GM_20_15_HCRH_Seg_D_Station_198_Rockfall_Recom_Memo.pdf

PDF 2 MB Posted Sep 10, 2015

Attached to: OR PFH 163(19), Historic Columbia River Highway Lindsey Creek to Starvation Creek Trailhead
Solicitation number: DTFH70-15-B-00012
Issued by: Department of Transportation Federal Highway Administration

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Geotechnical Memorandum 20-15 - Segment D

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Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

TO: Mike Odom, Project Manager Geotechnical Memorandum 20-15

FROM: Nathan Jenks, Geotechnical Engineer

DATE: June 17, 2015 In Reply Refer to: HFL-17, GM 20-15

SUBJECT: Historic Columbia River Highway State Trail OR PFH 163(19), Segment D Station 198+00 Rockfall Mitigation Recommendations

Introduction

The purpose of this memorandum is to provide geotechnical recommendations for the known rockfall site between approximate station 197+25 and 198+25 along Segment D of the Historic Columbia River Highway (HCRH) State Trail project. Segment D of the HCRH project is located between Lindsey Creek and Starvation Creek Trailhead, along the south side of Interstate 84 (I-84) between approximate mile post 53.5 and 55 in Hood River County, Oregon. A map showing the general location of the station 198+00 rockfall site is shown in Figure 1 and site photographs are shown in Figure 2.

The objective of the mitigation design recommended in this memorandum is to reduce the rockfall hazard to the proposed HCRH State Trail and adjacent I-84. The recommendations herein follow the previously developed design criteria for rockfall mitigation along the HCRH State Trail which is to prevent 99% of probable rockfalls from impacting (direct rockfall impact) the trail and I-84, as well as to prevent at least 90% of probable rockfalls from reaching the trail and I-84 through rolling.

Previous Geotechnical Studies

An assessment of the rockfall hazard and mitigation recommendations for the remainder of Segment D, along with HCRH Trail Segments A through C are contained in the report titled Preliminary Rockfall Mitigation Report, Historic Columbia River Highway State Trail Segments A through D, dated February 2015. This report was prepared by CH2M Hill and Cornforth Consultants for Western Federal Lands. The station 198+00 rockfall site (included in this memo) is not included in the February 2015 report.

Additional geotechnical information and recommendations for the proposed HCRH Segment D trail section is contained in a separate geotechnical report titled Historic Columbia River Highway State Trail - Segment D, dated December 2014.

Geotechnical Assessment

Field Review A site visit was completed on February 23rd, 2015 by Nathan Jenks, Doug Anderson, and Gabe Krumbein from WFL. The field work consisted of general field observations and measurements of slope geometry and other rockfall related elements. Rockfall source areas, slope materials, slope roughness, rock sizes, and other criteria were observed and recorded for use in rockfall trajectory modeling. Aerial photographs with project specific site survey were reviewed prior to the field visit, and were used as a base map during the site rockfall assessment.

ODOT Maintenance Interview A telephone conversation with ODOT Maintenance Coordinator, Brian Walker was conducted on March 5th, 2015 with both Nathan Jenks and Doug Anderson. Mr. Walker is responsible for the maintenance of I-84 at the location of the Station 198+00 rockfall site. General site information including rockfall history, frequency, and size were discussed. Some important information from the conversation includes:

• Historically, railroad ties and posts were placed up-slope for rockfall containment.

• The existing concrete Jersey barrier was placed in the late 1980s. Prior to placement of the barrier, rockfall would regularly reach the highway ditch and occasionally reach the interstate.

• About once every 5 years a rock will make it over the existing concrete barrier.

• A highway access control fence was placed between the barrier and the highway about 5 years ago. No rockfall has reached the highway since the access control fence was installed.

• Rock sizes are typically no larger than about 8 inches diameter with a few up to 18 to 24 inches.

• Rockfall consists typically of a few blocks at a time, no large scale events were reported.

• Rockfall is most common during freeze/thaw periods and heavy precipitation events.

• Oregon State Parks currently maintains the trail and removes rockfall from behind the concrete barrier. ODOT occasionally helps with maintenance on an informal basis.

Site Conditions

General At the station 198+00 rockfall site, the proposed trail is located on the HCRH roadway bench, which is approximately 5 to 6 feet above the adjacent interstate (Figure 2). The existing width of the HCRH bench is about 8 to 10 feet, and the final proposed width of the trail at this location is about 12 to14 feet, including foreslopes. A fill slope transitioning to a historic stone masonry wall is present on the fill side of the HCRH bench within this section and continues to the east. An approximate 1V:1H cutslope up to about 6 to 8 feet in vertical height is present on the cut side of the proposed trail location.

A concrete Jersey barrier has been placed along the outside edge of the existing HCRH bench to prevent rockfall from reaching the highway. The length of the concrete barrier is approximately 90 feet. Additionally a highway access control fence is located at the base of the HCRH roadway bench fill slope. While this is not a high-impact rockfall barrier, according to ODOT maintenance, this barrier has also helped to reduce rockfall from reaching the interstate, as discussed above.

Geologic Setting Geology of the area is dominated by the Columbia River Basalt Group. These Miocene age basalt flows were shaped by downcutting of the Columbia River and further eroded by a series of catastrophic floods during the late Pleistocene. Additional information on the regional geology is contained in the December 2014 geotechnical report referenced above.

Rockfall and Slope The source of rockfall at this location are Columbia River Basalt rock cliffs approximately 225 feet slope distance up-slope from the trail location (Figure 2). There is an erosional chute in the rock cliff that channels water during storm events along with rock and debris from further up-slope. At the base of the rock cliff and erosional chute, a relatively narrow channel consisting of coarse rock and debris continues downslope for about 125 foot slope distance at a slope angle of about 38 to 40 degrees. Material within and around the channel consists of coarse, angular scree and talus. The common size of this material up to about 8 inches with occasional larger blocks up to about 24 inches. This channel is well defined and does not have any vegetation present.

At the base of the scree and talus channel, the geomorphology changes to a debris fan that widens as it continues down slope for a slope distance of about 100 feet to the trail elevation. The slope angle of the rockfall debris fan is about 36 to 37 degrees. Material within the debris fan appears to consist primarily of talus in a matrix of silty sand with gravel. There is limited vegetation on the debris fan, primarily consisting of conifers, although the density and age of the trees on the fan area is more sporadic and younger as compared with the surrounding areas. The total width of what appears to be the active rockfall debris fan at the trail location is about 105 feet.

The most frequent type of rockfall at this location is interpreted to be individual rock blocks or a few near simultaneous rock blocks with sizes up to about 8 inches in diameter. Occasionally, the size of the rock blocks will be larger, up to approximately 24 inches in diameter. The shape of the rock blocks are typically blocky and angular and often discoidal.

It is apparent from the geomorphology of the area that larger scale events are possible. A larger event would likely take the form of a debris flow consisting of a saturated, fluid mass of rock and debris in volumes of 10’s of cubic yards and possibly up to 100’s of cubic yards. Based on historical information obtained from ODOT, a large scale event has not be observed within the last 20 plus years, and therefore the risk of this type of event is expected to be relatively low.

Rockfall Mitigation Alternatives Various rockfall mitigation alternatives were considered for the station 198+00 rockfall site. Both, vertical and horizontal alignment shifts were considered but the topography, close proximity to the interstate, and the presence of the existing historic stone wall did not allow for a practical shift of the alignment. Due to the constrains in the area, the final proposed trail width through this section will be slightly narrower as compared to the rest of Segment D.

Other alternatives were considered and discussed with the design team and partner agencies and were ultimately dismissed for various reasons. These alternatives included various cut-side wall concepts, with and without a rockfall barrier, a rockfall barrier at the upslope trail ditch location, and a rockfall barrier on the outside edge of the trail, which would protect the interstate but not the trail users.

The preferred rockfall mitigation alternative was a rockfall attenuator fence system placed upslope of the trail to reduce the likelihood of rockfall to reach the trail or interstate facility, and to limit the visual impacts of HCRH trail users. Aesthetics of the recommended rockfall mitigation alternatives were considered in the design and discussed with the Oregon Department of Transportation (ODOT), the Forest Service, and other interested parties for concurrence.

Rockfall Analyses

Rockfall trajectory was modeled for the site using the Colorado Rockfall Simulation Program (CRSP) version 5.0. Modeling was completed for both the existing condition (without the concrete barrier and access control fence) and for the inclusion of the rockfall attenuation barrier as discussed in the Recommendations section below. The results of the analyses are included in the attachment.

Rockfall modeling was used to estimate the kinetic energy, bounce heights, and rollout distances of falling rocks at various points along the slope. The slope was modeled using a cross-section developed from light detection and ranging (LiDAR) data. For each modeling simulation, 100 spherical shaped rocks were initiated for each slope condition (existing and mitigated) and two different rock sizes were modeled, 8 inches and 24 inches, respectively. This resulted in four separate analyses as shown in the attachment.

