Abstract
The ultimate causative factor for the failure is rapid removal of toe support of the slope due to unplanned mining accompanied with uncontrolled blasting. There is also a natural causative factor behind, a naturally formed highly weathered slip surface, where along the slope failure has taken place. Secondary discontinuity created along the well-developed foliation plane due to an earlier disturbance of rock mass along kinematically more unstable joint planes, is the inception. This has turned into a weaker plane by groundwater seepage for a very long period facilitated by drainage pattern of the area. Intense weathering features of failure zone, chert particles found from the slip surface are good indications for this factor. Furthermore, it was identified that, the shear strength of rock joints can conveniently and rapidly be determined using Rock Mass Rating System and Empirical Equations. Even though these methods provide more conservative values, results will be very useful in initial design work. Results show that the back analysis method is more reliable compared to above two but is conditional as a similar type of a failure need to occur in the same rock mass in order to employ this method. Moreover, it was revealed that Barton‟s theorem can be effectively applied for local rock masses in determining the shear strength of discontinuities and is reliable in using at lower stress levels. When considering the stability of remaining slopes of the same site, these are highly venerable for same type of failure at any moment. According to site geometrical parameters and shear strength parameters found out from back analysis reveals that the natural factor of safety is only around 1.0 for slopes that remain hanging at this site. Further, study reveals that, the most economical method of stabilizing these existing unstable areas in the site is by reduction of the slope height with the use of controlled blasting techniques. ENGINEER, Vol. 47, No. 03, pp. 49-58, 2014
Highlights
Landslides hazard is a major problem in the Central and Sabaragamuwa provinces of Sri Lanka, where high rainfalls are experienced throughout the year
Hydrogeological pattern of the area has further worsened the situation by stagnation of water on this disturbed rock mass, may have caused to seep large quantities of flow through opened up more unstable joint sets down to more stable well developed weaker foliation plane, which became as the slip surface of the case under consideration
This fact has been further proved by the presence of chert particles on the slip surface and excessive weathering caused to leach the mineral grains of the slip surface and the reduction of the shear strength of the slip surface
Summary
Introduction Landslides hazard is a major problem in the Central and Sabaragamuwa provinces of Sri Lanka, where high rainfalls are experienced throughout the year. Increase of pore water pressure in joints and discontinuities of rock masses and subsequent reduction in effective stress cause to reduce the shear strength of the failure plane. The main causative factors behind are the ignorance of geological structural features of rock mass, careless violation of rules and regulating conditions imposed by the mining regulating authorities, non-removal of the overburden soil mass prior to excavation, excavating and removal of toe support of slopes and improper designing of benches with poor drainage control. These may contribute either as singly or as combinations
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