If you are considering building a structure on a vacant lot then there are a number of different tests that you need to complete before you begin framing up your footings and pouring concrete.
One of the big things that many people run into is thinking that the lot they are building on is virgin soil, meaning no one has ever built on it before and it has never been used for any other reason.
Sometimes a property deed search can tell you that your site has never been built on but what if the owner had previously used the site for storing materials like clean fill, top soil and maybe construction materials. In this case it is still a requirement to dig a number of holes throughout the site even in areas not used for your foundation to understand if there are any problems.
Another situation is where you are buying a lot that in a subdivision. Many times these lots can be used for storage of materials but also they can be graded by the developer to provide drainage for existing homes. They can have their topsoil removed to replace soil on homes already built and basically they can be used for so many reasons that you often just can not trust what you are getting without inspection.
Soil Compaction Why Is It Necessary
When you are building on a site the first thing that you need to do is remove all of the topsoil that is present. Normally you will save this material to use later when you are grading your site after completion but the organic materials that are found in topsoil will break down over time naturally and they can not be used as a base for your footings.
Once you remove the topsoil you will be left with a gravely soil that can be made from a variety of materials. It can be sandy and it can be clay. It can have a large amount of small aggregate or it could be rock ledge. You never know until you inspect the soil. Additionally from thousands of years of water runoff, streams that might dry and other conditions you can have your soil properties change dramatically within 10, 20 or 50 feet.
If you want to build on soil that isn’t of the best quality then you will need to widen your footing. A normal footing might change from 12 inches wide to 24 inches wide and that stability of the concrete footing will allow your home to sit on the soil and be supported for many years.
If you know anything about mixing concrete you know that you normally have the driest mix that you can because when water is in the concrete and evaporates the voids or holes in the concrete will weaken it.
There is a similar situation in soil that when you compact it you must have enough moisture to hold it together but not so much that it turns to mud and mush. As you have less water in concrete you want to compact your soil to remove the air pockets between the particles.
What most people don’t understand is that you can actually over compact your soil where it can not absorb and hold enough water to get that perfect density and strength.
So now you know that there are many types of soil conditions and that for the highest strength you want to remove air but allow the soil to continue to absorb a specific amount of water so that it gains its highest strength.
Soil Compaction Site Testing
As we said there can be a variety of soil conditions on your property whether you have a small or large lot. Theoretical values for soil compaction can not be relied on and you need to get out and sample test throughout your building site and the areas around it.
Remember its not just conditions at your footing that matter it is also conditions where your driveway will be placed. Where your septic system will be placed and also any banks of dirt around the property such as large hills. You want to know that your entire property is stable and then after this you can take steps to make it better.
When you are laying out your foundation you will need a number of compaction tests performed so engineers can rely on calculations for your footing size.
It is a pretty easy test to perform. Basically a technician comes out with a metal pole that has affixed a slide hammer of a specific weight. The pole is incremented normally in inches and then in tenths of an inch between each inch. The slide hammer will be lifted and then released and allowed to drop on its own. It is not hammered or driven with force into the ground. The weight of the slide hammer impacts a flange that drives the steel pole into the ground. Calculations are taken to see how many strikes are necessary to drive the pole a specific number of inches into the ground. This will continue in steps until the pole is driven about 30 inches into the ground.
The fewer strikes of the hammer that it takes the lower the soil compaction rate. The more strikes of the hammer it takes the higher the compaction rate is in that exact location.
What is interesting is machinery, trucks and other things will normally compact the first foot of the soil so it is normally very difficult to get through but then lower down the soil can change. This is surface compaction and it must be accounted for. If it is a freshly dug trench you don’t have the same issues with false surface compaction issues.
If a rock is hit the technician may remove it or they may start over. There might be a variety of different conditions that could effect the test but as much information is collected as possible and then the data will be entered into software and a result will be provided with an engineering stamp.
Final Note
Soil testing for compaction is not perfect but you are better off having the tests performed than not having them done. It is good insurance that the area that you are building on is stable enough that its foundation will survive for many decades.
There are a few other methods including nuclear and cone method for compaction testing but as an average home owner you are not likely to have these tests performed on your property. Nuclear probe testing is when they use a rod to make a 3/4 inch hole and then drop a probe into it. Radiation is emitted and the results can give soil compaction and water level information. Another method is the cone test where a hole is dug about 6 inches deep and then sand is poured into the hole to measure the volume of the soil removed. That soil is then dried and compacted and you get a result of the maximum compaction rate of that mixture of soil. Neither of these methods take into account the changes in compaction over depth. The cone method is severely restricted as it only measures the top six inches of soil.
My personal opinion is that the slide hammer method is a better overall test.
In other howtos we will go over how you can compact soil and what you need to do to amend soil if the mixture quality is extremely low.
This how to is just an overview of what you might expect a civil engineer to do while reviewing and testing your soil compaction levels.