State Road 120/Union Road Interchange: California’s First Diverging Diamond Interchange

Delays are the bane of any project, but an approximately six-month delay on a new interchange that serves as a critical piece of a highly traveled roadway can really put things into perspective. Our team was tasked with providing construction management services for the $24-million State Road 120/Union Road interchange located in Manteca, California, which, when completed, will be the first diverging diamond interchange in the state.


Off to a Rough Start

The project got off to a rough start with an approximately six-month delay waiting for overhead utilities to be relocated. The delay had a significant impact on commuter traffic as well as on the project’s cost. One of the unique and challenging opportunities of diverging diamond interchanges is the staging. There are certain components of the staging that can get complex where alignments overlap, which was the case with this interchange project. There's a lot of field and public coordination that needs to happen, which was exponentially complicated through the utility delay. Along the way, additional staging challenges in the electric work, drainage, and signage presented themselves and had to be coordinated in real-time to keep all disciplines moving ahead.

In addition to the delay, one of the biggest challenges for the project was moving the excess dirt on the job. Due to the way the project was originally set up, there were several inefficiencies in the way the dirt was being moved. The proposed project plan included moving 275,000 cubic yards of imported borrow, which is dirt that is brought in to help construct the site.

Making Lemonade Out of Lemons

To solve the issues of the delay and dirt removal, the construction management team, guided by the city and led by Koosun Kim, acting director of public works, decided to re-sequence the staging of the project. This included closing one side of the over-crossing bridge as well as the westbound on-ramp and the eastbound off-ramp. While this forced detours for commuters, the decision allowed us to combine three phases of the construction process into one which had several benefits:

  • Reduced offhaul of roadway excavation material
  • Removed the inefficient balancing of dirt in each stage of construction. Our team was able to reduce the transported borrow quantity from 275,000 to 225,000 cubic yards.
  • Cut four months off of the project schedule
  • Provided a roughly $350,000 credit for the city following a value engineering change proposal that we conducted

In a value engineering change proposal (VECP), a contractor presents a cost and/or time-saving measure and shares the savings with the owner. In the case of the SR120/Union Road interchange project, the contractor initiated the VECP proposal and then our team reviewed it. Following our review, we vetted the proposal with the city, including their risk department and public relations departments, and negotiated the terms and dollars for the VECP. This proposal resulted in the $350,000 credit to the city of Manteca. The city and our team were very pleased to partially mitigate the utility delay by shaving four months off of the project and saving costs.


Carrying Momentum into the Future

Ultimately, there were several factors that played into our team’s success for the project. Keeping a good relationship with the contractor was instrumental in navigating some of the project’s challenges. It was also crucial that we included a formal partnering process on the project along with a professional facilitator. From our construction management team standpoint, keeping an open dialogue with all parties involved, maintaining a willingness to listen to the client and contractor, and the overall inter-team collaboration were all vital factors to our success. I suspect that we will see many more diverging diamond interchange projects in the state of California, and I am thrilled for my team to have had the learning experience and success from this project that will help propel us to execute at a similarly high level moving forward. 

  • Howard Zabell
    Howard Zabell
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