R.N. Gupta’s research while affiliated with National Institute of Rock Mechanics and other places

A database of ground vibration due to blasting at 27 limestone quarries, located in various parts of India, has been created. The database contains peak particle velocity (PPV), frequency, other vibration related and blast design parameters. Regression analysis of the data is carried out to derive site constants of the USBM predictor equation for individual quarries. It is found that these site constants are correlated with each other. By combining all the data, a generalised predictor equation is developed to assess and control ground vibration. In addition, mean zone of attenuation has been delineated using the predictor equations of the individual quarries. The dominant frequency of ground vibration with respect to distance and the possibility of modifying it by changing delay intervals in production blasts are also examined.

Every component in a hydel project is an important element. Delay in any one component will affect the project time and cost equally. Excavation of surge shaft is one such component which happens to be the most critical as it involves multiple stages for its completion. This operation as such is slow, difficult and if it encounters adverse geology the operation puts the entire project on the critical path. This paper discusses in detail the problems encountered during the entire excavation of a surge shaft for a 1020MW hydel project in Himalayan geology with special reference to the blockades. It deals with the methods adopted to clear the chokes at various stages of excavation and to achieve the accelerated progress by full face blasting.

Fragmentation measurements of ten different samples were carried out using a manual (photographic) and two image analysis systems. The photographic method was found as slow and the automatic netting facilities available with the image processing and analysis systems were not sufficient and required manual editing to improve the fidelity of the netting. Almost identical distributions were produced but the Fragalyst results deviated significantly with the merged, WipFrag, and manual analysis. The Rosin-Rammler curves fitted to the merged distributions showed close agreement between the two in case of manual and WipFrag analyses.

Most vibration standards have adopted frequency dependent peak particle velocity criteria for protection of surface structures. However, frequency categorisation in these standards is neither consistent nor universally acceptable. This paper summarises important studies on structure response conducted in India and abroad. Further studies on this subject have been conducted at a coal mine. It is found that structure response and damage potential of the structures is governed by the frequency of the ground motion. On the basis of structure responses, frequency has been categorised into low (<20 Hz), medium (20-50 Hz) and high (>50 Hz). The proposed frequency categorisation may be used in vibration standards.

The method adopted to controlling damage to rock mass while excavating an underground powerhouse cavern at Tala Hydroelectric Project is discussed. A 1020 Mega Watt hydroelectric project at Tala in Bhutan in under construction, as a joint venture project between the Royal Government of Bhutan and the Government of India. The project involves mainly the construction of concrete dam, diversion tunnel, head race tunnel, and tailrace tunnel. Controlled blast design have led to better wall rock profiles and half cast factor of about 70% in phyllitic quartzite rock and this seems to authentic the need for controlled blasting in underground caverns.

Various methods of transducer mounting provide varying degrees of coupling between the transducer and the measurement surface. The influence of four of these methods on vibration measurements was studied. For this purpose, the first transducer was placed freely on a horizontal surface, the second one was ‘sandbagged’, the third one was ‘spiked’ and the fourth one was completely buried in soil. These transducers were mounted side by side and ground vibrations were monitored for 14 blasts at an opencast coal mine.Ground vibrations in terms of peak particle velocity, peak vector sum and frequency with different mounting methods were analysed. Assuming the data of the buried transducer as the most acceptable one, relative values of other transducers were determined and plotted. For the given tolerance for instrumental and human errors, anomalous readings were found in some cases. The waveforms of the buried transducer were then compared with those of others. Clear distortion in the waveforms or a very low correlation coefficient between two waveforms was suspected poor coupling.The results indicate that decoupling is most likely with the surface transducer. However, the sandbagged and spiked transducers are also prone to decoupling. Decoupling can result in higher or lower ground vibration. Therefore, burial should be the preferred method for mounting of transducers in soil.

Utilising the vibration data available with NIRM, parameters of ground vibration due to blasting at different surface mines were examined. Comparison of the observed parameters with the permissible levels as per the DGMS standard revealed that coal and lignite mines are incurring heavy penalties in complying with the vibration limits because of the low frequency that cannot be usually controlled by blast design. The limestone mines and construction projects are in a better position because of the relatively high frequency. Without compromising the safety of surface structures, the DGMS standard may need to be revised in the interest of the industry.

The safety and stability of concrete and masonry dams is a great concern when blasting has to be conducted close to these dams in order to construct small hydro-electric projects. There is a danger of ground vibration amplification to those residential-type buildings that are built close to these dams. Responses of three concrete and masonry dams were measured directly by conducting a number of blasts and by monitoring vibration in the ground as well as on the dams. The amplitudes and frequencies of the motions were analysed and vibration attenuation relations were derived. These relations were used to compare the vibration levels on the dams with those in the ground. Because of close-in construction blasts that produced high frequency ground vibrations, there was no amplification of the ground vibrations by these dams. The measured amplitudes of ground vibration were comparable to those of the dams.

With the introduction of large diameter deep hole blasting in underground metal mines in India for extraction of ores, there was a serious concern for safety and stability of underground structures. This paper deals with vibration monitoring and damage assessment in five different metal mines. The damage was assessed by visual observation and the rock mass conditions were evaluated through RMR values. Attempts were made to correlate the extent of damage with peak particle velocity (PPV). Using the data generated, a generalised predictor equation was derived for prediction of ground vibration in other mines. The analysis of the field data indicates that vibrations produced by large and small blasts in a mine can be grouped ' together to derive a predictor equation for the same mine. The degree of damage to the rock mass vis-a-vis supports requirement depends on the intensity of ground vibration and rock mass quality. In general, falling of loose pieces' is expected at PPV greater than 200 mm/s and formation of new cracks' at PPV exceeding 300 mm/s.