What is a Foundation?
Types of Foundation
What is a Structural Foundation?
Structural foundation is the part of the building that is connects it to the ground and soil. It provides support for the structure that is above soil level. The main function is to transfer loads from the structure to the ground. An example is when slabs transfer their weight to girders which transfers that load as well as the load already applied to girders to beams. Beams transfer that load as well as the load already applied to beams onto columns. In return, columns transfer that load to the foundation. When designing foundations, you must consider the bearing capacity of the soil as well as the resistance to shear stress, overturning, and sliding.
Underpinning
http://www.understandconstruction.com/introduction-to-foundations.html
Underpinning, or foundation repair, is the act of strengthening a foundation. This occurs when the building is showing signs of sinking or cracking (indicating that the current foundation system is not able to bear the weight of the building), the building is being enlarged or converted (which results in heavier loading than the original design planned for), or if a large new building with deep foundations or basements is being constructed close to an existing building (causing problems for the existing building).
- Pit Method
This is done in small sections at a time. Depending on the size of the building, each section could be 8ft to 12 ft (approx. 2m – 4m). Steps on how to carry this out are listed below.
- A temporary foundation system is first constructed around the existing one.
- Then a pit is dug around and below the existing foundations (relieving them of load-carrying duties).
- A new foundation is built below the existing one by filling the pit with concrete.
- Once the concrete has set, the temporary foundations are dismantled, allowing the load to settle onto the improved foundation system.
- Needle Beam Method
This method has an advantage as it does not disturb the inside of the building. Steps on how to carry this out are listed below.
- First construct two micropiles at a specified distance from the building. This is done with a micropiling rig.
- Cut a hole just large enough to take the needle beam.
- Construct the RCC needle beam over the piles and inside the foundation wall.
- Once the beam has set, fill in the earth over the beam. The new foundation system can now take additional load.
- Micropiling Method
- Construct the micropile just below existing foundation at an angle.
- Excavate earth till the top of the micropile ensuring there is enough space for a worker to use their tools.
- Manually remove all earth between the pile and the existing foundation, creating a cone shaped void that rests on the micropile.
- Fill in the gap with concrete. Once it sets, the existing foundation can transfer forces to the micropile which increases the capacity.
- Repeat this process at regular intervals.
Types of Foundations?
Shallow Foundations
http://www.understandconstruction.com/introduction-to-foundations.html
Shallow foundations must be protected from freezing since water in the soil around the foundation can freeze and expand which damages the foundation. These foundations should be built below the frost line (level in the ground above which freezing occurs). If they cannot be built below this line, they must be protected by insulation (e.g. a little heat from the building will permeate into the soil and prevent freezing).
- Spread Footings / Individual Footings
This type of foundation is shallow (usually no more than 3 ft deep). It can support one column or multiple columns (called “joined footing”). It’s normally used for small loads and the top soil layers are not weak (e.g. flag poles, single story buildings). “To construct these types of footings, the required depth is excavated, reinforcing steel bars are placed according to design, framework is placed according to design dimensions, and concrete is poured. After the concrete reaches its maximum compressive strength, the excavation is filled with soil, and the soil is compacted.”
http://www.understandconstruction.com/introduction-to-foundations.html
- Strip Footings
This type of footing is commonly found in load-bearing masonry construction. It acts as a long strip that supports the weight of an entire wall. These are used when the building loads are carried by entire walls instead of isolated columns.
- Mat Footings
Mat foundations, also known as raft foundation, are used when the soil has low bearing capacity. It has multiple columns supported on one large foundation. Due to having a large foundation, when the loads are distributed, it reduces the stress exerted on the low-bearing capacity soil. Mat foundations are normally used in situations where piles cannot be constructed and spread footings are not practical (e.g. if the top layers of the soil are weak and you’re constructing a low-rise, multistory building).
