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HomeMy WebLinkAbout5815 Tilden St - Special Inspections/Engineering - 09/16/2014A. GO Wassenaar 2 180 South Ivanhoe Street, Suitt 5 Dcnvar, Colorado 80222-5710 303-759-8100 Fax 303-756-2920 Geotechnical and Environmental Consultants www.agwassenaar.com September 16, 2014 Meritage Homes of Colorado Inc. 6892 South Yosemite Court, Suite 1-201, Centennial, Colorado 80112 Attention: Mr. Michael Bird Subject: Soil and Foundation Study Proposed Residential Structure Lot 3, Block 6 Mail Creek Crossing Fort Collins, Colorado Project Number 143623 Purpose As requested, we have performed a soil and foundation study at the subject site. The purpose of our study was to observe subsurface conditions encountered and to recommend geotechnical design criteria for the design and construction of the foundation for the proposed residence. This letter presents a summary of our findings and recommendations. Subsurface Conditions The field exploration included drilling a 4-inch diameter auger boring near the center of the lot to a depth of approximately 20 feet. The subsurface materials encountered consisted of: 0' to 5' 5' to 12' 12' to 20' Date of drilling: Depth to water: Laboratory Testing Fill, clay, erratic moisture and stiffness, silty, sandy, mottled brown to dark brown Clay, medium stiff, silty, sandy, slightly moist to moist, brown to reddish brown Clay, stiff, silty, sandy, moist to very moist, rust to brown to gray to olive August 13, 2014 Dry at the time of drilling 19'/ feet 2 days after drilling Samples obtained during drilling were returned to the laboratory. They were visually classified and testing was assigned to selected samples in an effort to evaluate the engineering properties of the subsurface materials encountered. Site specific laboratory swell/consolidation tests exhibited no to low measured swell (0.2% at a depth of 4 feet and 0.0% at a depth of 9 feet) upon an increase in moisture content under a load of 1,000 pounds per square foot (psf). Based upon visual observation of the Meritage Homes of Colorado, Inc. Project Number 143623 September 16, 2014 Page 2 subsurface conditions encountered and laboratory testing for this and/or adjacent lots, it is our opinion that the subsurface materials generally exhibit low potential for expansion and moderate potential for compression. Refer to the Colorado Geological Survey Special Publication 43 for a description of expansive soils and their impact on structure performance. Site Preparation for Foundation Construction It is possible that after excavation for the basement foundation system, soft, rutting, unstable soils will be encountered which will not be suitable for support of the proposed construction. If soft, rutting soils become a problem, stabilization with rockfill may be necessary. Site preparation for the foundation system for the proposed structure may be accomplished by excavating the site to 1 or 2 feet below the proposed bottom of footing a distance of at least 2 feet beyond the edge of the footing. Our office should be retained to observe a proof -roll of the foundation system subgrade to determine if stabilization will be required. The soft, rutting soils should then be stabilized by placing 6-inch minus crushed rock or an equivalent material, which should be evaluated by our office prior to use. Crushed concrete should not be used. The material should be placed in approximately 8-inch loose lifts and rolled into the underlying soft or loose soils. This procedure should be observed by a representative of our firm and should continue until a front-end loader with loaded bucket, or equivalent, can be supported on the rock fill with no significant deformation. Subsequent gravel layers should be placed accordingly until foundation elevation has been obtained. A representative of our firm should verify stabilization by observing a proof -roll prior to forming footings. Foundation Recommendations Based on our evaluation of the subsurface conditions, the proposed residence may be founded upon spread or pad -type footings bearing below any existing fill on the natural undisturbed soils or on properly placed and compacted fill. The footings should be designed for a maximum bearing pressure of 1,200 psf. Footing dimensions and foundation structural elements should be determined by a structural engineer. Concrete in