Review of rock mass classification of tropically weathered limestone

Abstract

In tropical climate, limestone is frequnetly exposed to rain water which when absorbed by limestone forms carbonic acid. This further enhances process of dissolution resulting in change in geomechanical properties of limestone. With various geological discontiuties such as faults, folds, joints, water flows through these cavities. The weathering process along these cracks or cavities takes place at faster rate. In this paper, two case studies are reviewed. Rock mass is classified based on lithology, degree of hardness, degree of weathering, % of cavities, Rock Quality Designation (RQD)% and nature of joints. In another case study on limestone deposist from Thailand is classified based on Geological Strength Index (GSI) which enhances blast design. Limestone deposit at Aruwakkalu based on rock structure can be classified as (i) heavily cracked, frequent weak joints, weakly cemented layers (ii) Thin, well-cemented layers with tight joints (iii) Massive intact rock. This limestone deposit is also classified as bedding plane dipping into face, bedding plane dipping into cut and other cases. Existing system of rock mass classification at Sri Lanka is reviwed with case studies. Thus for Aruwakkalu limestone deposit, rock mass can be classified based on the types of rock structure, Blastability Index (BI), RQD%, degree of weathering and degree of hardness. Average powder factor of 0.15 kg/t can be correlated to BI.