Blasting overhangs close to a vibration-sensitive structure
By R. Farnfield, technical services manager, EPC-UK
In the summer of 2007, Blasting Services received a call from a civil engineering contractor asking if they could ‘come down and blast a couple of overhangs’. As is often the case in such situations, the work turned out to be much more challenging than originally anticipated with nearby vibration-sensitive structures and a general lack of knowledge about the use and impact of explosives.
Land suitable for housing development is in increasingly short supply and great encouragement is given by national and local authorities for the use of so-called ‘brownfield’ sites, such as old industrial areas. In this particular case the housing development was to take place on land previously used for granite quarrying, with part of the final void being filled with quarry waste to form a platform on which to build the houses. An overall view of the development area is shown in figure 1. The area to be filled with waste is indicated by the number ‘3’ in the figure. The waste platform was to be formed by the construction of a stone retaining wall with the waste compacted in thin layers as the wall was constructed.
In the area indicated by the number ‘1’ in figure 1, the quarrying operation had left a series of overhangs; these are shown in more detail in figure 2. It can be seen that the upper section of the face consists of sandstone with the lower portion comprising granite. The contact between the two rock types is an unconformity with the granite being considerably older than the sandstone. The contact surface was also known to dip into the excavation. Following a geotechnical assessment, the contractor considered that these overhangs formed an unacceptable risk to machine drivers operating beneath then on the retaining wall and fill platform construction, and hence the request for remedial blasting in this area.
Blast design process
A key part of the blast design process was to determine the limiting maximum instantaneous charge weight, given that there was a structure about 90m from the blast area. The stated vibration limit for this structure was 6mm/s, as monitored on the ground surface next to the structure. As there were no historical vibration data for this area, a ‘worst case’ scenario was applied by employing a regression analysis based on an extensive blast vibration database maintained by the drilling and blasting contractor. The use of this database gave rise to a limiting charge weight of 5kg with a confidence limit of 95%.
Although the quarry was no longer operational, it was decided to apply all the principles outlined in the Quarries Regulations in relation to drilling and blasting. To this end a full blasting specification was drawn up. This specification is effectively a time-line outlining the data that must be collected and recorded to allow a safe blast to take place, as outlined below.
- Plan showing the location of the blast.
- Drilling plan with the intended location of each hole together with depth, inclination and direction.
- Drillers log showing any cavities, broken ground etc.
- Drill hole survey showing the actual hole location, depth, inclination and deviation.
- Profiling to allow the burden to be determined (in conjunction with item 3 above) around each hole.
- Face inspection highlighting the geology and other features – normally via photograph.
- Hole loading diagrams giving details of the explosive loading and initiation for each hole.
- Plan showing the intended initiation sequence.
- Plan showing the danger zone.
For most blasting operations, the order in which these data are collected is generally in line with the order outlined above. However, for this job it was considered necessary to vary the order by profiling the face before drilling. This pre-profiling would allow the drilling plan to be exactly tailored to the face shape and conditions.
Face profiling in this case was carried out using a 3D photogrammetric system. Employing this system had the advantage of combining both the profiling information and photographs, thus allowing the explosive loading of the holes to be modified to suit the geological conditions in front of each hole. The input to the photogrammetry system consisted of two photographs of the face taken a few paces apart. In this case the images were taken from the far side of the quarry with a zoom lens. Figure 3 shows the left-hand photograph of one of the overhangs, and in the image the reference points (red disks) can be seen. These points are used to convert the two photographs into a 3D image. The same reference points are also subsequently used to set out the surface location of the blastholes. Having generated a 3D model, holes were interactively positioned in the model at a 1m spacing so as to allow the overhang to be cleanly blasted away. Figure 4 shows a screen image from the software with the 3D image colour-coded according to the burden around each blasthole. In this case the red colouration indicates that the burden is less than 0.5m. Figure 5 shows an example of a profile drawing through the overhang and indicates that the blasthole should drill into air at the bottom of the overhang. This was found to be a useful check of the accuracy of the system, as this could easily be verified in the field.