contact with the subsurface materials may be designed for negligible (SO) sulfate exposure as defined by ACI 318.4.3. Bearing materials loosened by machine excavation should be removed prior to placing footing concrete. Occasionally, pockets of loose soils are encountered in foundation excavations. If this condition occurs, the footings should extend to firm soils. Exterior footings should bear at least 3 feet below exterior grade for frost protection. The bearing materials beneath footings should be protected from freezing during construction. All footing excavations should be observed prior to placement of concrete to confirm the footings are bearing on suitable materials as anticipated for design purposes. The foundation walls backfilled with on -site materials should be designed for a lateral earth pressure based upon an equivalent fluid density of 65 pounds per cubic foot (pcf) for the "at rest" condition or 50 pcf for the "active" condition. The "active" condition should only be used where wall movements of at least 0.5% of the wall height are allowed. These values have been provided without considerations for sloping backfill, surcharge loading or hydrostatic pressures. Construction of a drain system and proper surface drainage as discussed later in this report may lower the potential of developing hydrostatic pressure in the backfill materials. Minor cracking of concrete foundation walls should be expected. Meritage Homes of Colorado, Inc. Project Number 143623 September 16, 2014 Page 3 Basement Floor Construction A basement slab performance risk evaluation was conducted in general conformance with industry guidelines for the local area. The risk assessment of a site for potential movement is not absolute; rather, it represents a judgment based upon the data available and our experience in the area. Movement of foundations and concrete flat work will occur with time in low to very high risk areas as the soil moisture content increases. On low and moderate rated sites, slab movements of up to 3 inches or more across the slab with slab cracking of up to %-inch or more in width and/or differential are considered normal. The damage generally increases as the risk assessment increases and as the depth of wetting increases. It must be understood, however, that assessing risk is an opinion. There is currently no type of testing or correlation of factors that will definitively predict the amount of heave that a floor slab will exhibit. Therefore, it may be possible that heaves less than or in excess of what is considered "normal" may be experienced. For sites with a risk assessment of high or very high, we recommend an interior floor system engineered for expansive soils be constructed. An alternative to the use of an engineered floor system, such as soil modification to reduce the risk assessment, may also be considered. In addition, an engineered interior floor system is recommended for all finished areas or any other areas where floor movements cannot be tolerated. Based upon our evaluation of the subsurface conditions at this site, it is our opinion that the slab performance risk for this site is low. If this risk of movement is not acceptable, engineered interior floors should be constructed or an alternative such as soil modification should be considered. If the Builder and/or Owner desires to construct a concrete slab -on -grade and accepts the risk of slab movement, slabs supported by the expansive subsurface materials should be constructed using the following criteria: Slabs should be separated from exterior walls and interior bearing members with a joint which allows free vertical movement of the slab. 2. Slab bearing partitions should be constructed with a minimum 2-inch void space. Stairways bearing upon the slab should be constructed in such a way as to allow at least 2 inches of slab heave. In the event of slab heave, the movement should not be transmitted directly through the partitions to the remainder of the residence. 