Abstract:
The largest open pit limestone mine in Sri Lanka which belongs to Holcim
Lanka Limited is located at Aruwakkalu, Puttalam. The sub-surface of this site is
comprised of an overburden of average thickness 24 m followed by a 9 m thick low
grade limestone layer and a high grade limestone layer at the bottom which extends
even below the seawater level.The overburden which is currently being excavated in
five benches of near vertical cuts becomes unstable during the rainy season, which is
a significant safety issue on this site. In this scientific investigation, the physical
properties of each layer of material are determined by laboratory testing, in order to
design optimum bench heights and slope angles for this open cast mine. Direct
shear tests are carried out on undisturbed soil samples collected from the
overburden, whereas rock-triaxial tests are carried out on the samples collected from
low-grade and high-grade limestone layers, in order to determine design
parameters: cohesion(c) and friction angle(ϕ) of these materials. The stability
analysis is carried out by means of “SLOPE/W” software. The analysis of data for a
three layer model reveals that the possible failures are confined to the overburden
based on the outcome of this study. The overburden mine slopes could be optimized
to three berms of each 8 m height, 6 m wide and of 700 slopes, having a sufficient
safety factor which will enhance the safety and productivity of the future mining
operations on this site.
