; it is intended to reduce total and differential movement. It is likely some cosmetic distress will occur such as drywall cracks, slab heave or settlement and associated cracks, etc. A portion of the concrete driveway or other improvements installed outside the sub -excavated area are likely to be affected by the underlying expansive soils. These improvements may heave, crack and/or exhibit other distress. It is critical that all recommendations in our report are followed to increase the chances that the foundations. and slabs-on- 351 Linden Street I Suite.140 1 Fort Collins, Colorado 80524 Telephone:970-206-9455 Fax:970-206-9441 1,0 . ; EXHIBIT C MITIGATION OF EXPANSIVE/COMPRESSIBLE SOILS AND EXPANSIVE BEDROCK HERITAGE TODD CREEK SUBDIVISION, FILING 1, AMENDMENTS 3 AND 8 THORNTON, COLORADO The soils and underlying bedrock found beneath much of the Front Range area of Colorado are expan- sive; they swell when wetted. Deep sub -excavation of these clayey soils and bedrock is considered an appropriate mitigation method in areas where expansive materials are present. Excavation destroys the fractures and non -uniform structure of the near -surface soils and bedrock. The excavation typically extends about 10 feet below anticipated foundation levels, although deeper or shallower sub - excavations are sometimes used. At Heritage Todd Creek Subdivision, Filing No. 1, Amendments 3 and 8, the excavation was planned to bottom at least 10 or 12 feet below basement foundations (Fig. 1). The excavated soils and bedrock are compacted in thin, nearly horizontal layers (Fig. 2). Water is mixed with the materials as they are placed to "pre -swell" the expansive soils and reduce potential future swell. On projects where deep sub -excavation has been employed, testing of the soils, bedrock, and backfill has demonstrated that significant reduc- tion in swelling characteristics can be achieved. Figure 1 Figure 2 Exhibit C-1 f q The sub -excavation approach mitigates the influence of expansive materials on man-made improve- ments and structures in several ways. The weight of the fill helps to resist potential heave of the underly- ing natural materials. The fill acts like a cushion to spread movement over a larger area, reducing damaging movements near the ground surface (Fig. 3). The fill also covers the site with a thick layer of compacted material that limits penetration of water to the underlying natural soils and/or bedrock. Figure 3 The sub -excavation process can provide greater stability for streets, sidewalks, driveways, and buried utilities (Fig. 4), if the sub -excavated layer extends beneath these improvements. Foundation drains around basements and under -drains below sanitary sewer mains (if installed within the subdivision) also help to collect irrigation and precipi- tation water that seeps into the ground. Home owner irrigation practices are critical to the overall performance of foundations and concrete flatwork and are discussed in more detail in Exhibit B. Exhibit C-2 B grade will perform satisfactorily. After construction, home owners must assume responsibility for maintaining the structures and use appropriate practices regarding drainage and landscaping. In summary, the subsurface conditions encountered in the boring on this lot consisted of 14 feet of clay and sand fill underlain by interbedded claystone and sandstone bedrock to a depth of 251/2 feet. The soils are predominantly low swelling. Use of a footing foundation system is recommended. Slab -on -grade construction can be used for the basement floor. Foundations and floor systems should not be constructed on the existing fill. The existing fill should be removed, moisture conditioned and recompacted. Further details are described in the following paragraphs. Groundwater was encountered at a depth of 11 feet or greater. Existing groundwater levels will affect below grade construction at this site. An interceptor drain and underdrain system are planned. The drains were not installed at the time of this letter. Basement construction is not recommended until the drain systems are installed. Based on our investigation, low swelling soils are present at depths that will likely influence the foundation and slab performance at this site. We believe there is low 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 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. Based upon results of laboratory tests and other factors, we judge basement slab performance risk for this lot to be low. 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. 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. RICHMOND AMERICAN HOMES 2 BELLA VIRA SUBDIVISION LOT 6, BLOCK 2 CTL I T PROJECT NO. FC06224.001-120 Considering the sul5i6rface conditions at this lot, we recommend construction of the proposed residence on a footing foundation system. Footings should be designed for a maximum allowable soil pressure of 3,000 psf and a minimum deadload 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 home 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. 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. 