3. Plumbing and utilities should be isolated from the slab. 4. Where a forced -air heating system is used and the furnace is located on the slab, we recommend provision for a collapsible connection between the furnace and the duct work to allow for at least 3 inches of slab heave. Utility connections should also be provided with flexible connections capable of accommodating the same magnitude of movement as specified above. Provide frequent control joints in the slab. Meritage Homes of Colorado, Inc. Project Number 143623 September 16, 2014 Page 4 Following these recommendations should reduce damage caused by movement of the floor slab; however, the void spaces recommended are not intended to predict total slab movement. Care should be taken to monitor and reestablish partition voids and flexible connections when necessary. We are available to provide further consultation regarding basement slab performance risk assessments. Crawl Space Construction The crawl space ground surface should be sloped to the perimeter drain system. Trenching or dishing out of the crawl space is not recommended unless a drain system is placed in these areas in such a manner to facilitate drainage. The recommended clearance from the crawl space ground surface to the engineered floor system should meet applicable codes. In addition, all plumbing lines should be isolated from the ground surface or foundation walls by at least 2 inches. During construction, the crawl space area should be checked for standing water or very moist conditions, construction debris, and other deleterious materials. If these conditions exist, the area should be evaluated and mitigated, as necessary. Crawl space areas should be constructed with consideration given to proper ventilation and moisture management. Provisions such as the installation of a vapor retarder should be utilized to reduce the amount of moisture (humidity) in the crawl space air. The Client and any future Owner should be aware that crawl space areas are subject to various air quality issues. A consultant specializing in ventilation and air quality control should be contacted to provide any additional recommendations. Such recommendations are beyond the geotechnical scope of this study. The environmental division of A. G. Wassenaar, Inc. is capable of providing such services. Refer to Homeowners Guide To Moisture Management" by Tri-County Health Department (Brochure Number S-323) for additional information. Subsurface Drainage As a minimum, we recommend providing a subsurface drainage system around the lowest below grade area. The purpose of the drain is to collect water which may become trapped on the surface of the excavation and enter the basement or crawl space areas. A drain should be constructed similar to the attached drain detail (Figure 1) and should be uniformly sloped to a positive gravity discharge or sump. If a sump pit is installed, it should be monitored for water accumulation and proper operation. The water level in the sump pit should not be allowed to rise above the bottom of the drain inlet pipe(s). If this occurs, a pump should be installed (if not originally equipped) or maintenance should be performed on the existing pump. Surface Drainage The wetting of foundation soils and/or bedrock materials may be reduced by carefully planned and maintained surface drainage. The following recommendations should be implemented during construction and maintained by the Homeowner after the residence is completed: Meritage Homes of Colorado, Inc. Project Number 143623 September 16, 2014 Page 5 Excessive wetting or drying of the open foundation excavation should be avoided as much as practical during construction. 2. The ground surface surrounding the exterior of the foundation should be maintained in such a manner as to provide for positive surface drainage away from the foundation. At completion of construction, we recommend a minimum fall away from the foundation of 6 inches in the first 5 feet. This slope should be continuous across the backfill zone. 3. Backfill around the foundation should be moistened and compacted in such a manner as to reduce future settlement. Areas which settle should be filled as soon as possible in order to maintain positive drainage away from the foundation. 4. If lawn edging is used around the exterior of the foundation, it should be constructed in a manner to prevent ponding of surface water in the vicinity of the backfill soils. 5. Drainage swales should be constructed and maintained a minimum of 5 feet away from the foundation on side yards and 15 feet away from the foundation on back and front yards. Natural drainage swales should maintain a slope of at least 2% off of the lot. Swales must not be blocked by fences, landscaping, paths or other Homeowner installed items. 