Why is there a shift from density to modulus based testing?

Nov 8, 2018 | Asphalt Pavement Investigation, Compaction Control, Intelligent Compaction, Rail Formation & Ballast Condition Assessment

Ammann soils intelligent compaction TX

Have you noticed an increase in the number of questions relating to modulus based testing for soils? We certainly have.

We are now wondering how soon this trend will gather pace in Australia and New Zealand for asphalt. It’s already well underway in Europe and the USA.

Current earthworks specifications often rely on the assumption that there is a direct correlation between density and modulus (i.e. the greater the density achieved, the higher the modulus of the compacted material). As a result, existing specifications often require that either the sand replacement test or Nuclear Density Gauge (NDG) test be conducted to demonstrate that adequate density was achieved within the earthwork layers. However, the assumptions used when converting density to modulus have been shown to be highly idealised, and they can be affected by the properties of the compacted fill, subgrade or base material. More importantly, testing has shown that a higher density does not necessarily indicate a higher strength or modulus (Mooney et al. 2003, Mooney et al. 2010).

Issues with a reliance on density testing and CBR results for QA purposes include:

  • Lag indicators – Several days / one week typical to complete. Contractor typically continues work and advances fill placement above the lift – before QA results are available. If non-conforming QA test results are then made available, there are significant costs associated with removing and replacing both the non-conforming material and the overlying material that has been placed whilst the contractor was waiting for the results.
  • Density Oversize correction – This applies when greater than 20% of material exceeds 19 mm or 38 mm for Mould A and B size, respectively. This is not consistently being applied across the industry, with a recent study showing that 22% of 235 samples examined not applying that correction (FSG Geotechnics).
  • Strength and modulus parameters – geotechnical and pavement designs are based on strength and modulus values. It is assumed during the design stage that a relationship exists between density and strength/modulus even though density is neither a strength nor a modulus parameter. The simple correlation between CBR and modulus (E) (e.g. E = 10 x CBR) often used in design is generic and there is a significant correlation error associated with its use.
  • The CBR test is not applicable when more than 20% of the material is retained on the 19 mm sieve. Such material is often discarded as part of the test according to the Australian Standards. Differences in material preparation Road Authority Standards would result in different CBR test values being determined and reported.

For more details on these issues, check out the latest reearch.

To conclude, modulus is the most accurate and independent means for judging deformation (stiffness) and, thus, a materials level of compaction. Modern geotechnical and pavement designs are based on in-situ modulus values.

However, in Australia, the current state of practice is to base the field-testing parameter selection on the result of a non-deformation parameter, such as DCP or CBR results from site investigation or density/moisture relationships in earthworks QA.

This means that there is currently a disconnect between design and construction quality control.

How to Bridge the Gap

Clients are looking to eliminate disconnects between the design specification parameters and the as built construction assessment parameters, leading to improved pavement performance. Importantly, performance based specifications are driving this.

Rollers fitted with Intelligent Compaction technology allows for the direct linkage between pavement design parameters (modulus), compaction (stiffness / modulus indication) and testing (field and laboratory). Intelligent Compaction is used for granular soils, cohesive soils, aggregate bases and subbase and asphalt materials and is a major aid to attain high compaction quality and uniformity, which in turn ensure long-lasting pavement performance.

Caterpillar Intelligent Compaction Perth

The Plate Load Test enables you to measure the in-situ static modulus. By measuring the modulus value, the Plate Load Test provides the direct link between the design specification (design modulus value) and the actual site condition (in-situ modulus value).

Plate Load Test Anix AX01 Wind Farm Ev2 Strain Modulus Modulus of Subgrade reaction K

This test is complemented by the Light Weight Deflectometer, another modulus based testing device, which is finding support and gaining popularity. The Light Weight Deflectometer (LWD) is used as the tool to calibrate the outputs from the Intelligent Compaction systems.

Light Weight Deflectometer Zorn & Intelligent Compaction

The LWD enables one to:

(a) reliably provide a direct measure of the strength or insitu modulus value; and

(b) offers significant time savings in turnaround time of QA test results.

Taken together, the design, indication of level of compaction from the rollers and the testing regime form an integrated eco-system.

We have found the PANDA to be an invaluable acquisition to our company, using it on projects ranging from Remote Community Housing in Central Australia to a NASA Rocket Launch Facility in Arnhem Land, NT.

Stephen Flux

Director / Principal Geotechnical Engineer - WANT Geotechnics

The ZFG 3000 is an excellent tool and should be included in any professional contractors equipment list of assets. This tool should have a similar impact on the construction industry as the laser level. The ability to accurately and reliably measure this aspect of ground condition will improve the quality of the work and remove doubt as to the progress of the project.

I bought my ZFG 3000 and on the same day less than two hours later, it had paid for itself. Using this instrument, I was able to demonstrate scientifically, repeatably and independently the quality of the work that we had completed for our client. Ending the disagreement and creating an improved working relationship. Most jobs are not as confrontational as this was however, most jobs are built to a standard. The contractor can now measure as required, to know on site, at the time of testing if this standard is being achieved.

I strongly recommend the ZFG 3000 Deflectometer to any earthmoving, civil construction contractor. Good luck with your projects.

Jim Bassett

Managing Director, Bassearth Pty Ltd

After scouring the market, we settled on the PANDA DCP test equipment as it offered the technical capability and, moreover, the flexibility required to work in the restricted environment our project presented; overhead clearance typically at 1500mm but as low as 1200mm.  We had a unique problem that required a unique solution and that’s precisely what we found through the team at Insitutek and the PANDA equipment.  The ease of assembly, intuitive operability and compact nature of the equipment, made light of the numerous soundings we required across a widely dispersed area.

Bjorn Schultz

Project Manager, Solar Farm, Australia

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