OR FLAP DOT CRGNSA 100(8)_Geotechnical Memorandum.pdf

M E M O R A N D U M

To: Kevin Bracy, PMP / David Evans and Associates, Inc. Date: March 24, 2023

(ISSUED: 05/10/2023)

GRI Project No.: 6783-A

From: Keith S. Martin, PE, GE; and George A. Freitag, CEG

Re: Geotechnical Design Recommendations Multnomah Falls Viaducts Repair

OR PFH 163(21)

Western Federal Lands Highway Division (WFLHD) Contract No. DTFH7015D00002L Task Order 69056718F000084 WFLHD Geotechnical Memorandum No. GM35-21 Multnomah County, Oregon

As a subconsultant to David Evans and Associates, Inc. (DEA), GRI is providing geotechnical-consultation services for the repair of two viaducts, the East Viaduct and West Viaduct, at Multnomah Falls, located between approximate mileposts (MPs) 17 and 18 on the Historic Columbia River Highway (HCRH) about 13 miles east of Troutdale, Oregon. The location of the viaducts along the HCRH is shown on the Project Alignment Map, Figure 1.

We understand DEA is providing 100% final plans, specifications, and cost estimates for repairs to the viaducts. GRI’s previous scope of work, as discussed in the previous version of this memorandum, dated December 7, 2021, was to (1) discuss the geotechnical impacts of restoring bearing underneath the existing viaduct spread footings; (2) discuss the geotechnical design recommendations for the design of keyed riprap slope stabilization under the East Viaduct at the existing collapsing walls; (3) discuss the geotechnical review of the outward-leaning rockery wall at the west abutment of the East Viaduct; and (4) to perform a site visit to evaluate changes since our original site visit in July 2014. The purpose of this memorandum, as outlined in Section VD.9.A of our current scope of services, is to revise our geotechnical memorandum to remove the keyed riprap alternative that had been previously considered for placement under the East Viaduct and add recommendations for installation of the temporary ballast path material on the slopes beneath the viaduct where erosion has impacted footings and in front of the short retaining walls, if quantities allow. Our work to date for this current phase of work has included a review of available information for the viaducts, a project team kickoff meeting held on March 2, 2023, discussions with DEA, and limited engineering analysis.

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum Page 2 March 24, 2023 (Issued: 05/10/2023)

This memorandum summarizes the work completed and provides our 100%-design, geotechnical-related conclusions and recommendations regarding repairs to the viaducts.

EXISTING INFORMATION

The following documents were reviewed as part of this current phase of work:

GRI memorandum, “Geotechnical Site Reconnaissance, Potential Geotechnical Issues, and Initial Recommendations for Resolution, Multnomah Falls Viaducts Repair, OR PFH 163(21), Task Order No. DTFH7014F10001, Multnomah County, Oregon,” dated July 9, 2014.

GRI report, “Pavement Report, Historic Columbia River Highway, Multnomah Falls Viaducts Repair Project, MP 17.7 to 18.6, Federal Highway Administration (FHWA), Western Federal Lands Highway Division, Task Order No. DTFH7015F19004, Multnomah County, Oregon,” dated December 18, 2015.

GRI report, Geotechnical Design Recommendations, Multnomah Falls Viaducts Repair, OR PFH 163(21), Western Federal Lands Highway Division (WFLHD), Contract No.

DTFH7015D00002L, Task Order 69056718F000084, WFLHD Geotechnical Memorandum No. GM35-21, Multnomah County, Oregon, dated December 7, 2021

David Evans and Associates, Inc., “Plans-In-Hand (70% Plans) with ODOT and FWHA Comments and Initial Responses,” dated March 20, 2019.

David Evans and Associates, Inc., “Special Contract Requirements (70%) with ODOT and FHWA Comments and Initial Responses,” dated March 20, 2019.

David Evans and Associates, Inc., “Final Design Plans Progress Print,” dated August 2, 2021.

David Evans and Associates, Inc., “Draft 100% Plans”, dated July 2022

Our July 9, 2014, draft memorandum is attached as Appendix A. A selected drawing from the final design progress plan set is attached in Appendix B.

PROJECT DESCRIPTION

The viaducts were constructed in 1916 along the lower portion of a steep, north-facing slope adjacent to the Union Pacific Railroad. During our ground-level reconnaissance completed as part of our 2014 work, we observed numerous spread footings, primarily under the East Viaduct, which were undermined; short retaining walls along the base of the slope under the East Viaduct in poor condition or partially collapsed; and an outward lean of the dry, stacked rockery wall at the west end of the East Viaduct along the north side of the road. As part of the project, we understand bearings will be restored beneath 23 spread footings under the East Viaduct and four footings under the West Viaduct and keyed riprap slope stabilization will be placed under the East and

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum Page 3

West viaducts to protect the slopes under the viaducts and to mitigate/replace the existing walls under the East Viaduct that have collapsed or are in poor condition. At this time, the leaning portion of the rockery wall at the western end of the East Viaduct will not be mitigated due to project cost constraints. We understand the viaduct repairs are not being designed for seismic loading.

SITE RECONNAISSANCE

A site visit was completed by a certified engineering geologist and licensed geotechnical engineer from GRI on June 24, 2021, to evaluate whether conditions had changed since our previous reconnaissance in July 2014 at the East Viaduct (see attached memorandum in Appendix A).

