mt-ep-yell2022-1-3_Rescue Creek Bridge Final Foundation Memo.pdf

158377846

M E M O R A N D U M

Western Federal Lands Highway Division

610 E. Fifth Street

Vancouver, WA 98661

DATE: November 5, 2024

TO: Nathan Jones

Project Manager

FROM: Eric Lim

Sr. Geotechnical Engineer

Ricardo Ayala

Geotechnical Engineer

Orion George

Engineering Geologist

SUBJECT: Geotechnical Memo GM2624

Final Foundation Recommendations

Rescue Creek Bridge Replacement

MT ERFO NP YELL 2022-1(3)

Yellowstone National Park, Montana

1.0 INTRODUCTION

The Federal Highway Administration (FHWA) Western Federal Lands Highway Division

(WFLHD) GEO team has prepared this memorandum to provide geologic review, limited analysis, and final foundation recommendations for replacement of the Rescue Creek Bridge.

No seismic analysis was done. The project is located approximately 1 mile southeast of

Gardiner, Montana off the former North Entrance Road (Highway 89), over the Gardner River in Yellowstone National Park. This memorandum supersedes Geotechnical Memo GM37-23

(WFL, 2024).

2.0 PROJECT DESCRIPTION

The former structure was a trail bridge with reported steel beams clad with timber, as shown in

Figure 1. The existing footbridge was washed off the concrete foundations during the June 2022 flood event, as shown in Figure 2. We understand that although the Park initially explored the possibility of replacing the bridge using National Park Service trail crews, the current design will require outsourcing to a contractor. The replacement bridge will be 8 feet wide to accommodate loaded pack mules, and skew of the bridge added to allow the pack mule train to cross and turn off the bridge onto the trail without unpacking their loads. In addition, WFL Hydraulics engineers recommend moving the east abutment away from the edge of the river as much as possible due to the potential for stream meander. The location of the east abutment will be the on the steep east bank of the Gardner River.

Figure 1 – Photo of the Former Bridge facing east (Yellowstone Explored, undated).

Figure 2 – Photo of the remaining foundations, facing east, June 2022.

3.0 GEOLOGIC SETTING

3.1 GEOLOGY

Site-specific subsurface explorations have not been completed specifically for the bridge replacement. However, geotechnical exploration work is ongoing in the vicinity for a new North

Entrance alignment. As part of this work, WFL geotechnical engineers and engineering geologists have performed literature review and reconnaissance of the area, including the immediate area around the proposed bridge replacement.

The geology of the site was documented by Dewey, Pierce, and Nicholas (2018) in their work titled Geologic Studies on the North Entrance Road: Phase 3 – Collaboration on Alternatives for the Road Corridor, Yellowstone National Park, and accompanying map, Surficial Geologic

Map Involving the Roads between Gardiner, Montana, and Mammoth, Wyoming (Pierce, Dewey, Nicholas, 2018). An excerpt of the geologic map is shown below in Figure 3.

Figure 3 – Excerpt from surficial geologic map (Pierce, Dewey, & Nicholas, 2018).

Approximate

Location of

Former Bridge

N

Alluvium

Gravel

Landslides north of Sliding Lake landslide

Pinedale flood deposits

Cretaceous gray shale and light beige sandstone

Table 1 – Geologic Units Present in Figure 3 (Pierce, Dewey, & Nicholas, 2018)

Unconsolidated

Deposits

Alluvium (Qal) - are mostly tan, fine-grained alluvium of sand, silt, and clay. In landslide areas, seasonal moisture fosters tall grasses. Along permanent drainages such as the Gardiner River, mostly gray gravel with a sand matrix.

Gravel (Qg) (Latest Pleistocene – Holocene) – are gravel on terraces lower than and are younger than Pinedale flood deposits.

