Scholarly record

BLASTING DEMOLITION DESIGN AND PROCESSES OF A CONCRETE BRIDGE

Laszlo Robert, Ciprian Jitea, Stefan Ilici, Aurelian Nicola, Levente Miklos

Abstract

Today, the demolition process employing blasting operations has a demanding technological application, demonstrating that the employment of the blasting method is a suitable alternative in terms of efficiency, quality, and safety. This article describes the practical way and safety conditions of realizing the demolition process of a concrete bridge built over a width river and located in the area of a water dump and a power plant. The bridge was designed as a structure that crosses the river, continued with an access viaduct on the right bank, having 7 spans with a superstructure length of 124.45 m and a total length of 133.05 m. In the immediate vicinity of the road bridge being demolished, the following objectives are located - the entrance to the neighborhood at 300 m, a railway line at 220 m, a high-voltage overhead power line pole at 150 m, a high-voltage overhead power line crossing the river upstream at 50 - 70 m, a medium-voltage overhead power line parallel to the riverbed at 75 m, two abutments with concrete and reinforced concrete elevations, six piers with frame-type elevations at 8 – 20 m, the first and second lines of the undercrossing for the water supply pipeline located at a distance of 10 - 11 m upstream of the bridge, the undercrossing for the telecommunications cables located at a distance of 10 m downstream of the bridge. The superstructure of the bridge consists of 3 monolithic beams, in cross-section, supported by metal bearing devices. The beams are rigidly connected to each other at the top with monolithic reinforced concrete slabs and are linked together with cross braces. The main span is crossed by a deck supported by two parallel arches made of reinforced concrete. The arches have a solid section at the base, with a width of 1.56 m and a thickness of 1.00 m, and at the crown, the cross-section is in the shape of an "H," with a height of 1.60 m and a width of 0.90 m, with flanges and a web of 0.30 m. Following a technical expertise, it was found that the bridge is in an unsatisfactory condition, with structural elements in an advanced state of degradation, requiring its demolition and replacement. The method of bridge demolition refers to the initial action of dismantling the bridge's superstructure from the main span through drilling & blasting works and its collapse into the river, followed by its subsequent removal using mechanical means, including the infrastructure elements that remain unblasted from the other six spans, such as abutments, piers, etc. Considering that the arched support of the bridge is subjected to compression while the beams together with the deck are subjected to bending, the collapse of the superstructure was achieved by acting on the load-bearing elements formed by the construction of the two arches, the tie rods connecting the arches and the longitudinal section beams, as well as the deck elements that rigidify the entire assembly. The demolition through blasting works was carried out in a single phase (action) and the order of initiation of the explosive charges was descending and from the middle of the bridge towards the banks, starting with the holes drilled in the brackets together with the two arches, followed by those in the tie rods and then those in the beams at the deck level.A risk assessment was also conducted regarding the effects of demolition by blasting works, which refers to seismic effects generated by the detonated explosive load, seismic effects upon the impact of the structure being demolished falling into the riverbed, overpressures in the front of the air shock wave, dust and gases resulting from the explosion, as well as the projection of material pieces under the action of the explosion. The novel aspect of the demolition project was in the methods used to prepare the building for destruction. The project's challenge was to identify solutions that would minimise the demolition effects due to the proximity of the aforementioned objectives, as well as to decrease the fragmentation size by blasting the bridge elements into dimensions that facilitate rapid mechanical evacuation while preventing obstruction of the river, which could impact the operation of nearby hydroelectric facilities. The demolition works were successfully carried out and thanks to the technical solutions and the adopted protection and security measures, no damages were recorded.

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