Between our 2014 and 2021 site visits, a large wildfire burned through the forest above the viaduct in 2017. Subsequent to the fire, a rock catchment fence was installed about 30 feet upslope of the viaduct. However, during our 2021 site visit, we observed some additional rock and woody debris had collected in areas between the cut slope and side of the viaduct since our 2014 visit, which we anticipate may have occurred prior to installation of the fence. Observation below the viaduct in 2021 was limited due to access considerations. At the east end of the viaduct in 2021, the vegetation was observed to be very thick and did not permit reasonable access for more than about 100 feet to 200 feet. However, the conditions beneath the viaduct in this area appear to be relatively unchanged since our 2014 visit. The remainder of limited observations along the north side of the viaduct in 2021 were completed from the roadway above. Based on our observations, deposits indicative of large-scale erosion beneath the viaduct were not obvious at the toe of the slope along the north side of the viaduct in 2021. We did observe areas where numerous cobbles are present on the relatively flat area between the viaduct and railroad. However, it was not clear if the cobbles were more recent than our 2014 visit. Obvious indications of deep-seated slope instability within the project limits were not obvious during our 2014 or 2021 visits.

CONCLUSIONS AND RECOMMENDATIONS

Spread Footing Repairs Based on the referenced plans and discussions with DEA, 23 footings under the East Viaduct and four footings under the West Viaduct that are undermined will be repaired by the placement of structural concrete backfill to restore uniform support beneath the footings. We understand no structural modifications to the existing footings are planned. A typical detail showing the intended concrete-backfill repair beneath footings for the East Viaduct is shown on the footing repair detail plan sheet included in Appendix B.

In our opinion, the planned repair is a reasonable approach to restore uniform bearing beneath the footings. Prior to placement of the concrete, we recommend the exposed subgrade in the area to receive concrete be stepped or benched to key the concrete backfill into the slope. The steps or benches should be horizontal and at least 18 inches to 24 inches wide, if feasible. Based on our observations, the slope below the viaducts, particularly the East Viaduct, is very steep and

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum Page 4 mantled with loose gravel and cobbles with sand and silt. Therefore, temporary shoring may need to be provided to prevent sloughing of excavation sides prior to placing the concrete. It may be feasible for the formwork for the concrete to serve as shoring. Soft or loose materials at subgrade should be removed prior to placing the concrete. Alternatively, if the subgrade consists of primarily granular soil and is loose, the surface may be compacted with lightweight compaction equipment, although this may not be feasible directly under the footing.

Retaining Wall Repair

General Gravity retaining walls consisting of stacked cobbles or railroad ties and timbers are present in areas between columns in the northern row along portions of the East Viaduct. The walls are generally 3 feet to 4 feet high and retain the lower few feet of the steep slope under the viaduct.

The backslopes under the viaduct behind the walls are as steep as 35° to 40°, with locally flatter and steeper areas. The condition of many of the walls is poor, and some of the stacked-cobble walls have collapsed. During project development, the collapsed walls and walls in poor condition were being considered to be replaced with new walls or potentially buttressed with new rock fill/inlay (keyed riprap) placed at the face of the existing walls and extending up the slope behind the walls. However, due to funding considerations, the keyed riprap will not be installed as part of this project. In-lieu of the riprap, the construction path ballast rock will be reused at the end of construction to regrade local areas of ground under the viaducts that have been oversteepened from erosion and have impacted footings, and potentially used to buttress the existing retaining walls under the east abutment, if quantities allow. Provided below is a written summary of potential wall replacement alternatives that have been discussed with the DEA team during project development. A discussion of the construction path ballast reuse alternative follows the retaining wall discussions and is currently the planned alternative for final design.

Stacked Concrete Blocks or Gabion Baskets Construction of a stacked-concrete-block wall alternative (Ultrablock or similar) or a rock-filled gabion basket wall alternative would consist of excavating into the toe of the existing slope in short sections of a few feet and placing stacked concrete blocks or construction rock-filled gabion baskets and placing granular backfill to retain the slope. If concrete blocks are used, we anticipate the blocks would receive an architectural facing. The advantages of commercially available concrete blocks, such as Ultrablock, are that they are relatively economical to procure and straightforward to handle. Materials for gabion baskets should be relatively economical but may require more handling and time to construct than a concrete block wall.

The existing slope under the viaduct is steep at or near the angle of repose of the existing soil in many areas, and a temporary cut required to place the new wall elements will need to be very steep so as not to undermine footings upslope. An unsupported, steep, temporary cut could result

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum Page 5 in sloughing of the cut face and potential instability extending up the slope that could adversely affect the viaduct footings. Once instability begins in steeply sloped, granular material, the failure can quickly advance uphill due to raveling and sloughing on the face of the cut and upslope surface. Where the soil contains a significant amount of fines, it may stand up steeply for short time periods to permit placement of the concrete blocks or gabion baskets, but this is difficult to ascertain without observations made in the field at the time of construction. In this regard, a temporary shoring system will likely be necessary to support the excavation during construction of the wall. Installation of a temporary shoring system on/in the steep slope composed of coarse, granular materials will be difficult and result in significant disturbance of the uphill slope supporting the footings, depending on how it is implemented.

