Loading...
HomeMy WebLinkAboutSEVEN GENERATIONS MULTIFAMILY - FDP230008 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORT (2) CTL|Thompson, Inc. Denver, Fort Collins, Colorado Springs, Glenwood Springs, Pueblo, Summit County – Colorado Cheyenne, Wyoming and Bozeman, Montana November 3, 2022 Black Timber Builders 417 Jefferson Street Fort Collins, Colorado 80524 Attention: Russell Baker, Principal Subject: Soil and Foundation Investigation Addendum Multi-Family Residential Building 3221 Eastbrook Drive Fort Collins, Colorado CTL|T Project Number: FC10564-120 L1 CTL|Thompson, Inc. previously provided a Soils and Foundation Investigation for 3221 Eastbrook Drive in Fort Collins under the subject project number, report dated September 20, 2022. We have been asked to provide reinforced concrete mat and post-tensioned slab (PTS) foundation design criteria for this site. Our investigation indicated predominantly low to moderate-swelling soils at the anticipated foundation depth for a reinforced mat or PTS foundation. We recommend that either foundation type be placed on a minimum 2 feet of over-excavated, moisture treated and properly compacted fill. Existing fill was encountered in two borings to depths of up to 3 feet. Deeper fill areas may be encountered during site development. The fill is of unknown origin and age. The fill presents a risk of settlement or heave to improvements constructed on the fill. We recommend any existing fill encountered be removed, properly moisture treated and recompacted in the building area. Reinforced Concrete Mat 1. Reinforced concrete mat foundations should be constructed on 2 feet of fill placed for an over-excavation as described in the Over-Excavation section below. The reinforced concrete mat foundation should be designed for a net allowable soil pressure of 2,000 psf if constructed on the properly compacted fill. The soil pressure can be increased 33 percent for transient loads such as wind or seismic loads. 2. Reinforced slabs are typically designed using a modulus of subgrade reaction. We recommend use of a modulus of 75 pounds per square inch per inch of deflection (pci). 3. The soils beneath mat foundations can be assigned an ultimate coefficient of friction of 0.4 to resist lateral loads. The ability of foundation backfill to resist lateral loads can be calculated using a passive equivalent fluid pressure of 250 pcf. This assumes the backfill is densely compacted and will not be removed. BLACK TIMBER BUILDERS 3221 EASTBROOK DRIVE, FORT COLLINS CTL | T PROJECT NO. FC10564-120 L1 2 Backfill should be placed and compacted to the criteria in the Backfill Compaction section of the report. A moist unit weight of 120 pcf can be assumed for natural soils and compacted fill. These values are considered ultimate values and appropriate factors of safety should be used. Typically, a factor of safety of 1.5 is used for sliding and 1.6 for lateral earth pressure. 4. The edges of the mats should be thickened or turned down for structural strength. 5. Materials beneath the mat foundation should be protected from frost action. We believe 30 inches of frost cover is appropriate for this site. 6. We should be retained to observe the completed excavations for mats to confirm that the subsurface conditions are similar to those found in our borings. Post-Tensioned Slabs (PTS) We believe PTS foundations can also be considered for the proposed apartment structures provided they are designed and constructed using the criteria presented below. PTS foundation design is based on a method developed by the Post-Tensioning Institute (PTI, 3rd Edition, 2008). Various climate and relevant soil factors are required to evaluate the PTI design criteria. These include the Thornthwaite Moisture Index (Im), suction compression index (γh), unsaturated diffusion coefficient (α), depth of probable moisture variation, initial and final soil suction profiles, percent fine clay fraction and PTI mineral classification. In the Fort Collins area, the Im is about negative 25. Based on laboratory test results and the 2008 PTI design method, we estimate γh of 0.03 and α of 0.005 are appropriate. The PTI design procedure is based upon soil movement that is primarily controlled by climate. The 2008 design estimates movements for a depth of wetting of 9 feet below the ground surface. Recent research by Walsh, Colby, Houston and Houston1 indicates there is a 90 or 95 percent probability that the wetting depth will not exceed 24 feet for this region. Ground movements can be estimated based on swell and/or soil suction test results. The PTI design procedure does not predict soil movement caused by site conditions such as irrigation or poor surface drainage that may lead to deeper wetting. If deeper wetting of the foundation soils occurs, the foundation movement may exceed the design movements in the PTI procedure. The PTI slab design includes evaluation of two mechanisms of soil movement (edge lift and center lift) based on assumptions that the wetting and drying of the foundation soils are primarily affected by the climate. Our experience indicates that the foundation soils will normally undergo an increase of moisture due to covering of the ground surface by the buildings, pavements, and irrigating around the buildings. Depending upon the surface drainage or the amount of available water, the differential movement, edge and center lift which control design, could approach total heave. The edge moisture variation distance can also