Environmentally-sensitive underwater blasting project in Columbia River meets stringent safety limits with HBM’s Genesis equipment
Two US contractors – Contract Drilling & Blasting along with Aimone-Martin Associates – have used HBM’s Genesis GEN7t high-speed data acquisition equipment to successfully monitor the underwater blasting of a one-mile long, environmentally-sensitive section of the Columbia River near St Helens in Oregon.
Managed by the US Army Corps of Engineers, the US$52 million project to increase the navigable depth of the river was unique because it was the first time data acquisition equipment and sensors had been used in such extremely demanding conditions.
The current in the Columbia River can reach eight to ten knots (11.5 mile per hr) while debris in the water frequently tangled sensor lines and caused high wear and tear on sensors and connections during the tight blasting period.
The Columbia River benefits from a well-established salmon industry that has been the focus of a great deal of environmental concern over the past several years. This made it imperative that there should be minimal risk to marine life during any blasting.
To meet these requirements, the US Army Corps of Engineers, as the project leader, stipulated a maximum allowable underwater overpressure of 70 psi (4.83 bars) at a distance of 140 ft (42.7 meters) from the closest blast hole. The blasting operations were undertaken outside of the salmon season to limit any possible damage to fish.
Controlled drilling and blasting
The project, which was completed at the end of February 2010, removed approximately 500,000 cubic yards (380,000 cubic meters) of basalt and bottom sediment and increased the river depth from 43 ft to 49 ft (13.1 m to 14.9 m) to enable transit by the latest generation of modern large-draft ships. Contract Drilling & Blasting of Bayonne, New Jersey, with over 18 years’ experience in controlled drilling and blasting on many major navigation channel and harbor expansion projects in North America and the Caribbean was responsible for preparing the river bedrock.
Aimone-Martin Associates, of Socorro, New Mexico, with over 35 years experience in blast monitoring and instrumentation for large construction, mining, and government projects deployed water-based sensors near the river bottom to record blast pressures.
Extreme testing conditions
The contract stipulated that the entire project had to be shut down if the pressure monitoring system ever failed to operate. This placed a great deal of pressure on the teams to provide a redundant system with multiple sensors in constant working order even though operating conditions were extreme and not conducive to the type of instrumentation used.
The Genesis HighSpeed GEN7t was used to record underwater pressure time histories for two different pressure sensors. The two sensors were deployed underwater 10 ft (3.0 m) off river bottom to meet contract specifications and at distances of 140 ft and 300 ft from each blast.
Use of Genesis HighSpeed
HBM’s Genesis GEN7t was selected because there was a need for multiple simultaneous acquisition channels running at 1 MS/s. The continuous direct-to-disk capability of the Genesis GEN7t, even at these sample rates,
enabled acquisition of the data without triggering. This meant there was little risk of missing any of the blasts.
A further advantage of the Genesis GEN7t was its real-time display capabilities which proved very helpful during pre-blast setup and application of the sensors.
Blasting and monitoring was kept to an extremely demanding schedule from November 1st, 2009, to February 28th, 2010, with operations running from 5 am to 8 pm six to seven days per every week during the project.
The drilling process involved using a drill barge with three drill rigs to drill a row of blast holes. Once a row was drilled, the barge moved back 10 to 12 ft (3.0 to 3.7 m) to drill a further row until a rectangular pattern had been drilled and was ready to load with explosives.
The Genesis GEN7t data acquisition and monitoring equipment was fitted on a separate work boat anchored up stream of the dredging site. The monitoring team was called out two hours before blast time so that tides and currents at the blast site could be checked, the sensor systems deployed, and the initial data quality checked. Data streaming was started at the five minute pre-blast warning with data collection running from the one minute warning. Recording was stopped about three seconds after the detonation was complete.
All data was displayed in real time and analyzed immediately after the blast with peak pressures transmitted to the drill barge within two minutes. A full report was completed within 15 minutes after each blast. This was uploaded to an FTP site that could be accessed by all participants.