Plate bearing testing

What does a plate bearing test measure?

It measures how much the ground settles under a known load applied through a rigid circular plate. Load is applied in increments by a hydraulic jack reacting against a heavy machine, and the settlement is recorded at each stage. The output is a load and settlement relationship, from which a bearing pressure, a stiffness modulus or a derived CBR can be calculated depending on what the design needs.

Which standard is plate bearing testing usually carried out to?

Plate bearing tests are usually carried out to the plate loading method described in BS 1377-9, with the plate size, load increments and maximum applied pressure set by your specification. Where the test is checking a working platform, the target pressures come from the platform design, now normally prepared to the Temporary Works Forum guidance on granular working platforms, with some specifications still naming the earlier BRE report BR 470. Tell us what the result has to demonstrate and we will confirm the method before booking.

When should we use a plate bearing test rather than LWD, DCP or a direct CBR test?

Use a plate bearing test when you need settlement under a known static load, or when the material is too coarse for a plunger CBR. The plate applies a sustained load close to the stress the real plant or structure will impose, which is why it suits working platforms, crane and piling positions and thick granular layers. Material containing stone over 20 mm usually rules out a direct plunger CBR, and a plate test with an agreed derived calculation is then the sensible route.

What plate diameter do you use, and how deep does the test assess?

Plates of 300, 450, 600 and 760 mm diameter are commonly used, and the choice controls how deep the test reaches. As a rule of thumb a plate stresses a depth of ground roughly twice its diameter, so a 600 mm plate is assessing over a metre of material rather than just the surface. Larger plates need more reaction load, so the plate is chosen from the layer you need to assess and the design pressure.

What reaction load do we need to have on site?

You will normally need to provide a machine heavy enough to resist the full test load without lifting, usually a tracked excavator. The weight required depends on the plate diameter and the maximum pressure being applied, so a small plate at low pressure needs far less than a 600 mm plate loaded to working platform pressures. We confirm the size of machine and the operator time to allow when the work is booked.

How should the test position be prepared before your engineer arrives?

The position needs to be level, trimmed flat and clear of loose material at the exact level you want tested. A plate that is not bedded evenly gives settlement readings that reflect the seating rather than the ground, so we bed it on a thin layer of sand where the surface is uneven. Standing water, a formation softened by rain or ground churned up by tracking will all show in the result.

How many plate positions should we allow for across the area?

That depends on the area, how variable the ground is and what your specification asks for. Positions are normally spread to cover each material type and each part of the area being signed off, with extra tests where the ground looks weaker or the plant loads are highest. For a working platform or a crane position they usually sit under the intended footprint rather than at random. Send a layout plan and we will suggest a sensible number.

How long does a plate bearing test take on site?

Allow roughly an hour per position for a standard test, covering setting up the plate, the loading cycles and repositioning the reaction machine. A longer sequence with several load and unload cycles, or higher pressures on a large plate, takes more. Several positions can usually be completed in a day, and the limit is normally how quickly the excavator can be moved and reset rather than the test itself.

What does the plate bearing test report contain?

The report gives the applied load and the settlement recorded at each increment for every position, presented as a load and settlement plot, with the plate diameter used, the test level and a description of the material. Where a modulus or a derived CBR has been agreed, the calculation behind it is stated so the reader can see how the figure was reached. Any deviation from the specified method is recorded.

Does the test damage the formation, and does anything need reinstating?

Damage is minor. The plate leaves a shallow impression and the bedding sand stays behind, and where the test is done in a pit that excavation needs backfilling and compacting once we have finished. The tested spot is disturbed by the loading, so a repeat test is offset slightly rather than done in the same footprint. Tracked plant moving across a soft formation usually does more damage than the test does.

Does your UKAS schedule cover plate bearing testing?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope, so it covers particular methods rather than every service we offer. Ask us to confirm whether the plate bearing testing you need is covered before you book, and we will say plainly if any part of the work sits outside the schedule.

What drives the cost of plate bearing testing?

Cost is driven by the number of test positions, travel to site, the plate size and whether you can provide the reaction machine, plus any standby, out of hours working or traffic management the site needs. To price the work we need the site postcode, the number of positions, the design load or plate size if you know it, your preferred dates and the specification if there is one.