IS 2911:2021 Part 3 is the Indian Standard (BIS) for design and construction of pile foundations - under-reamed piles. This standard provides guidelines for the design and construction of bored, cast-in-situ, under-reamed concrete pile foundations. It covers piles with single or multiple bulbs, which are particularly effective in expansive soils for anchoring against uplift and in soft soils for increasing bearing capacity.
Provides guidelines for the design and construction of under-reamed pile foundations, particularly suited for expansive soils.
Key design parameters, reinforcement details, construction tolerances, and load test acceptance criteria for under-reamed piles.
| Reference | Value | Clause |
|---|---|---|
| Min. Pile Diameter— For bored cast-in-situ piles with length up to 5 m. | 200 mm | Cl. 5.2.1 |
| Bulb to Stem Diameter Ratio (Du/D)— A ratio of 2.5 is commonly adopted. | 2.0 to 3.0 | Cl. 5.2.2 |
| Spacing between Bulbs (Centre-to-Centre) | 1.25 to 1.5 times Du | Cl. 5.2.3 |
| Topmost Bulb Location from Ground Level— In expansive soils, must be below the zone of seasonal moisture variation. | ≥ 2 Du | Cl. 5.2.4 |
| Pile Spacing (Multi-Bulb Piles)— Can be reduced to 1.5 Du with justification and for single bulb piles. | 2.0 Du | Cl. 5.3.1.1 |
| Pile Spacing (Single Bulb Piles) | 1.5 Du | Cl. 5.3.1.1 |
| Min. Concrete Grade | M 25 | Cl. 6.1.1 |
| Min. Cement Content (Severe Exposure)— For 20 mm nominal aggregate size. Refers to IS 456 Table 5. | 340 kg/m³ | Cl. 6.1.2 (Ref: IS 456) |
| Max. Water-Cement Ratio (Severe Exposure)— Refers to IS 456 Table 5. | 0.45 | Cl. 6.1.2 (Ref: IS 456) |
| Min. Clear Cover to Reinforcement | 60 mm | Cl. 6.2.3 |
| Min. Longitudinal Reinforcement | 0.4 % of c/s area | Cl. 6.2.4.1 |
| Min. Longitudinal Bar Diameter | 8 mm | Cl. 6.2.4.1 |
| Lateral Ties - Min. Diameter | 8 mm | Cl. 6.2.5 |
| Lateral Ties - Max. Pitch | 150 mm | Cl. 6.2.5 |
| Bearing Capacity Factor (Nc) for Bulbs— For bearing capacity calculation in cohesive soils. | 9.0 | Cl. 7.2.1.1 |
| Adhesion Factor (α) for Stiff Clays— For undrained cohesion (Cu) of 100-150 kN/m². | 0.3 - 0.4 | Cl. 7.2.1.2 (Table 1) |
| Tolerance in Verticality (Plumb) | 1.5 % of pile length | Cl. 8.4.1 |
| Tolerance in Position at Cut-off Level— For single piles or piles in a row. | 75 mm | Cl. 8.4.1 |
| Initial Test Load | 2.5 x Safe Load | Cl. 9.1.4.1 |
| Initial Test - Max Settlement— Whichever is less, at test load. | ≤ 12 mm or 2% of Du | Cl. 9.1.5.1 (a) |
| Routine Test Load | 1.5 x Safe Load | Cl. 9.1.4.2 |
| Routine Test - Max Settlement— At test load. | ≤ 12 mm | Cl. 9.1.5.2 (a) |
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
| Parameter | IS Value | International | Source |
|---|---|---|---|
| Under-ream / Bell Diameter Ratio (Du/D) | Typically 2.5. Can be up to 3.0 in special cases with permission. | No prescribed ratio; commonly up to 3.0 times shaft diameter, based on design requirements. | ACI 336.3R-14 |
| Vertical Spacing Between Bulbs | 1.25 to 1.5 times the under-ream diameter (Du). | Not specified, as multi-bell piers are uncommon. Design would be based on shear analysis between bells. | ACI 336.3R-14 |
| Factor of Safety on Bearing Capacity (from soil properties) | 2.5 to 3.0 | Generally 3.0 for end bearing and 2.0 to 3.0 for side resistance. | ACI 336.3R-14 |
| Factor of Safety on Bearing Capacity (from load test) | 2.5 for initial test; 2.0 for routine test. | Typically 2.0. | ACI 336.3R-14 |
| Minimum Longitudinal Reinforcement | 0.4% of pile stem cross-sectional area. | Typically 0.5% to 1.0% of the gross cross-sectional area (references ACI 318). | ACI 336.3R-14 / ACI 318 |
| Bell Slope Angle (from horizontal) | Not explicitly defined; formed by the cutting tool geometry. | Recommended to be at least 45° to 60° for stability, depending on soil type. | ACI 336.3R-14 |
| Concrete Slump (for Tremie/Underwater Placement) | 150 - 200 mm | 175 - 225 mm (7 - 9 inches) | ACI 336.1-19 |