The rockfall analyses results are expected to be conservative because spherical rocks were modeled rather than blocky or discoidal rocks. The results show that for existing conditions (without concrete barrier and access control fence), about 90% of the rocks reach the trail and between 50% and 80% of the rocks will reach the interstate. For the existing conditions, the analyses indicates that rockfall energy and bounce height at the trail are between 3 ft-kips and 55 ft-kips and 5 feet and 12 feet, respectively. For the existing conditions, rockfall energy and bounce height at the highway are between 2 ft-kips and 32 ft-kips and 2 feet and 8 feet, respectively. These values appears consistent, or possibly conservative, as compared with field observations and historical information from ODOT maintenance.

For the mitigated conditions, many rocks will be contained within the barrier and will not pass beneath the barrier. For any rocks that do pass through the barrier, their energy will be greatly reduced and the modeling results indicate that the rocks will likely be contained in the small trail-side ditch, or may stop on the slope above the ditch. The final mitigation analyses indicate that no rocks will cross the trail, and therefore no rocks will reach the interstate.

Rockfall Mitigation Recommendations

As mentioned above, the preferred rockfall mitigation recommended for the Station 198+00 site is an attenuation barrier placed up-slope from the trail. An attenuation barrier is a hybrid rockfall barrier that combines a rockfall catchment fence with a section of draped rockfall mesh (a tail) below the fence. The fence portion of the barrier is elevated relative to the slope to intercept falling and bouncing rocks. The high-strength mesh continues down-slope (as a tail) from the elevated portion of the barrier, lying on the ground surface to attenuate, or reduce, the energy of falling rock. The falling rock is allowed to exit the bottom of the system with very low velocity so that it stops on the slope below the barrier or is retained in the trail ditch.

Three locations were considered for placement of the attenuation barrier. An upper-slope location at the top of the debris fan, a mid-slope location on the debris fan, and a lower-slope location such that the tail of the rockfall mesh ended near the proposed trail elevation. Considering rockfall bounce heights, rollout distance below the barrier, aesthetic characteristics, and length of the barrier along the slope, the mid-slope barrier location is recommended.

The recommended length along the trail alignment for the attenuation barrier is 80 feet. This will require 3 support posts, one post on each end and one post in the middle. The maximum post spacing for the attenuation barrier is 40 feet. The starting station for the barrier is approximately 197+45 and the ending station is approximately 198+25. The locations of the posts on the slope should be at an elevation of approximately 153 feet. Final post locations should be made in the field during construction with the support of the WFL Geotechnical Section.

Attenuation Barrier Requirements The recommended post height for the barrier is 8 feet for all three posts. This is the vertical height of the post as measured from the ground surface to the top of the post. Posts will be steel W sections with a minimum post size for all end posts of W 8 by 40 and a minimum size for all intermediate post of W 8 by 35.

The recommended tail length of the attenuator, the portion of the rockfall mesh that is laying on the ground down-slope of the elevated mesh, should be 20 feet. Therefore the total mesh length, including the mesh supported by the posts should be 28 feet. A 30 foot overall mesh length is acceptable. The tail length was selected based on modeling results and research conducted by the Colorado Department of Transportation as contained in the Transportation Research Board report Colorado’s Full-Scale Field Testing of Rockfall Attenuator Systems, October 2009.

The rockfall mesh used in construction of the barrier should be constructed from either a 1x3 high tensile steel wire spiral rope, or 7x7, or 7x19 galvanized aircraft cable with a minimum nominal diameter of 5/16 inch and breaking strength of at least 9,200 pounds. The opening size in the mesh should be no larger than 12 inches by 12 inches. A secondary, double-twist wire mesh should be placed on the outside of the primary rockfall mesh.

A plan map, details, and specifications for the recommended rockfall attenuation barrier will be provided to the WFL Design group for inclusion in the final plan documents.

Attenuation Barrier Anchor and Foundation Recommendations A subsurface investigation at the recommended attenuator location was not completed. Based on field observations, ground conditions for the wire rope anchors and for the post foundation locations are expected to consist of strong, basalt cobbles and boulders in a matrix of silty sand and gravel.

Bedrock is not expected at the wire rope anchor and foundation locations. If bedrock is found during construction, the WFL geotechnical group should be contacted for revised recommendations.

Required pullout capacities for the wire rope anchors will be provided to the WFL Design group.

Required capacities will vary between approximately 20 and 60 kips per anchor depending on the location and type of anchor. Depth of the anchor will be determined by the contractor to meet the required pullout capacities. Wire rope anchors will require testing during construction.

At post foundation locations, it is recommended that five, hollow core, self-drilling ground anchors be used at each foundation. Three anchors on the uphill side of the post and two on the downhill side.

