The 1985 edition of IS 11315 Part 11 outlines a standardized procedure for the quantitative assessment of discontinuities within rock masses by measuring core recovery and calculating the Rock Quality Designation (RQD). This method is crucial for evaluating the integrity of rock cores obtained via drilling, supporting civil engineering, mining, and geotechnical site investigations. It serves as an essential reference for professionals engaged in foundation design, tunneling, and rock mechanics analyses.
Overview
The 1985 edition of IS 11315 Part 11 outlines a standardized procedure for the quantitative assessment of discontinuities within rock masses by measuring core recovery and calculating the Rock Quality Designation (RQD). This method is crucial for evaluating the integrity of rock cores obtained via drilling, supporting civil engineering, mining, and geotechnical site investigations. It serves as an essential reference for professionals engaged in foundation design, tunneling, and rock mechanics analyses.
Audience
Contents
Structure
Frequently Asked
According to IS 11315 Part 11, core recovery (CR) is determined by dividing the total length of all core pieces retrieved during drilling by the total drilled length, then multiplying by 100 to express as a percentage. When core fragments are present, their lengths should be estimated by fitting the pieces together. The final CR value is recorded to the nearest 2%. This measure applies to individual core runs or entire boreholes and provides insight into drilling success and rock mass quality.
The Rock Quality Designation (RQD) is defined as the percentage ratio of the sum of lengths of intact core segments at least 10 cm long to the total length drilled, multiplied by 100. RQD categories classify rock quality as follows: 90-100% indicates excellent quality, 75-90% good, 50-75% fair, 25-50% poor, and below 25% very poor. Additionally, RQD correlates approximately with the square of the ratio of P-wave velocity from seismic surveys to ultrasonic velocity from core samples, multiplied by 100.
To maintain accuracy during core drilling and handling, broken core pieces should be carefully reassembled if their combined length exceeds 10 cm and counted as one segment. Obvious weak materials, such as over-consolidated gauge, even if intact and longer than 10 cm, should be excluded. Utilizing a double tube core barrel of NX size (internal diameter 54 mm) enhances recovery, and supervision is necessary to reduce errors. Core length must be measured along the centerline to avoid underestimation due to parallel fractures. Core grinding should be minimized and recorded, and any zones of core loss must be marked with depth indicators. In limestone, sudden increases in drilling speed may help estimate cavity thickness.
IS 11315 Part 11 establishes a linear correlation between RQD and the volumetric joint count (Jv) expressed as: RQD = 115 - 3.3 × Jv for Jv values equal to or greater than 4.5. For joint frequencies below 4.5, RQD is considered 100%. This means that as the number of joints per meter increases, the RQD decreases, reflecting a decline in rock quality. This formula provides a useful preliminary estimate of rock mass quality based on joint density.
Yes, IS 11315 Part 11 permits indirect estimation of RQD when drilling is not feasible. One method uses the ratio of P-wave velocity from seismic refraction surveys to ultrasonic velocity measured on rock cores, with RQD approximated by squaring this velocity ratio and multiplying by 100. Another approach involves correlating RQD with observed jointing characteristics in the rock mass. These indirect techniques offer practical alternatives for assessing rock quality in challenging conditions.
Ask AI about any clause, requirement, or provision in IS 11315 PART 11. Get instant, clause-cited responses powered by our indexed library.
Free tier includes 150 queries (50 AI + 100 Reference) · No credit card required