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Types of Foundation Systems
 

(Figure 1)
The above figure shows different types of foundations. The four shown here are the most commonly used in the Denver Colorado area. There is a monolithic design used for detached garages and small buildings not shown here.

  1. The top left is a concrete footer with the foundation wall on top of the footer.


  2. Top right is a foundation wall poured on existing soil (no footer).


  3. Bottom left is a foundation wall poured on pads as specified by a professional engineer. Spacing of pads depends on soil tests load bearing properties.


  4. Bottom right is a concrete drilled pier that locks into bedrock.

(Figure 2)
Where expansive soil is found the type pictured below is most common. This type of wall sits on top of drilled caissons which extend to various depths into the ground depending on structure and soil type.

Between these caissons is a collapsible material known as verticell or void form which keeps a void of several inches between the foundation and soil. When expansive soil collapses this void material becomes so intense under the foundation wall that cracking starts appearing in the foundation wall. Cassions are designed to hold a structure up as well as hold it down from lifting up expansive soil conditions.
 
(Figure 3)
This picture shows where vertical void has closed and as a result this foundation wall lifted leaving the reinforced pier in the ground pulling the foundation wall apart.		  
 
(Figure 4)
When the above occurs this is what the outside looks like.		  
Cracking and Heaving of Concrete
Minor cracking in concrete is normal. However, when concrete sinks it is caused by setting of the soil. If concrete begins to heave and separate, swelling (expanding) soil is the problem. There are different types of swelling (expansive) soils. Figure #5 shows clay before and after expansion.

What Causes Soil to Swell?

(Figure 5)
In a sample of pure montmorillinite the plates may expand so much that the sample increases up to 15 times its original volume. While most montmorillinite soils will not exceed over 35-50 percent, the pressure and movements created on a building foundation or floor by this tremendous volume increase are responsible for most swelling soil damage.

The change in rock or soil volume is caused by the chemical and physical attractions of water molecules to the tiny clay plates that make up the expandable montmorillinite clay material often called "bentonite". As moisture or water comes in contact with the clay, the flat plates are pushed further apart. (Figure 5)
(Figure 6) This picture shows a room addition that is pulling away and dropping from the main part of the house. There are several reasons this happens: (1) No foundation under the room when it was enclosed on a slab which is a violation of the building Code in all jurisdictions. (2) Improper foundation system for the existing soil conditions (3) Poor drainage causing settling of lifting of the foundation system. (4) Allowing excessive moisture and excess drying after the structure was built.

Our structural engineers have designed corrections numerous times over the years for these problems. Over the years, Alltypes Concrete & Construction, Inc. has stabilized and successfully repaired many of these situations.
(Figure 7)
Several access holes were excavated next to the structure to either under pin or install helical piers.		 (Figure 8)
A helical pier is being installed
(Figure 9)
Many times top wall and wall bracing is installed to strengthen and stabilize the foundation wall. This is accomplished on the interior of the present concrete basement wall.
(Figure 10)
If the foundation wall is badly cracked a structural wood timber pressure treated wall is used from the inside of the basement wall. This system is designed by our structural engineer.
(Figure #11) A sister wall is another method occasionally used to reinforce an existing foundation wall that is a problem due to various problems.
(Figure #12) Here we are installing a foundation under a room that started settling because it was built on a concrete slab.
(Figure #13) A new foundation is completed and ready for water proofing. The last step is to back fill the excavated area.
Please contact Alltypes Concrete & Construction, Inc. if you have additional questions regarding Engineering or repairs.