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

IS 12070: Scope - Key Formulas, Tables, and Specifications

Scope Summary (Clause 4.1 & 6.2)

• Applies to determination of safe bearing pressure on rock foundations.
• Methods depend on rock quality and discontinuity spacing (Table 1).
• Rock quality categories:
  • Good rock with wide spacing (>1 m)
  • Rock mass with moderately close spacing (0.3 to 1 m)
  • Low strength rock with close spacing (5 to 30 cm)
  • Very low strength rock with very close spacing (<5 cm)

Key Formula for Safe Bearing Pressure (Clause 6.2)

[ q_s = \frac{N_J \times \sigma_c}{3 + \frac{S}{B_f}} ]

Where:

• Includes a factor of safety = 3
• Valid for (S > 30 \text{ cm}), discontinuity aperture < 10 mm, (B_f > 30 \text{ cm})

Table 1: Applicability of Methods

Table 4: Value of (N_J) (Clause 6.2)

IS 12070 - Definitions Summary

1. Reference Standards for Definitions:

   • IS 2809-1972: Rules for rounding off numerical values.
   • IS 11358-1986: Glossary of terms and symbols in soil engineering and rock mechanics.

2. Key Terms:

   • Rock Quality: Classified by discontinuity spacing (wide >1m, moderately close 0.3-1m, close 5-30cm, very close <5cm).
   • Safe Bearing Pressure: Maximum pressure a rock foundation can safely bear.

3. SI Units Used:

4. Applicability of Methods for Safe Bearing Pressure (Table 1):

Diagram: Rock Quality vs. Method Applicability

flowchart TD
    A[Rock Quality]
    A --> B[Good rock (>1m spacing)]
    A --> C[Moderately close (0.3-1m spacing)]
    A --> D[Low strength (<500 kg/cm²), close spacing]
    A --> E[Very low strength (<250 kg/cm²), very close spacing]

    B -->|Use| F[Rock mass classification (Clause 5)]

IS 12070: Design Considerations for Foundations on Rock

Key Design Factors (Clause 3.2)

• Rock Surface Conditions: Undulating surfaces affect stress distribution.
• Rock Mass Heterogeneity: Bearing capacity can vary up to 10× due to fractures, shear zones, weathering.
• Cavities: Solution and gas cavities reduce effective bearing area.
• Bottom Heave & Slope Stability: Must be checked to avoid foundation failure.
• High In Situ Stresses: Influence stress distribution and potential failure modes.

Adjacent Construction Effects

• Blasting: Use controlled methods (line drilling, cushion blasting, presplitting) to protect rock integrity.
• Excavation & Groundwater Lowering: Rarely affects igneous/metamorphic rocks but important for sedimentary.
• Seismic Response: Consider dynamic effects on foundation stability.

Other Effects

• Scour & Erosion: Critical for piers and abutments.
• Frost Action: May cause heaving and settlement issues.

Special Cases (Clause 7.1)

• For weak, highly fractured, or weathered rock, treat as granular soil and use soil mechanics principles for design.

Bearing Capacity Estimation (General Approach)

• Use rock strength parameters (uniaxial compressive strength, cohesion, friction angle).
• Adjust allowable bearing pressure for heterogeneity and discontinuities.
• Settlement prediction based on elastic modulus and rock mass classification.

Simplified Bearing Capacity Formula (adapted for rock)

[ q_{allow} = \frac{q_{ult}}{FS} ]

Where:

• ( q_{ult} ) = Ultimate bearing capacity (from rock strength tests)
• ( FS ) = Factor of safety (typically 2.5 to 3 for rock)

Summary Table: Rock Foundation Design Considerations

IS 12070: Applicability of Methods for Safe Bearing Pressure on Rock

1. Applicability Summary (Table 1, Clause 4.1)

Note: Each method can be used for all rock qualities if required.

2. Pressure Meter Method (Clause 7.2)

[ q_{ns} = vD + K_a (P_L - vD) ]

• (q_{ns}) = net safe bearing pressure (t/m²)
• (v) = unit weight of rock/soil (t/m³)
• (D) = foundation depth (m)
• (P_L) = limit pressure from pressure meter (t/m²)
• (K_a) = factor from Table 5 (depends on footing depth/radius)

Table 5: Values of (K_a)

3. Correction Factors (Clause 9.1)

• Safe bearing pressures from tables or tests should be multiplied by correction factors for geological conditions (except RMR classification).

