GM_27-15_Hard_Creek_Bridge_WA_SKAGIT_40076(1)_MSE_Wall_Recommendations.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

Date: July 09, 2015

In Reply Refer To: HFL-17 GM 27-15

From: Dustin Woods, Geotechnical Engineer Brian Collins, Sr. Geotechnical Engineer

To: Betty Chon, Project Manager

Copies: Chris Conrad, Design

Fabricio Quinonez, Bridge James Neighorn, Hydraulics Malcolm Ulrich, Geotechnical Craig Sanders, Construction

Subject: Hard Creek Bridge

WA SKAGIT 40076(1)

MSE Wall Recommendations

Geotechnical Memorandum GM 27-15

INTRODUCTION

This memorandum provides damage history, subsurface observations, and mechanically stabilized earth (MSE) wall recommendations for the proposed Hard Creek Bridge Repairs Project in Skagit County Washington.

PROJECT DESCRIPTION

Hard Creek Bridge is located at approximately milepost 12.7 on the Cascade River Road east of the town of Marblemount, Washington in the Mt. Baker – Snoqualmie National Forest. The bridge located here spans Hard Creek near the confluence of the Cascade River. See Figure 1.

Project Location Map.

This site has been the subject of multiple Emergency Relief for Federally Owned Roads (ERFO) projects over the last 20 years. They are:

FS ERFO 96-20(2): Prior to 1997, a 48 inch diameter corrugated metal pipe culvert conveyed Hard Creek at this location. A storm event in the winter of 1996 plugged the culvert with debris and the culvert was washed out during the storm. The ERFO project constructed a bridge to convey the creek and any debris coming down the slope during high rain events. The bridge was contractor designed with foundation recommendations from Dave Lofgren (retired 2011), WFLHD ERFO Geologist, to found the abutments a minimum of one foot into the bedrock. Recommendations can be read in Geotechnical Memorandum GM23-96. The bridge was constructed in 1997. The bridge deck was built with 7, 4 feet by 43 feet precast, prestressed concrete slabs set edge to edge. As-built plans of the structure were not made available to WFLHD.

During a site visit on May 2, 1997, Mr. 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. Mr. Lofgren’s recommendation was to lower the footing elevation.

Figure 1. Project Location Map

WA FS ERFO 2008(1)-20(1): November 2007, another storm event caused a debris flow at the same location, both overtopping and damaging the bridge constructed in 1997. The furthest upstream slab of the bridge deck took the brunt of the damage as well as undermining both abutments, notably Abutment 2. During an investigation of the damage it was realized that the Abutment 2 footing was placed on backfill material and not extended all the way to the bottom of the over-excavated bedrock.

This project removed the damaged upstream slab and underpinned both abutments by removing the fill material below the footings and filling the voids below the footings with shotcrete doweled into the underlying bedrock. Recommendations for this project can be read in Geotechnical Memorandum GM07-08 and Geotechnical Report GR07-08.

WA SKAGIT 40076(1): In January 2015 another storm event caused a debris flow similar to that of the 2007 storm. The storm caused a debris flow to overtop the road and damage the furthest upstream deck slab. This project will slightly realign the bridge to the downslope direction, replace the bridge deck, and improve the hydraulic opening of the bridge. Currently, the downhill side of the road has rockery walls supporting the road approaches to the bridge abutment. In order to accomplish the shift of the alignment in the downslope direction, the rockery walls will be removed and MSE retaining walls will be installed to support the bridge approaches.

GEOLOGY

A surficial geology map from the Washington State Department of Natural Resources (Appendix A) shows the underlying formation at the project site to be a Mesozoic-Paleozoic heterogeneous metamorphic rock, possibly overlain with a Quaternary alluvium.

SITE CONDITIONS

The bridge is located in very steep mountainous terrain in a coniferous forest typical of Western Washington. The average yearly precipitation is 76 inches per year (Climate Marblemount), the average precipitation, high, and low temperature by month is listed in Table

1. Site photographs provided by the National Park Service in January 2015 have been attached in Appendix B to try and give perspective for this project.

Table 1. Monthly Average Climate Data (Climate Marblemount).

