Overview & Structure
IS 2911 provides the code of practice for the design and construction of pile foundations. It is structured into four parts covering different pile types and testing: Preamble
| Part | Title | Latest Revision |
|---|---|---|
| Part 1 / Sec 1 | Driven Cast-in-Situ Concrete Piles | 2010 |
| Part 1 / Sec 2 | Bored Cast-in-Situ Concrete Piles | 2010 |
| Part 1 / Sec 3 | Precast Concrete Piles | 2010 |
| Part 3 | Under-Reamed Piles | 1980 |
| Part 4 | Load Test on Piles | 2013 |
When to Use Piles?
Pile foundations are adopted when:
- Hard strata is at a depth where shallow foundations become uneconomical
- Soil near the surface is weak (soft clay, loose sand, expansive soil)
- Heavy concentrated loads from columns
- Scour depth is deep (bridge foundations in rivers)
- Expansive soils causing heave/shrinkage
Pile Classification
Driven Cast-in-Situ
A steel casing is driven into the ground, concrete is poured, and the casing is withdrawn while leaving the concrete in place. Also called "shell-less" piles.
Part 1 / Sec 1
Bored Cast-in-Situ
A hole is drilled (bored) into the ground using rotary or percussion methods, reinforcement is placed, and concrete is poured. Most common in urban areas.
Part 1 / Sec 2
Precast Concrete
Piles are manufactured at a factory or casting yard and then driven into position using a hammer. Reinforced or prestressed. Suitable for marine and river works.
Part 1 / Sec 3
Under-Reamed Pile
Bored pile with an enlarged bulb (under-ream) at the base. Used in expansive soils (black cotton). Bulb increases end bearing and resists uplift.
Part 3
Piles can also be classified by load transfer mechanism:
- End-Bearing Piles: Transfer load through tip to hard stratum below.
- Friction Piles: Transfer load through skin friction along the shaft.
- Compaction Piles: Used to densify loose granular soils.
- Tension Piles: Resist uplift forces (anchor piles).
- Lateral Load Resisting Piles: Resist horizontal forces (retaining walls, dolphins).
Pile Capacity Estimation
The ultimate load capacity of a single pile is the sum of its end-bearing capacity and shaft (skin) friction capacity. Appendix A / Cl. 6
Ultimate Pile Capacity (Static Formula)
Qu = Qeb + Qs = (qb ร Ab) + (fs ร As)
Where qb = end-bearing pressure, Ab = cross-sectional area at toe, fs = unit skin friction, As = surface area of shaft.
For Cohesive Soils (Clay)
End Bearing in Clay
qb = 9 ร cu
cu = undrained cohesion at pile tip level
Skin Friction in Clay
fs = ฮฑ ร cu
ฮฑ = adhesion factor (0.3 to 1.0 depending on soil consistency). For very soft clay ฮฑ โ 1.0; for stiff clay ฮฑ โ 0.3.
For Granular Soils (Sand/Gravel)
End Bearing in Sand
qb = 0.5 ร ฮณ ร D ร Nq
ฮณ = effective unit weight, D = depth of pile tip, Nq = bearing capacity factor (depends on ฯ).
Skin Friction in Sand
fs = K ร ฯฬv ร tan ฮด
K = earth pressure coefficient, ฯฬv = average effective vertical stress, ฮด = angle of wall friction (โ 0.75ฯ for concrete).
Static Formula Limitations
The static formula gives only approximate capacity. It must always be verified with pile load tests (Part 4). The code recommends a minimum of one initial load test and one routine test per 100 piles or 2500 mยฒ of pile footprint area.
Part 1 / Sec 1: Driven Cast-in-Situ Piles
These are formed by driving a steel casing into the ground, filling it with concrete, and withdrawing the casing. IS 2911 (Part 1/Sec 1)
Construction Sequence
- Position the casing at the pile location using a template or guide.
- Drive the casing using drop hammer, diesel hammer, or vibratory hammer.
- Excavate soil from inside the casing (if necessary for cleaning).
- Pour concrete into the casing (dry pour or tremie method).
- Withdraw the casing gradually while maintaining concrete pressure.
- Install reinforcement cage before or after concreting (as applicable).
Key Specifications
| Parameter | Requirement |
|---|---|
| Diameter | 300 mm to 600 mm (common). Up to 1200 mm for large projects. |
| Length | Typically 6 m to 30 m. Beyond 30 m requires special provisions. |
| Concrete Grade | Minimum M25 for structural piles. |
| Clear Cover | 50 mm (minimum) for above-ground piles; 75 mm in aggressive soils. |
| Minimum Reinforcement | 0.4% of gross cross-sectional area. |
| Spacing of Ties | Not exceeding 300 mm or 12 times main bar diameter. |
Necking Problem
The most common defect in driven cast-in-situ piles is "necking" โ the cross-section reduces during casing withdrawal if concrete pressure drops. Always maintain sufficient concrete head above the casing tip during withdrawal.
Part 1 / Sec 2: Bored Cast-in-Situ Piles
These are formed by boring a hole into the ground and then filling it with concrete. Most widely used in urban construction due to low noise and vibration. IS 2911 (Part 1/Sec 2)
Boring Methods
- Rotary Drilling: Using auger bits with bentonite slurry for stabilization.
- Percussion Drilling: Chisel dropped repeatedly for hard rock / boulder soil.
- Grab / Shell: Cable-operated grabs for soft soils.
