It cannot be sufficiently stressed how important it is to start a building project with a good foundation. Here we will deal only with a slab on grade foundation. It can be residential or commercial. The steps are essentially the same.
So the first step is to see what the finished grade is on the project’s site plan. Also called the final grade. Next you have to know how thick the materials such as concrete will be going on top of the dirt. This will help you determine the grade of the dirt underneath the concrete. This is called the subgrade which is our primary concern.
You need to know the current elevations of the area where the foundation is located.
1. Set up stakes around your foundation. You will need those in order to mark where the final elevation will be.
We use a laser level (Topcon RL-H4C Self Leveling Horizontal Rotary Laser, Telescopic Rod and Tripod to find the elevations, which is far superior to the old method which was to use a manual level. It required two people, one to hold the rod and another to look through the level to determine the rod’s measurement.
2. Set up your tripod and place the laser on top. It is self-leveling. There is no viewfinder to look through. Follow the instructions for operating the laser level. Now all you have to do is go hold the rod. The receiver attached to the rod will display the elevation.
3. Find a known elevation in the area such as a manhole cover so you can set your elevation on the site based on the known elevation. The elevation of the manhole should be found on the site plan as well.
So let’s say the known elevation is 200 feet above sea level. The plan says your final elevation is 196 feet above sea level. You shoot an elevation at that known point of 200 feet using what is called a laser rod. A rod is really just a tall ruler. The laser will display the grade of the manhole cover on the rod. Let’s say it reads as 15 feet. The manhole cover becomes your temporary benchmark (TBM). You already know that there is a four-foot difference between the known elevation of 200 feet and your goal elevation of 196 feet. So, if the manhole cover, your TBM, is measuring a 15 on the rod then the final elevation of the foundation should read on the rod as 19. The final grade of the site will be lower than the manhole.
4. Now you move the rod to a point on the foundation. The laser level reads 23. That tells you that the foundation will have to be raised by four feet. In other words, you will need to bring in enough dirt for your subgrade to raise it four feet minus the thickness of the concrete. Typically, a house foundation has five inches of concrete. With that in mind you will need 3 feet and 7 inches of dirt for your subgrade. Next step would be to mark the stakes at the proposed elevation of the subgrade.
5. Strip grass and topsoil and pile on the side using a bulldozer. Now you are ready to move on to bringing in the additional fill.
To know when you have sufficient compaction you need to know that most sites require a 95 percent compaction. In order to achieve that there must be a firm surface on which to place the new fill. New fill is normally called “borrow excavation” or trucked in material.
1. Before you bring in the borrow excavation you need to proof-roll the existing dirt. You can do this by having a loaded tandem (15 yard) dump truck drive over the area in every direction. Look for any pumping, which is best described as dirt moving as the truck rolls over it.
If the proof- rolling produces no pumping and ruts then it is stable and you may proceed to bringing in lifts. However, if the dirt is pumping then you need to consult with the geo-tech inspector on site to determine what to do. Most likely you will have to excavate additional soil from the site and reprocess it. You then will have to refigure how much fill to bring in and adjust your price as necessary.
2. Once you get the good pad on which to start you have to know how thick the soil has to be placed and what you need to pack it in. This also entails knowing how wet or dry the soil is and how many lifts you will need. A lift is the incremental amount of material you place as you bring it in. Using a sheepsfoot roller each lift has to be compacted. The first lift will be about 12 inches.
3. After you have compacted the fill with the roller it is time for a test. The geo-tech inspector will conduct the test. The inspector will drive a hole of approximately 6 inches deep in the dirt using a rod and sledgehammer. He inserts the rod of his nuclear compaction machine into the hole. The fill to be imported was previously tested and he will input those test results into the machine. The machine calculates the compaction percentage and the dampness of the dirt. If the compaction is at least 95% then you can proceed to add additional lifts.
Generally, the remaining lifts will be 9 inches. When the nine-inch lift is compacted you end up with a finished lift of approximately six inches. Test will be conducted on each lift until you reach the correct elevation.
Another important thing to remember is that while you are preparing the site you do not want any foreign matter in your foundation soil. Examples of foreign matter include roots from existing trees near the site, topsoil, trash or any other matter that does not belong in the foundation.
You also must be prepared for possible rain showers. If you will not be able to complete the foundation before a rain then the site needs to be sloped in order to have positive drainage so the site will not be inundated with water. You should slope it so the water will drain to a small area so that upon your return you can pump the water out with a pump. We recommend (Drummond Electric Water Pump) or a Gas Powered Pump (we use a Titan Heavy Duty Trash Pump) for dewatering. Once drained, remove the wet soil and then continue to bring in your lifts.
When you have reached the correct elevation on your subgrade and passed all the compaction tests you are done. Invoice your client!