LWD testing

What does LWD testing measure, and how is the test carried out?

An LWD measures the stiffness of a layer by dropping a known weight onto a plate and recording how far the surface deflects. A 10 kg weight falling onto a 300 mm plate is typical, with a few seating drops to bed the plate followed by measuring drops that are averaged. The result is a dynamic surface modulus, reported in megapascals, calculated from the force applied and the deflection it produced.

Which standard or guidance is LWD testing usually carried out to?

There is no single British Standard for the lightweight deflectometer, so testing usually follows the equipment manufacturer's procedure alongside a recognised method such as ASTM E2583. On highway schemes the requirement is set by the specification rather than the test method, with performance testing of pavement foundations covered by Series 800 of the Specification for Highway Works. Tell us what your specification calls for and we will confirm how the test will be run.

What does the surface modulus figure mean, and how is it reported?

The result is a surface modulus in megapascals at each position, which your designer compares against the stiffness the specification requires. Pavement foundation design in CD 225 of the Design Manual for Roads and Bridges sets foundation classes by surface modulus, starting at 50 MPa, and your specification will say which class applies and how it has to be demonstrated. The report lists each position, the individual drops, the mean modulus, the plate size and the drop weight.

How deep into the ground does an LWD test assess?

The influence depth is shallow, generally between one and one and a half times the plate diameter, so with a 300 mm plate you are assessing roughly the top 300 to 450 mm. That makes it a good check on a placed layer and a poor way of finding out what is happening deeper down. Where the layer is thin, the reading is a composite of that layer and whatever sits beneath it.

Which materials does the test work well on, and where is it unreliable?

It works best on granular and recycled materials, capping, subbase and made ground, where the layer has a genuine elastic response to measure. Soft cohesive soils are the weak point, because clays deform plastically and give little elastic recovery for the equipment to read. Loose or uneven surfaces are also unsuitable, since the plate cannot seat properly. Describe the material to us if you are unsure whether it suits the method.

Can an LWD result be converted into a CBR value?

A CBR can be estimated from a surface modulus, but it is a correlation rather than a measurement and it has to be appropriate to the material. Dynamic stiffness and a direct plunger CBR are different properties, so a converted figure should be labelled as derived in the report and agreed with the designer before it is relied on. Where the specification asks for a CBR percentage, check first that a correlated value will be accepted.

Does the surface need preparing before the plate is set down?

Yes. The surface has to be flat, firm and clean enough for the 300 mm plate to make full contact, so the layer should be compacted and trimmed before we arrive. Loose stone, ruts or a freshly tipped surface stop the plate seating and produce scattered readings, and a thin sand seating layer can be used where the surface is coarse. The test leaves nothing behind, so no reinstatement is needed.

How many positions can you complete in a visit, and how are they chosen?

Each position takes only a few minutes, so a large number can be covered in a day and walking between them is usually the limit. That makes the method well suited to checking uniformity across a placed layer, on a grid or at set intervals along a route, rather than proving a single design load. Positions are normally spread across each placed area, with extra readings where compaction looks variable.

Does rain or a wet layer change the result?

Yes. Stiffness falls as a granular layer takes on water, so a layer tested after heavy rain gives a lower modulus than the same layer tested dry. The result describes the layer in the condition it was in on the day, which is why testing is normally done when the layer is at the moisture condition the works will be built on. Standing water or a frozen surface will stop testing.

When would a plate bearing test be a better choice than LWD?

Choose a plate bearing test when you need settlement under a sustained known load, or when the design has to be proved at the pressure the plant or structure will actually apply. The LWD applies a small dynamic pulse and reaches only the upper few hundred millimetres, so it is not a substitute for a static test on a working platform or crane position. The trade off is a heavy reaction machine and far more time per position.

Can an LWD result be issued as a UKAS accredited result?

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 applies to particular methods rather than to every service. Because the LWD is not covered by a single British Standard, it is worth asking us to confirm the status of the testing and the reporting before you commit to it.

What information do you need to quote for LWD testing?

Send the site postcode, the area or number of positions to be tested, the layer involved, your preferred dates and the specification if there is one. Tell us whether a modulus is enough or whether you need a correlated CBR figure, since that has to be agreed in advance. Cost is driven mainly by travel, time on site and the number of visits, as a return trip for each placed layer adds up.