Geotechnical_Report_07-08.pdf

Transmittal U.S. Department of Transportation Geotechnical Report No. 07-08

Federal Highway Western Federal Lands Highway Division Administration 610 East Fifth Street Vancouver, WA 98661-3801

Subject: GEOTECHNICAL INFORMATION:

Hard Creek Bridge – WA FS ERFO 2008(1)-20(1) Date: August 11, 2008 Geotechnical Report

From: Brian Collins, Geotechnical Engineer

Thru: Malcolm Ulrich, Acting Geotechnical Services Team Leader

To: Karl Gleason, Project Manager

Attached is Geotechnical Report No. 07-08, Hard Creek Bridge. This report provides the results of the geotechnical site investigation and provides recommendations for foundation repairs.

A copy of the report has also been uploaded to the WFLHD ftp site and can be downloaded from the following address: ftp://198.145.188.2/pub/Hard%20Creek%20Geotech/

Please contact me at (360) 619-7657 or Brian.Collins@fhwa.dot.gov if you have questions regarding the recommendations or interpretations presented in this report.

Attachment: Geotechnical Report 07-08 cc: Kevin Parker, Senior Designer Marc Veneroso, Bridge Design Engineer Sven Leon, Hydraulic Engineer Danny Capri, Environmental Specialist Betty Chon, Project Manager Geotechnical Files ftp://198.145.188.2/pub/Hard%20Creek%20Geotech/

Geotechnical Report No. 07-08

Hard Creek Bridge

WA FS ERFO 2008(1)-20(1)

MT. BAKER-SNOQUALMIE NATIONAL FOREST

SKAGIT COUNTY, WASHINGTON

August 11, 2008

Hard Creek Bridge Geotechnical Report No. 07-08

TABLE OF CONTENTS

Page

GEOTECHNICAL SUMMARY

INTRODUCTION

Project Location Project Background Scope of Work

FIELD EXPLORATION

Site Observations Subsurface Exploration

ANALYSIS

RECOMMENDATIONS

Boulder Removal Footing Stabilization Construction

Appendix

Boring Location Map Descriptive Terminology Boring Logs 2008-1 & 2008-2

GEOTECHNICAL SUMMARY

A debris flow undermined a portion of one of the spread footings supporting the Hard Creek Bridge.

The upstream ends of the footings are supported directly on bedrock and the downstream ends have up to about 7 feet of soil between the bottom of the footing and the underlying bedrock. The soil below the footings is susceptible to scour during future runoff events. We recommend constructing a concrete or shotcrete mass between the bottom of the footings and the underlying bedrock to ensure long-term stability of the bridge.

INTRODUCTION

Project Location This project is located approximately 12.7 miles east of Marblemount, WA, at milepost 12.7 on Cascade River Road in the Mt. Baker – Snoqualmie National Forest. Figure 1 below shows the project location.

Hard Creek Bridge

Figure 1. Project Location Map.

Project Background In November of 2007, a debris flow that occurred in the Hard Creek drainage caused damage to the bridge, resulting in closure of the bridge. A photograph of the bridge following the debris flow is shown in Figure 2. The bridge was covered with debris up to the top of the guard walls. A significant amount of debris exited the creek channel at the bridge location and was deposited off the shoulder of the road south of the bridge as seen in Figure 3 below.

Figure 2. Debris on Hard Creek Bridge.

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.

Geotechnical Memorandum 23-96 recommended that both footings be keyed a minimum of one foot into bedrock.

On May 2, 1997, during construction, WFLHD Engineering Geologist 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 lowered to elevation 992 feet. No As-build drawings of the bridge could be located.

The bridge deck consists of seven 4 feet wide by 43 feet long precast prestressed concrete slabs set edge to edge. The debris flow almost completely destroyed the upstream-most deck slab. The other six deck slabs were not damaged. The Abutment No. 2 footing was partially undermined by the debris flow. An minor amount of undermining occurred below the Abutment No. 1 footing.

