Underpinning is the process of strengthening or deepening the foundation of an existing structure, either to increase load-bearing capacity, to allow adjacent excavation, or to correct differential settlement. As India's urban infrastructure ages and new construction encroaches on older buildings, underpinning is an increasingly important specialty in geotechnical engineering.
When Is Underpinning Required?
- Foundation failure or settlement: Cracking, tilting, or subsidence due to weak or failed foundation
- Increased loading: Adding floors to an existing building without adequate original foundation
- Adjacent deep excavation: New construction, metro tunnelling, or basement adjacent to existing building — may undermine existing foundation
- Soil change: Soil shrinkage/swelling (expansive clays), washout, or ground subsidence
- Change in water table: Pumping of groundwater causes settlement; underpinning may be needed
- Heritage building rehabilitation: Old colonial-era buildings requiring structural upgrading
Methods of Underpinning
1. Mass Concrete (Pit) Underpinning — Most Common
The oldest and most widely used method in India. Suitable for shallow foundations on relatively stable soil.
Procedure:
- Expose existing foundation by excavating a pit alongside it in stages (never expose more than 1–1.5 m at a time)
- Work in alternating bays (typically 1–1.5 m wide): 1, 3, 5, 7... first; then 2, 4, 6... after first set gains strength
- Extend excavation below existing foundation to new bearing depth
- Pour mass concrete (generally plain concrete or PCC); build up to within 75–100 mm of existing foundation soffit
- Dry-pack (grout) the 75 mm gap under the existing foundation to eliminate any void and ensure full contact
- Proof load the new concrete before proceeding
Advantages: Simple, no heavy machinery, relatively cheap
Limitations: Slow (alternate bay sequencing); not suitable for deep underpinning (>6 m); not suitable in waterlogged conditions; requires temporary shoring of superstructure
2. Beam and Base Underpinning
A reinforced concrete needle beam is threaded through holes in the existing foundation wall at a higher level than the new foundation. The needle beam transfers the load to a new pair of concrete bases (piers) on either side of the wall.
Used when soil is too weak or deep for mass concrete pits; allows bridging over soft spots.
3. Mini Pile (Micropile) Underpinning
Small-diameter (100–300 mm) bored piles, drilled through the existing foundation, penetrating to a competent stratum well below. Steel tubes or threaded bars grouted in place.
Advantages:
- Minimal vibration — ideal for historic or sensitive structures
- Can be installed in confined spaces (low headroom)
- Works in any soil type; reaches rock or competent stratum
- Can carry significant loads (200–2000 kN per pile)
- Small footprint; no need for large excavation
Types: Cased (for loose soil), uncased (grouted), bracket type (piles installed at angle through the footings).
4. Piled Raft Underpinning
Multiple micropiles installed through or around the existing foundation; connected by a new reinforced concrete raft beam. Effectively creates a new piled foundation below the original. Used for large buildings with extensive settlement.
5. Chemical Injection / Ground Improvement
Grouting (cement, chemical, or compaction grouting) fills voids, strengthens weak soil, and lifts settled foundations without excavation:
- Permeation grouting: Low-viscosity chemical grout (sodium silicate, polyurethane) injected into permeable soils
- Compaction grouting: High-viscosity grout expands in soil, compacting surrounding material and lifting structure
- Jet grouting: High-pressure jet cuts soil and mixes with cement grout → columns of soil-cement
Safety Considerations in Underpinning
- Never expose foundation by more than 1.2 m at any one time (BOCW and BS 8004 guidance)
- Always work in alternate bays; allow concrete to reach minimum 5 N/mm² before starting adjacent bay
- Shoring/propping of superstructure walls and floors may be necessary before excavation
- Monitor structure continuously with crack gauges and precise levelling during the work
- Ensure dewatering if groundwater is present
- IS 3764 (excavation safety) and IS 4014 (scaffolding) apply to underpinning works
Design Considerations
- New foundation bearing depth must be below the depth of influence of the previous failure (e.g., below swelling/shrinking zone, below soft layer)
- Settlement of new underpinned foundation must be compatible with remaining original foundation settlement
- For mini piles: Design per IS 2911 Part 1 Section 2 (bored piles) adapted for small diameter
- Pile load tests (maintained load tests per IS 2911 Part 4) required for verification
Monitoring During Underpinning
- Precise levelling (≥1/week) on grid points across the structure
- Crack monitors (tell-tales) on major cracks
- Inclinometers if adjacent deep excavation is the cause
- Pre-agreed settlement alert and alarm levels triggering work suspension
Frequently Asked Questions
How long does underpinning take for a typical 3-storey building?
Mass concrete underpinning of a typical 3-storey building perimeter wall (say 40 m total) in 1.2 m bays alternating: approximately 8–12 weeks. Each bay requires excavation (0.5 day), concrete pour (1 day), curing (3 days), dry-packing (1 day), and inspection before proceeding — alternating means roughly 2 bays per week. Micropile underpinning can be faster if drilling access is good.
When is micropile underpinning preferred over mass concrete?
Micropiles are preferred when: the building is in use and vibration/disturbance must be minimal (hospitals, heritage buildings), when headroom is very limited, when soil is very weak requiring deep bearing, or when the building footprint prevents large excavations. Mass concrete remains preferred for straightforward shallow cases where cost is the primary driver.