LWD testing in Bristol.

Same-day and next-day dates are often available, subject to the schedule, but Bristol is around two hours from our laboratory in Milton Keynes, so next-day is the realistic one. An LWD visit needs no reaction vehicle, which makes it easier to schedule than plate bearing. Ring 0117 422 8206 with the postcode and the area to be covered.

A lightweight deflectometer measures how stiff the surface layer is, 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 in megapascals, calculated from the force applied and the deflection it produced. Each position takes a few minutes, so a large Avonmouth platform can be covered in a day.

Not on their own. An LWD assesses roughly the top 300 to 450 mm, and a working platform on the estuary is often thicker than that, over many metres of soft clay and peat. The platform is designed from the ground investigation and proved by plate bearing at the design pressure, normally to the approach in BRE 470. LWD is a useful addition to show the platform is uniform between plate positions.

Not reliably once the clay is wet. Soft cohesive soils deform plastically and give little elastic recovery for the equipment to read, so results scatter. At formation level on these clays an in-situ CBR is the better test. Use the LWD on the capping and sub-base above, where it gives quick, repeatable readings.

Your specification or designer sets it. For pavements, 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 slabs and yards the engineer may set a project value. Tell us the target and the report shows every position against it.

There is no single British Standard for the lightweight deflectometer. 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.

Roughly one to one and a half plate diameters, so the top 300 to 450 mm with a 300 mm plate. That is the single most important thing to understand about the method in Bristol. It is a test of the layer you placed. It says nothing about estuarine clay three metres down or old workings under a coalfield site.

Dozens. Each position takes a few minutes, and because Bristol is a two hour trip each way, covering the whole area in one visit is what makes the method good value here. Send the layout and we will suggest a grid, or follow the spacing in your specification.

It can be estimated, but dynamic stiffness and direct CBR are different measurements and the relationship depends on the material. Any conversion must be appropriate to the material and agreed for the project, and should be reported as derived. If the adopting authority wants a CBR, a CBR test is the cleaner route.

Very little access, because the equipment is hand carried. The surface should be reasonably flat, even and free of loose stone so the plate seats properly. Port and energy sites at Avonmouth usually have their own induction, so allow time for it. Tell us the material, the layer thickness and what is beneath it, because that affects how the results should be read.

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, and at two hours each way travel is part of the price. The way to get value is to cover the whole area in one visit and to combine LWD with any plate bearing, CBR or sampling you need. Send the postcode, the area to be tested and a firm ready date and we will quote.

No. Hixtra has one laboratory, in Milton Keynes, and every Bristol visit is run from there. The 0117 number rings through to the same team as our other numbers, and we keep no staff or depot in the city. Book with a firm ready date so the trip is not wasted.