Att5b_Addendum_to_the_Geotechnical_Investigation_Report_12-23-20.pdf
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This addendum provides additional geotechnical recommendations for the proposed construction on the alluvial delta at Phantom Ranch for the Trans-Canyon Water Distribution Pipeline Replacement Project at Grand Canyon National Park. It supersedes any conflicting information in the original July 2018 geotechnical investigation report. The addendum confirms that spread footings can be used rather than mat foundations to reduce concrete needs. It specifies minimum width and dimension requirements for continuous, isolated, and column spread footings. The allowable bearing capacity of 3,000 psf and differential settlement of 1 inch total and 0.5 inches differential are confirmed to be applicable for the delta site based on the sandy soil with cobbles and boulders observed. The minimum burial depth of 18 inches below grade for frost protection at elevations between 3,000 to 4,500 feet is also deemed acceptable, providing flexibility while still protecting infrastructure.
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2750 S. Wadsworth Blvd, Suite D-200 Denver, Colorado 80227
303.625.9502 www.LithosEng.com
December 23, 2020
Project No. 16066-1
HDR Engineering, Inc.
419 Canyon Avenue
Fort Collins, CO 80521
Attention: Mr. Rich Thornton, PE
Project Manager
Regarding: Addendum to Geotechnical Investigation Report
Trans-Canyon Water Distribution Pipeline Replacement Project
GRCA 190083
Grand Canyon National Park, Arizona
Mr. Thornton, Subsequent to the issuance of Lithos’ Geotechnical Investigation Report, dated July 6, 2018, HDR
Engineering requested further geotechnical recommendations regarding the proposed construction on the alluvial delta at Phantom Ranch. Following are the questions from HDR and Lithos’ responses.
Recommendations provided in this letter supersede any conflicting information the 2018 geotechnical report.
1. Can you provide the IBC / ASCE-7 seismic site class at the delta? There doesn’t appear to be seismic tests at that site in particular, and given the gravels, cobbles, and boulders at the delta, I don’t want to just assume that shear wave velocities would be the same as at Phantom Ranch.
Response: IBC/ASCE 7 Site Class A, Hard Rock is appropriate for this location.
2. We are designing for spread footings at the Phantom Ranch site rather than mat foundations to save concrete quantity.
Can you confirm this is acceptable, and provide:
a. Minimum width of continuous spread footings.
Response: 18-inch minimum width should be used. If this is infeasible for the location of the work, 12-inches could be used. Lithos requests to review drawings before they are finalized if 12-inches is used.
b. Minimum dimension for isolated (column) spread footings.
Response: 24-inch minimum square width should be used. If this is infeasible for the location of the work, 18-inches square could be used. Lithos requests to review drawings before they are finalized if 18-inches is used.
c. Confirm that 3,000 psf allowable bearing is acceptable for spread footings at the delta.
Response: 3,000 psf is acceptable.
HDR Engineering Trans-Canyon Water Distribution Pipeline Replacement Project Page 2 of 2
d. Confirm that 1 IN / 0.5 IN total / differential settlement is applicable for spread footings at the delta.
Response: This amount is applicable. Based on what we observed there, the soil appears to be mostly sand with cobbles and boulders. We do not anticipate above typical amounts of settlement here.
3. Coconino County has various frost depth requirements by elevation. For elevations between 3000 and 4500 FT, the minimum frost depth line is 12” below grade. The report recommends 24” and I wanted to see if you would object to 18” below grade? (I’d like save on some of the concrete we need to place down at that site.)
Response: 18-inches minimum burial is acceptable.
If you have any questions regarding the contents of this addendum, please contact the undersigned.
Sincerely, Lithos Engineering
Nate Soule PE
Vice President
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