INTRODUCTION

The rock mass dassessment system and rock characterization methods have been previously established on a single type of rock which was assumed as an isotropic and homogenous. However practitioners reported that none of the established systems are relevant to interbedded rock mass which was also known as a problematic or difficult ground (Mohamed, 2004). It was reported that it would be a very tedious exercises to predict the behavior of such rock mass without first carrying out a thorough and systematic physical and engineering assessment. The engineering problems of interbedded rock are due to the differences in rock lithology, complex mode of failures and non-homogenous behaviour. Since early 90’s, some research studies had been initiated into heterogeneous rock formation which is also known as composite rock mass. The rock mass engineering model can be predicted either by simple rock classification system; empirical model, numerical modeling or a combination of any of them.
A geotechnical research study has been carried out to characterize, classify and determine the engineering properties and behavior of weathered sandstones and shale named as composite material, dominating a Kenny Hill formation (Mohamed, 2004). From a rigorous experimental works the empirical strength models are established and later become a mean to further understand and predict the modulus and stiffness characteristics of the composite rock with respect to their respective weathering state. The advancement of powerful statistical software has helped to expedite the analysis and hence developed an empirical rock mass model. Shakor et al. (1991) and Ulusay et al. (1994) had developed empirical models for sandstone rock mass by using multiple regression analysis from petrography and geo-mechanics data. Meanwhile Habimana et al. (2002) characterized the catalyst of quartzite sandstone and phyllite schist by adopting the Hoek-Brown failure criterion. However, Gokceoglu et al. (2003) has predicted the empirical model of rock mass based on the ratio of rock modulus and its quality index to the weathering grade of weak rock. They had respectively emphasis on the success of precise prediction of a single rock mass behavior hence contributed to the reduction to the cost of site investigation.
This paper highlighted and discuss the development of an empirical relationship that express the strength and stiffness behavior of composite rock determine from the rigorous study on the typical rock formation of composite material (Kenny Hill rock mass). Figure 1 below illustrated four possible observed vertical profiles of weathered sandstone and shale by assuming the layers representing an ideal horizontal interbedding as recommended by Mohamed (2004).
The figure above clearly indicated the heterogeneity of composite Kenny Hill rock mass. Generally as the state of weathering changes from slightly weathered to highly weathered, the physical and mechanical properties also deteriorate and degrade accordingly. Thus any combination of rock mass profile is expected to be unique which explained the difficulties often encountered and reported by practitioners working with Kenny Hill rock mass that resulted to possible unexpected delay on geotechnical work

CHARACTERISATION CLASSIFICATION BY WEATHERING GRADE

The outcrop of sedimentary cut slope at Section 8, Shah Alam, Selangor was selected as a case study. The outcrop is geologically known as Kenny Hill rock mass. From the field mapping, the cut slope constituted of slightly to highly weathered sandstone and shale with traces of quartz veins. The two dominating materials were differentiated physically, where weathered sandstone is a fine grained gritty texture while weathered shale is smooth and powdery. It was very difficult to ascertain the grade of weathering through physical observation therefore some methods of testings had been used to characterize and classify the weathered sandstone and shale. The objective is to determine the degree of deterioration and disintegration of both materials hence explained the complex mode of slope failure observed in-situ. The details of finding have been discussed by Zainab 2004. Table 1 summarizes the physical classification of weathered sandstone and shale looking from the quality of weathering grade, hand sample hardness, rebound hardness, surface texture, dry density and slake durability. It was found to be a gradual physical degradation and deterioration of both materials with respect to the increase in grade of tropical weathering. Hence, the type of composite rock models as shown in Figure 1 , each is expected to has different strength and stiffness characteristics with any combination of the respected weathered materials.
Table 1: Physical properties of weathered sandstone and shale of Kenny hill formation