Capital Equipment News April 2016

CONSTRUCTION

O n a recent visit to a supplier we hit some traffic on Atlas Road because of road maintenance taking place to the side of an intersection. After the rains the potholes and road surface in many areas had deteriorated as a result of old repairs lifting and new ones developing, since hydraulic forces worked on any weak spot available. Although the traffic was an inconvenience, see repairs being carried out was heart-warming. As we drew abreast and waited for our turn to go I could not help but watch the work that was being carried out and I was dumb-struck at what was happening. The team had made a neat rectangular cut of approximately 5 m x 2 m and was breaking up the damaged layer, spreading it evenly in the repair area and then compacting this with a walk behind roller. On this, I assume a layer of tack and asphalt was to be placed as a sealer. That all seems to be correct you may think however, the layer being compacted, although relatively well graded and mixed, was being compacted DRY. So the repair was definitely not going to last much beyond the next rains as the basics of compaction were being ignored. What then are the basics of compaction? First, the reason for compaction is to reduce air voids and increase friction, thereby creating a higher bearing capacity and more stable material and thus a more reliable stable layer to traffic. What that asphalt was going to go on clearly did not meet those requirements. There are many factors that influence compaction, such as the soil type i.e. non- cohesive (gravel, sand, stone) or cohesive (mud, clay, silt) soils, particle size, shape and distribution and the fracture surface characteristics and finally moisture content at time of compaction. Moisture content has a decisive influence on compaction as it acts as a lubricant during compaction and is thus one of the more important aspects of compaction. The repair team was clearly not aware of this fundamental basic and although the material was a mixture of base material and damaged asphalt they were breaking it down creating a homogeneous mix, spread- ing it evenly and so forming a relatively THE BASICS of soil compaction

well graded layer ready for compaction. Compacting it dry was defeating all their worthy efforts and render- ing the repair, on comple- tion, short lived. Compaction needs to take place at OMC (Optimum Moisture Content). If the water content is too low, the lubricating effect is very low and the frictional resistance of the individual grains to each other is very high. The soil is then in fact very hard to compact.

If, on the other hand, the water content is too high, high water pressure builds up in the soil during compaction, which impedes or resists the compac- tive effort and again makes the soil difficult to compact.

If the water content of a soil is the same as the OMC the best compaction result can be achieved. This is when the water acts as a lubricant between the particles but does not interfere with com- paction. Carrying out all the desperately needed repairs on the smaller damaged pieces of our roads does not realistically allow for all the required material tests to be done in determining the soil characteristics and OMC. However, compaction does not have to take place at exactly OMC as there is an allowable range within which optimum compaction can take place.

The teams carrying out these repairs need to be educated in the Basics of Compaction and the importance thereof to ensure that the repairs are more durable than just last- ing from one rain to the next. The costs of continual repairs degrade the base layer so that it becomes unsuitable and ineffective as a stable base for surfacing and so the cycle will continue.

With acknowledgement to WIRTGEN SOUTH AFRICA

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CAPITAL EQUIPMENT NEWS APRIL 2016