The Talega Bridges in San Clemente, Calif., provide a vital backbone of transportation across an environmentally sensitive habitat

The city of San Clemente, Calif., boasts spectacularly dramatic coastlines known by surfers to produce some of the best waves in the world. This once-sleepy beach town, located in the southernmost point of Orange County, has evolved into a thriving community, attracting tourists to the 342 days of sunshine and providing a home to dedicated local residents and military service men and women stationed at Camp Pendleton. With growth projected to double over the next 20 years, the developmental expansion of San Clemente—in particular that related to transportation—poses a challenging quandary of balance between the nurturing of delicate coastal ecosystems, the sustenance of crucial natural canyon areas, and the completion of crucial efficient transportation systems.

As part of the Talega Master-Planned Community in San Clemente, providing a vital backbone of transportation and utility links across the major canyons while preserving wetlands and environmentally sensitive habitat areas presented a significant structural design challenge to the project team. To meet this challenge, RBF Consulting (RBF) provided bridge design services for three major bridges over the Segunda Deschecha Canada wetlands. The ultimate goal was to provide direct access for residents to the I-5 freeway, the Talega Village Center, the Talega Business Park, and provide an alternative route to Avenida Pico.

Sensitive structures

The bridges would carry traffic for Calle Saluda, Avenida Talega, and Avenida Vista Hermosa; each three span structures of 405 feet, 415 feet, and 586 feet long, respectively. The three bridges were required to address and protect environmentally sensitive areas, aesthetically blend with the Talega Community, satisfy the structural design challenges and criteria in this high-seismic region, and be cost-effective in order to keep the project economically viable. Schedule was also critical as completion of vital transportation routes were linked to the permitting of residential units for construction.

The longest of these structures is the Avenida Vista Hermosa Bridge, a three-span, cast-in-place, post-tensioned concrete box girder with an overall length of 586 feet and spanning 80 feet over the wetlands below. Its long, center-span length of 250 feet was required to avoid a meandering environmentally sensitive area regulated by the U.S. Army Corps of Engineers and the U.S. Fish and Wildlife Service. In addition, numerous environmental groups have actively sought to protect the area, which represents one of the last remaining alkaline wetlands along the southern coast of California. During construction, driven pipe-pile falsework supports were used within the wetlands to minimize temporary construction impacts.

A successful foundation

To support the enormous, 4,500-kip-loads imposed by each of the bridge’s six, 84-inch-diameter columns, two foundation alternatives were considered. Construction access and environmental constraints eliminated the use of conventional, driven-pile foundations with supporting pile caps. The bedrock materials were not conducive to driven-pile foundations and the footprint of the required pile cap at each column with the associated excavation was too intrusive to the canyon and habitat area. Additionally, portions of the canyon slopes, in a construction effort isolated from the bridge, required the use of tie-back systems to mitigate the slope stability concerns. For this reason, excavation into the canyon slopes for footing construction was not favored.

A pile-shaft foundation system was chosen. At each of the six columns, a large-diameter, 120-inch pile shaft was drilled as deep as 130 feet into the underlying bedrock and filled with reinforced concrete. This system allowed drilling of the foundation system very close to the wetlands area and helped significantly to reduce the required length of the main span, minimizing the structure depth contributing to the slender, graceful appearance of the bridge across the canyon.

The pile-shaft system presented several construction challenges, as well. The largest drill equipment available would need to have access to the canyon in order to drill the shafts to their ultimate depth, and high groundwater and caving would need to be mitigated to ensure sound concrete in the shaft foundations. A temporary construction road was permitted and graded into the canyon to allow access for the drill rig and a polymer slurry "drilling mud" system was utilized to prevent caving during drilling of the shaft and placement of the reinforcing cages. The polymer slurry was stored in tanks and introduced as the drilling progressed and would later be pumped out from the top of the shaft as concrete, tremied to the bottom of the shaft, and the slurry would be displaced. Prior to concrete placement, the reinforcing cage, weighing 90 kips, was lowered into the shaft. As concrete was placed, the polymer slurry was pumped from the shaft back to the storage tanks, avoiding any spillage into the wetlands. Prior to column construction, ultrasonic and gamma-gamma testing of the drilled shafts was performed to ensure competent concrete was placed for the full length of the shaft.

Due to the natural undulations of the canyon and the need to satisfy Caltrans Seismic Design Criteria for lateral-stiffness compatibility between adjacent supports, some of the columns had to be artificially lengthened by deepening the pile cut-off elevation and isolating the column from the confining effects of the soil by using steel casings around the bottom of the "shorter" columns. Eight-foot-diameter corrugated metal pipe isolation casings—15 feet in length—were used at the Pier 2 columns.

An accommodating design

In order to accommodate the numerous utilities serving the new subdivision, which includes more than 4,000 new homes, the bridges were used to carry the utilities across the canyon rather than having to trench through the sensitive wetlands habitat. As part of the seismic consideration, utility openings in the superstructure-end diaphragms were sized to accommodate transverse seismic displacements of up to 24 inches without damage to domestic waterlines.

Consistent with the Spanish ranch-style architecture within the development—deemed Ole Hanson-style architecture in recognition of the founder of San Clemente—unique architectural elements were integrated into the bridge design: A special motif of simulated timber hand-railing constructed from integrally pigmented pre-cast concrete beams, manufactured stone veneer specified on the exposed surface of the bridge abutments, and pre-cast concrete paving stones were used within the 28-foot-wide median to enhance the appearance of the bridge from the driver’s view. The slender, yet very long, bridge span rests gracefully into the Talega Valley within a sag-vertical curve of the road profile.

As privately funded, privately built bridges, which were required as part of a new master-planned community, the owner (a private developer) wanted to design and build the most aesthetically pleasing bridge possible at the lowest possible cost. With this directive and knowing the very challenging physical and environmental constraints imposed by the wetlands and canyon, the design engineer concluded that a cast-in-place, post-tensioned concrete box girder would be the most economical because of its familiarity to contractors in the region. As a result of the selection of the structure type and layout, and the economical use of commonly available construction methods and materials, the design and construction were completed ahead of a very aggressive schedule and for approximately half of the originally budgeted cost estimates.

Conclusion

The preliminary and final design phases progressed on an accelerated schedule to meet the demands of the real estate market. Through the use of a common structure-type in the region, there was interest from a large number of bidders (forcing aggressive bids) and their familiarity with the construction methods ultimately enabled a rapid construction schedule. Timely project delivery allowed home sales and occupancy to keep pace with the extremely high demands in this sought-after real estate market.

The unusual ecological problems surrounding projects in Southern California continue to challenge designers to find and use innovative structural engineering technology in maintaining the balance between the growing economic and social needs of our human population and sensitivity to our natural areas to sustain our ecologic future for generations to come.

Paul R. Young, S.E., P.E., is the vice president of RBF Consulting and has more than 25 years of professional engineering experience with a focus in the analysis, design, and construction support of major bridge projects and public works structures. During that time he has been involved in the planning, design, seismic retrofit, or widening of more than 150 bridges throughout Southern California. He may be reached at 949-855-3641or via e-mail at pryoung@RBF.com.

Design and construction team

Project name: Talega Bridges
Owner: Talega Associates, LLC, San Clemente, Calif.
Structural engineer: RBF Consulting, Irvine, Calif.
Contractor: Brutoco, Fontana, Calif.
Other agency: City of San Clemente, Calif.