Method for the quantitative description of discontinuities in the rock mass, Part 12: Drillcore study
1992 Edition

To achieve optimal core recovery critical for accurate discontinuity characterization under IS 11315 Part 12, utilize NX-sized cores (54 mm) with double or triple tube core barrels and bottom discharge bits, especially in fractured or softer rock formations. Employ short drilling runs, maintain controlled drilling speed and water flow, avoid over-penetration, and promptly extract tools if jamming occurs. Minimize drill string vibrations to reduce core loss and wear. Implement core orientation methods such as the Craclius technique, hardened steel groove with compass and photographic aids, or integral sampling with grouted azimuth bars to ensure core alignment and data quality.

As per IS 11315 Part 12 Clause 5.4.1.1, artificial fractures typically exhibit rough, brittle surfaces with fresh cleavage planes, often resulting from drilling or handling damage. Conversely, natural discontinuities have smoother or weathered surfaces and may display coatings or infilling materials such as talc, gypsum, chlorite, mica, or calcite. In foliated or schistose rocks, some splits may be ambiguous; the conservative approach is to classify doubtful breaks as natural. Grinding or rounded surfaces caused by core rotation complicate identification, but these should also be considered natural if uncertain. Maintaining separate records of artificial fractures helps refine rock quality assessments.

The quantitative description of discontinuities in drill cores includes several key parameters: Core Recovery (R), the percentage of recovered core length reflecting rock continuity; Discontinuity Frequency (F), the count of natural fractures per meter of core; and Rock Quality Designation (RQD), calculated as the percentage of core length consisting of sound pieces longer than 10 cm. Additionally, discontinuity orientation is measured relative to the core axis, with corrections applied for hole inclination. These metrics provide a basis for rock mass characterization but should be corroborated with field observations for engineering design.

Rock Quality Designation (RQD), defined in IS 11315 Part 12 Clause 5.4, is computed by summing the lengths of drill core pieces equal to or exceeding 10 cm and expressing this total as a percentage of the entire drilled interval length. The calculation excludes smaller fragments generated by natural jointing or weathering. Artificial fractures caused by drilling or handling should be discounted to avoid skewing results. When identification is uncertain, a conservative approach treats questionable breaks as natural. RQD serves as an index of rock mass integrity and must be combined with other parameters such as fracture frequency and core recovery for comprehensive evaluation.

IS 11315 Part 12 recommends combining visual inspections using borehole TV cameras or periscopes to classify discontinuities as open or tight with quantitative water injection tests, particularly Lugeon packer tests, to estimate permeability and infer aperture characteristics. The real aperture size often exceeds theoretical smooth-wall estimates due to roughness and tortuosity. Correlate water flow test results with core recovery percentages, RQD values, and fracture frequency to assess aperture openness. Supplementary hydraulic tests such as falling head and tracer analyses provide additional seepage data. Presenting Lugeon values alongside core recovery and RQD logs facilitates comprehensive seepage characterization.