ATT F - Geotechnical Engineering Report May 2025.pdf

GEOTECHNICAL ENGINEERING REPORT

US101: MP303.06 Emergency Culvert Repair Project Oregon Coast Highway (US101/Hwy 9), Curry County, Oregon Key No. M22022 Mile Point 303.06 EA : 3253204-031

Geo-Environmental Unit ODOT Region 3 Tech Center 3500 NW Stewart Parkway Roseburg, Oregon 97470

Published Date: May 2025 Bid Date: June 2025

Prepared By: Tomsen Reed, PE, GE, CEG Reviewed By: Brad Hayes, PE, GE

TABLE OF CONTENTS

VICINITY MAP ................................................................................................................ ii SITE MAP ...................................................................................................................... iii

1.0 INTRODUCTION

2.0 BACKGROUND REVIEW AND GEOLOGIC INVESTIGATION

3.0 SEISMICITY EVALUATION

4.0 HYDRAULICS INFORMATION

5.0 GEOTECHNICAL DESIGN RECOMMENDATIONS

5.1 Trenchless Culvert Installation

5.2 Horizontal Drain Installation

5.3 Stream Restoration, Backfill of Existing Culvert, and Backfill of Scour Related Voids

6.0 CONSTRUCTION RECOMMENDATIONS

6.1 Earthwork

6.2 Backfill of Existing Culvert and Backfill of Scour Related Voids

7.0 SPECIAL PROVISIONS

8.0 LIMITATIONS

9.0 REFERENCES

10.0 SIGNATURE PAGE

US101: MP303.06 Emergency Culvert Repair Project (Maintenance #M22022) Oregon Coast Highway (US101/Hwy 9), Mile Point 303.06 Geotechnical Engineering Report Date: May 12, 2025 ii

VICINITY MAP

Oregon Coast Highway (US101/Hwy 9) Curry County, Oregon Maintenance No. 22022 Mile Point 303.06 EA: E3253204-031

Figure modified from ODOT TransGIS (ODOT, 2025)

OREGON DEPARTMENT OF TRANSPORTATION – REGION 3 TECH CENTER, GEO-ENVIRONMENTAL UNIT

Geotechnical Engineering Report Date: May 12, 2025 iii

SITE MAP

Figure modified from ODOT TransGIS (ODOT, 2025)

OREGON DEPARTMENT OF TRANSPORTATION – REGION 3 TECH CENTER, GEO-ENVIRONMENTAL UNIT

Approximate Existing Culvert Location

GEOTECHNICAL ENGINEERING REPORT

1.0 INTRODUCTION

The proposed project is located approximately 1.5 miles south of Port Orford, Oregon in Curry County, and includes the installation of a new culvert to convey an unnamed creek below US101. As discussed in the Engineering Geology Report for the project (Cunningham and DeKoekkoek, 2025), the existing culvert crosses over the margin of a large existing landslide complex and under a large embankment fill and has had associated issues with shearing and rupturing since its construction in 1946. Planned construction activities will consist of replacing the failed culvert along a new alignment using trenchless installation methods, installation of horizontal drains to reduce groundwater levels in the embankment, and placement of Low Density Cellular Concrete (LDCC) in voids that have formed near the base of the existing roadway embankment as a result of piping erosion and scour, to be discussed further below.

2.0 BACKGROUND REVIEW AND GEOLOGIC INVESTIGATION

Discussion of regional and site-specific geology, including subsurface conditions near the proposed culvert alignment, are discussed in detail in the Engineering Geology report for the project (Cunningham and DeKoekkoek, 2025). A summary of salient points is provided below:

1. A subsurface study was previously performed at this site in the late 1990s to early 2000s as part of a project with the intent to mitigate the landslide movement along this stretch of highway between MP303.0 and MP303.5 (the Rocky Creek Landslide Complex). An inclinometer installed near the existing culvert during this previous study indicated slope movement below the base of the culvert (99303-02; see Geotechnical Data Sheets), which likely caused the shearing of the existing culvert.

2. A new subsurface study (performed in 2024 through 2025), that included the installation of several slope inclinometers and piezometers, suggests relative stability near the existing culvert inlet and the slope to the north of the existing outlet, shallow (less than 30 feet deep) movement within the roadway embankment fill prism, and deep-seated movement in the slopes near the existing culvert outlet.