The external stability of planned walls should be checked to confirm satisfactory external stability (e.g., sliding, overturning, eccentricity) can be achieved for long-term conditions. The required configuration and external stability of a possible concrete block or gabion basket wall can be evaluated assuming an active lateral earth pressure based on an equivalent fluid unit weight of 100 pounds per cubic foot (pcf). This active earth pressure criterion assumes drained conditions, a backslope angle of between 35° and 40°, an effective unit weight of about 125 pcf, and an effective internal angle of friction approximately equal to the backslope angle. Sliding along the base of the concrete or gabion-basket wall can be computed assuming nominal coefficients of friction of 0.45 and 0.55 between the bases of the concrete blocks and gabion baskets, respectively, and the underlying soil. It is assumed that the wall would be placed on a leveling course of angular, compacted, crushed rock. Ideally, the base of the wall would be embedded at least 1 foot below finished grades. However, this may not be feasible due to the creation of additional temporary cut height and resulting stability considerations of the temporary cut and slope and/or the presence of existing viaduct foundations. Excavations should not be made below the base of the existing footings to reduce the risk of undermining the footings. We understand the American Association of State Highway and Transportation Officials (AASHTO), Load Resistance Factor Design (LRFD) Bridge Design Specifications will be used for design of the project. In our opinion, the wall can be designed for a nominal bearing resistance of up to 6 kips per square foot (ksf) and 2 ksf for the Strength and Service limit states, respectively. LRFD resistance factors of

0.45 and 1.00 are appropriate for the Strength and Service limit states, respectively. Because gravity retaining walls tend to rotate outward slightly to achieve the active earth pressure condition, a slight inward batter should therefore be considered in design. In our opinion, a minimum of 5° to 10° inward batter would be reasonable.

Ultrablock is typically manufactured in two common lengths, 5 feet and 2.5 feet. The width and height of Ultrablock are typically 2.5 feet. The horizontal spacing between the bridge-support columns appears to be on the order of 8.5 feet. Further, there is a grade beam present that extends downslope (from south to north) through the existing footing located midway between the shear

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum Page 6 walls. Consideration will therefore need to be given regarding configuration of the blocks relative to the existing structure, and modifications may be needed to address “gaps” where the blocks do not fit snugly. Other commercially available concrete block products of different dimensions may be available for consideration with this alternative. Rock gabion baskets are also available in standard dimensions that may not fit ideally with the column spacing, requiring special-order lengths and/or other modifications to fit properly.

As an alternative to concrete blocks or gabion baskets, it may be possible to utilize a stacked-rockery wall to retain the slopes. However, the previously discussed construction considerations would also be applicable to construction of a stacked-rockery wall. Stacked-rockery walls should be designed and constructed in substantial conformance with the guidelines presented in the November 2006 Federal Highway Administration (FHWA) Publication No. FHWA-CFL/TD-06-006, Rockery Design and Construction Guidelines.

Mechanically Stabilized Earth (MSE) Wall Similar to a stacked-concrete-block, gabion basket, or stacked-rockery wall, a mechanically stabilized earth (MSE) gravity wall could potentially be constructed to retain the slopes. The previously discussed construction considerations would also be applicable to an MSE wall.

However, it is anticipated an MSE wall will require more effort and time to construct than placing concrete blocks or gabion baskets and likely a more extensive excavation into the slope to install the required lengths of reinforcement strips/grid. Placing the reinforced MSE backfill will take significantly longer than backfilling behind a concrete or gabion basket wall and require workers to enter the temporary cut area to place the layers of reinforcement. Consequently, this alternative may potentially result in more risk of inducing slope instability during construction than a concrete blocker or gabion basket wall due to the excavation extending farther into the hillside and the additional time the cut will need to be open (if unsupported) to place the required reinforcing strips and MSE backfill.

Drilled Cantilever Soldier Pile Wall A third alternative would be to install drilled, cantilevered soldier-pile walls. The walls will retain the soil with lagging placed between the piles. The embedded portion of the piles provides the lateral resistance from passive soil resistance in front of the piles. Due to the possible presence of coarse, granular soil, as observed at the surface, and/or the potential for encountering intact rock, we anticipate driving or auguring to install solider piles may be impractical. Additionally, driving piles will result in vibrations that could potentially affect the viaduct. Augering unsupported holes near footings could potentially result in sloughing and the undermining of footings. Limited overhead clearance under the viaduct may not permit the use of typical pile driving or auger equipment. Alternatively, it may be practical to install drilled micropiles for the solider piles. Typical rotary-drilling micropile rigs can drill through a variety of subsurface materials, including coarse, granular materials and rock, with the proper tooling and bits. A low-overhead micropile drill rig

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum Page 7 may be required to drill under the edge of the viaduct. A temporary pad of fill may be needed to elevate and level the drill rig for installation of the micropiles. It is anticipated that drilling for micropiles would result in considerably less excavation and disturbance to the slope than installation of a gravity wall (concrete block, gabion basket, rockery, or MSE wall types) in most locations, although some local shoring may be needed if raveling or sloughing occurs. Lagging for the wall could be placed on the front or back of the pile and an architectural facing placed on the front.

Micropile nominal diameters are commonly on the order of 5 inches to 8 inches, with nominal wall thicknesses of ⅜ inch to ½ inch. The micropile can be filled with cement grout (and steel reinforcement, if needed) to provide additional structural stiffness. Cuttings are typically expelled from the drilled hole using compressed air, which will require the contractor to provide containment around the drill rig to avoid ejecting drill cuttings against the underside of the viaduct and/or onto the adjacent railroad. Micropiles are installed by specialty foundation contractors, who commonly design the pile and select the installation means and methods based on criteria provided by the owner’s design team. However, for this project, we anticipate the pile will be designed by DEA and the micropile installer will select the installation means and methods.

