Intelligent compaction update

27/10/2009

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The traditional role of a soil compactor is to help create a stable base that will support a construction project.  However, the emergence and development of intelligent compaction technology may change that role.
Intelligent compactors feature advanced capabilities that may lend additional responsibilities to the traditional roles of soil compaction.  These additional roles will result in savings in time and rework expense, bolstering the importance of the compactor at the construction site. But how is this possible, that an ordinary soil compactor can suddenly become a value-driven tool at the modern jobsite?

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The materials utilised as the base course are of a specified soil type, grade and moisture content.  A soil compactor, when properly employed, will reduce or eliminate voids and compel the particles of the material to interlock with each other, creating a stable matrix that provides optimum load bearing strength. Understanding when this point has been achieved is difficult for the operator, even one with years of experience.

Conventionally, compactor operators roll an area back and forth, relying on their senses to provide an indication of compaction levels.  Some judge by appearance, some by the way the roller “feel” as it vibrates the compacted material.  Obviously, neither of these indicators can provide the kind of dependable feedback that helps the operator determine ideal and consistent compaction. This practice frequently results in the materials being over or under compacted.

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Parts of greater puzzle

Because stability of the base course is essential for the smoothness and longevity of the riding surface, site engineers have developed various means to judge the bearing strength of a prepared base.  Primarily, they utilize density gauges, and Proctor test.
       
While these tested provide some evidence of the stability of a material, they do not provide an adequate picture of the overall bearing strength of the material. Unfortunately, density is often mistakenly believed to be the same as bearing strength.  In fact, density is just an indicator – one of many other critical factors including soil type, gradation, moisture content, and level of confinement that affect the bearing strength.  The many factors that contribute to bearing strength make it unrealistic if not impossible to get an accurate estimate using density alone.
       
Density-testing devices and techniques also have several limitations that restrict their usefulness.  One limitation is that they are costly and time consuming to use.  This means that it is not practical to test a site with comprehensive, full-area coverage; an engineer can only test a few areas and then draw generalized conclusion regarding site-wide conditions based on the results.
       
Another limitation is that these tests do not measure very deeply into material, usually 305mm (12”) or less. A site that tests out a passing grade may hide a deficient soft area just below the maximum testing depth that could potentially weaken the new driving surface – to be found only after the pavement fails years later.
       
Much of the current intelligent compactor research and development is attempting to determine which indicator or combinations of indicators can provide the best evidence of material’s bearing strength. Presently, most intelligent compactor determine a measurement related to soil stiffness – the ability of a material to resist deflection – as an indicator of bearing strength. Soil stiffness is considered to be much better indicator of bearing strength than density.  However, moisture content and confinement also affect this measurement, so soil stiffness alone does not provide an absolute representation of load bearing strength.

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Looking for better way
       
The need for a better way to conduct on-site, in-place testing is well recognized, and intelligent compaction appears to have potential as a solution.
       
Soil compactors equipped with intelligent compaction systems have a number of advantages when it comes to providing in-place testing.  The large mass of an intelligent compactor does a better job of measuring deeper into the soil, as much as 1-2 m (3.2-6.4”) down.
       
This is important because a material can be very stiff on the top layer, but much less stiff at greater depths.  Like a plate of glass on top of a mattress, the stiff surface will perform well with lighter loads and record high densities with the small portable testing devices engineers use today.  But if a load is applied that is great enough to “shatter” that top layer, the whole wear surface will fracture, requiring costly repair and ruining the overall ride smoothness.
       
The intelligent compactor is equipped with a GPS mapping system so it can record these measurements and correlate them with a 3-dimentional job plan.  This provides a proof rolling record for every square foot of the project.  The ability to generate these proof rolls as the machine works also provides economy of time and alerts the operator to trouble spots that require immediate rework.
       
One of the most intriguing changes that could result from the development of intelligent compaction technology is the potential redefinition of the role of the soil compactor and its operator on the jobsite.  Traditionally, operating the soil compactor can be tedious, monotonous, dirty and noisy.  Often, the operator is a worker with minimal amount of experience or training, and therefore is responsible for little else besides compacting materials.

Intelligent compaction could change that.  In fact, it could turn a lowly “ground-pounder” into the quarterback of the entire earthmoving project simply by giving the machine the capability of mapping bearing strength indicators of the entire jobsite.  This would allow a compactor to assess the suitability of the job produced by the contractor (quality control, or “QC”), and provide the owner with documentation on a one-to-one testing basis (quality assurance, or “QA”).  The compactor operator could be responsible for identifying trouble spots that require rework, directing water trucks and making a final proofing pass – generally, controlling the pace of the job.
       
While the technology is still evolving and processes have not been fully developed regarding its use, current research by manufacturers and DOTs suggest that the value of intelligent compaction technology is quickly growing for both contractors and site engineers.  Recent tests conducted on actual jobsites have provided tantalizing insights into its future value.
       
For example, on soil compaction projects, CAT Vibratory Soil Compactors equipped with intelligent compaction technology have consistently been able to locate and map areas of poor compaction.  This allowed the contractors to rework the areas to meet specification before the driving surface was placed, eliminating more expensive rework in the future.  One operator with a minimal amount of training discovered that he could determine when the materials he was compacting required additional water simply by watching the display.
       
There are still many questions to be answered, but the future of intelligent compaction is promising.  If an intelligent soil compactor is accepted as the standard for in situ testing for a complete construction project, this ordinary machine may become the hub of jobsite activity and increase overall efficiency dramatically.