Performance evaluation of existing surface excavation assessment methods on weathered sedimentary rock

Abstract

This paper is intended as a contribution to the discussion of the overview of existing excavatability assessments and their reliability in assessing the state of tropically weathered rocks, specifically in Nusajaya, Johor, Malaysia. For this purpose, case studies were conducted on sedimentary rock masses of the Jurong formation, located in Nusajaya, Johor. The existing available rock mass excavatability assessments were first critically reviewed. Three case studies were then conducted in order to assess the performance of surface excavatability in these sedimentary rock masses by practical excavation tests on site. Comparisons were then made between the assessments of those assessors and that obtained through the practical excavation test. The conclusions made in the paper about the results of the study show many divergent opinions among the assessors. It emphasizes the limitations of existing available assessments in predicting the excavatability of tropically weathered rock masses. Further complication may arise in sedimentary rock masses where different layers of rock are interbedding, producing different weathering profiles. In view of these, a simpler graphical method, which considers only two parameters, namely point load strength and spacing of surface discontinuities, was introduced to overcome the problems, thus might be misjudged during early excavation assessment. This subjectivity becomes more important as the heterogeneity of a rock mass increases with increasing interbedding and inherent anisotropy of the rock material, giving rise to quite different evaluations, even for a rock with the same state of weathering. Most of the existing excavatability assessment methods are found to be less reliable because nature, lithology and weathering state at various rock mass layers is not being taken into account. A more practical and appropriate excavatability assessment method is thus required to economically assess the tropically weathered sedimentary rock during the preliminary stage.