Lateral loading on a cantilever, micropile, or soldier-pile wall can be evaluated assuming an active soil lateral earth pressure based on an equivalent fluid unit weight of 100 pcf acting against the back of the lagging. This lateral earth pressure assumes drained conditions. Additional lateral resistance due to passive earth pressures in front of the micropiles can be computed based on equivalent fluid having unit weights of 250 pcf and 500 pcf (nominal passive resistance) for the Service and Strength limit states, respectively. ASSHTO LRFD resistance factors of 1.0 and 0.5 should be applied to the passive nominal resistances for the Service and Strength limit states, respectively. The passive resistance acts over two pile diameters. The earth pressure distributions will be triangular, starting at the top of the ground surface and increasing with depth. We recommend ignoring the upper 1 foot of passive resistance in front of the piles due to disturbance during construction and/or potential for loose surface soil. The above criteria assume the piles will have a minimum center-to-center spacing of at least three pile diameters.

The lateral design of the micropiles can also be evaluated using the software LPILE, published by Ensoft, Inc. We recommend using the sand (Reese) soil model to analyze the embedded portion of the pile that will be providing resistance from passive soil pressure. We recommend an effective unit weight of 115 pcf to a depth of 5 feet below finished grades and 55 pcf below this depth, an internal angle of friction of 34°, and a soil modulus parameter, k, of 90 pounds per cubic inch (pci) to a depth of 5 feet and 75 pci below this depth. These parameters assume groundwater could rise to within 5 feet of the ground surface during extended periods of wet weather. A group p-modifier of 1 is applicable for a single row of piles with a center-to-center spacing of three pile diameters or greater.

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum Page 8

Wall Drainage and Backfilling Obvious indications of significant amounts of groundwater seepage emerging on the slope at the locations of the old walls were not observed during our 2014 reconnaissance. However, drainage should be considered in the wall design. The type of drainage will depend on the wall type and construction considerations. Installation of a perforated drainpipe and a fabric-wrapped granular drainage blanket behind walls, particularly gravity-block walls, may require additional excavation into the slope, increasing the difficulty of construction and risk of instability of the temporary excavation. For a concrete block, gabion basket, or dry-stacked rockery wall, we anticipate well-graded, crushed rock having a maximum particle size of 1½ inches and containing less than 5% passing the No. 200 sieve (washed-analysis methods) will be suitable for backfilling and provide adequate drainage to allow water to flow through the gaps between the wall blocks or rocks, reducing the risk of a hydrostatic condition developing behind the wall. If a grouted rockery wall is used, gaps in the rocks or weepholes near the base should be present to allow for drainage.

Using the aforementioned well-graded, crushed rock will also prevent fines from the existing soil from piping through the backfill to the face of the wall. However, for an MSE wall, it may be necessary to include a zone of open-graded, crushed rock and a perforated drainpipe. If the MSE wall option is selected, GRI can further evaluate drainage requirements with the MSE wall designer.

We anticipate placement of commercially available drainage board with weepholes at the base of the lagging will be a practical drainage alternative for a micropile wall. If utilizing drainage board, we recommend backfill behind the back of the wall consist of the aforementioned well-graded, crushed rock.

Backfill should be compacted using lightweight, vibratory, hand-operated equipment to about 93% of the relative dry density determined by ASTM International D698 (AASHTO T 99) to avoid inducing excessive lateral pressure against the wall.

Temporary Construction Path Ballast Reuse As discussed previously, placement of keyed riprap was planned to mitigate the walls in poor condition under the East Viaduct and for general slope stabilization under both the East and West Viaducts. However, due to cost considerations, the keyed riprap will not be placed as part of this project. In-lieu of the riprap, the construction path ballast rock will be reused at the end of construction to regrade local areas of ground under the viaducts that have been oversteepened from erosion and have impacted footings, and potentially used to buttress the existing retaining walls under the east abutment, if quantities allow. A typical detail showing the general configuration of the ballast placed on the slope under the viaduct is shown on the Ballast Slope Protection (Typical Detail), Figure 2 and on the Slope Stabilization and Footing Repair Detail by DEA in Appendix B. It should be possible to place angular ballast at finished slopes of up to about 1.5H:1V. It is anticipated that the placement under the viaduct will require small excavators to move the material and potentially some hand placement to assist in keying the pieces together.

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum Page 9

Due to the steep slope under the viaduct the base of the ballast slope will need to be established on a relatively flat bench(es) to reduce the risk of the ballast from raveling down the slope. We recommend the toe of the ballast slope at the base of the slope be embedded in a toe trench having a minimum depth below the existing ground surface of 1 foot and a width of at least 4 feet.

The ballast rock used should be angular, hard, durable, and manageable with the anticipated excavation equipment. We recommend the ballast be placed in lifts not thicker than 12 inches and tamped until dense and well keyed. In our opinion, a granular filter blanket or geotextile fabric will not be necessary between the ballast and the existing slope or walls (where present).

