Zone Load Testing (ZLT) of improved ground
A Zone Load Test (ZLT) loads a large footing or plate, typically several metres across, placed over a group of stone columns, rigid inclusions or other treatment points, so the test includes the improved soil between them and not only a single element. Load is applied in stages by kentledge or jacking against a reaction, and settlement is measured with time. Because the loaded area and stressed depth are closer to a real foundation, the result is a more representative check of the improved ground than a small plate test.
When the test is used
Typically used for…
- Verification of stone columns, rigid inclusions and vibro replacement.
- Checking that a treated area meets the settlement criteria under foundation-scale load.
- Calibrating design settlement predictions.
- Important structures where small plate tests stress too shallow a depth.
Limitations
- Large reaction loads are needed and the test is slow and costly, so only a few are performed.
- The test is still smaller than most foundations and is interpreted with the design.
- Long-term creep and consolidation are not fully captured in the test duration.
The test type, number and locations are agreed with the consultant for each project.
How the test works
A reinforced concrete footing or large steel plate is placed on the treated ground over a group of treatment points, usually with a granular bedding layer. Kentledge blocks or a jacking system against a reaction frame apply the load in increments.
Settlement is read on the footing with dial gauges or transducers referenced to beams supported outside the zone of influence. Each load step is held until settlement stabilises, up to the test load, followed by unloading.
Test planning inputs
- Design foundation pressure and settlement criteria.
- Treatment layout and the points included under the footing.
- Footing size and test load.
- Reaction method and site access.
- Loading and holding sequence.
Calibration and equipment checks
- Test procedure, load steps and stabilisation rules agreed before testing.
- Calibration of jack, load cell and gauges.
- Reaction capacity checked before loading.
Test execution controls
- Load applied and held to the agreed steps.
- Settlement readings at the agreed time intervals.
- Reference beam supports outside the zone of influence.
- Footing level and tilt monitored.
- Weather protection of instruments.
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
Test load and settlement acceptance criteria agreed with the consultant.
What was measured
Load-settlement and settlement-time curves.
What it shows
Comparison of measured with predicted settlement, in the test report.
Deliverables
- Test procedure
- Calibration certificates
- Load and settlement records
- Load-settlement and time curves
- Interpretation report
Discuss the testing your project needs.
Share the project stage, the ground conditions and the specification or acceptance criteria.