FOUNDATION

Well Foundation

Hollow shaft sunk through soft soil to firm strata, filled with concrete — used for major bridge piers in rivers.

Also calledcaissonmonolith
Related on InfraLens
CODES
IS 3955IRC 78
Definition

A well foundation (also called caisson or monolithic well) is a hollow structure with a sharp cutting edge at the bottom, sunk through soft soil to bear on firm strata at depth. Used primarily for major bridge piers in flowing water (rivers, sea), where shallow foundations would scour out and pile foundations would be inadequate. The Indian standards: IS 3955:1967 (revised) covers well foundation design and construction; IRC 78:2014 provides specific guidance for highway bridges; both reference IS 456 for structural concrete and IS 6403 for geotechnical aspects.

The construction sequence: (1) Pre-fabricate the well's cutting edge (steel + RCC) and place it at the founding location, typically pre-cast on a barge or floating platform; (2) Build the well's body in lifts of 1.5-3.0 m using forms — RCC walls 0.6-1.0 m thick, hollow inside; (3) Sink by excavation inside the well using grabs, dredgers, or compressed-air systems — gravity provides the sinking force; (4) Continue sinking and adding RCC lifts until the well reaches firm strata; (5) Plug the bottom with concrete (the 'plug') after dewatering; (6) Fill the well with sand and concrete to seal and provide weight; (7) Cap the well with a top slab to support the bridge pier or pier cap.

Well foundations are economical for: (a) major bridge piers in deep water (depth 10-50+ m), (b) heavy column loads where pile groups would be impractical, (c) sites with highly compressible upper strata over firm rock or dense gravel below. Drawbacks: slow construction (typical sinking rate 0.5-1.0 m/day), subject to tilting during sinking (must be corrected immediately), and require specialised contractors. Major Indian well-foundation projects: Howrah Bridge (1942 — wells 75 m deep), Mahatma Gandhi Setu (1982 — wells 35 m), Bandra-Worli Sea Link (2009 — well-and-pile combination), Bhopal Bypass Bridges. Modern Indian practice has largely shifted to pile foundations for new bridge construction, but well foundations are still used in deepwater scenarios where piles cannot be installed.

Typical values
Wall thickness0.6-1.0 m
Diameter (typical)5-12 m
Sinking rate0.5-1.0 m/day
Maximum depth20-75+ m
Plug thickness (after sinking)1.5-3.0 m
Cutting edge — steel angleISA 200×200×16 minimum
Where used
  • Major bridge piers in flowing water (rivers, estuaries, bays)
  • Industrial structures with heavy loads on weak upper strata
  • Submerged pump houses and gates of irrigation projects
  • Lighthouse and waterfront structures on deep weak strata
  • Historical Indian construction — Mughal and British Raj-era bridges
Acceptance / threshold
Per IS 3955 + IRC 78: structural design for sinking forces, hydrostatic pressure, soil pressure, and load from superstructure; cutting edge robust (steel angle ≥ ISA 200×200×16); bottom plug ≥ 1.5 m thick; verticality during sinking ≤ 1% of depth.
Site example
Site reality: the Mumbai-Nashik bridge project required well foundations 28 m deep through 18 m of soft marine clay to firm rock. The contractor's barge-based well construction was on schedule for 6 months until tilting reached 8% during the last 6 m of sinking. Corrective measures: forced lateral excavation on one side, additional ballast on the other, and underwater concreting of the corrected position. Total schedule slip 4 months. Well foundation construction is unforgiving — verticality monitoring must be continuous with corrective action immediately when deviation exceeds 1%.
Frequently asked
What is well foundation?
A well foundation (or caisson) is a hollow structure with a sharp cutting edge at the bottom, sunk through soft soil to bear on firm strata at depth. Used primarily for major bridge piers in flowing water. Sinking is by gravity and excavation inside the well; the bottom is plugged after reaching firm strata. Indian codes: IS 3955:1967, IRC 78:2014 for highway bridges.
How is well foundation different from pile foundation?
Pile foundations are cylindrical or other-shape members driven or cast into the ground, typically 0.3-3.0 m diameter and 10-60 m deep, in groups under a pile cap. Well foundations are large hollow structures (5-12 m diameter, 0.6-1.0 m wall thickness) sunk into the ground, single-piece. Wells are economical for deep water (>10 m depth) where piles cannot be installed. Modern Indian practice has shifted to piles for most new bridge construction; wells are reserved for deepwater scenarios.
What is the depth of well foundation?
Depth depends on geotechnical conditions — wells are sunk through soft strata until reaching firm bearing soil. Typical Indian projects: 20-50 m. Maximum depths achieved: 75 m (Howrah Bridge), 70 m (Sealdah-Howrah Suburban Bridge). Sinking rate ~0.5-1.0 m/day, so a 30 m well takes 30-60 days. Verticality must be maintained ≤ 1% of depth to avoid major corrective interventions.
Related foundation terms
Isolated Footing
Footing supporting a single column. Most common for low-rise buildings on soils with adequate bearing capacity.
Combined Footing
Single footing supporting two or more columns — used when columns are close (<2 m) or property line restricts.
Raft Foundation
Single thick slab covering the entire building footprint — for soft soil, heavy loads, or when isolated footings overlap.
Pile Cap
Reinforced concrete cap connecting tops of piles to the column. Designed as a deep beam or strut-and-tie.
Strip Footing
Continuous footing under a load-bearing wall — width = wall thickness + 150 mm projection on each side.
Stepped Footing
Footing with stepped/tapered profile — saves concrete on large footings while maintaining bearing pressure distribution.
Bored Cast-in-situ Pile
Pile constructed by drilling a hole and filling with concrete + reinforcement. Diameter 300-2400 mm, depth up to 60 m.