Compaction Equipment and Methods: Compaction of Gravel and Soil

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Nothing leads to more failures than improper compaction. Needless to say, compaction is one of the most important, if not the most important, aspect of any project. This begins and ends with having the right compaction equipment at your disposal for a project, identifying your sub-soils, knowing the right methods of compaction to apply to a project, using the right gravel for your project, and achieving optimal compaction at each stage of your project.

As contractors, we need to recognize the importance of preparing a proper base. At the same time, homeowners need to recognize that this is a costly step of any project that if not done properly will spell certain doom for your project and is the number one place where a contractor can cut corners because you will never see it or notice a difference until it is too late. The base preparation is where a significant portion of your cost for your project comes from and you will never see it in the end product.

Compaction begins with the subsoil and continues through the various lifts of the base material once a geotextile is installed on top of the compacted subsoil. In some soil conditions, an amendment may be necessary to stabilize and increase the bearing capacity of the subsoil. However, we are sticking with the focus of compaction equipment and methods in this post.

Compaction in Construction

Having the right foundation for any aspect of construction is important. If the foundation fails, everything else fails. That is why upwards of 70% of the time and labor is spent on the proper preparation of the base from excavation to final compaction.

As with any project, excavation is an important first step that is crucial to the strength of your foundation. When excavating, you need to remove organic material, excavating to an equal depth following a slope if necessary, and getting down to the undisturbed portion of the land that is your sub soil. We have clay and sandy sub soils with everything in between.

This is when compaction becomes a crucial component in construction which is basically the consolidation of a sub base with a base material consolidated on top (geotextiles used to separate subsoil from base) to provide a very hard structure. This eliminates sinking, shifting, dipping, voids, erosion, the collection of water that can freeze and expand, or the buckling and crumbling of the aggregate above.

How to Compact Soil

From the point to which you have completed the excavation to the undisturbed subsoil, you are going to compact that soil. You can begin the preparation for the compaction of the subsoil once you have identified the type of subsoil. A simple visual and hand test will allow you to identify the clay to sand mixture of the subsoil and allow you to proceed with what you do next.

Typically in our excavations we have some sort of clay material in our subsoil. In these situations, we like to do an amendment of that soil to get it to proper bearing ability. To do this, we typically pour a layer of 3/4″ clear stone through the excavation area. Additionally, we can also add a lime amendment to chemically change that subgrade or a Type S Mortar mix. This will release the water from that subsoil and add to the bearing ability of that subgrade.

Soil should also have the right amount of moisture content when being compacted. A quick test to know how much moisture content is in your soil on site is to pick up a handful, squeeze it into a ball, and drop it. If after squeezing it the ball crumbles in your hand, the soil is too dry. If you drop it and it stays in one piece or leaves water in your hand, it is too wet. If you drop it and it breaks into a few pieces then it is at a good moisture content to be compacted. You may need to add water to get to this stage or wait for the soil to dry.

Moisture will allow that subgrade to adhere and pack together well while also acting as a lubricant for the top layer so that when you are vibrating across while compacting with your equipment, the smaller particles will work their way around the larger particles with minimal friction and resistance. There are methods to test moisture content like the standard proctor compaction test or the nuclear density meter, though for preparing a hardscape you will be able to test the density of your base using a penetrometer if necessary.

Types of Compaction: Soil Compaction Methods

There are different compaction methods for different types of subsoil. Soils are cohesive materials like clay that move through various degrees of cohesiveness until you get into sand and gravel that are not cohesive. Clay soils will be difficult to identify the granules within it, whereas gravel is easy to identify the different granules within it. These are two different ends of the spectrum of soils.

• Compacting Clay Soil

Clay soils are smooth when rubbed between fingers and will not mix well with water. They can be rolled in between your hands to make long strands. They are cohesive soils, dense and tightly bound particles. The best compaction method for clay requires force rather than vibration.

When compacting clay soils, you would want to use a rammer or a vibrating Sheepsfoot as they provide excellent compaction for clay soil. A rammer is good for small and tight access areas, especially for small trenches for walls. Whereas a vibrating Sheepsfoot is best for larger areas. Both of these action provide an impact method of compaction.

Other good options for compacting clay soils would be a static Sheepsfoot or grid roller. Think of this compaction as a kneading action using downward pressure on these cohesive materials to achieve the highest level of compaction possible.

• Compacting Sandy Soil

Where clay soils are smooth, sandy soils are more coarse and the grains within them can be seen. They are gritty when rubbed between your hands and mix well with water. It will crumble easily when dry and has very little cohesive strength, meaning it does not stay together. In this case, we would move away from an impact method of compaction towards a vibratory soil compactor.

