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HomeMy WebLinkAboutGeotechnical Letter for 625 Boardwalk - September 2023Civil Engineering ● Geotechnical Engineering ● Land Surveying ● Construction Services ALLIED ENGINEERING 32 Discovery Dr. Bozeman, MT 59718 Ph: (406) 582-0221 www.alliedengineering.com September 25, 2023 Greg Matthews Studio, Inc. Attn: Greg Matthews 620 East Cottonwood Street, Suite 103 Bozeman, MT 59715 Email: greg@gregmatthewsstudio.com RE: Geotechnical Letter for 625 Boardwalk Avenue Bozeman, Montana Dear Mr. Matthews: This letter summarizes our geotechnical evaluation for the proposed commercial development at 625 Boardwalk Avenue, Bozeman, Montana. Presented herein is a description of the site’s soil and groundwater conditions and our geotechnical analysis and recommendations for foundation support and drainage. We understand the intent of the project is to construct a new architectural studio on a shallow basement (e.g., “garden-level basement”). DESCRIPTION OF PROPERTY The project site is located at 625 Boardwalk Avenue in the Ferguson Farm Subdivision on the west side of Bozeman. The legal description of the property is Lot 2 of Ferguson Farm Subdivision Block 3, Plat J-563, located in a portion of Section 10, Township 2 South, Range 5 East, Principal Meridian Montana, Gallatin County, Montana. The property is bound to the east by Boardwalk Avenue, to the south by Lot 3, to the west by Lot 4, and to the north by Lot 1. Lot 1 is also currently undeveloped, but Lots 3 and 4 are developed with multistory commercial buildings and parking areas. The topography at 625 Boardwalk Avenue (Lot 2) is flat. At the time of our investigations, the west half of the lot had already been paved as part of a large parking area shared by Lots 1-4, while the east half of the lot had already been stripped of topsoil and overlying fine-grain soils down to native gravels leaving a shallow “pond” about three feet deep. GEOLOGY OF THE SITE According to the geology map prepared by Lonn and English in 2002 for the eastern part of the Gallatin Valley (Figure 1), the project site is underlain by older alluvium of braid plains (Qabo). These deposits are described as rounded to well-rounded, dominantly cobble gravel with clasts as large as boulders, as well as sand, silt, and clay. Our test pit findings were consistent with the geologic mapping. SUBSURFACE EXPLORATIONS Subsurface conditions were investigated on December 9, 2022, by Mike Molloy, a staff engineering geologist with Allied Engineering Services, Inc. One test pit exploration, identified as TP-1, was excavated on the lot in the center of the unpaved east half using a tracked mini excavator provided by Elevation Greg Matthews Studio Geotechnical Letter for 625 Boardwalk Ave, Bozeman, MT September 25, 2023 AESI Project No: 22-172 32 Discovery Drive. Bozeman, Montana 59718. ● Ph: (406) 582-0221 ● Fax: (406) 582-5770 Page 2 Excavation. The test pit extended down to 7.0 feet. Please see Figure 2 for the approximate test pit location. During the exploration, soil and groundwater conditions were visually characterized, measured, and logged. The relative densities of the exposed soil profiles were estimated based on the ease or difficulty of digging and overall stability of the completed excavation. A copy of our test pit log is attached. This log provides assorted field information such as soil depths and descriptions, groundwater conditions, relative density data, and a sketch of the soil stratigraphy. Please be aware that the detail provided in the log cannot be accurately summarized in a paragraph; therefore, it is important to review the log in conjunction with this report. Following completion of the fieldwork, the test pit location was backfilled and cleaned up to the extent possible. The test pit was staked with a wooden lath that identified it accordingly. We anticipate that the test pit will underlie future site improvements and should be completely re-excavated and backfilled in properly compacted lifts to avoid undesirable settlements. SUBSURFACE CONDITIONS Provided below is a description of the soil conditions encountered. For a more detailed description of the on-site soil conditions, please refer to the attached test pit log. At the time of our exploration, the site was already stripped of topsoil and any surficial fine-grained deposits, with