Every new build or addition must start with a strong foundation. The stability of a strong foundation relies on the soil beneath it. To ensure the soilon your build sitehas ideal properties for concrete slab support, it is common practice for contractors to haul in new soilto be used as “fill”.
Fill dirt -also known asfill- is a type of dirt that’s used to create a sturdy base to support a concrete foundation. Bringing in fill dirtis not always necessary; withthe right foundation system, it is possible to use the native soil thatalreadyexistsonsite as the subgrade for your structure.
The Basics: Native Soil vs. Select Fill
Native soil is the soil that is originally present on an untouched building site.This means that thesoil that has not been previously disturbed and matches the properties ofothersoils in thelocal area. The qualities of native soils might not always be favorablefor building,andtheyvary distinctively from region to region.
If the conditions are not favorablefor construction,the project will require some subgrade preparation for concrete slabs. To achieve this,select fill or “fill dirt” isbrought onsite.This soilisbrought to the site for its desirable properties. Typically select fill will include granular materialslike sand, gravel, or crushed stonethatprovides thestrength and permeabilitythatis ideal for slab-on-grade projects.
Why Is "Fill Dirt" Often Used in Construction?
Expansive soilsare defined as any soils thatexpand and shrink depending on the presence of water.The use ofexpansive dirt under a concrete slab can bea big cause for concern. When these soilsexpand and shrink, they create movementthat causestress totheconcrete slababove them. Thisdifferential movement of soils is one of the most common causes ofserious structural damage.
If the native soils onsite exhibit these less-than-desirable traits, they are removed and replaced with select fill. “Fill dirt” will be free draining;meaning thatit will not retain water, thereforecannotexpand and contract below your concrete slab,causingfoundationfailure.
On hauling ofselect fill does improve the conditions of the site,butit addssignificant coststo the projectthat add up quick. There will be added cost for excavating, hauling, and disposing of the existing native soils along with purchasing and installing the new select fill.
These costscan add up to $25/SF depending on the severity and size of the site!Not to mention the time spent searching forquality select fill material in the quantity that is needed for the project.
Can You Use Native Dirt Under Concrete Slab?
It is possibleto avoid these costs andto utilize the existing site soil beneath your foundation.
In certain regions, the soils are ideal, free-draining,and fine aggregate, andonlyrequire evening-out. In this case, there is no need to haul in select fill. However, what about non-ideal scenarios? Can we avoid the cost of hauling in select fill?
The best way to avoid the need for fill soil is by choosing a concrete foundation solution that is designed to accommodate for less-ideal soil types.
Using a foundation system that can accommodate expansive soil conditions, like the Wafflemat Foundation System, is your best option. This system utilizes structured voids beneath your foundation slab that provide the expansive soil a pocket to expand into.Sincethe expansive soilsunder this foundationhave a place togo,they will notbe stressing the slabnor causeany structuralfailures.
Sometimes the best fill for a concrete slab is the type that will cost you nothing – no fill at all.
Instead of paying for select fill for your build site, choose a foundation system that will work with the soil that is already there.Thismethodbenefits the overall costand turnaround timeof yourproject,and itremoves the need to locate offsite fill soil thatcanbe difficult tofindin the quality and quantity needed.
StopPaying forSelect Fill with the Wafflemat Foundation System
The Wafflemat Foundation System is designed to accommodate expansive soils. Also known as a ribbed slab foundation, this foundation can be installed in “subpar” native soil conditions including expansive, rocky, or hydro-collapsible soilswithout the need for any fill dirt!
The signature difference in a Wafflemat foundation are the void forms. Void forms are placed above the native soils after subgrade preparation for concrete slab in a grid pattern. The forms are secured with patented brackets that double as a connection point for post tension cables.These void forms are made of plastic boxes ensuring their integrity for years to come.
Once the void forms are properly installed, concrete is poured directly over the forms to create your slab. The void space in the forms allows the native soils to expand beneath the slab without disrupting the resting location of yourfoundation. If soil moves beneath a concrete slab without this system in place, your building will be subject to structuralfailuresincluding cracks in walls and floors, jammed doors and windows, or irreparable foundation damage.
After a Wafflemat system is installed, it will appear exactly like a traditional slab-on-grade concrete foundation with the only notable difference in time and cost savings.Wafflemat canresultin savings ofup to 20%on concrete and steel materials. The Wafflemat Foundation Systemis usedin both residential and commercial applications.
If you’re a developer looking to save time and money by cutting out the need for select fill dirt on your construction sites, you need to check out Wafflemat.
Since 1993, Wafflemat has been used in over 30,000,000 square feet of foundations across Americawithout a single structural failure,proving its effectiveness against expansive and rocky soil conditions.
For more details on how theWafflemat Foundation Systemwill work for your project,please fill out the formon this page!
Written By: Brionna Marie
Brionna has a B.S. in Civil Engineering from Clarkson University and several years of design and construction experience serving both public and private clients.Today, she utilizes her industry knowledge to conduct research and write technical articles across a variety of civil engineering topics.
