Loading...
HomeMy WebLinkAboutOLD TOWN NORTH, 3RD FILING - PDP - PDP160017 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORT400 North Link Lane | Fort Collins, Colorado 80524 Telephone: 970-206-9455 Fax: 970-206-9441 PRELIMINARY GEOTECHNICAL INVESTIGATION OLD TOWN NORTH, BLOCK 6 FORT COLLINS, COLORADO Prepared For: GREELEY ASSOCIATES, LLC 2775 Iris Avenue Boulder, Colorado 80304 Attention: Terry Palmos Project No. FC07353-115 April 27, 2016 TABLE OF CONTENTS SCOPE ....................................................................................................................... 1 SUMMARY OF CONCLUSIONS ............................................................................... 1 SITE DESCRIPTION AND PROPOSED DEVELOPMENT ...................................... 2 GEOLOGIC HAZARDS .......................................................................................... 3 SUBSURFACE CONDITIONS ................................................................................... 4 DEVELOPMENT RECOMMENDATIONS ................................................................. 4 Site Grading ............................................................................................................ 4 Utility Construction .................................................................................................. 5 Preliminary Pavement Thicknesses ....................................................................... 7 Foundations and Floor Systems ............................................................................ 6 Basements.............................................................................................................. 6 Surface Drainage ................................................................................................... 7 General Design Considerations ............................................................................. 8 RECOMMENDED FUTURE INVESTIGATIONS AND TESTING ............................. 9 LIMITATIONS ............................................................................................................. 9 FIGURE 1 – LOCATIONS OF EXPLORATORY BORINGS FIGURE 2 – SUMMARY LOGS OF EXPLORATORY BORINGS APPENDIX A – LABORATORY TEST RESULTS APPENDIX B – GUIDELINE SITE GRADING SPECIFICATIONS 1 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 SCOPE This report presents the results of our Preliminary Geotechnical Investigation for Block 6 of Old Town North Subdivision in Fort Collins, Colorado. The purpose of our investigation was to identify geologic hazards that may exist at the site and to evaluate the subsurface conditions to assist in planning and budgeting for the proposed development. The report includes descriptions of the subsoils, bedrock, and groundwater conditions found in our exploratory borings and discussions of site development as influenced by geotechnical considerations. This report was prepared based upon our understanding of the development plans. The recommendations are considered preliminary and can be used as guidelines for further planning of development and design of grading. We should review final development and grading plans to determine if additional investigation is merited, or if we need to revise our recommendations. Additional investigations will be required to design building foundations and pavements. A summary of our findings and recommendations is presented below. More detailed discussions of the data, analysis and recommendations are presented in the report. SUMMARY OF CONCLUSIONS 1. No geologic or geotechnical conditions were identified which would preclude development of this site. The primary geologic hazard concerning development at this property is the presence of shallow groundwater. The risks associated with shallow groundwater can be mitigated with proper design and construction practices. 2. Soils encountered in our borings consisted of 4½ to 7 feet of sandy clay overlying sand and gravel. Claystone bedrock was encountered in all six borings at depths of 20 to 22½ feet. 2 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 3. Groundwater was encountered in all six borings during drilling at depths of 6 to 10 feet, and was measured several days later at depths of 6 to 7 feet. We recommend a minimum 3-foot separation from foundations and floor slabs to groundwater. Final grading plans should consider groundwater levels for the proposed development. 4. Spread footing foundations are likely appropriate for all of the lots at this site. Slab-on-grade basement floors are likely appropriate for all of the lots. We believe the risk of poor basement slab performance is generally low at this site. 5. Preliminary data indicates sandy clay will likely be present at subgrade levels for pavements. New fill is assumed to possess equal or better qualities as the native soil. Estimated pavement sections are provided in the report. Subgrade stabilization may be necessary to reduce expansive potential of the subgrade soils. Mitigation may include moisture conditioning or chemical treatment with lime or fly ash. 6. Overall surface drainage should be designed to provide rapid run-off of surface water away from the proposed residences. Water should not be allowed to pond near the crest of slopes, on or adjacent to pavements, or adjacent to structures. All permanent slopes should be re-vegetated to reduce erosion. 