North abutment settlement mitigation-TM.pdf

Memorandum

1100 112th Ave. NE, Suite 500

Bellevue, WA 98004

United States

T +1.425.453.5000 www.jacobs.com

North abutment settlement mitigation-TM 1

Subject North Abutment Settlement Mitigation

Project Name North Cascades Visitor Center, Skagit River Bridge

Attention John Davis

From Karen Dawson

Date February 10, 2021

Copies to Jim Mills, Bud Alto

1. Introduction

The purpose of this memorandum is to document site visit observations of both approach embankments of the North Cascades National Park Visitor’s Center Skagit River Bridge and provide recommendations for improvement as part of a previously-planned pavement restoration project. The work was authorized by a modification to an agreement between FHWA Western Federal Lands Division and Jacobs Engineering Group Inc. DTFH7015D00004L.

Pavement at the norther approach to the bridge is approximately 2.5 inches below the bridge deck. This is a 1.5-inch increase from the 1-inch settlement noted in the September 1, 2018 bridge inspection report (FHWA, 2018). In response to the increased settlement and accompanying pavement distress, Jacobs was asked to conduct a site visit and develop mitigation recommendations and design documents, and to evaluate the conditions on the south abutment, where settlement correction had been completed in 2017, but where bridge inspections have noted corrosion on wall facing elements.

1.1 Limitations

This technical memorandum has been prepared for the exclusive use of Western Federal Lands for specific application to settlement mitigation of the National Park Service Newhalem Visitor’s Center, Skagit River Bridge in accordance with generally accepted geotechnical engineering practice. No other warranty, express or implied, is made.

The recommendations are based on observations and measurements from a single site visit, without the benefit of complete bridge as-built drawings. Subsurface explorations were not conducted and assumptions – documented in this memorandum – were made about subsurface conditions. Field modifications to reflect unearthed bridge conditions should be anticipated. If conditions differing from those described in this memorandum are encountered during construction, the interpretations and recommendations provided herein must be reevaluated.

North Abutment Settlement Mitigation

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2. Bridge Structure

The existing 1958 one-lane concrete box girder bridge is supported on four piers – two in the river and two above ordinary high water. The bridge approach spans cantilever from Piers 1 and 4, so that the earthen approach embankments do not support the concrete structure. The bridge piers are supported on spread footings. It is not known if the footings are supported on soil or rock.

Multiple bridge drains are located across the span at both the upstream (east) and downstream (west) curb lines. These drains are open 4-inch diameter holes that discharge through the deck just outside of the box girders. There are no downspouts; drainage discharges into the river and onto the ground from a height ranging from about 10 to 25 feet. The effects of bridge drain discharge may be increased because debris builds up along the edges of the roadway on the bridge, temporarily blocking drainage and allowing water to pond on the deck. When the water level gets high enough, debris is forced through the drain holes and the resulting flow is a heavy spray of water rather than a continuous but smaller drip.

A 2-inch cast-iron water line held within an insulated corrugated metal pipe is mounted to the outside of the upstream bridge curb. Four conduits are mounted on the outside of the sidewalk on the downstream side of the bridge.

A plan, profile, and typical section from the original bridge construction are provided in Appendix A.

In 2017, bridge rehabilitation was performed which included a “deep patch” in the southern abutment and epoxy overlay of the deck. The deep patch is labeled in the 2017 drawings as a geogrid wall over existing gabion baskets. This notation is believed to be in error or is misleading terminology because the material visible below the bridge girder is soil behind hardware cloth and a welded wire face, which is most likely to be a mechanically stabilized earth wall rather than gabion baskets. The age of the MSE wall is unknown but is not shown in the 1958 drawings.

3. Site Visit Observations

Karen Dawson, a geotechnical engineer from the Puget Sound area, visited the site and observed both abutments on February 1, 2021.

3.1 North Abutment Observations

Key observations at the north abutment are as follows:

1. The pavement has settled 2.5 inches below the bridge end (photo 1)

2. The stability of the embankment fill at the bridge ends is controlled by erosion of the slope in front of (river side of) the end of the bridge box girders and an end wall constructed for a short cantilever section of the sidewalk and curb. Relative to “undisturbed” soil beneath the center of the box girder, erosion of the slope behind Pier 1 is about 3.5 feet deep at the downstream edge of the girder and as much a 7.5 feet at the downstream drip line. Approximately 6.5 feet of soil has been lost on the downslope side of Pier 1 (Sketch 1). Pier 1 has rotated toward the river, making the bearing appear to have rotated in the opposite direction (Photo 2); it is unclear whether this rotation occurred during construction backfill as active earth pressured developed behind the pier wall or if it is a result of additional rotation due to uneven loss of soil between the upslope and downslope sides. Photo 3 also shows erosion around Pier 1.

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3. Soil beneath the bridge end wall has raveled to expose the bottom of the end wall and girder. As incrementally more erosion occurs, soil from behind the end wall moves downward resulting in settlement at the road surface. Photos 4 shows the exposed bottom of the end wall.

