GM_40-15_Skagit_River_Bridge__WA_NPS_NOCA_20(1)_Final_Geotechnical_Recommendations_9Dec2015.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

Date: December 7, 2015 In Reply Refer To: HFL-17

From: Diann Morehouse, Geotechnical Engineer

To: Betty Chon, Project Manager

Copies: Matthew Hinshaw, Design Benn Oltmann, Bridge

Geotechnical Memorandum GM 40-15

Subject: Final Geotechnical Recommendations Skagit River Bridge

WA NPS NOCA 20(1)

Introduction

This memorandum summarizes observations made during a site visit and provides geotechnical recommendations for the proposed Skagit River Bridge Repair project in North Cascade National Park, Skagit County, Washington.

Project Description

The project is located approximately 1 mile west of Newhalem, Washington along State Route (SR) 20 about 60 miles driving distance east of Burlington. A Google aerial photo shows the project location (Figure 1). This bridge crosses the Skagit River and provides access to the NPS visitor center and campground. The bridge is orientated in a north/south direction with the Skagit River flowing generally from the east to the west at this location. The proposed project involves mitigating subsidence and pavement distress occurring within the south bridge approach.

Skagit River Bridge Repairs

GM40-15 Page 2 of 3

Geology

Surficial geology is Quaternary-aged alluvium consisting of unconsolidated or semi-consolidated alluvial clay, silt, sand, gravel, and/or cobble deposit. The Quaternary–aged alluvium overlies Tertiary-Cretaceous-age weakly to strongly foliated orthogneiss. (Dragovich, 2002)

Site Conditions

A site visit was conducted November 10, 2015. Pavement subsidence and transverse cracking was observed within the south bridge approach extending about 20 feet from the end of the bridge (Pictures 1 and 2). The bobtail cantilever concrete box girder bridge cantilevers the bridge deck to meet the approach fill. A gabion retaining wall with 18-inch high and 18-inch deep gabion baskets was constructed under the bridge deck to retain the abutment fill. As-built drawings for the gabion wall are not available, so the bottom of the wall elevation is unknown.

Four layers of gabion baskets are visible. Viewed from under the bridge three layers are visible, the bottom layer is mostly buried and underlies the next two visible gabion basket layers (Picture 3). A fourth gabion layer is visible from the west (downstream) side of the approach fill (Picture 4).

Gabion layers two and three, extend to about the same vertical plane as the east (upstream) outside edge of the bridge endwall; however, these two layers stop about 3 feet short from the west side outer edge of the bridge end wall (Picture 5). A void is present above the west end of the third gabion layer. The fourth layer, a non-standard gabion basket, is offset from the lower basket layer about 1.5 feet towards centerline. A pile of over-steepened, loose, silty sand with gravel and cobbles is present in the approach fill where the gabions were not constructed (Picture 6). It is interpreted that water entering through the pavement cracks is migrating through the abutment fill resulting in soil piping and loss of pavement structure support.

The north approach also shows evidence of pavement subsidence within 2 feet from the bridge end (Picture 7). At the time of the field review, repair of the north approach was not included in the scope of work.

Recommendations

A deep patch is recommended to mitigate the pavement subsidence. Subexcavate the deep patch site approximately 16 feet wide and a depth of 27 inches measured vertically below subgrade elevation, sloping the bottom of the excavation towards the underdrain. Deep patch reinforcement consists of three layers of Class 1 Type A stabilization geotextile with a 9-inch vertical spacing.

The geotextile is lapped back 3 feet at the slope face (north) and along each side (east and west) of the deep patch. Ensure a close contact with the backfill, eliminating gaps and wrinkles in the geotextile. Back fill the deep patch with select borrow. Place a thin layer of gravel between the lap and the overlying geotextile layer to prevent direct contact between adjacent geotextile layers.

Construct the face of the deep patch to the vertical plane of the bridge terminus. Construct the sides of the deep patch to the vertical plane of the curb outside edges. If the existing gabion baskets extend beyond the end of the bridge, leave the gabions in place and form the deep patch face tightly against the gabion baskets. Use solid corrugated drainage tubing to outlet the underdrain system on one side of the approach embankment.

GM40-15 Page 3 of 3

Closing

This memorandum provides final recommendations for the repair of pavement subsidence at Skagit River Bridge repair project. Please contact Diann Morehouse with any questions regarding these recommendations at Diann.Morehouse@dot.gov or (360) 619-7609.

References

Dragovich ,J., et al.(2002) Geologic Map of Washington Northwest Quadrant, Division of Geology and Earth Resources, Geologic Map GM-50, Sheet 1.

Attachments

Figure 1: Google Earth aerial photo of project area.

Picture 1: Facing north across the bridge towards SR 20 Picture 2: Facing north at the south approach.

Picture 3: Facing south under the bridge looking at the south abutment Picture 4: Facing west under the bridge at the south abutment.

Picture 5: Facing east from under the west side of the south abutment.

Picture 6: North approach appears to be a similar issue.

Figure 2: Section Views Figure 3: Section View A-A’ Figure 4: Section View B-B’ mailto:Diann.Morehouse@dot.gov

Skagit River Bridge Repair

Geotechnical Memorandum GM40-15 December 2015

ATTACHMENTS

Geotechnical Memorandum GM 40-15 Page A-1

Figure 1: Google Earth aerial photo of Newhalem, Washington and proposed Skagit River bridge repair location.

Picture 1: Facing north across the bridge towards SR 20. (11/10/2015)

Geotechnical Memorandum GM 40-15 Page A-2

Picture 2: Facing north at the south approach. (11/10/2015)

Picture 3: Facing south under the bridge looking at the south abutment. (8/14/2014)

Sprayed foam on water pipe for frost protection

End of gabion basket

Geotechnical Memorandum GM 40-15 Page A-3

Picture 4: Facing east from under the west side of the south abutment. (11/10/2015)

Picture 5: Arrow shows the bridge endwall. (11/3/2015)

Geotechnical Memorandum GM 40-15 Page A-4

Picture 6: Facing west under the bridge at the south abutment. (11/10/2015)

Picture 7: North approach appears to be a similar issue. (11/10/2015)

Over-steepened loose silty sand with gravel

Geotechnical Memorandum GM 40-15 Page A-5

Assumed Stationing: 10+22.5

A A’

B

B’

Figure 3: Section View Overview

N

Diann.Morehouse Typewritten Text

Diann.Morehouse Typewritten Text

Diann.Morehouse Typewritten Text

Diann.Morehouse Typewritten Text Page A-6

Geotechnical Memorandum GM 40-15 Page A-7

Picture 1: Facing north across the bridge towards SR 20. (11/10/2015)

Figure 4: Section View B-B’