3. Elevated groundwater levels (above the shear surfaces indicated by the inclinometers) appear to correlate with increased movements of the landslide and embankment fill. Measured maximum groundwater

Geotechnical Engineering Report Date: May 12, 2025 elevations ranged from 13 feet and 119 feet; but were generally between 77 and 119 feet in and around the roadway embankment.

4. The subsurface materials anticipated along the trenchless culvert alignment include embankment fill and mélange. The embankment fill is anticipated to consist primarily of a mixture of medium stiff to stiff clays and silts and loose to medium dense sands, with the possibility of wood (up to 2.5 feet thick, assumed to be trees) and trace gravels. The Mélange is anticipated to consist primarily of a sheared siltstone/sandstone matrix with inclusions of more resistant blocks, ranging in strength from extremely weak to weak (R0 to R2).

During a field reconnaissance subsequent to the publication of the Engineering Geology Report, we observed scour and piping erosion near the toe of the embankment that likely occurred as a result of a collapse of the existing culvert. Our understanding of these events are summarized below:

1. Prior to March 16, 2025, water was flowing consistently through the entirety of the existing 42” CMP culvert, in spite of the sheared portions of the culvert. Based on culvert video inspection, the sheared portions of the culvert exposed angular rock bridged over the pipe.

2. On March 16, 2025, an intense winter storm produced almost 10 inches of rain at a gauge in Port Orford (NOAA, 2025), generating significant runoff and infiltration.

3. On March 18, 2025, we visited the site and observed that water had, at some point, backed up behind the inlet of the existing culvert to a height of approximately 13 feet above the pipe invert. This observation was based on a line that had formed, below which all of the vegetation was covered with silt and clay particles, while all of the vegetation above this line was free of soil particles. However, at the time of the visit, water was flowing consistently through the culvert and the water levels had receded back to near base levels.

4. On April 4, 2025, we visited the site and observed that water was again backing up behind the inlet of the culvert. The inlet was not visible, suggesting that the ponded water was over 13 feet deep. Only a small amount of water was flowing through the outlet and loose rocks and soil were observed along the invert of the pipe that had not been observed previously. Additionally, a small (6-8” diameter) CMP which had not been observed conveying any water previously (and was filled with sediment in February, 2024), was clear and conveying water. Our interpretation of these observations is that during the intense storm event on March 16 water flowed out of the sheared zones of the existing pipe, potentially dislodging and disturbing the bridged rocks above the

Geotechnical Engineering Report Date: May 12, 2025 pipe. Following this, the rocks fell into the pipe and blocked a majority of the flow, causing the subsequent backing up of water above the inlet.

5. Between April 4 and April 17, water continued to back up and increase in height behind the embankment, which triggered slope movements that displaced pavement, especially in the southbound lane of US101. Water flowed up through the sheared portion of the culvert to the ground surface and created a pond . The water flowed out of the pond and eroded the nearby slopes. Eventually, it appears the water eroded material from beneath the embankment fills, generating large voids near the downslope toe of the embankment. We visually estimated the main body of this void to be approximately 50 feet long by 20 feet wide and 10 feet deep. Another void above the existing culvert appears to be approximately 15 feet wide by 15 feet long by 10 feet deep (although the depth is potentially greater; we could not observe the base of the void because it was still full of murky water at the time of our visit). At the northern end of the main void, we observed a deep natural shaft, approximately 20 to 30 feet deep and about 5 to 10 feet in diameter and could see water flowing at the bottom of this natural shaft. This water observed at the base of the shaft flowed underground and daylighted approximately 100 feet away. The erosion also caused numerous trees on the slope to fall over, as soil support was removed from their roots.

6. Between April 10 and April 23, Maintenance crews installed a temporary pumping system to pump the water from behind the embankment into an adjacent culvert. We understand that by April 23, the water behind the embankment had been pumped down to base flow levels and continues to be pumped as needed to prevent ponding of water.

3.0 SEISMICITY EVALUATION

The proposed structure is considered a buried structure by AASHTO; therefore, per Section 12.6.1 earthquake loads should only be considered if the structure crosses an active fault. The geology evaluation did not identify any active faults within the project limits.

