by Paul McClendon – Chicago Pneumatic South Central US Sales Manager and Ride On Roller National Sales Manager
Those of us that are involved in the road building process know that it is not an easy job – rather it is one that takes patience, experience, and a great amount of knowledge. It is not something we leave up to novices or greenhorns, only highly trained, experienced individuals and for good reason. Road building is not cheap, and there is a bucket full of things to remember on the job. In the world of road building, it is necessary to get the job right the first time, and it all starts with one of the most critical processes: Compaction.
Why Soil Compaction
Soil compaction is a process that increases the density of the soil. Before we even lay asphalt or concrete, the soil is the base layer. At one point or another, you have likely driven on a highway or interstate road and noticed cracks running across it, or the road is buckled or bulging. That is mainly due to the base of the dirt not being prepped correctly and poorly compacted. It is critical to get this first step right.
Soil compaction does many things, but mainly it increases load-bearing capacity and stability that allows traffic to drive over it, prevents soil settlement and frost damage, and reduces water seepage.
Soil and Moisture
First off, it is important to know what kind of soil you are dealing with – different soil types feature different maximum densities and moisture levels. Cohesive (like clay), granular (like sand), and organic (for planting) are the three basic soil groups and only two of the three soil groups are suitable for compaction – cohesive and granular. Cohesive soils have particles that stick together, granular soils crumble easily, while organic soils are not fit to be compacted. Each soil has it’s own unique characteristics from absorbency to compaction difficulty.
It is not only important to determine the soil type, but also the moisture content in the soil before starting to do compaction work. There is a happy medium to discover in terms of moisture in the soil – too much moisture weakens stability, but too little moisture will result in poor compaction. A quick and simple hand test will show the operator what they’re working with. The test requires you to pick up a handful of soil, squeeze it in your hand, and then open your hand and drop the soil. When doing the hand test, you will find the ideal moisture will mold when you squeeze it and break into a few pieces when you drop it (not too many pieces however). To determine if there is too little or too much moisture in the soil is rather easy. A dry soil will come out powdery and break into fragments when it’s dropped. On the other hand, if the soil leaves moisture in your hand and the mold of the soil remains intact when you drop it, it has too much moisture to be compacted. The right amount of moisture is important not just for the foundation, but it will end up saving you money in the long run by trimming down the amount of compaction work necessary in getting the results you want.
Poor compaction could and likely will result in settlement of the soil. This creates future maintenance issues that could have been avoided or in extreme cases, the road cracking, buckling, or bulging, resulting in complete structural failure. Compaction equipment is made for turning the loose soil at the beginning of the job into compacted soil, giving the right soil consistency before laying down asphalt or concrete.
There are different types of compaction equipment; rammers (some refer to as ‘jumping jacks’), forward/reversible plates, walk-behind rollers, ride-on rollers and trench rollers. Rammers and trench rollers are best utilized on cohesive soils, while forward/reversible plates, walk-behind rollers and ride-on rollers are better suited for granular soils.
In dealing with small compaction equipment, users need to be aware that it weighs, in some instances, about 1/5th of the compaction force it is putting out. For example, the user has a 185-pound rammer that they are using in an 11-foot by 11-foot area and the rammer is outputting nearly 3,000 pounds of force, so you can only imagine what that piece of equipment is going through. While compaction is a critical process for roads, the same can be said for a good compaction maintenance program.
Whether you are renting compaction equipment or it’s part of your fleet, it is important to have a service interval plan in place. Make sure the rental shop regularly maintains the equipment, or if you purchased equipment, that you have a reliable and trained, certified technician servicing the equipment annually, at minimum. It is important to check the oil, make sure the bolts are tightened and that you or your technician are inspecting the machine with a fine-toothed comb. Compaction is the first step to road building, so the last thing you want to lose out on is time and productivity because the equipment is acting up.
Biggest Mistakes of Compaction
It is not uncommon to see compaction mistakes, even for the most experienced road-building contractor. It is important to take your time through this step and not rush, or it will end up haunting you later. The biggest mistakes typically seen on the job are either overcompaction or undercompaction. Poor compaction can lead to many unwanted results, from cracks in the slab, buckling or bulging to erosion in the foundation and potential pipe leakage and breaks, among others.
Soil can experience overcompaction if the operator makes too many passes with their compaction machine in one direction, therefore reducing the soil density. Undercompaction occurs when the operator does not make enough passes with the compactor, so the particles in the soil are too soft and not getting the cohesion they need to create the right amount of density.
Another important tip is to make sure you are getting the right amount of lift, which is the depth of the soil layer. As you fill in a trench, you want to make sure you put the correct amount of dirt in there and are providing the right amount of compaction force to the area. When the soil becomes more compacted, the impact on the compacting machine has less distance to travel and more energy returns to the machine, making it come off the ground higher. An incorrect lift – one that has too much dirt and is too deep of a layer – will create a loose layer of dirt, resulting in undercompaction.
Nuclear density tests determine the density of the compacted soil using core samples. After laying an asphalt road, many times you will see road workers bore about a one-inch hole in different areas right through the asphalt into the ground. What they are doing is checking the density in those areas to see if everything remained compacted the way it should be after laying the asphalt on top of it.
In summary, there are many steps and considerations to take into account while doing compaction work. The most significant step is to have your compaction equipment regularly serviced. Compaction products work hard and hit the ground with much greater force than their static weight. Soil has a way of being unforgiving, and while compaction can be a long process, it is important to take your time and do it right the first time. It will wind up saving you a lot of headaches and money in the future.