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. RICHMOND AMERICAN HOMES 3 BELLA VIRA SUBDIVISION LOT 6, BLOCK 2 CTL I T PROJECT NO. FC06224.001-120 emu■ 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 THOMPSON, INC. Spencer Schram, PE Project Engineer G, 46576 RICHMOND AMERICAN HOMES BELLA VIRA SUBDIVISION LOT 6, BLOCK 2 CTL I T PROJECT NO. FC06224.001-120 E! EXHIBIT A E; „ _�;, SLAB PERFORMANCE RISK EVALUATION, INSTALLATION AND MAINTENANCE As part of our evaluation of the subsoils and bedrock, samples were tested in the labora- tory using a swell test. In the test procedure, a relatively undisturbed sample obtained during drilling is first loaded and then flooded with water and allowed to swell. The pressure applied prior to wetting can approximate the weight of soil above the sample depth or be some standard load. The measured percent swell is not the sole criteria in assessing potential movement of slabs -on -grade and the risk of poor slab performance. The results of a swell test on an individu- al lot are tempered with data from surrounding lots, depth of tests, depth of excavation, soil profile, and other tests. This judgment has been described by the Colorado Association of Geotechnical Engineers' (CAGE, 1996) as it relates to basement slab -on -grade floors. It can also be used to help judge performance risk for other slabs -on -grade such as garage floors, driveways, and sidewalks. CTL Thompson also performs potential heave calculations to aid in our judgment. The risk evaluation is considered when we evaluate appropriate foundation systems for a given site. In general, more conservative foundation designs are used for higher risk sites to control the likelihood of excessive foundation movement. As a result of the Slab Performance Risk Evaluation, sites are categorized as low, mod- erate, high, or very high risk. This is a judgment of the swelling characteristics of the soils and bedrock likely to influence slab performance. REPRESENTATIVE MEASURED SWELL AND CORRESPONDING SLAB PERFORMANCE RISK CATEGORIES Slab Performance Risk Category Representative Percent Swell* (500 psf Surcharge) Representative Percent. Swell* (1000 psf Surcharge) Low 0 to <3 0 to <2 Moderate 3 to <5 2 to <4 High 5 to <8 4 to <6 Very High > 8 > 6 *Note: The representative percent swell values presented are not necessarily measured values; rather, they are a judgment of the swelling characteristics of the soil and bedrock likely to influence slab performance. The rating of slab performance risk on a site as low or high is not absolute. Rather, this rating represents a judgment. Movement of slabs may occur with 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 areas. 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 considerednormal and we believe in; the majority of instances, movements of this magnitude constitute reasonable slab performance, more heave can occur. Slabs can be affected on all sites. On lots rated as high or veryhigh risk, there is more likelihood of need to repair, maintain or replace basement and garage floors and exterior flatwork. 3"Guideline for Slab Performance Risk Evaluation and Residential Basement Floor System Recommendations", Colorado Associa- tion of Geotechnical Engineers, December 1996. Exhibit A-1 CTL I Thompson, Inc. recommends use of structurally supported basement floors, known as "structural floors," for lots rated as high and very high risk. We also recommend use of structural basement floors on walkout and garden level lots rated as moderate, high or very high risk. 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. The home buyer should be advised the floor slab in the basement may move and crack due to heave or settlement and that there may be maintenance costs associated during and after the builder warranty period. A buyer who chooses to finish a basement area must accept the risk of slab heave, cracking and consequential damages. Heave or settlement may require maintenance of finish details to control damage. Our experience suggests that soil moisture increases below residence sites due to covering the ground with the house and exterior flat - work, coupled with the introduction of landscape irrigation. In most cases, slab movements (if any) resulting from this change occur within three to five years. We suggest delaying finish in basements.with slab -on -grade floors until at least three years after start of irrigation. It is possi- ble basement floor slab and finish work performance will be satisfactory if a basement is fin- ished earlier, particularly on low risk sites. For portions of the houses where conventional slabs -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. 1. Slab -on -grade floor construction should be limited to areas such as garages and basements where slab movement and cracking are acceptable to the builder and home buyer. 2. The 2006, 2009 and 2012 International Residential Code (IRC R506) states that a 4-inch base course layer consisting of clean graded sand, gravel, crushed stone or crushed blast furnace slag shall be placed beneath below grade floors (unless the underlying soils are free -draining), along with a vapor retarder. Instal- lation of the base course and vapor.retarder is not common in this area. Histori- cally, there has been some concern that installation of clean base course could allow wetting of expansive soils to spread from an isolated source. IRC states that the vapor retarder can be omitted where approved by the building official. 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 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 va- por retarder and the floor slab. 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 American Concrete Institute (ACI) Committee 302, "Guide for Concrete Floor and Slab Construction (ACI 302.R-96)". Exhibit A-2 S,.- .. I �"� I'A :• �" it r �� '_.2 3. Conventional slabs should be separated from exterior walls and interior bearing members with a slip joint that allows free vertical movement of the slabs. These joints must be maintained by the home buyer 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 waterlines leading to slab - supported appliances should be constructed with flexibility. The homebuyer must maintain these connections. 