943
Physical modeling of arch action in undercut slopes with actual engineering practice
to Mae Moh open-pit mine of Thailand
Modélisation physique de l'effet de voûte dans les pentes en déblai en suivant la pratique de
l'ingénieur pour la mine à ciel ouvert à Mae Moh en Thaïlande
Pipatpongsa T.
Global Scientific Information and Computing Center, Tokyo Institute of Technology
Khosravi M.H., Takemura J.
Department of Civil and Environmental Engineering, Tokyo Institute of Technology
ABSTRACT: In the field of mining engineering, a stable arch formed across a pit is beneficial to the design of an undercut slope;
therefore, prediction of the maximum stable undercut width under which the slope does not collapse is needed. The relation between a
stable width and an inclined angle has been obtained experimentally to confirm the developed theoretical relations. A series of simple
experiments using a block of compacted moist sand confined by parallel rigid walls has been conducted by varying the thickness,
width and length. The actual engineering application was immediately tested at the Mae Moh open-pit mine in Thailand. The factor of
safety in fully saturated condition with hydro-static water pressure on bedding shear was evaluated. It is concluded that this novel
procedure in 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.
RÉSUMÉ : En génie minier, la formation d'une voûte stable au-dessus de l'excavation fournit un avantage pour la conception d'une
pente en déblai; par conséquent, la prédiction de la largeur du déblai maximale, qui ne provoque pas d'effondrement, est nécessaire.
La relation entre une largeur stable et un angle d'inclinaison a été obtenue expérimentalement pour confirmer les relations théoriques.
Une série d'expériences simples utilisant un bloc de sable humide compacté confiné par des mur rigides parallèles a été menée en
variant l'épaisseur, la largeur et la longueur. La pratique de l'ingénieur est immédiatement appliquée pour la mine à ciel ouvert à Mae
Moh en Thaïlande. Le coefficient de sécurité dans un état complètement saturé avec la pression hydrostatique sur la zone de
cisaillement a été évalué. Il est conclu que cette nouvelle procédure de l'exploitation minière est réalisable, entraînant une réduction du
volume d'excavation, du transport et du déversement de la masse rocheuse instable, ainsi qu'une économie de temps et d'argent.
KEYWORDS: arching effect, undercut slope, excavation, physical model, open-pit mining.
1 INTRODUCTION
Evaluating the stability of slopes is one of the most important
activities in geotechnical engineering. The existence of a stable
scarp in some slope failures along oblique faults can be
evidence of an arching effect in those slopes. Pipatpongsa et al.
(2009) reported the existence of some stable scarps in huge
slope failures in the Mae Moh coal mine. Exposed scarps of a
slope failure remains stable if the material has a sufficient
strength to resist the load transferred to the stable adjoining
parts. This phenomenon of load transfer from the yielding part
of the material to the adjacent stationary parts is known as the
arching effect (Janssen 1895 and Terzaghi 1936).
300 m
Unstable area
A
A’
Figure 1. Unstable Area 4.1 (as of September 2010)
The relation between a stable width and an inclined angle has
been obtained experimentally to confirm the developed
theoretical relations (Khosravi 2012) for (a) a strip arch with
slip failure in laterally supported sand blocks, (b) a segmented
arch with arch-shaped failure in mild undercut slopes and (c) a
circular arch with buckling failure in steep undercut slopes. For
the design purpose of undercut mining, this particular relation
provides the maximum span of the undercut where load could
laterally be transferred to vertical planes of a neighboring rock
mass. A case study of an undercut slope at the Mae Moh open-
pit mine in Thailand is presented.
2 SITE DESCRIPTION
The actual engineering application of the developed theory was
immediately tested at the Mae Moh open-pit mine in Thailand.
The Mae Moh open-pit lignite mine primarily supplies coal to
generate electrical power in Thailand. The mine, under the
operation of the Electricity Generating Authority of Thailand
(EGAT) since 1952, is located approximately 630 km north of
Bangkok in Lampang province. Currently, the annual
production of the mine is about 16−17 million tons/year with a
volume of excavated overburden of around 60−80 million
m
3
/year. Its pit has a maximum width of about 4 km and a
maximum length of about 9.5 km. Green clay in the bedding
shear zone has caused problems of various scales. Slopes in the
Mae Moh mine are prone to plane failure once they dip out of
the slope face and strike parallel to the bedding shears.