7. Further investigations are required to make design recommendations for foundations, floors, and pavements. SITE DESCRIPTION AND PROPOSED DEVELOPMENT The project area is located north of Osiander Street, east of Blondel Street in Fort Collins, Colorado (Figure 1). The area is planned for residential development, including associated roadways and utilities. We have assumed a mix of single-family and multi-family residences having one or two stories. 3 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 GEOLOGIC HAZARDS Colorado is a challenging location to practice geotechnical engineering. The climate is relatively dry and the near-surface soils are typically dry and relatively 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 called expansive soils. Other soils can settle significantly upon wetting and are referred to as collapsing soils. Most of the land available for development east of the Front Range is underlain by expansive clay or claystone bedrock near the surface. Laboratory tests on soil and bedrock samples from this site show low to moderate swell potential. The soils that exhibit collapse are more likely west of the Continental Divide; however, both types of soils occur all over the state. FIELD AND LABORATORY INVESTIGATIONS Subsurface conditions were investigated by drilling six exploratory borings at the approximate locations shown on Figure 1. The test holes were drilled to a depth of 25 feet using a truck-mounted drill rig with 4-inch diameter, continuous- flight augers. Our field representative observed drilling, logged the soils and bedrock found in the borings and obtained samples. Summary logs of the borings, including results of field penetration tests, are presented on Figure 2. Samples recovered from the test holes were returned to our laboratory and visually classified by the geotechnical engineer for this project. Laboratory testing included moisture content, dry unit weight, swell-consolidation, particle- size analysis, and water-soluble sulfates tests. Swell test samples from the site were wetted at a pressure approximating the weight of the overburden soils. Laboratory test results are presented in Appendix A and summarized in Table A- I. 4 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 SUBSURFACE CONDITIONS Soils encountered in our borings consisted of 4½ to 7 feet of sandy clay overlying sand and gravel. Claystone bedrock was encountered in all six borings at depths of 20 to 22½ feet. The overburden clay classified as medium stiff to stiff based on field penetration test results. Swell testing was conducted on four samples; results indicated 0.2 percent compression to 2.2 percent swell. The sand and gravel classified as medium dense to very dense. Gradation testing of two samples indicated fines contents (percent passing No. 200 sieve) of 14 and 25 percent. Based on laboratory testing and our experience, we judge that the sand and gravel soils are non-expansive to low- swelling. Groundwater was encountered in all six borings during drilling at depths of 6 to 10 feet, and was measured several days later at depths of 6 to 7 feet. We recommend a minimum 3-foot separation from foundations and floor slabs to groundwater. Final grading plans should consider groundwater levels for the proposed development. DEVELOPMENT RECOMMENDATIONS Site Grading The properties of fill will affect the performance of foundations, slabs-on- grade, utilities, and pavements. The ground surface in areas to be filled should be stripped of vegetation, scarified, moisture-conditioned, and densely compacted. The on-site soils can be used as site grading fill provided they are substantially free of organics and other deleterious materials. Permanent cut and fill slopes should be no steeper than 3:1 (horizontal to vertical) and be 5 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 seeded or mulched to reduce erosion. Use of flatter (4:1) slopes is preferable. Our experience has shown clay fill moisture treated to optimum moisture content or above will exhibit lower swell than drier material. Clay fill should be moisture-conditioned to between optimum and 3 percent above optimum moisture content and compacted to at least 95 percent of standard Proctor maximum dry density (ASTM D 698). Sand fill should be moisture-conditioned to within 2 percent of optimum moisture content. The placement and compaction of site grading fill should be observed and density tested by our representative during construction. Guideline grading specifications are presented in Appendix B. Permanent slopes should be terraced and re-vegetated to reduce erosion. Utility Construction We believe excavations for utility installation can be performed with heavy-duty trenchers, excavators, or large backhoes. Groundwater will likely be encountered in utility trenches. Bracing and/or temporary dewatering may be required during utility construction. Dewatering may be accomplished by sloping excavations to occasional