Figure 3. Air-photo of Hard Creek Bridge (looking north).

Brian Collins of the WFLHD Geotechnical Team visited the project with Marc Veneroso, WFLHD Bridge Design Engineer, and Sven Leon, WFLHD Hydraulics Engineer, on April 30, 2008. We met Peter Wagner - National Forest Service, Craig Holmquist - North Cascades National Park Service, and Dave Sheridan, Jeff Miller, and Chris Seidel with Skagit County on site. Our observations and preliminary recommendations were summarized in Geotechnical Memorandum 07-08, dated May 6, 2008.

Scope of Work The scope of work for this project is to remove the damaged deck slab section and construct support and protection for both footings to ensure stability during future runoff events. Repair recommendations for the bridge deck were presented in a letter from Marc Veneroso, WFL Bridge Design Engineer, dated May 28, 2008. A hydraulic site assessment with recommendations for scour protection was provided by Sven Leon, WFLHD Hydraulics Engineer, on May 14, 2008. The Geotechnical Team was tasked with providing a mitigation plan to repair the void below the Abutment 2 footing and protect both footings from scour.

FIELD EXPLORATION

Site Observations The Hard Creek site is located in mountainous terrain. The section of roadway adjacent to the bridge ends was likely constructed by cutting the uphill side of the road and filling the downhill side.

The Hard Creek channel has a steep gradient (>10 degrees) and is incised 20 to 40 feet into schist bedrock above and below the bridge location. Bedrock outcrops were observed near all four corners of the bridge. See Figures 4 through 7 below.

A large boulder measuring about 10 x 10 x 6 feet is leaning against the damaged bridge slab.

Another 6 x 6 x 5 foot diameter boulder is pinned upslope on the 10 foot boulder. See Figures 4, 6, and 8 below.

BOULDERS

Figure 4. Sketch Showing Locations of Observed Bedrock Outcroppings.

ABUTMENT NO. 1

Figure 5. Abutment No. 1.

BEDROCK

BEDROCK

BEDROCK

BEDROCK

BEDROCK

BEDROCK

BEDROCK

BEDROCK

BEDROCK

BEDROCK

BEDROCK

BEDROCK

ABUTMENT

NO. 2

ABUTMENT NO. 1

ABUTMENT

NO. 2

BEDROCK

10’ BOULDER

BEDROCK

BEDROCK BEDROCK

BEDROCK

BEDROCK

BEDROCK BEDROCK

BEDROCK

Figure 6. Looking Upstream.

BEDROCK VOID

Figure 7. Abutment No. 2.

Below the bridge, the soil above the bedrock consists of mostly cobble and boulder-sized particles in the creek, with smaller cobbles, gravel, and sand in areas further away from the creek. The upstream ends of both footings appear to be in direct contact with the underlying bedrock. The bedrock outcrops below the bridge on the downstream side are 5 to 8 feet below the bottoms of the footings.

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.

Scour at Abutment 1 undermined less than one square foot of material from below the footing. It appears that most of the upstream half of the footing is in direct contact with bedrock. The bedrock elevation 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, gravel, and cobbles.

Figure 7, above, 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 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 is located directly below the ladder on the left side of Figure 7. The material exposed below the footing is sand and gravel with cobbles and boulders. Bedrock outcrops on the downstream end of the bridge were up to 8 feet below the bottom of the Abutment 2 footing.

Subsurface Exploration Two borings were completed on July 9, 2008. The borings were drilled 1 foot off of each end of the bridge slab and penetrated through the footings in order to determine the bedrock depth below the footings. The borings were drilled with a truck-mounted BK-81 drill rig. Hollow-stem augers were used to advance the borings to the top of the footings. Then a HQ core barrel system was used to continue the borings through and below the footings. Borings 2008-1 and 2008-2 were advanced to depths of 20 and 25 feet, respectively. Standard Penetration Tests (SPTs) were performed in the soil layers with a 2 inch O.D. split tube sampler. 2.5 inch diameter core samples were taken in the concrete and bedrock layers. The samples were visually classified in the field and a log of each boring was created to summarize the subsurface conditions encountered.

