Plate Load Testing (PLT) for bearing and stiffness verification
Plate Load Testing (PLT) applies load in increments to a rigid steel plate on the ground while settlement is measured. The load-settlement response gives the stiffness (modulus) and bearing behaviour of the ground within a depth related to the plate size. It is commonly used to verify compacted fills, improved ground and working platforms.
When the test is used
Typically used for…
- Verifying compacted fills, platforms and improved ground.
- Checking bearing behaviour for shallow foundations and slabs.
- Acceptance testing against modulus or settlement criteria.
- Verifying working platforms for cranes and piling rigs.
Limitations
- The depth of influence is roughly 1.5 to 2 times the plate diameter, so deeper layers are not tested.
- A reaction load, such as kentledge or a heavy vehicle, is required.
- Each test represents one location; the number of tests must represent the area.
The test type, number and locations are agreed with the consultant for each project.
How the test works
A rigid steel plate is bedded on a levelled surface. A hydraulic jack between the plate and a reaction load applies load in steps, each held until settlement stabilises.
Settlement is measured by gauges on a reference beam supported outside the zone of influence. Loading and unloading cycles give the load-settlement curve, from which stiffness moduli are calculated.
Test planning inputs
- Purpose and acceptance criteria (modulus, or settlement at a given load).
- Plate diameter and maximum test load.
- Load steps, cycles and hold criteria.
- Test locations and number of tests.
- Applicable standard.
Calibration and equipment checks
- Jack and load cell calibration certificates checked.
- Reaction load confirmed to exceed the maximum test load.
- Surface preparation and plate bedding checked.
Test execution controls
- Load increments and hold times as per the procedure.
- Settlement readings at set intervals until stable.
- Reference beam supports outside the zone of influence.
- Test protected from vibration, direct sun and disturbance.
- Test location surveyed.
Interpretation and reporting
The test plan, the raw record and the interpretation are kept separate, so it is always clear what was specified, what was measured and what it shows.
What is specified
Plate size, loading procedure and acceptance criteria agreed with the consultant.
What was measured
Load and settlement readings for each increment and cycle.
What it shows
Load-settlement curve and calculated moduli compared with the criteria.
Deliverables
- Test procedure
- Calibration certificates
- Load-settlement records and curves
- Calculated moduli
- Test report with photographs
Common questions
What does a plate load test show?
The stiffness and bearing behaviour of the ground near the surface, from the settlement of a plate under stepped load.
How deep does a plate load test reach?
Roughly 1.5 to 2 times the plate diameter. Deeper layers are not tested.
What reaction load is needed?
A kentledge or heavy vehicle that safely exceeds the maximum test load.
Discuss the testing your project needs.
Share the project stage, the ground conditions and the specification or acceptance criteria.