Two test sections were constructed in Cell 1 to study the use of QB to fill the voids of large primary crusher run aggregates for increased stability and lower settlement potential. Such rockfill
applications of PCR aggregates are common on top of very soft subgrades (e.g. CBR = 1%). They are built in thick lifts to act as construction working platforms on top of which the pavement sections are constructed. However, these open-graded aggregate layers contain large voids and are prone to variable settlement due to the high porosity. The goal of the constructed test sections was to study the incorporation of QB1 into the voids between PCR aggregates to provide increased stability. They
were placed by adding the QB from the surface in one single 21 in. (530 mm) lift or in two 10.5 in. (265 mm) lifts and with the help of vibratory action from a vibratory compactor.
From the UIUC packing box study of PCR/QB1 discussed in Chapter 3, the target QB1 quantity to be added was 25% by weight of the large rocks. This recommendation was based on single 10.5 in. (265 mm) lift compacted on top of the subgrade with CBR = 1%, and with a QB1 moisture content of 2.5%. Similarly in the field, representative samples of QB1 were first collected and tested for their field
moisture contents, and the average moisture content was found to be 3.2%. The field moisture content was even higher than the moisture content obtained during the laboratory tests.
During the construction of the first lift in the two-lift test section (C1S1), the large rocks were placed first and then the full amount of QB1 (25% QB materials by the dry weight of the PCR lift) were evenly distributed on the surface using a skid-steer loader. A vibratory compactor was used to shake the QB into the inherent voids in the PCR skeleton. This resulted in the formation of a densely-packed layer of QB on the surface, which slowed down QB percolation, and prevented any further filling of the voids in the PCR skeleton. This is assumed to be a result of the compactive effort, which was
considerably higher than that applied in the laboratory box study. To overcome this issue in the first constructed layer in the two-lift test section, the QB were uniformly intermixed with the large rocks using the teeth of an excavator bucket (See Figure 4.8).
To prevent the formation of a densely-packed thin QB lift on the surface of subsequent lifts, the QB were slowly and incrementally spread using shovels on the surface of the large rocks, in smaller increments. The construction procedure is illustrated in Figure 4.8. The QB were then vibrated into the voids of the PCR using a larger (and heavier) standard size vibratory roller, and the process was repeated several times until the full quantity of QB was added (25% by dry weight of the PCR).
However, for section C1S2 constructed in a single lift, the maximum possible amount of QB that could be packed was 16.7% by the dry weight of the PCR using this approach described. A possible reason for this can be the higher moisture content of the QB used in the field construction or the more challenging single-lift percolation goal, which might have prevented the QB from percolating the full depth of the 21 in. (530 mm) single-lift layer.
The successful construction demonstrated when the QB was added in smaller increments, followed by a vibratory action, suggests that the future field practice of vibrating QB from the surface into the inherent voids of the PCR skeleton may require developing an automated technique to spread the QB uniformly and more slowly on the surface, accompanied with continuous and strong vibration. The construction for the PCR/QB1 test sections for Cell 1 were done simultaneously for Cell 1N and Cell 1S (i.e. both paved sections and construction working platform sections). Following the
construction of the PCR/QB1 aggregate layers, these sections were capped with a 3 in. (76 mm) dense-graded CA06_R, mixed and compacted at the optimum moisture content. The capping layers were constructed last for all Cell 1 test sections at the same time. Nuclear gauge dry densities measured with a back-scatter technique for the capping layer ranged between 126.0–138.0 pcf (19.8–21.7 kN/m3), which translates to a satisfactory 97.8%–107.1% relative field compaction.
Construction of the first lift in the two-lift PCR/QB1 test section (C1S1)
QB1 placed on top of PCR (before compaction) PCR/QB1 blends (after compaction)
Uniform spreading and incremental placement
of QB1 on top of PCR lift Final compacted surface of C1S1
Figure 4.8. Construction of PCR/QB1 sections in Cell 1.
To evaluate the quality of the constructed test sections, the stiffness of the PCR/QB1 test sections were measured using a Light Weight Deflectometer (LWD). A minimum of nine LWD drops, including three seating drops, were carried out at the two measuring points in each section; both before and after the placement of the capping layer. To ensure the uniformity of the surface, a layer of sand was placed at the locations of LWD drops. This was especially necessary on top of the PCR/QB1 layers, where the surface elevation could vary widely. The results from LWD testing on top of the measuring points for the construction platform and low volume roads test sections (i.e. Cell 1S and Cell 1N, respectively) are presented in Figure 4.9. All test sections indicated a significant increase in stiffness upon the addition of the capping layer. Before the placement of the capping layer, the sections constructed in two lifts had lower moduli compared to the sections constructed in one lift. C1S1 constructed in two lifts in Cell 1N had the lowest back-calculated modulus. As it will be discussed later
in this report, trenching results for Cell 1N indicate that this lowest modulus is largely due to the lower layer thickness for the second measuring point in this section, which resulted in a noticeably lower average modulus for this section.
(a) (b)
Figure 4.9. Average composite surface moduli measured by LWD for C1S1 and C1S2 for (a) Cell 1S construction platform and (b) Cell 1N low volume road sections.