5. Use of 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 that 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 slab movement is not transmitted to the ductwork. These connections must be maintained by the home buyer. 7. Roofs that overhang a patio or porch should be constructed on the same founda- tion as the residence. Isolated piers or pads may be installed beneath a roof overhang provided the slab is independent of the foundation elements. Patio or porch roof columns, may be positioned on the slab, directly above the foundation system, provided the slab is structural and supported by the foundation system. Structural porch or patio slabs should be constructed to reduce the likelihood that settlement or heave will affect the slab by placing loose backfill under the struc- turally supported slab or constructing the slab over void -forming materials. 8. Patio and porch slabs without roofs and other exterior flatwork should be isolated from the foundation. Movements of slabs should not be transmitted to the foun- dation. Decks are more flexible and more easily adjusted in the event of move- ment. 9. Frequent control joints should be provided in conventional slabs -on -grade to re- duce problems associated with shrinkage cracking and curling. Panels that are approximately square generally perform better than rectangular areas. We sug- gest an additional joint about 3 feet away from and parallel to foundation walls. Exhibit A-3 I:I 011,711 11111 i7 SURFACE DRAINAGE, IRRIGATION AND MAINTENANCE Performance of foundations and concrete flatwork is influenced by the moisture condi- tions existing within the foundation soils. Surface drainage should be designed to provide rapid runoff of surface water away from proposed residences. Proper surface drainage and irrigation practices can help control the amount of surface water that penetrates to foundation levels and contributes to settlement or heave of soils and bedrock that support foundations and slabs -on - grade. Positive drainage away from the foundation and avoidance of irrigation near the founda- tion 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. The home buyer should maintain surface drainage and if an irrigation system is installed, it should sub- stantially conform to these recommendations. 1. Wetting or drying of the open foundation and utility excavations should be avoid- ed. 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 con- structed slope of at least 12 inches in the first 10 feet (10 percent) in landscaped areas around each residence, where practical. We do not view the recommendation to provide a 10 percent slope away from the foundation as an absolute. It is desirable to create this slope where practical, be- cause we know that backfill will likely settle to some degree. By starting with suf- ficient slope, positive drainage can be maintained for most settlement conditions. There are many situations around a residence where a 10 percent slope. cannot be achieved practically, such as around patios, at inside foundation corners, 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. We believe it is acceptable to use a slope on the order of 5 percent perpendicular to the founda- tion in these limited areas. For lots graded to direct drainage from the rear yard to the front, it is difficult to achieve 10 percent slope at the high point behind the house. We believe it is ac- ceptable to use a slope of about 6 inches in the first 10 feet (5 percent) at this lo- cation. Between houses that are separated by a distance of less than 20 feet, the con- structed slope should generally be at least 10 percent to the swale used to con- vey water out of this area. For lots that 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 quickly and does not pond for extended periods of time. We suggest minimum slopes of about 2 to 2.5 percent in grassed areas and about 2 percent where landscaping rock or other materials are present. If slopes Exhibit B-1 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 back - fill. Splash blocks and/or extensions should be provided at all downspouts so wa- ter discharges onto the ground beyond the backfill. We generally recommend against burial of downspout discharge. Where it is necessary to bury downspout discharge, solid, rigid pipe should be used and it should slope to an open gravity outlet. Downspout extensions, splash blocks and buried outlets must be main- tained by ,the home owner. 6. The importance of proper home owner irrigation practices cannot be over- emphasized Irrigation should be limited to the minimum amount sufficient to maintain vegetation• application of more water will increase likelihood of slab and foundation movements. Landscaping should be carefully designed and main tained to minimize irrigation. Plants placed close to foundation walls should be limited to those with low moisture requirements. Irrigated grass should not be lo- cated within 5 feet of the foundation. Sprinklers should not discharge within 5 feet of foundations. Plastic sheeting should not be placed beneath landscaped areas adjacent to foundation walls or grade beams. Geotextile fabric will inhibit weed growth yet still allow natural evaporation to occur. 7. The design and construction criteria for foundations and floor system alternatives were compiled with the expectation that all other recommendations presented in this report related to surface and subsurface drainage, landscaping irrigation, backfill compaction, etc. will be incorporated into the project. It is critical that all recommendations in this report are followed. Exhibit B-2