Company:
City of Laguna Hills
Status:
Awarded
Awarded:
Roadway & Highway Project of the Year
Additional Files
Initial pour for the tie back wall.
Placement of the rock pattern.
Reinforcement within the wall.
Placement of tie backs.
Face of wall reinforcement.
Final forms for pour of the wall face.
Concrete forms for wall face.
Peeling back the wall rock pattern stencils.
Loop ramp landscaping installation.
Final loop ramp landscaping installation.
Additional Information

Project Lead – City of Laguna Hills
Design Team – Parsons Brinckerhoff
Contractor – SEMA Construction
Construction Manager – Athalye Consulting

City of Laguna Hills - La Paz Road at Interstate 5 Improvements

Project Location:

La Paz Road at Interstate 5, Laguna Hills, CA

Project Description:

La Paz Road is a primary arterial in the cities of Laguna Hills and Mission Viejo and the surrounding communities. It intersects I-5 at the northeast boundary of the City of Laguna Hills. The interchange configuration prior to improvements was a modified partial cloverleaf with an elongated northbound loop on-ramp, and a southbound loop on-ramp. La Paz Road is a six-lane divided roadway for most of the area but it narrowed to four lanes just underneath I-5 which created a choke point. 

The choke point not only had major traffic impacts to the intersections of La Paz Road at Cabot Road and La Paz Road at Muirlands Boulevard, it also created traffic congestion on the I-5 ramps, as well.  The worst traffic condition occurred on the southbound off-ramp to La Paz Road during evening peak hours which have a projected volume of 1750 vph in the year 2025.

The City conducted a study to determine solutions to traffic congestion along La Paz Road and I-5.  The following project elements were determined necessary to improve traffic flow and to help alleviate traffic congestion:
• Add a fourth lane to the southbound off-ramp to La Paz Road for right-turn only;
• Add a second left turn lane for westbound La Paz Road to southbound Cabot Road;
• Add a right-turn lane for eastbound La Paz Road to the northbound on-ramp;
• Add a right-turn lane for westbound La Paz Road to the southbound on-ramp.

One of the most significant design and construction features for the project included a retaining wall along the southbound off-ramp and tie-back walls underneath the undercrossing bridge to accommodate the additional lanes. The construction cost of the project was approximately $6,000,000 and construction was completed over a 15-month timeframe. The project was accepted as complete on November 26, 2013.

Project Justification:

The City of Laguna Hills recognized a serious and significant point of traffic congestion on La Paz Road and I-5 and sought a cost effective short-term solution while maintaining full freeway operation and providing an aesthetically pleasing and environmentally sensitive solution. The Design Team representing the City of Laguna Hills, and in cooperation with Caltrans and the City of Mission Viejo, designed a project to substantially increase the traffic carrying capacity of the interchange and provide relief for the through traffic movements which is also a route to school. Much of the project was constructed at night to avoid construction traffic impacts.

An improvement strategy for widening of La Paz Road was the construction of the tie-back walls underneath the undercrossing bridge to accommodate additional lanes on La Paz Road. This work created a new lane out of otherwise unusable space under the freeway bridge and was an innovative solution to a tight cross-section. These types of retaining walls are excellent in addressing two major challenges associated with street widening under highway undercrossing or overcrossings.  These challenges are:  Lateral support of existing abutment structure and tight construction envelope. The existing abutments of La Paz Road undercrossing are supported on deep foundations consisting of vertical 16” cast-in-drilled hole (CIDH) piling.  The tieback wall was built in vertical lifts, starting from the top, allowing for the needed lateral support for the adjacent slope and existing structure foundation during construction.  Most importantly, this wall type was capable of resisting laterally induced loads from the bridge structure.  The retaining wall height ranged from 12 to 14 feet.  A conventional retaining wall would have been very difficult and costly to construct because it would necessitate the construction of temporary shoring to provide slope/existing foundation lateral support and to allow for the footing construction.  The tieback wall had a very small footprint (no more than 3 feet.) This allowed for getting as close as possible to the existing abutment while maximizing the horizontal envelope for the street widening.  The construction also included tight overhead clearances (to existing bridge soffit) for drilling tieback holes and the spacing of tiebacks to avoid the existing pilings.  These challenges were overcome during the design which avoided construction conflicts.

