Evd value | ZORN ZFG Light Weight Deflectometer (LWD)

Soil compaction using compaction equipment such as rollers, vibratory plates or vibratory rammers serves to improve the subsoil, as does soil consolidation or soil replacement. By reducing the proportion of air voids in the soil as well as by mechanically interlocking the rock particles, the aim is to reduce subsequent soil settlement and thus improve the stability of structures. Today, soil compaction is a standard procedure in the construction of both buildings and infrastructure objects like roads, railway lines and trenches.

The degree of compaction is an established criterion for assessing the quality of compaction work. As a rule, the term refers to the ratio of the dry density of a soil measured on a construction site to the assumed maximum achievable dry density for this soil. This maximum is usually determined before the start of construction work in specialized soil testing laboratories using the so-called Proctor test. It is referred to as Proctor density (D_pr). The degree of compaction is given as a percentage (%).

The Proctor density (D_pr) is the highest density of a soil that can be achieved with a defined compaction work at an optimum moisture content. It is determined in a standardized laboratory test.

A soil sample from the construction site is installed in a round metal cylinder, the so-called Proctor mold, with a prescribed number of compaction-blows using a special pressure hammer. The sample is then weighed and its density determined. This process is carried out at least five times with different water contents in the soil sample.
If the determined density values are plotted in a diagram against the respective water content, a curve is obtained from which the maximum density achieved and the corresponding water content can be read. The maximum density of this curve is referred to as the Proctor density and the associated water content indicates the assumed optimum for compaction.

The test therefore also illustrates the relationship between the water content of a soil and its compactability.

The test and its measurements are named after US engineer Ralph Roscoe Proctor, who was instrumental in the development of the test in the 1930s. Today, the test exists in various versions and is standardized in many countries. The most important national standard is the North American ASTM D698.

IMPORTANT! It is important to point out a common misconception. Proctor Density is often mistaken for the actual maximum density of a soil achievable through compaction. This interpretation is incorrect.

The term 'maximum achievable density of a soil with optimum water content at a defined compaction effort' refers to the energy introduced into a small sample by the compaction effort in the laboratory test. However, the energy introduced in the laboratory test is considerably less than the energy achieved by the compaction work of modern compaction equipment. As a result, the degree of compaction that can be achieved on site today is regularly in excess of 100% Proctor Density.

This background is important, among other things, for understanding the empirical relationship between the degree of compaction and the deformation modulus of a compacted soil as determined by plate load testing (static and dynamic).

The deformation modulus of pavement materials such as soil and aggregate, but also asphalt, is based on a material property known from classical physics as elasticity. Under load, a solid body can absorb forces within a certain range without permanently deforming. The body therefore returns to its original shape when the load is removed.

The deformation modulus is therefore ideal for assessing the load bearing capacity and durability of pavements with their varying loads from vehicles of all types. Put simply, the higher the value of the modulus of deformation, the better a road or track will be able to withstand the load from vehicles travelling over it. This applies to all pavement layers, i.e. the natural soil as the lowest layer, the soil and aggregates placed above it as substructure and base layers, and the surface layer on top.

Regular laboratory and field test methods are now available to determine the different deformation moduli depending on the material and layer. Which method is used varies regionally and nationally. This depends on the building materials available, the state of the technical equipment, but also simply on certain traditions. The unit of measurement for the different moduli is always MPa or MN/m².

ZORN INSTRUMENT is the world's leading supplier of equipment for the dynamic plate load test method and for determining the dynamic deformation modulus E_vd.

The static plate load test (also known as the load bearing test) is an in-situ test to determine the load-bearing capacity of soils and aggregate.

The layer to be tested is repeatedly loaded and unloaded in stages by a standardized load plate (typically with a 150 mm radius) using a pressure device. The required stress of up to 0.5 MPa is usually applied with a hand pump using a hydraulic cylinder under a lorry or other heavy construction vehicle as a counterweight. The mean normal stresses under the load plate and the corresponding settlements at the individual load stages are the test variables. Most of the time, two passes are carried out. The load plate is loaded comparatively slowly and the entire test procedure takes up to 30 minutes. Hence the term ‘static’ plate load test.

In the course of the test evaluation, the stress and settlement values are displayed in a corresponding graph as a pressure-settlement curve, from which the deformation modulus E_v and the subgrade reaction modulus ks can be determined.

The value E_v2, i.e. the deformation modulus for the second load cycle, is decisive for assessing the compaction quality. Secondly, the ratio of the values E_v2 : E_v1 is also important.

Calculation of the static deformation modulus as follows:
E_v = 0.75 * (slab diameter D in mm) * (difference in normal stress transferred from the slab to the ground between the two load stages)

The dynamic plate load test is a rapid in-situ test method in which a soil or aggregate layer is subjected to an impact load of the maximum force F_max produced by a drop weight on a circular load plate. This plate has a defined radius r and is assumed to be rigid. The standard version of the Light Weight Deflectometer (aka Light Drop-Weight Tester) used for this purpose has a load plate with a radius of 150 mm and a drop weight of 10 kg. The instrument is calibrated so that the normal stress σ_max under the load plate is 0.1 MN/m².

There is an acceleration sensor on the load plate which records the movement, i.e. the penetration of the load plate into the layer being tested. The duration of the measurement process is about 17 ms, which is comparatively short. Hence the term 'dynamic' plate load test.

The load plate displacement is calculated from the acceleration signal using the rules of mechanics with double integration. From this value, the dynamic deformation modulus E_vd is then calculated, taking into account other soil mechanical parameters.

Calculation of the dynamic deformation modulus as follows:

E_vd     = stiffness modulus (MPa)
f         = plate rigidity factor (use 2 as the default)
σ        = maximum contact stress (kPa)
r         = plate radius (m)
µ        = Poisson’s ratio (use 0.5 as the default)
s         = maximum deflection (mm)

Both the degree of compaction D_pr and the deformation moduli E_v1/E_v2 and E_vd are nowadays used in parallel as criteria for assessing the quality of compaction work. In practice, the comparability of static and dynamic plate load test results and the degree of compaction for a particular site is therefore a common issue. For example, a certain E_v2 value may be specified as the target for compaction, but the contractor may want to test with a Light Weight Deflectometer.

In fact, no mathematical correlation has been established yet between the characteristic values E_v1/E_v2, Evd and D_pr.

However, empirical correlation values do exist for various soil classes. These can be found in technical guidelines such as the German ZTV E-StB (Additional Technical Contract Conditions and Guidelines for Earthworks in Road Construction). For a greater degree of safety, it is advisable to create a test area on site to establish the relationship between the E_v2 and the E_vd. In some cases, this may even be mandatory. Information on this can then be found in the contract documents or site specification.

To establish the relationship between the degree of compaction and the dynamic plate load test, a new laboratory test method has recently become available in accordance with ASTM E3331-22. For this test there are deflection measurements made directly on the Proctor test specimens using a modified Light Weight Deflectometer device. The measured deflections are then compared with the Proctor test values. This allows target values to be set for subsequent dynamic plate load testing on site.