Landslides north of Sliding Lake landslide (Qnls) – are all earthflows. Subdued hummocky debris rich in clay. Vegetated with sparse grassland and low sagebrush. Surface roughness of 2.4° to 3.3°

(Nicholas, 2018). Landslides are laterally interspersed with Cretaceous shaley bedrock and Pinedale sub-glacial surface.

Pinedale flood deposits (Pfd) – are buff, coarse boulder gravel, poorly to moderately sorted, poorly bedded. Boulders mostly of Absaroka basaltic andesite, Quaternary basalt, and Precambrian granitic rock.

Bouldery flood deposits may cover less boulder glacial outwash gravels.

Bedrock

Formations

Cretaceous gray shale and light beige sandstone (Kbr) – now called the Landslide Creek Formation are upper cretaceous shale and conglomeritic sandstone, some sections containing bentonite beds that total 26 feet in thickness.

As can be seen by the Dewey et al. (2018) mapping the site is marked by water deposited sediments. The west side of the river at the bridge location is mapped as Qg, or Gravel. The Qg is described as alluvial (water transported) gravel on flood terraces lower than and younger than glacial Pinedale flood deposits (Pfd); the Pinedale deglaciation was approximately 13,000 –

14,000 years ago and numerous large glacial outburst floods have been interpreted from that period (Pierce et al., 2014). The east side of the river is mapped as more recent Qal, or Alluvium.

The Qal is described as sediments along intermittent streams within and draining landslide areas, mostly tan, fine-grained sand, silt and clay. Alluvial fans and broad bands of alluvium, including broad gently sloping alluvial flats just inside Yellowstone National Park south of town of Gardiner, form downstream from landslide areas as well. Below cliffy slopes of Mount Everts, forms boulder debris flow deposits. Along permanent drainages such as the Gardner River, mostly gray gravel with a sand matrix can be found (Pierce, Dewey, Nicholas, 2018).

The area west of the North Entrance roadway is mapped as earthflow landslides. Consideration of these landslides and their impact potential is not part of the scope of this document. Since the former structure did not have significant impacts from landsliding and there is a considerable distance to the larger landslides west of the bridge, we estimated a relatively low risk of impacts from landslide hazards at the proposed bridge site.

3.2 SITE CONDITIONS

The site was visited by WFLHD Engineering Geologist Orion George on July 10, 2023 and again

July 13, 2023. The July 13 site visit had numerous Yellowstone road team and WFLHD design team staff in attendance. A subsurface investigation was not completed for the site; however, numerous fresh river cut banks along this reach of the Gardiner River gave good exposure of bank slope materials.

We performed a geologic reconnaissance of the stream banks in the bridge area in the summer of 2023. We noted ground conditions on the west side of the river were generally unconsolidated gravels, cobbles, and boulders up to six feet in size with a sandy matrix consistent with the geologic map. This west side of the river is generally flatter and lies within the flood plain. The ground conditions on the east bank of the river were generally unconsolidated, poorly graded gravel with silt and sand and abundant cobbles and boulders up to six feet in dimension. This is also consistent with the geologic mapping. Figure 4 is a photograph of the bridge site from downstream looking south.

3.3 GROUNDWATER

Due to the coarse gradation of the river bank soils, groundwater levels should be considered to be approximately the same as the adjacent levels of the Gardner River. We did not observe active seepages in the east river bank at the time of our reconnaissance.

3.4 SEISMICITY

The project is located within an area of active seismicity due to its proximity to the Yellowstone caldera. Though no fault traces are mapped as underlying the projects, several fault zones are located within 20 miles from the site. Seismic analysis was not performed.

Figure 4 – Photograph of the bridge site looking south. Arrows point out the trail head sign on the west bank and the trail switchback on the east bank.