Having determined the optimum drilling pattern this was drilled with an Atlas Copco D7 remote-control drill rig employing 102mm holes. After drilling, each hole was surveyed and any variations from the design were entered into the 3D model. After careful examination of the profiles, drilling log and 3D face photographs, a design was determined for each blasthole with a combination of 40g/m detonating cord and 28mm diameter dynamite. In simple terms, the criteria outlined in table 1 were used.
Where the burden in front of a hole was greater than 2.0m, consideration was given to adding an additional hole between the existing hole and the free face.
It is worth noting that the explosive loading outlined in table 1 results in a very low consumption of explosive per unit volume of rock, ranging from 80 to 400g/m3. In reality the design relies on the charges being closely spaced and, therefore, producing a fracture plane that allows the overhang to fall away with the aid of gravity.
Use of explosives banned
Having drilled all the holes, designed the blast and ordered the explosives, the client then decided that the nearby structure was in such a poor state of repair that the use of explosives could not be permitted. However, the drilling and blasting contractor decided not to walk away from the contract at this point and instead sought to try to help the client to find alternative methods. During the following couple of months, a number of alternative rock-breaking methods were examined or trialled on site, including:
- Mechanical breaker – this option was not considered to be safe as the machine would be required to stand on the overhang. There was also a risk of toppling when the rock finally gave way under the impact of the hammer.
- Pyrotechnic rock-breaking cartridges – trials were carried out with this type of product with little success. This was due to the fact that the holes were drilled for the use of conventional explosives and were too far apart and too large a diameter for these pyrotechnic-based products.
During this period, work at the site slowed and eventually stopped, as machinery needed to work underneath the overhangs. Also during this period, all communications from Blasting Services to the client noted that ‘we remain convinced that the only safe and secure way of removing the overhangs is with the use of explosives’.
Eventually the client accepted that the only sensible way forward was to agree to the use of explosives, but with a reduced vibration limit level of 4.0mm/s. Steps were also taken during this period to stabilize the problem structure by replacing the roof and pulling its gable walls back into vertical. It was also agreed with the owners of the structure that it should be evacuated for each blast and not reoccupied until it had been declared safe by a structural engineer.
Updated blast design
Following the decision to allow the use of explosives, it was agreed that a small initial blast should be carried out to allow site-specific vibration data to be collected. Fortunately, such a blast had already been drilled to remove a small portion of rock at the end of the bench furthest from the structure. This blast confirmed that a maximum instantaneous charge weight of 2.5kg could be used while keeping to the new 4.0mm/s vibration limit. The only way to achieve this was to introduce double-decking in holes that were likely to exceed the 2.5kg limit.
Figure 6 shows the final hole loading design for the overhang shown in figure 4. It can be seen that the loading varied considerably from hole to hole, and this resulted in the charging process taking several hours to complete. There was some concern that there would not be enough energy in the blast to detach the overhang and, therefore, it was decided to also load the holes with dry sand in an attempt to ensure that the explosive energy was efficiently used. Each deck of explosive was initiated with an electronic detonator employing 5ms between successive charges.
The hole loading design was also complicated by the fact that an entire box of dynamite needed to be used together with a complete reel of detonating cord, as it was not possible to store any explosives on site.
Results of blast
The overhang blast was a complete success with the rock being detached from the face to leave a clean and stable surface, as can be seen in figure 7. The blast gave a peak vibration level of 2.4mm/s at the sensitive structure and the building remained in the same condition it was in before the blast.
Following the successful firing of the first overhang blast, two additional blasts were approved to remove the remaining overhangs and these were accomplished without any problems and with a maximum vibration level of 3.0mm/s being recorded at the sensitive structure.
As a result of the successful remedial blasting operations, the client’s confidence in both the use of explosives and the drilling and blasting contractor increased to such an extent that they requested an additional 17 blasts on site to generate rock for use in facing the retaining wall. These blasts were all designed to minimize vibration levels at the various structures in the area, employed a maximum instantaneous charge weight of 5kg and were all fired with the aid of electronic detonators
The fundamental problem in this contract was the client’s fear of the use of explosives. In reality, this was due to a lack of knowledge and education within the UK civil engineering industry with regard to drilling and blasting. However, by persevering with this contract in a patient and professional manner, the drilling and blasting contractor was able to turn what could have been a very difficult situation into a successful and profitable operation.