CCPR with Subgrade Stabilization

Given that the subgrade is rather weak, it is worthwhile to consider CCPR with subgrade stabilization. It is assumed that there is available site nearby where an CCPR plant can be set up. Since the percent passing the #200 is less than 15%, and the PI is less than 6, CCPR-FA or CCPR-EA are feasible recycling strategies.

For the subgrade, it is assumed that the PI is greater than 20 since the detailed USCS test results were not available for this analysis. For actual designs please obtain the actual test results and design accordingly.

Following the design guide, with a PI greater than 20, it is recommended to use a combination of lime and cement, first to modify the subgrade, then to stabilize the subgrade to provide a cement stabilized soil (CSS) layer with a target design strength of 300 psi.

The proposed design is shown in the figure below. In this example, the goal is to design the thickness of the HMA layer. Note that the CCPR layer is assumed to be 1.1 ft, which is the combined thickness of the existing HMA and AB multiply by a swell factor of 1.07. A minimum of 0.5 ft AB is typically required over the CSS layer to prevent shrinkage cracks in the CSS layer from reflecting to the upper layer.  In this case the CCPR layer is believed to be able to stop the reflection of shrinkage cracks so no new AB is needed above the CSS layer.

To evaluate this option, save a copy of the baseline trial for Section 3 as “Section 3: CCPR and CSS”. After removing the existing HMA and AB layer, and adding the new layers, the project inputs are shown below:

Several iterations were run to determine the thicknesses the intermediate HMA layers to carry the design traffic for the design life. The results are provided in Table 6. The results show that the minimum required thickness of the intermediate HMA layer is 0.20 ft. The dominant failure in this design has changed from rutting in other alternatives to fatigue cracking in this case.

Table 6: CCPR and Subgrade Stabilization CalME Results

                                                  *: shaded row indicates the optimal design