GM_07-08.pdf

Subject: Hard Creek Bridge Debris Flow Damage Date: 05/06/2008 Site Observations and Preliminary Geotechnical Recommendations

From: Brian Collins Geotechnical Engineer

Through: Dave Lofgren Acting Geotechnical Team Leader

To: David Hilgendorf ERFO Coordinator Distributed electronically

Introduction

The site is located on Cascade River Road east of Marblemount, WA, in the Mt. Baker- Snoqualmie National Forest. In November of 2007, a debris flow caused damage to the bridge over Hard Creek at milepost 12.7, resulting in closure of the bridge.

On April 30, 2008, a meeting was held at the site to assess the damage to the bridge. The following people were present:

Brian Collins – Western Federal Lands Marc Veneroso – Western Federal Lands Sven Leon – Western Federal Lands Peter Wagner – National Forest Service Dave Sheridan – Skagit County Jeff Miller – Skagit County Chris Seidel – Skagit County Craig Holmquist – North Cascades National Park Service

This memorandum summarizes our site observations, recommendations for further investigation, and potential mitigation options.

Memorandum

GEOTECHNICAL MEMORANDUM 07-08

WESTERN FEDERAL LANDS HIGHWAY DIVISION

610 EAST FIFTH STREET

VANCOUVER, WA 98661-3801

Project Background

The Hard Creek Bridge was constructed by Oneway Construction as an ERFO project in 1997 [WA FS ERFO 96-20(2)]. The bridge replaced a 48 inch corrugated metal pipe that was plugged with debris and washed out during a flood event. The bridge is supported on spread footings that were supposed to be keyed a minimum of one foot into bedrock (GM23-96).

On May 2, 1997, during construction, WFLHD Dave Lofgren visited the site and found that the contractor had over excavated several feet into weathered bedrock below the planned bottom of footing elevation at the southeast corner of Abutment 2. The footing elevation was subsequently lowered to elevation 988 feet at Abutment 2. The bottom of footing elevation at Abutment 1 was modified to elevation 992 feet. No As-build drawings of the bridge could be located.

Site Observations

Figure 1, below, shows the amount of debris that was deposited on the bridge. The debris was removed from the bridge prior to our site visit.

Figure 1. Debris on Hard Creek Bridge.

The damage that was sustained to the bridge deck can be seen in Figure 2, below, taken after the debris was cleaned off of the bridge.

Figure 2. Hard Creek Bridge After Removal of Debris.

Note the large boulder leaning against the bridge on the right side of Figure 2. The boulder is about 10 feet in diameter and it is granodioritic, i.e., from a different formation than the schist outcrops observed adjacent to the bridge, indicating that it traveled some distance before coming to rest against the bridge. Based on a review of geologic and topographic maps, it appears the boulder may have traveled more than half a mile from its point of origin. A six foot diameter boulder is pinned upslope from the 10 foot boulder.

Figure 3, below, shows the void that was scoured below the footing of Abutment 2. Damage from passing debris can also be seen on the top of the footing and the abutment stem wall. The damage to the footing was generally cosmetic. The void measured 13 feet wide at the bottom of the footing, was up to 3 ½ feet deep horizontally below the footing, and extended 1 to 3 feet vertically below the footing. The upstream side of the footing adjacent to the void was in direct contact with bedrock. The bedrock can be seen directly below the bottom of the ladder on the left side of Figure 3. The material exposed below the footing and adjacent to the downstream side of the footing appeared to be colluvium or fill material. It appears that bedrock may be as deep as 10 to 15 feet below the bottom of the downstream side of the footing.

BOULDER

Figure 3. Abutment No. 2.

Figure 4. Abutment No. 1.

BEDROCK

BEDROCK

Figure 4, above, was taken from the downstream side looking at Abutment 1. Scour in front of Abutment 1 undermined less than one square foot of material from below the footing. The yellow dashed line in Figure 4 shows the limits of exposed bedrock that was observed below Abutment 1. It appears that most of the upstream half of the footing is in direct contact with bedrock. The bedrock contact on the downstream side of Abutment 1 is likely less than 5 feet below the bottom of the footing. The soil below the footing consists of sand and cobbles.

