Friday, July 1, 2011

Vibratory Roller

The vibratory roller is a commonly used piece of equipment at construction sites around Newfoundland. It employs a steel drum that vibrates while it is pushed and pulled over the soil. It is especially effective at compacting gravel material.

  • contains a vibratory unit that vibrates the roller as it is pulled/pushed over the soil
  • vibration frequencies range from 1500-1200 cycles per minute
  • effective in compacting clean sands and gravels

Sheepsfoot Roller

The sheepsfoot roller is another piece of equipment that is not common in Newfoundland as it is best used on soil types that are not present here. It gets its name from the imprint left on the soil after it has passed over; it looks like a herd of sheep had just walked over it. Working on a site where the soil is being compacted by a sheepsfoot roller makes it difficult to walk around, the indents left behind are about 6" in diameter and about 4" deep. It is like walking around on a large 'Bat the Mole' game.


  • drum with metal knobs called feet
  • feet reduce contact area and therefore increase pressure
  • P=F/A
  • good for compacting silts and clays

Pneumatic Roller

Pneumatic rollers are not all that common to Newfoundland; the soil types here do not often require this type of equipment. The machine is best used on clay, where the soil here in Newfoundland is rocky and gravel-like. The machine is a self-driven (has an engine) piece of equipment that uses several highly inflated rubber tires as a compaction surface. The weight of the machine is used to provide compaction and extra weight may be added by filling boxes on the ends with sand or rocks.


  • machine consists of a number of rubber tires, highly inflates
  • vary from small rollers to very large and heavy rollers
  • may be self-propelled but the larger ones are usually towed
  • some have boxes mounted above their wheels so that material may be added to provide more compaction
  • used on 'clayey' and silty soils
  • effective in compacting granular material containing small amounts of fines

Smooth Wheel Roller

Smooth wheel rollers are one of the most common types of compaction equipment used. We've all seen them in action on a highway during the summer. They are a standalone piece of equipment that have an engine, move very slowly and their weight is used to provide compaction. They are typically used for base layers of soil or for asphalt pavement.

  • 2-3 smooth metal rollers
  • useful in compacting base layers and paving mixtures
  • also used to provide a smooth finished grade
  • generally are self-propelled, i.e. has an engine
  • compaction is provided by the weight of the machine


Tampers are small devices used to compact soil. It is likely you've seen one before; they can be rented for a few hours at a time and are often seen where sidewalks and walkways are being built. The tamper uses a plate that is moved up and down a small amount very quickly to compact the soil.

Tampers are typically used in places where large rollers cannot be used. They are versatile machines and can fit into some tight spaces.

When soil is being compacted with a tamper, it is placed in smaller layers that are slightly thinner, 6" before compaction rather than 8" as we previously discussed.

The slides above show a heavy duty tamper that runs on gasoline or compressed air and a light duty tamper (plate tamper) that runs on gasoline.


  • tampers are devised that compact soil by delivering a succession of light, vertical blows
  • they are held in place and operated by hand
  • tampers are powered pneumatically (compressed air) or by gasoline engines
  • they are limited in scope and compacting ability - good for small jobs and tight spaces
  • layers are limited to 6" or 150mm with a tamper

Disk Plow

A disk plow is just what it sounds like - a plow with a series of disks that churn the soil and mix it with water. It is typically an attachment that is towed by a larger piece of equipment.


What is the purpose of a disk plow?

Field Compaction

It is difficult to compact all of the soil that is placed on a construction site at once. The thicker the layer, the less effect compaction has on the soil at the bottom of the layer. Soil is typically placed in layers and each layer is compacted. The layers start out at about 8" in depth and once compact it is about 6" in depth.

If the weather is warm and sunny, it is common practice to have a water truck sprinkle the soil with water. The soil is then mixed with the water using a disk plow. A disk plow will be shown and explained further in the next section.

Likewise, if the soil is too wet, a disk plow can be used to expose wet parts of the soil that are under the surface. This helps the soil to dry faster.

After each layer is compacted, it is good practice to 'rough up' the top of the smooth layer so that it bonds to the next layer of soil. Depending on the size of the area to be compacted, tampers and rollers are used.

  • normally soil is compacted in layers
  • usually each layer starts as 8" or 200mm of loose material which is then compacted to 6" or 150mm
  • if the soil is too dry, water can be added with a water truck and mixed into the soil by disk plowing
  • if the soil is too wet, moisture can be reduced by aeration, i.e. spreading it out, disk plowing to constantly turn out wet soil, and allowing to dry
  • the surface of each layer should be 'roughed up' by disk plowing or scraping to provide bonding between the layers
  • work is usually done by tampers and rollers
  • tampers and rollers usually achieve their maximum compaction withing 6 - 10 complete passes over the compaction area

The Standard Proctor Test

The Standard Proctor Test is a test developed by the American Society for Testing and Materials (ASTM). The test, which can be referred to as the 'laboratory compaction test', is performed on a soil sample to determine the soil's optimum moisture content (OMC) and maximum compaction density. The maximum compaction density is also known as the maximum dry unit weight.

The apparatus consists of a two-piece cylinder and baseplate, and a 'hammer' that is dropped on the soil in the cylinder to compact it. The soil is dried in an oven and then weighed. The weight of the soil is used to determine the amount of water to add for the test. If you had a 1.0kg dry soil sample, and you were testing for 10% moisture, then you would add 100g of water to the sample. Typically, the same soil type is tested with multiple moisture contents to determine which one provides the highest level of compaction.

