Monitoring, and thus quantifying, the effects of blasting during hard-rock tunnel excavation can be important for several reasons. As demonstrated by the accompanying article describing work undertaken in China, predetermination of likely blasting effects based on factors such as rock conditions, dimensions and blasting procedure, can also be of great value in planning drill-and-blast excavation so as to avoid damage to surrounding ground and adjacent structures. The reasons for devising blasting procedures and monitoring may include:
Contractual limits on vibration article velocity and acceleration at pre-determined locations, prompted by generally increased environmental concerns; Determining most cost-effective drilling procedures and use of blasting agents; Minimising blast vibration on adjacent structures that might otherwise be damaged, increasing project costs and delays; Demonstrating actual vibration levels to local authorities, residents and business owners that might otherwise perceive vibration and ‘shock’ to be larger and have concerns about improbable damage.
In addition to any ‘third’ party’ concerns, any delay, stoppage or excessive restriction to blasting would be generally intolerable for limited face underground tunnelling. The public may mistake blast vibrations for natural seismic activity, and vice versa, but they are of different frequencies from different source locations. Vibration monitoring can also detect the differences and sources. The extraction of large volumes of underground rock, especially in mining, could trigger seismic events as the pressures and stresses on the remaining rock changes. Faulting can accentuate this.
Vibration monitoring is based on measuring units of peak particle velocity (PPV) and acceleration at the selected location, perhaps near a structure that may be causing concern. This allows a comparison with known threshold levels above which damage to structures is known to occur. Of course this is also dependent on the existing security of the structure causing concern. As shown in the accompanying article, monitoring values can be simulated by appropriate software to predict the threshold of possible ‘third party’ damage and to design appropriate blasting procedures before any actually takes place.
Blast monitoring includes recording field data using a specialised digital seismograph and geophone and recording events, such as blasting times and charge, that may be relevant to the vibration readings.
Extended permanent blast monitoring has been is use at Australia’s Kalgoorlie Super Pit since 1998 to check on vibration generated by the Mount Charlotte underground mine. The system includes eight monitors that generate data used to manage and minimise possible impacts, including redesign of the blasting to produce the lowest vibrations. Special explosives and detonators have also been manufactured for this purpose.
In ‘civil’ tunnelling perhaps the greatest need for blast vibration monitoring is in urban situations underlain by hard rock; a situation that is common in regions such as Scandinavia and Asian cities such as Hong Kong and Singapore particularly for complex or cavernous excavations. The alternatives of roadheaders and hard-rock TBMs may be discounted due to, respectively, excessively hard rock or planned drives of relatively short length.
In addition to protection of adjacent structures and the public from the effects of tunnel blasting, it is often necessary to protect existing tunnels, whether from blasting, impact ground consolidation or most frequently, piling. In Singapore Instantel dealer Absolute Instruments supplied 42 Instantel DIN geophone instruments with 21 Minimate Plus 8-channel monitors to the Woodsville Interchange Upgrade Project to check on construction vibrations reaching the underlying existing metro tunnels as well as nearby residents. Instantel, a division of Xmark, has been a part of the Stanley Black & Decker group since 2008. It manufactures vibration instrumentation and software under the series names Minimate and Blastmate.
One of several projects in Sweden involving Orica Group’s Nitro Consult’s blast vibration is the Stockholm City Line (Citybanan). Nitro Consult has also been involved with ground investigations, noise monitoring, structure-borne sound and risk analysis. The project requires about 1.5 million m3 of rock to be excavated sensitively by drill and blast. Two of the tunnels are underwater and the rest of the excavations, including two 260 x 220 x 220m underground stations, are beneath city structures.
A recent Nitro Consult introduction, used on the Citybanan, is the NCVIB web-based system to make the results of vibration measurements more useable. It presents the results of vibration and airborne shockwave measurements together with other useful parameters such as temperature, noise, stresses and groundwater levels.
The blaster can receive up-to-the-minute information from the last round blasted and can therefore quickly adjust the drilling and charging parameters for the next blast. Warnings of exceeding vibration limits can be built in
