GM29-15_Ivars_Bridge_Prelim_Foundation_Recommendations.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

Date: August 6, 2015 In Reply Refer To: HFL-17

From: Brian Collins, P.E.

Senior Geotechnical Engineer

To: Tom Lonergan, Project Manager

Copies: Adam Fisher, Bridge Engineer Jack Doucey, Roadway Designer Carolyn Sourek, Hydraulics Engineer Megan Chatfield, Materials Engineer Robert Kraig, Geotechnical Engineer Malcolm Ulrich, Geotechnical Manager

Subject: Preliminary Foundation Recommendations

Ivar’s Bridge

AK IRR FORT 1002(1)

Geotechnical Memorandum No. 29-15

Introduction This Memorandum provides preliminary foundation recommendations for Ivar’s Bridge and discusses the primary geotechnical risks that have been identified for this project.

Ivar’s Bridge is also known as the Sucker River Bridge. The bridge replaces a functionally obsolete crossing composed of a series of culverts capped with aggregate bedding and a series of concrete slabs. The crossing provides access primarily to subsistence activities for the residents of the village of Fort Yukon as well as connectivity to other communities.

The preliminary design for Ivar’s Bridge consists of a 150-foot long steel plate girder superstructure supported on a geosynthetic reinforced soil – integrated bridge system (GRS- IBS) at each abutment.

The primary geotechnical risks that have been identified at this site include increased risk of post-construction settlement due to the presence of permafrost at the site and increased risk of problems dewatering during construction due to the presence of groundwater in free-draining gravel layers within the limits of proposed excavation. Our understanding of these risks is discussed in more detail below.

Site Investigations In 2007 three geotechnical borings were drilled by Shannon & Wilson as part of a Geotechnical Study for replacement of Ivar’s Bridge (Carroll, 2007). The borings are in relatively close proximity to the current proposed bridge location: Boring 07-1was drilled on the west side of the river near proposed Abutment 1 and Boring 07-3 was drilled on the east side of the river near proposed Abutment 2. Boring 07-2 was also drilled on the east side of the river bank but was located about 100 feet from the river’s edge.

The borings encountered between 2.5 to 8.0 feet of sandy silt and silty sand overlying layers of sandy gravel, gravelly sand, sand, and sandy silt. Permafrost was encountered in Boring 07-2 and Boring 07-1 at depths below 6.5 and 25 feet, respectively. Permafrost was not encountered in Boring 07-3. Groundwater was encountered in Boring 07-1 and Boring 07-3 at depths of 7 feet and 0.5 feet, respectively. The borings penetrated to depths of 40 feet.

Four test pits were conducted by WFLHD at the locations of the proposed GRS-IBS abutments on June 20, 2015. Test pits TP-1 and TP-2 were near proposed Abutment 1 and TP-3 and TP-4 were near proposed Abutment 2. A locally available Case 580 loader/excavator was used to dig the test pits. The excavator encountered refusal in frozen ground at depths of 4 and 5 feet in TP-2 and TP-1, respectively. TP-3 and TP-4 were terminated at depths of 6 and 7 feet, respectively, because the gravel encountered below the water table continuously caved during excavation.

The test pits encountered organic silt, silt, and sandy silt layers to depths of 3 to 6.5 feet overlying gravel, sand with gravel, and gravel with sand that extended to the termination depths of the test pits of 4 to 7 feet. The elevation of the contact between the silty layer and the underlying sand and gravel layers was approximately 986.5 near proposed Abutment 1 and about 983.5 near proposed Abutment 2.

Frozen ground was encountered in TP-1 and TP-2 below a depth of 2 feet. The frozen ground extended to the termination depth of the test pits which was determined by excavator refusal at depths of 4 to 5 feet. This layer is within the active frost zone so it may only be seasonal frost or possibly be indicative of underlying permafrost. In TP-3 and TP-4 frozen ground was encountered below a depth of 1 foot and the excavator penetrated through the frozen silt and sandy silt layer at depths of 4.5 to 6.5 feet and penetrated into an unfrozen gravel layer.

Groundwater was encountered in the gravel layer below the frozen layers in TP-3 and TP-4.

The groundwater filled the test pit holes to approximately the elevation of the Sucker River which was stagnant and estimated to be between elevation 987 and 988 feet at the time of the test pit investigations.