the ground surface starting approximately 3 feet below the original native existing grade. The soil conditions exposed in our test pit consisted of native alluvial gravel consisting of dense, brown, sandy fine to coarse (well graded) gravel with scattered rounded cobbles up to 10 inches in diameter. The alluvial gravels extended to the bottom of the test pit at a depth of 7.0 feet, becoming more gravelly and less sandy with depth. The test pit depth was limited by shallow groundwater as discussed below. Target bearing for the proposed commercial building is within the native alluvial gravels. Groundwater Conditions Soils were moist, becoming wet at a depth of 4.5 feet with groundwater observed beginning at 5.0 feet. We installed a groundwater monitor in TP-1 and continued to monitor groundwater levels throughout the 2023 groundwater season. A summary of these records is provided in Table 1 below. The high-water level was recorded as 3.2 feet below ground surface at the test pit location (on May 17 and June 5, 2023). Table 1: Groundwater Monitoring Data Summary Date Water Depth BGS (feet) 12/9/2022 (date of test pit excavation) 5.0 4/4/2023 (beginning of 2023 monitoring season) 5.25 5/2/2023 3.4 5/17/2023 3.2 (peak) 5/25/2023 4.1 6/5/2023 3.2 (peak) 7/6/2023 3.6 Greg Matthews Studio Geotechnical Letter for 625 Boardwalk Ave, Bozeman, MT September 25, 2023 AESI Project No: 22-172 32 Discovery Drive. Bozeman, Montana 59718. ● Ph: (406) 582-0221 ● Fax: (406) 582-5770 Page 3 GEOTECHNICAL RECOMMENDATIONS Foundation Support Design We understand that the proposed commercial building will include a half-basement (e.g., a “garden level basement”) with perimeter footings/walls and interior spread footings. We suggest bearing footings for the structure directly in the native alluvial gravels or structural fill being on the native gravels. The native subgrade soil should be proof-rolled and/or vibratory compacted to a dense, unyielding condition prior to the placement of structural fill or footings. Exterior footings should bear a minimum of 4.0 feet below the adjacent exterior finished grade for frost protection. Based on our 2023 groundwater monitoring data, we suggest that a lower basement slab bear no deeper than the current bottom of the excavated “pond” (about three feet below the adjacent parking lot elevation). This would place groundwater levels during a typical year about three feet below the slab and still provide some buffer in the event of an abnormally wet year where groundwater levels rise further, which is always a possibility. Anything below that, and some thought would need to be given to waterproofing. The City of Bozeman does not allow permanent groundwater dewatering systems. However, we still suggest a sump and pump within the basement to protect against the possibility of an unusually high groundwater season (higher than that documented in our 2023 monitoring) to prevent moisture problems or basement flooding. An appropriate bearing pressure for the design assuming conventional spread and continuous footings is 3,000 pounds per square foot (psf). We estimate the total settlements to be under 0.75 inches with differential settlements of approximately half that amount. Based on our on-site explorations and knowledge of the underlying geology, the seismic site class for this project should be Site Class D (as per criteria presented in the 2012 IBC). Granular structural fill under footings should consist of either 3-inch-minus sandy pit run or 1.5-inch-minus crushed road mix may be used. We recommend that fill materials be sourced and placed in accordance with the Montana Public Works Standard Specifications. We suggest the placement of six inches of 1-inch minus clean crushed rock and a 15-mil vapor barrier under interior slabs. The vapor barrier should be placed over the clean crushed rock and directly under the slab. Lateral Earth Pressures All foundation walls that will be fixed at the top prior to the placement of backfill should be designed for an “at rest” equivalent fluid pressure of 55 pounds per cubic foot (pcf). In contrast, cantilevered retaining walls may be designed for a lower, “active” equivalent fluid pressure of 40 pcf, provided either some slight outward rotation of the wall is acceptable upon backfilling or the wall is constructed in such a way that accommodates the expected rotation. These “at rest” and “active” design values are only applicable for walls that will have backfill slopes of less than ten percent and which will not be externally loaded by surface pressures applied above and/or behind the wall. If there will be any slopes greater than 10 percent Greg Matthews Studio Geotechnical Letter for 625 Boardwalk Ave, Bozeman, MT September 25, 2023 AESI Project No: 22-172 32 Discovery Drive. Bozeman, Montana 59718. ● Ph: (406) 582-0221 ● Fax: (406) 582-5770 Page 4 within 10 feet of the wall or significant surface loads within 10 feet of the wall, we should be consulted to provide appropriate lateral earth pressures for design. These lateral earth pressures also assume proper subsurface drainage provisions are installed to prevent the development of hydrostatic pressures. Lateral forces from wind, earthquakes, and earth pressures on the opposite side of the structure will be resisted by passive earth pressure against the buried portion of the foundation wall and by friction at the bottom of the footing. Passive earth pressures in compacted backfill should be assumed to have an equivalent fluid pressure of 280 pcf; while a coefficient of friction of 0.4 should be used between cast-in- place concrete and the native soils or granular structural fill. Actual footing loads (not factored or allowable loads) should be used for calculating frictional resistance to sliding along the base of the footing. Please be aware that the friction coefficient has no built-in factor of safety; therefore, an appropriate safety factor should be selected and used in all subsequent calculations for each load case. Foundation Wall Backfill Exterior wall backfill can consist of any excavated foundation soil, other than topsoil, if it is not overly moist, highly plastic, or too rocky in composition. All select backfill materials should be placed in multiple lifts and properly compacted to 95 percent of its ASTM D-698 (Standard Proctor) density. To prevent damaging foundation walls during the backfilling process, only hand-operated, compaction equipment is recommended within 3.0 feet of walls that are not buried on both sides. The level of care (with respect to the selection of dry backfill materials and the compactive effort that is used) should be increased significantly in those areas along the foundation wall that will either receive concrete/asphalt surfacing or that will support a retaining wall to minimize the potential for future settlement problems. Finally, the re- use of topsoil as backfill should be limited to the uppermost 4 to 6 inches in landscape areas. Subsurface Drainage and Moisture Protection Recommendations While high groundwater levels during a typical year appear to be about three feet lower than the current excavated “pond” elevation, we still suggest the installation of a sump and pump in the half basement in the event of a very wet year. The sump and pump should be connected to a network of 3-inch perforated drainpipe installed within the crushed drainage rock under the slab to capture and pump out groundwater. We also suggest the installation of a footing drain (encased in crushed drainage rock and wrapped in a non-woven geotextile fabric) around the perimeter of the building and tied to the sump and pump. As discussed earlier, a 15-mil vapor barrier (Stego vapor barrier or equivalent) should be installed directly under the concrete slab. The vapor barrier should be properly attached to footings/walls and sealed at the seams as per manufacturer recommendations. Surface Drainage Recommendations No surface water should be allowed to accumulate against or flow along the exposed foundation walls. In landscape areas, adjacent soils must be graded to drain away from the foundation at a minimum slope of 5 percent for a horizontal distance of at least 10 feet. In hardscape areas, the minimum grade away from the building is 2 percent. Greg Matthews Studio Geotechnical Letter for 625 Boardwalk Ave, Bozeman, MT September 25, 2023 AESI Project No: 22-172 32 Discovery Drive. Bozeman, Montana 59718. ● Ph: (406) 582-0221 ● Fax: (406) 582-5770 Page 6 REFERENCES 1. Lonn, J.D., and English, A.F., 2002. “Preliminary geologic map of the eastern part of the Gallatin Valley, Montana,” Montana Bureau of Mines and Geology Open-File Reports MBMG- 457, Scale 1:50,000. P:\2022\22-172 625 Boardwalk Geotechnical Explorations and Letter\05 Design\Geotech\Letter\Geotechnical Letter for 625 Boardwalk - September 2023.docx © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS FIGURECivil Engineering Geotechnical Engineering Land Surveying 32 DISCOVERY DRIVE . BOZEMAN, MT 59718 PHONE (406) 582-0221 . FAX (406) 582-5770 www.alliedengineering.com 625 BOARDWALK AVE VICINITY MAP BOZEMAN, MONTANA 1 N © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS FIGURECivil Engineering Geotechnical Engineering Land Surveying 32 DISCOVERY DRIVE . BOZEMAN, MT 59718 PHONE (406) 582-0221 . FAX (406) 582-5770 www.alliedengineering.com 625 BOARDWALK AVE QUADRANGLE MAP BOZEMAN, MONTANA 2 N © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS FIGURECivil Engineering Geotechnical Engineering Land Surveying 32 DISCOVERY DRIVE . BOZEMAN, MT 59718 PHONE (406) 582-0221 . FAX (406) 582-5770 www.alliedengineering.com 625 BOARDWALK AVE TEST PIT LOCATION MAP BOZEMAN, MONTANA 3 TP#N TP-1 © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS FIGURECivil Engineering Geotechnical Engineering Land Surveying 32 DISCOVERY DRIVE . BOZEMAN, MT 59718 PHONE (406) 582-0221 . FAX (406) 582-5770 www.alliedengineering.com 625 BOARDWALK AVE GEOLOGY MAP BOZEMAN, MONTANA 4 N © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS FIGURECivil Engineering Geotechnical Engineering Land Surveying 32 DISCOVERY DRIVE . BOZEMAN, MT 59718 PHONE (406) 582-0221 . FAX (406) 582-5770 www.alliedengineering.com 625 BOARDWALK AVE GROUNDWATER MAP BOZEMAN, MONTANA 5 N MOISTURE CONTENT DETERMINATION (ASTM D-2216) Project: 625 Boardwalk Ave Project Number: 22-172 Sample Identification: Varies Soil Classification: Sandy GRAVEL Date Sampled: 12/9/2022 Date Tested: 12/12/2022 Tested By: MJM Sample Identification: S1-A S1-B S1-C Exploration Location: TP-1 TP-1 TP-1 Sample Depth (ft): 1.5 5.0 7.0 Container Number: DD RR MM Weight of Container: 30.85 31.32 31.06 Container + Wet Soil: 151.91 185.47 201.31 Container + Dry Soil: 145.95 171.39 189.22 Weight of Water:5.96 14.08 12.09 Weight of Dry Soil: 115.10 140.07 158.16 Moisture Content:5.2%10.1%7.6% Sample Identification: Exploration Location: Sample Depth (ft): Container Number: Weight of Container: Container + Wet Soil: Container + Dry Soil: Weight of Water: Weight of Dry Soil: Moisture Content: Reviewed By: 32 Discovery Drive Bozeman, MT 59718 Phone (406) 582-0221 Fax (406) 582-5770    LIMITATIONS OF YOUR GEOTECHNICAL REPORT      GEOTECHNICAL REPORTS ARE PROJECT AND CLIENT SPECIFIC      Geotechnical investigations, analyses, and recommendations are project and client specific.  Each project  and each client have individual criterion for risk, purpose, and cost of evaluation that are considered in  the development of scope of geotechnical investigations, analyses and recommendations.  For example,  slight changes to building types or use may alter the applicability of a particular foundation type, as can a  particular client’s aversion or acceptance of risk.  Also, additional risk is often created by scope‐of service  limitations imposed by the client and a report prepared for a particular client (say a construction  contractor) may not be applicable or adequate for another client (say an architect, owner, or developer  for example), and vice‐versa.  No one should apply a geotechnical report for any purpose other than that  originally contemplated without first conferring with the consulting geotechnical engineer.  Geotechnical  reports should be made available to contractors and professionals for information on factual data only  and not as a warranty of subsurface conditions, such as those interpreted in the exploration logs and  discussed in the report.      GEOTECHNICAL CONDITIONS CAN CHANGE      Geotechnical conditions may be affected as a result of natural processes or human activity.  Geotechnical  reports are based on conditions that existed at the time of subsurface exploration.  Construction  operations such as cuts, fills, or drains in the vicinity of the site and natural events such as floods,  earthquakes, or groundwater fluctuations may affect subsurface conditions and, thus, the continuing  adequacy of a geotechnical report.       GEOTECHNICAL ENGINEERING IS NOT AN EXACT SCIENCE       The site exploration and sampling process interprets subsurface conditions using drill action, soil sampling,  resistance to excavation, and other subjective observations at discrete points on the surface and in the  subsurface.  