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