4.0 HYDRAULIC RECOMMENDATIONS

Hydraulic recommendations were provided in Rescue Creek Trail Bridge Waterway Design

Hydraulics Memo, dated September 26, 2024, by Jenn Mora, WFLHD Hydraulics Engineer. The following recommendations were provided:

• Minimum low chord elevation (project datum) 5353.1 feet + 5 feet for ice and woody debris potential = 5358.1 feet

• 170-feet waterway width, channel spanning structure (no piers in channel), vertical abutments, and shallow foundations set back far enough to mitigate erosion potential on steep slope along right (east) bank.

• Riprap is recommended to protect the west abutment due to avulsion hazards.

5.0 FOUNDATION RECOMMENDATIONS

We understand that only shallow foundation concepts were considered for support of this trail bridge at this location. Site conditions, mapped subsurface conditions, scour potential, design loading, constructability, structure type/usage, and cost were the major factors and risks considered when evaluating foundation alternatives. Based on input from NPS, they would typically use a pre-cast concrete shallow spread footing to support a bridge of this type.

Therefore, we will provide geotechnical design parameters for use in design of conventional shallow spread footings.

5.1 Preliminary Structure and Foundation Parameters

Preliminary bridge data were provided by the WFLHD Structural Group on September 9, 2024 and are summarized below:

• 8 feet wide

• 170-foot span (bearing to bearing)

• Low Chord at West Abutment = 5360.0 (min low chord = 5358.1)

• Bridge sloped up from west to east at 3% (max. 5% grade recommended for equestrian traffic)

• Treated Timber Deck with timber running planks (3” x12”)

• Truss fabricated from weathering steel

• Foundations shown are concrete spread footings (8’ x 14’ x 2.5’)

• Preliminary foundation loads at bottom of each abutment (includes abutment deadload):

o Service Limit State = 220 kips o Strength Limit State = 305 kips

5.2 Resistance Factors

Per Table 10.5.5.2.2-1 of the AASHTO BDS, the following resistance factors were used for

Strength Limit States are shown in Table 2 below:

Table 2 – AASHTO LRFD Resistance Factor for Shallow Foundations

Method / Soil / Condition Resistance Factor

Bearing Resistance b Semi-empirical methods (Meyerhof, 1957), all soils 0.45

Sliding

Precast Concrete placed on sand 0.90

Cast-in-Place Concrete on sand 0.80 ep Passive earth pressure component of sliding resistance 0.50

As discussed in AASHTO BDS section 10.5.5.1, all resistance factors for Service Limit States shall be taken as 1.0, except as provided for overall stability in Article 11.6.2.3.

5.3 Foundation Design Recommendations

Bearing resistance for shallow foundations was estimated using judgement of the geotechnical engineer based on subgrades consisting of medium dense gravel in a silty sand matrix.

Recommended shallow foundation design parameters are shown in Table 3.

These bearing resistances are based on medium dense (compacted) subgrade footings that are a minimum 2 feet in depth from the ground surface. Due to river bank slope stability concerns, the edge of the footing closest to the river should bear below a 1V:2H plane (27 degrees from horizontal), from the edge of the river bank at ordinary high water (see Slope

Stability Analyses section below). We expect that due to a combination of this slope setback requirement, Hydraulic scour concerns, as well as trail geometry and bridge skew considerations, the east bridge footing will be near the top of the river bank above the existing trail switch back. WFLHD GEO should review the final bridge abutment location for concurrence with our recommendations.

The west bridge abutment footing location will likely have to be in the relatively flat floodplain on the west side of the river. This area appears to be flat enough to not require much special consideration beyond the being out of the ordinary high water zone. An embankment fill will likely be located on this side to access the west end of the bridge, which is anticipated to be raised to meet minimum grade requirements from west to east for the pack animals.

