The Landslide

Photo 1: Landslide on April 12, 1995, completely dammed the North Fork of the Virgin River in Zion National Park in Utah.

On the night of April 12, 1995, the North Fork of the Virgin River, swelling with spring runoff, touched off a massive landslide on the rain-saturated west slope of Zion Canyon. The river flow was completely dammed by a mass of soil and rock.

Zion National Park (ZNP) emergency response teams evacuated campgrounds and notified the town of Springdale, Utah, and others of the possibility of a flash flood, which fortunately did not occur. Finding its way around the landslide mass, the river flowed over the park's access road. Within two hours, 180 meters (m) of roadway had been washed away along with buried power, telephone, and sewer lines and 450 m of Springdale's water system.

Within 24 hours and with the assistance of local contractors, the National Park Service (NPS) had pioneered a narrow one-lane roadway around the washout, evacuated 450 stranded visitors from the canyon, and restored temporary water and sewer service to the park, using fire hoses. The river course had shifted about 60 m, and because of lingering fears of further canyon instability, the access road was closed.

The economic impacts to Springdale and the rest of southwest Utah were significant, but they could have been even more devastating if it were not for the rapid, coordinated, cooperative effort of federal, state, and local agencies in conjunction with highway contractors and designers.

The Initial Response

In the days that followed, engineers from the Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) visited the site. They were joined on very short notice by roadway construction contractors, working on other CFLHD-administered projects at nearby Bryce Canyon and Arches National Parks. Although neither contractor was guaranteed a contract or reimbursement for travel, they responded immediately and freely shared valuable insight and expertise.

On April 21, nine days after the slide, a scope of work for a site visit and a proposed emergency landslide repair project was delivered to Boyle Engineering Corporation and Woodward-Clyde Consultants, architectural/engineering (A/E) design consultants under contract with CFLHD. CFLHD's project manager returned to the park with the consultants and discussed potential long-term and short-term solutions.

A team was established with representatives from ZNP, CFLHD, A/E consultants, the park concessionaire, and others. Team members understood the emergency nature of the situation and the positive effort required to ensure successful and rapid completion of the project.

NPS submitted an application for federal emergency relief funds to assist with the needed repairs and reconstruction of the flood-damaged roadways within ZNP. The preliminary cost of repairs was estimated by NPS at between $750,000 and $1 million. After the damage site reviews and appropriate reports, FHWA authorized an initial program approval of $1,039,000 for engineering and reconstruction efforts.

In an effort to mitigate potential impacts to the local economy, ZNP Superintendent Don Falvey informed the community of Springdale that the roadway would be open by Memorial Day weekend in late May. Practically speaking, the road would have to be open to the ZNP, Zion Lodge, and concession staffs a few days prior to reopening this part of the park to the general public. This allowed only 26 days to establish a serviceable two-lane highway.

Due to the complexity and time constraints of this project, a consulting engineering firm assisted in design preparations. Final design and construction were consistent with NPS needs and requirements within the framework of the federal emergency program. The final cost of repairs amounted to $140,000 for NPS work and $2,706,000 for work undertaken by CFLHD.

CFLHD decided that the long-term solution would take considerably longer than 26 days. Therefore, the decision was made to break the project into two phases, thereby making good on promises to open the road by Memorial Day (Phase 1), while still delivering a high-quality, properly engineered, long-term solution (Phase 2).

Phase 1

Photo 2: Excavation of slide mass replaced with keyed buttress material to stabilize the slide prior to other "at risk" activity.

Mapping, site investigations, and preliminary long-term design concepts were pursued by the A/E firm. Concurrently, CFLHD pursued a time and materials contract for Phase 1 construction. Only through a special Small Business Administration program was it possible to immediately enter into negotiations with a highly qualified and respected contractor. This program, commonly referred to as the 8(a) program, allows sole-source, negotiated contracting when specific criteria are met. A Colorado company that was familiar with the unique requirements of park road construction, having successfully completed a project in Arches National Park, was selected. The contractor was expected to complete both phases of the work to ensure continuity and reduce costs. This contracting flexibility allowed the construction contractor to become involved early in the design phase to ensure proper consideration of all potential issues prior to final plan development.

