Case History: Broadcast Tower Anchor Stabilization, Portland, Oregon

On Superbowl Sunday 2000 an extreme windstorm with sustained gusts of 80 km/hr induced random and potentially damaging cable motions in the supporting cables for a 350 m high, 680 tonne broadcast tower. Observed amplitudes in the 50 mm diameter support cables were reported to be on the order of up to 10 m. The tower is supported at the base on a spread footing and by three cable anchor systems placed 120 degrees apart. Relatively high moisture contents in the soft silt and clay were encountered within the foundation zone for the piles at the northeast anchor. Similar conditions were observed in the backfill soils covering the deadman anchor on the south. A simple pullout analysis indicated that the northeast and south anchor would fail under the design loading. Selection of the appropriate remedial option was primarily driven by the need for uninterrupted broadcast operations. The support cables and anchors are essential for tower support, and could not be dismantled without disassembling the tower itself. Consequently, consideration of remedial options required the ability to effect repairs without disturbance to the tower support system. A remedial system consisting of a precise compaction grouting pattern for in-situ ground improvement combined with load transfer by pin piles to bedrock was implemented at the northeast anchor. Compaction grouting was used at the south deadman anchor to add weight and increase soil strength properties within the active earth fill wedge.