Civil Engineering January/February 2021 | Vol 29 No 1

36 January/February 2021 Civil Engineering the degradation of the subgrade and sub- base layers. A 25 kJ three-sided impact com- pactor was applied directly on top of the existing chip and spray road surface. Settlements were monitored as the number of passes increased. No significant increase in settlement was found after 15 to 20 passes, and it was decided to limit the number of passes to 20. Settlement of between 50 mm and 100 mm was measured. The construction process was thus changed to the following: 1. Levels were taken at 20 m intervals on the compacted surface on the centreline and at points 3 m and 4.5 m from the centreline. These were used to regrade the road such that approximately 50% of the existing base material could be stockpiled for use in the new subbase layer. Tests had indi- cated that a 50% blend of this material mixed with an imported G5 material adequately met the requirements for the subbase layer. 2. The subbase layer was constructed with the 50% blend which was chemi- cally stabilised to provide a C3 subbase layer. 3. A 150 mm G1 layer was constructed, followed by a 40 mm continuously graded asphalt layer. The use of impact compaction offered several benefits, including: Q Q Construction costs for the layerworks were reduced by approximately 40% Q Q Construction time for the earthworks was reduced by 35% to 40% Q Q Water usage was substantially reduced Q Q No additional effort was required to accommodate the inclusion of the increased road width (from 3.3 m to 4.7 m) Q Q Permeability was reduced by reducing the void ratio, thereby reducing mois- ture ingress into the outer wheel-track Q Q The road’s life expectancy was increased by a factor of between 10 and 50 Q Q The reduced variable strain signifi- cantly reduced long-term maintenance costs. Quality control in the form of Dynamic Cone Penetrometer (DCP) and plate load tests were conducted to monitor the compaction process. A typical DCP result is shown in Figure 1. Falling weight deflectometer (FWD) tests conducted before and after construc- tion indicated the following: FWD deflections Ave BLI Ave MLI Ave LLI Before construction 432 157 38 After construction 155 60 21 Acceptable deflection (Dr E Horak) 200 100 50 We have also developed and patented a system whereby the deflections achieved by the impact compactor are measured and recorded. This is a significant im- provement as the deflections are reported in the form of an e-modulus for every square metre compacted. CONCLUSION The innovative use of new technologies such as impact compaction not only offers time and cost saving, but also produces a strength balanced subgrade which im- proves the long-term performance of the road and can save the South African road construction industry billions of Rands. Similarly, more roads can be constructed or rehabilitated for the for the same financial outlay. However, this technique is perhaps better suited to the construction or reha- bilitation of low-volume roads where the percentage of savings can be even higher. A paradigm shift in the road design philosophy can further add to these savings without compromising basic engineering principles.  CBR 100 80 60 40 20 0 850 770 750 710 670 650 590 540 510 450 370 320 290 240 180 160 130 100 70 40 Depth in mm Before After Figure 1 Typical CBR values derived from DCP tests before and after impact compaction Settlement achieved after impact compaction