Table 3 -– Recommended Soil Parameters for Abutment Foundation Design

Description Value

Nominal and factored bearing resistance Service limit state: 3 ksf for approximately 1 inch of settlement

Strength limit states: 6 ksf including a 0.45 resistance factor

Design Subgrade Soils and Preparation Notes Medium dense GRAVEL with silty sand matrix

Prepare foundation soils per FP-14 Section

Minimum Depth 2 ft

Minimum Depth for Scour Not considered

Assumed design depth for bearing, Df 2 ft

Soil Angle of Internal Friction for backfill, ’ 32 deg

Moist Unit Weight, T 120 pcf

Active Equivalent Fluid Pressure 35 psf/ft

At-Rest Equivalent Fluid Pressure 55 psf/ft

Passive Equivalent Fluid Pressure 300 psf/ft

Friction angle, , between precast concrete and subgrade 17 deg

Description Value

Nominal coefficient of sliding, tan , between precast concrete and subgrade 0.31

Friction angle, , between cast-in-place concrete and subgrade 26 deg

Nominal coefficient of sliding, tan , between cast-in-place concrete and subgrade 0.50

The above values are unfactored.

If overexcavation of wet, loose, organic, or otherwise unsuitable foundations soils in necessary, Foundation Fill, per FP-14 subsection 704.01 should be used within a 1V:1H splay beneath foundations. Structural Backfill per FP-14 Subsection 704.04 should be used as backfill behind abutment foundation walls and wing walls, if any. Geocomposite sheet drains connected to a perforated footing drain pipe should be installed behind abutment/walls, if any, and drained to daylight to ensure adequate drainage. The soil parameters listed in Table 3 may be used to determine the lateral earth pressures exerted and sliding resistance of the abutment foundation and wing walls. The parameters listed in Table 3 assume a vertical abutment/wall with level backfill.

5.4 Slope Stability Analyses

Due to the use of shallow foundations on or adjacent to existing slopes at the east river bank, we performed slope stability analyses of the existing slope and proposed bridge footings. An analysis of the proposed cut on the east side trail alignment was also evaluated. SLIDE2 software by Rocscience was used with auto refine circular failure surfaces analyzed using

Bishop, GLE/Morgenstern-Price, Janbu simplified, and Spencer methods. The overall model, soil parameters, and groundwater conditions are illustrated in attached Figure A-1. The existing slope conditions were analyzed as shown in attached Figure A-2 and resulted in a relatively stable factor of safety of 1.3. The addition of the preliminary 3 ksf foundation loads (using 305 kip Strength limit state loads over a 8 ft by 14 ft foundation area), were analyzed as shown in attached Figure A-3, and resulted in an insignificant, no net increase in factor of safety of 1.3.

As mentioned previously, this analysis assumes that the edge of the footing closest to the river will bear below a 1V:2H plane (27 degrees from horizontal) from the edge of the river bank at ordinary high water. Significant slope cuts on the east side of the trail alignment were analyzed specifically at station 200+45, which has the steepest slope along the trail alignment. Figure A-

4 shows the factor of safety of the slope before the cut and Figure A-5 shows the factor of safety after creating a 1.75H:1V cut, which both resulted to a relatively acceptable factor of safety of 1.94 and 1.28.

6.0 REFERENCES

Dewey, Craig S., Pierce, Kenneth L., and Nicholas, Grace E., 2018, Geological Studies on the

North Entrance Road: Phase 3 - Collaboration on Alternatives for the Road Corridor, Yellowstone National Park, Slickenside Consulting, LLC, Vancouver, Washington.

Lim, Eric, & George, Orion, 2024, Geotechnical Memo GM3723 – Preliminary Foundation

Recommendations – Rescue Creek Bridge Replacement, Western Federal Lands Highway

Division, Vancouver, Washington.

Pierce, Kenneth L., Dewey, Craig S., and Nicholas, Grace E., 2018, Surficial Geologic Map

Involving the Roads between Gardiner, Montana, and Mammoth, Wyoming, Slickenside

Consulting, LLC, Vancouver, Washington.

