5 Things to Know About the Light-Weight Deflectometer (LWD)

1. What it is

A Light-Weight Deflectometer (LWD) is a fantastic tool for testing the prepared formation of your pitch or a stone foundation which supports the turf system of your pitch. It works equally well for the assessment of the base of an athletic track or court.

Where an engineer utilises soils testing protocols such as CBRs, Plate Bearing Tests or Cone Penetration test methods to test soil or the sub-base of a sports facility foundation, the LWD tool replaces expensive and time consuming tests with a quick assessment tool for in-situ use. An LWD is a portable device for measuring the Surface Modulus of in-situ material. The measurement made is a ‘composite value' with contributions from all underlying layers - meaning it takes into account all the layers underlying the device plate which is in contact with the ground. The Deflection measured through the velocity transducer and applied stress through the load cell allows a Surface Modulus to be calculated.

The falling mass, which is normally dropped from variable heights, gives a resultant 5 - 120 kPa stress when set up in conventional format and generates a load pulse duration of 15 - 25ms. This is what stresses the materials and allows stiffness to be measured. The device under use can be seen in photographs provided.

2. How it works
The LWD uses a completely different approach to measuring the bearing capacity, compaction, and/or deformation of a prepared surface. Using an anvil (10Kg) weight dropped from a height onto a set of buffers and load cell, the energy pulse emitted is transferred through a base plate and onto the surface under testing. This pulse of energy stresses materials by effectively squashing them and the response to this stress is measured by a geophone. The stress dependency curves are interpreted to provide an indication of compaction, deformation and bearing capacity.

3. What it tells us
The construction of a properly performing pitch foundation requires the constituent materials to have a sufficient stiffness and resistance to deformation, so it is essential that base materials are adequately compacted at their optimum moisture content to perform appropriately under load. The in-situ measurement of stiffness and state of compaction are measured using an LWD.

An LWD device can be used in several modes, but the most common is the mode which generates a stiffness value. A target value is normally specified for the materials under test, then in-situ materials are measured against the target. This is a great method for controlling the quality of base installations. An LWD device can also be used to assess deformation from the curves which are generated by the software. Contractors often use an LWD for compaction control of materials. The versatility and speed of an LWD is one of its strong points.

4. It can improve base constructions for artificial sports surfaces

Sports Labs adopted a protocol which is based on Key Stage Inspections (KSI), which is one approach for improving base construction for an artificial sports surface. You can find out more about the process in this article

Key Stage Inspections: The Stealthy Way to Ensure Quality

5. More information is available

If you're interested in the subject, there are further reference documents available. Here are two:

(a) Research Paper; Fleming, Frost, and Lambert
A review of the lightweight deflectometer (LWD) for routine in-situ assessment  of pavement material stiffness.

(b) BRITISH STANDARD BS 1924 update

Hydraulically bound and stabilised materials for civil engineering purposes draft for consultation.