The same soil compaction equipment used to compact clay soils will not necessarily provide optimal results when compacting sandy soils. Sand is best vibrated using a plate compactor, vibrating roller, or vibrating Sheepsfoot. This vibrating action will move the particles in place to achieve the best compaction results.

• Compaction of Gravel

As we move from clay soil which is cohesive as the particles stick together easy to more granular soils like sandy soils which are not cohesive, we arrive at gravel which is used for the preparation of a base.

Whether you are compacting sand, compacting crushed rock, or compacting gravel of any sort, we need to recognize the best piece of equipment to achieve optimal compaction for your project.

Much like sandy soils, gravels are not cohesive and therefore need to be vibrated into place. This means the best pieces of equipment are vibratory plate compactors or rollers. The larger the piece of equipment is, the more productive you can be.

But the size of the machine is not just about how high your lifts can be when you are preparing a base, but also the size of aggregates you are compacting. The larger the particles you are compacting, the larger the machine you will need to obtain a lower frequency as larger particles require lower frequencies.

Over Compaction

Compaction itself is basically the action of applying a downward force for consolidation, taking larger and smaller materials and letting them fall and work their way into place. There is a risk of over compaction as you could, especially with larger gravel, break the granules down into smaller pieces and destroying the composition of that gravel. In addition to that, there is also greater risk of displacing material that has already settled into its place with over compaction. Not to mention the wasted time and labor in over compaction.

So how do you know when you have reached optimal compaction?

There is soil compaction testing equipment that you can purchase and other compaction testing equipment that comes built in to some compactors.

If you want to properly test the compaction of your soils, you would use what is called a nuclear density gauge. What this involves is a piece of equipment that is baseplate, a drill rod, and you would accompany that with a heavy handheld mallet or a sledge hammer. This gauge is going to give you a moisture content reading and it’s going to give you a density reading.

To use this, you would pick a spot in your compacted area and sit the base plate down. You would find a nice flat area and slither the base plate down. On the base plate are two holes, one of which you are going to put the drill rod down that has markings every two inches. This would be hammered down to a maximum of half your base depth and remove it.

From here, you would trace the outside of the base plate and put it away. Then the actual measuring device would be placed on top of that traced area. The device would also have a rod that falls into the same place, turn on the device and give it 60 seconds of testing. This would give your readings of moisture and density that would then be compared to the lab results of the standard proctor test to measure compaction.

This is a complex process that requires expensive equipment. Another alternative would be to use a dynamic cone penetrometer to do a similar test which would involve simply lifting the weight that is on the rod of the penetrometer and dropping it multiple times and measuring how much that rod has penetrated the base. A rule of thumb may be 10 hits takes you down 2.5 inches providing good compaction.

In addition to the compaction of your subgrade and base materials, you can also investigate using a geotextile or geogrid to help stabilize your base.

 

Compaction Equipment and Techniques

Compaction is mechanically increasing the density of a soil or base material and force is the dead weight of the machine being used to apply a downward force compressing the particles below it. When it comes to your plate compactor compaction depth, the rule of thumb is that you need 1,000 pounds of capacity force for every one inch of lift with lift referring to the height or depth you are filling the material. Therefore, if you have a base depth of 8 inches, you would need a machine that has at least 8,000 pounds of force to properly consolidate that material in one lift. Otherwise, you will need to compact in separate lifts.

After understanding the amount of force your machine has and how many lifts you are going to need to complete for your project to achieve optimal compaction levels, you will then begin to prepare the lift by screeding the material into a smooth surface so that you are not riding up and down slopes with your mechanical plate compactor.

When it comes to compacting, you would start at one corner and work the outer area, then on your next pass move inward while overlapping the first pass by about 1/3 of the width of the baseplate of the machine. Continue these passes until you have reached the innermost area of the project and the entire area has been compacted. By doing this, you have come to a center point and are actually pinching the material inward towards each other rather than pushing it outwards.

Once that first path has been completed, you can then do a second pass that has you running stripes left to right or up and down the project. A third pass would depend on your compaction levels.

What Size Compactor Do I Need

Besides what has been discussed above and throughout this article from choosing the right equipment for the job and deciding what power of force is needed for your projects, other variables to consider to provide you with the most force would be the engine power, weight, and footprint of the machine.

