Spot Assessment Report of
two quarries located near proposed tunnel no. 1 near bairabi, KOLASIB DISTRICT, MIZORAM
By: F. Lalnuntluanga, Geologist
In persuance of this department office order No. 16 of 2012 Dated the 31st January 2012, two officer Pu F.Lalnuntluanga, Geologist Jr. and Pu Lalremmawia, Asst. Controller of Mines visited the two quarry sites located near Bairabi on 7th February, 2012. The assessment report and suggestion for futher course of action are given below:
1. BACKGROUND
New Railway line starting from Bairabi Station upto Sairang near Aizawl City stretching about 57 km is proposed. The present site near Bairabi, Tunnel No. 1 is one of the 23 tunnels going to construct to fulfill the new railway line. This tunnel No. 1 is the longest of the 23 tunnels proposed and has located between two quarries permited to Pu R.Lalbiaktluanga of Zarkawt, Aizawl and Pu Lalthangmawia of Meidum village.
Though the tunnel floor is about 140m below the existing quarry floor, any kind of execution of explosives in any form or manner can causes or shortened the life of the tunnel or at the worst cases it may colapse due to shock waves generated by the explosion even during tunelling work.
Taking cares of all possible senario, Railway engineer and District Administrator of Kolasib District alongwith some other official have already visited the site and assessment the situation and proposed the Technical input of Geology and Mineral Resources Department regarding Blasting Impact Assessment.
2. Status of two quarries
2.1 The quarries are owned by Pu R.Lalbiaktluanga of Zarkawt, Aizawl and Pu Lalthangmawia of Meidum village.
2.2 The two quarries are operational and holding valid permit.
2.3 Pu R.Lalbiaktluanga’s quarry permit will expired on 31st March 2012 where Pu Lalthangmawia’s quarry permit will expired on 30th September 2012.
2.4 Both the quarry site are under Garden Pass issued by Bairabi Village Council, Bairabi, Mizoram.
2.5 Pu R.Lalbiaktluanga quarry is 145m north of the railway line whereas Pu Lalthangmawia quarry site is 100m south of the railway line as reported by the Railway Engineers.
2.6 The existing floor of the two quarries are about 140m above the proposed tunnel formation level.
3. Geological Features of the area
3.1 The area is mainly composed of medium-grained brown sandstone, silty sandstone and shale. The shale beds are highly fractured and loosed. The rock beds are over lained by thick over burden residual top soil.
3.2 Geophysical Survey report shows that there is possibility of present of weathered sandstone below 12.47m.
4. GEOTECHNICAL PARAMETERS
4.1 The rock quality and strength of the two quarry are checked by using Smidtch Hammer N-Type and the data collected are converted and calculated the rock strength using Singh et. al (1983) conversion formula, the average Uniaxial Compressive strength of the rock is rangng from 56.40MPa to as high as 88.80MPa.
4.2 Majority of the rock exposed are low grade in term of UCS test.
4.3 The rock are dissected by fractured and small joints.
5. Blasting Impact
There are four different possible causes of annoyance, which may create uneven post-blast disturbances, they are- Ground vibration, Flyrock and air overpressure.
5.1 Ground Vibration: As only about 20-30% of energy of commercial explosive used in Mines is utilised for fragmenting rocks, the rest of the energy is transmitted through the earth in the form of Ground Vibration resulting in damaged to the surrounding structure. When explosive ( eg. ANFO, Powergel etc) are detonated a seismic waves pass through a medium ( rock mass or soil), they cause particle of the medium to move which is termed as ground vibration. The velocity at which particles of the medium moved is called particle velocity and its maximum value is peak particle velocity ( ppv). Ground vibration is one of the most undesirable side-effect of rock blasting. It can produce structural damage to residential building, tunnel etc in the vicinity of the mining operation. So, Directorate General of Mines Safety ( DGMS) has stipulated the threshold values of vibration for the safety of buildings and structures like tunnel, bridges etc which can be seen at Table-1.
Table-1 : Permissible Peak Particle Velocity (ppv) at the foundation level of structures in Mining/Quarrying Areas in mm/s ( DGMS Cir. Tech. 7/ 1997).