At Abutment 1, Boring 2008-1 encountered bedrock immediately below the footing. At Abutment 2, Boring 2008-2 encountered bedrock 7 feet below the bottom of the footing, at approximately the same elevation as the bedrock outcropping below the center of the bridge on the downstream side as seen in Figures 5 and 6. The bedrock encountered in the borings consisted of the same schist that was observed in outcroppings adjacent to the bridge.

A boring location map and boring logs are included in the Appendix.

ANALYSIS

We were unable to verify the width of the footings and the analysis was completed assuming both footings are 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. We also verified that there is an adequate factor of safety for global stability.

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

RECOMMENDATIONS

Although the footings are currently stable, the soil layers below the footings are susceptible to scour, and loss of additional material may create an unstable condition. Therefore we recommend constructing a protective layer of shotcrete or concrete below the bottom of the footings tied into the underlying bedrock to ensure long-term stability.

Boulder Removal We recommend removing the two boulders adjacent to each other leaning against the upstream side of the bridge deck.

The boulders measure approximately 10 x 10 x 6 feet and 6 x 6 x 5 feet. The boulders should not be blasted. The boulders should be broken up into manageable pieces using a mechanical breaker, boulder buster, expansive chemical expansive grout, or other method approved by the CO. The boulders should be removed before completing any work below the bridge.

10’x10’x6’ BOULDER

6’x6’x5’ BOULDER

Figure 8. Boulders to be Removed.

Footing Stabilization At least a portion of each of the footings is lying directly on bedrock. Based on observations of bedrock outcroppings and the results of the borings, it appears that portions of the footings may have up to about 7 feet of soil between the bottom of the footing and the underlying bedrock.

In areas where the footings are not directly bearing on bedrock, we recommend using steel fiber reinforced shotcrete to create a structural seal and protective layer between the bottom of the footings and the underlying bedrock Steel fibers in the shotcrete will significantly increase the impact resistance. The mix should include 2 percent steel fibers by volume. The shotcrete should be structurally tied into the footing and the underlying bedrock with dowels.

Details showing how the repairs should be completed are shown in Figures 9 through 11 below.

Figure 9. Proposed Repair – Abutment 1.

Figure 10. Proposed Repair – Abutment 2.

Figure 11. Dowel Installation and Drain Pipe Details.

Construction Constructability is a key issue on this project due to the limited headroom below the bridge. Most of the excavation work will need to be completed by laborers with hand tools. Access from the upstream end has limited clearance between the bridge slab and bedrock outcrops, and access from the downstream is limited since the stream drops steeply after passing below the bridge. It may be possible for equipment to reach below the bridge from the flat area directly south of the bridge. It may be necessary to improve access by temporarily removing the rockery wall adjacent to the abutment.

It will be necessary to stage the construction so that the footings are not destabilized during the repairs. Each footing should be repaired in no less than 3 stages. The length of each stage, measured perpendicular to centerline, should be 10 feet or less. Both of the footings can be repaired concurrently. The void area on Abutment 2 should be repaired in the first stage.

After removing the boulders, it will be necessary to divert the stream away from the work areas so the work can be completed in the dry.

Shotcrete should be placed in lifts no more than 6 inches thick unless the contractor demonstrates in test panels that thicker lifts can be used without sloughing.

APPENDIX

Boring Location Map

Descriptive Terminology

Boring Logs 2008-1 and 2008-2

BORING LOCATION MAP

HARD CREEK BRIDGE

BORING 2008-2

BORING 2008-1

7/9/08

ELEV:

Hard Creek Bridge, WA FS ERFO 2008(1)-20(1)

S A

M P

LE

01.75 in O.D. SPLIT TUBE SAMPLE

OTHER:

WATER CONTENT (%)

BLOWS PER FOOT

(140 lb mass, 30 in drop)

S A

M P

LE

HQ CORE

20.0

20/0"

4-6-4 (Rec: 9in)

11-10-8 (Rec: 11in)

8.0

10.0

Bottom of Hole Elevation = 982.2 ft..