4. Plate Load Test (Clause 3.4 & 8)

• Recommended for very low strength rock or talus/soil.
• Use 12 mm settlement criterion for bearing pressure recommendation.

This structured approach ensures

IS 12070: Safe Bearing Pressure Estimates from Classification Tables

1. Safe Bearing Pressure (qns) from Rock Mass Classification (Table 2 - Clause 5.2)

2. Safe Bearing Pressure Based on Rock Mass Rating (RMR) (Table 3 - Clause 5.3.1)

• Note: Use average RMR over a depth equal to foundation width.
• If upper rock quality is lower in ¼ width depth, use that lower value or remove inferior rock.
• Do not increase qns if foundation is embedded.

3. Correction Factors (Clause 9.1)

• Multiply qns from Table 2 or other methods by correction factors based on geological conditions.
• Exception: No corrections for RMR-based qns.

4. Applicability of Methods (Table 1 - Clause 4.1)

IS 12070: Estimation of Safe Bearing Pressure from Core Strength (Clause 6.2)

Key Formula:

[ q_s = \frac{N_J \cdot \sigma_0}{3 + \frac{S}{B}} ]

Where:

• ( q_s ) = safe bearing pressure (gross)
• ( \sigma_0 ) = average uniaxial compressive strength of rock cores
• ( N_J ) = empirical coefficient based on discontinuity spacing (from Table 4)
• ( S ) = spacing of discontinuities (cm)
• ( B ) = footing width (cm)
• Factor of safety included = 3

Table 4: Value of ( N_J )

Notes:

• Valid for discontinuity spacing > 0.3 m, aperture < 10 mm (or 15 mm if filled), footing width > 0.3 m.
• Rock surface parallel to foundation base, no tangential load, no open discontinuities.
• ( S ) and ( B ) must be in cm for the formula.

Conceptual Visualization:

graph LR
A[Rock Core Strength \(\sigma_0\)] --> C[Calculate \(q_s\)]
B[Discontinuity Spacing & Aperture] --> C
D[Footing Width \(B\)] --> C
C --> E[Safe Bearing Pressure \(q_s\)]

This formula helps estimate safe bearing pressure for foundations on rock masses with favorable conditions, incorporating discontinuity effects through ( N_J ).

Safe Bearing Pressure from Pressure Meter Test (IS 12070: Clause 7.2)

Formula:

[ q_{ns} = vD + K_a (P_L - vD) ]

Where:

• (q_{ns}) = Net safe bearing pressure (t/m²)
• (v) = Unit weight of soil/rock (t/m³)
• (D) = Depth of foundation (m)
• (vD = ) Overburden pressure (t/m²)
• (P_L) = Limit pressure from pressure meter (t/m²)
• (K_a) = Constant from Table 5 (depends on footing depth/radius)

Table 5: Values of (K_a)

* Equivalent radial dimension of footing

Notes:

• Factor of Safety ≈ 3 is embedded in the formula.
• This method accounts for rock mass discontinuities and weathering effects.
• Use this for poor rock or rock with close discontinuities (<30 cm).

flowchart TD
    A[Pressure Meter Test] --> B[Determine Limit Pressure \(P_L\)]
    B --> C[Calculate Overburden Pressure \(vD\)]
    C --> D[Select \(K_a\) from Table 5 based on footing depth]
    D --> E[Apply formula \(q_{ns} = vD + K_a (P_L - vD)\)]
    E --> F[Obtain Net Safe Bearing Pressure \(q_{ns}\)]

This concise approach ensures safe foundation design on poor or weathered rock masses using IS 12070 guidelines.

Determination of Safe Bearing Pressure from Plate Load Test (IS 12070)

Key Formulas (Clause 8.3):

For a given footing settlement ( S_f ), calculate plate settlement ( S_p ):

• Massive/Sound Rocks: [ S_p = S_f \times \frac{B_p}{B} ]

• Laminated/Poor Rocks: [ S_p = S_f \times \frac{B_p (B + 30)}{B (B_p + 30)} ]

Where:

• ( S_p ) = Plate settlement (mm)
• ( S_f ) = Footing settlement (mm)
• ( B_p ) = Width of plate (cm)
• ( B ) = Width of footing (cm)

Safe Bearing Pressure:

• Obtain from pressure-settlement curve corresponding to calculated ( S_p ).
• Permissible settlement ( S_f ) = 12 mm (Clause 3.3), even for large loaded areas.
• For rigid structures, permissible settlement may be increased judiciously.

