Project Background

Superbowl Sunday 2000 offered more excitement for Portland, Oregon’s local television broadcasters than just the game. A freak windstorm with sustained gusts of 50 miles per hour induced random and potentially damaging cable motions in the supporting cables for the 1,100-foot, 750-ton broadcast tower. Observed amplitudes in the 2-inch diameter support cables were on the order of 30 to 40 feet. The tower is supported at the base on a spread footing and by three cable anchor systems placed 120 degrees apart.

Uplift loads on each of these anchors are on the order of 575 kips, and are transferred via five support cables at each leg. The northeast and northwest anchors consist of a group of three 5-foot diameter drilled piers, approximately 30-foot deep, with a 6-foot thick pile cap. The south anchor is supported by a 35 foot long by 12 foot wide by 6 foot thick deadman anchor at a depth of 18 feet below ground surface. Subsurface conditions encountered at the northeast anchor consists of soft silt and clay of Willamette Silts Formation underlain by weathered basalt at 41 feet below ground surface.

The soil /anchor system was analyzed using finite element methods to determine the maximum deflection and zones of stress concentrations in order to identify soil zones for stabilisation. Selection of the appropriate remedial option was primarily driven by the need for concurrent broadcast operations. Consequently, a ground treatment system consisting of a precise compaction-grouting pattern for in-situ ground treatment combined with load transfer by pin piles to bedrock was selected.

Project Description

Moore & Taber engaged in a design/build venture with AMEC Earth & Environmental, Tower Engineering Constructors, and Tri-Land Group to address this emergency repair project.  Compaction grouting was used at the south deadman anchor to add weight and increase soil strength properties within the active earth fill wedge. High-capacity pin piles were installed around the existing Northeast drilled pier group and connected to it by a structural system to transfer loads to underlying bedrock. The soils around the pin piles and CIDH piers were treated using low-slump compaction grout to stabilise alluvial and residual soils, improving load transfer characteristics and factor of safety with respect to uplift. A monitoring program is also underway using tiltmeters to monitor the anchors on a quarterly basis. So far, the monitoring program indicates a successful installation.

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