- Steve Davidson
Pavement that takes the cake – advice on conditioning gravel
We’ve recently been out in the Tanami Desert helping manage a road upgrade. While it would be tempting to think that 280km north west of Alice Springs you can get away with building a sub-standard road…that’s not the case. Every day, it is a lifeline for the communities and local mine that haul in bulk fuel and other supplies.
One of the most critical aspects of any road is the pavement. For this project, the team had to use a natural gravel as the cost of importing a quarry-won product was, in a word, horrendous. From a structural perspective, it meant we were starting with something less than gold-plated. Whatever we did, it had to be done correctly.
Building good pavement is like baking a cake: very few people see the hard work that goes into it, but they all appreciate the output. And just like baking a cake, quality pavement needs the right mix of products. Of course, for our process, we were mixing in water to condition the gravel, knowing the optimum water content provides the densest pavement and, as a result, the stiffest and most resilient road. Too wet, and there would be soft spots…too dry, and the materials wouldn’t be compactable (aka densified to their optimum value).
The key to getting pavement mix just right is to be consistent with every part of the process. Here are some critical tips that are just as relevant to road building in urban settings as they are to remote project sites:
Place – ensure all trucks are carrying a consistent tonnage and the drivers know exactly how to dump. This gives you the best chance of success by starting off on the right foot. We would typically specify the number of loads per 25m along the road.
Balance – balance the material as a critically important step before any water is added. Balancing is the term we use to ensure there is sufficient, evenly-placed material on the road waiting to be mixed. After a grader has spread out the piles, use GPS control or stringlines to confirm that the depth of gravel is the same the whole way along the road. Failing to get a consistent depth of gravel will result in moisture changes because a water cart generally waters at a consistent rate for the entire length of the mix. If some of the mix is 200mm deep, and other areas are 150mm deep, the shallow areas will be very wet or the high areas too dry. Either way would result in a poor product.
Mix – ensure the water cart drivers understand the process and, most of all, that they drive at a consistent speed. Watch out at the ends of each mixing run. These areas are often where trucks slow, adding more water per meter and resulting in wet patches at the joins.
Cure – let natural gravels cure for at least 24 hours if at all possible, before they are laid out. This does wonders for evening out moisture content.
Schedule – consider if mixing on nightshifts is possible. This normally helps to minimise water lost to evaporation. It’s also typically the same crew every night, which increases the likelihood of a consistent result.
These simple practices are commonly overlooked, maybe because they seem trivial or even time-wasting and overkill. The proof, however, is in the final product and a pavement that serves its purpose – for the life intended. Quality pavement ‘drives’ productivity and value in many other areas. If you want a strong project, consider this one of the foundations to get right.