Key Construction Checks
1. Bentonite Slurry: Maintains borehole stability by hydrostatic pressure. Properties: density 1.04โ1.10 g/cmยณ, viscosity 30โ60 s (Marsh cone), pH 9.5โ12.
2. Cleaning the Base: After boring, the pile base must be cleaned of loose debris. Allowable debris: โค 50 mm thickness.
3. Tremie Concreting: For piles below water table or bentonite. Concrete placed through a tremie pipe with its bottom always submerged in fresh concrete. Prevents segregation.
4. Concreting Duration: Concreting should commence within 2 hours of boring completion. If delay exceeds 4 hours, re-clean the bore.
Specifications
| Parameter | Requirement |
|---|---|
| Diameter | 450 mm to 1500 mm (common). Up to 3000 mm for bridge foundations. |
| Concrete Grade | Minimum M25 (or M30 in corrosive environments). |
| Slump | 150โ200 mm for tremie concrete. |
| Clear Cover | 75 mm (minimum) in soil; 100 mm in aggressive/marine conditions. |
| Longitudinal Steel | 0.4% to 2.5% of gross area. |
| Transverse Ties | โฅ 8 mm bars at spacing โค 300 mm or half shaft diameter. |
Part 1 / Sec 3: Precast Concrete Piles
Piles manufactured offsite and transported to location for driving. Suitable for marine structures, bridge foundations, and projects requiring large quantities of identical piles. IS 2911 (Part 1/Sec 3)
Advantages
- Quality control in factory conditions
- Fast installation โ no curing time at site
- Can be inspected before driving
- Can be prestressed for higher capacity
Disadvantages
- Difficult to transport long piles
- Cannot adjust length easily after driving
- Requires heavy driving equipment
- Noise and vibration during driving (problematic in urban areas)
Driving Formulae
The pile capacity can be estimated from driving records using dynamic formulae:
Engineering News Record (ENR) Formula
Qu = (W ร H) / (S + C)
W = weight of hammer, H = height of fall, S = final set (penetration per blow),
C = constant (25 mm for drop hammer, 2.5 mm for single-acting hammer).
Hiley Formula
Qu = (ฮทh ร W ร H) / (S + C/2)
ฮทh = efficiency factor accounting for energy losses. More accurate than ENR.
Dynamic Formulae Are Approximate
Driving formulae give only a rough estimate. They assume that pile capacity correlates with driving resistance, which is affected by many factors (soil setup, pore pressures). Always verify with static load tests.
Part 4: Load Testing of Piles
Load testing is the most reliable way to determine pile capacity. IS 2911 (Part 4):2013 defines two types: IS 2911 (Part 4)
Types of Load Tests
Initial / Routine Load Test
Performed on test piles before production piling begins. Determines the actual load-settlement behaviour and verifies design assumptions. Load: up to 2.5 times the design load (or until failure).
Section 5.1
Working / Proof Load Test
Performed on working piles (production piles) to confirm they meet the design capacity. Load: typically 1.5 times the design working load. Settlement should be within permissible limits.
Section 5.2
Maintained Load Test Procedure
- Apply load in increments of 25% of design load.
- Hold each increment until settlement rate falls below 0.1 mm/hour (or 2 hours, whichever is greater).
- Record settlements at the pile head using dial gauges (min 4, at 90ยฐ intervals).
- After reaching maximum test load, hold for 24 hours (or until settlement stabilises).
- Unload in equal decrements. Record rebound at each stage.
Permissible Settlement
| Pile Type | Max Settlement at 1.5ร Working Load |
|---|---|
| Single pile (isolated) | 12 mm |
| Pile group (10+ piles) | 25 mm |
| Bridge foundations | As per IRC provisions |
Soil Setup / Freeze
In driven piles, the soil around the pile undergoes remoulding and excess pore pressure generation during driving. Capacity increases with time as pore pressures dissipate (called "setup" or "freeze"). Wait at least 7 days after driving before conducting a load test in clay.
Group Pile Effects
Piles are rarely used individually. They are arranged in groups with a pile cap. The capacity of a group may differ from the sum of individual pile capacities. Cl. 5.5
Group Efficiency (ฮทg)
Efficiency of Pile Group
ฮทg = Qgroup / (n ร Qsingle)
Where n = number of piles, Qsingle = capacity of single pile tested alone.
Spacing Rules
- Minimum Centre-to-Centre Spacing: 2.5ร pile diameter (friction piles) or 2ร diameter (end-bearing piles).
- Preferred Spacing: 3ร diameter for friction piles to ensure group efficiency โ 1.0.
Key Principle: In clay, closely spaced friction piles may have ฮทg < 1.0 due to overlapping stress zones ("block failure"). In sand, ฮทg is often > 1.0 due to densification from driving adjacent piles.
Lateral Load Capacity
Piles must also resist horizontal forces (wind, seismic, earth pressure). IS 2911 references the IS code for lateral load tests. Appendix B
Methods of Analysis
- IS Code Method: Based on Broms' theory โ assumes soil as elastic continuum and pile as flexible beam.
- p-y Method: More advanced, used in software (LPILE, REPUTE). Defines soil reaction (p) as a function of lateral deflection (y).
- Empirical Approach: Lateral capacity โ 1% to 3% of axial capacity for typical piles in medium dense sand.
Practical Guidance
For most building projects, bored cast-in-situ piles of 600โ900 mm diameter provide adequate lateral capacity through passive resistance on the shaft. Raked (battered) piles are an option if horizontal loads are very high.
Related Codes
Pile design requires coordination with several other codes: