Bear down

Understanding exactly what the customer needs is an important part of providing what the customer wants. It might sound simplistic, but it’s true. If it’s not followed, the customer could end up leaving with equipment that isn’t appropriate for the job. They aren’t likely to blame themselves for this, human nature being what it is. Instead, they’ll blame you or your staff.

Knowing the basics of compaction science can help your staff to provide your customers with what they need. Photo appears courtesy of Wacker Neuson.

Compaction equipment is a big part of the bottom line for many rental stores. That’s why it’s absolutely vital that your staff understands various compaction needs, and what equipment is right for each job.

Understanding compaction begins with understanding soil. Dirt may look boring, but for those in the know it can be full of interest. However, it isn’t necessary to become an expert on soil to become knowledgeable about compaction. From the point of view of the rental operator, there are basically three factors to consider. They are the type of soil, how much moisture it contains, and the effort required to compact it effectively.

Soil is incredibly complex. Luckily, a lot of that complexity doesn’t really matter when it comes to compaction. The only aim is to increase the soil density, which leads to increased strength and decreased permeability, providing a better footing for foundations or roads. Since that’s the only goal, we can ignore a lot of the differences between soil types, and only look at them as broad categories.

One of the more common types of soil classification systems is the Unified Soil Classification System (USCS). The USCS has three major classifications:

• Coarse-grained soils (sands and gravels)
• Fine-grained soils (silts and clays)
• Highly organic soils (peat)

All that we really need to know about highly organic soils is that they’re unsuitable for compaction. The other two types require us to dig a little deeper.

Coarse-grained soils have a number of defining characteristics and are known for their water draining properties. These soils have very little plasticity or cohesion, and will crumble easily. If it feels gritty on your fingers, the sample is probably mostly coarse-grained. Incidentally, this classification can even be applied to things that are not technically soil. Asphalt, for example, is considered to be coarse-grained because its component particles are of mixed sizes.

Fine-grained soils are generally much denser, with a smooth, greasy feel to them. They have high strength when dry, as anyone who has ever tried to dig in clay can tell you. Compaction of this type of soil will usually require impact or pressure.

The way that the soil responds to moisture, and how much moisture it contains, can be very important when it comes to compaction. Too little moisture will lead to inadequate compaction, because the individual particles don’t have enough lubrication to move past each other. Too much can leave pockets of water, lowering the soil’s load-bearing capability.

There are a number of lab and field tests that will accurately determine soil density and moisture content. However, a rough guide can be obtained through hand testing the soil. It’s relatively simple to do. Pick up a glob of dirt. Squeeze it. If the soil won’t take the shape of your hand, or if it shatters when you drop it, then it’s too dry. If it leaves water on your hand, or doesn’t break at all when you drop it, then it’s too wet. If it takes the shape of your hand, and only breaks into a few pieces when dropped on the ground, then the soil is suitable for compaction.

Each type of soil has its own requirements, and it is these requirements that determine which type of compaction machine is best to use.

No matter the type of soil, there are essentially two forces at work in compaction. They are static and vibratory. Static compaction is caused by the weight of the machine, and is limited the soil’s upper layers. The only way to change it is to use a heavier or lighter machine.

Vibratory compaction adds mechanical force to the static weight of the machine. This is really what comes to mind when we’re talking about compaction forces. The actual mechanism is often a rotating, eccentrically balanced weight. In the case of rammers, the mechanism is usually a combination of piston and spring.

Earlier we mentioned the smooth, greasy feel of fine-grained soils. This is because the particles in the soil like to stick together. In turn, this means the customer will need a machine that can deliver high levels of impact force. A rammer, or a pad-foot vibratory roller, is recommended for these situations.

When it comes to coarse-grained soils, vibratory plates or vibratory rollers are the way to go. Coarse-grained soils don’t stick together the way fine-grained soils do, and need vibration to get them moving. The exact type of machine that will be best for your customer is largely dependent on particle size. Small particles need high frequency vibration. The larger the particles get, the lower the frequencies you need to get them moving. Larger equipment delivers lower frequencies and higher compaction forces. The larger the particles being compacted, the larger the compaction equipment required for effective compaction.

Of course, the second we leave the world of theory, we land squarely in the real world. Soil is almost never entirely composed of one type, but is a mixture of both in varying proportion. Because of this, determining which type of compaction equipment will do the job best can be difficult. As a rough guide, it’s best to choose the machine most suited to the larger percentage of material in the soil.

Two main factors determine the actual force delivered. They are frequency and amplitude. Frequency is often listed as vibrations per minute (vpm), essentially the rate at which the shaft turns or the machine jumps. There is an optimum frequency for each machine, at which it will supply the maximum amount of force. Amplitude can be defined as the absolute value of the maximum displacement from a zero value during one period of an oscillation. In other words, amplitude is the maximum amount the shaft, spring/piston combination or other mechanical force delivery system moves away from its axis.

The amplitude of any particular piece of equipment will change depending on conditions. This doesn’t just apply to the condition of the soil when compaction begins. Amplitude increases as the soil becomes denser due to compaction.

The depth of the soil layer – also known as lift height – can affect machine performance, as it changes during the compaction cycle. Force is directed down by the machine but it will sooner or later reach a hard surface and bounce back up. As the soil becomes more and more compacted, the force returning to the machine will continue to increase, lifting it higher off the ground. If the lift height is too deep, the soil will not be compacted effectively. Not only will it take longer, but a layer in the soil will not be compacted. This can lead to subsidence and other problems down the road.

Soil can also become over-compacted if too many passes are made. When this happens it also wastes time. There’s a very good reason that rental operators should care about preventing this. It means unnecessary wear and tear on a machine that you expect to rent again. Improper technique will sooner or later take dollars out of your pocket.