Strip Foundations
Strip foundations are a type of shallow foundation, also known as strip footings. They are usually used before constructing low—to medium-rise domestic buildings. They can be conveniently and quickly laid on grounds where stable conditions and good load-bearing capacity exist.
This guide covers the uses, types, benefits, drawbacks, and installation methods of strip foundations. Moreover, it provides the depth and width requirements and other important considerations for the safe installation of strip foundations.
Uses of Strip Foundations
Strip foundations are laid to provide the necessary support to the load-bearing walls and closely grouped columns. The ground conditions are carefully evaluated before installing the strip foundations, and any weaker areas or other problems, such as access restrictions, water flow, etc., are identified. These types of foundations are suitable for new builds or extension projects at the domestic level.
Types of Strip Foundations
There are different types of strip foundations, and a particular type can be selected for a specific project based on the access method, budget, ground conditions, timeline, and type of building being constructed.
- Deep Strip Foundations are laid by digging a wider trench, pouring concrete at a lower level, and building masonry walls while ensuring the efficacy of the ground’s load-bearing capacity.
- Offset Strip Foundations can be installed in areas where a new structure is to be built close to an already-constructed building or in hard-to-access areas.
- Trench-fill Strip Foundations are laid by digging and filling the trenches with concrete just below the ground level. This requires the installation of a damp proof course, more concrete pouring, and the addition of steel reinforcements to ensure complete structural stability for the future.
Wide Strip Foundations can be used in soft soils with low load-bearing capacities. These foundations are spread across a larger area, followed by reinforcement with steel for enhancing the bearing capacity of reinforced strip foundations
Advantages of Strip Foundations
Construction specialists and labourers understand the methods for laying reinforced and concrete strip foundations, making it easy to find the experts for your project. This technique is relatively affordable and cost-effective in terms of construction techniques and required materials.
It involves the formation of simple formwork structures followed by a simple reinforcement installation, making the design and construction methods simple, straightforward, and quick. Moreover, these foundations efficiently transfer weight to stable ground conditions and contribute to ensuring structural stability with reduced risks of structural damage and settlement issues in the future.
Disadvantages of Strip Foundations
Strip foundations are not suitable for installation in irregular or weak soil conditions as they can lead to severe structural integrity problems or collapse in extreme conditions. Moreover, these cannot be used on sloping areas or sites where a retaining wall is to be constructed.
Installation Method for Strip Foundations
The installation of strip foundations involves these steps:
- Identification and marking out of the location where load-bearing walls need to be constructed.
- Excavation of trenches by taking the centre line of each wall as a reference point.
- Digging trenches to a greater depth, if required and backfilling them with the compact hardcore.
- Constructing formwork for providing the required support to poured concrete.
- Using mesh panels or rods for positioning the steel reinforcement which leads to the formation of a reinforced strip foundation.
- Pouring and levelling consistent quality of concrete into the trench work for even coverage throughout.
- Allowing the concrete to set for a certain time before the removal of formwork.
- Backfilling the area from where formwork is removed with the excavated soil completes the process of installing concrete strip foundations.
Depth & Width Requirements for Strip Foundations
To fulfil the British Building Regulations, some important guidelines have been provided by the UK Government in section 2E of the approved document A. According to the provided guidelines, the minimum depth for strip foundations should be 0.45m on their underside to avoid frost action. The minimum depth for strip foundations can be increased by more than 0.25m in areas which are subjected to long periods of frost for efficient load transfer to stable ground strata.
The depth of strip footings should be taken to a point where no impairment is expected due to ground movements in clays, which are subjected to volume change after drying. The depth of strip footings’ underside should be at least 0.75m, 0.9m, and 1.0m for low, medium, and high shrinkage clay soils, respectively, but this depth can be increased if weight is to be transferred to stable ground conditions.
Important Considerations for Strip Foundations Installation
Different soils, such as clay, gravel, sand, and silt, have their own characteristics, weaknesses, and varying bearing capacities. Therefore, it is crucial to undertake a geotechnical survey of the construction site to confirm the adequacy of the ground’s load-bearing capacity before the project for strip foundation installation starts.
An SPT (Standard Penetration Test) should be performed at the construction site to identify the soil’s composition and type. The results of this test will also be helpful in the designing phase, ensuring the selection of a suitable type of foundation for the project. Moreover, the relevant standards for foundation depth and width should also be strictly followed throughout the design phase.