CFLHD enabled a quick start to the Phase 1 work by authorizing the contractor to begin mobilization during processing of the section 404 permit application prepared by the NPS. Acting for the U.S. Army Corps of Engineers under local emergency provisions, the Utah State Engineer's office (USE) immediately approved portions of this application allowing emergency fill and minimal excavation (widening) of the new river channel. The restricted permit defined the limits of the Phase 1 temporary road project.

Using the combined experience of a seasoned contractor and a CFLHD project engineer, many technical requirements and acceptance parameters were established in the field. This cooperative effort, focusing on the temporary nature of the Phase 1 work and the practical need to open the roadway for the peak tourist season, expedited the process. Through the outstanding, cooperative efforts of NPS, the A/E firm, the contractor, CFLHD, and various state and federal regulatory agencies, the temporary roadway was open to the public on May 25, 1995 _ two days early.

Phase 2

Study and Treatment of the Slide Mass

CFLHD and A/E designers faced several issues in determining how best to treat the landslide on the west side of the river:

The constriction of the river caused by the landslide required widening to restore the hydraulic capacity of the channel.

• Partial removal of the landslide was needed to allow reconstruction of a two-lane roadway.
• NPS regards the landslide as a significant event in the overall geologic history of Zion Canyon, and requested that the slide mass be preserved for interpretive purposes.
• The Utah State Engineer's Office held the position that the slide mass should be stabilized to prevent secondary slide activity as a result of the river undercutting the slide mass, and to prevent long-term siltation of the river.

The west side of the river could not be accessed by road, and thus, engineers could conduct only limited sampling of the slide mass material. Additional material samples were obtained from the east side of the canyon and were assumed to be representative of the slide mass. The slide mass material was estimated to have an effective angle of internal friction of 33 degrees and a unit weight of 2080 kg/m3. The body of the slide mass was determined to be sandstone and siltstone blocks in a matrix of sand and silt. Material near the failure surface was determined to be soil-sized material with properties typical for sand and silt.

The amount of proposed slide mass excavation on the west side of the canyon was related to the configuration of the proposed roadway on the east side of the canyon. Adequate hydraulic capacity for the river channel had to be maintained.

Photo 3: Completed wall viewed from the reconstructed parking area to the north. Remaining work includes stripping, application of stain to integrate wall coloring with surroundings, and placement of dimensioned masonry facing consistent with historical improvements elsewhere in the park.

The A/E geotechnical engineers performed analyses to investigate the stability of the slide mass. Spencer's method and UTEXAS3 software were used to determine factors of safety (FSs) under steady-state, long-term conditions at two design sections of the slide mass for each of four cases:

• Existing conditions.
• Alternative I _ Excavate the toe of the slide mass and construct a stabilizing buttress on the west side of the channel and a retaining wall on the east to support the roadway section.
• Alternative Ia _ Leave the slide mass as is, allowing for eventual natural erosion of the toe of the slide mass by river flow.
• Alternative II _ Excavate the toe of the slide mass to enable construction of the roadway on a sloped earth embankment.

It was determined that, while the slide mass was stable in its then-present state, allowing the river to naturally erode the base of the slide mass (Alternative Ia) would result in unacceptable FSs. Alternative II would force the river channel further into the slide mass, and was also determined to produce unacceptable FSs. Alternative I was the only case studied that resulted in acceptable FSs.

Buttress Design

As envisioned for this project, the buttress would be a solid mass of rock designed to retain and stabilize the landslide. The buttress would require rock gradation and would measure approximately 5.5 m high, 4.6 m deep, and 125 m long. Additional protective armoring of the upstream side of the landslide included an additional 21 m of buttress. In all, nearly 3060 m3 of rock were placed on the west side of the river.

A black basalt rock was readily available at a local quarry, and it met the hardness and density requirements. ZNP staff did not want to use black rock, preferring red sandstone to blend with natural rock formations found in Zion Canyon. However, large, intact sandstone rocks were difficult to find, and sandstone did not meet the hardness specification for buttress rock. A compromise was reached. The core of the buttress was constructed using the basalt, and a 0.6-m-deep layer of smaller sandstone rock was used to plate the buttress.