Deep Foundations
- Pile Foundations
This type of foundation can be made of reinforced concrete, steel, or wood. They are large columns that reach into deep layers of the spoil which act as fixed ends for the structures that they support.
http://www.understandconstruction.com/types-of-foundations.html
They are used when there is a layer of weak soil at the surface. Since the layer of soil cannot support the weight of the structure, the loads of the structure bypass this layer and are transferred to the layer of stronger soil or rock that is below the weak layer. They are also used when the building has very heavy, concentrated loads (e.g. high rise structure). Pile foundations can take higher loads than spread footings.
http://www.understandconstruction.com/types-of-foundations.html
- End Bearing Pile
The bottom end of the pile rests on a strong layer of soil or rock. The pile acts as a column as the load of the structure is transferred through the pile onto the strong layer. The important thing is that the bottom ends rests on the surface which is the intersection of a weak and strong layer so the load can bypass the weak layer and is safely transferred to the strong layer.
- Friction Pile
The load of the building is transferred to the soil through the full height of the pile by friction. For visualization, imagine pushing a solid 4mm metal rod into a frozen tub of ice cream. Once the rod is pushed in far enough, it is strong enough to support some of the load. The deeper the rod is placed into the tub of ice cream, the more load it can support. The amount of load that a friction pile can support is directly proportionate to its length.
https://lassiterexcavating.com/blog/deep-foundations-types-and-uses/
- Caissons
These are hollow substructures that can be constructed near or on the ground surface which are sunk to the desired level as a single unit. They can carry enormous loads and are commonly used for bridges.
- Cylinders
These are small caissons with only a single cell.
https://lassiterexcavating.com/blog/deep-foundations-types-and-uses/
- Basement Foundations
The hollow substructure provides storage or working space below the ground level. The usage of the space controls the structural design. They’re constructed in open excavations.
https://lassiterexcavating.com/blog/deep-foundations-types-and-uses/
- Buoyancy Rafts
“Buoyancy rafts, also known as hollow box foundations, are designed to create a semi-buoyant or buoyant substructure below which the loading on the soil is decreased to the required low intensity.”
They are more expensive than other foundations so they are mainly used on sites that are on silts, soft sands and other sedimentary deposits that are very deep, or where loads can be kept concentric.
https://lassiterexcavating.com/blog/deep-foundations-types-and-uses/
- Shaft Foundations
Shaft foundations are good for numerous seismic retrofit projects as they are under low overhead conditions and can be constructed near existing structures. This foundation is constructed by drilling a cylindrical hole within a deep excavation and subsequently placing a concrete or another prefabricated load-earing unit in it. It is difficult to install them under certain conditions such as soils with boulders, soft soil, loose sand, and sand underwater.
House Foundations
https://www.thespruce.com/types-of-house-foundations-1821308
Basement: Full and Daylight
A full basement has structural foundation walls that are supported by foundation footings. In modern homes, they are made with poured concrete or mortared concreate block. Footings extend below the frost line. If desired, it can be heated or cooled like the rest of the house. A full basement can act as a living space with a height that is at least 6ft tall.
A daylight basement is when the foundation is built against a slope and it has one or more sides that are completely embedded in the ground from floor to ceiling. As the slope descends, certain sides of the foundation are exposed to light allowing large windows and doors to bring daylight in.
Modern basements are made with poured concrete or mortared concrete block whereas basements located in old homes can have stone walls.
Pros:
- https://edensstructural.com/a-look-at-the-different-types-of-home-foundations/
- Space for storage and additional living space
- Natural ventilation
Cons:
- https://edensstructural.com/a-look-at-the-different-types-of-home-foundations/
- Expensive
- Lead to more maintenance issues if not properly cared for as basements are known for having issues relating to flooding, moisture, and mold
https://www.thespruce.com/types-of-house-foundations-1821308
Crawlspace
A crawlspace has foundation walls that stand on footings. The walls are short (can be 4ft tall for storage) creating a space where you must crawl to get through. These spaces are unheated but can be ventilates to allow a small amount of airflow. It is less expensive than full basements because they require less excavation, foundation wall material, and labour to build. Typically, crawlspaces are made with poured concrete or mortared concrete block.
