Abstract：The successful identification of interfaces of simple weathered granite ground in Hong Kong has encouraged its further use of instrumented drilling system as a ground investigation tool in intricate weathered granite strata. In this tested ground，the boreholes reveal that the weathered granite alternately changes between moderately and strongly. AR–20 rotary-hydraulic drill rig is instrumented at the appropriate locations with a digital drilling process monitoring(DPM) system and data acquisition can be automatically carried out during the drilling process. The qualitative and quantitative analyses of penetrating parameters indicate that effective thrust force，rotary speed，flushing pressure，penetrating rate and displacement of the bit fluctuate at ground interfaces. It shows that the parameters get well response with the change of rock strength at the interfaces，which can reveal the change of the intricate granite formation. Besides，a variable-slope method has been established for identification of dominative and subsidiary interfaces in the granite site. The result from t test shows that the confidence of the instrumented drilling system in identification of the geotechnical interfaces is 99%.
