Have you been told by your architect or structural engineer that your building needs a pier foundation? Wondering what exactly is a pier foundation and why it has been recommended for your construction project? This guide will answer all your questions on pier foundations and help you understand the basics. We will take an in-depth look at what a pier foundation is, the different types, where and when they are used, their pros and cons. It is important to have a clear understanding of pier foundations and be able to make an informed decision about using it for your building.
What is a Pier Foundation?
A pier foundation is a type of shallow foundation that is used to support larger structures when the topsoil layer is unsuitable or unstable for conventional foundation systems like footings. In simple terms, a pier foundation refers to a group of tall columns or piers constructed out of materials like concrete, brick or steel that transfer the load of the building directly to the stronger soil layers below the surface. These piers act individually or together as a system to evenly distribute the weight of the building to the ground.
The key things to note about a pier foundation are:
- It uses vertical columns called piers to support the building instead of continuous foundation walls or footings.
- These piers are constructed by digging holes (manually or mechanically using augers) and installing precast concrete, brick or steel sections in them.
- The piers extend downwards until they reach a competent soil stratum that can handle the building loads.
- On top of the piers, beams are placed to distribute the load, on which the floor slab or flooring sits.
So in summary, a pier foundation transfers the loads of a building to deeper and stronger soil layers using a system of tall vertical columns called piers.
Types of Pier Foundations
There are different types of pier foundations used depending on the soil and loading conditions. Let’s take a look at the four main types:
Straight-shaft end-bearing pier – Supports loads through end-bearing on a hard stratum. Friction in overlying soil is ignored.
Straight-shaft sidewall-shear pier – Develops support through side friction by penetrating into bearing stratum, ignoring overburden soil friction.
Straight-shaft pier with both sidewall shear and end bearing – Combines side friction and end bearing for support.
Under-reamed or belled pier – Features a wider base that increases bearing area and capacity to carry more loads.
Among these, under-reamed piers are suitable for highest loads while end-bearing piers are used where only minimal loads are involved. On the other hand, sidewall piers are an intermediate option. The type is chosen based on project specifications and subsurface conditions.
Where Are Pier Foundations Used?
Now that we understand the basic concept, let’s look at the common situations where a pier foundation proves advantageous:
Expansive or compressible soils: Soils prone to swelling or shrinking put excessive pressure on continuous foundations and so piers are used to transfer loads bypassing such problematic soils.
Sloping sites: Unlike footings, piers can negotiate uneven terrain easily and maintain structural stability on slopes.
Under or around existing structures: Retrofitting calls for minimal disruption which piers allow via their narrow boreholes.
Flood-prone areas: Raising the structure above on piers protects it from floodwaters and buoyancy forces.
Shallow bedrock: Where bedrock lies close to the surface, piers can gain direct support from it within reasonable drilling depths.
Large buildings: Tall structures concentrating heavier loads demand piers that can channel them effectively to deeper competent soils.
Heritage buildings: Preserving old structures need unobtrusive piers instead of invasive conventional foundations.
Advantages of Using Pier Foundations
Listed below are some key benefits that make pier foundations a practical choice in many construction situations:
- Requires minimum excavation as only narrow boreholes need digging to install piers. This causes less disruption.
- Well suited for difficult ground conditions like compressible soils where continuously placed footings may not work.
- Each pier can be precisely placed and loaded independently to maintain structural stability on sloping or uneven terrains.
- Flexible design allows adding or removing piers easily as per the structure’s needs without disturbing other parts.
- Raised construction protects structures from floodwaters, standing water issues and soil moisture problems.
- Piers offer direct load transfer so less material is needed relative to other deep foundations. This makes them relatively economical.
- Changes in design can be incorporated during construction by adding or extending piers as required.
- Minimal vibration and noise during installation compared to driven piles. Safer near existing buildings.
- Allows unobstructed access to utility lines beneath for repairs by leaving space between ground and structure.
Disadvantages of Pier Foundations
While pier foundations provide clear benefits in a number of applications, they also have some disadvantages to be aware of:
-Prone to moisture problems if proper damp-proofing is not provided in the form of sealants, membranes or venting. Excess moisture leads to issues like rot, mold growth, pest infestation in the enclosed area under the structure over time.
-Requires experienced contractors and constant supervision during installation to ensure piers are of uniform size, placed correctly and constructed as per engineering design.
-More expensive than conventional spread footing foundations due to involvement of specialized drilling, concreting and construction work.
-Lack of continuous foundation walls can cause some vibrations and creaking in the floors over the lifecycle of the building since the load gets transferred to ground via discrete piers only.
-Occasional inspection and maintenance is needed to check for any issues periodically in the long run. And issues may not be visible externally.
-Scaffolding or drilling equipment needs space for maneuvering which could be a constraint in tight urban construction sites.
Types of Materials used for Piers
The most common pier materials include:
Reinforced Concrete Piers – Provides highest strength for large loads in all soil types. Requires skilled labor.
Precast Concrete Piers – Factory made above ground for ease; transportable as hollow sections then grouted.
Cylindrical Concrete Piers – Simplest circular section poured on-site below ground level. Suitable for residential construction.
Timber Piers- Economical and sustainable option suitable for lighter buildings, used historically too but less protective than concrete.
Steel Shell and Concrete – Hollow steel shell driven/drilled then filled with high-strength concrete; resists high bending and compression.
Masonry Piers – Stacked cut stone, brick or block piers stable enough to support smaller buildings; traditional historical method.
Some specialized options also used are micropiles, helical piers, composite piers etc. Options are selected based on structural needs, soil conditions, material availability and costs.
Choosing Between Pier and Pile Foundations
There is some conceptual overlap between pier foundations and another similar deep foundation system called pile foundations. While both involve installing columns into the ground to transfer building loads below the surface, there is an important difference:
Piles are installed using prefabricated sections that are driven or vibrated into place using hammers/vibrators.
Piers are constructed on-site by excavating boreholes and pouring/placing concrete, steel, precast components within.
So in summary – piles are pre-made sections driven into ground whereas piers are built by placing material into holes. They are generally smaller (less than 1m diameter) while piers are larger. Piers on the other hand offer more design flexibility in difficult soils compared to standard pile lengths. The two can be used based on specific project needs, ground conditions and loading requirements.
Types of Materials used for Piers
The most common pier materials include:
Reinforced Concrete Piers – Provides highest strength for large loads in all soil types. Requires skilled labor.
Precast Concrete Piers – Factory made above ground for ease; transportable as hollow sections then grouted.
Cylindrical Concrete Piers – Simplest circular section poured on-site below ground level. Suitable for residential construction.
Timber Piers- Economical and sustainable option suitable for lighter buildings, used historically too but less protective than concrete.
Steel Shell and Concrete – Hollow steel shell driven/drilled then filled with high-strength concrete; resists high bending and compression.
Masonry Piers – Stacked cut stone, brick or block piers stable enough to support smaller buildings; traditional historical method.
Some specialized options also used are micropiles, helical piers, composite piers etc. Options are selected based on structural needs, soil conditions, material availability and costs.
Conclusion
To summarize, a pier foundation involves constructing tall load-bearing columns using different materials in narrow dug holes to transfer the weight of structures safely down to competent strata beneath unstable ground conditions. The four main types are end-bearing, sidewall, combined and under-reamed piers. Applications include expansive soils, slopes, existing buildings and flood-prone areas. Advantages of piers are minimal disturbance, flexibility and raising structures above ground level, while moisture and specialized work are potential disadvantages. When you understand their basic concept, applications and differences from pile foundations, it helps you in making informed choices regarding foundation systems for your construction projects.