sumps where water can be removed by pumping. The sumps should be several feet below the bottom of the excavations so that water is pumped down through the soils rather than up through the bottom of the excavations to reduce the potential that the support capacity of the subsoils will be compromised. Utility trenches should be sloped or shored to meet local, state, and federal safety regulations. Excavation slopes specified by OSHA are dependent upon soil types and groundwater conditions encountered. Seepage and groundwater conditions in trenches may downgrade the soil type. Initial trench excavations may be unstable and require flatter slopes than required by OSHA. Contractors should identify the soils encountered in the excavations and refer to 6 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 OSHA standards to determine appropriate slopes. Excavations deeper than 20 feet should be designed by a professional engineer. Water and sewer lines are usually constructed beneath paved roads. Compaction of trench backfill can have a significant effect on the life and serviceability of pavements. Our experience indicates compacted trench backfill may settle 1 to 2 percent. More settlement may occur if wetting of the fill occurs. Trench backfill should be placed in thin loose lifts, moisture-conditioned and compacted as recommended in the Site Grading section of this report. The placement and compaction of backfill should be observed and tested by our firm during construction. Foundations and Floor Systems Low swelling clays were encountered in the upper 4½ to 7 feet of our borings. Spread footing foundations and slab-on-grade basement floors are likely appropriate for all of the lots at this site. If a design level investigation encounters areas of higher swelling clay, shallow over-excavation may be recommended. Basements Groundwater was encountered in all six of our borings at depths of 6 feet and lower. We recommend a minimum 3-foot separation from foundations and basement floors to groundwater. Meeting this separation will limit the depth of basement excavations or require raising grades by several feet. Foundation drains will be recommended around basement walls. Basement walls should be designed to resist lateral earth pressures. 7 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 Preliminary Pavement Thicknesses The subgrade will likely consist of sandy clay classifying as A-6 and A-7-6 according to AASHTO criteria. We have assumed any fill placed at the site will have properties equal to or better than the native materials. We judge that the risk of pavement distress due to swelling soils is considerable over the majority of the site. Swell mitigation will likely be recommended. This may include moisture conditioning and/or chemical stabilization, such as fly ash or lime of the subgrade. Preliminary pavement thickness alternatives are presented in Table 1 below. These thicknesses should be used for planning purposes only. We recommend a subgrade investigation and final pavement design after site grading is complete. TABLE 1 PRELIMINARY PAVEMENT THICKNESS ALTERNATIVES Classification HMA* + ABC* PCC* Local Standard Residential (EDLA = 10) 4"-5.5” HMA + 6”-8” ABC 6”- 7” PCC Residential Collector (EDLA = 50) 5.5”- 6.5" HMA + 6"-10” ABC 6”- 7" PCC *HMA = Hot Mix Asphalt; ABC = Aggregate Base Course; PCC = Portland Cement Concrete Surface Drainage The performance of foundations will be influenced by surface drainage. The ground surface around proposed residences should be shaped to provide runoff of surface water away from the structure and off of pavements. We generally recommend slopes of at least 12 inches in the first 10 feet where 8 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 practical in the landscaping areas surrounding residences. There are practical limitations on achieving these slopes. Irrigation should be minimized to control wetting. Roof downspouts should discharge beyond the limits of backfill. Water should not be allowed to pond on or adjacent to pavements. Proper control of surface runoff is also important to limit the erosion of surface soils. Sheet flow should not be directed over unprotected slopes. Water should not be allowed to pond at the crest of slopes. Permanent slopes should be re-vegetated to reduce erosion. Water can follow poorly compacted fill behind curbs and gutters and in utility trenches. This water can soften fill and undermine the performance of the roadways, flatwork, and foundations. We recommend compactive effort be used in placement of all fill. General Design Considerations Exterior sidewalks and pavements supported above the on-site clays are subject to post construction movement. Flat grades should be avoided to prevent possible ponding, particularly next to buildings. Positive grades away from the buildings should be used for sidewalks and flatwork around the perimeter of the buildings in order to reduce the possibility of lifting of this flatwork, resulting in ponding next to the structures. Joints next to buildings should be thoroughly sealed to prevent the infiltration of surface water. Where concrete pavement is used, joints should also be sealed to reduce the infiltration of water. Since some post construction movement of pavement and flatwork may occur, joints around the buildings should be periodically observed and resealed where necessary. 