Important Notes:

• Plate load test per IS 1888-1982.
• Account for heterogeneity if site has rock and soil/talus (Clause 3.4).
• Use 12 mm settlement criterion for both soil and rock for consistency.

Summary Table: Settlement Conversion

This method ensures safe bearing pressure is derived from plate load test data, adjusted for rock type and footing size.

Key Factors Affecting Foundation Design on Rock (IS 12070:1987)

1. Geological and Excavation Factors

• Undulating rock surface: Causes uneven stress distribution.
• Heterogeneity: Bearing capacity varies up to 10× due to fractures, shear zones, weathering.
• Solution/gas cavities: Reduce bearing capacity locally.
• Bottom heave: Upward movement due to excavation.
• Slope instability: Potential sliding affects foundation safety.
• High in-situ horizontal stresses: Influence stress distribution.

2. Adjacent Construction Activities

• Blasting: Use controlled methods (line drilling, cushion blasting, presplitting) to prevent damage.
• Excavation and groundwater lowering: May cause settlement or instability.
• Seismic effects: Consider undesirable seismic response in design.

3. Other Effects

• Scour and erosion: Critical for piers and abutments.
• Frost action: Can cause uplift or heave.
• Seismic response: Must be evaluated for dynamic loads.

Design Recommendations:

• For weak, highly fractured rock, treat as granular soil and apply soil mechanics principles (Clause 7.1).
• Check footing shear failure using pressure-settlement curve (Clause 8.5).

Typical Bearing Capacity Estimation on Rock

Mermaid Diagram: Factors Influencing Foundation Design on Rock

graph TD
    A[Foundation Design on Rock] --> B[Geological Factors]
    A --> C[Construction Activities]
    A --> D[Other Effects]

    B --> B1[Undulating Surface]
    B --> B2[Heterogeneity]
    B --> B3[Cavities]
    B --> B4[Bottom Heave]
    B --> B5[Slope Instability]
    B --> B6[High Horizontal Stress]

IS 12070: Treatment of Foundations — Key Points & Formulas

1. Foundation Treatment Overview (Clause 10)

• Excavation: Remove filled soil up to 80% of area level; backfill with lean concrete.
• Rock Foundations: Excavate rock to pre-selected foundation level.
• Heterogeneous Rock/Soil: Special attention and remedial measures needed, especially on rock slopes.

2. Reinforcement (Clause 9.8)

• Provide circumferential reinforcement in ring foundations on heterogeneous soil/rock to improve stability.

3. Reporting Requirements (Clause 11)

Include in foundation report:

• Geology, uniaxial compressive strength, RMR, unit weights.
• Safe bearing pressure from various methods.
• Correction factors.
• Recommended net and gross allowable bearing pressures.

4. Bearing Capacity & Settlement (Clause 3.1)

• Both bearing capacity and settlement must be checked.
• Settlement often governs design to avoid structural distress.

5. Basic Bearing Capacity Formula (Terzaghi’s for Rock/Soil)

[ q_{ult} = cN_c + qN_q + 0.5 \gamma B N_\gamma ] Where:

• (c) = cohesion,
• (q) = overburden pressure,
• (\gamma) = unit weight,
• (B) = width of foundation,
• (N_c, N_q, N_\gamma) = bearing capacity factors.

Summary Table: Foundation Treatment

flowchart TD
    A[Site Investigation] --> B[Excavate Fill Soil]
    B --> C{Is Rock Present?}
    C -- Yes --> D[Excavate Rock to Foundation Level]
    C -- No --> E[Backfill with Lean Concrete]
    D --> F[Provide Circumferential

IS 12070 - Reporting of Results (Clause 11):

When reporting foundation investigations on rock, include:

• a) Geology of the site: Description of rock types, discontinuities, weathering.
• b) Table of key parameters:
  • Uniaxial Compressive Strength (UCS)
  • Rock Mass Rating (RMR)
  • Geological parameters (discontinuity spacing, condition)
  • Unit weights (γ, t/m³)
• c) Safe bearing pressure from various methods (rock mass classification, core strength, pressure meter, plate load test).
• d) Correction factors applied (e.g., depth, footing size).
• e) Recommended net allowable bearing pressure (q_ns).
• f) Recommended gross allowable bearing pressure.

Key Formula (Pressure Meter Method, Clause 7.2):

[ q_{ns} = \gamma D + K_a (P_L - \gamma D) ]

Where:

Table 5: Values of (K_a)

*Radius = equivalent radial dimension of footing.

Table 3: Net Safe Bearing Pressure Based on RMR (t/m²)