Can You Compact Wet Dirt?

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Soil compaction may seem like a very simply task, but when you come down to actually performing the same, you are likely to find that there are a lot of intricacies involved in the process. So much so that there are actually many studies on the topic – especially the question of what you can compact wet dirt with.

Whether you’re compacting dirt for a patio or paver or to lay a foundation within, it is important to do it right. A plate compactor can be extremely useful to compact dirt, but then again, a question remains. Is it better to compact dirt as is? Can you compact wet dirt?

A straightforward answer; yes. Not only can you compact wet dirt, it is actually recommended in most cases that you do so. This is because water reduces the friction between dirt particles, effectively reducing the time it takes for dirt to compact. We mentioned previously how long it takes for dirt to settle on its own and the table within showed that wet dirt settles more quickly. But there’s more to this than meets the eye.

Can You Compact Wet Dirt – How It Works

Wet dirt compaction

Soil in the UK often remains generally wet compared to some states in the US and other equatorial regions (such as Middle Eastern countries). The wet weather means that while the soil ends up compacting much more easily, there are several challenges that fieldwork may present.

On the other hand, where the soil is relatively dry, there are concerns regarding the soil not settling enough and therefore not being suitable for laying foundations.

Whether you’re looking to plant in the soil or create a structure on top of it, it is usually a good idea to wet your soil before you start working it. Normally, there are a lot of gaps between soil grains. By introducing water into the mix, you are basically filling those voids with water. The cohesive nature of water means that it pulls grains together, facilitating their motion toward each other.

This way, wet dirt compaction is much easier than dry dirt compaction.

However, before rushing out and gushing water on top of your dirt, remember that there is a technique of doing so. Too much and you will end up making the soil too soggy. As you compress, the water will rise up, effectively leading to a loose top layer. You will have to drain the excess water before compacting the topsoil.

Add too little water and you run the risk of uneven compaction. As you continue compacting using a plate compactor or even an excavator, it might seem like the surface is even but as time passes and the dirt continues to settle, you will find that in some places your surface will depress, while in others, it will remain the same.

Remember, soil wetness is inversely proportional to void spaces in the soil. The more water you add, the fewer the voids there will be and the more your soil will compact. However, you need to make sure that you don’t end up adding too much water.

By default, most soils have a much larger ration of air in between grains. The optimum moisture content (OMC) is 50%, and that is when soil compacts best.

Practical Applications of Wet Dirt Compaction

As mentioned above, the best compaction happens only when the moisture content is optimal. In the field, industrial workers use a modified Proctor test (ASTM D 1557) to determine the moisture level. Different settings are used for different purposes. The test also takes into consideration numerous other factors such as material grade, environment, and other fine details.

Another method used in the field is the measurement of bulk density.

For DIY purposes, just keep an eye on the soil and continue watering it. Once you see that water has stopped seeping in, you’re done. Optimally, you should keep the water level slightly below this point, but without a test, this is the best way to do it.

Another method you can use – a relatively more reliable one – is to pick up the uncompacted, wet dirt and see whether it can be created into a ball or not. Create the ball and see if it retains the shape. If it does, the soil is ready to be compacted.  

How to Wet Dirt for Compacting

Now that we’ve covered the basics, let’s consider how you can wet dirt to compact it. Remember, compacting methods are relatively similar; all you have to do is sue your excavator or plate compactor (don’t forget to wear wet shoes!) and you’re done.

The goal should be to decrease soil porosity, meaning the number of open pores between soil particles. The more compact your soil is, the fewer pores it has. Water has a way of eliminating these air pockets by ‘opening new doors’ for these grains.

Before you get into wetting dirt, however, it is important to remember that it is more difficult for plant roots to spread in compact dirt than in loose soil.

To begin, you will need:

  1. A rake
  2. A garden hose. If it has a spray nozzle, that will perform much better. A sprinkler will do just fine here as well
  3. (Optional) A soaker hose
  4. A plate compactor. If you’re simply looking to compact dirt for your lawn, a lawn roller will do just fine as well. You can also go with a hand tamper

Naturally, the more effort you put in, the better the results you’ll get. To pack the dirt:

  • If the soil isn’t already poured in, pour it. If you’re compacting dirt for a lawn, fill it an inch, two at most, above the desired level. If you’re looking to lay pavers or a foundation for your patio, it is a good idea to fill soil 6-8 inches above the required level.  
  • Rake the soil with your rake. A bow rake is much better here as it can help you cover more ground quickly.
  • Once the soil is flattened and all debris is removed, spray the soil gently. Use the spray nozzle for better results. Keep spraying until you find that the bottom layer is retaining moister. A slow trickling sprinkler system will have the same effect, but you may not be able to achieve uniform wetness.
  • Allow one hour for water to drain deep into the soil. You’ll find that some soil will settle by the one-hour point.
  • Spray water again to the point where water starts to pool up slightly on the surface. Wait a while for the water to drain and softly pat the dirt. Do not apply pressure. At this point, your only concern is to ensure that water seeps properly.
  • Repeat several times until the water doesn’t drain anymore. Every time you add water, remember to add a little less than last time and to spray it evenly across the area.
  • Once water has stopped draining, the soil automatically won’t settle anymore. Now it’s time to move in with your lawn roller, plate compactor, or hand tamper. Apply an equal amount of pressure across the soil. Make sure you have a measuring wedge driven into the soil so you know how much to compact.
  • As you compact, water will either start collecting on top or seeping below. Don’t add any more water; just keep on compressing until you have the desired level of compactness.
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About Charlie D Paige

Charlie is a massive DIY fan, with dozens of DIY projects under his belt - ranging from tiling to electrics, and concrete pads to walls. Charlie loves tinkering, seeing how things works, the outdoors and playing with power tools... so is it any wonder that he's completed so many DIY jobs over the years?

Charlie loves spreading his hard-won DIY experience with the world via this blog.