Introduction To Foundation Details

A foundation detail refers to the specific design and construction specifications of a foundation system for...
Foundation Details

The primary function of foundations is to provide structural support to a building by transferring its loads through the walls and into the surrounding soil. In the case of a timber frame structure, foundations also serve the purpose of safeguarding the timber from moisture infiltration by elevating the members above the ground level.

The foundation’s essential task is to safely transmit the combined dead and imposed loads of a building to the ground.

Different types of foundations are employed in construction:

  1. Strip foundation: This is the most common and preferred choice for low-rise housing. It involves a concrete strip located beneath all load-bearing walls.
  2. Pile foundation: Long concrete members are used to transfer the building’s load through weak soils until they reach load-bearing strata.
  3. Pad foundation: Primarily used under point loads, such as columns, pad foundations can also be utilized beneath ground beams to distribute loads.
  4. Raft foundation: A concrete raft is employed to spread the loads evenly across the entire ground floor. Raft foundations are used when building loads are substantial or ground conditions are unfavorable.

…but what is a foundation detail?

A foundation detail refers to the specific design and construction specifications of a foundation system for a building or structure. It provides a comprehensive plan that outlines the dimensions, materials, and construction methods necessary for the successful implementation of the foundation. Foundation details typically include technical drawings, diagrams, and written instructions that guide contractors and builders during the construction process.

A foundation detail encompasses various aspects, including the type of foundation (e.g., strip, pile, pad, raft), the dimensions and layout of the foundation elements, reinforcement requirements, concrete mix specifications, excavation and trenching details, waterproofing and drainage provisions, and any additional considerations specific to the project.

The purpose of a foundation detail is to ensure the structural integrity and stability of the building by accurately conveying the necessary information for constructing a strong and durable foundation. It serves as a vital component of the overall architectural and engineering plans, allowing contractors to implement the foundation system according to the specified design requirements and local building codes.

Foundation detail types

In the following sections, we will provide a brief overview of each type of foundation and present examples of their respective details.

Strip foundations

Strip foundations are a popular choice for residential construction projects. They involve a reinforced concrete strip located centrally beneath a load-bearing wall. The width of the concrete strip is determined based on the requirements of the structure it supports, as well as the bearing capacity of the soil, ensuring safe transmission of loads to the ground.

To create strip foundations, trenches are excavated to reach a level of undisturbed compacted soil where the concrete strip will be placed. The width of the excavated trench should allow for the bricklayer to construct the wall on top of the strip, typically with a minimum width of 600mm. According to Building Regulation requirements, the minimum trench depth is 450mm. However, this depth is increased to 700mm in areas prone to frost heave.

For cost-effective solutions, a trench fill method can be employed in areas where sands and gravels are capable of supporting low-rise housing with a narrower concrete strip width of 400-500mm. This alternative approach eliminates the need for additional working space and can be a more economical option. In the trench fill method, a deeper concrete fill is poured closer to ground level, where the wall is subsequently constructed.

Pile foundations

Pile foundations are employed when strip foundations are unsuitable for a construction project. They are commonly used in situations where trees or shrubs are located near the proposed structure, or when a site has been cleared of existing trees and may experience volume changes. Pile foundations serve to transfer the building’s load to a depth that can adequately support the entire structure. However, this process tends to be more expensive due to the involvement of specialized subcontractors.

To implement pile foundations, piles are driven into the ground at specific positions. Once in place, the tops of the piles are cut to the desired level to create a solid base for casting a concrete beam over them. This beam extends from one pile to another, eliminating the need for support from the soil beneath it. This design ensures stability and structural integrity for the walls of the building.

Pad foundations

Pad foundations are typically composed of a square concrete pad that serves as a support for ground beams, upon which structural walls are constructed. Unlike pile foundations, pad foundations do not extend to significant depths. Instead, the width of the pad is adjusted to distribute the loads over a larger area. Excavation pits are dug to the necessary depth, and the concrete is then poured to create the pad foundation. Brick or concrete piers are subsequently built or cast on top of the pad foundation, extending up to the underside of the concrete beams that provide support for the walls.

Raft foundations

Raft foundations come into play when the soil conditions, such as very soft clay, or other factors make strip, pad, or pile foundations unsuitable. These foundations are designed to distribute the loads of the building across the entire area under the raft, resulting in minimal or no settlement. There are two primary types of raft foundations: the flat slab raft and the wide toe raft.

The flat slab raft is typically utilized for smaller buildings with relatively light loads. It consists of a uniform thickness reinforced concrete slab, with reinforcement present both at the top and bottom of the slab.

In cases where the loads on the foundation necessitate a thicker slab, the wide toe raft is employed. The wide toe raft features a reinforced stiffening edge beam that includes a toe at the edge. This toe serves as a base for the outer leaf of a cavity wall, providing additional stability and support.

Both types of raft foundations offer a comprehensive solution for distributing building loads and accommodating specific structural requirements based on the size and weight of the structure.