LWD testing in Cambridge.

Same-day and next-day dates are often available, subject to the schedule. Cambridge is about an hour from our laboratory in Milton Keynes, so it is one of the easiest cities for us to reach at short notice, and an LWD visit needs no reaction vehicle, only a technician and the equipment. We do not guarantee an attendance time. Ring 01223 298718 and we will tell you what is possible.

A lightweight deflectometer drops a known weight onto a plate and records 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 in megapascals, calculated from the force applied and the deflection it produced. On a Cambridge capping layer that takes a couple of minutes per position.

Not reliably when it is wet. Clays deform plastically and give little elastic recovery for the equipment to read, so results on a soft Gault, boulder clay or Ampthill Clay formation scatter and are hard to interpret. At formation level on these clays an in-situ CBR is the better test. The LWD comes into its own on the capping and sub-base placed above.

Your specification or pavement designer sets it. Pavement foundation design in DMRB CD 225 defines foundation classes by surface modulus, starting at 50 MPa, and the specification says which class applies and how it must be demonstrated. For building slabs the engineer may set a project-specific value. Tell us the target and the report will show each position against it.

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 comes from the specification, with performance testing of pavement foundations covered by Series 800 of the Specification for Highway Works. Tell us what your specification calls for.

Roughly one to one and a half plate diameters, so about the top 300 to 450 mm with a 300 mm plate. That makes it a good check on a placed layer and a poor way of finding out what is happening deeper. On the fen edge north of Cambridge, where a thin platform sits over soft ground, the reading is a composite of the layer and what is beneath it.

A CBR can be estimated from a surface modulus, but dynamic stiffness and direct CBR are different measurements and the relationship depends on the material. Any conversion has to be appropriate to the material and agreed for the project, and the report should make clear that the CBR is derived. If an adopting authority wants a CBR at formation, a CBR test is usually the cleaner answer.

Many. Each position takes a few minutes, so dozens of readings in a visit is normal. On a laboratory or data centre slab in Cambridge that means a proper grid across the footprint, which shows up soft spots that a handful of plate tests could miss. Send the drawing and we will suggest a spacing, or follow the one in your specification.

For checking that a layer is uniform, often yes. For proving a piling platform or a crane hardstanding, usually no, because those designs are checked against a plate bearing result at the design pressure. A sensible arrangement is plate bearing at the critical positions and LWD across the rest. Check with your designer before relying on LWD alone.

Very little. The equipment is portable and carried to each position by hand, with no reaction vehicle, so finished areas, plots between buildings and positions a lorry cannot reach are all testable. The surface needs to be reasonably flat and even so the plate seats properly. Loose or rutted surfaces are trimmed or lightly bedded first.

Hixtra holds UKAS accreditation to ISO/IEC 17025 under schedule number 8996. Accreditation is granted method by method, not to the company as a whole, so it applies only to the tests listed on our current schedule. Ask us to confirm the accreditation status of LWD testing when you enquire, particularly if your specification requires accredited results, and we will tell you before you book.

Every visit is quoted from the site postcode. Because there is no reaction vehicle and each test is quick, the cost per position is low compared with plate bearing, and Cambridge is close enough that travel is a small part of it. The price depends mainly on the time on site. Send the postcode, the area to be covered and your dates and we will quote.

No. Hixtra has one laboratory, in Milton Keynes, and every Cambridge visit is run from there. The 01223 number rings through to the same team as our other numbers. Cambridge is only about an hour away, so that makes little practical difference to dates or cost.