Wall Selection

Uniquely restrictive site conditions affected the selection of wall material: The road needed to remain open, with major construction activities completed by Memorial Day. Spring construction meant that the contractor would have to contend with rising water levels in the North Fork of the Virgin River, which are often unpredictable due to variability in snowmelt and precipitation.

The wall would need to withstand the erosive effects of river flow velocity estimated to be more than 6 meters per second.

The wall would need to be aesthetically pleasing and have an appearance consistent with other structures in the park.

Once determined that a wall was the appropriate design solution, several wall types were considered. While a conventional mechanically stabilized earth (MSE) wall can be constructed relatively quickly, its appearance is generally not consistent with other structures in the park, and an MSE wall is susceptible to scour. A cast-in-place concrete cantilever retaining wall was too expensive at this location due to local material costs. Crib wall and timber wall were removed from consideration based on aesthetics.

CFLHD had considered the use of precast concrete units during Phase 1 emergency reconstruction of the roadway but opted instead for a temporary rock and earth embankment. During the design of Phase 2 reconstruction, attention was refocused on precast concrete units as a possible permanent design solution. A number of such products are commercially available, and several were considered. CFLHD and A/E designers then contacted The Neel Company to discuss the possible application and use of their T-Wall product.

The product is a precast concrete T-shaped unit. When installed, it resembles a crib-type wall, but its design and function are based on modified MSE design principles. The product promised the ability to construct a wall quickly with only minimal work on the channel bottom. The wall could withstand the scour effects of high water velocity, and its appearance could be made to blend with other structures in the park.

Wall Design

CFLHD and A/E designers worked closely with the wall vendor in developing design plans, which also provided the contractor with the option of submitting alternative wall designs. The wall alignment, dictated by the roadway alignment, is nearly continuously curvilinear with curvature ranging from 6.8 to 27.8 . The wall itself measures approximately 143 m long and 7.3 m high (maximum), and it uses more than 600 precast concrete units.

Scour at the toe of the wall was a design issue that required special attention. The toe of the wall was embedded approximately 1.5 m below the channel bottom. The toe of the wall was lined with large riprap, consisting of individual boulders measuring 1 to 1.2 m in diameter and weighing approximately 3630 kilograms each. Smaller rock was keyed into voids between the boulders.

Hydrostatic pressure behind the wall was relieved through joints between the faces of adjacent units. A geotextile filter cloth was used behind vertical and horizontal wall joints.

The wall vendor did not require free-draining backfill between the stems of the T-units. However, to ensure that the wall backfill would remain free-draining _ especially during rapid drawdown of the river water surface _ CFLHD materials engineers required that the backfill satisfy stringent gradation limits. All backfill material was required to pass the 7.6-centimeter sieve, and no more than 5 percent was allowed to pass the No. 200 sieve.

The lowest course of precast units rests on a nonstructural concrete leveling pad. The excavation for and pouring of the leveling pad were the only construction activities that needed to be accomplished under dry conditions.

NPS and ZNP staff expressed concern regarding the aesthetics of the retaining wall. A/E designers worked closely with NPS and ZNP staff to develop a wall design that was responsive to their concerns, yet still eligible for federal emergency funding. The resulting wall design features a relief texture on each wall unit face resembling a sandstone masonry finish, real sandstone masonry on the roadway side of and on top of the wall parapet, and staining of the precast concrete portions of the completed wall to match sandstone masonry work located elsewhere in the park. (Photos used in this article were taken before staining.)

Shoring Design

Lacking other viable solutions, CFLHD and A/E designers jointly decided to pursue the design of a temporary shoring system. Under most circumstances, the design of temporary shoring systems is left to the contractor; however, CFLHD staff believed that site conditions were unique and special enough that designers needed to demonstrate the feasibility of temporary shoring as part of the overall permanent wall design solution.

Local soils at depth were rocky enough to eliminate steel sheet piling or I-beams, and timber lagged in consideration. Based on a review of available alternatives, attention focused on a soil-nail wall as a viable temporary shoring system. A/E geotechnical engineers developed the design for the soil-nail wall, which included approximately 345 individual "nails" at 1.5 m of nominal spacing. Each "nail" installation involved a 5.8-m threaded bar, inserted into a drilled and grouted hole. A bearing plate was then attached to each bar at the face of the temporary cut slope. The entire "nailed" surface was covered with nearly 745 m2 of wire mesh and 68 m3 of shotcrete. The temporary soil-nail wall was constructed incrementally with crews working on benched earth cuts.