Special Circumstances:

Water Quality – This project met the definition of new construction for an interchange improvement project, located in an urban area and the climate is suitable to allow for establishment of new vegetation. Flow velocities within the project area were not high enough to cause scour. For these reasons, biofiltration was selected as the treatment Best Management Practice for this project. Planting was designed within the project area to provide the biofiltration features.  Also, the project was designed with infiltration basin sites located at the loop on-ramps of the project.

This project was designed to be aesthetically pleasing in keeping with the standards of the City of Laguna Hills and surrounding communities. The tie-back wall and other retaining walls were faced with poured in place granite stone formliners after which each stone was individually painted to create a lifelike appearance, an extensive use of granite stones were used to line the raised median island on La Paz Road and for slope face protection on the northerly side of the La Paz underpass area and the entire project site was irrigated with recycled water and planted with low water use plantings. The addition of recycled water for irrigation was a City contribution to reducing the use of potable water by Caltrans to irrigate freeway plantings.

Traffic Capacity Improvements - Both Cabot Road and Muirlands Boulevard intersections operated at an acceptable Level of Service D during the AM peak. However, PM peak hour volumes are much higher, causing the Cabot Road intersection to experience severe congestion (LOS F), while the Muirlands Boulevard intersection operated at LOS E.  After the proposed improvements were put in place, the Cabot Road and Muirlands Boulevard intersections now both operate at an improved LOS C during the AM peak.  For the PM peak traffic operations at the Cabot Road intersection, the level of service is improved but it could not be made to improve above LOS F even with the additional lanes. However, the Muirlands Boulevard intersection now operates at an improved LOS D. Although the interim improvements to La Paz Road will not prevent the Cabot Road intersection from operating at LOS F during the PM peak, there are significant improvements in traffic operations at the intersection that should be noted.  The intersection delay, while still at LOS F, is reduced a projected 38.7 seconds in 2015 and 43.3 seconds in 2030.  There will be a considerable amount of delay savings at Muirlands Boulevard, as well.  The proposed changes will reduce the intersection delay by 27 seconds in 2015, and by 36.2 seconds in 2030.

The improvements also solve the existing blocking problems at the Cabot Road and Muirlands Boulevard intersections, and the I-5 southbound off-ramp.  The additional lane on the SB off-ramp and proposed left-turn lane on westbound La Paz Road at Cabot Road will provide enough additional capacity to eliminate overflow of these turning bays and the blockage caused by the queues that existed.  These blockages not only hinder the performance of the Cabot Road intersection, but the I-5 and Muirlands Boulevard intersection.  The extended loop on-ramp lane will also improve traffic operations on La Paz Road by reducing the number of vehicles at the undercrossing.  The eastbound movement is the heavier movement through the interchange, and the extended lane will take approximately a third of the vehicles off of La Paz Road during the peak periods.

Project Attachments:

See information provided below.

Award Citation::

La Paz Road at Interstate 5 interchange improvements were implemented to increase the traffic carrying capacity of the southbound La Paz Road off-ramp and the east and west through traffic on La Paz Road utilizing tie-back wall technology in a very tight cross-section and improving water quality and aesthetics in this important regional interchange.

Suggested Award Summary:

The City of Laguna Hills, in cooperation with Caltrans, directed the improvement of the La Paz Road interchange with Interstate 5 to relieve traffic congestion, create a fourth southbound off ramp lane, create a new east bound to north bound exclusive freeway access lane utilizing tie-back wall technology, create a second La Paz Road east bound to south bound left turn lane to Cabot Road, replace traffic signals, provide irrigation with recycled water, enhance water quality and add aesthetic features to this regional interchange. The project was mostly constructed at night over a 15-month construction period to reduce traffic impacts at a cost of approximately $6,000,000. An improvement strategy for widening of La Paz Road was the construction of the tie-back walls underneath the I-5 Bridge to accommodate additional lanes on La Paz Road. This work created a new lane out of otherwise unusable space under the freeway bridge and was an innovative solution to a tight cross-section. This project was designed to be aesthetically pleasing in keeping with the standards of the City of Laguna Hills and surrounding communities. The tie-back wall was faced with poured in place granite stone formliners after which each stone was individually painted to create a lifelike appearance, an extensive use of granite stones were used to line the raised median island on La Paz Road and for slope face protection on the northerly side of the La Paz underpass area and the entire project site was irrigated with recycled water and landscaped with low water use plantings. The addition of recycled water for irrigation was a City contribution to reducing the use of potable water by Caltrans to irrigate freeway plantings.

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