The recommended pullout capacity of each uphill anchor should be a minimum of 40 kips, and the pullout capacity of each downhill anchor should be 50 kips. The final length of the anchor will be determined by the construction contractor to meet the required pullout capacities. Ground anchors will require testing during construction.

Aesthetics All attenuation barrier components, including the primary and secondary mesh, steel posts, and all associated hardware must be colored or stained with an approved color, stain, or weathering agent.

These special contract provisions will be provided to the WFL Design Team.

Only those trees within the area of the attenuation barrier should be cut, including any trees within the area of the mesh tail. Any cut trees should be cut flush with the ground surface. No other trees should be cut for construction access or any other reason. The remaining trees will provide screening of the barrier from some locations.

Construction, Maintenance, and other Considerations

Construction Access for construction of the barrier may require specialized equipment. Temporary access roads to the barrier location will not be permitted. Based on the limited width of the trail, it is unlikely a crane will be able to reach the barrier location. Therefore, equipment such as portable drills and/or wagon drills, supported upslope by ground anchors and cables, will likely be required. A drag line or other system to transport material up-slope will also likely be required. Hand work for foundations is expected. Specialty rockfall mitigation contractors are familiar with this type of work and access restrictions.

Subsurface materials consist in large part of hard basalt cobbles and boulders. This will result in difficult drilling conditions. The contractor should be prepared to use a variety of drilling methods.

Temporary casing to maintain an open hole should be expected. The contractor should also be prepared to mitigate grout loss. This may include grout additives, grout set and secondary drilling, grout socks, or other methods as approved by the contracting officer.

Maintenance In most applications, attenuation barriers are relatively self-cleaning of rock debris. Meaning that rockfall escapes through the tail of the barrier and comes to rest below the barrier in a ditch or other safe location. Due to the angle of the slope at this location, and the energy of expected rockfall, only some of the rockfall will pass through the barrier. The remainder of the rockfall will be contained beneath the mesh. Therefore, the rockfall barrier will require occasional maintenance and cleaning.

If rock debris accumulates to the point it is pulling on, or stressing the mesh, the attenuation barrier’s ability to attenuate the energy of falling rocks will be significantly reduced and may cease to function effectively.

Other The recommended attenuation barrier at this location is designed to attenuate and capture rockfall up to 24 inches, and likely larger. It is not designed to contain a larger scale event such as a debris flow, which is possible at this location as discussed in the Site Conditions section above. The attenuation barrier may reduce the impacts of a debris flow to the trail and highway, but may also be overwhelmed and destroyed by such an event. If a large debris flow event occurred, the trail would likely be impacted and possibly also the interstate highway.

Conclusion

This memorandum was prepared to provide geotechnical information and recommendations for the rockfall site near station 198+00 of Segment D on the HCRH State Trail project. Recommendations presented herein are applicable only to this project and this location should not be used for other purposes. Limited site investigation and no subsurface investigations were completed at the site.

Potentially unfavorable subsurface conditions may exist which could impact the project. If site conditions during construction are not consistent with those described in this geotechnical memorandum, the WFL Geotechnical Group should be contacted for revised recommendations.

Figures and Attachments Figure 1 - Location Map Figure 2 – Site Photos

Attachment – Colorado Rockfall Simulation Program Modeling and Analyses Results

Copies: Gabe Krumbein, Roadway Designer Doug Anderson, Engineering Geologist Malcolm Ulrich, Geotechnical Team Lead Geotechnical File

Figure 1 – Station 198+00 Rockfall Site General Location

Station 198+00 rockfall site, looking west

Station 198+00 rockfall area, note 8 to 18 inch block sizes behind concrete barrier

Looking up-slope from trail elevation at debris fan

Top of debris fan and narrow upper channel

Looking down toward trail from narrow channel area just above the debris fan

Source area, erosional chute in rock slope, and channel beneath rock cliffs

Colorado Rockfall Simulation Program--CRSP 5.0

CRSP Rockfall Analysis

PROJECT INFORMATION

Company Name: WFLHD Project Name: HCRH Top of Slope 2 ft Rocks Station: 197+75 Location: HCRH State Trail - Seg D Analysis By: NJ and DA Analysis Case: Top Origination - 2 ft spherical Date: 3/19/2015

CRSP was used to determine the expected kinetic energy and bouncing height of falling rocks at various points along the slope. In the analysis, it was assumed that rounded boulders with a maximum diameter of 2.0 feet would impact the rockfall retaining system. For each simulation, the model rolled 100 spherical shaped rocks from a source area at 290 - 370 feet. A slope surface roughness of 0.1 - 1.0, and hardness of 0.1 - 0.6 were used.