The 1987 edition of IS 11315 Part 10 outlines a standardized procedure for the quantitative assessment of block size within rock masses, formed by intersecting discontinuities. This guideline assists engineers and geologists in systematically evaluating block dimensions utilizing joint spacing, number of joint sets, and persistence, essential for analyzing rock mass behavior in projects like tunneling, slope stability, and foundation engineering.
Overview
The 1987 edition of IS 11315 Part 10 outlines a standardized procedure for the quantitative assessment of block size within rock masses, formed by intersecting discontinuities. This guideline assists engineers and geologists in systematically evaluating block dimensions utilizing joint spacing, number of joint sets, and persistence, essential for analyzing rock mass behavior in projects like tunneling, slope stability, and foundation engineering.
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Structure
This section of IS 11315 Part 10 defines the scope referencing terminology from IS 11358-1986 and specifies that numerical results should be rounded following IS 2-1960 rules. It outlines that no direct formulas or tables are included in this clause. The scope ensures uniform understanding by referencing established definitions and rounding protocols.
Definitions adhere to IS 11358-1986 standards. Block size classification is based on volumetric joint count (Jv), categorizing blocks from very large (<1 joint/m³) to very small (>30 joints/m³). A Jv exceeding 60 indicates a crushed rock zone. This classification aids in systematic rock mass characterization.
Block size is influenced by joint spacing, number of joint sets, and persistence. Larger blocks tend to form stiffer rock masses with favorable arching and interlocking, while smaller blocks exhibit soil-like behavior with different failure mechanisms. Quantitative parameters such as the block size index (Ib) and volumetric joint count (Jv) are introduced to describe these characteristics.
Block dimensions (length, width, height) are measured along discontinuity intersections using tapes or laser devices. Shape is characterized by dimension ratios to classify blocks as cubic, tabular, or elongated. Multiple measurements ensure statistical reliability, providing quantitative inputs for engineering analyses.
The block size index (Ib) is derived from averaging visually selected typical block dimensions with an accuracy of ±10%. For sedimentary rocks with three joint sets, Ib can be calculated by averaging the modal joint spacings. Modal block size indices (Ip) for various rock domains should be recorded alongside the number of joint sets and joint persistence.
Rock mass blockiness is classified into categories such as massive, blocky, tabular, columnar, and crushed based on joint spacing and block shape. The block size index formula provided helps quantify block dimensions, while recognizing that average spacing calculations can be misleading when more than three joint sets are present.
Sampling lengths of 5 or 10 meters perpendicular to joint strikes are recommended for joint counts. Joints per meter are calculated by dividing the joint count by the sampling length, and the volumetric joint count (Jv) is the sum over all joint sets. Random joints may be included under certain spacing conditions. Proper orientation and consistent measurement techniques ensure reliable data.
Although primarily related to prestressed concrete, IS 11315 Part 10 aligns with IS 11358-1986 for terminology. It includes calculations for prestressing force, accounting for losses, and permissible stresses following IS 1343. The guidelines emphasize the importance of considering all factors in design and ensuring accurate force estimation and anchorage.
Numerical outcomes must be rounded in accordance with IS 2:1960. Rules include rounding up or down depending on the digit following the last significant figure, with special consideration when the digit is exactly 5. This ensures consistency and precision across reported measurements.
Definitions are sourced from IS 11358-1986, while numerical rounding follows IS 2-1960. IS 11315 Part 10 is part of a series covering various test methods, and cross-referencing these standards ensures consistent terminology and procedures.
Frequently Asked
As per IS 11315 Part 10 (1987), block size in rock masses is quantified by measuring the dimensions of blocks formed at the intersections of discontinuities using a tape of at least 3 meters with millimeter calibration. The measurement reflects the geometric arrangement of the discontinuity sets, enabling a field-based, precise evaluation of block dimensions critical for engineering analyses.
Joint sets determine the shape and size of rock blocks through their number and spatial orientation, as they form the block boundaries. Persistence, which refers to the continuity length of these joints, influences block size by controlling how extensive the blocks are. Longer, continuous joints lead to larger, well-defined blocks, whereas shorter joints result in smaller or irregularly shaped blocks. Both factors are essential to understanding block size, as described in IS 11315 Part 10.
Volumetric joint count (Jv) is measured by counting the number of joints intersecting sampling lengths of typically 5 or 10 meters oriented perpendicular to the strike of each joint set. The joint count per meter for each set is calculated by dividing the number of joints by the sampling length, and these values are summed across all joint sets to obtain Jv. This method, recommended in IS 11315 Part 10, provides a rapid and reliable estimate of joint density for rock mass characterization.
IS 11315 Part 10 classifies rock mass block shapes into several categories: Massive (few, widely spaced joints with equidimensional blocks), Blocky (blocks formed by multiple joint sets, generally equidimensional), Tabular (one dimension significantly smaller, plate-like blocks), Columnar (one dimension much larger, pillar-like blocks), and Crushed (highly fragmented blocks resembling sugar cubes). These classifications assist in understanding rock mass stability and mechanical behavior.
Block size significantly impacts rock mass mechanical behavior and stability. Large blocks typically result in a less deformable rock mass exhibiting arching and interlocking, which enhance underground stability. Smaller blocks tend to behave more like soils, with failure modes shifting to circular or rotational mechanisms. Extremely small blocks may behave like a flowing shear zone. Therefore, accurate block size measurement is vital for predicting rock mass response and designing safe engineering structures.
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