The information obtained by doing this test is used in the field by comparing the moisture content in the soil to the test values to determine if the soil is near the desired compaction level and, typically, the maximum value is desired.

To see videos of the Standard Proctor Test in action, click here for Part 1, Part 2, Part 3, Part 4.

  • soil is compacted in a standard mold using a standard hammer dropped from a standard distance
  • Standard Proctor uses a 5.5lb hammer dropped from 12". Modified Proctor uses a 10lb hammer dropped from 18"
  • take four samples, dry them in the oven
  • add varying water contents to the samples making sure the water contents bracket the OMC
  • compact each sample in the mold using specified numbers of blows per layer
  • determine the wet unit weight of the sample
  • dry in oven and determine water content
  • find the dry unit weight
  • plot moisture vs. dry unit weight
  • peak of the curve gives OMC and maximum compaction density

Will soil that is overly wet or overly dry compact well?

How many samples are normally tested during a Standard Proctor Test?


To achieve effective, long lasting compaction the soil should be slightly wet when being compacted. The water in the soil acts as a lubricant allowing the soil particles to slide against each other and move very tight together. Very dry soil doesn't compact very well as the particles do not bond to each other as they slide. Very wet soil doesn't compact very well either - the water fills the little gaps or voids in the soil and isn't able to be pressed out during compaction.

It takes the correct amount of water in the soil to achieve the highest amount of compaction. The amount of water varies depending on the soil type and location, and can be determined by performing tests on the soil.

  • soil too dry - doesn't compact well
  • soil too wet - doesn't compact well
  • soil just wet enough - compacts well

Effects of Poor Compaction

In the slide above, you see six examples of structures that have been compromised because of poorly compacted soil.

In the first example, a structure has cracked due to the soil below it failing due to lack of compaction. In the second example, a concrete slab has cracked due to poor soil underneath it. The third example shows the joint in a pipeline which could separate due to poor supporting soil. The fourth example shows a foundation cracking and falling away due to the soil settling. The fifth example shows a concrete block designed to support a structure tipped to one side as the soil is not supporting it evenly. The last example shows a trench settling, which jeopardizes the pipe that runs through it.


What is the basic meaning of compaction?

How is compaction normally produced in soil?

How does compaction differ from consolidation?

What are some effects of poor soil compaction?

Effects of Soil Compaction

When soil is compacted, the density increases. Picture a bucket full of loose soil, its heavy but can be lifted easily. Now picture that same bucket of soil, but now you've stepped on the soil to compact it and more soil has to be added to fill the bucket to the top. The bucket with compacted soil is heavier and can't be lifted quite as easily.

The compacted soil takes up the same amount of physical space as the loose soil; however, its weight is much heavier. This means the compacted soil has a higher density than the loose soil.

Soil is compacted in order to increase its suitability to support a structure such as a building or road. The three most important effects achieved by compaction are that the soil can withstand larger shear forces, settlement (or building creep) is significantly decreased, and the soil is less susceptible to water saturation.

In general, the higher the level of compaction that can be achieved, the greater the benefits experienced by the three effects we just mentioned.

  • compaction increases density and produces three important effects:
  • - an increase in the soil's shear strength
  • - a decrease in future settlement of the soil
  • - a decrease in permeability
  • these three effects are beneficial for various types of construction, such as highways, airfields, etc.
  • general rule of thumb - the greater the compaction, the greater the benefits


The word 'compact' refers to pressing things closely together, which makes the object physically smaller as it takes up less space. With this in mind, and the knowledge that soil is composed of various-sized soil grains, soil compaction occurs when these soil grains (or particles) are pressed together. When this happens, air is expelled from the soil and the compacted soil takes up less room than non-compacted soil.

To get an idea of the difference between non-compacted soil and compacted soil, picture a set of footprints left behind by someone walking over loose dirt or that pesky cat next door that keeps walking across your flower garden.

For the purpose of constructing large buildings, houses, roads, and just about any type of structure, soil is compacted deliberately and quickly with the use of heavy compaction equipment. We will discuss this equipment later in the lesson.

Consolidation is another way in which soil gets smaller; however, it results from water being removed from the soil, rather than air. Consolidation is not a part of this lesson, but it is important for you to be aware of the basic difference between compaction and consolidation.

  • the general meaning of the word 'compact' is "to press closely together"
  • in soil mechanics, it means to press soil particles tightly together by expelling air from the void space
  • compaction is normally produced deliberately and rapidly and often by heavy compaction rollers
  • compaction differs from consolidation, which results from extrusion of water rather than air, and is not rapid


At the end of this lesson, you should be able to:
  • define compaction
  • describe effects of compaction
  • have an understanding of the Standard Proctor Test
  • be able to distinguish between the types of tampers
  • differentiate between the different types of rollers used for compaction
Before continuing, as yourself the following question:

What happens to the soil when it is compacted?


Welcome to the lesson blog for students taking CA2800, Soil Mechanics I. Nancy Barnes, Sara Sexton, and Andrew Spearns will be writing the blog and preparing the in-class instruction.

Please feel free to leave a comment or start a class discussion.