How to choose the types of foundations for houses

Selecting the type of foundation of your house project is the first major decision to take when you are either planning to construct your own house or when you are going to design a house as an engineer.lt is the most important step as it affects the future construction type and method, and it is difficult to change once the project has went forward significantly.

There are generally two main features of the selection of the type of foundation. The first is the type of structure of the building, and the second is the suitability of the foundation regarding the site conditions of the plot where the house will be constructed.

 Hence, when designing the house foundation, one weighs the geotechnical conditions against one’s preferences for house type and costs.

Factors that affect the selection of the type of foundation

1.Type of building, Structural Foundation Form Type

-Slab on Grade 

The house is placed on a foundation made of a slab.  It is common in warm regions

–Crawl Space    

The house has space under it for storage and maintenance uses. These are mostly found in temperate areas and sloped terrain

–Basement 

The house has an underlying basement, and it is used for keeping the building warmer and for providing more rooms. They are frequent in cold regions, like Scandinavia. 

2.Type of Ground Support Condition

–Shallow foundations are suitable for stable ground consisting of rock, gravel, or well-packed soil.

–Deep foundations are chosen for soft clay with a high risk of settlement. It is necessary for unstable soil layers that piles or piers support and transfer the loads down to firm ground.

–A mat or raft foundation is selected for clayey silt, silty clay, silt, and firm clay. Raft foundations are good for softer ground and distribute the load over a larger area. Many of the houses with slabs on the ground have raft foundations to avoid piles and save cost and time.

In addition to the types of soil, groundwater presence, frost level, and other environmental conditions are considered in assessing the geotechnical conditions for choosing the right foundation type.

Ground Investigation and Assessment

 A ground investigation is not needed if the available information about the soil’s condition is sufficient for a safe assessment, and inspection on-site confirms that the ground is not made up of weak soils. Ground Inspections are carried out by professional geotechnicians.

 Geotechnical investigations are a requirement of building regulations and must be carried out for all new constructions, even for residential buildings.

 During such an investigation, available information about soil, rock, and groundwater conditions is collected, and a site report is prepared as a preliminary source of information.

In addition, a soil survey may need to be carried out with test drilling in the ground and possibly sampling of soil material for lab tests.

As a result of the geotechnical investigation and inspection, possible types of foundation constructions are assessed, primarily in terms of bearing capacity but also regarding drainage, frost insulation, ground water level, and others.

What Types of foundation does my house need?

Generally, there are three main categories of foundation types: shallow, deep, and raft foundations.  

1. Shallow foundation or Firm ground

 A shallow foundation is suitable when the ground consists of bedrock or well-compacted soils such as moraine, gravel, sand, and in some cases, silty sand. The ground should remain stable and should not experience movement caused by changes in soil moisture, insufficient bearing capacity, or the effects of frost.

Hence, the structural loads can be safely transferred down the earth to the very near-surface stable soil layers, rather than to deeper soil or rock layers. It is typically characterized by having a width greater than its depth (embedment).

Foundations built on bedrock, once regarded as the safest option, can also present certain drawbacks. Ground vibrations, such as those caused by blasting during nearby construction, may induce stresses in masonry structures and lead to cracking. Additionally, constructing on bedrock can complicate water drainage, making it necessary to implement measures for moisture protection.

 Nowadays, it is common to build rock foundations on a bed of blasted rock or well-compacted gravel. This bed will also serve as an elastic layer for load distribution and as a water-draining layer.

 Both houses with basements and slab-on-ground houses have a higher risk of future problems if the foundation is not done properly. The key is ensuring that water can drain away, proper thermal and moisture insulation is used, and that the blasted rock bed is thoroughly compacted.

Common Types of Shallow Foundations:

Applications

• Commonly applied to small to medium-sized buildings like houses, garages, sheds, and some larger structures like bridges or towers.

Advantages of Shallow Foundations:

• Lower construction cost compared to deep foundations.
• Easier and faster to build.
• Require less excavation and no heavy equipment.
• Quality control is simpler and can often be visually inspected.

2. Deep Foundation or supported ground

Deep foundation is used when the ground surface does not offer sufficient load-bearing capacity. The foundation is supported by piles or piers down to firmer material or bedrock. The depth-to-width ratio of a deep foundation is typically greater than 4 to 5, distinguishing it from shallow foundations, which are placed closer to the surface.

For economic reasons, piling often goes down to firm soil layers instead of bedrock. The foundation is then made on reinforced concrete caps that bind together a set of such piles or piers.

There are two types of piles: end-bearing piles and friction piles, based on how the load is transferred from the piles to the ground. End-bearing piles reach the firm bottom and transfer the loads through their tips. 

For large depths, this can be uneconomical. It is better to use the forces that arise along the surface of the piles, known as skin friction. With friction piles, the load is transferred by friction between the soil and the pile. In cohesive soils, the loads are taken up through adhesion that occurs between soil particles; these piles are called cohesion piles.

Friction soils include gravel and sand; cohesion soils are clay and silt. So it is not uncommon to employ soil friction and cohesion to avoid uneconomical end bearing pile lengths.

Materials commonly used include concrete, steel, timber, or composites, depending on the type and site requirements

Types of deep foundations include:

• Pile foundations: Long, slender columns (often steel, concrete, or timber) driven or drilled deep into the ground to transfer loads by end bearing or skin friction.
• Caisson (well) foundations: Large, watertight structures (often cylindrical) sunk into the ground or underwater, commonly used for bridges and piers.

Drilled Piers (Drilled Shafts)

These are Large-diameter, cast-in-place concrete columns drilled into the ground, often extending to bedrock or stable soil. They are used for Heavy structures like skyscrapers, bridges, or towers requiring high load capacity.

Micropiles (Mini-Piles):

They are Small-diameter (typically 12 inches or less), high-capacity piles drilled and grouted into place, often reinforced with steel. They are used for retrofitting existing foundations, stabilizing slopes, or construction in areas with restricted access.

Helical Piles:

They are Steel piles with helical plates screwed into the ground, providing load-bearing capacity through friction and bearing. Mostly used to support Light to medium structures, temporary structures, or areas with soft soils

Applications of deep foundations:

• The upper soil layers are weak, compressible, or highly variable.
• The building imposes heavy or uneven loads (e.g., skyscrapers, bridges).
• Site constraints or environmental factors (such as high groundwater, risk of scour, or seismic activity) make shallow foundations impractical.

Advantages of deep foundations:

• Provide high load-bearing capacity by reaching stable strata.
• Minimize settlement and increase structural stability in weak soils.
• Offer flexibility in design for complex site conditions.
• Improve resistance to seismic and lateral forces.

3. Mat or raft foundation for flowing ground

A raft foundation (also known as a mat foundation) is a large, thick, reinforced concrete slab that extends over a wide area, often the entire footprint of a building

Raft or mat foundation is applied where there is no firm ground to support the loads—on clayey silt, silty clay, silt, and firm clay—where the structure is subjected to the movements of the soil because of settlement, water level incursion, or frost effects.

The mat foundation should be as stiff as possible because it is prone to settling across the entire foundation. Masonry structures are especially sensitive to settling, as visible and damaging cracks may form. Timber construction, on the other hand, is more forgiving and therefore well-suited for houses using this type of foundation.

Source- Wikipedia Commons

Applications: They are commonly used for residential and commercial buildings, industrial structures, storage tanks, and high-rise buildings, especially where the subsoil is weak or compressible.

Types: Common types include flat plate mats, mats thickened under columns, two-way beam and slab rafts, piled rafts (where piles are used to further reduce settlement), and cellular (rigid frame) rafts.

Advantages:

• Provides stability on weak soils.
• Reduces differential settlement.
• It can be more economical than deep foundations in certain conditions.

Limitations:

• Not suitable for very deep soft soils or where there are large differences in load across the building.
• Requires careful design and quality control during construction.

In summary, a raft foundation is a robust solution for distributing structural loads over a large area, making it ideal for sites with challenging ground conditions.

