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HomeMy WebLinkAbout216 Tigercat Way - Special Inspections/Engineering - 05/09/2017A.. VJJ May 9, 2017 CalAtlantic Group, Inc. 6161 South Syracuse Way Suite 200 Greenwood Village, Colorado 80111 Subject: Soils and Foundation Summary Letter East Ridge Phase 1 Lot 23/24, Block 3 Fort Collins, Colorado CTLIT Project No. FC06953.004-120 R2 `� 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. performed a Soils and Foundation Investigation for lots within East Ridge Subdivision, (Project No. FC06953.004-120 R2; report dated April 10, 2017). 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 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 pattems, 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 1 feet of fill and 16 feet of interlayered clay and sand underlain by sandy gravel to the maximum depth explored of 29 feet. Groundwater was encountered at a depth of 14 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 described in following paragraphs. Expansive soils 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 eliminated 400 North Link Lane I Fort Collins, Colorado 80621 Telephone: (970) 206-9455 Fax: (970) 206-9441 by careful design, construction and maintenance procedures. We believe the recom- mendations 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 sails. Colorado Geological Survey Special Publication 431 was designed to provide this information. Laboratory tests were performed on samples from this lot and nearby lots. Samples from this lot compressed 0.3 percent or swelled 0.2 to 0.4 percent when wetted. 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 recommenda- tions. We performed calculations of total potential ground heave and basement heave as part of our study. The calculated potential heave for this lot is less than 0.5 inch at the ground surface and less than 0.5 inch at the basement level. It is not certain this move- ment 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 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. We recommned over -excavation, moisture treatment and recompaction to a minimum depth of 4 feet below non -basement footings, or extending all footings to basement level. 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 "'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. CALATLANTIC GROUP, INC EAST RIDGE SUBDIVISION PHASE 1 LOT 23/24, BLOCK 3 CTL I T PROJECT NO. FC06953.004-120 R1 be maintained by the home owner. 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- marily 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 I THOMPSON, INC. )J �AL_ Spencer Schram, PE Geotechnical Department Manager CALATLANTIC GROUP, INC EAST RIDGE SUBDIVISION PHASE 1 LOT 23/24, BLOCK 3 CTL i T PROJECT NO. FC06953.004-120 R1 EXHIBIT A SLAB PERFORMANCE RISK EVALUATION, INSTALLATION AND MAINTENANCE As part of our evaluation of the subsurface soils and bedrock, samples were tested in the laboratory 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 individual 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. 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, moderate, 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. 3"Guideline for Slab Performance Risk Evaluation and Residential Basement Floor System Recommendations", Colorado Association of Geotechnical Engineers, December 1996. EXHIBIT A-1 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 considered normal 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 very high risk, there is more likelihood of need to repair, maintain or replace basement and garage floors and exterior flatwork. 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 flatwork, 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 possible basement floor slab and finish work performance will be satisfactory if a basement is finished 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. 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. Slabs should be placed directly on the exposed subsoils or properly moisture conditioned, compacted fill. The 2009 International Building Code (IBC) or 2009 International Residential Code (IRC) requires a vapor retarder be placed between base course or EXHIBIT A-2 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. 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)". 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. iacu_3rrM 7. Roofs that overhang a patio or porch should be constructed on the same foundation 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 structurally 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 foundation. Decks are more flexible and more easily adjusted in the event of movement. 9. Frequent control joints should be provided in conventional slabs -on - grade to reduce problems associated with shrinkage cracking and curling. Panels that are approximately square generally perform better than rectangular areas. We suggest an additional joint about 3 feet away from and parallel to foundation walls. EXHIBIT A-4 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. 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 foundation 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 homebuyer should maintain surface drainage and install an irrigation system that substantially conforms to these recommendations. 1. 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 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, because we know that backfill will likely settle to some degree. By starting with sufficient 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 foundation 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 acceptable to use a slope of about 6 inches in the first 10 feet (5 percent) at this location. EXHIBIT B-1 Between houses that are separated by a distance of 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 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 advocate a minimum slope of about 2.5 percent in grassed areas and 2 percent where landscaping rock 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. Downspout extensions, splash blocks and buried outlets must be maintained by the homebuyer. s. The importance of proper homeowner 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 maintained to minimize irrigation. Plants placed 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. 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. EXHIBIT B-2