Soil composition and site constraints should govern your equipment choices.
By Kenneth A. Hooker

Properly compacted soil is a key component of virtually every construction project. It provides the firm, dense base needed to support footings and foundations, slabs, and pavements. When the underlying soil isn’t solid or dense enough, excessive soil settlement can lead to a variety of structural problems such as basement walls that crack and leak, pipes that leak and break, slab cracks, and foundation erosion.

There are a lot of options when it comes to compaction equipment, but finding the right machine for a particular project can help ensure satisfactory results. The choice depends on two main criteria: The first is the composition and properties of the soil to be compacted; the second is the amount and expanse of material to be compacted. The first criterion determines the type of compaction force required, and the second determines the size of machine needed to do the work efficiently.

Soil types
Different types of soil have different characteristics that affect its maximum density and optimal moisture content. Soils are typically classified by grain size, which is found by passing soil through a series of progressively finer sieves. A well-graded soil features a wide range of particle sizes, with smaller particles that fill in voids between the larger ones. Such soils develop a suitably dense structure when compacted.

The American Association of State Highway and Transportation Officials classifies 15 different soil groups, and soils in nature generally represent a combination of types. The types fall within three broad categories: cohesive, granular, and organic. Only the first two are capable of compaction.

Cohesive soils are mainly composed of the smallest particles—silts and clays that range from 0.00004 to 0.002 inch and are tightly bound together through molecular attraction. These soils are dense, and the individual particles are not visible to the naked eye. They can be molded when wet, and become very hard when dry.

Granular soils are mainly composed of sand and gravel particles—sand ranging from 0.003 to 0.08 inch and fine-to-medium gravel from 0.08 to 1 inch. The individual grains are visible, and the soil feels gritty when rubbed between fingers. Water drains readily through granular soil.

The table below rates these soil types for their properties and suitability for some construction applications.

Compaction specifications
Project documents include a soil compaction specification, based on the loads the engineer expects the soil will need to support. This spec most often requires that a particular soil density be achieved. ASTM D 1557 is used to determine the characteristics of a soil sample subjected to measured compaction forces in the laboratory. The method, called the Proctor or Modified Proctor, establishes a maximum possible density for the compacted soil, as well as the optimum moisture content needed to achieve it. Most specs require the contractor to meet some percentage of the Proctor maximum density, usually 90 or 95 percent.

Traditional compaction specifications often spelled out the compaction method to be used, including the type of equipment, the maximum lift height, the number of passes, etc. More common now are “end-result” or “performance” specifications, which give the contractor flexibility in achieving the desired soil density.

New soils engineering research, in conjunction with innovations in compaction equipment, will likely lead to further changes in the typical soil compaction spec, according to David J. White, Ph. D., director of the Center for Earthworks Engineering at Iowa State University in Ames. White says there will eventually be specifications based on the soil’s stiffness modulus, rather than soil density. Soil stiffness is a property that can be monitored automatically by some advanced compaction equipment, and the results are better than those from nuclear density sensors. “The latest highway bill contains a reference to intelligent compaction equipment and points to [stiffness] modulus-based quality control/quality assurance,” White says.

Compaction machines
Compaction machines apply forces to increase soil density. Some equipment applies only static forces, which include pressure and kneading produced only by the dead weight and configuration of the machine itself. Such static forces affect only the top layers of material, and their impact is felt only to a limited depth. Other equipment also applies the dynamic forces of vibration or impact to increase its compactive effect. These machines use a mechanism, usually engine-driven, to add a downward force beyond the machine’s static weight.

One or more rotating eccentric weights are normally used as a vibrating mechanism. Vibration moves through the soil, setting the particles in motion and moving them closer together to produce the highest possible density. This affects both the top layers and deeper layers. The magnitude of a machine’s dynamic compaction force depends on both the frequency of the eccentric shaft’s rotation or machine jumps (expressed in vpm) and the amplitude of its movement, that is, the distance it moves from its axis.

There are three basic types of compaction machines: rammers, vibratory plates, and rollers.

Rammers. Rammers apply high-impact compaction forces generated by a small gasoline or diesel engine that powers a piston with two sets of springs. The machines are inclined slightly so as to travel forward with each bounce, guided by the operator. Because they have a relatively small footprint, rammers are used mostly in narrow excavations, up to about 24 inches wide, or adjacent to structures.
Delivering high-amplitude blows at frequencies of 500 to 750/minute, rammers provide the shear forces needed to compact cohesive soils.

Vibratory Plates. Vibratory plates produce low-amplitude, high-frequency vibrations best suited for compacting granular soils. They use gas or diesel engines that drive one or more eccentric weights to generate vibration at frequencies ranging from 2500 to 6000 vpm.

The most basic vibratory plate compactors travel only forward, their forward motion driven by the vibrating plate itself. The plate width on these lighter duty machines ranges from 14 to 20 inches. They are generally used to compact smaller areas and shallow lifts under 12 inches.

There are also larger and heavier vibratory plate compactors. These larger units are typically reversible, which means they use two eccentric weights and so can transition smoothly from forward to reverse travel, or maintain force while stopped to provide spot compaction. The heavier plates and dual weights also increase their compaction force, so they can be used effectively on semi-cohesive as well as granular soils. Plate widths range from about 16 to 28 inches.
Rollers. Rollers also comprise several categories that serve various purposes. There are walk-behind and ride-on models, static and vibratory rollers, smooth-drum and padded or sheepsfoot designs.

Walk-behind or ride-on, smooth-drum vibratory rollers are a good choice for compacting granular or mixed soils for larger areas such as driveways, parking lots, and roadways. They are also suitable for roller-compacted concrete projects. These machines provide high-frequency (about 4000 vpm), low-amplitude vibrations generated by eccentric shafts within the drums or mounted on the frame.

Padded rollers, also known as trench or sheepsfoot rollers, are designed for compacting cohesive soils and are often used in the confined space of utility trenches. These machines feature large eccentric weights to produce the high impact force and relatively high amplitude needed to compact cohesive clay and mixed soils.

Padded rollers are powered by diesel engines and use either hydraulic or hydrostatic steering and operation. Some units can be remote controlled.

Compactor trends and innovations
A feature to look for when considering compaction equipment is design that reduces the transmission of machine vibrations to the operator’s hands and arms. Such vibration can cause a repetitive strain injury called Raynaud’s syndrome, with symptoms that include numbness and tingling in the fingers, and ultimately loss of sensation and muscle control in the fingers and hands. Although there are currently no regulations limiting vibration exposure in the U.S., such standards have been imposed in Europe, and manufacturers that supply both markets make machines that meet the European requirement for reduced vibration.

Another innovative feature available on some compaction machines is a monitoring system that uses sensors to measure soil stiffness and indicate compaction progress. These systems provide real-time feedback to help prevent over-compaction, which not only wastes time and fuel but can also result in reducing soil stiffness.

Buy or rent?
Compaction equipment is available for rent, and many contractors opt to go that route. With so many variables affecting the suitability of equipment for any particular project, you may not want to keep and maintain many different machines. Manufacturers say there’s really no such thing as a general purpose machine for soil compaction.

If there’s a type of work you do regularly, and the soil in your geographic area is fairly consistent, you might buy equipment that fits that bill and rent when your project calls for something different. In any case, you’ll get the best results by using equipment that’s best suited for the type of soil present and the size of the area that requires compaction.
Kenneth A. Hooker is a freelance writer based in Oak Park, Ill.

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