Duane Miller & Associates Arctic & Geotechnical · PDF fileDuane Miller & Associates Arctic &...
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May 4, 2006 Tetra Tech/KCM, Inc. 1971 First Avenue Seattle, WA 98101 Attention: Don Beard, P.E. Subject: Geotechnical Investigation Juneau Seawalk - Taku Fisheries to Miner’s Wharf Juneau, Alaska DM&A Job No. 4169.003
This letter presents the results of our geotechnical exploration for the proposed seawalk in Juneau, Alaska, for the City and Borough of Juneau (CBJ). The work was performed in accordance with our proposal to you dated June 2, 2005. The object of our work was to evaluate the soil conditions along the proposed walkway alignment and to develop conclusions and recommendations regarding foundation design. Static and seismic slope stability was specifically excluded from our scope of work. During the work we consulted with Mr. Don Beard, P.E., of Tetra Tech/KMC, Inc., and Mr. Skye Stekoll of CBJ.
The proposed walkway is located along the eastern shore of the Gastineau Channel, at the south end of the community. The seawalk is expected to be a pile-supported structure approximately 16 feet wide and 750 feet long. The walkway will extend north from the Franklin Dock to the Taku Smokeries lot, providing safe waterfront access for both local residents and tourists. The project area is shown on Plates 1. Existing Data
We began this investigation by reviewing existing geotechnical information for this area of Juneau. The following paragraphs summarize the results of previous work.
Duane Miller & Associates Arctic & Geotechnical Engineering 5821 Arctic Boulevard, Suite A Anchorage, AK 99518-1654 (907) 644-3200 Fax 644-0507
Juneau Seawalk – Taku Fisheries to Miner’s Wharf Duane Miller & Associates May 4, 2006 Page 2
R&M Engineering Inc., Tripp Dock Geotechnical Investigation and Pile Driving, 1990. This report contains borehole logs and pile installation records for the design and construction of the Tripp Dock on South Franklin Street. The boreholes logs and pile records show the thickness of glacio-marine deposits on the floor of Gastineau Channel and the depth to bedrock.
R&M Engineering Inc., South Franklin Cruise Dock, Subsurface Exploration 1990. This report follows geotechnical investigation for the intermediate vessel floating dock that is adjacent to the proposed seawalk at the north end of the seawalk project. The drawings, borehole logs, and pile records show the depth below mean lower low tide to mud line and bedrock. The elevations where driven piles encountered refusal are shown on Plate 2. Subsurface Exploration
On May 31 through June 1, 2006, Ms. J.J. Brown, geologist with DM&A, conducted field exploration along the proposed seawalk alignment. A total of four boreholes, B-1 through B-4 (numbered in order of drilling), were drilled to depths of 30.2 to 40.5 feet depending on soil and/or bedrock conditions. The boreholes were drilled using a CME-75 truck-mounted drill supplied and operated by Discovery Drilling. The rig was equipped with 8-inch O.D. hollow stem augers, a 140-pound automatic hammer with a freefall of 30 inches, and a 1.4-inch I.D. split barrel sampler. The automatic hammer results in about 60% of the blows per foot compared to a standard SPT test using a cathead and rope.
The boreholes were logged and sampled during drilling. Soil samples were collected by drive sampling with the split barrel sampler and by grabbing cuttings from the augers . The samples were sealed to prevent loss of moisture and were shipped to our laboratory in Anchorage for further examination and testing. The boreholes were backfilled with cuttings and fill provided by CBJ.
Table 1 - Summary of Boreholes Boring Elevation Total Depth Fill Depth Rock Depth Rock Elev.
B-1 25’ 30’ 25’ 25’ 0’
B-2 23’ 35.5’ 23’ 33’ -10’
B-3 23’ 40.5’ 30’ 40’ -17’
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B-4 26’ 42’ 33’ 40’ -14’
The boreholes are summarized in Table 1 (elevation is based on mean lower low water). Borehole locations are shown on Plate 2 and the borehole logs are presented on Plates 3 through 6. The soil conditions are classified in accordance with the Unified Soil Classification System presented on Plate 7.
In the laboratory, the samples were reexamined to confirm field classifications and to select samples for further testing. Laboratory tests were conducted to determine natural moisture content and grain size distribution. Moisture contents are shown graphically on the logs and are tabulated on Plate 8, the Summary of Samples. The results of particle-size analysis are presented on Plate 9. Photographs of the project area and drill sites are shown on Plates 10 through 12. Soil Conditions
This region is typically underlain by fairly shallow bedrock. At low elevations, the bedrock is blanketed by soils deposited during the last glacial period and more recent deposits of alluvial and colluvial material. In the downtown area of Juneau, steep slopes tend to end abruptly at the waterline. For this reason, near the waterfront, ground for parking and structures are commonly constructed of thick embankments of man-made fill. Structures have been supported on both shallow spread footings and driven piles.
The placement of fill to reclaim land in front of Franklin Street (Taku Smokeries area) is thought to have started between the two World Wars. The material used for fill was taken from the AJ Gold Mine rock dump, which was developed between about 1920 and the end of active mining during World War II. The wasted rock provided a granular fill material of angular, hard cobbles and boulders with a matrix of sand and silty sand. In some areas, however, the granular material is mixed with large amounts of organic soil, debris, trees, and sawdust.
Fill sections south of the Taku Smokeries building and within the Franklin Subdivision are more recent than most in the downtown area. Based on historic
Juneau Seawalk – Taku Fisheries to Miner’s Wharf Duane Miller & Associates May 4, 2006 Page 4
air photos, these embankments were constructed (or at least considerably enlarged) within the last fifteen years.
The boreholes drilled during this investigation revealed three discrete fill section profiles along the proposed seawalk alignment. At the north end near Taku Smokeries the fill is retained by a retaining wall with a steep slope downhill. Boring B-3 at Station 1+40 revealed that the fill is granular and about 30 feet thick, and is underlain by sand to a depth of 40 feet where hard bedrock was encountered. The retaining wall ends at Station 1+60.
Borings B-1 and B-2 are in the central area of the project (approximate Stations 2+80 and 4+70). Both borings encountered thick layers of sawdust and debris fill under a granular fill about 10 feet thick. The sawdust and debris were present to depths of up to 23 to 25 feet. Weathered bedrock was encountered directly beneath the sawdust in Boring B-1, and the drill was able to penetrate nearly 5 feet into the weathered bedrock surface before auger refusal. In Boring B-2, the sawdust was underlain by a 10-foot thick layer of loose, silty sand over relatively competent bedrock.
Boring B-4, at the south end of the project (Station 5+90), did not encounter any sawdust fill. Loose to medium dense granular fill was present to a depth of 33 feet. The fill was oil soaked below 25 feet (about sea level). Medium dense gravel was present between the base of the fill and the bedrock surface at a depth of 40 feet.
Discussion and conclusions
All four of the borings were drilled on the flat surface of the parking lots behind the fill slope. The proposed seawalk will be constructed over the slope. Preliminary design shows the top of boardwalk at an elevation slightly above the parking lots. The boardwalk elevation of 26 to 29 feet (mean lower low water) allows room for framing above high tide under the walkway. The mean higher high water elevation is about 17 feet, and the highest recorded water level is 23.9 feet. The existing fill slopes are steep with an average inclination of about 1.5 horizontal to 1 vertical (1.5 H : 1 V). As shown in the photographs presented on Plates 10 through 12, all of the slopes are covered with slope protection.
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The boardwalk will need to be supported on a pile foundation. A pipe pile can be used provided that slope protection at each pile location is removed before the pile is installed. The piles will develop their vertical downward capacity by end bearing on the bedrock that underlies the site. The soils between the fill and the bedrock were generally thin and not dense enough to result in shallower end bearing. The piles should have a heavy wall (0.5-inch or greater) and should be driven open-ended with internal tip reinforcement.
The depth to bedrock depends on the distance of the boardwalk from the top of slope. At the boring locations, the elevation of bedrock varies from 0 to -17 feet (mllw). Bedrock is exposed on the uphill side of South Franklin Street for the full length of the project area. Bedrock elevations are also interpreted from the piles driven to refusal along the floating dock that is west of the slope. The slope of the bedrock from the borings back to the east side of South Franklin is close to 30% (3 feet of rise per 10 feet of horizontal). Under the area of the boardwalk the bedrock slope is steeper and more variable and appears to drop at a slope of 50% to 70%.
Some uplift resistance will be developed along the skin of the piles in the weak soils that are present over the bedrock. However, the uplift will be very small in those areas underlain by sawdust fill (Borings B-1 and B-2) and should probably be ignored. Tension capacity can be developed by installing an anchor into the rock beneath the depth of end bearing.
Where mineral soil is present, such as at Borings B-3 and B-4, a pile will develop uplift resistance from friction along the length of embedment. For a pile installed on the slope at mean lower low water, the embedment to rock could be as shallow as 10 feet. For that case the uplift resistance for a 16-inch diameter pipe pile is 4000 pounds with a factor of safety of 2. Longer embedments will result in more uplift resistance.
Lateral loads can also be resisted by batter piles which probably is more cost effective than rock anchors. Batter piles have been installed at inclinations as steep as 2 vertical to 1 horizontal. Piles that are in compression and driven to end bearing on bedrock will develop downward resistance equal to their full structural capacity.
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An experienced inspector should be present during the pile installation and should submit pile driving records to us. If soil conditions different than those found during this investigations and past pile installations are encountered, we should be contacted. The inspector should also submit pile-driving records to us. Very truly yours,
Duane L. Miller, P.E. Attachments: Plate 1, Project Area Plate 2, Borehole Locations Map Plate 3 through 6, Logs of Test Holes Plate 7, Soil Classification Chart Plate 8, Summary of Samples Plate 9, Particle Size Data Plates 10 through 12, Site Photographs