4.0 HYDRAULICS INFORMATION

Based on our discussions with the project team, we understand the following hydraulic constraints for the project:

Geotechnical Engineering Report Date: May 12, 2025

1. The proposed culvert should be designed and constructed with a minimum slope of 2 percent, maximum slope of 10 percent.

2. The inside diameter of the culvert pipe should be 42 inches minimum for a welded steel pipe.

3. Stream restoration will be required at the inlet and outlet to direct water into the new culvert and from the outlet back into the original stream channel before reaching the beach below.

5.0 GEOTECHNICAL DESIGN RECOMMENDATIONS

Design recommendations are based on our review of the proposed project objectives, as well as available existing information, including the Engineering Geology Report and recent field observations.

5.1 Trenchless Culvert Installation

Open trench excavation was considered as a potential option for this project in an earlier stage, but we recommend against this option for the following reasons:

1. The proposed culvert will have a burial depth of approximately 65 feet at its deepest point, and therefore temporary excavations to allow installation of the culvert would need to be greater than 65 feet, in poorly compacted embankment fills, mélange, and possibly landslide deposits.

2. An excavation of this magnitude would most likely necessitate the construction and use of a temporary bridge for traffic, adding costs and risk to the project.

3. Based on the location of the fill adjacent to/overlying a preexisting deep-seated landslide, replacement of the excavated area with full-weight compacted fills has the potential to exacerbate movements of the landslide, which could impact the proposed culvert outlet area. We note that the excavation could be backfilled with lightweight fill but anticipate the volumes of backfill would be very large and expensive.

Placement of the proposed trenchless crossing alignment has been selected to reduce the potential for the pipe to cross existing landslide scarps/boundaries and also to reduce environmental impacts and the potential for permanent retaining walls or structures to reroute water flow towards the existing stream channel. We note that instrumentation data and topographic data / surficial geomorphic expression suggest that the proposed alignment will be located outside the limits of the larger Rocky Point Landslide Complex but may cross over or be near the limits of the road prism/embankment failure near the outlet (see Engineering Geology Report). The proposed inlet

Geotechnical Engineering Report Date: May 12, 2025 in a location that appears to be stable, both based on our recent instrumentation data and the performance of the existing culvert east of the existing shear zone. The proposed outlet location was partially constrained by environmental permitting concerns that the water eventually end up near the same location as the existing outlet and also partially constrained by landslide limits. The proposed outlet was moved as far to the north as possible without necessitating permanent shoring, while still making the construction of a riprap lined channel feasible for directing water from the proposed outlet to the existing stream channel.

5.2 Horizontal Drain Installation

Because of the three-dimensional nature of the Rocky Point Landslide Complex and the associated road prism failure, it is challenging to locate the landslide/failure scarps precisely, relative to the proposed pipe alignment. As discussed in the Engineering Geology Report, it is our opinion that the instrumentation data supports the assumption that the outlet is located beyond the limits of the Rocky Point Landslide Complex, but there is some uncertainty as to whether the pipe will be located within the road prism failure or not. Because of this uncertainty, we recommend the installation of horizontal drains to lower groundwater levels in and around the embankment to reduce the potential of future movements of the road prism. Due to the emergency nature of this project and the complex nature of the Rocky Point Landslide Complex and its interaction with the roadway embankment, we did not conduct a stability analysis, but rather reviewed groundwater data to assess the potential effectiveness of horizontal drains at lowering groundwater levels. The scope of this project does not include full landslide mitigation of the Rocky Point Landslide Complex and that the intent of these horizontal drains is not to mitigate those movements, but simply to reduce the risk and frequency of movement of the embankment. We anticipate that continued movement of the Rocky Point Landslide Complex will eventually displace the embankment, regardless of groundwater levels in the embankment fill.

Groundwater data collected from Test Holes 24015-01, 04, 06 and 07 was reviewed when developing recommendations for the locations of the horizontal drains. Prior to the March 16 storm and the associated blocking of the existing culvert, groundwater levels in these 4 test holes ranged between 87 and 119 feet in elevation.

