Input Test Details
Formula Used
Plate area: Circular area = πd²/4. Rectangular area = width × length.
Applied pressure: q = P / A, where P is stage load and A is plate area.
Corrected pressure: qc = q × correction factor × groundwater factor.
Allowable pressure by ultimate: qa,u = qult / FOS.
Allowable pressure by settlement: pressure interpolated at the selected allowable settlement from the measured load-settlement curve.
Safe bearing pressure: qsafe = minimum of the ultimate-based value and settlement-based value.
Coefficient of subgrade reaction: k = q / s, using pressure at the chosen settlement and settlement converted from mm to m.
How to Use This Calculator
- Choose the plate shape used during field testing.
- Enter plate dimensions and the trial footing width.
- Set the allowable settlement and the modulus settlement point.
- Enter your safety factor and any design correction factors.
- Fill in staged load values and observed settlements.
- Press the calculate button to view bearing pressure results.
- Review the governing criterion and export the result table if needed.
Example Data Table
| Stage | Load (kN) | Settlement (mm) | Pressure for 0.60 m Circular Plate (kN/m²) |
|---|---|---|---|
| 1 | 20 | 0.45 | 70.74 |
| 2 | 40 | 0.95 | 141.47 |
| 3 | 60 | 1.70 | 212.21 |
| 4 | 80 | 2.90 | 282.94 |
| 5 | 100 | 4.80 | 353.68 |
| 6 | 120 | 8.50 | 424.41 |
FAQs
1. What does a plate load test measure?
It measures how soil reacts to incremental surface loading. The recorded load and settlement curve helps estimate bearing pressure, stiffness, and settlement behavior for shallow foundation design.
2. Why is settlement-based pressure important?
A soil layer may support load without immediate shear failure, yet still settle too much. Settlement-based pressure limits design values to serviceable movement levels.
3. What is the subgrade modulus used for?
The modulus of subgrade reaction is often used in raft, slab, pavement, and mat analyses. It represents contact pressure per unit settlement over the selected range.
4. Why does the calculator ask for correction factors?
Field plate tests may need engineering adjustments for footing size, groundwater level, and local design practice. Those factors help align raw observations with design assumptions.
5. Can I use only one load reading?
No. At least two readings are needed to create a meaningful load-settlement relationship. More stages improve interpolation and interpretation reliability.
6. Does this replace a geotechnical report?
No. It is a design aid. Final bearing capacity and settlement decisions should consider stratification, seasonal moisture, groundwater, and the geotechnical engineer’s recommendations.
7. Why is safe bearing pressure the lower value?
Design must satisfy both strength and serviceability. Using the lower value ensures the soil is not overstressed and settlement remains within the target limit.
8. Which footing area is used for the estimated load?
This version estimates load for a square footing using the entered footing width. Multiply safe bearing pressure by your actual footing area when geometry differs.