Team Approach to Design

The limited schedule for design activities required a substantial amount of coordination between CFLHD staff, A/E designers, NPS staff, ZNP staff, the wall vendor, regulatory agencies, and the contractor. The overall objective was to ensure that the design of key project features was continually refined to respond to the concerns of all involved. Several examples of the team approach to overall project design can be cited.

FHWA was cautious to use a wall product that was very new to CFLHD materials, geotechnical, and structural engineering staff. The wall vendor responded to several technical inquiries from CFLHD and was able to address concerns over the use of the T-Wall system as an option in the design plans.

A/E designers were able to accommodate NPS requests to reduce the amount of riverbank work to retain natural vegetation and to specify appropriately colored rock for the buttress.

CFLHD set aside additional funds to enable ZNP staff to assist in special restoration and landscaping activities.

At the preliminary design level, plans and specifications were submitted for contractor review, comment, and input. Suggestions on ways to reduce costs and speed construction were voiced in several working meetings with the contractor during design and in the plans, specifications, and estimates (PS&E) review.

Construction Activities

Mendez Inc. remobilized on Dec. 22, 1995, for phase 2 construction. ZNP staff again maintained a strong desire to have major construction activities completed and the roadway open by Memorial Day 1996.

Initial activities included partial excavation of the slide mass and construction of the rock buttress on the west side of the river. It was crucial to accomplish this work before snowmelt and spring runoff would raise the water levels in the river. Access to the west side of the river was provided by a temporary haul road across the river. Approximately 4740 m3 of material (about 2 percent of the total landslide) was removed from the slide mass.

Attention then focused on excavation for the permanent structure and construction of the temporary soil-nail wall. Once excavated, the water at the bottom of the cut area was removed using a rock and earth cofferdam and pumps. After completion of the soil-nail wall, soil conditions at the bottom of the cut offered sound material on which the permanent wall structure could be founded. The concrete leveling pad was installed, and wall erection began. Total erection time for the permanent wall was approximately 90 days, including excavation, shoring, and backfilling operations. The precasting and delivery of the T-units were critical path activities.

Major construction activities were completed just before Memorial Day. ZNP staff was pleased that the road could again be open to two lanes of traffic for the kickoff of Zion Canyon's busy tourist season. Paving and restoration activities were completed immediately after the Memorial Day weekend.

Conclusion

The many challenges of the Valley Floor Highway reconstruction project required substantial flexibility in what is considered the normal FHWA design process.

The success of the project is attributable to the positive attitude brought to the project by each agency and firm involved. It is an excellent example of the ability of the Federal Lands Highway Office to productively serve client agencies such as NPS, especially in the aftermath of natural disasters. The project is also a model of joint and cooperative design efforts by the public and private sectors. The staffs of ZNP, NPS, NPS Water Resource Office, United States Geological Survey, Utah State Historic Preservation Office, Utah State Engineer's Office, and other agencies are commended for their efforts to ensure the accomplishment of this project within the time line _ especially in a year of partial government shutdowns, other national weather-related disasters, and reduced budgets.

Kevin M. Mentz is a senior consultant with BRW Inc. in Denver. While with Boyle Engineering Corp., he was in charge of technical highway design activities on the Valley Floor Highway reconstruction project. He has nine years of experience and is a licensed professional engineer in Colorado and Wisconsin. He received his bachelor's degree in civil engineering from The Pennsylvania State University.

Eric Worrell is a design engineer with the FHWA/CFLHD in Denver. He was the contracting officer's technical representative (COTR) and design team leader on the Valley Floor Highway reconstruction project. He has 18 years of experience, including 15 years with FHWA. Eric received his bachelor's degree in civil engineering from the University of Colorado and is a licensed professional engineer in Colorado.

F. Dave Zanetell is a construction operations engineer with the FHWA/CFLHD in Denver. He was responsible for administration of the construction contract. He has been with FHWA for nine years and is a licensed professional engineer in Colorado. He received his bachelor's degree in civil engineering from the Colorado School of Mines and a master's degree in civil engineering from the University of Colorado.