How it Works

Principles of Measuring Compaction

For a highway to perform well over the long term, its soil and aggregate layers need to provide a stiff, stable foundation. Inspections are required during construction to ensure that pavement foundation materials have been compacted enough to ensure this condition. Light Weight Deflectometer’s (LWD’s) are used to determine the stiffness of unbound materials (subgrade/subsoils and base layers, granular layers & backfilling materials) during construction. The device measures a deflection and estimates a modulus value (Evd) based on the force required to generate a given deflection for that soil type. Modulus is the most accurate and independent means for judging deformation (stiffness) and, thus, a material’s level of compaction. By measuring the modulus value, the Light Weight Deflectometer (LWD) provides the direct link between the design specification (modulus value) and the actual site condition (modulus value).

Stiffness is the relationship between stress and strain in the elastic range or, in layman’s terms, how well a material is able to return to its original shape and size after being stressed. In general, the more resistant to deformation a subgrade is, the more load it can support before reaching a critical deformation value.  Three basic subgrade stiffness/strength characterizations are California Bearing Ratio (CBR), Resistance Value (R-value) and elastic (resilient) modulus. Resilient Modulus (Mr) enjoys widespread use in pavement design.

Load bearing capacity. The subgrade must be able to support loads transmitted from the pavement structure. This load bearing capacity is often affected by the degree of compaction, moisture content, and soil type. A subgrade that can support a high amount of loading without excessive deformation is considered good.

Sub-grade materials are typically characterized by (1) their resistance to deformation under load, in other words, their stiffness or (2) their bearing capacity, in other words, their strength. In general, the more resistant to deformation a subgrade is, the more load it can support before reaching a critical deformation value.

How the Light Weight Deflectometer Equipment Works Technically

A mechanical impact on a circular steel plate produces a deformation on the soil surface. A light weight is dropped from a standard height and, after release, moves down a rod to a dumping spring that transfers the force to the centre of the load plate.

The load plate contains an accelerometer that measures movement and sends a signal to the control unit. The first integration of acceleration shows the velocity of the plate. And the second integration gives the deflection. The deflection and the velocity of the plate are calculated by double integration of the acceleration. The dynamic deformation modulus is calculated from these results provided that all further parameters like the contact pressure are constant. This simple approximation leads to stable results.

Using the 10kg falling weight, the Light Weight Deflectometer (LWD) measures in the range of 15 -70 MN / m². Using the 15kg falling weight (1.5 times the impact load), the measuring range extends from 70-105 MN / m².

The measuring procedure starts with three pre-loading drops for good contact with the ground. Then, three measuring drops are executed for registration and calculation of the average value and dynamic deformation modulus Evd. The results are stored on a SD-card or printed out directly. With the aid of a SD-card, the data can transferred to a PC.

The print out results are presented in this format:1. Sinking in speed 2. s/v: to evaluate compression 3. Deflection curves 4. Date and time of test 5. Deflections 6. Mean values 7. Evd: dynamic deformation modulus

Thinking behind the LWD Equipment Design

The principle of the Light Weight Deflectometer ZFG 3000 and ZFG 3.0 is based on the principle of a truck loaded with 10 Tonnes driving at 80km / hour and passing over 1 square metre. This is a typical dynamic process and the ZFG 3000 simulates these conditions. Under the rear double tyre, you have a pressure of approx 0.1MN/m2. When the 10kg weight drops on the 300mm diameter plate, you exert the same pressure of 0.1MN/m2 under the plate. (Plate diameter: 300 mm, area 0.07068 m² that means the falling weight must produce a force of 7.068 kN to get 0.1 MN/m²). For the 15kg falling weight, a force of 10.6 kN results in a pressure of 0.15 MN/m² under the plate. The time of this test is given by the spring under the falling weight.

• 0.1MN/m2 = 10.2 Tonnes over 1 m2

If the test is done on a chip sealed pavement, the chip seal acts like an elastic band, as the binder holds the material together. Hence the results are not as direct / accurate as if the test was done on top of the sub-grade.