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HomeMy WebLinkAboutAFFINITY AT FORT COLLINS - PDP - PDP150010 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORT (5)4396 GREENFIELD DRIVE WINDSOR, COLORADO 80550 (970) 545-3908 FAX (970) 663-0282 www.earth-engineering.com June 5, 2015 Inland Group 1620 North Mamer Road, Building B Spokane, Washington 99203 Attn: Mr. Mark Ossello (marko@inlandconstruction.com) Re: Subsurface Exploration Report – Addendum No. 1 Proposed Affinity at Fort Collins Apartments 2600 East Harmony Road Fort Collins, Colorado EEC Project No. 1142094 Mr. Ossello: Earth Engineering Consultants (EEC) conducted a subsurface exploration study for the Affinity at Fort Collins Apartment Development project at the referenced site in December of 2014. At that time, EEC prepared a report detailing our findings and geotechnical engineering design recommendations based on the subsurface conditions encountered. For further information, please refer to our “Subsurface Exploration Report” dated December 15, 2014, EEC Project No. 1142094. Based on the subsurface conditions encountered, the type of construction planned for the apartment complex at that time, recommendations for a 6-foot over-excavation and replacement concept to address the moderate to high swelling subsoils on-site were provided, and the use of conventional type spread footings bearing on a zone of moisture conditioned/engineered on-site subsoils. The developers and design team members recently requested site-specific recommendation for possibly using a post-tensioned-slab (PTS) foundation system instead of the conventional type spread footings as previously identified. This report provides recommendations for the site work preparation and PTS foundation design criteria for the current design approach. ANALYSIS AND RECOMMENDATIONS Swell – Consolidation Test Results The swell/consolidation test is performed to evaluate the swell or collapse potential of soils or bedrock to assist in determining foundation, floor slab and pavement design criteria. In this test, relatively undisturbed samples obtained from the California sampler or thin-walled tubes are placed in a laboratory apparatus and inundated with water under a predetermined load. The Earth Engineering Consultants, LLC Affinity of Fort Collins - Addendum Report EEC Project No. 1142094 June 5, 2015 Page 2 swell-index is the resulting amount of swell or collapse after the inundation period expressed as a percent of the sample’s initial thickness. After the inundation period, additional incremental loads are applied to evaluate the swell pressure and consolidation response. As a part of our 2014 laboratory testing, we conducted fourteen (14) swell/consolidation tests on samples of the native subsoils at inundation and pre-loading pressures of 150 and 500 psf. The swell index values for the samples analyzed revealed low to high swell characteristics on the order of (+) 1.2 to (+) 10.8%. The (+) test results indicate the soil or bedrock materials swell potential characteristics. The Colorado Association of Geotechnical Engineers (CAGE) uses the following information to provide uniformity in terminology between geotechnical engineers to provide a relative correlation of slab performance risk to measured swell. “The representative percent swell values are not necessarily measured values; rather, they are a judgment of the swell of the soil and/or bedrock profile likely to influence slab performance.” Geotechnical engineers use this information to also evaluate the swell potential risks for foundation performance based on the risk categories. Recommended Representative Swell Potential Descriptions 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 < 2 Moderate 3 to < 5 2 to < 4 High 5 to < 8 4 to < 6 Very High > 8 > 6 Based on the laboratory test results, the samples analyzed for this project were within the low to high range. As further discussed in the “Site Preparation” section of this report, due to the relatively dry in-situ moisture contents of the overburden subsoils as well as the moderate to high swell potential exhibited in the laboratory, an over-excavation and replacement procedure will be necessary to reduce the potential for movement/heave in the foundation alignments, floor slab areas, pavements and exterior flatwork areas. Earth Engineering Consultants, LLC Affinity of Fort Collins - Addendum Report EEC Project No. 1142094 June 5, 2015 Page 3 Site Preparation Based on our understanding of the proposed development, it appears small cuts and/or fills may be necessary to achieve design grades. After stripping and completing all cuts to top of subgrade and foundation bearing elevations, and prior to placement of any fill and/or site improvements, we recommend at a minimum the top two (2) feet of the existing relatively dry, moisture deficient, stiff, moderately to highly expansive cohesive subsoils be removed below all grade beam and floor slabs within the building foundation areas, below all exterior concrete flatwork, and below top of subgrade within all pavement areas. The over-excavated dry cohesive subsoils should be replaced as moisture conditioned/engineered fill material. The 2-foot overexcavation below all PTS foundations elements would require over- excavation in the grade beam alignments on the order of at least 4 to 5-feet below supporting final grades. Consideration could be given to over-excavating the entire building footprint areas to an elevation equal to the 2-feet over-excavation below the foundation grade beam bearing elevations. The overexcavation(s) should extend laterally in all directions a like distance beyond the building grades beams and beyond pavement/flatwork