Following the March 16 storm, the groundwater elevations increased in several of test holes, especially Test Hole 24015-06 in which the groundwater increased from 87 feet to 113 feet. These groundwater increases were accompanied by marked movement at the pavement surface, as well as displacement in the inclinometers. The groundwater response following the March 16 storm was relatively rapid, especially in 24015-06, suggesting that the embankment materials are relatively permeable and will be conducive to draining via horizontal drains.

Geotechnical Engineering Report Date: May 12, 2025

We recommend that the horizontal drains be installed near the base of the existing embankment, which can be accomplished using the existing access road that was constructed for installation of the instrumentation, following anticipated improvements to the road required to access the outlet of the proposed trenchless culvert. We anticipate that the horizontal drain drilling pads will be located between 72 and 76 feet in elevation, with collar elevations between 75 to 79 feet. To maximize potential drawdowns and avoid conflicts with the proposed trenchless culvert installation, we recommend that these drains be installed at an inclination of 2 degrees from horizontal. Plans have been prepared presenting the proposed azimuth and locations of the drains.

5.3 Stream Restoration, Backfill of Existing Culvert, and Backfill of Scour Related

Voids The proposed inlet and outlet locations will necessitate some temporary excavation and stream restoration to direct water into the proposed inlet, and then back toward the existing stream channel after exiting the outlet of the proposed culvert. We note that the hydraulics engineer has prepared recommendations and plans for stream restoration; however, we provide the following supplemental recommendations from a geotechnical perspective:

1. Because the proposed inlet is upstream of the existing inlet, the existing stream channel between the proposed inlet and existing inlet will need to be filled to direct the stream water into the new inlet. We recommend the stream channel be over-excavated at least 2 feet below the surface of the existing streambed and backfilled with a low permeability material to reduce the risk of water percolating back down toward the existing inlet.

2. To construct the proposed inlet, permanent excavation slopes will be required. We recommend that permanent slopes at the proposed inlet be no steeper than 2H:1V.

We recommend the existing culvert be backfilled with a low permeability, flowable material such as CLSM or grout, to prevent the collapse of and seepage through the existing pipe. We do not recommend the use of sand or any potentially permeable materials for backfill of the existing culvert. The existing culvert has been sheared in a distributed zone, starting approximately 210 feet from the inlet, and continuing for at least 20-30 feet through the pipe. Available records indicate the existing culvert is a 36-inch diameter corrugated metal pipe (CMP), but field reports from maintenance personnel suggest the culvert may be 42-inches diameter.

Geotechnical Engineering Report Date: May 12, 2025

As discussed in Section 2.0, a large void has formed as a result of water flowing up and out of the sheared portion of the culvert and down through erodible embankment fill materials. This void is located near the margin of the Rocky Point Landslide Complex; therefore, we recommend the use of a lightweight backfill to reduce additional loading at the margins of the active landslide. Also, due to the emergency nature of the project and the potentially challenging access/safety concerns of getting personnel and equipment to the void area, we recommend the use of a flowable, self-compacting material to backfill the voids, such as Low Density Cellular Concrete, either Type I or Type II.

6.0 CONSTRUCTION RECOMMENDATIONS

The project specifications require several construction submittals with geotechnical elements, such as (but not limited to) trenchless culvert installation, horizontal drain construction, and Contractor designed temporary retaining walls. These submittals should be provided to the Geotechnical Engineer/Designer for review.

6.1 Earthwork

The Contractor is responsible for temporary excavations and shoring design. For planning purposes, we anticipate that the onsite soils will classify as OSHA Type B soils and that temporary cut slope inclinations should not exceed 1 horizontal to 1 vertical (1H:1V), subject to OR OSHA requirements. The contractor may construct temporary shoring to limit the excavation extents.

It is recommended that earthwork be perfomed during the drier portion of the year, typically between the months of June and September to avoid excessively wet soils and inclement conditions.