East Viaduct Existing Leaning Rockery Wall During our 2014 reconnaissance, we observed the upper 5 feet to 6 feet of the approximately 15-foot-tall, dry, stacked-rockery wall along the north side of the road at the west end of the East Viaduct had a slight lean outward that resulted in an estimated horizontal gap of 3 inches to 4 inches between the rockery wall and the northerly viaduct column at Bent 1. Additionally, the curb along the top of the wall appears to be offset to the north several inches relative to the curb on the viaduct. The amount of curb offset observed during our June 2021 site visit appeared to be about the same as observed during our July 2014 site visit. Photographs showing the outward lean of the wall and the offset curb are provided on the East Viaduct Leaning Rockery Wall, Figure 3 and on East Viaduct Offset Curb, Figure 4, respectively. The mechanisms that resulted in the offsets observed in the wall and curb are unknown but could be the result of excessive lateral pressure/load against the back of the wall or, possibly, settlement of the wall resulting in a slight outward lean. It is not known how long the wall has exhibited the observed lean and gap or if movement is continuing. As part of our 2015 pavement investigation, three borings were completed to a depth of about 6.5 feet in the roadway at locations varying between 50 feet and 200 feet to the west of the west end of the East Viaduct. All three borings disclosed sandy and silty gravel below the pavement section to the bottom of the borings. As-built drawings for the wall are not available, and the internal configuration and composition of the wall and backfill at the location of the leaning area of the wall are unknown.

As discussed previously, we understand reconstruction of the leaning portion of the wall to mitigate observed deformations is not currently planned due to project cost constraints. However, if in the future reconstruction is initiated, it should be understood that the backfill conditions behind the wall are unknown, as is whether removal of the rock at the face of the wall will expose backfill and require backfill removal. If removal of existing backfill is required, it may be practical to reuse the backfill during reconstruction. However, for planning purposes, we recommend assuming imported backfill will be placed behind the wall for the restoration. The imported backfill material should consist of well-graded, crushed rock with a maximum particle size of 1½ inches and containing less than 5% passing the No. 200 sieve (washed analysis). We anticipate actual requirements may need to be confirmed and modified at the time of construction based on actual conditions disclosed.

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum Page 10

RENEWS: 12/2023

LIMITATIONS

This memorandum has been prepared to aid the project team in the planning and design of this project. The scope is limited to the specific project and location described herein, and our description of the project represents our understanding of the significant aspects of the project relevant to the geotechnical design and construction of the proposed viaduct repairs. The conclusions and recommendations submitted in this memorandum are based on the data obtained from our geologic and geotechnical reconnaissance of the sites and other sources of information discussed in this memorandum. Subsurface information in the immediate area of the project is very limited, and our interpretation of potential subsurface conditions for development of our recommendations is solely based on visual examination of surface features made during previous reconnaissances and available geologic literature. The actual nature and extent of variation in subsurface conditions may not become evident until construction. If, during construction, subsurface conditions are different than assumed in this memorandum, we should be advised at once so we can observe and review these conditions and reconsider our recommendations where necessary.

Please contact the undersigned if you have any questions.

Submitted for GRI, George A. Freitag, CEG Keith S. Martin, PE, GE Principal Senior Engineer

6783-A MULTNOMAH FALLS VIADUCTS REPAIR MEMORANDUM

Enclosures: Figure 1, Project Alignment Map Figure 2, Ballast Slope Protection (Typical Detail) Figure 3, East Viaduct Leaning Rockery Wall Figure 4, East Viaduct Offset Curb Appendix A, GRI, “Geotechnical Site Reconnaissance, Potential Geotechnical Issues, and Initial

Recommendations for Resolution, Multnomah Falls Viaducts Repair, OR PFH 163(21), Task Order No. DTFH7014F10001, Multnomah County, Oregon,” dated July 9, 2014

Appendix B, Slope Stabilization and Footing Repair Detail Plan Sheet

This document has been submitted electronically.

RENEWS: 02/2024

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MULTNOMAH FALLS, OREG. (2014)

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EAST VIADUCT

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DAVID EVANS AND ASSOCIATES, INC.

MULTNOMAH FALLS VIADUCT REPAIRR IG

PROJECT ALIGNMENT

MAP

SEP. 2021 JOB NO. 5594-D FIG. 1

0 2,000 FT1,000

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N

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USGS TOPOGRAPHIC MAP

BRIDAL VEIL, OREG. (2014)

MULTNOMAH FALLS, OREG, (2014)

PROJECT ALIGNMENT

MAP

MAR. 2023 JOB NO. 6783-A FIG. 1

DAVID EVANS AND ASSOCIATES, INC.

MULTNOMAH FALLS VIADUCTS REPAIR

MAR. 2023 FIG. 2JOB NO. 6783-A

BALLAST SLOPE PROTECTION

(TYPICAL DETAIL)

4 FT MIN. WIDTH

(2) TOE TRENCH

1.5

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EXISTING SLOPE

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WHERE PRESENT (IF PERMITTED)

1 FT MIN. DEPTH (2)

TOE TRENCH

(1) SLOPE AT NO STEEPER THAN 1.5H:1V

(2) DO NOT EXCAVATE BELOW EXISTING FOOTING

MULTNOMAH FALLS VIADUCTS REPAIRSR IG

EAST VIADUCT

LEANING ROCKERY WALL

LEANING ROCKERY

WALL

EAST VIADUCT

SEP. 2021 JOB NO. 5594-D FIG. 3MAR. 2023 JOB NO. 6783-A FIG. 3

DAVID EVANS AND ASSOCIATES, INC.

