Method for the quantitative description of discontinuities in the rock mass, Part 3: Persistence
1987 Edition

IS 11315 Part 3 (1987) outlines a systematic approach to quantify discontinuity persistence by recording the termination points of each discontinuity and measuring the trace length between these ends in meters. When the full size distribution is available, statistical techniques such as extreme value statistics are applied to estimate the likelihood of various discontinuity sizes. Persistence is expressed as the proportion of continuous length within the rock mass. Conservatively, engineers often assume near 100% persistence to ensure safety. The persistence measurement influences the characterization of failure modes, distinguishing between plane failures at full persistence and stepped failures when less than 100%, with shear strength from intact rock bridges estimated as approximately one-sixth of the uniaxial compressive strength.

The classification of discontinuity sets by their persistence involves three categories: Persistent (systematic) discontinuities are long and continuous, typically exceeding 10 meters in trace length; Sub-persistent (sub-systematic) discontinuities have moderate lengths, generally between 1 and 10 meters; and Non-persistent (non-systematic) discontinuities are short, usually less than 1 meter in length, often terminating quickly within the rock or against other discontinuities. This classification assists in evaluating the rock mass structure and is often supported by visual aids such as field sketches and block diagrams.

Persistence significantly impacts stability evaluations by dictating the potential scale and nature of failure surfaces. In rock slopes and dam foundations, high persistence of adverse discontinuities facilitates the formation of extensive failure surfaces, increasing the risk of large-scale collapse due to uninterrupted propagation through multiple rock blocks. Persistence also governs the development of tension cracks behind slope crests, affecting overall slope stability. For tunnels, failures are often localized, where persistence over a few blocks is critical especially when combined with factors like smooth or clay-filled joints and multiple joint sets. Measuring persistence accurately is challenging, and estimates often rely on dip and strike length data combined with probabilistic analyses.

The standard advises using measuring tapes of at least 10 meters in length to measure discontinuity trace lengths on rock exposures. Visual identification and classification of discontinuities into systematic, sub-systematic, or non-systematic categories are essential. Measurements of dip and strike lengths are encouraged when feasible, to support probabilistic estimations of persistence. Observing and recording termination points where discontinuities end against intact rock or intersect other discontinuities is crucial. These combined methods ensure a reliable assessment of persistence despite challenges posed by limited exposure sizes.

Persistence data should be documented by recording the measured lengths of discontinuities along with termination information for each end. Persistence estimates, including those for stepped failure surfaces, must be rounded upward to the nearest 10% to maintain a conservative approach, often assuming 100% persistence when uncertain. Interpretation should consider that persistence less than 100% may result in stepped failures with intact rock bridges contributing cohesion. The cohesion of these intact bridges can be approximated as one-sixth of the uniaxial compressive strength. Final results should adhere to rounding rules specified in IS 2-1960. Presentation using diagrams and flowcharts aids in conveying the reliability and implications of persistence data effectively.