Why Deep Foundations Are Required
In many projects, shallow foundations are sufficient to support the loads of a structure because the surface soils have adequate bearing capacity and settlement characteristics. However, in other cases, near-surface soils may be too weak, compressible, or variable to provide reliable support. This is particularly evident in areas with soft clays, loose sands, marine deposits, or fill materials that cannot sustain the imposed structural loads. In such conditions, relying on shallow footings can lead to excessive settlement, tilting, or even structural failure. For projects with high load demands, such as high-rise buildings, bridges, or waterfront structures, the use of deep foundations becomes essential.
Deep foundations are specifically designed to transfer loads to stronger strata or rock layers located at greater depths. This design approach ensures stability and long-term performance where shallow solutions are unsuitable. By embedding the foundation elements deeper into the ground, engineers can bypass weak surface soils and reach competent materials, achieving the necessary bearing capacity and reducing risks associated with ground movement.
Types of Deep Foundations
Deep foundations are used when surface soils are too weak or unpredictable to support structural loads at shallow depth. By extending into stronger soil or rock at greater depth, they provide stability and prevent excessive settlement. The choice of foundation depends on soil conditions, load requirements, groundwater level, site constraints, and the required construction methods.
Pile Foundations
Image source: https://en.wikipedia.org/wiki/Piling
Pile foundations are one of the most widely used deep foundation types. They consist of long, slender elements made from reinforced concrete, steel, or timber piles, installed by driving, boring, screwing, or augering into the ground.
There are several pile foundation types, including:
- End-bearing piles – transfer loads directly to a strong layer or rock, making them ideal for sites where poor soil conditions exist near the surface but competent strata lie at depth.
- Friction piles – transfer loads through skin friction between the pile surface and the surrounding soil, particularly useful in soft clays and silts where no stable stratum is accessible at shallow depth.
- Replacement piles (bored piles) – constructed by removing soil and replacing it with reinforced concrete; suited for urban areas where vibration from driven piles must be avoided.
- crew piles (helical piles) – steel piles with helical blades that are rotated into the ground; used in expansive soils or where minimal disturbance is required.
Pile foundations are commonly chosen in complex soils such as compressible clays, loose sands, or variable fill materials. They are also frequently used in infrastructure projects, retaining walls, and flood protection works. The construction process often includes installation of a steel reinforcement cage, use of temporary casing, or concreting under slurry to maintain stability during pile construction.
Drilled Shafts and Caisson Foundations
Drilled shafts, also called bored piles or drilled piers, are large-diameter cast-in-situ foundations formed by drilling and filling with reinforced concrete. With diameters from under 1 m to several metres, they can support very heavy vertical and lateral loads. A key advantage is the ability to inspect soil conditions during installation, making them suitable for variable ground.
In marine and river settings, these foundations are built as caissons, watertight shafts sunk through soft or saturated soils until reaching rock or stable strata, then filled with concrete. Caissons minimise disturbance and perform well in alluvial and coastal conditions.
Together, drilled shafts and caissons provide reliable deep foundation solutions for bridges, towers, and marine structures where high capacity and stability are required.
Pier Foundations
Pier foundations are vertical, column-like structures that transfer building loads down to firm strata. They are particularly effective in areas where upper soil layers are too weak or inconsistent to provide reliable support, making them a dependable solution for bridges, towers, and other heavy structures built on complex soils.
Drilled Shafts (Bored Piles)
Drilled shafts are large-diameter concrete shafts bored deep into the ground. They are designed to carry very high-capacity loads, which makes them suitable when shallow foundations cannot cope with soil variability. Because they can penetrate through weak or unstable soils to reach more competent layers, drilled shafts are widely used in high-rise buildings, bridges, and industrial facilities.
Shaft Foundations
Shaft foundations, including caissons and well foundations, involve creating deep cylindrical excavations that are filled with concrete. These are especially useful in waterlogged or highly variable soils where stability is a major concern. Their ability to resist both vertical and lateral forces makes them an effective choice for marine structures, bridges, and foundations in areas with difficult geotechnical conditions.