MULTNOMAH FALLS VIADUCTS REPAIR

EAST VIADUCT

LEANING ROCKERY WALL

MULTNOMAH FALLS VIADUCTS REPAIRSR IG

EAST VIADUCT

OFFSET CURB

PHOTO FROM GOOGLE EARTH PRO, 2018

TOP OF ROCKERY WALL

CURB OFFSET

EAST VIADUCT

SEP. 2021 JOB NO. 5594-D FIG. 4MAR. 2023 JOB NO. 6783-A FIG. 4

DAVID EVANS AND ASSOCIATES, INC.

MULTNOMAH FALLS VIADUCTS REPAIR

EAST VIADUCT

OFFSET CURB

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum

APPENDIX A

GRI, “Geotechnical Site Reconnaissance, Potential Geotechnical Issues, and Initial Recommendations for Resolution, Multnomah Falls Viaducts Repair, OR PFH 163(21), Task Order No. DTFH7014F10001, Multnomah County, Oregon,” dated July 9, 2014

M E M O R A N D U M

To: Kevin Bracy, PMP / David Evans and Associates, Inc. Date: July 9, 2014

GRI Project No.: 5594

From: Keith Martin, PE, GE and George Freitag, CEG

Re: Geotechnical Site Reconnaissance, Potential Geotechnical Issues, and Initial Recommendations for Resolution Multnomah Falls Viaducts Repair

OR PFH 163(21)

Task Order No. DTFH7014F10001 Multnomah County, Oregon

As a subconsultant to David Evans and Associates, Inc. (DEA), GRI is providing geotechnical consultation for repair of two viaducts at Multnomah Falls, located between approximately MP 17 and 18 on the Historic Columbia River Highway (HCRH) about 13 miles east of Troutdale, Oregon. The purpose of our work was to evaluate existing conditions at the east and west viaducts sites and identify geotechnical-related issues that may need to be addressed as part of the restoration of the viaducts. Our work to date has included review of available information for the viaducts; a project team kick off meeting on June 26, 2014; and a ground-level reconnaissance by a geotechnical engineer and engineering geologist from GRI on July 1, 2014. Available drawings associated with the viaducts are attached. This memorandum summarizes our observations at the site and preliminary conclusions regarding the significant geotechnical-related issues associated with the viaducts and provides a list of potential geotechnical issues and initial recommendations for resolution.

EAST VIADUCT

The east viaduct was constructed in 1913. The west end of the east viaduct is located about 1/4 mi. east of Multnomah Falls Lodge and is situated along the lower portion of a steep, north-facing slope composed of Columbia River Basalt. An active railroad is located immediately north of the viaduct. The viaduct is a reinforced, cast-in-place concrete structure with a total length of about 860 ft and a width of about 20 ft out-to-out. The viaduct carries two approximately 9-ft-wide lanes (one lane in each direction). The deck of the viaduct is 15 to 20 ft above the ground surface along the north side of the structure and is supported by 87 bents. The two end bents (abutments) and 85 intermediate bents result in a total of 86 10-ft-long spans.

A concrete railing is located on the north side of the deck. On the south side of the structure, the viaduct is established in a cut, and an approximately 2-ft-high concrete curb wall is present along the entire length of the structure. In some locations, the curb wall retains the steep cut slope above the viaduct. In other locations, a gap or space is present between the steep cut and viaduct structure.

DRAFT

We understand the original construction consisted of two columns (north and south side) at each bent supported on square spread footings measuring 3 ft on a side. Grade beams were constructed transverse between footings in the same bent line. Based on repair plans (attached drawing No. 1725) and our observations at the site, the structure appears to have been modified/strengthened by adding a new column and footing at the center of each intermediate bent and concrete shear walls between the north and south columns (transverse) at every other intermediate bent and enlarging some original footings and columns. Based on our observations at the site, additional remedial work that appears to have been completed during the life of the structure consists of slope paving with cobbles and mortar around and below some footings to mitigate slope raveling and widening of some of the columns supporting the south side of the structure.

Several geotechnical-related issues identified during our ground-level reconnaissance of the east viaduct are discussed below. For the purpose of discussion, the bents are numbered 1 through 87 (west to east).

In general, the slopes beneath the east viaduct are relatively steep, estimated to be on the order of 35 to 45, with locally flatter and steeper areas. The soil exposed at the ground surface primarily consists of relatively loose, angular gravel and cobbles with varying amounts of sand and silt and appears to be at or near the natural angle of repose in many areas. Raveling of the granular soil appears to have reduced the embedment of many footings, and the bottom of some footings are exposed on the downhill side. As discussed previously, at some time during the life of the structure, the slope around some of the footings was paved with cobbles and mortar to prevent further raveling of soil on the slope from undermining the footings. In some locations, raveling has undermined the mortared cobbles. The raveling of the slope is likely primarily due to wind, wetting and drying, and freezing and thawing mechanisms rather than surface water flow. Retaining walls consisting of stacked cobbles or railroad ties are present at the base of the slope between the north columns along much of the viaduct alignment. The walls are about 3 to 4 ft high and appear to be retaining the lower few feet of the slope under the viaduct. The condition of the many of the walls is relatively poor, and some of the rockery walls have collapsed. At the west abutment of the east viaduct, the top 5 to 6 ft of the rockery wall appears to have a slight lean outward, resulting in a space of 3 to 4 in. between the rockery wall and the northerly viaduct column at bent 1.

