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HomeMy WebLinkAboutTHE SUMMIT ON COLLEGE PARKING GARAGE - MJA/FDP - FDP130056 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORT (3)Terracon Consultants, Inc. 1901 Sharp Point Drive, Suite C Fort Collins, Colorado 80525 P [970] 484-0359 F [970] 484-0454 www.terracon.com December 4, 2013 Capstone Development Partners, LLC 402 Office Park Drive, Suite G50 Birmingham, Alabama 35223 Attn: Mr. Walker P. May, LEED AP Senior Development Manager P: (205) 949-5055 E: wmay@capstonemail.com Re: Supplemental Geotechnical Engineering Recommendations Parking Garage at The Summit on College 1721 Choice Center Drive Fort Collins, Colorado Terracon Project No. 20135039 Previously, Terracon Consultants, Inc. (Terracon) prepared a Geotechnical Engineering Report (Project No. 20135039; report dated October 29, 2013) for the project referenced above. We were contacted by the structural engineer for the project (Mr. Hoshi Engineer, P.E., S.E. with Desman Associates) who requested we provide additional geotechnical recommendations and design criteria for drilled pier foundations planned for support of the proposed parking garage. This letter provides supplemental geotechnical recommendations for lateral load considerations, pier spacing and corresponding reduction factors, and lateral earth pressure criteria. Laterally Loaded Drilled Pier Foundations In our initial Geotechnical Engineering Report, we provided geotechnical parameters for input to LPILE to complete lateral load analysis of drilled pier foundation systems bottomed in bedrock. We were requested to provide alternative geotechnical parameters to include horizontal modulus of subgrade reaction for this project. For the purpose of drilled pier design, we recommend assigning the following values for the soil types encountered at this project site: Soil Type Horizontal Modulus of Subgrade Reaction, kh (tcf) Clay, sand, and gravel (above groundwater) kh = (20 x z)/d Clay, sand, and gravel (below groundwater) kh = (10 x z)/d Bedrock kh = 400/d Notes: z=depth (ft) and d=pier diameter (ft) Ignore the modulus values for the upper 2 feet of soil during lateral load analysis and design Supplemental Geotechnical Engineering Recommendations Parking Garage at The Summit on College ■ Fort Collins, Colorado December 4, 2013 ■ Terracon Project No. 20135039 Responsive ■ Resourceful ■ Reliable 2 Pier Reduction Values In our Geotechnical Engineering Report, we recommended constructing the proposed parking garage on a drilled pier foundation system bottomed in bedrock. We also recommended that piers should be considered to work in group action if the horizontal spacing is less than three pier diameters and indicated a minimum practical horizontal clear spacing between piers of at least three pier diameters should be maintained. As part of our supplementary recommendations for the project, Terracon believes piers should be considered to work in group action if the horizontal center-to-center spacing (rather than clear spacing) is less than three pier diameters based on the largest diameter pier. We understand there may be several locations within the proposed parking garage that will require construction of drilled pier foundations within three drilled pier diameters. A minimum spacing of three (3) pier diameters is recommended for piers subject to axial loading. For piers spaced at one (1) pier diameter (piers touching), the skin friction load reduction factor for both piers would be 0.5. End bearing values would not be reduced provided the piers bottom in bedrock at roughly the same elevations. The skin friction reduction factor can be interpolated for situations where pier spacings are between one (1) and three (3) pier diameters. When analyzing piers constructed in-line with the direction of lateral loads, we recommend piers be spaced a minimum of six (6) pier diameters (center to center) based upon the larger pier. If closer spacing is required, the modulus of subgrade reaction for initial and trailing piers should be reduced. At a spacing of three (3) pier diameters, the effective modulus of subgrade reaction for the first pier can be estimated by multiplying the given modulus by 0.6. For trailing piers in- line at three-diameter spacing, the factor is 0.4. Linear interpolation can be used for spacing between three (3) and six (6) pier diameters. At a spacing of less than three (3) pier diameters, the pier group should be analyzed as a single unit with the full effective modulus of subgrade reaction applied for the first pier and neglected for any trailing piers within three (3) pier diameters. Reductions to the modulus of subgrade reaction can be accomplished in LPILE by inputting the appropriate modification factors for p-y curves. Reducing the modulus of subgrade reaction in trailing piers will result in greater computed deflections for these piers. In practice, a grade beam or rigid pier cap can force deflections of all piers to be equal. Load-deflection graphs can be generated for each pier using the appropriate p-multiplier values. The sum of the lateral load resistance for the piers at selected deflections can be used to develop a total lateral load versus deflection graph for the system of piers. For lateral loads perpendicular to the line of piers, a minimum spacing of three (3) pier diameters can be used with no capacity reduction. At one diameter (piers touching) the piers can be analyzed as one unit. Interpolation can be used for intermediate conditions. Supplemental Geotechnical Engineering Recommendations Parking Garage at The Summit on College ■ Fort Collins, Colorado December 4, 2013 ■ Terracon Project No. 20135039 Responsive ■ Resourceful ■ Reliable 3 Lateral Earth Pressures Reinforced concrete walls with unbalanced backfill levels on opposite sides should be designed for earth pressures at least equal to those indicated in the following table. Earth pressures will be influenced by structural design of the walls, conditions of wall restraint, methods of construction and/or compaction and the strength of the materials being restrained. Two wall restraint conditions are shown. Active earth pressure is commonly used for design of free-standing cantilever retaining walls and assumes wall movement. The "at-rest" condition assumes no wall movement. The recommended design lateral earth pressures do not include a factor of safety and do not provide for possible hydrostatic pressure on the walls. EARTH PRESSURE COEFFICIENTS Earth Pressure Conditions Coefficient for Backfill Type Equivalent Fluid Density (pcf) Surcharge Pressure, p1 (psf) Earth Pressure, p2 (psf) Active (Ka) Granular - 0.33 Lean Clay - 0.46 40 55 (0.33)S (0.46)S (40)H (55)H At-Rest (Ko) Granular - 0.50 Lean Clay - 0.63 60 75 (0.50)S (0.63)S (60)H (75)H Passive (Kp) Granular - 3.0 Lean Clay - 2.1 360 250 --- --- --- --- Applicable conditions to the above include:  For active earth pressure, wall must rotate about base, with top lateral movements of about 0.002 H to 0.004 H, where H is wall height;