Weathering not only modifies the chemical, mineralogical, texture and fabric properties of rock and soil, but also decreases the strength of the materials. In order to have a correct design of foundation and prevention of landslide, a good understanding as well as an accurate assessment of the effects of weathering on both geological and geotechnical properties of weathered materials are very important. For the geological aspects, the chemical and mineralogical properties were investigated by means of X-ray fluorescence (XRF) and X-ray diffraction (XRD) respectively. Six-fold material classification scheme, which is adopted in Hong Kong to classify the degree of weathering on decomposed material, were reviewed and its relationship to a number of chemical weathering indices were examined...[ Read more ]

Weathering not only modifies the chemical, mineralogical, texture and fabric properties of rock and soil, but also decreases the strength of the materials. In order to have a correct design of foundation and prevention of landslide, a good understanding as well as an accurate assessment of the effects of weathering on both geological and geotechnical properties of weathered materials are very important. For the geological aspects, the chemical and mineralogical properties were investigated by means of X-ray fluorescence (XRF) and X-ray diffraction (XRD) respectively. Six-fold material classification scheme, which is adopted in Hong Kong to classify the degree of weathering on decomposed material, were reviewed and its relationship to a number of chemical weathering indices were examined as well. The texture and fabric properties of both decomposed granite and tuff materials were investigated by means of microscopic observation, crack counting, mercury intrusion porosimetry (MIP) and scanning electronic microscopy (SEM). Magnetic susceptibility of both decomposed granite and tuff materials in different decomposition grades were also characterized. For the geotechnical aspects, physical properties including specific gravity, particle size distribution as well as in-situ dry density were examined. Mechanical properties of both decomposed granite and tuff soil materials were investigated by means of drained and undrained triaxial compression tests .

Based on the results of chemical analyses and comparisons with current six-grade material classification scheme (GCO 1988), various quantitative chemical indices were examined. It was found that WPI, LOI and I_mob are shown to be good indicators of the degree of weathering for both volcanic and granitic rocks found in Hong Kong. Mineralogical analyses showed that quartz is the most resistant mineral during weathering. On the other hand, alkali feldspar and plagioclase gradually decompose with increase of weathering. Weathering product such as kaolin mineral usually forms in a well-drained condition but illite and smectite usually find in hindered drainage condition. Texture and fabric analyses showed that both granite and tuff materials become porous with increasing degree of weathering. Most of the microcracks and pores of decomposed granite and tuff are in the range of 0.1 to 10 microns. Physical properties such as dry density also shows a gradual decreasing trend with increasing degree of weathering. In order to identify the effects of weathering on engineering properties of soil materials into a much more sophisticated scale, a ten zones (vd, wd, xd, yd, zd, vl, wl, xl, yl zl) refinement classification was established for both decomposed granite and tuff soil materials. d and I represent the zone having a relatively higher and lower dry density respectively and the transition from level v to z stands for increasing degree of weathering. The shear strength, stress-strain relationship as well as dilatancy of both decomposed granitic and volcanic soil materials within each zone were characterized. During transition from zone vd to zl (increasing degree of weathering), it was found that peak stress ratio and friction angle (φ’) generally decrease. On the other hand, soil becomes more contractive during this transition for both granite and tuff soil materials. Particle size distribution of each zone was also characterized. During transition from zone vd to zl, it was found that granitic soil material changed from a very silty/clayey coarse soil to very sandy/gravelly fine soil. On the other hand, sandy fine soil is transformed into slightly sandy fine soil for tuff soil material.