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Alternative Foundation Design Reduces Construction Time, Saves Money and Enhances Safety

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Project Background

METRORapid Silver Line

The METRORapid Silver Line project, recently launched in collaboration with the Uptown Houston Management District, is a $192.5 million signature project focused on creating a beautifully landscaped pedestrian environment, preserving existing automobile access, and substantially improving transit access along Post Oak Boulevard.

METRORapid Silver Line is part of Metro’s $7.5 billion long-range plan, and is the first of many rapid transit projects planned in Houston. The line directly connects two major transit centers and 17 Metro bus routes. Much like a light-rail, trips are much faster because the dedicated bus lanes are separate from regular traffic, and GPS systems on the busses communicate with traffic signals to reduce the need for stops. Riders have efficient and easy, ADA compliant entry and exit from 14 elevated platforms that sit at the same level as the bus floors. (Source: Houston Chronicle)

Practice: Geotechnical Engineering

Sector: Road, Transit

Location: Houston, Texas

Services: Foundation Design, Pavement Design

 

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The Problem

The project's fundamental difficulty lay in constructing bus platforms in a busy urban area quickly and with little disruption of Uptown's heavy traffic flow. One of the signature features of the project is 14 bus platforms designed to provide convenient access to the express buses. The plan called for building elevated platforms that sit at the same level as the bus floors, providing for efficient, easy entry and exit and ADA compliance. The original platform foundation concept was drilled shafts due to the potential for shrink-swell movement. Due to an extensive network of existing utilities in the area, additional extended lane closures outside the planned construction zone would be needed creating substantial traffic delays during construction.

The Goal

  • Address the challenges of growth and mobility in the Uptown Houston area
  • Substantially improve transit service while enhancing the pedestrian experience
  • Relocate buses to dedicated express lanes in the median while preserving all existing traffic lanes, protected left turns, and current traffic signal operations
  • Minimize construction-related delays

Our Solution

Aurora Technical Service turned to HVJ to help mitigate these potential impacts. We determined a critical factor for station performance was maintaining a matching level between the platform and the bus floor level within close tolerances. While a drilled shaft foundation would limit shrink-swell movements of the platform, the dedicated bus lane pavement also would be subject to such movement. This risk meant a drilled shaft platform foundation could increase the risk of differential movement between the platform and bus floor. To reduce this risk, we proposed a mat foundation. By preparing the soils beneath the mat in a similar fashion to the pavement subgrade, the risk of differential movement was reduced, preventing tripping hazards and expensive future maintenance to correct the problem.

 Working with Lockwood, Andrews, and Newnam, mat foundations were designed, which further reduced tripping hazard. Future, unplanned maintenance risk was reduced by the installation of dowel bars between the platform and a thickened pavement section adjacent to the platform. This solution allowed the platform level to remain level relative to the bus floors.

The Results

Our solution eliminated utility conflicts, reduced construction cost, decreased construction time, and eliminated foundation construction-related lane closures. These lane closure reductions allowed the Uptown District to deliver on its commitments regarding available traffic lanes during the construction period.

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