4. The sidewalk on the downstream side and the curb on the upstream side of the bridge cantilever approximately 30 inches past the end wall. The cantilevered end of the sidewalk can be seen in Photo 1 and a gap that has developed under the cantilever is shown in Photo 5.

5. Contributors to slope erosion appear to be both oversteepening of the slope due to loss of material into the river at the toe and erosion from runoff which is concentrated into point sources by the bridge drains. Photo 6 shows how the drains penetrate the deck on the sidewalk side of the bridge; erosion can be seen in the splash zone below the drains.

6. Excavation to mitigate settlement will need to work around the existing conduits and water line as they transition from buried to bridge-supported. Photo 7 shows how the conduits exit the slope at the end of the sidewalk cantilever. The water line exits in a similar manner, but has the added complication of entering an insulated corrugated metal pipe.

7. Rounded rock at the river’s edge ranges from gravel to 5 feet in maximum dimension. Angular riprap is sporadically present in the 1- to 4-foot size range. As is visible in Photo 3 and Photo 8, the sand and finer material has been eroded from the ground surface below Pier 1 on the downstream side except for a small ridge of soil not exposed to rainfall, bridge drain runoff, or raveling from erosion at the river’s edge. The remaining surface material is primarily rounded 6 to 12-inch rock with the occasional larger sizes. In Photo 9, looking upstream, the rock sizes are larger and erosion has not occurred.

8. Soil near the roadway surface at the abutment appears to be primarily sand and gravel with fines in the range between 5 and 20 percent and approximately 5 percent cobbles by volume. It is difficult to predict the material composition below the surface, but it is likely to be similar.

3.2 South Abutment Observations

Observations from the south abutment can be summarized as follows:

1. Settlement of the pavement at the bridge end is less than ½-inch.

1. Differing from the north approach, curb and gutter contains roadway runoff and routes it to a culvert that exits on the abutment slope (Photo 10). The slope below the culvert outlet appears to be stable.

2. As shown in Photos 10 and 11, the silt fence from the 2017 paving improvements is still in place.

Although the runoff area is small, this fence tends to concentrate and channel surface runoff below the bridge where vegetation cannot stabilize the slope; it appears to be contributing to minor amounts of erosion.

3. The welded wire facing of the MSE wall below the 2017 deep patch shows some signs of corrosion (Photo 12). The maximum vertical exposure is 39 inches. We do not have as-builts for or know the age of this wall

4. While there is perhaps 0.5 to 1-foot of visible soil loss near the abutment and extending down to Pier 4, the slope down to the river is more heavily vegetated and not as heavily eroded as the

4 North abutment settlement mitigation-TM north abutment. The edge of the water line insulation in Photo 13 gives some indication of the amount of material loss that has occurred since the 2017 rehabilitation project.

4. Recommendations

4.1 North Abutment Recommendations

There are options for repairs that can be undertaken, each with increasing cost but increased longevity:

1. Grind and Overlay - Fill the pavement gap at the bridge end by paving only; this is a temporary solution that would have to be repeated, perhaps yearly depending on the allowable bump at the bridge end.

2. Abutment Reconstruction - Overexcavate a portion of the abutment and replace it with walls or blocks that can be extend below the anticipated depth of soil loss for some acceptable time period.

3. Slope Erosion Repair and Protection - Stabilize the near-surface as described in number 2 above and reconstruct and provide armoring on the slopes of the abutment to limit additional erosion.

We understand that a more long-term solution than option 1 is desired. There are funding and permitting limitations to the repairs and restorations that can be completed so that option 3 cannot be pursued at this time, leaving option 2 as the preferred solution. However, both options 2 and 3 are discussed below to facilitate planning for the future.

4.1.1 Abutment Reconstruction

Pavement settlement can be mitigated by excavating the soil at the abutment and replacing it with a “cohesive mass” that extends at least 2 feet below the long-term slope elevation. The “cohesive mass” should be a material that is stable with vertical slopes so that it does not depend on support from the steeply-sloped material between the abutment and the river for stability. The cohesive mass could be a block of lean concrete or gravity block walls, but the recommended approach is a welded wire-faced mechanically stabilized earth wall with facing on 3 sides, as shown conceptually in Sketch 2.

The MSE wall approach is similar to the repairs that were constructed at some unknown time on the south abutment, except we recommend extending the MSE wall to the pavement base instead of ending it at the elevation of the bottom of the girder. The MSE wall is preferred over the other means of creating a cohesive mass because:

1. The welded wire face and reinforcing are relatively easy to cut and shape in the field, which will be necessary because of the constrained space between the cantilevered sidewalk and curb at the bridge end.

2. The existing abutment soil is likely to be suitable as backfill in the MSE wall, minimizing material import and wasting. Select excavation and hand removal of cobbles larger than 6 inches comprising up to 15 percent of the total volume should be anticipated.

3. While Hilfiker is a well-known manufacturer of the welded wire facing and reinforcing, welded wire faced MSE walls are not a proprietary product.

