Consideration of gas pipeline safety against vibration of blasting; case study: excavation in Arak-Khorramabad freeway route

The ground vibration is one of the blasting negative effects on the environment, which could cause damage to various structures in some cases. Allowable vibration criteria in both group of environmental criteria and structural criteria have been presented which are generally based on the peak particle velocity. Environmental criteria concern the adverse effects of vibration on the human comfort and in the structural criteria the vibration effects on the stability of different structures. Gas pipelines are lifeline structures and their safety is critically important during their operation. There are some instructions and guidelines for blasting and explosion in the vicinity of gas pipelines. In recent years with the development of construction along the existing gas pipelines, some case studies have explored the possibility of an explosion near the existing pipelines and relevant restrictions has been considered. Considering all these research works, the limit of 50 millimeters per second has been used in many pipeline projects. In the current research, vibration due to the explosion in the route of the Arak - Khorramabad freeway in the vicinity of a gas pipeline (minimum distance of 25m) is calculated by the use of empirical methods, and the calculated vibration values were compared to the local seismic recorded motions during a few controlled trial explosions. The site location geologically consists of andesitic rocks from Jurassic period which were strongly altered and converted to serpentinite. The initial blasting plan for excavation of trenches in freeway route consists of 64 mm diameter holes with average depth of 3 m and horizontal distance of 3 m. Usual number of explosive hole is about 60 holes per blast. ANFO explosive material is used and the amount of explosive material in each hole is about 4 kg, hence, the total average amount of explosive material in each blasting is about 240 kg. Based on five empirical relationships, peak particle velocities against the distance were calculated for the conventional blasting plan of the project (240 kg of explosive material). In the critical distance of 25 m, the average predicted peak particle velocity was about 390 millimeters per second which is much higher than the allowed amount of 50 millimeters per second. At the same distance and based on the empirical relationships, the maximum allowable explosive charges have to be up to 13 kg. The acquired data were controlled by the use of seismic data monitoring. In this way two short period seismographs with 3 components 2 Hz sensors and 24 bit digitizer are used during 3 trial blasts. The seismographs were mounted so that the two horizontal components of the seismograph records, namely, the radial and tangential vibrations of the blast were acquired. Totally 18 seismic records (6 three-component records) were obtained during data acquisition. The recorded data were processed using Seisan software after the prior implementations. As expected, the vertical and radial components had the maximum amplitudes and the tangential component had the lowest range. Also the general ranges of vertical and radial components were close to each other. Ground vibration measurements showed that vibration amplitude at a distance of 15 meters for the explosion with charge of 4 kg was about 19 millimeters per second and for the explosion with charge of 8 kg ti was about 21 millimeters per second. Comparison generally showed that the measured values of amplitudes in recorded vibrations were lower than the predicted motions by the empirical relationships. It also showed that vibration values derived from calculations based on empirical relations were generally conservative at this site, and a local seismic data monitoring would be necessary to optimize the blasting program in civil engineering projects.