Analysis of Rock Mass Surface Colour using CIELAB and Munsell Chart

Abstract

Rock mass surface colour is crucial for weathering assessment. CIELAB colour space and Munsell chart are methods to express the surface colour of rock masses. This study analysed the L*a*b* value from both approaches and examined the effectiveness and reliability of rock slope weathering assessment method. The Munsell Rock chart is a conventional method primarily used in practice. However, due to the constraint of individual vision and variation in the optical environment, errors and bias in rock surface colour can easily occur using the conventional method. The colourimeter was used to determine the CIELAB colour values of the samples, while the rock colour chart determined the colour coordinates and Munsell values. The Munsell hue and chroma were assigned using linear interpolation by Munsell conversion software CMC 2.1 to convert Munsell colour systems to L*a*b* value. A total of six granitic rock slopes were investigated. For quantitatively method, the Munsell optical system was less effective in assessing the rock mass surface colour. The positive correlation of a* and b* with R2=0.9407 demonstrated the efficiency of CIELAB system. The a*b* result of the rock slope indicated that most of the slopes were considered medium to strong with a moderately weathered surface. This result is strongly supported by a geomechanical test of Schmidt-UCS and index properties (Id2) with a range of 50-100 MPa and 2-4 MPa respectively. The finding verified mineralogical assessment which showed the correlation of rock mass surface colour to mineral composition. The study recommended any geotechnical and technical purpose in enhancing the method of weathering assessment by using the CIELAB colour space to rapidly and efficiently measure the rock surface colour.