Climate Data

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Avg High (°F)

43 47 52 58 65 70 76 77 71 58 47 41

Avg Low (°F)

32 32 35 39 44 49 52 52 48 42 36 32

Avg Precip

(in)

11.69 6.61 6.89 4.76 4.09 3.11 1.65 1.77 3.7 8.43 13.78 9.53

The stream channel near the bridge substructure consists mostly of schist bedrock with outcrops and alluvium containing cobbles and boulders. The alluvium, cobble, boulder mixture is likely washed downstream during high water events and replaced with new material being transported from upslope. As seen in the past projects, the upstream hydraulic opening of the bridge is too small to convey large debris flows coming down Hard Creek in heavy rain or runoff events.

SUBSURFACE CONDITIONS

During the development of WA FS ERFO 2008(1)-2-(1) two subsurface borings were completed, one at each abutment. Boring 2008-1, located at Abutment 1, was drilled to a depth of 20 feet. Bedrock was encountered directly beneath the footing.

Boring 2008-2, located at Abutment 2, was drilled 25 feet deep and encountered bedrock after 7 feet of sandy gravel below the footing. The borings support both the geological map and Dave Lofgren’s 1996 comments in GM 23-96. The site contains a thin layer of sandy gravel alluvium over metamorphic schist bedrock. The boring location map and boring logs from the 2008 drilling contract are attached in Appendix C.

ANALYSIS

The design of the geogrid reinforced, welded wire faced MSE wall was completed by Geotechnical Engineer Eric Lim using the software MSEW 3.0. Mechanically Stabilized Earth Walls (MSEW) 3.0 is part of the ADAMA Engineering suite and checks the following:

Internal stability, external stability, bearing capacity, direct sliding, eccentricity, geogrid strength, and pullout. The completed analysis does take into account that guardrail posts will be installed through the top 2 layers of reinforcement.

The bottom of wall elevations were set such that the wall will be founded on bedrock or a relatively thin layer of granular material overlying bedrock.

MSE DESIGN RECOMMENDATIONS

The existing abutments and foundation elements are wide enough that it is possible to shift or realign the bridge in the downstream direction without constructing new abutments or foundations. However, due to the steep terrain downslope, in order to shift the roadway approaches to the bridge it is necessary to remove the existing rockery walls and construct new retaining walls further downslope. This reason has led to the decision to construct a MSE wall adjacent to both abutments. The MSE walls will support the new road approaches and allow installation of guardrail off each end of the bridge.

The following is a summary of our recommendations for a MSE wall with uniaxial geogrid reinforcement using FP-14 Specifications:

The wall was designed with a vertical face (0 batter).

Use a 4”x4” W5xW5 welded wire form with number 4 wire support struts spaced 16 inches on center.

The facing should be wall facing fill between 6 and 10 inches thick when measured perpendicular to the face.

The reinforced fill should be select granular backfill with a nonwoven class 2 type A geotextile filter separating the facing fill from the select granular backfill.

All reinforcing elements should be a type 2 geogrid with 18 inch vertical spacing.

All wall segments should include a 6 inch perforated pipe underdrain system.

The minimum horizontal distance between the back of the guardrail posts and MSE wall face is 3 feet.

Wall heights and reinforcement lengths vary by station; see Table 2 for details.

Table 2. MSE Wall Heights and Reinforcement Lengths.

Wall Station Wall Height, H (ft)

Reinforcement Length, L (ft) Start End

100+00 100+04.53 4.5 8

100+04.53 100.13.58 6 8

100.13.58 100.27.11 9 8 199+96 200+05.18 12 11

200+05.18 200+10.32 10.5 10

200+10.32 200+15.4 7.5 8

Specific design details and MSE wall layouts were provided to the WFLHD Design Section.

CONSTRUCTION CONSIDERATIONS

Temporary slopes and/or shoring will be required during excavation of MSE walls. Most of the excavated material is expected to classify as Type C (granular soil) according to the Occupational Safety and Health Administration (OSHA) definitions. Schist bedrock may be encountered in the excavations.

