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HomeMy WebLinkAbout012-Geotechnical Addendum Letter for Bob Wards Addition May 2023Civil Engineering ● Geotechnical Engineering ● Land Surveying ● Construction Services ALLIED ENGINEERING 32 Discovery Dr. Bozeman, MT 59718 Ph: (406) 582-0221 www.alliedengineering.com May 9, 2023 Woodbury Corporation 2733 East Parleys Way, Suite 300 Salt Lake City, UT 84109-1662 Attention: Davin McNeil Email: d_mcNeil@woodburycorp.com RE: Geotechnical Addendum Letter - Proposed Addition to Bob Wards Bozeman, Montana To Whom It May Concern: Per our signed contract dated April 27, 2023, this letter is our addendum to the Geotechnical Report for Gallatin Center (which Bob Wards is part of) dated October 10, 1999. The original geotechnical report is attached to this letter. Where needed, we have provided additional thoughts/recommendations and modifications to the original report within this letter. We understand the intent is to construct an addition to the west side of Bob Wards. The addition will measure just under 5,000 square feet. We anticipate a slab-on-grade construction with perimeter frost walls/footings and interior spread footings directly under the slab. The structural connection to the existing store is unknown. However, we anticipate the continuation of the existing east-west running perimeter walls/footings. We are unsure if an additional wall/footing is proposed running north-south directly adjacent to the west wall of the existing building. Some thoughts regarding the connection points are provided below. Previous Geotechnical Work The nearest geotechnical exploration to the addition completed back in 1999 (TP-3) encountered about two feet of random fill overlying silty clay topsoil and medium stiff, brown clay. Native dense alluvial gravel suitable for foundation bearing was found at about 4.5 feet. The test pit was dry to the bottom of the exploration at 5.5 feet. Note that we found groundwater in several other explorations completed in 1999 with the shallowest at about 5 to 6 feet. These levels may reach 3 to 4 feet during the run-off in the spring/early summer. Depending on the time of year, some groundwater dewatering may be required to install the structural fill under the addition (discussed below) and/or the installation of the re-routed water, sewer, and storm sewer facilities. Geotechnical Addendum Letter - Proposed Addition to Bob Wards Bozeman, Montana Project Number: 23-050 May 9, 2023 Bozeman Office: 32 Discovery Drive. Bozeman, Montana 59718. ● Ph: (406) 582-0221 ● Fax: (406) 582-5770 Page 2 Additional Foundation Recommendations In our original geotechnical report, we had recommended the removal of all overlying random fill, topsoil, and clay under footings. Suitable foundation bearing was either directly in the native gravels or structural fill bearing on the native gravels. While we were not involved in the construction inspection of the original Bob Wards building, we understand the adjacent Target was mass over-excavated within the footprint of the store down to the native gravels and backfilled with compacted granular structural fill. In addition, the nearby Ross store (which we did inspect) was mass excavated to the native gravels and backfilled with granular structural fill. While we do not have any specific information regarding Bob Wards, we suspect this building was also mass excavated to the native gravels. At a minimum, the existing footings should sit either directly in the native gravels or on compacted structural fill bearing on the native gravels. Given the small size of the addition and the likelihood of interior spread footings directly under the slab, we recommend the mass excavation of all random fill, old wall backfill (from the original construction) topsoil, and lean clay down to the native gravels. The excavation should be backfilled in thin lifts back up to footing grade with granular structural fill compacted to 98 percent of its Standard Proctor density based on ASTM D-698. Note in the 1999 report, we had recommended compaction to 97 percent of the Standard Proctor density. Design foundation bearing pressures provided in the original report are still suitable. The excavation should extend a minimum of three feet outside the outside edge of perimeter footings. The connection to the existing footings is perhaps the most complex portion of the design/construction of the addition. Based on our nearby exploration (TP-3), we anticipate the native gravels are found below the elevation of the existing footings and structural fill was used in the original construction to bring the subgrade up from the native gravels to the design level of the footings. Therefore, if the addition is mass- excavated, we may risk undermining the existing footings along the west wall of the store. This may be especially problematic if an additional footing line will be run north-south directly adjacent the existing west wall/footing. Generally, we suggest maintaining a 1H:1V excavation line from the edge of existing footings down to the native gravels to limit the potential for undermining. Structural connections may be made to the existing foundation via thickened walls and grade beams designed to span this gap. If the new footings lines are to be constructed within this excavation zone, we suggest underpinning of the existing west wall footing to limit the potential for undermining. This may be done via closely properly designed helical piles, grouted micropiles, etc. Note that the underpinning should be close enough to limit soil spillage from between the underpinned locations. Any soil loss from under existing footings between underpinned locations runs the risk of impacting the performance of the interior slab of the existing store. Further, we suggest spacing the underpinning locations sufficiently close to one another so that the existing footings can span between them even if there is a loss of soil support from under the foundation. Granular Structural Fill One modification from the original report are the granular structural fill types that may be used. Two available options include: Geotechnical Addendum Letter - Proposed Addition to Bob Wards Bozeman, Montana Project Number: 23-050 May 9, 2023 Bozeman Office: 32 Discovery Drive. Bozeman, Montana 59718. ● Ph: (406) 582-0221 ● Fax: (406) 582-5770 Page 3 • Sandy (Pitrun) Gravel - Sandy (pitrun) gravel is a granular structural fill option for placement under and/or behind footings, slabs, and walls. This material shall be a non-plastic, well-graded mixture of clean sand and gravel with 100 percent of its fragments passing a 4-inch screen and less than 10 percent of its particles (by weight) finer than the No. 200 sieve. In addition to these material and gradation recommendations, it should meet all other applicable specifications as presented in Section 02234 of the Montana Public Works Standard Specifications (MPWSS) for uncrushed sub-base course gravel. • Crushed (Road Mix) Gravel - Crushed (road mix) gravel is another granular structural fill option for placement under and/or behind footings, slabs, and walls. This material shall be a non-plastic, well-graded mixture of clean sand and gravel that is processed (crushed) such that 100 percent of its fragments pass a 1-1/2-inch screen and less than 10 percent of its particles (by weight) are finer than the No. 200 sieve. It should also meet all other specifications as presented in Section 02235 of the MPWSS for crushed base course gravel. Seismic Design Factors Based on our on-site explorations and knowledge of the area’s geology, the project site class is Site Class D as found in the 2021 IBC. Note that this is not the Default Site Class D. To obtain site-specific seismic loading and response spectrum parameters, a web-based application from the USGS Earthquake Hazards Program can be used. The link to their web page is as follows: https://earthquake.usgs.gov/hazards/designmaps/. This web page has links to three third-party interfaces that can be used to obtain seismic information. The user needs to enter the design code reference document, site soil classification, risk category, and site latitude and longitude. Subsurface Drainage Assuming the interior slab elevation is higher than the final exterior grade at all locations, and a slab-on- grade foundation with perimeters walls and footings is designed, subsurface drains may be eliminated. Pavement Section Recommendations Two modifications for the design pavement section are provided below depending on the application. Note that these options differ slightly from the options provided in the original report. Table 1: Pavement Section 1 - Light-Duty Streets and Parking Areas MATERIAL COMPACTED THICKNESS (IN) Asphalt 3 (placed in 1 lift) Base Course Gravel 6 Sub-Base Course Gravel 15 TOTAL SECTION DESIGN THICKNESS 24 Geotechnical Addendum Letter - Proposed Addition to Bob Wards Bozeman, Montana Project Number: 23-050 May 9, 2023 Bozeman Office: 32 Discovery Drive. Bozeman, Montana 59718. ● Ph: (406) 582-0221 ● Fax: (406) 582-5770 Page 4 Table 2: Pavement Section 2 - Heavy Truck Traffic Areas MATERIAL COMPACTED THICKNESS (IN) Asphalt 4 (placed in 2 lifts) Base Course Gravel 6 Sub-Base Course Gravel 18 TOTAL SECTION DESIGN THICKNESS 28 The sub-base and base course materials that comprise the granular parts of the pavement section shall consist of 4-inch minus uncrushed sandy (pit run) gravel and 1-1/2-inch minus crushed (road mix) gravel, respectively. Both gravel courses shall meet the material and gradation specifications presented in MPWSS, Sections 02234 and 02235. Under normal circumstances, the gravel products should be placed in lifts not exceeding 10 inches in thickness (depending on the size of the compactor) and compacted to at least 95 percent of the maximum dry density as defined in ASTM D-698. If widespread unstable subgrade conditions are encountered, we ask that we be retained to view the subgrade. At that point, we may consider increasing the subbase section or adding a woven geotextile fabric (Mirafi 600X or equivalent) for subgrade stabilization. If highly unstable subgrade conditions are encountered, we may consider adding a geogrid (Tensar TX-190L) for subgrade improvement. Both pavement sections listed above assume that the subgrade is stable and shows no signs of rutting or deflection prior to placing the subbase gravel. AESI FUTURE INVOLVEMENT We suggest that we be retained during the design and construction of the addition to Bob Wards to assure that the recommendations provided herein are followed properly. We further suggest that we be allowed to view the foundation excavation to ensure that the appropriate target bearing materials have been reached prior to the placement of structural fill. LIMITATIONS This report provides our geotechnical recommendations for the proposed addition to Bob Wards located in Bozeman, Montana. These recommendations are based on our previous observation and evaluation of the site’s surface and subsurface conditions back in 1999, our review and interpretation of available geologic information, and our previous geotechnical engineering experience in the project area. If during earthwork construction, soil and groundwater conditions are found to be inconsistent with those described in the original report or this addendum, we should be advised immediately such that we can analyze the situation and modify our recommendations if need be. Please be advised that the original report and addendum are only applicable for the above-referenced project and shall not be used for any other nearby sites. Since geotechnical-related conditions can change in a short distance, we recommend that all properties be evaluated on a site-specific basis. All individuals associated with this project should consult this report during the planning, design, and construction of the site improvements. It should be made available to other parties for information on factual data only and not as a warranty of actual subsurface conditions such as those interpreted herein. Geotechnical Addendum Letter - Proposed Addition to Bob Wards Bozeman, Montana Project Number: 23-050 May 9, 2023 Bozeman Office: 32 Discovery Drive. Bozeman, Montana 59718. ● Ph: (406) 582-0221 ● Fax: (406) 582-5770 Page 5 We appreciate the opportunity to perform our geotechnical services. Please call if you have questions. Sincerely, Allied Engineering Services, Inc. Craig R. Madson, PE President / Principal Geotechnical Engineer enc: 1999 Geotechnical Report for Gallatin Center Limitations of Your Geotechnical Report qa/qc: ES P:\2023\23-050 Bob Wards Addition\05 Design\Geotech\Geotechnical Addendum Letter for Bob Wards Addition May 2023.docx    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.