Madrid-based demolition firm MAXAM has used controlled demolition techniques to demolish the concrete towers of a thermal power station in Spain. MAXAM’s Fernando Reartes says the company was in charge of the design and execution of the blasting operation, which he describes as “one of the most complex tasks of the dismantling of this plant.”
The project in Puertollano, Spain, involved the boring of 892 holes to host some 275 kilograms (606 pounds) of Riodin dynamite. The blast sequence also entailed the use of some 1,000 electronic detonators.
The Elcogas thermal power plant had used integrated gasification combined cycle technology before shutting down in 2016. Elcogas hired Recifemetal España S.L. to oversee the subsequent dismantling project.
The demolition of the site’s concrete towers (a cooling tower and a gasification tower) was one of the most complex tasks of the dismantling process, with Recifemetal deciding to demolish them by means of a controlled blast. The design and execution of the blasting operation was awarded to MAXAM, which has been involved in more than 100 such controlled demolition projects.
Blasting demolition is a technique that consists in using small charges of explosive to break load-bearing elements in critical points of a structure, thus resulting in its collapse, according to MAXAM. The explosive is confined in bore holes, so the air overpressure caused by its detonation is minimal and does not cause any damage to the surroundings. This technique is usually applied to demolish high, thin buildings consisting of a structure made up of reinforced concrete beams and pillars.
A sizable task
The cooling tower was 400 feet high and was made up of a reinforced concrete slab with a thickness of from 7 to 7.5 inches. The slab base progressively widened starting at 7.5 inches and reaching a maximum of nearly three feet wide near the slab's bottom. The slab was supported by 56 inclined cylindrical pillars more than 30 inches in diameter and 27 feet high.
The gasification tower was a rectangular prism with sides that measured 73 feet by 22 feet. Its highest point reached 266 feet. Its walls were made of reinforced concrete with a thickness of nearly 16 inches. Inside the tower there were several walls harboring the staircases and elevator.
The cooling tower was supposed to implode keeping the rubble approximately within the same area originally occupied by the tower. The gasification tower was supposed to fall in a north-westerly direction, using its long side (73 feet) as a hinge line.
The main concern of the three companies involved in the project was the safety of the working crew, as well as of any person who wished to watch the demolition.
Another priority was to mitigate the four main environmental effects caused by the demolition's blast: the possible generation of vibrations, air overpressure, flying debris and dust. Several structures located in the towers' surroundings required protection during the blast.
A tailor-made solution
A total of 466 individual loads of Riodin explosives were used in the cooling tower and 426 in the gasification tower. These loads were confined in blasting boreholes stemmed with polyurethane foam. The explosive charges were initiated using Riotonic DT electronic detonators designed to ensure a correct and precise initiation.
The 600 pounds of Riodin explosive and more than 1,000 detonators were employed in the demolition of both towers. The timing of the blast lasted 285 milliseconds in the cooling tower and three seconds in the gasification tower.
Before initiating bore hole drilling in both towers, structural engineering studies were conducted to certify stability during drilling and other required preparatory work before the blasts.
After several coordination meetings with companies in the area, neighbors and local and provincial authorities, a safety and communications plan was prepared. The plan was designed to explain in detail the safety and evacuation procedure to follow during the blasts.
A safety perimeter was designed with a radius of more than one kilometer (about 0.6 miles). This could only be accessed by the explosives engineers who would be in charge of firing the shot from a safe position provided with emergency escape routes. The evacuation perimeter was properly secured by the relevant authorities.
Four seismographs were installed: one in the electrical substation, two in the solar power plant and one in the closest house. These were installed to make sure the vibration levels would not exceed the limits established by Standard UNE 22-381-93. Bore holes were stemmed with polyurethane foam in order to reduce the air blast.
To avoid flying fragments, a layer of single-twist wire mesh and another layer of geotextile mesh were installed around each explosive charge. Lastly, dust collectors and hydrants were installed to trap part of the dust. In addition, a study of the area's prevailing winds was conducted in order to establish the best time for blasting.
The cooling tower fell vertically, and the resulting debris remained in the planned area, concentrated inside the pit and near the tower’s base. After eliminating the pillars, the structure fell nearly 30 feet vertically. The top, already weakened in its thicker parts due to the explosive placed in the bore holes, impacted against the floor, generating large transverse and longitudinal cracks in the thinner areas of the slab. These cracks instantly caused the structure to collapse entirely.
The gasification tower fell in the planned direction thanks to the breaking wedge generated by the explosive. By reducing its maximum height to 32 feet above floor level, the tower became more accessible and safer to be demolished using mechanical means.
Seismographs records reflected lower vibration values than those established in Standard UNE 22-381-93.
In the gasification tower, fragments flew to a distance of approximately 330 feet, which was far from where the closest people were located (at more than 3,000 feet). The only infrastructure that was hit by any flying debris was the CR504 road when the Cooling Tower was shot. This road was blocked to traffic and was quickly cleaned after the blast.
The adopted dust control measures were successful, as the dust that was not contained by the hydrants and collectors travelled northeast toward the neighboring hills and road. The electrical substation and the thermal power station’s solar panels were not affected at all, as subsequently confirmed by their respective owners.