6. Roof downspouts and drains should discharge well beyond the limits of foundation backfill. 7. Watering adjacent to the foundation should be reduced as much as practical. Landscaping which requires excessive watering should not be located within 5 feet of foundation walls. Sprinkler lines, zone control boxes, and drains should be located outside the limits of the foundation backfill. Sprinkler heads should be positioned such that the spray does not fall within 5 feet of foundation walls. 8. Plastic membranes should not be used to cover the ground surface immediately surrounding the foundation. These membranes tend to trap moisture and prevent normal evaporation from occurring. We recommend the use of a weed suppressant geotextile fabric. Limitations We believe the professional judgments expressed in this report are consistent with that degree of skill and care ordinarily exercised by practicing design professionals performing similar design services in the same locality, at the same time, at the same site and under the same or similar circumstances and conditions. No other warranty, express or implied, is made. The location of the test boring drilled and the laboratory testing performed for this study were designed to obtain a reasonably accurate picture of subsurface conditions for design purposes. Variations in subsurface conditions not indicated by the boring are possible and expected. Therefore, we should be retained to observe the foundation Meritage Homes of Colorado, Inc. Project Number 143623 September 16, 2014 Page 6 excavation and construction in order to verify or revise our recommendations. If unexpected subsurface conditions are observed by others during construction, we should be called to review our recommendations. This report was prepared for the exclusive use of our Client for the sole purpose of providing geotechnical design criteria for the subject structure based upon the existing site conditions as encountered. The conclusions and recommendations contained in this report shall not be considered valid for use by Others without written authorization from A. G. Wassenaar, Inc. In addition, the state of practice in geotechnical engineering is constantly evolving. Therefore, findings presented in this report should be reviewed and revised, if necessary, prior to actual construction. If we can be of further service in discussing the contents of this letter or in analysis of the proposed structure from the soil and foundation viewpoint, please call our office. Sincerely,' A. G. WAS ENAAR, I1; r"'€oa Michael R. Conner, Senior Engineer:�<•f=���..a �LJ,` py, ammo• ^ ^°®�;��'J�. Reviewed by: onald L. Taylor Jr., E. Presi M RC/D LT/sa r Attachment: Figure 1 Statement of Services A.G. Wassenaar Geotechnical and Environmental Consultants so NOTES: 1. DRAIN MUST SLOPE TO A POSITIVE GRAVITY OUTLET AND/OR TO SUMP WHERE WATER CAN BE REMOVED BY PUMPING 2. SLOPE BOTTOM OF TRENCH AND PIPE AT A MINIMUM OF 1/8" PER FOOT (i.e. 1%) OR AS APPROVED BY THE GEOTECHNICAL ENGINEER 3. 4-INCH DIAMETER RIGID PERFORATED PVC PIPE (ASTM D2729 MINIMUM SCHEDULE 20), OR SUBSTITUTE APPROVED BY GEOTECHNICAL ENGINEER 4. GRAVEL SPECIFICATION: 2" MINUS WASHED ROCKIGRAVEL, POORLY GRADED WITH NO MORE THAN 30% PASSING THE 3/8" SIEVE AND NO MORE THAN 10% PASSING THE #4 SIEVE, OR AS APPROVED BY THE GEOTECHNICAL ENGINEER SLIP JOINT CONCRETE SLAB ON GRADE BACKFILL DO NOT EXCAVATE WITHIN A 1:1 LINE EXTENDING DOWN AND AWAY FROM EDGE OF FOOTING. MAINTAIN THIS ANGLE OF EXCAVATION TO A DEPTH OF TWICE THE FOOTING WIDTH. 1NDATION MINIMUM 10 MIL POLYETHYLENE BARRIER OR WALL EQUIVALENT APPROVED BY GEOTECHNICAL ENGINEER. ATTACH TO WALL AFTER DAMPPROOFING AND EXTEND AT LEAST 1 FOOT UP ON WALL AND BENEATH FLOW LINE OF PIPE. BACKFILL NON -WOVEN GEOTEXTILE FILTER FABRIC (MIRAFI 140N OR EQUIVALENT APPROVED BY GEOTECHNICAL ENGINEER) PLACED ACROSS ENTIRE WIDTH OF DRAIN GRAVEL. DRAIN GRAVEL (SEE NOTE #4) GRAVEL SHOULD FILL ENTIRE TRENCH AND EXTEND LATERALLY TO TOP OF FOOTING. GRAVEL COVER ABOVE PIPE SHOULD BE AT LEAST 4" o;;p;: ;o; : Q;;:a;^•.:: . .' "•,'�V o•' ' EXTEND POLYETHYLENE TO OUTSIDE EDGE OF BOTTOM OF TRENCH. \ DRAIN PIPE - (SEE NOTES #2 & 3) 8" AT LOCATION OF HIGH POINT, _ _ \ MINIMUM ESTABLISH BOTTOM OF DRAIN PIPE AT LEAST 4" BELOW BOTTOM OF FOOTING OR PAD FOOTING FOUNDATION TYPICAL EXTERIOR DRAIN DETAIL SEPTEMBER 2007 F3EXT- FIGURE 1 00