As discussed previously, the south side of the east viaduct is located in a cut. Between about bents 12 and 30 and bents 66 and 87 (east abutment), the viaduct is embedded into the uphill slope and retains an estimated 4-ft thickness of the steep slope above the road (2-ft curb wall plus 2 ft of embedment below the deck). Between bents 1 and 12 and bents 30 through 66, a gap or space estimated to be approximately 2 to 5 ft wide and up to about 8 ft deep is present between the side of the viaduct and the steep cut made to construct the viaduct. In some locations, raveling in the area between the cut and viaduct has resulted in the deposition of several feet of soil behind some of the uphill columns. There is some uncertainty as to whether the viaduct is adequately designed to resist these soil loads. Several relatively large boulders are lodged in the gap between the cut and the south side of the viaduct. It was not obvious how long the boulders have been present behind the viaduct or whether the boulders raveled out of the cut or rolled down the slope from farther above the viaduct. There is currently no form of catchment above the viaduct to prevent large debris upslope from impacting the viaduct.

Near the east end of the viaduct, two creek channels are present on the slope above the viaduct. The channels are oriented approximately perpendicular to the roadway and terminate at top of the short curb wall on the south side of the viaduct. Water was not observed in the channels at the time of our reconnaissance. We anticipate water flows in the channels primarily during periods of intense or prolonged precipitation common during the wet fall through spring months. The outlet of an approximately 10- to 12-in.-diameter plastic pipe was observed emerging from behind the wall at the east end of the viaduct. An inlet, or opening, in the pipe was observed at the base of one of the channels. Due to debris in the channel, an inlet into the pipe for the other channel was not obvious. Indications of erosion from water flowing from these channels and extending beneath the viaduct were not obvious. It appeared that water in the channels was either going into the drain pipe and/or onto the roadway.

However, localized areas of damp soil were observed under the upper part of the viaduct, primarily beneath the areas where the curb wall retained the slope. Although eroded channels were not present in the slope under the viaduct, it appeared that some groundwater seepage has likely contributed to localized erosion under the curb beam and around some footings at the top of the slope where the viaduct was retaining portions of the slope.

Obvious indications of deep-seated slope instability were not observed at the site at the time of our reconnaissance.

WEST VIADUCT

The west viaduct was also constructed in 1913. The east end of the viaduct is located about 500 ft west of Multnomah Falls Lodge and is situated along the lower portion of a steep, north-facing slope composed of Columbia River Basalt. An active railroad is located immediately north of the viaduct. The viaduct consists of a reinforced, cast-in-place concrete structure with a length of about 400 ft and a width of about 20 ft out-to-out and supports two approximately 9-ft-wide lanes (one lane in each direction). The deck of the existing viaduct is up to about 15 to 20 ft above the ground surface on the north side and is supported by the two end bents (abutments) and 19 intermediate bents, resulting in total of 20 20-ft-long spans. A concrete railing is located on the north side of the roadway. On the south side of the roadway, the deck of the viaduct is established in a cut that is retained by a 3- to 5-ft-high (exposed height) concrete retaining wall. We understand the original construction consisted of two columns (north and south side) at each bent that are supported on a square spread footing measuring 3 ft on a side. We understand that at a somewhat later date, the viaduct was strengthened by increasing the cross sectional area of the columns beneath the north side of the viaduct and extending a grade beam between the north and south footings at each bent.

Based on our observations at the site, the slopes beneath the west viaduct are relatively steep, on the order of 30 to 40 with locally flatter and steeper areas, and are mantled primarily with angular gravel and cobbles with varying amounts of sand and silt. An outcrop of relatively hard basalt is present on the upper portion of the slope under the viaduct between about bents 3 and 5. The ground under the viaduct slopes downward to the north at a relatively uniform inclination to the railroad tracks. Unlike the east viaduct, retaining walls are not present at the base of the slope. The curb beam beneath the south side of the structure appears to be set back a horizontal distance of about 1 to 1.5 ft from the edge of the slope.

However, the northerly edges of several footings under the south side of the structure are located at the top of the slope, and the bottom edge of the footing is exposed; undermining of footings was not observed. It appears some raveling and loss of ground beneath the west viaduct has occurred since the original construction; however, there were no obvious indications that foundation support has been significantly affected. At bent 1 (west abutment), the footing for the northerly column is founded within the lower portion of the rockery wall. It appears some of the rock pieces on the north side of the footing are missing.

Along the top side of the viaduct, a concrete railing is present along the north side of the structure. On the south side of the structure, a 3- to 5-ft-high (exposed height) concrete retaining wall is present. The ground surface above the retaining wall slopes steeply upward to the south and is vegetated with brush, shrubs, and some large trees. In general, the retaining wall appears to be in relatively good condition with the exception of an approximately 3-to 4-ft-long section at the west end that has cracked and settled. Similar to the east viaduct, there is no form of catchment above the viaduct to prevent debris from rolling down the slope and impacting the viaduct. Soil creep on the slope above the wall is evident based on the curved trunks of several trees above the wall. Soil creep is the very slow, gradual creep-like movement of the upper few feet of the soil mantle due to gravity. Obvious indications of deep-seated slope instability were not observed at the site at the time of our reconnaissance.

