Web exclusive: Rebuilding the historic Chisholm Trail, Texas, US

According to recent government census data the total population for the Dallas/Fort Worth area in the year 2009 is 6.6 million. This number is projected to grow to about 9.1 million by the year 2030.

This growth rate is at a magnitude never before experienced in this region. It is estimated that in 2007 the local economy lost about US$ 4.3billon due to traffic congestion in the Dallas/Fort Worth area.

By 2030, the cost to the local economy is estimated to increase to US$ 6.8 billion. Consequently, more efficient transportation systems are needed to accommodate existing and future traffic demand in the Dallas/Fort Worth area.

Though major reconstruction of the freeway system is occurring, many of the local and rural streets are forced to serve as thoroughfares, creating congested and unsafe conditions.

Without major improvements, including the disruptive reconstruction of the current roadway network, the existing system will remain inadequate to handle this tremendous burden.

Prolonged decision to rebuild the Old Trail

Although the need for a direct and continuous thoroughfare to relieve traffic congestion to the central Fort Worth business district was first identified over 45 years ago, funding shortages since the mid-1970s prevented this project from moving forward.

The Southwest Parkway is a proposed six-lane tollway with an estimated construction cost of US$ 1.14 to 1.24 billion to be built and maintained by the NTTA and the TxDOT. Design of the Parkway is in its final stages and environmental clearance has been received.

When the Parkway opens in 2011, it will provide a direct and continuous southwest-to-central route to the residents of Fort Worth and the surrounding area thereby providing the benefits of:

● Improved regional mobility, air quality and safety

● Improved response time of public safety and emergency vehicles

● Reduced burden on the local transportation system

● Incorporation of context sensitive design with mobility goals

● Infrastructure that complements Fort Worth's Trinity River Vision Plan

Southwest Parkway Interchange: The heart of a new trail

In January 2006, NTTA selected architectural, engineering and consulting company HDR to design a new, five-level interchange at state highways 183 and 121, and Interstate 20. The project has an estimated construction cost of US$ 314 million and represents the largest of five segments comprising the Parkway.

HDR's segment involved design of 2 miles (3.2 km) of six new lanes on the Southwest Parkway with localized frontage roads, more than one mile of reconstruction for SH 183 with new frontage roads, and direct connectors to the I-20.

The bridge design for this fully-directional interchange included the design of numerous overpass bridges, reconstruction of one cross-street underpass and nine direct connector bridges (several thousand feet-long ramp structures to provide uninterrupted access between freeways).

The ramps and portions of the direct connectors near the take-off and landing points were typically comprised of fairly standard prestressed concrete beam spans; however, the portions over the heart of the interchange were tightly curved with longer spans driven by the need to keep pier locations away from the overpassed roadways.

These units had to be designed as curved steel plate-girder structures, which is inherently complex. There were over 200 bents (the concrete supporting elements for the bridges), many uniquely configured, which had to be designed and detailed.

These included many unique bents of various configurations - some as tall as 28 m - and many wide enough to straddle across underpassing roadways (some requiring steel post-tensioning to provide additional capacity). Due the extensive amount of bridge related structures, over half the cost of construction is associated to bridges.

HDR's project team quick to react

For the cowboy, driving cattle up the trail was full of many challenges and dangers. Hostile Native Americans, dangerous river crossings and the unforgiving Texas weather required the cowboy to be quick to react under harsh conditions.

A thunderstorm near Waco in 1876, for instance, caused a herd of 15000 longhorns to plunge into a steep ravine, killing several thousand cattle and injuring many riders.

HDR's Southwest Parkway design team would face several similar unique design challenges that could have potentially driven HDR's project and schedule down these figurative ravines.

However, through the quick and skillful leadership of HDR's trail boss, project manager Dan Ruth, all such potential pitfalls were quickly addressed and resolved to meet the client schedule and demands.

