Context
As more land comes under settlement, physical infrastructure is being developed in closer proximity to residential areas. Environmental management of the impacts of these projects during construction is getting prominence due to higher number of complaints from quarrying, cutting, filling and compacting. These activities raise vibrations, noise and dust.
Construction of the Kenyan Standard Gauge Railway was a watershed moment for these environmental concerns. Sections of the right of the rail went through residential areas, causing unprecedented impacts. Complaints of cracks in buildings, punctured roofs, damaged boreholes, drying hand dug wells, silted dams; dead poultry and aggravated hypertension in humans were reported. This resulted in disputes, with claims running into hundreds of millions.
Residents of Oloosirkon area of Rongai complained that blasting works at a quarry in the area and a cut section had caused cracks in their houses. The SGR contractor engaged Georama Engineering to conduct an assessment of their procedure and establish whether it was these activities that caused the cracks, or whether there were other unrelated causes.
Since there was no baseline survey conducted to establish the status of the buildings before start of the works, the consultant had to conduct measured tests of the blasting and vibration compaction processes to establish whether safe limits were exceeded. Scientific methods were used in determining whether the impacts could be attributed to SGR activities or not, and to inform of what could be done to minimize such impacts in the future.
Peak particle velocity (PPV) resulting from a blast can cause structural damage and human and animal discomfort if a certain limit is exceeded. The NEMA permissible ppv is 0.5 cm/s at a distance of 30m from the source of the vibrations.
Vibration survey set up
Vibration monitoring was done with a triaxial velocity sensor. A calculated explosive weight was used in a blasting program designed to monitor the ppv variation with distance. The monitoring device was placed at a known distance from the centre of the blast array so that the relationship could be used to relate the distance to the charge mass.
where v is the PPV (m/s); D is the scaled distance (), which is defined as the ratio of distance from charge point, R (m), to the square root of charge mass, Q (kg), expressed in TNT net equivalent charge weight, i.e.
; k and b are site constants
The blast charge was laid in 50 holes, each 8m deep, in an area of 450 square meters, measuring 30m x 15m and the blasting agent consisted of 900 kg of ammonium and 25 kg of gelignite. The monitoring point was 1190m from the blasting site, at one of the affected homes, at an altitude of 1704m compared to 1695m amsl at the blasting point.
Results
At the moment of the blast, one person out of four at the testing site felt the vibration, but the rest did not, hence the vibrations were clearly mild at the point. The maximum velocities recorded on the 3 axes and their relative frequencies were as follows:
- X – 0.35 cm/s; Freq. 210.526 Hz
- Y – 0.38 cm/s; Freq. 235.294 Hz
- Z – 0.19 cm/s; Freq. 48.193 Hz
The data shows that the vibration level was within the NEMA regulation of 0.5 cm/s.The calculated ppv was 0.31 cm/s, which was lower than but within the range of X and Y velocities of 0.35 and 0.38 cm/s, respectively. When the model input UCS was replaced by an actual tested specimen of a trachyphonolite, the calculated ppv was 0.36 cm/s.
It was found out that some buildings nearer than others to the blasting site were less affected compared to those that were farther. While the structural soundness of their construction could contribute to this discrepancy, a natural cause that was identified was that buildings that were founded on or situated near rock outcrops were affected more than others were. This was attributed to the velocity damping effect caused by a thick overburden compared to outcropping rock. Also, blast vibration velocity is affected by elevation between blast and impact point and the slope.
Conclusion
Some of the structural damage experienced by homeowners would have been minimized or avoided if the contractor had developed a blast vibration velocity model for the quarry site. This would have enabled them to plan for material production to meet their demand rate, while protecting against negative environmental impacts.