GEOLOGIC HAZARDS

The ground-reconnaissance discussed above focused primarily on slopes under and immediately adjacent to the viaducts. However, it is recognized that the viaducts are located at the base of a very steep slope that extends several thousand feet upward to the south. Areas of the slope above the viaducts contain large, tall rock cliffs and oversteepened slopes that can be the source of rockfall. Rock debris was observed in the concrete ditch on the uphill side of the east viaduct. Damage from rock impacts was observed on the uphill side of the concrete railing on the east viaduct. Pavement damage from rock impacts was observed on the east viaduct.

In addition to geologic hazards from upslope, the viaducts could experience relatively strong shaking from a seismic event.

POTENTIAL GEOTECHNICAL ISSUES AND INITIAL RECOMMENDATIONS FOR RESOLUTION

Indications of foundation settlement or horizontal deformation of the viaduct structures were not observed at the time of our reconnaissance, and the viaducts generally appeared to be in good condition. However, the potential issues identified during our ground-level reconnaissance as discussed above could potentially have an adverse effect on the structures. The viaducts are located in a confined area between the railroad to the north and the steep slope to the south, which will need to be considered when developing mitigation alternatives.

Based on our reconnaissance and review of available information, we have developed the following list of potential geotechnical issues and recommendations for resolution.

Issue: Soil raveling around east viaduct footings.

Recommendation: Detailed mapping of raveling areas. LiDAR or detailed conventional survey to determine slope configuration beneath viaducts.

Geotechnical investigation of areas requiring mitigation. Develop design options for mitigation.

Issue: Aged/collapsed retaining walls at lower portion of slope under east viaduct, outward leaning rockery wall at west abutment of east viaduct, and missing rock pieces at west abutment of west viaduct.

Recommendation: Detailed mapping of retaining wall areas. LiDAR or detailed conventional survey to determine slope configuration beneath viaducts.

Geotechnical investigation of areas requiring mitigation. Develop design options for mitigation.

Issue: Uphill soil loading on east viaduct.

Recommendation: Geotechnical/structural analysis of existing conditions. Geotechnical investigation of areas requiring mitigation. Develop design options for mitigation.

Issue: Rockfall hazard to east and west viaducts.

Recommendation: Additional reconnaissance and detailed mapping of rockfall areas above viaducts and impact areas on viaducts to establish limits. Develop options for mitigation.

LIMITATIONS

This memorandum has been prepared to aid in the evaluation and conceptual design of repairs to the Multnomah Falls viaducts. The information provided in this memorandum is preliminary and is intended for project planning and scoping purposes and is based on a limited ground-level reconnaissance of the viaducts and other sources of information described herein.

Please contact the undersigned if you have any questions.

Submitted for GRI, George A. Freitag, CEG Keith S. Martin, PE, GE Associate Senior Engineer

5594 RECON MEMO

GRI 6783-A – Multnomah Falls Viaducts Repair Memorandum

APPENDIX B

Slope Stabilization and Footing Repair Detail Plan Sheet

WESTERN FEDERAL LANDS HIGHWAY DIVISION

FEDERAL HIGHWAY ADMINISTRATION

U.S. DEPARTMENT OF TRANSPORTATION

OREGON

NO. DATE BY REVISIONS DESIGNED BY DRAWN BY CHECKED BY SCALE PROJECT TEAM LEADER

AS SHOWN ON PLANS

NO. DATE BY REVISIONS

BRIDGE DRAWING

of

DATE DRAWING NO.

NATIONAL SCENIC AREA

COLUMBIA RIVER GORGE

FLAP DOT CRGNSA 100(8)OR

STATE PROJECT

NUMBER

SHEET

D. ALTENBURG A. BLANKENSHIP

MARCH 2023

RENEWS: 12-31-2023

OREGON

E R

G

IS

TE

RED ROP FESSION

A

LE

N

GINEE

R

72495PE

2 ,21 REBM

E

V O

N

PI

HSNEKNALB EIRAM A

DN

A

M A

Existing ground

" = 1'-0"8

Scale:

undisturbed in-situ ground.

Cast grout against

Grout (East Viaduct).

Pay limits of existing footings

Bottom of existing footing existing ground below to lowest elevation of

Excavate level bench

VIEW A-A

" = 1'-0"16 Scale:

Protect in place.

Existing strut, typ.

Existing footing, typ.

A

A ground

Existing beneath footing

Approximate void

Viaduct)

Grout (East limits of

Placement

FOOTING REPAIR

EAST VIADUCT

directed by CO.

any excess ballast material as centered on footing repair.Place material 10-feet along station, repair. Extend placement of locations of East Viaduct footing stabalize the slope at all the exit and place as shown to control stabalized construction access path and Soil erosion from the Temporary ballast base, grading c (railroad ballast)

Temporary ballast Aggregage deck project work, remove the

After the completion of below

(East Viaduct)

Structure excavation pay limits sheet C.3.

access path. See

Temporary ballast

Grout pay limits

1.5 Min.

1.5 Min.

utility pipe in place

Bury existing abandoned

A. CALCAGNO E. FERLUGA 24 RG3040-X

FOOTING REPAIR DETAILS

SLOPE STABILIZATION AND

I.24 level bench

2'-0" Min.

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M in o v e r e x tg g r o u n d

M in c o v e r

4'-0"

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