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).

IS 12070

: 1987

Code of Practice for Design and Construction of Shallow Foundations on Rocks

CurrentSpecializedCode of PracticeGeotechnical · Rock Mechanics

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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

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

Practical Notes

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.