The type of foundation your building requires depends on many variables: soil type, building load, and environment to name a few. Sometimes the surface soil is very loose but below it is rock or very solid soil. Taller, heavier buildings require stronger foundations, while a shorter, wider building spreads the load over a larger patch of ground and can use a shallower foundation.
Two common foundations are mat foundations (shallow) and pile foundations (deep).
A mat foundation, sometimes called a raft foundation, looks like it sounds. It is a “mat” of concrete that sits on or just below the ground; in other words, a shallow foundation. You may have heard it referred to as a slab foundation, a common choice for steel buildings.
Mat foundations are appropriate for light pre-engineered metal buildings with multi-span rigid frames and flexible walls. And they are suitable for building on poor soil (excluding peat and organics) that is uniform in consistency.
Heavy column and wall loads are distributed across the building area to lower the contact pressure, more so than with footings. Mat foundations are popular in areas where basements are common.
Mat foundations are cost effective choices for individual column footing areas totaling an area covering more than half the total footprint of the completed building. It is at its least expensive when the top of the mat is at floor level; meaning slab-on-grade is not needed.
During the design phase, you must consider the following:
- Mat stiffness
- Edge conditions
- Variability of column loads
Mat foundation design
Mat foundations are best for buildings of moderate size with a regular layout. It’s possible to place the center of the mat at the center of the vertical loads held by the columns provided a uniform soil pressure is present. Horizontal column actions to the outside are canceled within the mat while inward reactions are mitigated by the friction of the mat on the soil. You usually don’t have to worry about uplift pressure simply due to the sheer weight of the concrete. The best designs minimize the effects of settling between the columns.
The downsides of mat foundations
Some of the disadvantages of mat foundations are:
- The need for heavy reinforcing in certain regions.
- Frost heave can damage it if the mat is at grade.
- It doesn’t accommodate trenches or deep pits well.
- The design can become very complex.
Again, rather like it sounds, this is a foundation built on piles, columns made of strong materials. A pile foundation is a deep foundation meant to support buildings on ground that is of inappropriate soil near the surface but where uniform and supportive soil is found deeper. They are highly complex structures and require geo-technical engineering expertise.
The piles are either driven into the ground or cast in place. Usually, piles are installed in tripods, groups of three. This provides a sturdy foundation that is relatively tolerant to imprecise column placement and/or unequal column loading.
A pile foundation has two components:
- Pile cap
- Single pile or group of piles
The pile cap is a platform that covers the top of a group of piles and on which to place the supporting members of the structure. The piles are long, thin members (up to 15m or more) reaching from the pile cap to the supportive layer of soil. The foundation is classified based on the construction material of the pile, the type of soil, and the load transmitting characteristic of the piles.
Made of wood, steel, or reinforced concrete, pile foundations are vertical or leaning columns that are either prefabricated or cast in place. Prefabricated piles can be hammered, grouted, vibrated, or twisted into place. Those that are cast in place may or may not have a shell, can be created by many different technologies, and have multiple construction steps.
Piles act as laterally braced columns that pierce through the weaker soil into more suitable ground. The load is transferred to the soil through end bearing and/or skin friction. Friction is better against uplift since end bearing only has the weight of the piles themselves to hold the foundation.
Piles actually resist lateral loading by bending, either like a cantilever or like a beam on an elastic foundation. However, to provide the bending behavior the top of the piles must displace significantly. This may not bode well for brittle building exteriors. In earthquake country, pile caps are required which are interconnected by ties able to transmit tension or compression forces of at least 10% of column loading.
Since there is less and less land available for construction, builders count on pile foundations when placing structures on softer ground, where a mat foundation cannot be used.
Types of pile foundations
- End-bearing piles are those in which the bottom end of the pile rests on a layer of rock or very solid soil. The building load is transferred through the pile onto the stronger substrate layer and acts as a column.
- Friction piles transfer the building load to the soil across the entire length of the pile. The friction of the soil against the the whole pile works to transfer the load to the soil. The load is not completely supported by the end.
- Micropiles or minipiles are small piles used in areas where pile-driving is restricted. A boring machine drills a hole in the soil into which the micropile is grouted into place.
- Helical piles have spiral blades attached that allow them to act like a drill, another alternative to regular piles.
- Under-reamed piles have enlarged bases that are formed mechanically, up to six meters in diameter. The bottom of the pile looks like an inverted cone. A good choice for expansive soil, under-reamed piles allow greater load-bearing capacity than a typical pile.
The downsides of pile foundations
Pile foundations are expensive, made moreng expertise.
Pile driving is extremely noisy and can send shockwaves through the surrounding soil. The result can be structural damage to a nearby building or disruption of sensitive work.
Considering the expense of a building collapsing due to a faile
As you can see, mat and pile foundations have very little in common and are appropriate for very different buildings. However, if the building will be light and the soil is uniform on the surface over the area of the building footprint, mat foundations are perfectly acceptable, cost-effective foundations. For larger, heavier buildings, pile foundations, laid properly, will ensure that the building will last for decades.