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IS 12070:1987 is the Indian Standard (BIS) for design and construction of shallow foundations on rocks. This code provides guidelines for the design and construction of shallow foundations on various types of rock. It covers site investigation procedures, methods to determine bearing capacity (including presumptive values), settlement analysis, and construction requirements to ensure a safe and stable foundation.
Code of Practice for Design and Construction of Shallow Foundations on Rocks
! The presumptive bearing values in Table 1 are for sound rock. The actual capacity is highly dependent on rock mass characteristics like RQD, jointing, and weathering, which must be assessed via site investigation.
! Ensure the foundation base is thoroughly cleaned of all loose fragments, soil, and weathered rock to achieve proper seating on a sound rock stratum before placing concrete.
! For important structures, presumptive values should only be used for preliminary design. Final design should be based on in-situ tests like the Plate Load Test or rock mechanics principles.
EN 1997-1:2004+A1:2013CEN (European Committee for Standardization), Europe
MediumCurrent
Eurocode 7: Geotechnical design - Part 1: General rules
Covers general principles for geotechnical design, including shallow foundations on rock, within a Limit State Design framework.
BS 8004:2015+A1:2020BSI (British Standards Institution), UK
HighCurrent
Code of practice for foundations
Provides detailed UK-specific guidance on the design and construction of foundations, including those on rock, supplementing Eurocode 7.
Canadian Foundation Engineering Manual, 4th EditionCanadian Geotechnical Society (CGS), Canada
HighCurrent
Canadian Foundation Engineering Manual
Comprehensive design manual providing detailed methods for bearing capacity and settlement analysis of foundations on rock.
AS 5100.3-2017Standards Australia, Australia
MediumCurrent
Bridge design - Part 3: Foundations and soil-supporting structures
Specifies design requirements for foundations on rock, primarily for bridges, but principles are widely applicable.
Key Differences
≠IS 12070 uses an Allowable Stress Design (ASD) philosophy with a global factor of safety, whereas modern standards like Eurocode 7 use a Limit State Design (LSD) approach with partial factors for loads and material resistances.
≠IS 12070 provides presumptive bearing capacity values based on descriptive rock classifications ('good', 'fair', etc.). International codes emphasize calculating bearing capacity based on quantitative rock mass rating systems (e.g., RMR, GSI) and rock mass strength criteria (e.g., Hoek-Brown).
≠Settlement limits in IS 12070 are prescribed as absolute values (e.g., 12 mm). Eurocode 7 and related standards define serviceability limits based on project-specific requirements, often focusing on angular distortion rather than a fixed total settlement value.
≠The Indian code provides a simple empirical formula for allowable pressure based on UCS and a joint factor. International practice involves a more rigorous process: deriving rock mass strength parameters (c' and φ') from classification systems and then using general bearing capacity equations.
Key Similarities
≈All standards recognize that the properties of the rock mass (discontinuities, weathering, jointing) are more critical than the intact rock strength for foundation design.
≈There is a common emphasis on the importance of in-situ tests, particularly plate load tests (PLT), to verify the bearing capacity and deformability of the rock mass, especially for major structures.
≈All codes stress the critical importance of proper construction and preparation of the foundation base, including removal of all loose or weathered material and ensuring a clean, sound surface for placing concrete.
≈The fundamental inputs for design are similar across all codes, requiring investigation of rock type, structure (RQD, joint spacing/orientation), and strength (UCS of intact rock).
Parameter Comparison
Parameter
IS Value
International
Source
Design Philosophy
Allowable Stress Design (ASD) with a global Factor of Safety (typically 2.5-3.0).
Limit State Design (LSD) with partial safety factors on loads (γ_F) and resistances (γ_M, γ_R).
EN 1997-1
Permissible Total Settlement (Isolated Footings)
12 mm
Not prescribed as a fixed value; determined by structural tolerance to angular distortion (e.g., 1/500).
EN 1997-1 / BS 8004
Minimum Foundation Depth in Rock
Minimum 300 mm into fresh, sound rock.
No specific minimum numerical value; excavation must extend to sound, unweathered stratum as determined by a geotechnical engineer.
BS 8004:2015
RQD for 'Good' Quality Rock
75% - 90% (as per common Indian practice, e.g. IS 11315)
75% - 90% (based on original classification by Deere)
Canadian Foundation Engineering Manual
Treatment of Rock Discontinuities
Accounted for using an empirical reduction factor 'C_j' (0.1 to 0.4) applied to intact UCS.
Quantified using rock mass classification systems (RMR, GSI) to determine overall rock mass strength parameters.
Canadian Foundation Engineering Manual
Foundation Base Preparation
Clean surface of loose material with air-water jet; roughen surface; fill seams with grout or concrete.
Foundation bed to be cleared of all loose debris, soft spots and water before placing concrete.
BS 8004:2015
⚠ Verify details from original standards before use
Key Values5
Quick Reference Values
Minimum Factor of Safety for Bearing Capacity3.0 (Clause 5.3)
Minimum depth of exploration below foundation1.5 times the width of the footing (Clause 4.3.2)
Presumptive SBC for massive crystalline bedrock (Granite)10000 kN/m² (Table 1)
Presumptive SBC for soft rock (Shale)440 kN/m² (Table 1)
Typical allowable total settlement12 mm (common practice, Clause 6.1 notes it's generally low)
Key Formulas
S = [q * B * (1 - v^2) * I] / E — Elastic settlement of shallow foundation on rock
Tables & Referenced Sections
Key Tables
Table 1 - Presumptive Values of Safe Bearing Capacity of Rocks
What is the presumptive safe bearing capacity for sound granite rock?+
10,000 kN/m². This value from Table 1 must be confirmed by site-specific investigation.
What is the minimum recommended factor of safety for bearing capacity on rock?+
A minimum factor of safety of 3 is recommended against the ultimate bearing capacity (Clause 5.3).
How deep should the site investigation boreholes be for a rock foundation?+
To a depth of at least 1.5 times the width of the footing below the proposed foundation level (Clause 4.3.2).
Is settlement a major concern for foundations on rock?+
Generally no, settlement is usually low and uniform. However, it must be evaluated for highly fractured/weathered rock or for structures highly sensitive to settlement (Clause 6).