Pro:
- https://edensstructural.com/a-look-at-the-different-types-of-home-foundations/
- One of the most durable foundations in areas with a high groundwater level
- Better airflow underneath the house in the hot months
- Storage for outdoor items
Cons:
- https://edensstructural.com/a-look-at-the-different-types-of-home-foundations/
- More difficult to heat the home in the winter due to the open space below the house
- Stagnant water and moisture can become trapped in a crawl space and cause health issues
- If not insulated and sealed properly, issues will occur
Pier Foundation
https://edensstructural.com/a-look-at-the-different-types-of-home-foundations/
“Pier foundations are constructed using small circular or square pads of concrete that are located in strategic points along the outer perimeter of the home. These pads are inserted and secured into the ground using treated wood posts or steel rods. These posts or rods are then connected to the home for support.”
Pros:
- https://edensstructural.com/a-look-at-the-different-types-of-home-foundations/
- Ideal for plots of land where traditional foundation is difficult to lay
- Strong and durable
- Excellent support and doesn’t come with the cons that full basements and crawlspaces have
Cons:
- https://edensstructural.com/a-look-at-the-different-types-of-home-foundations/
- Expensive
- Foundation can lead to creaking and sagging floors
- Associated with poor ventilation
Concrete Slab-On-Grade
https://www.thespruce.com/types-of-house-foundations-1821308
This type of foundation is used in climates that do no experience freezing and thawing (otherwise it would lead to cracks in the concrete and shifting of the foundation). It is a solid piece of concrete that rests on the ground. Grade refers to the ground-level and slab refers to the monolithic concrete pad.
https://edensstructural.com/a-look-at-the-different-types-of-home-foundations/
The thickness of a slab can range from 4 inches – 8 inches and are reinforced with steel rods and drainage pipes.
Pros:
- https://www.thespruce.com/types-of-house-foundations-1821308
- Offers better protection against termite infestation (more common in warmer climates) since there is no airspace in between the home and the foundation.
- https://www.thespruce.com/types-of-house-foundations-1821308
- Less expensive than a full basement and crawlspace
Cons:
- https://www.thespruce.com/types-of-house-foundations-1821308
- More susceptible to warming due to a lack of ventilation between the home and the slab
- Little protection against floodwater
- https://www.thespruce.com/types-of-house-foundations-1821308
- Offers no storage or living space
- Concrete slab must be cut into to access the pipes.
https://www.thespruce.com/foundation-footings-code-basics-1822269
Building Code Foundation Requirements:
The following requirements listed below are from the International Building Code (IBC) for 1 – 2 story residences.
Site Grading
The slope of the ground immediately adjacent to the foundation footings must be a minimum of 5% for 10 ft.
Load-Bearing Value of Soil
Foundation must rest on undisturbed soil because it is stronger than disturbed soil. Undisturbed soil is soil that has never been turned over, tilled, graded, hoed, dumped, dug, scraped, compacted, amended, or anything of that nature by human or machine. The code refers to load-bearing values (LBVs) as “presumptive”. This means that a test must take place to determine if the soil is disturbed or not. Disturbed soil can be reclassified as undisturbed soil after a certain time.
TITLE: A chart displaying how much square footage is needed for testing depending on the type of soil
Depth of Footings
https://www.thespruce.com/foundation-footings-code-basics-1822269
In general, footings should extend to a minimum depth of 12 inches below undisturbed soil as well as 12 inches below the frost line or must be frost protected. Certain structures such as sheds or decks have different regulations and requirements.
Width of Footings
https://www.thespruce.com/foundation-footings-code-basics-1822269
“Footing width can vary according to the structure, site, and conditions. Under code, generally, one-story buildings with footings on undisturbed soil with LBVs between 1,500 and 4,000 should have a minimum width of 12 inches. Two-story buildings require a minimum of 15-inch-wide footings for 1,500 LBV soil. For 2,000 LBV soil or greater, two-story buildings may have 12-inch minimum wide footings.”
Levelness
- Top of the Footings
The top of the footings must be level.
- Bottom of the Footings
Preference is that the footings are level but if you’re building on a sloped grade, you can have a 10% slope (one unit vertically per 10 units horizontal).
Strength of Footing Concrete
The minimum strength of the concrete for foundations and slabs (except garages) should be 2500 psi. Vertical walls that are exposed to the weather may require a stronger minimum strength of 2500 psi – 3000 psi depending on weather conditions.
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