946
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
measuring at the G1 clay seam level which is located at a depth
of 11
12 m. Stage 1 is subdivided into 1A and 1B. Despite a
crack along the existing fault zone with a length of about 50 m,
observed in May 2012 at the left side corner of the pit, the
process of excavation and mining for stage 1A to a width of 120
m was successfully achieved over December 2011 to July 2012.
At the end of stage 1A, the total movement towards the pit at
the clay seam level measured by an inclinometer was 24 mm
and the slope had already been mobilized along the bedding
shear plane. For stage 1B, an increment width of 60 m was
excavated during July to October 2012 across a rainy season,
while the slope on the right side of the excavated pit was being
dumped with overburden “claystone” from the pit up to 100,000
m
3
covering a length of about 60 m to provide a counterweight.
Supported dump was on an incline plane, limestone rock bunds
were constructed underneath dumped material for reinforcement.
Due to excessive movement more than 60 mm in September
2012, the inclinometer tube was deformed and not functioning.
Fig.6 shows the beginning of stage 1B observed in October
2012, when the total excavated width reached 190 m.
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6 CONCLUSIONS
The slope stability problem in the Mae Moh lignite mine in
Lampang province in Thailand has been briefly reported. The
presence of a weak shearing zone in the clay seam between the
layer the shale caused trouble in the northeast pit. Area 4.1 in
the northeast pit is one of the potential failure slopes; part of the
lignite and rock mass had been left in front of this slope as a
counterweight to prevent a huge landslide. Mining in the
unstable slope was considered expensive. The newly developed
moving-pit mining method based on the physical model and
theoretical developments was introduced as an applicable
method for mining in Area 4.1. In order to apply this method,
two stages of excavation were planned. The process of
excavation and in-pit dumping must be done in sequence. At the
clay seam level, the total excavated width of 190 m with an
exposed width of 130 m and an area of dumped claystone of 60
m with limestone rock bunds underneath was found stable. It is
concluded that this novel procedure for mining is practically
realizable and results in reductions in massive excavation,
transportation and dumping of unstable rock mass, as well as
saving an amount of time and expense.
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7 ACKNOWLEDGEMENT
This research work was funded mainly by the Electricity
Generating Authority of Thailand (EGAT) under the research
project grant “Stabilization of Alternative Excavations and
Strengthening of Supporting Materials for Pit Wall in Area 4.1”.
In addition, the financial support provided by JSPS KAKENHI
Grant Numbers 23760441 and JSPS Asian CORE Program is
truly appreciated. The authors would like to express their
gratitude to all staffs of Mae Moh Geotechnical Department of
EGAT and particularly acknowledge Mr. Prajuab Doncommul,
EGAT and Dr. Cheowchan Leelasukseree, Chiang Mai
University for their collaborations and contributions.
Wangsa R., Wongsiriworakul J., Mungpayabal N., Pipatpongsa T. and
Wattanachai P. 2012. Residual shear strength of Mae Moh clay
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Elev. +170
Elev. +181
Elev. +192
Elev. +203
11 m
11 m
11 m
Exposed width 130 m
Dumped claystone
width 60 m
Figure 6. The total excavated width of 190 m, consisting of an exposed
width of 130 m and an area of a dumped claystone of 60 m measuring at
the level of clay seam (as of October 2012)
