Design and Types of Rafts or Mat Foundation.

Raft or Mat Foundation

Rafts have other names such as mat foundation, ribbed slabs, or slab on grade. Rafts take loads came from superstructure of building and spread the loads into a more uniform contact pressure, which eliminates the need for individual footing and may be quite useful in weak soils. 

Rafts can used for larger building 30 to 50 stories. Concrete rafts for such buildings can be 3 to 12 feet thick with extensive steel reinforcement.

Rafts can used instead of piles and the designers compare the cost of drilling very deep piles in a medium to soft soil to that of constructing a stiffened raft on or near the surface. Also, we can use raft instead of piles in construction building nearby structures or infrastructures to avoid pile driving vibrations effect 

- Types of raft foundation.

- Stiffened rafts.

Stiffened rafts sometimes referred as a waffle foundation. It is not a uniform thickness throughout but has thinner slab panels that are bounded by stiffener beams. While stiffener can be above the main slab, they are most found below the main slab where they can be formed against earth.

Stiffened rafts used in:

• Residential construction
• Mid-size commercial in areas where soil condition warrant such use.
• May used on a softer soil that may be subject to settlement or bearing failure in case of reduce the cost of concrete because the stiffening beams are placed to provide the necessary stiffness to the raft without as much concrete as uniform thickness raft.

- Uniform thickness raft.

Used in heavy loaded building structure. Raft thickness ranging from 6 to 24 inches thick are often used for residential and light commercial construction. In this type of rafts used more concrete and reinforcing compare with stiffened rafts.

- Design considerations.

- Soil stress distribution.

Soil stress distribution depend on the rigidity of the raft as well as the type of soil. For example, if placed a very rigid raft on a deep sand deposit will have higher reaction forces in the center of the raft than around the edges. This because visualized by the sand tend to crawl away around the edges. This because the sand does not have as much confinement as that in the middle.

But if placed a very rigid raft on clay soil would tend to have higher stresses in the middle, this because the clay acts more like an elastic continuum than sand. And the change in deformation between the outside of the building and the edge of the foundation is greatest around the exterior and less on the interior.

In case of a completely flexible raft, the edges would go down more than the center in sand soil. And in clay soil, the center would go down more than the edges. Real raft are somewhere between completely flexible and completely rigid; thus, designer should take these variables into account when design the raft.

- Low strength soils.

In case of soil at site have low bearing capacity which required very large spread or strip footing to properly distribute the loads to the soil, the use of a continues raft either stiffened or uniform thickness might be more efficient.

In low strength soils often have varying strength across the site, so using raft will distribute the forces and not permit as much differential deflection across the structure. 

- Expansive soils.

Moisture content in expansive soil changes, so the soil volume will change as well. Increase or decrease in moisture content will result in an increase (swelling) or decrease (shrinkage) in volume, respectively.

Shrinkage is different than settlement. Settlement is a decrease in volume changes due to air or water being squeezed out from the soil and is generally not recoverable.

Shrinkage of an expansive soil is caused by the removal of water from between the clay particles allowing the particles to be closer together. Shrinkage is generally recoverable, because once the water returns to the expansive soil, the clay particles will be pushed apart again. 

Raft on expansive soils designed assuming wetting or drying around the perimeter of the foundation. When the soils under the raft’s perimeter dry out and shrink, the edges of the raft lose support and deflect downward. While this type of movement is confusingly referred to as “center life” in the engineering literature, a more appropriate description of this type of movement would be “edge drop”. The weight of the structure contributes to the downward movement of the edges of the raft.

When the soils under the raft’s perimeter wet up, this type of movement referred to as “edge life” in the engineering literature. While the weight of the structure resists the upward movement of the edge of the raft. Most residential and light commercial structures are relatively light, and therefore this resistance may not be significant.

- Reinforcement of the concrete in raft foundation.

Reinforcement of the concrete in raft foundation whether the raft is stiffened or uniform thickness, can be conventional reinforcing steel or post-tensioning cables.

- Mild steel reinforced raft.

Reinforcement in raft is based on the need for the addition of strength to maintain the stiffening effect required for the site condition and to reduce cracking due to concrete during shrinkage.

Mild steel in the slab portion of a stiffened raft with 4 or 5 inches thick slab for the purpose of eliminating concrete drying shrinkage typically ranges from # 3 bars at 10 inches on center each way to # 4 bars at 16 inches on center each way.

Stiffener beams in a stiffened raft will typically have steel ranging from # 4 bars at top and bottom to three # 8 bars top and bottom.

- Post-tensioned reinforced raft.

Flexural or bending stresses place the concrete in tension at either the top or bottom face, and the pre-stress force must be overcome before the tensile strength of the concrete is engaged.

The cables or pre-stress tendons are usually 7 strand, high strength steel of nominal ½ inch diameter, encased in a plastic sheath to prevent bonding of the tendon to the concrete. Steel anchors are located on each end of a cable to transfer the pre-stressing force to the concrete. The inherent tensile strength of concrete is typically on the order of 300 psi.

An advantage of post-tensions foundation is that:

Required less labor to install the reinforcement cables than that required to install steel reinforcement in mild steel raft.

Required less quantity of steel reinforcement which led to decrease the cost of raft.

- Installation.

Since raft is one type of shallow foundation types which rely on the sub-soil immediately below the foundation for support, its important that the support be adequate and available in the right places and also have the required bearing capacity to can carrying the structure loads. It can either compact the original soil and tested it or replace the original soil (if it has low bearing capacity) by approved fill materials.

Then it is important to have a proper water proofing system to prevent floor from ground water or moisture. After that place steel reinforcement or cables in case of post-tensioned raft then cast the approved concrete type.