When planning basement construction, several key factors must be considered to determine the most suitable type, including basement depth, ground conditions, water table level, site access, and intended use.
At Harban Redwood, our extensive experience in basement design enables us to provide tailored guidance, ensuring each project meets safety standards while delivering cost-effective solutions.
In this post, we outline the main types of basements and the scenarios in which each is most appropriate.
- Piled Basement
Piled basements offer one of the most, though often the most costly, solutions. They are typically used on sites with poor or clayey soil or in proximity to rivers. In these systems, piles transfer building loads deep into the ground until they reach a stable soil stratum that can support the structure.
Piles may act as cantilevers in their permanent condition or be temporarily propped during construction until they are permanently restrained by structural elements, such as floors or slabs.
Some typical drawings of different pile basement examples are shown below:
Common Pile Types
The most commonly used types of piles in this sort of basement include contiguous piles, secant piles, soldier piles, and sheet piles. For more information on these types of piles, please refer to: https://drive.google.com/file/d/14MSECMbkPA8rDO6qiPuoUl0l1QEV_zfk/view?usp=drive_link
Pros and Cons of Piled Basements
Pros:
- Provide adequate ground support throughout construction.
- Have a minimal excavation footprint.
- Reduce the need for temporary support or bending in the surrounding ground that may impact nearby structures.
- Piles act as support for the basement slab to resist buoyancy from groundwater.
Cons:
- Vibrations from piling works may affect surrounding structures.
- Temporary propping can be costly, particularly when building large basement schemes.
- Raft Basement
Raft basements use a battered excavation to reach the formation level to construct a reinforced concrete (RC) raft slab covering the entire footprint of the building. Concrete basement walls are then built off the slab using integrated starter bars left out when pouring the RC to form the basement wall, providing a stable base in poor soil conditions.
Typical Basement Materials for Walls:
RC Walls: Reinforced concrete walls using a framework are the most common form of construction of basement walls.
Stepoc Walls: Stepoc blocks are precast and act as the formwork to help restrain the wet concrete before setting.
ICF Walls: Insulated Concrete Formwork (ICF) blocks provide insulation and receive in-situ concrete.
Drawings of these wall types can be seen below:
Pros and Cons of Raft Basement
Pros:
- Eliminates the need for using specialist piling equipment.
- Usually more cost-effective than piled solutions.
Cons:
- This type of battered excavation utilises more space, so it may not be suitable for sites with strict access constraints
- Backfilling and drainage behind basement walls requires careful management.
- Careful consideration is needed regarding buoyancy and uplift from groundwater.
- Underpinned Basement
Underpinned basements are ideal scenarios where a basement is to be installed below or adjacent to an existing structure. This method involves underpinning existing foundations to maintain structural stability during the excavation.
When creating an underpinned basement, it’s imperative to ensure the stability of the neighbouring structures throughout the construction.
The vertical load on the top of the underpinning wall provides lateral restraint to the underpinned wall portion.
Pros and Cons of Underpinned Basements
Pros:
- These basements ensure the stability of neighbouring structures
- Safe excavation is possible once the surrounding structure is supported and the underpinning is completed
Cons:
- Underpinning is a complex and time-consuming process, typically associated with relatively high costs.
- Underpins need to be installed in a hit-and-miss sequence, meaning that each new underpin must be securely tied to the underpin.
See the example of underpinning the basement wall below.
Each basement construction method comes with unique advantages and challenges. The optimal solution depends on the site’s specific conditions and requirements.
Early involvement of a structural engineer is crucial for evaluating factors such as soil conditions, nearby structures, access limitations, and the intended use. And will help to procure a safe, efficient and cost-effective outcome.
At Harban Redwood, we collaborate closely with clients, architects, and contractors to deliver bespoke basement designs that meet technical, aesthetic, and budgetary goals, ensuring long-term success.