Groundwater was not encountered in the borings; however, it may be encountered in the MSE wall excavation. Perched water is likely to be present in permeable soil layers overlying bedrock. Water elevations are likely to fluctuate seasonally, particularly during spring thaw and following periods of heavy precipitation. Excavation dewatering may be needed. Any water that enters the MSE wall excavation should be removed promptly. At the end of each day’s operations, adjacent surfaces should be sloped to drain away from the MSE wall excavation.

It is possible that schist bedrock will be encountered when excavating for the MSE wall.

During conversations with Dave Lofgren before his retirement in 2011, he recalled the 1997 contractor being able to rip the bedrock with a relatively large excavator. If bedrock is encountered it must be removed from the limits of the reinforcement shown in the plans.

A relatively large excavator, rock breaker, or drilling and using expansive grout may be required to remove the rock outcrop from the waterway upstream of the bridge to the grades shown in the plans. Blasting is not allowed.

Special Contract Requirements for this project have been provided to the WFLHD Design Section and include the following:

Section 255 Mechanically Stabilized Earth Walls, Section 563 Painting, Section 705 Rock, and Section 725.24A Weathering Agent

SUMMARY

This memorandum presents geotechnical recommendations for the Hard Creek Bridge repairs.

Contact Dustin Woods (Dustin.Woods@dot.gov, 360-619-7541) or Brian Collins (360-619-7657, Brian.Collins@dot.gov) with any questions regarding this memorandum.

Professional judgments and recommendations are presented in this report. They are based partly on evaluation of the technical information gathered, partly on historical reports, and partly on the WFLHD Geotechnical Section’s general experience with subsurface conditions in the area. It should be noted the borings might not represent potentially unfavorable subsurface conditions between borings. The observed water levels and/or conditions indicated on the boring logs are as recorded at the time of the exploration. The water levels and/or conditions may vary considerably, with time, according to the climate, rainfall, and other factors and are otherwise dependent on the duration of, and methods used in, and interpretations made from the exploration program.

If, during construction, soil conditions are encountered that vary from those discussed in this memorandum or historical reports, or if design loads, project stationing, and/or configurations change, the WFLHD Geotechnical Section should be notified immediately in order that we may evaluate effects, if any, on the recommendations contained in this memorandum. The recommendations presented in this report are applicable only to this specific site. These data are not to be used for other purposes.

REFERENCES

Climate Marblemount - Washington. (n.d.). Retrieved July 6, 2015, from http://www.usclimatedata.com.

Lofgren, D. (1996). Mount Baker-Snoqualmie National Forest, WA FS ERFO 96-20(2) (Cascade River Road); ERFO Repair Sites MP 2.7, 4.5, and 12.7: Geotechnical Design and Construction Recommendations. Geotechnical Memorandum GM23-96.

Collins, B. (2008). Hard Creek Bridge Debris Flow Damage Site Observations and Preliminary Geotechnical Recommendations. Geotechnical Memorandum GM07-08.

Collins, B. (2008). Hard Creek Bridge - WA FS ERFO 2008(1)-20(1). Geotechnical Report

GR07-08.

OSHA Technical Manual (OTM) | Section V: Chapter 2 - Excavations: Hazard Recognition in Trenching and Shoring. (n.d.). Retrieved July 6, 2015.

Subsurface Geology Information System, Division of Geology and Earth Resources - Washington's Geological Survey. (n.d.). Retrieved July 6, 2015.

Attachments: Appendix A, Hard Creek Geology Map Appendix B, Hard Creek Site Photographs Appendix C, Hard Creek Boring location Map and Boring Logs

Appendix A

Hard Creek Geology Map

Hard Creek Geology Map

Bridge Location

Appendix B

Hard Creek Site Photographs

Photo 1. Hard Creek Bridge Looking Northwest1.

Photo 2. Hard Creek Bridge Looking Northwest 2.

Photo 3. Looking North

Photo 4. Looking Southeast.

Photo 5. Looking West1.

Photo 6. Looking West 2.

Photo 7. Abutment 1.

Appendix C

Hard Creek Boring Location Map and Boring Logs

BORING LOCATION MAP

HARD CREEK BRIDGE

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

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: