Controlled Blasting for the Construction of an Underground Power House

摘要

Panama’s current economical growth demands an increase in electric power supply; projected energysupply for 2012 exceeds the demand by as little as 2%. In an effort to comply with the demand, severaldifferent hydroelectric projects, promoted by the government, are currently under construction and eachof them vary in generating capacity, construction design and complexity.Blasting is heavily involved in most of these projects and ranges from surface blasting for access roads,forming a base for a dam and trenching for water channels, to underground blasting for waterconduction tunnels, shafts and power houses.Currently we are working on the construction of an underground power house planned to be built withinan 18,000 cubic meter cavern, all of which will be excavated through drill and blast methods. Aroundthe cavern area there are other tunnels related to the power house that need to be excavatedsimultaneously with the cavern. Some of them overlap or run parallel to the cavern at distances notgreater than 10 meters.The vibration limit set by the project designer is 152 millimeters per second, monitored at the closesttunnel or structure of concern. Continuous vibration monitoring is mandatory for the blasting works.Seismographs are installed in niches built into the tunnel walls. This makes it possible to mount theseismographs’ geophones directly to a steel plate, which is in turn bolted into the rock in a protectedarea.Another criterion observed by the project’s designer is over-break due to blasting, especially in criticalareas like the water induction tunnel which will transport the water from the main tunnel to the powerhouse. Over-break analysis is determined by comparing the designed area with the area obtained afterthe blast. Final design for this tunnel includes a cast in-situ concrete lining using precast forms;therefore any over break must be filled with concrete, incurring extra costs for the customer.Thoughtful analysis of the recovered vibration information is a must in order to efficiently managemodifications in pattern geometry, advance per round, changes in timing design, as well as type ofinitiation system to be applied within the project to successfully comply with the set restrictions.