Even the best designed, best constructed, and best preserved roadway has a discrete service life and can reach a point of no return where we may be forced to dig it out and start over.  More realistically, many local agency roads, particularly rural ones, were not designed at all, may have sparse subbase, and may be on weak subgrade or suffer from poor drainage.  When the subbase begins to fail, the structural framework of the roadway follows and a familiar pattern of patches within patches begins to emerge.  When the area and extent of structural distresses in the pavement require full-depth patching of more than about 15% of the road section area, the economics of continued patching can quickly decline.

The traditional response was to excavate the poor subbase materials down to suitable subgrade, haul them away, and bring in virgin materials for a new roadway section.  Loaded haul trucks taking out bad material and bringing in virgin aggregate beats up the other local roads and contributes to unnecessary fugitive emissions and expense.  Full depth reclamation (FDR) offers an attractive alternative that can add 15-25 years of service life.  We recently visited with some FDR projects and built videos for our YouTube channel that can help you understand the steps involved in full depth reclamation.

So how does FDR work?  There are variants for sure, but the common elements begin with pulverization of the wearing course and some or all of the subbase (even some of the subgrade at times) to reduce the largest aggregate size below a certain size and homogenize the mix.  Often, but not always, stabilizing agents and additives are then mixed in, and the material is regraded and compacted.  Usually, but again not always, a surface treatment like chip seal or asphalt is added as a wearing course.

Sometimes, just re-homogenizing the material to a 6-12” depth will restore the base materials to some of their former glory.  More often, mechanical and/or chemical stabilization is used to “kick it up a notch.”  Knowing what to do with your specific road is the first key to optimal FDR economics and an experienced geotechnical engineer is your first stop.  They will core the existing road cross section at numerous locations to measure the thickness of asphalt and subbase materials and characterize the type and condition of the subgrade material.  For example, they may find 2½” asphalt over 7” of aggregate, with a lean clay subgrade.

The geotechnical engineer will estimate the equivalent single axle loads (ESALs) for a pavement design life of, say, 20 years.  For a rural residential road, the value might be about 20,000 ESALs and imply a required California Bearing Ratio (CBR) of greater than 100%.  Depending upon the characteristics of the existing in-situ materials and their thicknesses, the design structural strength may be achievable with mixing of the asphalt and subbase materials, followed by some wearing course.

More commonly, however, mechanical stabilization (addition of new aggregate or recycled materials) and/or chemical stabilization (cement, lime, fly ash, etc.) are necessary to support the load cycles the road is expected to experience during the design life.  The geotechnical engineer may then experiment with some of the soil samples, adding some additional aggregate, some different size aggregate, cements, lime, or other combinations to test in various ways and determine the best balance for the project.

The geotechnical engineer will usually provide a report that outlines the finding of the core samples, design targets for the rehabilitated roadway, and whatever specifications that should be made (e.g., depth of pulverization, application rate of cement or other materials, target compressive strength for payment, etc.).


A contractor with appropriate (and well-maintained) equipment and experience is then selected.  After establishing temporary traffic control managed with certified flaggers, the contractor will begin by pulverizing the wearing surface and subbase to the required depth, which can be 9-12” or even deeper (to some maximum size material, such as 2½”).  Typically, a large machine known as a reclaimer (think of a ridiculously oversized rototiller) is used to pulverize the material.  A target distribution will often require the bulk of the material to be closer to ¾”-1”.  The business end of a reclaimer is shown here and you can see how it could “put a serious hurtin’” on in-situ materials.  Prior to any stabilizing aggregate or chemicals, the pulverized mix is usually wetted.

Recycled asphalt pavement (RAP) or crushed aggregate may be added at this time, if material stabilization is required.  For example, if the cores of the in-situ material show an abundance of sand and other small aggregate, the design may require the addition of some slightly larger aggregate.

Also at this time, chemicals (if used) are spread over the pulverized mix.  Commonly, this would be in the form of hydraulic cement, but some designs use lime, fly ash, kiln dust, and calcium or magnesium chloride.  Even bituminous materials (e.g., emulsified asphalt or foamed asphalt) is sometimes used.  Hydraulic cement (and other chemical additives) is usually applied as a dry powder through a metered application head, although they can also be added in a slurry form.

Still other variants on the process would have the materials placed into a paver (perhaps with a milling machine) and placed in a mat.

Regardless, it is important that the required material application rates be achieved (commonly in the range of 40-90 #/SY for Portland cement) and that the stabilizing materials are then fully homogenized with the in-situ materials, again using several passes of the reclaimer.  A padfoot roller (aka, sheepsfoot roller) is typically used at this point to knead the materials together and achieve the bulk of compaction.  Vibratory mode may be used to enhance compaction, depending upon the characteristics of the in-situ soils.


A motor grader will then establish the surface profile and a smooth drum vibratory roller will complete compaction and seal off the surface until (normally) a wearing surface is applied.  If hydraulic materials, such as Portland cement, were added, the roadway is normally wetted at least once per day with a water truck to assist in the cure of the materials.

With a little bit of experience and well maintained equipment, the process is otherwise a bit forgiving, as long as the material is pulverized to the correct distribution and well mixed, the stabilizing agents are applied uniformly and at the correct rate, the full cross section is mixed to a homogenous consistency, the materials are well compacted and properly graded, and water (if required) is generously applied during the cure period.

There are some excellent resources where you can learn more about FDR.  For some overview, see the Pavement Preservation & Recycling Alliance (PPRA) Treatment Toolbox we introduced to you late last year. It is available as a free, online resource for your pavement preservation planning and in their Treatment Resource Center is some in-depth guidance on full depth reclamation (FDR) for pavements.

The Portland Cement Association (PCA) also has dedicated resources for FDR, including a webinar on the topic, fact sheets, case histories, and an excellent Guide to Full-Depth Reclamation with Cement.

Hopefully, the bulk of your jurisdictional roads have strong cross sections for the traffic they experience and you keep the surface wearing course in good condition through a variety of pavement preservation techniques (e.g., crack seal, fog seal, microsurfacing, slurry seal, etc.) to ward off the need for milling and paving, but if you do find that a road has just laid down and died, consider full depth reclamation as an alternative that can provide a suitable road base for, literally, decades.

The Delaware T2/LTAP Center’s Municipal Engineering Circuit Rider is intended to provide technical assistance and training to local agencies and so if you have pavement management questions or other transportation issues, contact Matt Carter at matheu@udel.edu or (302) 831-7236.

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