As with trail bosses of old, Mr Ruth enlisted the services of design team members that had the experience and savvy to react quickly to unique design challenges and unforeseen obstacles. His core team included Mark Borenstein, lead structures engineer, and Vicki McCullough, lead roadway engineer, who together with Mr Ruth, have a total of 82 years of roadway/bridge design experience.

This core group would be faced with the task of assembling a design team across several different US cites, meeting the constant need of both client and public demands for an environmentally pleasing project and the design challenges associated with a project of this complexity.

(See the box story - The Southwest Parkway: collaborative effort - right for more details

Treading lightly on the old trail

During the planning phase it was evident that treading lightly on the Old Trail would be a high priority to all Tri-Party Project partners, but especially so with the city of Fort Worth as evidenced by the establishment of its Citizens Advisory Group (CAG).

This committee facilitated ongoing community involvement for the city of Fort Worth during the PS&E process. The goal of the CAG was to create a safe and attractive parkway by minimizing the construction impact on local communities while enhancing the environmental quality of the project area.

Their efforts led to the development of the Southwest Parkway Nature and Character Plan, which was included in the Final Environmental Impact Statement and implemented in the Southwest Parkway Corridor Master Plan.

These documents ensured that the architectural and landscape design of the parkway improved the local community's character, met the community's needs and preserved the natural environment.

Included in the master plan were the requirements for a transportation facility with a visually narrow cross section with integrated structural elements with park-like landscaping and architectural treatments.

These aesthetics treatment were to reflect the local vernacular styles found in Fort Worth's Zigzag Moderne style of Art Deco. These treatments helped establish a common architectural thread throughout the corridor at intersection bridges, toll plazas and specific mainlane elements.

These treatments would include:

• Landscaped medians and areas between the parkway and neighborhoods
• Recessed roadways to minimize visual and noise impacts
• Bridges, including architectural monuments, railings and pilasters
• Sound and retaining walls with architectural copings and planter walls

The main challenge to roadway and bridge designers was to incorporate these aesthetics and environmental constraints into every facet of design. "HDR's completed design brought to fruition the intent of the Southwest Parkway project - that being to maintain the parkway feel," stated Project Manager Dan Ruth.

Deep Canyons on new Trail

Deep canyons along the rugged Texas Landscape were obstacles that all cattle driving cowboys tried to avoid, however, they were at times inevitable.

As with most roadway projects of this complexity there was a need to lower a segment of roadway. For Southwest Parkway, this occurred at the crossing of SH183 and SH121 causing all design team members to refer to this segment of the project as "the Canyon."

Construction of the "Canyon" retaining walls in this cut location presented several challenges that needed to be accounted for during design. These challenges included placement of new utilities in front and behind the walls, new bridge foundations carrying the frontage road over SH183, foundations for future direct connection ramps and construction of walls with traffic staging since SH183 was required to remain open during construction due to the existing retail facilities in the area.

These walls are approximately 1 km long and with a overall wall height varying along the length from an at-grade elevation at the beginning and ends to about 17 m in height near the Northbound SH121 Frontage Road.

The average wall height is about 11 m along SH183. The subsurface bearing strata, where these two large cut walls would be located consisted of good quality gray limestone and gray shale.

Therefore, a soil/rock nail wall was selected as the preferred wall type in these cut locations. The nailed walls consisted of a shot-crete concrete facing with drilled and grouted anchors at a five foot spacing horizontally and vertically into the bearing strata.

A steel connection device will be used to connect the grouted nails to the front aesthetic wall panels with a reinforced concrete closure pour wall.

In addition, the designers needed to modify the foundation types for the direct connector bridges, located directly behind the walls, to reduce or eliminate the lateral pressure applied to the walls from these foundations.

The tiebacks for the walls were spaced accordingly to straddle or miss the drilled shafts for these foundations.

New trail, same hope

Over a hundred years have past since the days of the Chisholm Trail, but the dust hasn't settled on the beaten trail, nor has the need for this trail to once again assist the state of Texas and the populations of the Dallas/Fort Worth area.

This project will improve the quality of life of all those who's history and life have been touched by this historic trail.