SCHIST, light gray, moderately weathered, close to moderately close joints, medium strong (R3).

CONCRETE, Abutment No. 1 footing.

FILL, GRAVEL with Silt and Sand, medium dense to loose, brown, fine to coarse grained, subangular to subrounded.

BORING 2008-1

LIQUID LIMITPLASTIC LIMIT

WATER LEVEL

2 in O.D. SPLIT TUBE SAMPLE (SPT)

3.25 in O.D. SPLIT TUBE SAMPLE (D & M)

BEGAN:

CORE

8 in H-S AUGER

RECOVERY (%)

BK 81

PROJECT:

STATION, OFFSET:

Clear, 70s (F)

10+60, 3.5 ft RT

SAMPLE PENETRATION RESISTANCE

DRILLER:

FEDERAL HIGHWAY ADMINISTRATION

VANCOUVER, WASHINGTON

GEOTECHNICAL SECTION

BORING LOG (English Units)

G R

A P

H

IC

L O

G

BLOW

AUGER

RQD (%)

DRILL:

COUNT

WEATHER:

NQ CORE

3 in SHELBY TUBE

COMPLETED:

Mike Corn

Sheet 1 of 1

1002.20

7/9/08

D E

P

TH

(f t)

D E

P

TH

(f t)

6 in H-S AUGER

DESCRIPTION

6 16-22-11 (Rec: 4in)

7-7-20/0" (Rec: 4in)

12-8-8 (Rec: 2in)

4-5-6 (Rec: 5in)

5-5-6 (Rec: 9in)

19-13-9 (Rec: 9in)

1.75 in O.D. SPLIT TUBE SAMPLE

5-5-3 (Rec: 4in)

S A

M P

LE

3-5-5 (Rec: 5in)

4-6-6 (Rec: 6in)

10-35-40 (Rec: 6in)

Bottom of Hole Elevation = 976.8 ft.

SCHIST, light gray, moderately weathered, close to moderately close joints, medium strong (R3).

Sandy GRAVEL, possible Fill, dense to loose, moist, brown, fine to coarse grained, subangular (schist fragments).

CONCRETE, Abutment No. 2 footing.

FILL: Silty GRAVEL with Sand, medium dense, moist, brown, fine to coarse, subangular to subrounded.

25.0

20.0

13.0

>>11.0

3 in SHELBY TUBE

BK 81

SAMPLE PENETRATION RESISTANCE

LIQUID LIMIT

CORE

7/9/08

2 in O.D. SPLIT TUBE SAMPLE (SPT)

RECOVERY (%)

PROJECT:

STATION, OFFSET:

Clear, 70s (F)

3.25 in O.D. SPLIT TUBE SAMPLE (D & M)

8 in H-S AUGER

11+08, 8.5 ft RT

BEGAN:

20 40

PLASTIC LIMIT

WATER LEVEL

NQ CORE

ELEV:

50 100

BLOWS PER FOOT

(140 lb mass, 30 in drop)

WATER CONTENT (%)

OTHER:

Hard Creek Bridge, WA FS ERFO 2008(1)-20(1)

HQ CORE

BORING 2008-2

BORING LOG (English Units)

6 in H-S AUGER

D E

P

TH

(f t)

DESCRIPTION

Mike Corn

WEATHER:

FEDERAL HIGHWAY ADMINISTRATION

VANCOUVER, WASHINGTON

GEOTECHNICAL SECTION

7/9/08

D E

P

TH

(f t)

1001.80

RQD (%)

S A

M P

LE

Sheet 1 of 1

G R

A P

H

IC

L O

G

BLOW

COUNT

AUGER

COMPLETED:

DRILL:

DRILLER: