Step 01
Excavate to founding level
A test pit is cut to the design founding depth and the base is trimmed level and undisturbed, with any loosened material removed by hand.
Plate Load Test
Bearing capacity measured rather than inferred. A rigid plate is loaded in increments against a reaction load while settlement is read at each stage, producing a load settlement curve for the founding stratum itself.
Plate Load Test: what it actually tells you
Most bearing capacity figures are calculated from SPT or laboratory shear strength through a correlation. The plate load test skips the correlation. It loads the actual soil at the actual founding level and records how much it settles, which is why approving authorities and heavy plant designers often ask for it specifically.
The test is run at founding level, so the pit is excavated to the design depth first. A plate of 300 mm to 750 mm is bedded on levelled soil and loaded in increments, each held until the settlement rate falls below a set threshold. Dial gauges on independent datum bars read settlement to 0.01 mm.
The limitation is depth of influence. A plate stresses only about twice its own width of soil, so a 600 mm plate tells you about the top 1.2 metres. Where a weaker layer sits below that, the plate result must be read alongside the borehole log rather than instead of it. We say so in the report when it applies.
At a glance
- Plate size300 mm, 450 mm, 600 mm or 750 mm rigid steel
- Depth of influenceAbout twice the plate width
- LoadingIncremental to failure or to twice design load
- DurationTypically one working day per test point
How we run it
The sequence we follow on site and in the laboratory, and the point of each stage.
Step 02
Set up reaction
Kentledge or a reaction frame anchored against the ground provides the load, with a hydraulic jack and calibrated load cell between the reaction and the plate.
Step 03
Seat and instrument
The plate is bedded on a thin layer of fine sand or plaster of paris, and two or four dial gauges are mounted on datum bars founded outside the zone of influence.
Step 04
Load in increments
Load is applied in stages of about one fifth of the estimated safe load, each held until settlement is under 0.02 mm per minute or for the specified minimum time.
Step 05
Plot and interpret
The load settlement curve is plotted, ultimate load identified, and safe bearing capacity derived with the settlement criterion applied and corrected for foundation size.
What you receive
- Load settlement curve for the founding stratum
- Ultimate bearing capacity
- Safe and allowable bearing capacity with the factor of safety stated
- Modulus of subgrade reaction for raft and pavement design
- Settlement corrected from plate size to foundation size
When this test is the right one
- Heavy machinery and crane foundations
- Raft foundations where subgrade reaction is needed
- Pavement and hardstanding subgrade acceptance
- Where a calculated bearing capacity needs independent confirmation
- Where an authority specifically requires an in situ load test
Methods and standards
Every result is reported with its method reference and the calibration status of the instrument used.
ASTM D1194 bearing capacity of soil for static load on spread footingsASTM D1195 and D1196 repetitive and non repetitive plate load testingIS 1888 method of load test on soilsBS 1377 Part 9 in situ tests
Common questions
What engineers most often ask us about plate load test.
No, and this matters. The plate only stresses soil to about twice its width, so it says nothing about a soft layer three metres down that a real foundation would load. Use it together with boreholes, not instead of them.
Through the standard size correction relationships, which differ for granular and cohesive soil. In sand, settlement scales with the ratio of footing to plate width; in clay it scales close to directly with width. The correction used is stated in the report.
Enough to reach roughly twice the design load without lifting. For a modest footing that is commonly 20 to 40 tonnes of kentledge, which needs planning for delivery and space on site.
Often ordered alongside
Most investigations combine several of these. We scope them together so you are not paying to mobilise twice.
Need plate load test on your project?
Send us the location, the structure and your programme. You get a scoped quotation with the standard, the number of tests and the turnaround written into it.