Lake tap blasting poses a great challenge because of the innate intricacies involved. A high level of skill and meticulous execution is needed to carry out a final tap blast that not only produces an opening as required by the project, but also ensures that existing overlying structures are not disturbed.

Previous experience of lake tap blasting of a rock plug at the Koyna Hydroelectric Power Project, (Stage-IV) India, with Norconsult proved very beneficial. However, extreme caution is crucial in such an operation as no remedial measures can be taken after the blast due to water ingress.

The Andhra Pradesh Power Generation Corporation (APGENCO) of India, recently carried out a lift irrigation scheme at the Srisailam Left Bank Canal Project (SLBC) for the government of Andhra Pradesh. The project included installation of four 4 x 25,000hp pumps to lift 2,400m3/sec of water from the Nagarjuna Sagar reservoir for regional irrigation purposes (Figure 1).

In order to protect the underground machinery and workings from an inrush of water while main excavation and lining works were carried out, a rock plug was left and blasted at the end of the project. The plug, designed by Central Mining Research Institute, India, was situated about 10m from the upstream side portal of the intake tunnel, taking into consideration the stability of the rock mass. A cofferdam was constructed to allow excavation of a 10m length of the tunnel from the lake side. The cross section area of the tunnel was 40m2.

Because there were nearby structures, a controlled blast strategy in phases was developed for regular tunnel construction blasts, prior to the final plug blasting. A trial plug blast was conducted on a dummy plug at a nearby location, designed to ascertain different variables and the final blast design including charge concentrations and delay sequencing. Vibration and damage characteristics were also ascertained to refine the blast design for the final plug.

A great deal of published material currently exists on controlled blasting, and this was referred to before finalising the blast design. Raina et al (2000) have summarised most of the existing literature, however, different strategies suggested and adopted by Adhikari (1999), Berry and Dantini (1981), Holmberg (1980), Holmberg (1982), Worsey (1984) and Worsey (1986) were also consulted in order to achieve better results.

Geology and rock mass properties

The rock formation of the plug was medium hard fresh granitic gneiss with a RQD varying from 60% to 85%. The strata above the plug was mildly weathered granitic gneiss with a RQD ranging between 50% and 60%. The weathered formation resulted in water seepage near the plug. In situ permeability tests carried out in a blast hole near the rock plug yielded a water discharge of 1 to 1.5 litre/min.

There are two major joint sets, one is horizontal with 0.1 to 1.5m spacing and the other is inclined (semi-vertical), with 0.25 to 2m spacing. Rock samples were collected from the upstream and downstream sides of the rock plug for testing various engineering properties (Table 1).

All the rock properties were thoroughly examined in order to determine the optimum thickness for the rock plug.

Rock plug design

The standard method (static load approach) was used for the plug design, in consonance with the rock mass condition and other factors (Figure 2). The width of the rock plug was 4.5m. This maintained a standard factor of safety of 10, in view of the critical nature of adjacent structures and personnel involved in construction activities.

The rock plug comprised of granitic gneiss with an unconfined compressive strength of 38MPa. However, the rock slope at the portal of the intake structure constituted moderately weathered granitic gneiss with a slope angle of 50° to 60°. These geotechnical aspects prompted an adoption of different peak particle velocity (Vmax) limits for the rock plug and the slope.

The safety concern for the slope was greater, as any failure could result in a rock slide that could block the entire intake structure and prove extremely detrimental to the project execution.

Blast vibration data (Vmax) was monitored during regular tunnel blasts for the intake construction before the trial plug blasting, to predict possible damage to the periphery of the tunnel and to avoid damage to the surrounding structures and outside rock slope.

Safe blast design for the trial rock plug was thus developed based on the results of the regular intake blasts. A maximum limit of Vmax =25mm/s (Jimeno et al, 1995) and 50mm/s (Siskind, 1998) for rock slide and concrete structures, respectively, was adopted to work out maximum charge per delay.

Trial plug blasting

A trial blast at the outlet (surge pool side) of the intake tunnel was carried out before blasting the final plug. Since the strata are similar (except for the watery conditions near the final plug), trial plug blasting was considered suitable for assessing the volume of the material blasted, the spread of the blasted rock material from its original position, fragmentation and damage.

The desired throw was up to 15m and good fragmentation was expected. The objective of fixing the throw and fragmentation was to ensure adequate cross section of opening, and clearance of blasted debris, to allow 2,400m3/sec of water for pumping.

Vibrations measured during the trial plug blasting were 15.5mm/s at the surface (exactly on the top) and 11.7mm/s at 35m in one of the existing tunnels between the power house and surge pool. Visual inspection surveys confirmed that there was no disturbance to the rock mass or nearby structures. The profile of the periphery of the tunnel after the trial plug blast was smooth.

Rock slide control

Vibration was also monitored to predict rockslides at the inlet of the proposed rock plug prior to final plug blasting. For this purpose three trial blasts were conducted in the intake tunnel 42m to 48m from the proposed rock plug.

Details of explosive quantity and distance from vibration monitoring stations are given in Table 2. All the data was summarised and a predictor equation was worked out based on the analysis of the data (Figure 3).

(Vmax = 2967.8(SD)-2.2809; R2 = 0.7693; where SD = Scaled Distance i.e Distance/Charge 0.5 and R is the correlation coefficient)

Safe maximum charge per delay for the final rock plug blast was calculated based on the attenuation characteristics (Figure 3). The maximum charge used to control rockslides is given in Table 3.

Final plug blast

Based on the blast performance of the trial rock plug and vibration data generated during the blast and other nearby blasts, some modifications were made to the final plug blast design:

  • Specific charge was increased from 1.23kg/m3 to 1.33kg/m3 to improve the throw and the fragmentation.

  • Only gelatine explosive was used in the holes since the blast holes were wet due to water inflow.

  • Based on previous experience, dummy holes at a spacing of 0.3m were drilled to restrict the ground vibrations, avoid rockslides at the tunnel portal, and to minimise rock mass damage. This practice was not followed in regular blasts, as the charge in this case was much less than that used for the final plug blasting. These helped to limit Vmax near the critical structures.

  • Bottom row holes were completed before the crown holes to avoid under-breakage of rock from the design and easy breakage of corner rock mass.

  • A muck pit (or sump) to accommodate the blasted rock was initially planned, but could not be constructed because of time constraints. Therefore, charge in baby cut holes (0 and 1 in Fig 4) in both horizontal and vertical directions was increased to maximise the throw of muck.

  • A borehole, made from the top, was used to carry lead wires for detonation of the blast holes. Figure 4 shows the design details used for the final plug blasting.

Performance

Structures at a distance of 20m to 30m on the surface and vertically above the rock plug were undisturbed after lake tapping. The Vmax recorded was 22.5mm/s and 17.2mm/s at 20m and 25m respectively, from the rock plug, indicating no damage to the adjacent structures or the rock mass.

Figure 5 compares the predicted and observed values of Vmax from regular, trial plug blast and final plug blast. It is evident from the figure that the Vmax has been controlled well below the expected value in case of the plugs. Local variation in geology might have played a role but significant departure from the predicted values is apparent and is attributed to controlled blasting methodology adopted while blasting the plugs.

Successful lake tapping meant that lift irrigation started on schedule on 31 August 2001 and is now operating successfully.

Related Files
Figure 4: blast design for rock plug at SLBC Lift scheme
Figure 3: Relationship between Vmax and scaled distance at SLBC Lift scheme
Figure 1: Section of the overall project
Figure 2: Static load approach method used for plug design
figure 5: Comparison of the observed and predicted Vmax in different blasts