Glossary & FAQ


LWD Light Weight Deflectometer (ZFG 3000) Glossary & FAQ

What is Evd?

Evd is the dynamic deformation modulus (also known as the dynamic deformation module, dynamic modulus of deformation, dynamic stiffness modulus, composite stiffness, effective stiffness or surface modulus). Modulus is the most accurate and independent means for judging deformation and, thus, a material’s level of compaction. Evd is used to measure bearing capacity and compaction quality for subsoil’s and subgrade materials, unbound base layers, granular layers, backfilling materials, soil stabilisation with lime, cold recycling materials and pavements, cycle tracks and footpaths.

The soil receives an impact of maximum force Fs transmitted through the fall of the drop weight onto a circular plate of radius r, which is assumed to be rigid. When the device is calibrated, the force is selected such that the maximum normal stress σ under the load plate is 0.1MN/m2.

To calculate Evd, the deformability of the soil under a vertical load, as described above, with a settlement amplitude s (in mm) and the duration of impact ts:

Evd (MN/m2) = 1.5 x r x σ /s

For the 10kg drop weight and 300mm diameter plate Evd = 22.5/s
For the 15kg drop weight and 300mm diameter plate Evd = 33.75/s

What is the relationship between Evd and Ev2?

The relationship between the static deflection modulus Ev2, measured by the static plate bearing test, and the dynamic deflection modulus Evd depends on the kind of soil and the degree of compaction.

Experience shows that the ratio Ev2/Evd lies in the range 1.0 to 4.

For densely compacted soils Ev2/Evd ~ 2.3

On average the following relation between the dynamic deflection modulus Evd, and the static deflection modulus Ev2, can be used:

This relation does not hold for limiting values.

What is s/v and why is it important?

s/v is a measure of the degree of compaction of soil. The ZFG 3000 has an acceleration sensor attached to the base plate. The first integration of acceleration shows the velocity of the plate. And the second integration gives the deflection.

  • If the value of s/v is lower than 3.5 ms means: soil is compacted and it is possible to compact to higher compaction.
  • If the value s/v is higher than 3.5 ms: incompact like in-situ soil or if after compaction, it is not possible to compact to higher values.

The value of 3.5 ms is an empirical value, resulting from practical experience. In contrast to the static load plate, you get more information on the properties of the soil from dynamic load plate test.

What is the maximum grain size that can be tested?

The test procedure is suited, in particular, for coarse-grained and mixed-grained soils wth a maximum grain size of 63mm. For aggregate, bedding sand is used to ensure complete contact between the base plate and the surface being measured.

A “wiggling” sign was observed in the tip of the curves. What does this mean?

The wiggling shows that the material is VERY hard and the wiggling is like a vibration of the plate because of the hardness of the material (like a hammer hitting a piece of steel when you can feel the vibration).

In the “Statistic Dyn” spreadsheet, what does the value Q represents?

This is based on the required minimum you set (Required minimum quantile:) and is a calculated value Q(Evd) = (Arithmetic average of spot-check Xm(Evd) – Required minimum quantile) / Standard deviation s (Evd)

The test (Q>0,88) has failed. This comment identifies whether the test is within acceptable parameters compared with what you have specified (e.g. 25 MN/m²) or not. Again, it’s a calculated value.

How do stiffness values correlate with density?

Typically, stiffness values measured with the Light Weight Deflectometer (LWD) correlate poorly with density. Fundamentally, soil density and stiffness are two very different physical properties.

Density is a static behavior depending on grain density, grain size distribution etc. Density is of a measure of volume.

Stiffness is similar to bearing capacity or deformation modulus and is a measure over a given area.

Further, to meaningfully compare measurement methods, one must ensure the compacted area is homogeneous. This means that at every measuring point, the conditions must be the same material properties, moisture, grain size, thickness of layers, materials etc.

What is the relationship between stiffness and field moisture content?

Typically, there is  close relationship between the stiffness values measured with the Light Weight Deflectometer (LWD) and the field moisture content. The influence of moisture is much higher because of the capillary effect of water so the results are sensitive to insitu field moisture.

The optimum moisture from the Proctor Test will yield the best results, as one would expect. Every sand and gravel mixture has an optimum water content depending on grain size distribution.


Light Weight Deflectometer also known by a large number of different names that include:

  • Deflection survey
  • Deflection testing
  • Deflection test method
  • Drop weight tester
  • Dynamic Light Drop Weight Tester
  • dynamic plate test
  • dynamic load plate test
  • dynamic plate load test
  • Falling Weight Deflectometer
  • FWD
  • FWD-Light
  • German dynamic plate
  • Impulse deflection testing
  • Light Falling Weight Deflectometer
  • Light FWD
  • Light Weight Deflectometer
  • Light Drop Weight
  • Light Drop Tester
  • Light Drop Weight Tester
  • LDWT
  • Light Weight Hammer Drop Tester
  • Light Weight Deflectometer
  • LWD
  • Load deflection test
  • Load plate test
  • Portable Falling Weight Deflectometer
  • Portable FWD
  • PFWD
  • Portable Light Weight Deflectometer
  • Portable LWD
  • PLWD
  • ZFG2000
  • ZFG 2000
  • ZFG02
  • ZFG 02
  • ZFG3000
  • ZFG 3000
  • ZFG 3000 GPS
  • ZFG 3000 ECO
  • ZFG3000 GPS
  • ZFG3000 ECO
  • Zorn
Evd dynamic modulus Light Weight Deflectometer LWD Schematic Zorn ZFG 3000