A building’s foundation forms the critical link between the structure and the earth. It supports the entire structure, distributes building loads into the soil beneath, and ensures long-term structural stability. Selecting the appropriate type of foundation depends on various factors, including soil conditions, structural demands, and cost.
At Douglas Partners, we provide expert geotechnical engineering services, including site investigations and soil testing, for a range of clients. From defence and healthcare to property & buildings and transport sectors. This article explores the key factors affecting foundation selection and outlines the different types of foundations commonly used in construction.
What influences foundation type selection?
The choice between a shallow or deep foundation depends on a combination of geotechnical, structural, and logistical factors.
Soil conditions
The strength and characteristics of the soil beneath the structure play a major role. Factors such as soil bearing capacity, presence of fill, moisture sensitivity, and compressibility determine whether a foundation should rest near the surface or be driven deeper to a more stable bearing surface.
Building loads and design
The weight and layout of a structure influence the type of foundation required. Smaller structures may only need shallow footings, while larger buildings with heavy structural loads or taller height typically require deep foundations to maintain structural stability and resist both vertical and lateral loads.
Site limitations
Space constraints, adjacent buildings, or difficult access conditions may limit the ability to construct wide footings. In such cases, deep foundations like piles or caissons may be more suitable.
Groundwater and moisture variation
High water tables or fluctuating moisture levels can undermine the integrity of shallow foundations. Deep systems allow engineers to bypass problematic zones and anchor into stable strata.
Budget and construction schedule
Shallow foundations are often faster and more affordable to construct, especially on strong soils. However, deep foundations may reduce risk and future maintenance costs in areas with poor ground conditions.
Types of foundations in construction
Foundations are generally classified as either shallow foundations or deep foundations, each with distinct subtypes suited to specific soil and loading conditions.
Shallow foundations
Shallow foundations are among the most widely used foundation systems in construction. They transfer structural loads to the soil at a relatively shallow depth; typically within the upper three metres of ground level. These foundations are generally wider than they are deep, allowing the load to spread evenly across the bearing surface.
They are best suited to sites with strong soil near the surface and are commonly used for residential buildings, light commercial structures, and other smaller-scale projects. Shallow foundations are typically easier and more economical to construct, especially when soil conditions are favourable.
One of the most common forms of shallow foundation in residential construction is the concrete slab foundation, where a reinforced concrete slab is poured directly onto the prepared ground to support the structure above.
Here are some other common types of shallow foundations:
Strip footings
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Strip footings are continuous strips of reinforced concrete placed beneath load-bearing walls. They are widely used in residential buildings and help distribute loads evenly into the soil. This type of foundation is suitable where loads are relatively uniform and the soil has adequate bearing capacity.
Pad footings (Isolated footings)
Pad footings, also referred to as isolated footings, are used to support individual columns. Each footing is typically square, circular or rectangular and is placed directly beneath a column. They are effective when loads are concentrated and soil conditions are consistent.
Combined footings
Combined footings are used when two or more columns are located close together and individual footings would overlap. A single rectangular slab supports both columns. This design is often used when space constraints limit footing layout flexibility.
Strap footings
A strap footing connects two isolated footings with a rigid concrete beam known as a strap beam. This configuration helps balance loads between columns when they cannot be aligned centrally on their footings, especially near property boundaries.
Raft or mat foundations
A raft foundation, also called a mat foundation, is a large reinforced concrete slab that supports the entire foundation of a structure. This type is especially useful on sites with low bearing capacity or poor soil uniformity. Mat foundations support load-bearing walls and columns together, reducing the risk of differential settlement across the building.
Shallow foundations are generally easier to construct and cost-effective under favourable soil conditions. However, they can be unsuitable for sites with high groundwater, weak or variable soils, or where deeper support is required to avoid excessive movement.
Deep foundations
Deep foundations are used when surface soils cannot provide sufficient bearing capacity. These systems transfer building loads to deeper, more stable soil layers or bedrock. They are essential in supporting heavy structures, resisting uplift and lateral loads, and ensuring performance in difficult soil conditions.
