Open-bottom arch structures on the new Twin Peaks Road
accommodate drainage and provide safe wildlife crossings.

PROJECT CASE STUDY:
Precast arches solve drainage and wildlife crossing design challenges and speed construction in the Sonoran Desert.

As published by CE News Magazine.

By Sal Boenzi, PE

Project
Twin Peaks Road, Marana, Ariz.

Participants
Town of Marana, Ariz.
Psomas
Borderland Construction

Product application
The CON/SPAN precast concrete arch system from CONTECH Construction Products Inc. solved time and environmental challenges for an Arizona roadway.

In Marana, Arizona engineers, contractors, and suppliers collaborated to develop a method of utilizing precast concrete arch structures to span nine drainage and wildlife crossings in the construction of three miles of new roadway in the undisturbed Sonoran Desert. This collaborative relationship on the Twin Peaks Road Project resulted in innovative approaches to protect precious habitat, while saving the town of Marana time and money.

Because of limited project budget and complexities identified in the planning stage of the Twin Peaks Road Project, the town elected to take the Construction Manager at Risk (CMAR) approach, even though design was 90 percent complete. This brought the contractor on board early to work with the owner and the designer as the roadway and drainage design was finalized. The result was a collaborative working relationship that minimized the risks normally associated with the design-bid-build delivery method. This collaborative spirit led to cost savings and improved product features, and resulted in the development of new construction methods and creative solutions to design challenges.

Environmental challenges

Engineers were confronted with the following two major environmental challenges as they considered the design of the drainage crossings:

• accommodating the movement of indigenous amphibians, reptiles, and large mammals, and
• minimizing permanent disturbance to natural waterways as required by the Clean Water Act of 1972.

A study performed by the Arizona Department of Game and Fish identified numerous species of indigenous amphibians, reptiles, and large mammals that use washes and drainage ways throughout the alluvial fan topography traversed by the roadway alignment. Engineers were tasked with matching technical design with protection of wildlife and natural habitat. Since the migration paths through the desert would be interrupted by the roadway construction and later operation, the animals would either be totally blocked from migration destinations or placed at perilous risk if they tried to cross over the roadway surface. The solution to this challenge was to create suitable crossings under the roadway and to direct particular species to the crossings by means of fences.

Although the streambeds and washes in the Sonoran Desert are dry most of the time, they are still considered tributaries to navigable waters of the United States and therefore are subject to the jurisdiction of the U.S. Army Corps of Engineers and limitations imposed by the Clean Water Act. This means that a 404 Permit is required and the type of permit – general or individual – is determined by the amount of surface area that is permanently disturbed as a result of the project. Since the individual permit takes much longer and brings with it more mitigation requirements, the best approach was to minimize permanent disturbance to the streambeds.

Consequently, the project team determined that open-bottom arch structures were the best solution for nine of the 27 crossings to meet the challenges presented by the wildlife crossings and Clean Water Act requirements.

Design challenges and solutions

With a tight construction schedule driven by the desire for a simultaneous opening with a new traffic interchange linking the roadway to Interstate 10, the town of Marana opted for a precast concrete solution. A responsive, collaborative attitude by the supplier allowed the team to meet configuration challenges previously considered to be impossible to meet, thus reducing land acquisition needs as well as area of disturbance.

With approximately 360 precast sections to be installed, expeditious offloading and efficient installation rates during construction were critical to control costs.

Based on previous positive experience, Psomas turned to CONTECH Construction Products Inc. and its CON/SPAN precast arch system. The CON/SPAN system includes precast wing walls and headwalls, allowing a complete crossing to be assembled in a relatively short period of time, often in a single day for many applications.

While the simplest configuration of the system includes an end section aligned 90 degrees to the flow line, CONTECH routinely produces skews as great as 10 degrees. However, because of the wide, multiple-span crossings required by this project, the company was challenged to accommodate skew angles of 30 degrees or more to keep headwalls parallel to the roadway, minimize encroachment beyond the right-of-way, and reduce the total volume of precast sections. The firm’s project engineers responded by developing a new method of mechanically attaching pie-shaped sections and utilizing cast-in-place headwalls.

With approximately 360 precast sections to be installed, expeditious offloading and efficient installation rates during construction were critical to control the cost of labor and the total time needed for a 250-ton crane and equipment operators at a rental rate of $480 per hour. To speed up the process, the supplier and the contractor, Borderland Construction, worked closely to coordinate delivery of sections to match specific installation schedules. Additionally, the rigging crew developed a system to upend the sections in a single lift as they were hoisted from delivery trucks and set into place without “double handling” each load.

The next challenge involved constructability of the arch system. While the low profile of the arches allows for a smooth ride for traffic, backfill of stem walls penetrating below the scour line would be extremely difficult. An inverted “U” foundation proposed by the contractor solved this problem and reduced the cost of earthwork, concrete formwork, and precast quantities. A shallower excavation reduced the width of the temporary disturbance area within the washes, further reducing environmental impacts.

Lessons learned

The benefits of the CMAR process that the town of Marana expected from its experience on previous projects were realized to a high degree because of the professionalism of team members and the working relationships they developed.

According to Morris J. Reyna, construction division manager for the town of Marana, “Team building was a primary focus from the beginning, and was key to project success. Camaraderie and alignment of goals inspired the highest level of trust and cooperation.” He pointed out that the commitment developed from this relationship “drove the team’s desire to exceed expectations, which grew and strengthened throughout the construction process, creating a hugely successful and enjoyable project experience.”

At a time when the economy had dealt a severe blow to the construction industry throughout southern Arizona, more so than most other parts of the country, the Twin Peaks Road Project illustrates how a cooperative spirit and hard work can result in a highly successful project of which everyone can be proud.

Sal Boenzi, P.E., is the Arizona manager of the Psomas Program/Construction Management Team.

More about this project.