Current Design and Construction Practices For Micropile Supported Foundations of Electrical Transmission Structures in North America

This paper discusses innovative micropile foundation designs utilizing advanced steel and concrete caps constructed on several recent transmission projects. It details the foundation design selection process, 3D finite element simulation and design, steel cap testing procedures, construction challenges encountered, and specific benefits that foundation design provided to the projects.

Many of the construction sites detailed in the paper were located in remote, rugged terrain and required helicopter-only construction methods. At these sites, a design-build micropile foundation system was proposed and ultimately employed as an alternative to traditional concrete drilled shaft foundations. Typical micropile foundation construction included a circular array of battered micropiles installed to depths of approximately 35 feet at each tower leg or pole base. A concrete or steel pile cap was then selected to transfer the load from the stub angle to the micropile array.

These advancements in micropile foundation design and construction have been instrumental in the success of several large scale transmission projects in North America. 

June 1, 2014 » DOWNLOAD

Foundation Design Minimizes Environmental Impact

Transmission line projects are facing higher levels of scrutiny than ever before, and the Tehachapi Renewable Transmission Project is no exception.  The new 500 kV line crosses the Angeles National Forest, where The U.S. Forest Service has strict requirements regarding land disturbance.  An innovative design solution helped owner Southern California Edison gain approval for the project.

Standard drill shaft foundations were considered in the initial design phases, but involved construction elements that did not comply with environmental restrictions.  Helicopter-supported micropile foundations were selected as the optimal alternative.  This paper will discuss the foundation design selection process, as well as technical challenges encountered during implementation.  Basic advantages of micropile foundations will also be illustrated,  including minimized impacts and the ability to adapt to individual site conditions.

September 1, 2011 » DOWNLOAD

Micropile Transmission Foundations in Environmentally Sensitive, Challenging Terrain

A 13.6 mile portion of the 500 kV Tehachapi Renewable Transmission Project crosses the mountains of the Angeles National Forest. Access and environmental constraints precluded road development for most transmission tower foundations, tower erection and conductor stringing operations. Because of the access constraints, helicopter-supported micropile foundation construction was used for this steep and environmentally sensitive portion of the project. The general methodologies incorporated on the project represent innovative implementations of micropile design criteria,preconstruction micropile installation and verification testing, on-site geotechnical characterization, micropile and pile cap installation, and proof load testing. Challenges in constructing the 224 foundations for 56 transmission towers provide lessons learned related to transmission lattice steel tower specific design constraints, construction challenges (environmental and geotechnical), and construction schedule. 

August 2, 2010 » DOWNLOAD

Integration of Optimum, High Voltage Transmission Line Foundations

High voltage electric power transmission lines span various regions and geology, and with these variations in venue and subsurface conditions, comes the need for efficient foundation designs to control construction costs.  In most locations, conventional deep foundations prove to be economical for design and construction.  In some locations, however, site conditions make conventional foundations too expensive or impractical, and micropile foundations become the prime solution.

This paper will introduce and illustrate the basic advantages and disadvantages of both conventional and micropile foundations in this industry and how they are being integrated to provide design solutions. A brief history of the introduction of micropiles into this industry will also be discussed.

February 17, 2009 » DOWNLOAD

Horizontal Geotechnical Investigations for Tunneling

Conventional vertical investigations still make sense in many instances, particularly for tunnels planned beneath urban settings where truck access is convenient and where the tunnel horizon is not readily accessible via a portal or shaft.  However, equipment is presently available to conduct horizontal geotechnical investigations at reasonable production rates and cost.  Advantages include continuously sampling the tunneling horizon and minimizing “access drilling” to get the zone of interest at the underground construction horizon.  Horizontal boring lengths of up to approximately 800 m are considered feasible and those in the range of 150 to 300 m are now routine.

This paper is intended to provide an overview of applicable concepts and technologies involved in executing horizontal borings for tunneling; highlighting both benefits and potential shortcomings or areas of future development needed to achieve more widespread application.  Several case histories are presented to emphasize the concepts, technologies, benefits and different types of applications possible.

June 1, 2005 » DOWNLOAD

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