A plate load test records applied load and plate movement to characterise load-settlement response at a test location. The arithmetic is straightforward, but interpretation depends on plate geometry, test setup, stress range, seating, ground conditions and the selected method.
Topic guide: For background, method selection and related checks, see Plate Load Testing.
Applied contact pressure
For applied load \(P\) and plate area \(A\):
$$ q=\frac{P}{A} $$For a circular plate of diameter \(D\):
$$ A=\frac{\pi D^2}{4} $$For a rectangular plate:
$$ A=BL $$Settlement increment
For two readings:
$$ \Delta s=s_2-s_1 $$ $$ \Delta q=q_2-q_1 $$The load and settlement readings used must be identified. Seating movements and unload-reload cycles should not be removed or combined without following the nominated interpretation method.
Modulus of subgrade reaction
A secant or incremental modulus of subgrade reaction may be written as:
$$ k_s=\frac{\Delta q}{\Delta s} $$The units are pressure divided by displacement, such as kPa/mm or kN/m3.
The selected stress interval must be reported. A single \(k_s\) value is not an intrinsic soil property. It depends on plate size, stiffness, stress level and test conditions.
Worked example
Assume:
- plate area \(A=0.25\) m2
- applied load \(P=100\) kN
- settlement from the nominated origin \(s=5\) mm
Pressure:
$$ q=\frac{100}{0.25}=400\ \mathrm{kPa} $$Using \(s=0.005\) m:
$$ k_s=\frac{400}{0.005} =80{,}000\ \mathrm{kN/m^3} $$This may also be written as 80 MN/m3.
The result applies to the selected origin and stress interval. It is not automatically a design subgrade modulus for a different foundation or slab.
Load-settlement curve
The calculation record should preserve every stage:
| Stage | Time | Load | Pressure | Settlement | Corrected settlement |
|---|
Useful outputs may include:
- pressure at each stage
- settlement increments
- secant values over nominated ranges
- unload and reload response
- residual settlement
- plot of pressure versus settlement
- test-stage duration
Important corrections and controls
The interpretation may need to consider:
- plate seating
- datum and reference-frame movement
- reaction-system movement
- time-dependent settlement
- plate rigidity
- non-level contact
- disturbance during test preparation
- groundwater and moisture
- nearby boundaries
- stress bulb relative to layer thickness
These should not be hidden inside an unexplained correction.
Scale effect
A plate tests a limited volume of ground. A larger footing or slab influences a larger and deeper zone. Directly transferring a plate response to another loaded area requires an appropriate method and engineering review.
Calculation validation
The tool should:
- verify plate dimensions and area
- use compatible load and area units
- retain raw readings
- identify seating correction
- require the selected stress interval
- display the resulting units clearly
- flag zero or negative settlement increment
- prevent an unexplained conversion to bearing capacity
- distinguish measured curve data from derived values
Related resources
Authoritative references
Plate-load interpretation and transfer to design require the nominated method and review by a suitably qualified geotechnical professional.