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For welded wired faced walls, some agencies (e.g., WSDOT) require either rock at the face which is separated from the soil backfill by a filter fabric or vegetated soil with a hardware cloth (20-gauge screen) at the face for long-term applications. Vegetation will not grow under the bridge at the abutment and this abutment is a non-critical structure, so soil backfill directly against the hardware cloth is suitable.

If slope erosion repair and protection are implemented as described in the next section, the bottom of the MSE wall would extend 4 feet below the bottom of the girder and end wall. If the erosion repair and protection is not pursued, we recommend locating the bottom of the MSE wall 6 feet below the girder and end wall.

Additional considerations for the MSE wall include:

1. Care must be taken to protect and support the existing conduits and water line during excavation for the wall. The temporary slope of the excavation should be scaled back as much as possible to reduce load that could cause the conduits or pipe to slough into the excavation. Temporarily scaling back the slope may allow the wall construction to occur without temporary shoring.

Portions of the guardrail will also have to be removed and replaced.

2. The insulation around the water line may have to be extended to the north. Having insulation materials on-site or readily available will minimize delays during backfill.

3. A galvanized welded wire face is recommended. Though galvanizing can be a source of zinc that is detrimental to fish in aquatic environments, the wall will be well above river or other standing water levels and therefore zinc leaching should be minimal.

4.1.2 Slope Erosion Repair and Protection

The root cause of the abutment settlement is erosion of the slope. Slope erosion may also be causing the Pier 1 rotation and bearing rotation. In order to arrest and mitigate for the erosion, we recommend an application of heavy riprap on the slope below Pier 1 and a smaller riprap or large quarry spalls on the slope under the bridge above Pier 1. Slopes outside of the bridge dripline and bridge drain shower zone above Pier 1 should be revegetated. As part of the surface protection, the lost material on the downslope side of the bridge should be replaced so that the groundline is relatively constant between the upstream and downstream side of Pier 1. A hydraulic evaluation is recommended to size the riprap within the flood zone; however based on the size of existing rock, a maximum size of 5 feet and median size of 3 feet might be appropriate.

The existing ground between the bridge end and Pier 1 is sloped at approximately 1.5H:1V. The only way to apply stable fill and stone to slopes this steep is to start at the toe, where the ground is more gently-sloping, and work upward placing material in horizontal lifts. Ideally, a filter layer would be placed between the existing ground and added riprap or spalls, however, placement of a filter layer would require further regrading and disturbance of the area. Instead of a filter we recommend overbuilding of the erosion protection layer and accepting some soil loss.

It is our understanding that this work will not occur as part of the pavement restoration project. Detailed recommendations should be revised based on hydraulics and local material availability at the time that the work is planned.

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4.2 South Abutment Recommendations

Although erosion at the south abutment is less severe than at the north abutment, erosion is still occurring. In the long-term, slope protection – particularly between the abutment and Pier 3 – should be considered.

The welded wire faced wall below the bridge girder shows signs of corrosion. The face of an MSE wall only takes lateral soil load from each 2-foot soil lift; the larger loads will be on the buried wires. The amount of corrosion on the internal reinforcing wires is unknown. The next bridge inspection should include sampling and examination of one of the loose lap wires to assess the amount of strength loss and the amount of material loss should be tracked with time. Because the MSE wall is relatively short and “failure” of the wires would be unlikely to cause damage to the bridge or life safety risk to the public, in our judgement, the best course of action is to continue to monitor the condition of the facing and wait for repairs until soil loss starts to occur or other major work is contemplated.

The silt fence left after the 2017 repairs is concentrating runoff and causing erosion. It should be removed.

5. References

Federal Highway Administration (FHWA), 2018. Bridge Inspection Report, Skagit River Bridge Newhalem Campground Access Rd Over Skagit River, North Cascades National Park. Structure No 9470-015P.

Inspected September 1. 24 pp.

Federal Highway Administration (FHWA), 2017. Plans for Proposed Project WA NPS NOCA 21(1), NPS PMIS No. 169228, Skagit River Bridge Repair. 15 pp.

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Photos and Sketches a

Photos and Sketches

Photo 1. A 2.5-inch vertical drop from the edge of the bridge deck to the pavement at the north abutment

Sketch 1. Measured slope differences at Pier 1 b

Photo 2. Pier 1 top of the bearing has rotated toward the abutment and the pier wall has rotated toward the river

Photo 3. Erosion on downstream side of Pier c

Photo 4. Exposed bottom of end wall at north abutment

Photo 5. Raveled surface beneath sidewalk cantilever at north abutment d

Photo 6. Bridge drains on sidewalk side from north abutment e

Photo 7. Conduit transition from buried to bridge-supported at north abutment

Photo 8. Looking downstream from under bridge, north bank f

Photo 9. Looking upstream from under bridge, north bank

Photo 10. Roadway drainage outlet, south abutment g

Photo 11. Silt fence concentrating runoff on non-vegetated slope at south abutment h

Photo 12. Exposed MSE wall face at south abutment i

Photo 13. Exposed and insulated portion of water line at south abutment j

Sketch 2. Approximate MSE facing location at north abutment

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Appendix A. Bridge As-Builts