Spherical weathering body is one of complex geological strata, and drill and blast method can be used to precondition the boulders. To mitigate the adverse effects caused by boulder blasting, the particle velocities in the water-intake tunnel segments of Taishan nuclear power plant were measured. The measurement results indicated: (1) The particle velocity at the tunnel segments close to the blasting source is 1.4 times higher than that of tunnel segments away from the blasting source. (2) The particle velocity of the tunnel segments is 1.65 times greater than that of the rock mass. (3) The peak particle velocity (PPV) of the tunnel segments is lowered by 40% when the distance from the blast source to the measurement point increases from 38.85 m to 53.74 m and decreased by 70% when the distance increases from 31 m to 75 m. Therefore, attention and the particle velocities must be paid to an area 30 m close to the blast source.
Spherical weathering body is one of complex stratums as shown in Figure 1 and it often causes a serious problem in TBM excavation, because the uniaxial compressive strength (UCS) of boulders is usually greater than that of the rock mass surrounding boulders. According to previous studies, this difference led to extremely abnormal cutter wear and tunnel face stability problem (Bilgin, 2016), and the mixed ground conditions with boulders gave rise to squeezing and blocking of the TBMs or even caused complete failures of the segments and abandoning of the tunnel (Gong et al. 2016). As a result, advance time of tunnel boring machine (TBM) and project costs would be increased. Thus, necessary measures must be taken to precondition the boulders before TBM excavation.
Drilling and blasting from the ground to fragment the boulders is a common method, and it has been used in underground projects such as Guangzhou metro (Zhu et al. 2007), Shenzhen metro (Liu et al. 2011), sewers of Hitachi City in Japan (Du, 1990) and Taishan Nuclear Power Station etc. In this method, the specific charge is much higher than that in an ordinary blasting because the fragment sizes after blasting must meet the special requirement of the TBM muck haulage system. If not, large fragments can block muck movement in the cutterhead chamber as well as in the conveyor system (Gong et al. 2016). In addition, the adverse effect such as high vibrations on neighboring structures may increase. Thus, how to evaluate and control the adverse effect of boulder blasting on the neighboring structures, especially on the nearby tunnel segments, is important.
