Two different questions
CPT asks: how has the ground changed with depth? It gives a continuous profile through the treated zone and beyond.
PLT asks: how stiff is the ground near the surface under load? It gives the load-settlement response beneath a plate.
Side-by-side comparison
| CPT | PLT | |
|---|---|---|
| What it measures | Cone resistance, sleeve friction and pore pressure | Settlement of a plate under stepped load |
| Depth covered | Continuous, through the full treated depth | Near surface: about 1.5 to 2 plate diameters |
| Best used for | Verifying densification with depth; comparing before and after | Verifying surface stiffness and bearing behaviour |
| Main limits | Refusal in very dense or cemented layers; no sample | Shallow; one location per test; needs a reaction load |
Using them together
A typical verification scheme uses CPT to confirm improvement through the treated depth, including at positions between treatment points, where improvement is lowest, and PLT to confirm the stiffness of the finished surface.
A plate is much smaller than a foundation, so it loads a much shallower zone. Where a large footing or tank is involved, a larger zone load test or settlement monitoring may be specified in addition.
When to test
In sands, measured cone resistance can continue to increase for some time after treatment. A waiting period between treatment and testing is therefore commonly agreed in the specification, so that tests are not carried out too early.
Link the criteria to the design
Acceptance values should trace back to the performance the structure needs (bearing pressure and allowable settlement) rather than being fixed numbers copied from another project. Where CPT cannot penetrate, pressuremeter testing can provide parameters at depth.
Related services
Standards referenced
- ISO 22476-1, Geotechnical investigation and testing: electrical cone and piezocone penetration test
- DIN 18134 / ASTM D1196: plate load test methods