Phone: (707) 441-8855 Email: [email protected] Web: shn-engr.com 812 W. Wabash Avenue, Eureka, CA 95501-2138
CIVIL ENGINEERING • ENVIRONMENTAL SERVICES • GEOSCIENCES • PLANNING • SURVEYING Reference: 023202
December 1, 2023
Andrew Leighton, Engineering Project Manager
City of Crescent City
377 J Street
Crescent City, CA 95531
Subject: Geotechnical Evaluation, Foundation Design Recommendations, Proposed Surge Tank Project, South Bank Road, Crescent City, California
This letter report presents SHN’s geotechnical evaluation of subsurface conditions at the location of the proposed surge tank to be located on South Bank Road in order to provide foundation design recommendations. Our scope of services included reviewing the surge tank project plans prepared by AECOM, dated November 2, 2023, and performing a limited geotechnical investigation to assess subsurface soil conditions at the site.
Based on our review of the project plans prepared by AECOM, we understand that the proposed surge tank will be supported on a 13.5-foot-by-13.5-foot foundation that is 2 feet 1 inch thick underlain by 12 inches of aggregate base rock compacted to 95 percent relative compaction.
Our limited subsurface investigation consisted of excavating a shallow hand-dug test pit (TP-1) to a depth of 3.5 feet in the area of the proposed surge tank foundation, and a small excavator-dug test pit (TP-2) to a depth of 7 feet on the other side of the control building from the proposed surge tank. The results of our limited subsurface investigation indicate that the site is blanketed by loose to medium dense silty gravel to a depth between 1.7 and 2.3 feet, then medium dense well-graded sandy cobble gravels to the maximum depth explored.
Based on the results of our limited subsurface investigation, we believe the proposed surge tank base shown on Detail 3/P-5 will provide adequate support for the proposed surge tank. However, we recommend that the 12 inches of aggregate base rock compacted to 95 percent relative compaction below the tank base consist of crushed aggregate base rock; river-run aggregate base rock should not be used.
Provided the tank base is constructed as shown on Detail 3/P-5 and crushed aggregate base is used, the proposed surge tank foundation may be designed for the following allowable bearing pressures:
Dead plus long-term live load 2,500 pounds per square foot (psf) All loads, including wind and seismic 3,750 psf
The new footing excavations should be maintained in a moist condition prior to pouring concrete to avoid soil shrinkage.
City of Crescent City
Geotechnical Evaluation, Foundation Recommendations, Proposed Surge Tank Project, South Bank Road, Crescent City, California
December 1, 2023
P:\Eureka\2023\023202-SurgeTankGEO\PUBS\rpts\20231201-SurgeTankGeotech.docx Provided the surge tank foundation is constructed in accordance with these recommendations, we estimate that total post-construction settlement will be 0.5 inch or less under static conditions; the differential settlement will be about half of the total settlement.
The resistance to lateral loadings may be calculated using a friction factor of 0.45 between the bottom of the footing and the engineered crushed aggregate base fill soils. Where the footings are poured neat and the adjacent ground surface is paved or covered with concrete slabs, a passive resistance of 300 pounds per cubic foot (pcf) equivalent fluid weight may be developed between the footing and the adjacent soil.
Where the adjacent ground surface is not paved, the upper 1 foot should be neglected in determining the available passive resistance.
We trust this letter report provides the information you require at this time. Please call me at (707) 459- 4518 if you have any questions.
Thank you for the opportunity to be of service to the City of Crescent City for this project.
Respectfully,
SHN
John H. Dailey, PE, GE
Senior Geotechnical Engineer
Appendix 1: Test Pit Logs
JHD:ame
12/1/23
Test Pit Logs 1
GB
0 to
2'
GB
2.5 to
3 41.3
39.5 100
200
GM
GW
(GM) SILTY GRAVEL, medium dense to loose, moist, brownish gray, subrounded to subangular gravels to 3 inches maximum, low to medium plasticity, low cohesion, well graded gravel. (GW) WELL GRADED SANDY COBBLE GRAVEL, medium dense, moist, brownish gray, subrounded and subangular clasts to 9 inches maximum, fine to coarse sand, non plastic, non cohesive.