Pile foundations
Pile foundations consist of long, slender columns driven or bored deep into the ground. They can be made of timber, steel, or reinforced concrete. The choice between different types of piles depends on the load requirements and soil profile.
- Friction piles transfer load through skin friction between the pile surface and surrounding soil.
- End-bearing piles transfer loads to a solid layer, such as rock, at the pile tip.
- Concrete piles are common in infrastructure and high-load applications due to their strength and durability.
- Screw piles are particularly useful in sites with limited access or variable soil conditions. They are often used in residential, light commercial, and modular building projects.
Pile foundations are preferred when dealing with weak surface soils, expansive clays, or sites with inconsistent soil strata. They offer excellent load capacity and can be tailored to meet both vertical and lateral structural demands.
Caisson foundations
Caisson foundations, also known as drilled shafts or bored piers, involve drilling a deep hole, placing reinforcement, and pouring concrete to form a high-capacity column. This method is typically used in large infrastructure or commercial projects requiring deep, stable support under tall structures.
Caissons are ideal for resisting heavy building loads, and they provide versatility in varying soil types. Their large diameter and depth enable them to handle high axial loads and moments with great efficiency.
Choosing Between Shallow and Deep Foundations
The decision between shallow and deep foundations depends on the interaction between site-specific conditions and design requirements. Shallow foundations, such as strip footings, pad footings, and raft foundations, are often suitable for smaller structures on strong, near-surface soil. They are easier and faster to build, and cost-effective when soil bearing capacity is high at shallow depths.
In contrast, deep foundations, such as pile or caisson foundations, are more appropriate when dealing with poor or compressible soils, high groundwater, or when the building requires support at a greater depth. They offer better long-term performance, especially under heavy or complex load scenarios. Soil types like clay or sand play a key role in determining which foundation system is most suitable.
Comparison of Shallow and Deep Foundation Types
Comparison table showing differences between shallow footing foundations and deep foundation systems used in construction.
Shallow vs Deep Foundations – Geotechnical Comparison
| Criteria | Shallow Foundations | Deep Foundations |
| Depth | Typically within the top 3 metres of soil profile | Extend to deeper strata or bedrock (>3 metres), depending on geotechnical conditions |
| Common Types | Strip footings, pad (isolated) footings, raft slabs, strap footings, combined footings | Bored piles, driven piles, screw piles, caissons (drilled shafts) |
| Load Consideration | Suitable for relatively lighter structures with lower bearing requirements | Required for high axial loads, uplift resistance, or lateral stability in large or tall structures |
| Soil Requirement | Well-suited to strong, uniform, near-surface soils with high bearing capacity | Used where soils are weak, compressible, highly variable or reactive near the surface |
| Geotechnical Investigation Needs | Shallow soil sampling and classification; may include hand augers or boreholes <3 m | Deeper boreholes, in-situ testing (e.g. SPT, CPT), and advanced lab testing to assess deeper strata |
| Groundwater Sensitivity | More sensitive to seasonal groundwater changes and moisture variability (e.g. reactive clays) | Can bypass problematic zones like high water tables, organic clays, or fill, anchoring into stable strata |
| Construction Access | Typically easier to construct in open and accessible sites | Preferred in constrained or urban sites where surface space is limited or adjacent structures are present |
| Examples of Application | Low-rise residential dwellings, light industrial buildings on good ground | Bridges, high-rise buildings, infrastructure assets, or projects on soft soils or reclaimed land |
How Douglas Partners can help
Selecting the right foundation system is not just a matter of preference, it’s an engineering decision based on detailed analysis. At Douglas Partners, we conduct site classification,laboratory testing, and geotechnical reporting to inform the foundation design for each unique site. We aim to ensure that each structure is supported by a foundation that is safe, durable, and appropriate for the site conditions.
Final thoughts
Understanding the factors that affect the choice of foundation type is essential for every construction project. From shallow foundations like strip footings and raft slabs to deep foundations such as piles and caissons, the selection must align with soil conditions, structural requirements, and long-term performance goals.