Footing Load Tests: The Ultimate Proof of Foundation Performance
1. What is a Footing Load Test?
A Footing Load Test (FLT) is an on-site test where a prototype footing is loaded while we measure how much it settles. It directly reflects the real soil–foundation behaviour including soil layering, groundwater effects, and construction quality.
FLT provides:
- Ultimate bearing capacity
- Modulus of subgrade reaction (k)
- Full load–settlement curve
2. Why do a Footing Load Test?
Perform FLT when:
- The structure is heavy/critical (industrial foundations, chimneys, heavy machines).
- Soil conditions are highly variable.
- You need a reliable, site-specific k-value.
- Verifying ground improvement.
- A final proof test before large construction.
3. Footing Load Test vs Plate Load Test (short)
Plate Load Test uses small plates (300–750 mm) and estimates behaviour near the surface. Fast but limited.
Footing Load Test uses a large RCC footing (1.5–3 m). It loads the full influence zone (1.5–2× footing width), giving realistic results.
In short: PLT predicts, FLT confirms.
4. How a Footing Load Test is done (step-by-step)
- Prepare the test footing RCC footing 1.5–3 m square. Test load ≈ 2–2.5× design load.
- Install instruments & references Dial gauges/LVDTs on stable supports outside the influence zone.
- Provide a reaction system Kentledge, loaded platform, or reaction truss with anchor piles.
- Apply load in increments Typically 20–25% of design load per increment.
- Record settlements Measure settlement at each increment; unload in stages.
- Plot and interpret curve Determine k-value, yield point, ultimate load and allowable pressure.
5. Typical instruments and setup checklist
- Hydraulic jack with calibrated gauge
- Kentledge / reaction truss / platform
- Dial gauges or LVDTs
- Reference beams outside influence zone
- Data logger/readout
- Groundwater level record
6. Loading sequence & criteria
- Load increments: 20–25% of design load
- Hold until rate of settlement < 0.02 mm/min
- Continue loading to 2–2.5× design load or failure
- Unload in steps and record rebound
Longer hold time is essential on clays due to consolidation.
7. Reading & interpreting results
- Elastic zone
- Linear portion; used to find the k-value.
- Yield point
- Curve deviates from straight line.
- Ultimate load
- Rapid settlement indicating shear failure.
k-value formula:
k = q / s
Where q = applied pressure and s = settlement.
8. Factors that strongly affect results
- Soil variability and layering
- Groundwater table
- Influence zone (depth ≈ 1.5–2× footing width)
- Footing casting/contact quality
- Boundary effects (nearby structures)
- Loading rate and duration
9. When to use a Footing Load Test
- Heavy machine foundations
- Industrial load-bearing foundations
- Sites with unpredictable soil
- Ground improvement verification
- Proof-test before full construction
10. Common mistakes & practical tips
- Never fix gauges on the footing—use independent reference beams.
- Check reaction frame stiffness.
- Record groundwater level.
- Do not rush settlements—especially in clays.
- Correlate curve behaviour with borehole logs.
11. Quick worked example (k-value)
Given:
- q = 200 kN/m²
- s = 5 mm = 0.005 m
Compute:
k = 200 / 0.005 = 40,000 kN/m³
So, k = 40,000 kN/m³.
12. References & further reading
- IS 1888:1982 — Load Test on Soils
- IS 2950 — Raft Foundations
- IS 9214 — Modulus of Subgrade Reaction
- Bowles — Foundation Analysis & Design
- Das — Principles of Foundation Engineering
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