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HomeMy WebLinkAboutGEOTECH 2020-08-13 Terracon Consultants, Inc. 21905 64 th Ave. W, Suite 100 Mountlake Terrace, WA 98043 P (425) 771 3304 F (425) 771 3549 terracon.com LETTER HEAD August 13, 2020 U-Haul / AMERCO Real Estate Company 2727 N Central Avenue #9N Phoenix, Arizona 85004 Attn: Mr. Andrew T. Saltz P: (509) 308-5574 E: andrew_saltz@UHaul.com Re: Review of Updated Site Layout and Previous Geotechnical Recommendations Port Townsend U-Haul Facility 14082 Airport Cutoff Road Port Townsend, Washington Terracon Project No. 81135003 Mr. Saltz: This letter summarizes our review of the current proposed layout of the U-Haul facility at the intersection of Airport Cutoff Road and Highway 20. We previously conducted a geotechnical investigation of the site in early 2013 and prepared geotechnical recommendations for design and construction of the site layout being considered at that time; however, that design effort was halted and construction did not occur. The layout considered seven years ago differs from the current site layout; however, we find that the explorations advanced during that study are relevant to the current layout and the recommendations we developed in 2013 are valid for design and construction of the current layout. It is the purpose of this letter to revisit that previous geotechnical report and determine whether the current design is consistent with our previous recommendations. We have included a drawing with this letter that illustrates the positions of the 2013 explorations and the proposed 2020 layout. Previous Geotechnical Investigation and Previous Site Layout We presented geotechnical exploration and design recommendations for the project site in our report to you dated March 5, and 2013, plus a subsequent report memorandum on September 5, 2013. As part of our previous investigation of the site, we advanced 15 test pits into the subsurface at locations distributed throughout the project footprint. We also completed 4 pilot infiltration tests. Except in western portions of the site where soft soils had apparently been stripped during original site grading, soil conditions generally consisted of between 1 to 3 feet of loose or soft fine sand and silt (with organics) overlying a sequence of medium dense to dense sands and gravels and interbedded hard silt/clay layers. Based upon these conditions, we recommended stripping of Review of Updated Site Layout and Previous Geotechnical Recommendations Port Townsend U-Haul ■ Port Townsend, Washington August 13, 2020 ■ Terracon Project No. 81135003 Responsive ■ Resourceful ■ Reliable loose surficial layers to allow conventional spread footings to bear on competent soils at gre ater depth. Previously, proposed site development included ten storage buildings, approximately 28,100 square feet of covered RV/boat parking beneath canopies, paved surfaces across much of the site, stormwater infiltration, and landscape buffers. The existing building would be retained and remodeled. The previous site layout reportedly included considerable earthwork to provide building pads; we understand that the current site layout represents a moderate reduction in earthwork. Previously proposed grading was not available to us for review. Current Site Layout The current site layout includes a total of 8 new storage buildings, with six buildings - Buildings A, B, C, D, E, and F - situated across the northern portion of the site. Two buildings - Buildings G and J - would occupy the central area of the south half of the site. In the southeast portion of the site, fill up to approximately 7 feet deep would be placed to create a gravel-surfaced equipment parking and vehicle storage area. The building layout, gravel parking/storage, and other site features are shown on the drawing that accompanies this letter. The current site layout appears to be situated appropriately to accommodate existing grades and reduce the amount of earthwork needed for building pad construction. Current proposed grading cuts appear to largely accommodate our previous recommendation to lower grades where thicker accumulations of soft/loose soils are expected; these cuts are advised in order to situate proposed buildings and pavement on competent native soil. Cutting occurs predominantly in the northern portion of the site (where most buildings will be situated); filling occurs mostly in the southeastern portion of the site, where a gravel-surfaced storage area will be constructed (in the previous plan, covered storage was located here). Proposed site plans show that up to several feet of soil will be removed to create a level building surface below existing grades across much of the northern portion of the site , with the deepest cut (approximately 5 feet) occurring in the northeast corner. Buildings B, C, and D would be single- story structures founded in native soils below the existing grade in this area. Building A would be “split-level”, with the eastern half having a finish ground floor elevation approximately 10 feet higher than the ground floor of the western half; the building floor would descend westward to meet the existing downward slope to the west. A basement wall would be constructed to retain soil for this daylighted western portion of the building. Buildings E and F would include minor cutting below existing grades, and minor filling may be necessary along their southern halves to create a level building pad. Buildings G and J would be constructed upon