areas. After the initial over-excavation as described above, the exposed subgrade soils, where practical depending upon the selected over-excavation limits, beneath should be scarified to a minimum depth of 9-inches, adjusted in moisture content to within (-) 1% to (+) 3% of the material’s standard Proctor optimum moisture content and compacted between 95% to 98% of the material's standard Proctor maximum dry density as determined in accordance with ASTM Specification D698. Fill soils to develop the building, pavement/flatwork and site subgrades should consist of approved materials, which are free from organic matter and debris. It is our opinion the on- site cohesive soils could be used as fill in these areas, provided adequate moisture treatment and compaction procedures are followed. We recommend the fill soils be placed in loose lifts not to exceed 9 inches thick and adjusted in moisture content and compacted as recommended for the scarified soils. If the site lean clay soils are used as fill material, care will be needed to maintain the recommended moisture content prior to and during construction of overlying improvements. Earth Engineering Consultants, LLC Affinity of Fort Collins - Addendum Report EEC Project No. 1142094 June 5, 2015 Page 4 Care should be exercised after preparation of the subgrades to avoid disturbing the subgrade materials. Positive drainage should be developed away from the structures and pavements to avoid wetting of subgrade materials. Subgrade materials becoming wet subsequent to construction of the site improvements can result in unacceptable performance. As presented on the boring logs and laboratory test results of the Subsurface Exploration Report prepared by EEC in December of 2014, moderate to high swelling soils are present on this site. This addendum report provides recommendations to help mitigate the effects of soil shrinkage or expansion. Even if these procedures are followed, some movement and at least minor cracking in the structures should be anticipated. The severity of cracking and other cosmetic damage such as uneven exterior flatwork and pavements will probably increase if any modification of the site results in excessive wetting or drying of the site soils. Eliminating the risk of movement and cosmetic distress may not be feasible, but it may be possible to further reduce the risk of movement if significantly more expensive measures are used during construction. We would be pleased to discuss other construction alternatives with you upon request. Post-Tensioned Slab Foundation Systems The results of our field exploration and laboratory testing completed for this study in December of 2014 indicate the upper cohesive clay subsoils exhibited low to high swell potential and low to moderate bearing capabilities. Based on the subsurface conditions encountered, we anticipate the proposed apartment complex building having slab-on-grade structures could be supported by post-tensioned slabs (PTS) that are supported/bear on a minimum 2-foot zone of engineered/controlled fill materials placed and compacted as outlined in the “Site Preparation” section of this report. Overexcavation and backfill procedures are recommended below all buildings to develop subgrades for the post tension foundations. We recommend a consistent layer of at least 2-feet of low volume change fill be constructed below all foundation bearing levels and floor levels for all proposed structures. The design parameters provided below assume subgrade materials outlined under “Site Preparation” to mitigate the near surface higher swell soils. Outlined below are the post tensioned slab (PTS) design criteria based on the subsurface conditions and information provided in the 3rd Edition of the Post-Tensioning Institutes design Earth Engineering Consultants, LLC Affinity of Fort Collins - Addendum Report EEC Project No. 1142094 June 5, 2015 Page 5 manual. Post-tensioned slabs, thickened or turn-down edges, and/or interior beams should be designed and constructed in accordance with the appropriate design criteria. Post-Tensioned Slab (PTS) – 3rd Edition Design Parameters Maximum Allowable Bearing Pressure, psf 1500 Edge Moisture Variation Distance, em Center Lift Condition, ft. 8.2 Edge Lift Condition, ft. 4.2 Differential Soil Movement, ym Center Lift Condition, Inches 0.5 Edge Lift Condition, Inches 1.0 Slab-Subgrade friction coefficient,  on polyethelene sheeting 0.75 on cohesionless soils – (sands) 1.0 on cohesive soils – (clays) 2.0 All other geotechnical engineering recommendations, especially those related to pavement design and construction, provided in our December 14, 2015 should be followed and adhered to. GENERAL COMMENTS The analysis and recommendations presented in this report are based upon the data obtained from the soil borings performed at the indicated locations identified in our May 13, 2010 report and from any other information discussed in this report. This addendum report does not reflect any variations which may occur between borings or across the site. The nature and extent of such variations may not become evident until further exploration or construction. If variations appear evident, it will be necessary to re-evaluate the recommendations of this report. It is recommended that the geotechnical engineer be retained to review the plans and specifications so that comments can be made regarding the interpretation and implementation of our geotechnical recommendations in the design and specifications. It is further recommended that the geotechnical engineer be retained for testing and observations during earthwork and foundation construction phases to help determine that the design requirements are fulfilled.