The data is then interpreted by the engineer, who applies professional judgment to render  an opinion about over‐all subsurface conditions.  Actual conditions in areas not sampled or observed may  differ from those predicted in your report.  Retaining your consultant to advise you during the design  process, review plans and specifications, and then to observe subsurface construction operations can  minimize the risks associated with the uncertainties associated with such interpretations.  The conclusions  described in your geotechnical report are preliminary because they must be based on the assumption that  conditions revealed through selective exploration and sampling are indicative of actual      Allied Engineering Services, Inc. ● 32 Discovery Drive.  Bozeman, Montana 59718 ● Ph: (406) 582‐0221 Page 2 conditions throughout a site.  A more complete view of subsurface conditions is often revealed during  earthwork; therefore, you should retain your consultant to observe earthwork to confirm conditions  and/or to provide revised recommendations if necessary.  Allied Engineering cannot assume responsibility  or liability for the adequacy of the report’s recommendations if another party is retained to observe  construction.      EXPLORATIONS LOGS SHOULD NOT BE SEPARATED FROM THE REPORT      Final explorations logs developed by the consultant are based upon interpretation of field logs (assembled  by site personnel), field test results, and laboratory and/or office evaluation of field samples and data.   Only final exploration logs and data are customarily included in geotechnical reports.  These final logs  should not be redrawn for inclusion in Architectural or other design drawings, because drafters may  commit errors or omissions in the transfer process.     To reduce the likelihood of exploration log misinterpretation, contractors should be given ready access to  the complete geotechnical report and should be advised of its limitations and purpose.  While a contractor  may gain important knowledge from a report prepared for another party, the contractor should discuss  the report with Allied Engineering and perform the additional or alternative work believed necessary to  obtain the data specifically appropriate for construction cost estimating purposes.        OWNERSHIP OF RISK AND STANDARD OF CARE      Because geotechnical engineering is much less exact than other design disciplines, there is more risk  associated with geotechnical parameters than with most other design issues.  Given the hidden and  variable character of natural soils and geologic hazards, this risk is impossible to eliminate with any  amount of study and exploration.  Appropriate geotechnical exploration, analysis, and recommendations  can identify and reduce these risks.  However, assuming an appropriate geotechnical evaluation, the  remaining risk of unknown soil conditions and other geo‐hazards typically belongs to the owner of a  project unless specifically transferred to another party such as a contractor, insurance company, or  engineer.  The geotechnical engineer’s duty is to provide professional services in accordance with their  stated scope and consistent with the standard of practice at the present time and in the subject geographic  area.  It is not to provide insurance against geo‐hazards or unanticipated soil conditions.        The conclusions and recommendations expressed in this report are opinions based our professional  judgment and the project parameters as relayed by the client.  The conclusions and recommendations  assume that site conditions are not substantially different than those exposed by the explorations.  If  during construction, subsurface conditions different from those encountered in the explorations are  observed or appear to be present, Allied Engineering should be advised at once such that we may review  those conditions and reconsider our recommendations where necessary.      RETENTION OF SOIL SAMPLES      Allied Engineering will typically retain soil samples for one month after issuing the geotechnical report.  If  you would like to hold the samples for a longer period of time, you should make specific arrangements to  have the samples held longer or arrange to take charge of the samples yourself.