The 1987 code offers detailed guidelines for engineering shallow foundations on rock substrates, focusing on safe bearing pressure estimation by evaluating rock properties, discontinuities, and geological factors. It serves as a vital reference for professionals designing stable and reliable foundations in diverse rock conditions.
Overview
The 1987 code offers detailed guidelines for engineering shallow foundations on rock substrates, focusing on safe bearing pressure estimation by evaluating rock properties, discontinuities, and geological factors. It serves as a vital reference for professionals designing stable and reliable foundations in diverse rock conditions.
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Contents
Structure
This section defines the extent of the standard, focusing on methods for determining safe bearing pressures on rock foundations based on rock mass quality and discontinuity spacing. It includes empirical formulas and tables categorizing rock quality and corresponding methods.
Key terms related to rock quality, bearing capacities, and units of measurement are clarified with references to related IS codes, ensuring clarity in interpretation and application.
Highlights critical factors such as rock surface irregularities, heterogeneity, cavities, and in-situ stresses that influence foundation behavior, along with guidance on handling effects from adjacent construction and seismic influences.
Outlines the suitability of different testing and classification approaches depending on rock conditions, including rock mass classification, core strength tests, pressure meter tests, and plate load tests.
Provides tabulated safe bearing pressure values based on rock type and Rock Mass Rating (RMR), with notes on corrections and method applicability.
Presents a formula incorporating uniaxial compressive strength and discontinuity spacing, with empirical coefficients, to estimate gross safe bearing pressure.
Describes the calculation method using limit pressures from pressure meter tests, including correction factors related to footing depth and radius.
Explains procedures to convert plate settlement readings to footing settlements and obtain allowable bearing pressures, with formulas adjusted for rock type.
Discusses geological variations, excavation effects, blasting precautions, seismic impacts, scour, and frost action influencing foundation stability.
Covers excavation protocols, backfilling, reinforcement recommendations especially for heterogeneous deposits, and essential reporting elements.
Details the required content for foundation investigation reports including geological data, strength parameters, bearing pressure values, correction factors, and allowable pressure recommendations.
Frequently Asked
IS 12070 outlines specific methods tailored to rock mass conditions for estimating safe bearing pressure. For high-quality rock with wide discontinuity spacing (1-3 m or more), rock mass classification is recommended. Moderately spaced discontinuities (0.3-1 m) warrant core strength testing with an empirical formula involving an empirical coefficient (N_J). For low to very low strength rock masses with close discontinuities (5-30 cm) or highly fragmented/weathered rock, pressure meter testing is advised. Very weak rock with very close discontinuities (<5 cm) should be assessed using plate load tests. The core strength method includes a safety factor of three and uses discontinuity spacing to adjust the bearing pressure estimate.
According to IS 12070, the orientation and spacing of rock discontinuities are crucial in foundation design. Favorably oriented discontinuities aligned to resist applied loads enhance foundation bearing capacity. Wide spacing of discontinuities typically indicates stronger, more stable rock masses suitable for foundation support. Conversely, closely spaced or unfavorably oriented discontinuities reduce rock strength, potentially causing the rock mass to behave like granular soil, necessitating soil mechanics-based design approaches. Detailed mapping and assessment of discontinuity apertures and mechanical properties are essential for safe and economical foundation design.
The code prescribes a maximum permissible settlement of 12 mm for calculating safe bearing pressure for shallow foundations on rock, applicable across all loaded areas including large foundations. For rigid structures such as reinforced concrete silos, this limit may be increased cautiously. When foundations are situated on mixed strata involving talus or soil over rock, plate load tests on the softer layers should also apply a 12 mm settlement criterion to maintain safety and performance.
IS 12070 recommends thorough identification and mapping of discontinuities and cavities within the foundation zone, including their apertures and orientations. For rock masses that are highly fractured, weathered, or of low strength, the foundation design should treat them as granular soils using soil mechanics principles. When significant solution cavities cause uneven bearing surfaces, foundations should extend to depths ensuring at least 80% rock area contact beneath the foundation to avoid overhangs, thereby maintaining stability. The standard also emphasizes considering water-related effects on strength and excavation stability.
The standard advises treating very weak, heavily fractured, or weathered rock as granular soil and designing foundations accordingly. In cases where solution cavities cause uneven foundation surfaces, the foundation depth should be adjusted to guarantee a minimum of 80% rock contact area, avoiding unsupported overhangs. For heterogeneous soil-rock interfaces, circumferential reinforcement in ring foundations is recommended to enhance stability. Controlled blasting techniques such as line drilling, cushion blasting, and presplitting are prescribed during excavation to protect adjacent structures and preserve rock integrity.
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