be more than the design values provided in the PTI manual. Considering the limitations of the current PTI design, we believe a conservative approach with reasonable engineering judgment is merited in PTS foundation design. We estimate heave at current ground surface for the building could be up to 1” Method for Evaluation of Depth of Wetting in Residential Areas” by Walsh, Colby, Houston and Houston, Journal of Geotechnical and Geoenvironmental Engineering, ASCE February 2009. BLACK TIMBER BUILDERS 3221 EASTBROOK DRIVE, FORT COLLINS CTL | T PROJECT NO. FC10564-120 L1 3 1½ inches without additional soil modification. To reduce the risk of differential movement, we recommend the upper 2 feet of soil below the PTS be removed, moisture conditioned and recompacted (over-excavation) prior to construction of the slab. PTS foundation design is based on the potential differential movement of the slabs due to both settlement and heave of the soils. Our investigation indicates that predominantly sandy clay or clayey sand will be present at depths likely to influence foundation performance. The following criteria should be used to design and construct the PTS foundation. 1. The PTS foundation is suggested to be constructed on a 2-foot over-excavation. Where soil is loosened during excavation or in the forming process, or if any existing fill, soft or loose soils are exposed in excavations, the soils should be removed or moistened and compacted prior to placing concrete. 2. The foundation should be designed for a maximum allowable soil pressure of 2,000 psf if placed on the recommended over-excavated mat of soil. 3. For the PTI design method, we recommend a differential movement (ym) of 1.1 inches for edge lift conditions and 1.0 inches for center lift conditions. 4. Based on the 3rd Edition PTI Manual, edge moisture variation distance (em) of 5 feet for the edge condition and 9 feet for the center condition should be used in design. 5. We understand the PTI design method assumes the slab is somewhat flexible. The above-grade construction, such as framing, drywall, brick, and stucco, is not as flexible. We are aware of situations where minor differential slab movement has caused distress to finish materials. One way to enhance performance would be to place reinforcing steel in the bottom of stiffening beams. The structural engineer should evaluate the merits of this approach as well as other potential alternatives to reduce damage to finish. 6. Stiffening beams may be poured “neat” into excavated trenches. Soil may cave or slough during trench excavation for the stiffening beams. Disturbed soil should be removed from trench bottoms prior to placement of concrete. Formwork or other methods may be required for proper stiffening beam installation. 7. Exterior stiffening beams should be protected from frost action. Normally, 30 inches of frost cover is assumed in the Fort Collins area. If exterior patios are incorporated into PTS, we believe the stiffening beams around the patios should be as deep as those around the building exterior to increase the likelihood they will perform similarly to the rest of the PTS. 8. For slab tensioning design, a coefficient of friction value of 0.75 or 1.0 can be assumed for slabs on polyethylene sheeting or a sand layer, respectively. A coefficient of friction of 2.0 should be used for slabs on clayey sand or clay and/or clay, clayey sand fill. We believe use of polyethylene is preferable because it serves as a vapor retarder which helps to control moisture migration up through the slabs. BLACK TIMBER BUILDERS 3221 EASTBROOK DRIVE, FORT COLLINS CTL | T PROJECT NO. FC10564-120 L1 4 9. A representative of our firm should observe the completed excavations. A representative of the structural engineer or our firm should observe the placement of the reinforcing tendons and reinforcement prior to placing the slabs and beams. Over-Excavation We believe the risk of potential movements due to the swelling soils can be reduced by placing the reinforced mat or PTS foundation on 2 feet of over-excavated, moisture treated and properly compacted soil. The over-excavation should extend at least 2 feet below the PTS foundation subgrade and at least 2 feet beyond the perimeter of the footprint of the structure. The existing on-site soils and the existing fill are suitable for re-use as fill material provided debris or deleterious organic materials are removed. Areas to receive fill should be scarified, moisture-conditioned and compacted to at least 95 percent of standard Proctor maximum dry density (ASTM D 698, AASHTO T 99). Clay fill soils placed below the building should be moisture conditioned between optimum and 3 percent above optimum moisture content. The fill should be moisture-conditioned, placed in thin, loose lifts (8 inches or less) and compacted as described above. We should observe placement and test compaction of fill during construction. Fill placement and compaction activities should not be conducted when the fill material or subgrade is frozen. The construction alternatives discussed in this letter are given with the assumption that all other recommendations in our report, including those on irrigation and maintenance practices, are followed. We appreciate the opportunity to work with you on this project. Please do not hesitate to contact us with any other questions or concerns. Sincerely, CTL|THOMPSON, INC. Trace Krausse, EI Spencer Schram, PE Geotechnical Project Engineer Geotechnical Project Manager