Blasting in twin tunnels with small spacing and its vibration control

Abstract

There are some parallel twin tunnels with spacing less than the recommended value in design specifications recently in China. These twin tunnels are so closely spaced that the blast vibration in one tunnel may cause damage to the other. Therefore close attention must be paid to the tunneling safety in drilling and blasting. This paper presents an excavation project of urban highway twin tunnels completed in Shenzhen city, China. The project was divided into two stages. In the first stage, the north tunnel of the twins was constructed eight years earlier and opened to traffic onwards. In the second stage, the south tunnel, which ran parallel to the north one and was 2437.5 m long, was to be constructed with a cross section of 11.28 m (width) by 8.34 m (height). The two parallel tunnels were driven in weathered lava with a 25.5 m distance between centers. An excavation damage assessment had been carried out using finite element analysis. The calculation results showed that the rock in the web was still in the state of elasticity and the support parameters adopted earlier could meet the demands of stability under the static loading condition except some tensile stress appeared in the toe and waist of the concrete arch at the initial stage. The 2D numerical simulation of the blast vibration field in north tunnel was conducted. The results of simulation proved that the blast vibration would not affect the stability of the north tunnel as long as PPV value is controlled within the limit of 8 cm/s. In the initial 50 m of the excavating tunnel, a series of trial blasts were carried out. At the moment of blasts, vibration monitoring in the existing tunnel was taken, by positioning a few geophones and accelerometers at the surface of concrete lining and ceiling. The microscopic observation at the concrete lining and ceiling was conducted before and after the blasting to determine whether there were new cracks. After a certain number of trial blasts, it had been ascertained that no new cracks formed and the old cracks did not expand, and thus a vibration velocity limit of 6 cm/s was drawn up for the concrete lining of the existing tunnel. Tunneling practices indicated that cut type, hole-depth, initiation sequence and delayed time played a great role in control of the blasting vibration. The cut holes should be fired in delayed intervals to reduce the vibration and to improve the blast efficiency. The time interval between the successive detonations should be about 50 ms or greater to avoid the superposition effect. It was also an effective way to cut down the vibration level by decreasing the driving footage and the maximum charge per delay. The limit of 6 cm/s for the concrete lining was reasonable as no visible crack-expansion or abnormal water seepage had been found in the concrete wall and the ceiling of the north tunnel after a full year's blasting operation in the south one. The key to the success of driving the south tunnel was the vibration monitoring. The monitoring data helped to timely modify the blast design, especially the amount of charge per delay. Except the values of trial blasts, the PPV was kept under a value of 6 cm/s and no damage occurred at the surface of concrete lining during the excavation, which proved the measures of reducing blasting vibration was effective. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.