Micropiles
Micropiles are small-diameter piles, typically 100–300 mm, that are drilled and grouted into the ground. Despite their size, they can carry substantial loads and are reinforced for added strength. They are particularly advantageous in sites with limited access, difficult drilling conditions, or where soil variability makes conventional piles impractical. Micropiles are also commonly used in retrofitting and seismic upgrades for structures built on complex soils.
Deep Foundations in Complex Soils
Understanding Complex Soils
Complex soils refer to ground conditions that are difficult or unsuitable for conventional shallow foundations. These soils often include highly variable stratigraphy, such as alternating layers of clay, silt, and sand, each with different engineering properties. In many cases, the soils are soft and compressible, leading to excessive settlement when subjected to load. Expansive clays that shrink and swell with moisture changes, collapsible soils such as loess, or man-made fills with uncertain composition add further risks.
Marine and alluvial deposits present additional challenges because of their low shear strength, high water content, and susceptibility to liquefaction during seismic events. Even in locations where stronger strata are present at depth, groundwater pressures or obstructions such as cobbles and boulders may complicate foundation installation. For these reasons, the design of a deep foundation system in complex soils must be based on a detailed understanding of ground conditions and soil–structure interaction.
Key Factors in Designing Deep Foundations for Complex Soils
1. Soil Strength and Variability
The variability of soil strength is one of the most critical factors in deep foundation design. In layered profiles, some strata may be competent enough to transfer loads through end-bearing piles, while weaker layers may require reliance on skin friction or shaft resistance. Geotechnical investigation through boreholes, cone penetration testing (CPT), and laboratory analysis is essential to map out these variations and determine the appropriate pile type and depth.
2. Settlement Behaviour
Settlement is not only a matter of bearing capacity failure but also long-term performance. Even soils with adequate strength may undergo primary and secondary consolidation over time. Excessive or differential settlement can compromise structural serviceability. Settlement analysis therefore considers both immediate settlement and time-dependent consolidation, ensuring that tolerances are met for the design life of the structure.
3. Groundwater Conditions
Groundwater can greatly influence foundation design and construction. High water tables reduce effective stress, weakening soil shear strength and increasing the risk of base heave or piping during excavation. Pile installation in saturated soils may also require temporary casing, bentonite or polymer slurry, or specialized dewatering methods. Engineers must also consider long-term hydrostatic pressures, which may reduce effective pile capacity or cause uplift forces.
4. Structural Load Demands
The type and magnitude of structural loads directly dictate the foundation system. High-rise buildings, industrial facilities, and bridges typically require deep pile foundations or drilled shafts to safely distribute loads. In contrast, lighter structures may be supported by hybrid or shallower deep foundations, provided soils allow. Load transfer mechanisms—end-bearing, shaft friction, or a combination; must align with the load profile and the variability of soil strength.
Figure: Axial pile capacity illustrating load transfer through shaft friction and base resistance
5. Construction Constraints
Practical site constraints often influence the choice of deep foundation system. Restricted access or low headroom sites may preclude driven piles, favouring bored piles, micropiles, or CFA (continuous flight auger) piles. In urban areas, vibration and noise restrictions often make bored pile foundations preferable to driven systems. Construction methods must therefore balance technical suitability with practicality and site-specific limitations.
6. Cost, Time, and Environmental Factors
Project budget and timelines also play a decisive role. While buoyancy rafts or large-diameter caissons may be technically feasible, they are often less economical than driven or bored piles. Environmental considerations, such as noise, vibration, disposal of spoil, and impact on surrounding structures, must also be factored into the foundation selection process. The most suitable solution is one that balances safety, durability, environmental performance, and cost-effectiveness.
Conclusion
Deep foundations provide the essential connection between structures and challenging ground conditions. In complex soils, determining the right solution requires more than standard practice. It demands a thorough understanding of subsurface conditions.
At Douglas Partners, our geotechnical specialists focus on comprehensive site investigations, laboratory testing, and advanced analysis to deliver accurate data and insights. This critical information supports engineers and designers in selecting the most suitable foundation systems, whether piling, drilled shafts, or alternative methods, ensuring safe, economical and practical outcomes for every project.