Executive Summary
Keifang Quarry is located 3 km away from Keifang village toward Champhai. This quarry has been operational for the last 20 years or so and was executed by Boarder Road Organisation. The main uses of this rock are road construction purposes and in some cases local peoples are using for building materials at a very small quantity. This quarry has located at the road-side of Aizawl-Champhai road which is the main communication link between Aizawl and Champhai district. Due to extensive excavation of rock from this area, the road along this quarry has damage and sometimes block the traffic. In recent year, the public in general and vehicle owners in particulars awares the risk poise by this quarry due to frequent rockfall. The strange incident in this quarry is that no rockfall or collapse of rock from the quarry has never been take place during raining period. This is reported by local people working in the area for their livelihood.
Their is a public outcry this year because of blockage of road due to massive rockfall in June 23, 2007. Such type of blockage also happended during 2005 and 2006. Knowing the role plays by this quarry for road construction, The joint committee of all NGO covering Champhai District issue a press statement demanding suitable measure for public and traffic safety.
1 KEIFANG QUARRY Road Alignment : 250O Strike : 10O Dip : 100O Dip amount: 42O Joint:
1. 40O/50O/South-East(130O) 2. Bedding Joint 3. 270O/6 0O/East 4. 95O/75O/East
5. 210O/70O/310O 6. 280O/6 0O/310O 7. 145O/8 0O/Sub vertical 8. 275O/6 5O/320O1.
INTRODUCTION
1.1 Location: Keifang Quarry is one of the most important rock quarry along the Aizawl- Champhai road section located at 3 km away from Keifang Village toward Champhai that is 76 km from Aizawl. This quarry is operated by Border Road Organization for several years for road construction purposes.
1.2 Background: This quarry plays an important role in making of road development and construction particularly for Seling to Champhai road section. This is the main sources of rock for this road section. At the same time, due to many factors and circumstances, this quarry causes many hardships to people who are traveling through this site because of rock falls and rock blockage every year. With the implementation of “The Mizoram Minor Minerals Concession Rules 2000”, the operation of quarry and safety of quarrying come into the picture of Geology & Mining Wing since 2000. Taking care of the safety and standard of quarrying operation, the Geology & Mining Wing considered Keifang Quarry is one of the most high risk in term of the quarrying method, traffic that ply the site and human life. At the same time, this quarry has a potential of rock for road construction purposes for many years. Considering the potential hazard and risk that this quarry created to traffic and human life, the department detailed Pu R.Zirtluanga, Geophycisist Jr and Pu F.Lalnuntluanga, Geologist Jr. detail to conduct site study and assessment of the area.
1.3 Previous Work: Pu Lalhmachhuana, Geologist Sr. and Pu Lalnunhlua, Asst Mine Inspector has done Site assessment during 2002. They reported that the quarry has a potential a 7,29,000 cu3 m as possible reserve. During 2006, Pu Lianngura Pachuau has done assessment of rockfall after a huge rockfall happended during June 2006. According to his report, there is no major or minor fault 2within the quarry area. He concluded that the rockfall are due to extensive use of explosive and suggested proper scientific mining and minimal uses of explosive for extraction of rock. 1.4 General Geology: This area falls within Middle Bhuban formation hav- ing a composition of Sandstone-Siltstone-shale as major lithological compo- nent. The sandstone-siltstone alteration with a thin layers of shale beds par- ticularly along a fracture/weak plane are the main lithological features of this quarry. The over-burden materials is approximately 5-10m thick having loose, brown sandy materials (Fig-1).
1.5 Structural Geology: The important structural features in this quarry is present of three(3) sets of joint (Fig.2) which are cutting across to each other. This joint sets created the rock formation vurnarable to rockfall. The other joint set is bedding joint dipping towards southeast(100o ) has an im- portant role in destabilizing the rock beds. The joint spacing is very small(2inches) to as large as 10 ft.
1.6 Description of the area: BRTF people are working in the area for the last 20 years or so and extracted rock for road construction. The area is composed of Sedimentary rock of sandstone-siltstone formation having thin layers of shale beds mainly along the weak zone. This quarry covers more than 90m along the road and the vertical heigh is as high as 70m.
2. GEOTECHNICAL OBSERVATION:
Ensuring the stability of highway rock cuts, whether new or old requires an evaluation of the structure of the rock. Failure results in blocked roads, property damage, injury, and loss of life. Failures usually initiate and follow pre-existing discontinuities rather than break through intact rock (Fig.3). Failures are typically triggered either by seismic activity, high groundwater pressures, or progressive deterioration of the intact rock because of weath- ering. Analysis of potential instability can range from limiting equilibrium analysis to on-site engineering judgment of an experienced specialist. Rock determined to be loose with the potential for failure must be removed or restrained in some way. 3Vertical discontinuities paralleling the rock are however typically hidden, and can often not be identified by observation. Failures in rock cuts occur as a result of several mechanisms. Figure 4 shows raveling, undercutting. failures. These types of mechanisms are in general not conducive to predic- tive calculations or modeling, but rather require engineering judgment to determine whether they are likely to fail or to remain stable. Rock-fall hazard results from two processes: detachment of a rock volume from the upper part of the slope (local failure of the slope), and propaga- tion of this volume along the slope. A deterministic evaluation of rock-fall hazard in an extended area would consist of determination of potentially unstable rock volumes, their failure times and their trajectories. Unfortu- nately, the detailed knowledge of rock slope structure and of failure and propagation processes is not generally sufficient(needs instrumentation and laboratory test) in most areas to make possible such a deterministic evalua- tion.
1. The upper rockslope was kinematicly unstable with major blocks dipping out of slope that required reinforcement with post-tensioned rock bolts or dowels.
2. The adverse geologic structure such as four(4) prominent joint sets are present which exacerbated the excessive backbreak and rockfall.
3. The joint plane act as a triggering zone where rain water pass through all through-out the years.
4. Blasting causes destabilization of rock particularly along joint plane.
5. The joint plane are dipping/facing toward the road and the dip amount si very high. Along this joint plane, all rockfall and sliding took place.
6. Rock excavation and extraction have been done primarily from the bottom of the rock formation (Toe cutting). This type of extraction and excavation triggered and exacebated the rockfall.
7. The present of loose, fracture shale beds between sandstone and siltstone beds causes instability. 8. Along the shale beds, weathering and loosening of rock are much more faster that could have triggered any kind of instability.