9 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 RECOMMENDED FUTURE INVESTIGATIONS AND TESTING Based on the results of this investigation and the proposed development, we recommend the following investigations be performed by our firm: 1. Review of site grading plans once the plans are available; 2. Subgrade Investigation and Pavement Design after grading; 3. Design-level Soils and Foundation Investigation for each lot after grading; 4. Construction testing and observation during site development, and building construction, including compaction testing of grading fill, utility trench backfill and pavements, and foundation installation observations. LIMITATIONS Our exploratory borings were located to obtain preliminary subsoil data indicative of conditions on this site. Although our borings were spaced to obtain a reasonably accurate picture of subsurface conditions, variations in the subsoils not indicated in our borings are always possible. We believe this investigation was conducted in a manner consistent with that level of skill and care ordinarily used by members of the profession currently practicing under similar conditions in the locality of this project. No warranty, express or implied, is made. This report was prepared from data developed during our field exploration, laboratory testing, engineering analysis, and experience with similar conditions. The recommendations contained in this report were based upon our understanding of the planned construction. If plans change or differ from the assumptions presented herein, we should be contacted to review our recommendations. 10 GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 If we can be of further service in discussing the contents of this report or in the analysis of the building and pavement from the geotechnical point of view, please call. Very truly yours, CTL | THOMPSON, INC. Brendan P. Moran, EI Spencer Schram, PE Staff Geotechnical Engineer Project Manager BPM:SAS (2 copies) APPENDIX A LABORATORY TEST RESULTS APPENDIX B GUIDELINE SITE GRADING SPECIFICATIONS GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 B-1 GUIDELINE SITE GRADING SPECIFICATIONS 1. DESCRIPTION This item shall consist of the excavation, transportation, placement, and compaction of materials from locations indicated on the plans, or staked by the Engineer, as necessary to achieve preliminary street and overlot elevations. These specifications shall also apply to compaction of excess cut materials that may be placed outside of the development boundaries. 2. GENERAL The Soils Engineer shall be the Owner's representative. The Soils Engineer shall approve fill materials, method of placement, moisture contents and percent compaction, and shall give written approval of the completed fill. 3. CLEARING JOB SITE The Contractor shall remove all vegetation and debris before excavation or fill placement is begun. The Contractor shall dispose of the cleared material to provide the Owner with a clean, neat appearing job site. Cleared material shall not be placed in areas to receive fill or where the material will support structures of any kind. 4. SCARIFYING AREA TO BE FILLED All topsoil and vegetable matter shall be removed from the ground surface upon which fill is to be placed. The surface shall then be plowed or scarified until the surface is free from ruts, hummocks or other uneven features, which would prevent uniform compaction. 5. COMPACTING AREA TO BE FILLED After the foundation for the fill has been cleared and scarified, it shall be disked or bladed until it is free from large clods, brought to the proper moisture content (0 to 3 percent above optimum moisture content for clays and within 2 percent of optimum moisture content for sands) and compacted to not less than 95 percent of maximum dry density as determined in accordance with ASTM D 698. GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 B-2 6. FILL MATERIALS Fill soils shall be free from organics, debris or other deleterious substances, and shall not contain rocks or lumps having a diameter greater than six (6) inches. Fill materials shall be obtained from cut areas shown on the plans or staked in the field by the Engineer. On-site materials classifying as CL, CH, SC, SM, SW, SP, GP, GC, and GM are acceptable. Concrete, asphalt, organic matter and other deleterious materials or debris shall not be used as fill. 7. MOISTURE CONTENT AND DENSITY Fill material shall be moisture conditioned and compacted to the criteria in the table, below. Maximum density and optimum moisture content shall be determined from the appropriate Proctor compaction tests. Sufficient laboratory compaction tests shall be made to determine the optimum moisture content for the various soils encountered in borrow areas. FILL COMPACTION AND MOISTURE REQUIREMENTS Soil Type Depth from Final Grade (feet) Moisture Requirement (% from optimum) Density Requirement Clay 0 to 15 feet 0 to +3 95% of ASTM D 698 Sand -2 to +2 