Rock Mass Classification for the Assessment of Blastability in Tropically Weathered Limestones

Abstract

Rock mass classification systems have been developed in various applications of rock engineering design such as tunnelling, foundation, slopes, rippability, and excavatability. These systems are reviewed and gaps with respect to blastability are identified. Geomechanical properties and geological features of the rock mass are known to have a great and direct influence on the effects of blasting operations. The tropical climate which is characterized by heavy rainfall of about 2000 mm/year and wide temperature variation resulted in thick weathering profile consist of heterogeneous zones. In addition, tropical climate and post tectonic impacts on the rock mass often cause severe and deep weathering in complex rock formations. The uniqueness of tropical influences on the geoengineering properties of a rock mass leads to significant effects on blast performance, especially in the developmental blasting stage. This study aims to determine the significant factors of tropically weathered rock mass properties that influence blast performance and to develop a rock mass classification system for blastability in tropically weathered limestone. The studied limestones are classified into four classes, namely W1, W2, W3 and W4 depending on the extent of weathering W1 type limestone is absent in Sri Lanka. (Marine) sedimentary, sedimentary and metamorphosed limestones are found in Sri Lanka, Cambodia and Thailand respectively. Uniaxial compressive strength (UCS) of limestones tested is 55 MPa and 77 MPa. Rock mass properties (Range of rock quality designation (RQD), range of cavity (%) for limestones at the studied sites are (15–50%, 0%), (30–85%, 0–14%) and (50–90%, 0–12%) respectively. Range of geological strength index (GSI) varies from 40–70, 30–60, 20–50 and 15–45. Blastability index (BI) is a better parameter as it considers joint orientation and other rock mass properties as compared to GSI. After reviewing various blastability systems, W1 and W2 limestone are comparable with intermediate spaced joints of blast quality system (BQS) for joint spacing 0.1–1 m as rock mass description - ‘blocky’ matches. On the other hand, W3 and W4 type limestone are comparable with closely spaced BQS for joint spacing less than 0.1 m as rock mass description - ‘powdery/friable’ matches. Karst classification systems for engineering or cone and tower are not suitable for blastability as these systems compare the change in karst topography over a couple of centuries and wider area. Rock mass classification system for blastability is proposed for tropically weathered limestone based on limestone density, degree of weathering, RQD, cavities, porosity, water absorption, point load strength index and BI.