No free groundwater observed.
Note: Part of hand excavation was in backfilled utility trench. Backfill material was: (FILL) POORLY GRADED SAND / GRAVEL, loose, damp, brownish gray, subangular 1/4 inch clean sand / very fine gravel, exposed to depth of 2 feet BGS, then moved over and continued excavation. Total depth of trench backfill not observed.
Refusal at 3.5 feet.
Bottom of test pit at 3.5 feet.
1.7 ft
3.5 ft
NOTES Excavation advanced with hand tools
GROUND ELEVATION 43 ft
LOGGED BY A. Call
EXCAVATION METHOD Hand Sample
EXCAVATION CONTRACTOR Crescent City
CHECKED BY
DATE STARTED 11/16/23 COMPLETED 11/16/23 TEST PIT SIZE
AFTER EXCAVATION ---
GROUNDWATER DEPTH
AT END OF EXCAVATION ---
AT TIME OF EXCAVATION --- DEPTH (ft)
0.0 2.5 SAMPLE TYPE NUMBER
PAGE 1 OF 1
TEST PIT NUMBER TP-1
PROJECT NAME Surge Tank Geo
PROJECT LOCATION South Bank Rd, Smith River
CLIENT Crescent City
PROJECT NUMBER 023202 GENERAL BH/TP EUREKA COPY - GINT STD US.GDT - 11/17/23 14:36 - \\EUREKA\GEOGROUP\GINT\LIBRARY\BENTLEY\GINTCL\PROJECTS\PROJECT_FILES\2023\023202-SURGETANKGEO.GPJ RECOVERY % U.S.C.S. MATERIAL DESCRIPTION GRAPHIC LOG ST
0.5-1
GB
3-3.5
40.7 39.0
36.0 100
(100)
171
4 GM
GM
GW
(GM) SILTY GRAVEL/ GRAVELLY SILT, soft / loose, damp, dark brown, subrounded gravels to 3 inch maximum, slightly plastic, low cohesion. (GM) WELL GRADED COBBLE GRAVEL WITH SILT, medium dense, damp, grayish brown, subrounded to subangular cobbles and gravels to 10 inches maximum, non plastic, non cohesive.
(GW) WELL GRADED SANDY COBBLE GRAVEL, medium dense, brownish gray, moist, subangular to subrounded cobbles and gravels to 10 inch maximum, non plastic, non cohesive, caving from 3 feet BGS to bottom of excavation.
Becomes wet at 7 feet BGS.
Caving at 7 feet preventing excavation from getting deeper. HALT.
Refusal at 7.0 feet.
Bottom of test pit at 7.0 feet.
2.3 ft
4.0 ft
7.0 ft
NOTES
GROUND ELEVATION 43 ft
LOGGED BY A. Call
EXCAVATION METHOD Mini-excavator
EXCAVATION CONTRACTOR Crescent City
CHECKED BY
DATE STARTED 11/16/23 COMPLETED 11/16/23 TEST PIT SIZE
AFTER EXCAVATION ---
GROUNDWATER DEPTH
AT END OF EXCAVATION ---
AT TIME OF EXCAVATION --- DEPTH (ft)
0.0 2.5
5.0 SAMPLE TYPE NUMBER
PAGE 1 OF 1
TEST PIT NUMBER TP-2
PROJECT NAME Surge Tank Geo
PROJECT LOCATION South Bank Rd, Smith River
CLIENT Crescent City
PROJECT NUMBER 023202 GENERAL BH/TP EUREKA COPY - GINT STD US.GDT - 11/17/23 14:48 - \\EUREKA\GEOGROUP\GINT\LIBRARY\BENTLEY\GINTCL\PROJECTS\PROJECT_FILES\2023\023202-SURGETANKGEO.GPJ RECOVERY % (RQD) BLOW COUNTS (N VALUE) U.S.C.S. MATERIAL DESCRIPTION GRAPHIC LOG