By Brant Roeming
What do you think is the most important part of your house? If you asked each subcontractor on your job site, you likely would receive a different answer each time you asked the question. Personally, I think it’s the foundation of a home. But, in order for a properly designed foundation to last a lifetime, it must be placed on solid ground. This isn’t a new idea. In the book of Luke, in the New Testament of the bible, there is a story that says, “The rain came down, the streams rose and the winds blew and beat against that house; yet it did not fall, because it had its foundation on rock. But everyone who hears these words of mine and does not put them into practice is like a foolish man who built his house on sand.” Even 2,000 years ago builders knew the importance of having a foundation bear on something solid. “Rock” isn’t always conveniently located and many times neither are level building sites, so the use of fill material is a frequent necessity.
Many builders use fill, so what is the big deal? Once soil is removed from its original location, it tends to increase in volume because there is less pressure holding the soil particles together. This soil is no longer compact virgin soil but fill now that it has been disturbed. When the soil is later placed as fill to make a grade change it is still in a “loose” state and unless additional steps are taken during the filling operation, it will take a very long time for the soil to compact itself naturally. That’s fine, if your house isn’t sitting on top of it. But if your home’s footings are resting on loose soil, then there is likely to be settlement in the fill area producing differential movement in your foundation over a relatively short period of time. The effect of fill consolidation only takes a few years before it develops as a foundation problem. This post construction settlement of the soil can be overcome by compacting the soil while it is being placed. Before discussing placement, let’s talk about what makes good fill.
Good fill should be free of organic material such a roots, vegetation and other deleterious debris. These materials will decay over time causing settlement. Other undesirable objects include large rocks and other large difficult to compress chunks. Fill material should meet specifications created by a qualified professional engineer for a particular project regarding the type of material and the manner in which it should be placed. Often not only is a material specified by the engineer but also the amount of moisture needed to reach optimum compaction listed as well.
Tests have shown that soil placed within 3-5 percent of its optimal moisture content enables the fill to be packed more closely together. Having a small amount of water surrounding the particles helps them slide closer together allowing a tighter fit. If more moisture is needed, a water truck is driven over the compaction area if it is large or for smaller areas, a hose can be used to raise the moisture content of the fill. If the soil remains dry, the particles will lack the lubrication necessary for proper compaction. On the other hand, if the conditions become too wet, the excess water will only increase the volume causing a condition called “pumping.” A pumping subgrade is difficult to compact because it’s like trying to improve the density of a soil that’s covering a mattress.
Once a satisfactory moisture content is achieved, what’s next? The soil must be spread in sufficiently thin layers to permit water and air to be expelled easily. These “lifts” or layers usually range from 6- to 12-inches in thickness, depending on the type of soil used and then is driven over by some type of compaction machinery. This is generally achieved with the use of a compaction roller or heavy rubber tired vehicle like a fully loaded tandem axle dump truck. Typically, as each lift is placed, it must be thoroughly compacted to between 90 and 105 percent of the Standard Proctor Density as determined or to the specifications set by your engineer.
On site density tests should be conducted by a geotechnical engineer or his representative. Density tests are done in the compacted layer below the disturbed soil at the surface. As each layer of loose fill is compacted, density tests should be done as directed by the geotechnical staff. If the lift being tested fails to meet density specifications, that lift should be reworked until it meets the desired specifications.
The three ingredients to attain proper compaction must be achieved for positive results. These include the use of clean moist fill material, fill placement in thin lifts and testing by a geotechnical engineer. Following these steps will help assure that the majority of settlement will take place before the foundation is poured not five years after completion.
Brant Roeming is a Risk Manager for Bonded Builders Warranty Group. You can reach him by calling (800) 749-0381, Ext. 3660, or by emailing firstname.lastname@example.org.Share