How to do a raft foundation?
Building a raft foundation involves a structured process covering site preparation, layer installation, reinforcement, and concrete pouring.
A raft foundation, also known as a mat foundation, is a large concrete slab that covers the entire area of the building footprint. It's often used when soil conditions are poor or unstable, or when column loads are very heavy, distributing the weight over a larger area.
Based on the provided reference steps, here is how a raft foundation is typically constructed:
The process follows a specific sequence to ensure the foundation is stable, properly drained, insulated, and reinforced.
Step 1: Site Preparation and Excavation
The initial phase involves clearing the site of vegetation and debris. Excavation is then carried out to the required depth for the foundation slab, ensuring the area is level and compacted appropriately to provide a stable base for the subsequent layers.
Step 2: Stone Drainage Bed Installation
Following excavation, a layer of stone drainage bed is installed. This bed typically consists of crushed stone or gravel and serves to provide drainage beneath the slab, preventing water buildup that could affect the foundation's stability or lead to damp issues.
Step 3: Building Formwork
Formwork is erected around the perimeter of the excavated area. This formwork, often made of timber or steel, creates the mold that will contain the concrete and define the shape and thickness of the raft slab. It must be robust and accurately set out according to the design plans.
Step 4: Placing Insulation Layer
An insulation layer is placed within the formwork, typically on top of the drainage layer. This step is crucial, especially in colder climates, to prevent heat loss through the ground and improve the energy efficiency of the building. Rigid insulation boards are commonly used.
Step 5: Polyethylene and Sand Installation
Next, a layer of polyethylene sheeting is laid over the insulation and drainage layers. This acts as a damp-proof membrane to prevent moisture from rising from the ground into the concrete slab. A thin layer of sand is often spread over the polyethylene to protect it during the placement of reinforcement.
Step 6: Reinforcing Steel Placement and Concrete Pouring
Reinforcing steel, usually in the form of steel mesh or rebar cages, is carefully placed within the formwork, supported by spacers to ensure it is correctly positioned within the concrete slab. This steel provides tensile strength to the concrete. Finally, concrete pouring commences. The concrete is placed within the formwork, ensuring it is evenly distributed and vibrated to remove air bubbles and achieve full compaction around the reinforcement. Once poured, the concrete is finished and allowed to cure properly.
These steps outline the fundamental process for constructing a raft foundation, ensuring a solid and stable base for the structure above. For more detailed information on this process, you can refer to resources like the one provided: How to Build a Raft Slab Foundation.