4. Compensation Foundation

When the subsoil is soft, compressible soils, or where there is a high water table that cannot be expected to bear the loads from the house, and one does not want to switch to deep foundations, a compensated foundation can be applied. Here, Archimedes’ principle is used, and as much soil as the house weighs is excavated and replaced with lighter fill material. The house “floats” in the soil like a boat in water.

A compensated foundation is designed to minimize settlement by balancing the weight of the structure with the weight of the soil excavated from the site. The principle is that if the weight of the building (including the foundation and any basement) is approximately equal to the weight of the soil removed, the net increase in stress on the underlying soil is negligible. This results in the minimal additional settlement because the load imposed by the building is “compensated” by the relief of stress from the excavation

This method is most suitable for foundations with crawl spaces or basements, as they normally involve the excavation of a large quantity of soil. It can be calculated how much soil needs to be removed to counterbalance the weight of the house. 

Applications

• The technique is particularly suitable for tall buildings, swimming pools, or other structures where traditional foundations would cause excessive settlement.

Advantages:

• Reduces or eliminates settlement in soft soils.
• Can be integrated with other foundation systems, such as piling, for additional support.
• Construction is relatively straightforward and has minimal impact on adjacent structures.

How to choose foundation types when the Subsoil condition varies

Often, the issue is not that the subsoil is poor under the entire house. For example, there might be bedrock under part of the house, while another part rests on clay, 

When the subsoil varies beneath a house—such as having bedrock under one part and clay under another—the risk of differential settlement is significantly increased. This occurs because bedrock provides stable, unyielding support, while clay is compressible and can expand or contract with moisture changes. Over time, the section of the house on clay will settle more than the section on bedrock, causing the structure to move unevenly.

Differential settlement can lead to:

• Cracks in walls, especially at 45-degree angles around doors and windows.
• Stair-step crack patterns in exterior brick or block.
• Uneven floors and doors or windows that stick or don’t close properly.
• In severe cases, structural damage or even uninhabitability.

It is unsuitable to build foundations in two different ways under the same house because each section will respond differently to loads and soil movement, increasing the likelihood of settlement damage. The best practice is to ensure that all parts of the foundation are supported consistently. For example, if part of the house must rest on clay, that section should be underpinned—using steel or concrete pilings—to provide support equivalent to the bedrock, thus minimizing movement.

To prevent differential settlement:

• Conduct a thorough geotechnical study before construction to understand soil conditions and design appropriate foundations.
• Use underpinning or deep foundations where needed to ensure uniform support.
• Avoid mixing foundation types (e.g., shallow on clay, direct on rock) under the same structure.

Ignoring these precautions can result in costly repairs, structural instability, and safety risks over the building’s lifetime.

Selecting a foundation for Extensions

When building an extension, the foundation type must match that of the existing house. If your house is built on a pile foundation, the extension should also use piles; if it is on a floating slab or raft, the extension should use the same method. This consistency is necessary because different foundation types settle and move differently over time. If the extension and the original house move differently, it can cause cracking and structural problems at the joint between the two structures.

Key reasons for matching foundations:

• Differential Movement: Different foundation types (e.g., piles vs. slabs) will settle or react to soil changes at different rates, leading to stress and potential damage where the extension meets the existing house.
• Vibration Risks: If piling is required for the extension but not the original house, the piling process can cause ground vibrations that might affect the existing structure.
• Structural Integrity: Using the same foundation type ensures the load is distributed similarly, reducing the risk of cracks or separation at the joint.

Expansion Joints:
At the connection between the old and new structures, it is essential to include expansion joints (elastic joints). These allow for slight independent movement between the two parts, preventing cracks and structural damage in brickwork or concrete at the joint.

Foundation Depth

The depth at which a house foundation should be placed depends primarily on local frost depth, soil conditions, and environmental factors. The general rule is to place the foundation below the frost line to prevent frost heave, which can cause the soil to expand and damage the foundation. This frost line varies widely by region, from just a few inches in warm southern areas to several feet in colder northern climates.