Best Bang for the Buck: Controlled Implosion Yields Greatest Net Environmental Benefit From Dismantling the Bay Bridge Old East Span

Dragomir Bogdanic, Caltrans
Khalil Abusaba, Wood Consultants
Topic Area
Mitigation for Transportation Projects

Dismantling of the old east span of the Bay Bridge commenced immediately upon completion of the new east span on Labor Day, 2013. The environmental benefits of removing the old structure are reduced hazards to navigation, reduced net fill in the Bay, and removal of potential pollutant sources contained in the old structure. A significant challenge was removing submerged concrete pier foundations with the least environmental disturbance. Planning studies showed the greatest net environmental benefit of removal was by means of controlled implosion, rather than conventional mechanical means, which require large coffer dams, much longer work durations and potentially greater impacts and risks.
Results of the Pier E3 pilot test supported the removal of 19 more piers between 2016 and 2018. Monitoring throughout the project documented the effectiveness of mitigation measures including: materials collection and containment plan; Attenuation of sound and pressure wave; and water and sediment quality effects.
Innovative technologies applied in a setting with a robust environmental baseline demonstrated the net Project's environmental benefit.

Plume mapping verified the predictions of minimal, temporary pH increases following controlled implosion of underwater bridge piers. This innovative technology has been applied to a variety of stationary discharges, but implosion of underwater structures in a tidal estuary presents a unique challenge.
Implosions are timed for slack water, to maximize the performance of blast attenuation systems. Following implosion, affected water masses travel and disperse in response to tide and wind driven currents. Tracking vessels use continuous monitoring devices and GSP-enable current trackers to follow the affected water mass.
The maximum pH was as 9, and attained the water quality objective (8.5) within an hour. Turbidity effects are indistinguishable from background. Grab samples showed detectable changes in certain metals (e.g., chromium) but well below thresholds of concern. Stationary monitoring near eel grass beds showed no significant effects.
Innovative technologies applied to 18 different implosions under very different conditions show that while effects can vary depending on wind and tide conditions at the time of the event, water quality effects are minimal and transient, as predicted.

Investigating impacts to benthic sediment quality in the Bay following pier removal via controlled implosion requires a robust baseline of ambient conditions in the Bay. Samples were collected using a random stratified sampling approach. Sediments showed no significant changes in chemistry or toxicity as a result of controlled implosions.
Sonar surveys of the bottom provided detailed bathymetry before and after the implosion. A pre-existing scour crater around Pier steadily accumulated sediments after the implosion, showing that the former erosional environment had shifted towards depositional with removal of the pier.

Abstract Keywords
Benthic
sediment
water quality