6.2 Backfill of Existing Culvert and Backfill of Scour Related Voids During the March 16 storm, water backed up behind the culvert inlet and has subsequently flowed through the sheared opening in the pipe and onto the embankment downslope of the roadway. We note that this flow regime suggests that the materials above the sheared portion of the culvert have a relatively high permeability. The Contractor should be prepared to backfill the culvert in stages and/or utilize viscous backfill materials to reduce the amount of grout loss into the soil. We recommend backfilling the existing culvert prior to backfilling the scour related voids, as we anticipate that some of the scour related voids are connected to the sheared portions of the existing

Geotechnical Engineering Report Date: May 12, 2025 culvert; consequently, void backfill materials could potentially travel down to and through the existing culvert if it is not previously backfilled

We recommend that the scour related voids be backfilled prior to installation of the trenchless culvert pipe to reduce potential construction problems associated with drilling through possible voids and fluid/grout loss into the voids. .

We note that backfilling of the scour voids will likely need to be completed in stages to reduce the potential of material loss onto adjacent slopes. We also recommend that the Contractor visually monitor the slopes and existing stream channels for evidence of backfill intrusion and cease backfilling immediately if any intrusion is observed.

7.0 SPECIAL PROVISIONS

The following sections have been modified for project needs through Special Provision, and are included in the project contract documents:

1. Special Provision 00406 – Tunneling, Boring, and Jacking

2. Special Provision 00431 – Horizontal Drains and Discharge Systems

3. Special Provision 00444 – Low Density Cellular Concrete

4. Special Provision 00490 – Work on Existing Sewers and Structures

8.0 LIMITATIONS

The analyses and recommendations presented in this report are based on the data obtained from the subsurface explorations performed at the locations indicated and from other sources of information discussed in this report. The subsurface explorations have provided detailed information at specific locations in the project area. However, variations in soil and rock conditions may exist between the explorations and groundwater levels may fluctuate periodically. The data shown in the exploration logs applies only to the particular exploration listed on the dates indicated and is not intended to be conclusive as to the character of any material or conditions between or around the explorations (see Special Provision 00120.25). Any interpretation or evaluation of this report by individuals outside of the Oregon Department of Transportation is done so at the sole risk of the individuals.

The nature and extent of any variations in subsurface materials or conditions may not become evident until construction. If subsurface conditions different than those identified in the expolorations are observed or

Geotechnical Engineering Report Date: May 12, 2025 encountered during construction, or appear to be present beneath or beyond excavations, the Geotechnical Designer/Engineer should be advised at once so that the Engineer can observe and review these conditions and reconsider the design recommendations where necessary. Groundwater elevations can vary significantly spatially and temporally as a result of precipitation, seasonality, variable subsurface conditions, and other factors.

Conclusions and recommendations made in this report are based on current design and location data. In the event that any changes in the basic design take place, the Geotechnical Designer/Engineer should be given the opportunity to review the changes and to modify or reaffirm in writing the design recommendations.

It is recommended that construction operations relating to earthwork, trenchless culvert installation, horizontal drains, and backfilling of voids and the existing culvert be observed by the Geotechnical Designer/Engineer to determine if the work is proceeding in accordance with the intent of the geotechnical recommendations and to allow for design changes as necessary.

Geotechnical Engineering Report Date: May 12, 2025

9.0 REFERENCES

Cunningham, C.E., and DeKoekkoek, J., (2025). Engineering Geology Report, Culvert Repair Investigation, US101 Oregon Coast Highway, MP 303, Curry County, Oregon, Key 22022, EA E1243204-031. April.

National Weather Service, National Oceanic and Atmospheric Administration [NOAA], (2025). Climate NOWData, accessed on May 2 from https://www.weather.gov/wrh/Climate?wfo=mfr.

Oregon Department of Transportation [ODOT], (2024). Oregon Department of Transportation, Geotechnical Design Manual (GDM). May.

Oregon Department of Transportation [ODOT], (2025). Oregon Department of Transportation, Geographic Information Services Unit ODOT TransGIS. Accessed on May 2 from https://gis.odot.state.or.us/transgis/.

https://www.weather.gov/wrh/Climate?wfo=mfr

Geotechnical Engineering Report Date: May 12, 2025

10.0 SIGNATURE PAGE

Prepared by: Tomsen Reed, PE Region 3 Geotechnical Design Engineer

Reviewed by: Brad Hayes, PE, GE Region 5 Senior Geotechnical Engineer

HWYE31D

2026 Stamp