If you have a small machine, in reference to the baseplate of the machine, you are going to have to move around the compaction area many more times that you would if you had a larger baseplate. This is where a larger machine will save you in time. The weight and engine power will allow for more force from that machine increasing the height of your lifts and also increasing your efficiency. However, small machines are still handy when moving in tight areas and definitely still have their uses.

However, you need to decide which size best suites your business or project. Do you have the space or carrying capacity for a large machine? Is a smaller machine a better investment with understanding that the tradeoff is inefficiency? Would you be better off renting a larger piece of equipment when needed? These are all questions that need to be considered when choosing the right size and type of compactor.

Compaction Equipment

Besides what has been mentioned above, there are other factors you should be considering when purchasing or even renting compaction equipment. For one, mobility is a plus and if it has a built-in wheel kit then it is much easier to get off a truck or trailer and across the job site when needed.

Having a throttle control available to you at the handle is another plus as it reduces the amount of times you have to bend down while that machine is still moving to reduce the throttle when necessary. If you have a larger piece of equipment then having a properly balanced hoist is important to be able to strap or belt it to move it around.

A reputable engine is always something that should be high on your list as this is the most important part of the longevity of your investment. Having a kill switch near the handle is also an excellent safety feature.

In terms of the plate, cast iron is better than steel plates. This is because when it comes to compaction, cast iron will deliver and transmit the vibration and the weight created by that plate downward for deeper consolidation. It is much more efficient and effective than a steel plate. Steel will actually absorb vibration and work contradictive to what you deliver.

Here are the various types you should be considering for your business:

• Forward Plate Compactor

These compactors move in one direction, forward. When you get to the end of a run, you have to either turn around on its axis or rotate and pivot the handle around to turn it. This makes tight access areas difficult to maneuver with forward plate compactors. However, they are popular because they are lightweight and fairly inexpensive.

These are good for granular soils and asphalt compaction. In asphalt compaction, they are equipped with a water tank and sprinkler system so that the plate does not stick to the asphalt.





• Reversible Plate Compactor

Reversible plate compactors are a bit heavier than forward plate compactors and provide the option to go forward and reverse. This makes it much easier to control and work around obstacles, simply reverse at the end of a run, getting into corners or tight situations, and are great for large scale areas due to their size.





Because of their size and weight, they are ideal for non-cohesive and semi-cohesive soils from sandy soils to granular soils. The ability to be able to stop on the spot to compact a specific area and to feather the machine forward and reverse is a great feature for these machines to be precise in your compaction. The downside would be their weight and size if you are unable to accommodate this.

That being said, Bartell Global does have a mini plate compactor that is reversible available in their lineup which is quite impressive.





• Compaction Rammer

A rammer compactor delivers high impact force which is great for those cohesive and semi-cohesive soils from clay to sandy soils that require that high impact. These pieces of equipment are powered by an engine powering a piston set with two sets of springs on a plate that is angled forward so that as the machine jumps it travels forwards. These compactors cover impact, vibration, and kneading types of compaction.





• Sheepsfoot Roller Compactor

Sheepsfoot roller compactors are vibratory or static rollers that are excellent for cohesive soils like clay. They have protruding lugs from the roller that allow for the kneading of subsoil to compact the bottom layers while churning over the top layer.

• Hand Compaction Tools

The only time a hand compaction tool would be used would be in those really tight access spaces or when limited compaction is required as they do not provide the same force that an engine-powered machine would provide. The action provided by them are similar to rammers with the impact created.

 

Consolidation of Joint Material

Much like the consolidation of subgrade and base materials, when pavers or slabs have been installed the joints are filled with some form of jointing material. Whether that is polymeric sand or regular jointing sand, it needs to be consolidated with a vibratory compactor. Typically a small forward plate compactor is used for this application to prevent the cracking and crumbling of the paver material, but providing enough vibration to consolidate the jointing material.

This ensures that that the jointing material reaches the bottom of the paver providing excellent locking of the pavers and allowing for the proper installation of the joints. Plate compacting pavers is more about consolidating that joint material, but also helps to smooth out minimal highs and lows in the material though it should not be relied upon for this.

For most paving products, you will need to attach a polyurethane mat to the bottom of the plate compactor or some sort of protection to prevent the scuffing of the surface of the material. Sweeping a layer of sand on top of the pavers will not prevent this. An alternative for this can be laying plywood or carpet on top of the pavers and compacting that to consolidate the sand.

An alternative to this would be using a roller forward compactor that can be run on the surface of pavers or even 1″ square cut flag without breaking the product while still consolidating the jointing material.