Figures 5 and 6, below, are sketches summarizing our site observations. Figure 5, below, is a plan view showing the locations where exposed bedrock was observed. The observed bedrock is consistent with the schist described in GM23-96.

Figure 5. Sketched Plan View of Hard Creek Bridge.

Figure 6, below, is a profile sketch showing observed bedrock outcrops along the upstream and downstream sides of the bridge.

Figure 6. Profile Sketch of Hard Creek Bridge.

Figure 7, below, is a photo looking upstream adjacent to Abutment 1. Note the limited clearance between the bridge deck and bedrock on the upstream side near Abutment 1 (left side of photo).

Figure 7. Photo Looking Upstream Adjacent to Abutment 1.

BOULDER

Below the bridge, the soil above the bedrock consists of mostly boulder-sized particles in the creek, with finer grained cobbles and sand in areas further away from the stream. Bedrock outcrops were observed near all four corners of the bridge.

The bridge footings are 28 feet long, 7 feet wide, and 2 feet thick. The stem walls measured 6 feet and 10 feet high on Abutment 1 and Abutment 2, respectively.

Analysis

The discussion below is generally in reference to Abutment 2, which was undermined much more relative to Abutment 1. Note that we were unable to verify the width of the footing and the analysis was done assuming it is 7 feet wide as shown on the construction plans submitted by Oneway Construction, dated 5/13/97.

The void below Abutment No. 2 is about 16 percent of the total footing area. Due to the size of the footing, even when the reduced contact area at the bottom of the footing is accounted for, the bearing capacity of the underlying soil and rock is much greater than the pressure exerted at the bottom of the footing. There is also adequate sliding resistance along the base of the footing.

Due to the bridge layout, overturning, global stability, and settlement were not affected by the scour below the footing.

Recommendations

Although both of the footings are stable in their current configuration, we recommend mitigating the void below Abutment 2 and providing protection below Abutment 1 to ensure that future runoff or debris flow events to not continue to undermine the footings and compromise the bridge. The depth of bedrock below the footings needs to be determined so that the most cost effective and efficient mitigation can be selected.

Based on our analysis, up to 25 percent of the length of the footings can be underpinned at one time without compromising the capacity of the footing. The underpinning section should not extend beyond the centerline of the footing. Thus the underpinning will need to be done in 4 sections.

If the bedrock contact is less than 5 feet below the bottom of the footing, we recommend removing the soil above the bedrock and underpinning the footings with shotcrete as shown in Figure 8 below. The shotcrete should be doweled into the bedrock and the bottom of the footings. In addition, we recommend placing a reinforcement mat below the exterior surface to assist in protection of future scour events.

Figure 8. Preliminary Underpinning Concept.

If the depth to bedrock is greater than 5 feet, it may not be feasible to tie the underpinning directly into bedrock. It may be more cost effective to clean out the voids between the larger boulders with water or compressed air and grout a mass of boulders together below the footing.

Then a similar concept of shotcreting can be applied to fill any remaining void directly below the footing.

Subsurface Investigation

We recommend drilling 2 soil borings on the outside of each bridge abutment along the downstream side of the bridge to determine the depth to bedrock below the footing. The total drilling depth is estimated to be 50 feet for both borings and it will take one to two days to complete. It will be necessary to have the road closed to drill the borings at the desired locations.

We estimate the cost of the subsurface investigation will be on the order of $20,000.

Summary

A debris flow event that occurred late in November of 2007 scoured a hole below approximately 16 percent of the footing supporting Abutment No. 2 of the Hard Creek Bridge. No significant scour occurred below the Abutment No. 1 footing. The scour has not compromised the integrity of the footings or affected their ability to support the applied loads. We recommend underpinning both abutments to ensure long-term stability and scour protection during future runoff or debris flow events. Upon completion of the subsurface investigation we can develop detailed footing repair/underpinning details. If you have any questions feel free to contact Brian Collins at (360) 619-7845.

e-copies: Karl Gleason Dave Lofgren Marc Veneroso Rich Barrows Sven Leon