Pierce, K.L., J.M. Licciardi, T.R. Krause, and C. Whitlock, 2014, Glacial and Quaternary geology of the northern Yellowstone area, Montana and Wyoming in Shaw, C.A. and B. Tickoff, eds., Exploring the Northern Rocky Mountains: Geological Society of America Field Guide 37, doi:

10.1130/2014.0037(09).

United States Geological Survey (USGS, 2024B), United States Quaternary Faults, Retrieved August 29, 2024. https://doi.org/10.5066/F7S75FJM.

Yellowstone Explored website, Rescue Creek Trailhead, https://www.yellowstoneexplored.com/rescue-creek-trailhead, Retrieved December 15, 2023.

7.0 CLOSING

Please contact Eric Lim at (360) 619 7826 or eric.lim@dot.gov and/or Ricardo Ayala at (360)

619 7620 or ricardo.ayala@dot.gov with any questions regarding the contents of this memorandum.

Attachments: Figure A-1 – SLIDE Foundation Model

Figure A-2 – SLIDE Existing Stability Analysis

Figure A-3 – SLIDE Foundation Loads Analysis

Figure A-4 – SLIDE Trail Alignment-East Side – Station 200+45 – Before Cut

Figure A-5 – SLIDE Trail Alignment-East Side – Station 200+45 –1.75H:1V Cut

CC: Douglas A. Anderson, GEO Functional Manager

Evan Garich, Geotechnical Team Lead

Mark Swank – GEO Engineering Geologist Team Lead

Jenn Mora, Hydraulic Engineer

Benn Oltmann, Bridge Functional Manager

Nicole Garcia, Highway Design Engineer

Melissa Hogan, Environmental Protection Specialist

Geotechnical File https://doi.org/10.5066/F7S75FJM https://www.yellowstoneexplored.com/rescue-creek-trailhead mailto:eric.lim@dot.gov mailto:ricardo.ayala@dot.gov

Figure A-1 - SLIDE Foundation Model

Figure A-2 - SLIDE Existing Stability Analysis

Figure A-3 - SLIDE Foundation Loads Analysis

1.941.941.941.94

Ru

Value

Water

Surface

Phi

Cohesion

(psf)

Strength

Type

Unit Weight

(lbs/ft3) Color

Material

Name

0None340 Mohr-

Coulomb

Glacial

Outwash/

Min

FS

Method Name

2.00Bishop simplified

1.94Janbu simplified

2.00Spencer

2.00 GLE / Morgenstern-

Price

Safety Factor

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

4.50

4.75

5.00

5.25

5.50

5.75

6.00+

-60 -40 -20 0 20 40 60 80

Scenario Master Scenario

Group Station 200+45 - Before Cut

Company Western Federal Lands Highway Division

Drawn By

RVA

File Name Trail Alignment-East Side.slmd9/19/2024

Project

Rescue Creek Bridge Foundations

SLIDEINTERPRET 9.033

Ricardo.Ayala Text Box Figure A-4 - SLIDE Trail Alignment - East Side - Station 200+45 - Before Cut

1.281.281.281.28

Min

FS

Method Name

1.35Bishop simplified

1.28Janbu simplified

1.35Spencer

1.35 GLE / Morgenstern-

Price

Phi

Cohesion

(psf) Strength Type

Unit Weight (lbs/ ft3) ColorMaterial Name

Mohr-

Coulomb

Glacial Outwash/

Kame

1.75

Safety Factor

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

4.50

4.75

5.00

5.25

5.50

5.75

6.00+

-30 -20 -10 0 10 20 30 40 50 60

Scenario Master Scenario

Group Station 200+45 - 1.75H:1V Cut

Company Western Federal Lands Highway Division

Drawn By

RVA

File Name Trail Alignment-East Side.slmd9/19/2024

Project

Rescue Creek Bridge Foundations

SLIDEINTERPRET 9.033

Ricardo.Ayala Text Box Figure A-5 - SLIDE Trail Alignment - East Side - Station 200+45 - 1.75H:1V Cut