However, when a Dynapac representative says something about compaction it pays to listen, because Dynapac’s secret weapon is its compaction research laboratory, which opened in Sweden in 1948 and now goes under the guise of the International High Comp Centre. It has allowed Dynapac to develop an encyclopaedic knowledge of the compaction efficiency of different types of equipment on literally thousands of soil types and asphalt mixes, with the CompBase database freely available to Dynapac customers via their local branch.The results are available for standard and modified Proctor density measurements, and for average and minimum density readings. The minimum density reading indicates that the density reading does not fall below the minimum figure at any point throughout the layer, and using the recommended number of passes indicated on this chart ensures consistent density throughout the layer while avoiding over-compaction. Flow-on benefits are reductions in stone damage, density failures and incidences of sub-base or sub-grade collapse.It is from this research that the effectiveness of tandem rollers on base materials became known and could be quantified. The theory behind the findings is that in base materials with a high fines and moisture content, the fines and moisture provide similar lubrication to what emulsion does in asphalt, and allows soil particles to be compacted with high-frequency, low-amplitude vibration. The studies have shown that this provides good compaction throughout the mix, even in lifts as deep as 200mm. Base material mixed in a pugmill and cement-treated base material have been given as examples of the type of base material that could be effectively compacted by a tandem vibratory roller.As an example of the findings of the research, the results of a Dynapac CC224HF tandem roller (7.7-tonne static weight) and CA302D smooth single drum roller (12.75t static weight) can be compared in base course material with a fines content of 2-7%, a clay content of less than 5% and optimum water content. For a 100mm thickness and using the minimum scale, the CC224HF can achieve 100% modified Proctor density in four passes, whereas the CA302D requires eight passes. In consulting the average Proctor table, the CC224HF again requires four passes, while the CA302D still requires six passes. This is an indication that the tandem roller produces more even compaction throughout the layer, with the single drum roller requiring additional passes to achieve the same result.Even for a 200mm thickness, both rollers can achieve 100% modified Proctor in 16 passes, though a larger roller would generally be used in these circumstances. The 10.5t static weight CC424HF tandem roller could achieve 100% modified Proctor in 12 passes. This is quite respectable when compared to the 20.75t static weight CA612D smooth single drum roller, which can achieve the same result in eight passes.As can be seen, the results are not insignificant. Using the CompBase data, a project manager could balance the layer thickness with the compaction pass requirements to optimise the costs of laying and compacting base material without necessarily requiring the largest compaction equipment available. If ever there was a case for compacting “by the book”, this is it. The natural tendency is to expect that less passes are required by a single drum roller with a higher static weight. In some sectors there still seems to be a carryover of static rolling mentality, where it is assumed that extra passes lead to better compaction or at worst, can’t do any harm. Over-compaction is extremely costly, as it destroys material cohesion and requires over-compacted material to be dug out and replaced. If that material is left in place, early road failure is inevitable.When a database of compaction requirements for virtually any soil type is available, it seems churlish to rely on instinct.