95% of ASTM D 698 Clay Greater than 15 feet -2 to +1 98% of ASTM D 698 Sand -2 to +1 95% of ASTM D 1557 The Contractor may be required to add moisture to the excavation materials in the borrow area if, in the opinion of the Soils Engineer, it is not possible to obtain uniform moisture content by adding water on the fill surface. The Contractor may be required to rake or disk the fill soils to provide uniform moisture content through the soils. The application of water to embankment materials shall be made with any type of watering equipment approved by the Soils Engineer, which will give the desired results. Water jets from the spreader shall not be directed at the embankment with such force that fill materials are washed out. GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 B-3 Should too much water be added to any part of the fill, such that the material is too wet to permit the desired compaction from being obtained, rolling and all work on that section of the fill shall be delayed until the material has been allowed to dry to the required moisture content. The Contractor will be permitted to rework wet material in an approved manner to hasten its drying. 8. COMPACTION OF FILL AREAS Selected fill material shall be placed and mixed in evenly spread layers. After each fill layer has been placed, it shall be uniformly compacted to not less than the specified percentage of maximum density. Fill shall be compacted to the criteria above. At the option of the Soils Engineer, soils classifying as SW, GP, GC, or GM may be compacted to 95 percent of maximum density as determined in accordance with ASTM D 1557 or 70 percent relative density for cohesionless sand soils. Fill materials shall be placed such that the thickness of loose materials does not exceed 12 inches and the compacted lift thickness does not exceed 6 inches. Compaction as specified above shall be obtained by the use of sheepsfoot rollers, multiple-wheel pneumatic-tired rollers, or other equipment approved by the Engineer for soils classifying as CL, CH, or SC. Granular fill shall be compacted using vibratory equipment or other equipment approved by the Soils Engineer. Compaction shall be accomplished while the fill material is at the specified moisture content. Compaction of each layer shall be continuous over the entire area. Compaction equipment shall make sufficient trips to ensure that the required density is obtained. 9. COMPACTION OF SLOPES Fill slopes shall be compacted by means of sheepsfoot rollers or other suitable equipment. Compaction operations shall be continued until slopes are stable, but not too dense for planting, and there is not appreciable amount of loose soils on the slopes. Compaction of slopes may be done progressively in increments of three to five feet (3' to 5') in height or after the fill is brought to its total height. Permanent fill slopes shall not exceed 3:1 (horizontal to vertical). 10. PLACEMENT OF FILL ON NATURAL SLOPES Where natural slopes are steeper than 20 percent in grade and the placement of fill is required, benches shall be cut at the rate of one bench for each 5 feet in height (minimum of two benches). Benches shall be at least 10 feet in width. Larger bench widths may be required by the GREELEY ASSOCIATES, LLC OLD TOWN NORTH BLOCK 6 CTL | T PROJECT NO. FC07353-115 B-4 Engineer. Fill shall be placed on completed benches as outlined within this specification. 11. DENSITY TESTS Field density tests shall be made by the Soils Engineer at locations and depths of his choosing. Where sheepsfoot rollers are used, the soil may be disturbed to a depth of several inches. Density tests shall be taken in compacted material below the disturbed surface. When density tests indicate that the density or moisture content of any layer of fill or portion thereof is not within specification, the particular layer or portion shall be reworked until the required density or moisture content has been achieved. 12. SEASONAL LIMITS No fill material shall be placed, spread or rolled while it is frozen, thawing, or during unfavorable weather conditions. When work is interrupted by heavy precipitation, fill operations shall not be resumed until the Soils Engineer indicates that the moisture content and density of previously placed materials are as specified. 13. NOTICE REGARDING START OF GRADING The Contractor shall submit notification to the Soils Engineer and Owner advising them of the start of grading operations at least three (3) days in advance of the starting date. Notification shall also be submitted at least 3 days in advance of any resumption dates when grading operations have been stopped for any reason other than adverse weather conditions. 14. REPORTING OF FIELD DENSITY TESTS Density tests made by the Soils Engineer, as specified under "Density Tests" above, shall be submitted progressively to the Owner. Dry density, moisture content, and percentage compaction shall be reported for each test taken. 15. DECLARATION REGARDING COMPLETED FILL The Soils Engineer shall provide a written declaration stating that the site was filled with acceptable materials, and was placed in general accordance with the specifications.