Permafrost Permafrost was encountered in Boring 07-1 (near proposed Abutment 1, west side of river) below 25 feet. Permafrost was not encountered in Boring 07-3 (near proposed Abutment 2, east side of river) above the bottom of the boring at 41 feet below ground surface. A report published by the USGS (Ball et al, 2011) indicates most of Fort Yukon and the surrounding area is underlain by permafrost to a depth greater than 300 feet; unfrozen ground (taliks) is generally found below lakes and rivers. Through-going taliks (indicating the entire depth of permafrost is thawed) only appear to occur below larger lakes and rivers. Interpretation based on those studies suggests that Boring 07-3 would probably have encountered permafrost if it had been drilled deeper.

Permafrost may contain layers of pure ice. If those layers thaw it results in significant amounts of ground settlement. Thaw of permafrost, especially ice-rich permafrost, has damaged much infrastructure across Alaska. Permafrost along the proposed alignment will begin to thaw near the ground surface after removal of vegetation.

The subsurface investigation conducted at this site was limited and the borings that were advanced were not at the exact locations of the currently proposed abutments. Our interpretation of the subsurface conditions is that they are generally consistent across the site, comprising of an upper layer of 4 to 6 feet of silt overlying layers of sand and gravel with a low percentage of fines (material passing the No. 200 sieve). Permafrost is known to exist at moderate depth near Abutment 1 (west) and may be present near the proposed Abutment 2 (east). However, layers of ice were not encountered in the borings near those locations.

Foundation Consolidation and Settlement The proposed GRS-IBS abutments will bear in the sand and gravel layers. Our interpretation is that these layers should generally be thaw-stable and not experience significant thaw consolidation. Minor amounts of consolidation will likely go unnoticed because the abutment (GRS-IBS substructure) and bridge superstructure will settle together. Minor amounts of settlement can occur without damaging the bridge. As a precaution, it is recommended to detail the abutments so that the superstructure can be jacked back into position to correct differential settlements.

An alternative to using the GRS-IBS system and eliminate the settlement potential would be to utilize a pile foundation system. The pile foundation system would need to be installed an adequate depth into the permafrost layer to provide enough skin resistance to mitigate frost jacking of the piles. Predrilling the piling would be required. Use of pile foundations would require mobilization of additional materials and equipment and would add substantial costs to the project. This alternative can be considered if the risk associated with thaw consolidation of the permafrost is not acceptable; however, additional risks would need to be addressed if a pile foundation alternative is chosen.

Groundwater Groundwater was encountered in the sand and gravel layers that the GRS-IBS foundation will bear in. The sand and gravel layers have a low percentage of fines (material passing the No.

200 sieve) and therefore have a relatively high permeability if not frozen. The groundwater table is expected to fluctuate with the level of the Sucker River. At the time of the test pit investigations on June 20, 2015, the Sucker River was estimated to be at an elevation between 987 and 988 feet, and the recommended scour elevation is 981 feet (Sourek, 2015).

The high permeability of the sand and gravel layers presents an increased risk of problems dewatering excavations during construction. Depending on the level of the Sucker River at the time of construction, it may be necessary to excavate several feet below the groundwater table.

When the soils are thawed, water will flow freely into excavations extending below the groundwater table. In addition, the sand and gravel soils will cave when excavated below the water table. It is also possible that heaving conditions may be encountered in the excavations.

Shoring and dewatering should be designed with these considerations in mind. A cofferdam and layer of seal concrete may be needed to dewater the excavation. Excavation of the foundations at a time when the ground is frozen may be a viable alternative to prevent groundwater infiltration, but larger and more powerful equipment will be necessary to excavate frozen soils.

Preliminary GRS-IBS Design Recommendations The bottom of the GRS-IBS foundations should bear at or below the design scour elevation of 981 feet. This elevation is below the maximum depth of silt and sandy silt layers encountered in the test pits at the proposed abutment locations.

The preliminary bearing resistance at the top surface (bottom of concrete abutment slab elevation) of the GRS-IBS abutments is as follows:

Factored bearing resistance, service limit state = 4000 psf Factored bearing resistance, strength limit state = 6000 psf

Laboratory testing on the proposed GRS-IBS backfill has not been completed. Upon completion final bearing resistance values will be provided.

References Ball, L.B., Smith, B.D., Minsley, B.J., Abraham, J.D., Voss, C.I., Astley, B.N., Deszcz-Pan, M., and Cannia, J. (2011) Airborne Electromagnetic and Magnetic Geophysical Survey Data of the Yukon Flats and Fort Wainwright Areas, Central Alaska, June 2010, USGS Open-File Report 2011-1304.

Carroll, N.M. (2007) Final Geotechnical Study, Proposed Sucker River Bridge, Fort Yukon Alaska, submitted to PDC Inc. by Shannon & Wilson, Inc., Fairbanks, Alaska.

Sourek, C. (2015 July 20) Final Bridge Scour Analysis Summary