fill of variable depth, with fill from between up to about 5 to 6 feet deep. Portions of these building footprints would be in areas of grade lowering. Review of Updated Site Layout and Previous Geotechnical Recommendations Port Townsend U-Haul ■ Port Townsend, Washington August 13, 2020 ■ Terracon Project No. 81135003 Responsive ■ Resourceful ■ Reliable The gravel pad in the southeast part of the site will consist of up to 7 feet of fill, with the fill perimeter inclined at about 2H:1V. Current site layout includes bioretention cells south of the gravel parking and Building J. During field exploration in 2013, Terracon conducted pilot infiltration tests at four locations across the site, including one pilot infiltration test (designated “FIT-2” in the 2013 exploration program) a short distance to the north of the current proposed bioretention cells. The results of the infiltration tests indicated steady-state short-term infiltration rates ranging from 2 and 5 inches per hour, as presented in the 2013 report. These tests were reportedly conducted at depths of one foot below existing grades; we do not know how 2013 grades differ from the bottom of proposed bioretention cells. Conclusions and Recommendations The current proposed layout is broadly consistent with the previous geotechnical site design recommendations we presented in the March 2013 report and the September 2013 memorandum. Recommendations presented previously for the support of shallow building foundations, concrete slab-on-grade floors, basement retaining walls, earthwork, and infiltration are valid for the current design. We understand that the permitting process for this project was suspended several years ago, and that permitting will resume according to building codes that were current when the permitting process was initiated. As such, we understand that the 2009 version of the International Building Code is being retained as the standard for structural permitting, including determination of seismic ground motions for design, and seismic site classification. Previously conducted infiltration tests do not coincide with the layout of current stormwater bioretention cells. However, these test results can still be considered in stormwater design, provided that appropriate rate factors are applied according to the 2019 edition of the Department of Ecology’s Stormwater Management Manual for Western Washington. Based upon subsurface variability encountered during exploration, the 2013 report recommended long-term infiltration rate of 0.3 inches per hour. This rate reduction between short-term and long-term appears to be appropriate, given site variability. We re-iterate our recommendation that Terracon be retained for geotechnical review of drawings and specifications prior to construction. We also recommend that we be retained for construction- phase observations and documentation of the condition of footing and pavement subgrades, the placement of fill, backfill placement behind basement walls and footings, and other geotechnical construction-phase services discussed in our previous report and memorandum. Closing Review of Updated Site Layout and Previous Geotechnical Recommendations Port Townsend U-Haul ■ Port Townsend, Washington August 13, 2020 ■ Terracon Project No. 81135003 Responsive ■ Resourceful ■ Reliable The discussion presented in this letter is based upon our understanding of the previous and current site development layouts, plus review of previous explorations and our geotechnical recommendations. This letter nor our previous reporting does not reflect variations that may occur between explorations, across the site, or due to the modifying effects of construction or weather. The nature and extent of such variations may not become evident until during or after construction. If variations appear, we should be immediately notified so that further evaluation and supplemental recommendations can be provided. The scope of services for this review does not include either specifically or by implication any environmental or biological (such as mold, fungi, and bacteria) assessment of the site, nor any identification or prevention of pollutants, hazardous materials or conditions. If the owner is concerned about the potential for such contamination or pollution, other studies should be undertaken. This review letter and our previous reporting have been prepared for the exclusive use of our client, for specific application to the proposed project site, and in accordance with generally accepted geotechnical engineering practices. No warranties, either express or implied, are intended or made. Site safety, excavation support, and dewatering requirements are the responsibility of others. In the event that changes in the nature, design, or location of the project as outlined in this letter and previous reporting are planned, the conclusions and recommendations contained in these documents shall not be considered valid unless Terracon reviews the changes and either verifies or modifies these conclusions of in writing. Sincerely, Terracon Consultants, Inc. Chad T. McMullen, P. E. David A. Baska, P.E., Ph.D. Geotechnical Project Engineer Principal Attachment: 2020 Site Layout with 2013 Explorations Overlay (next page) Review of Updated Site Layout and Previous Geotechnical Recommendations Port Townsend U-Haul ■ Port Townsend, Washington August 13, 2020 ■ Terracon Project No. 81135003 Responsive ■ Resourceful ■ Reliable 2020 Site Layout with 2013 Explorations Overlay North