How to Construct a Raft Foundation

A raft foundation, also known as a mat foundation, is a large reinforced concrete slab that spreads across the footprint of a building and supports both the structure’s walls and columns. Unlike isolated or strip foundations that transfer loads at specific points, a raft foundation distributes the building’s entire load over a wide area of soil, reducing the pressure on weak or unstable ground.

The key purpose of a raft foundation is to create a stable, solid base where soil conditions are not strong enough to support conventional foundations. By acting as a “raft” floating over the soil, it ensures uniform settlement and minimizes the risk of cracks or structural damage.

Raft foundations are commonly used in:

• High-rise buildings or structures with heavy loads.
• Sites with poor or loose soils where differential settlement is likely.
• Basements, industrial plants, and storage tanks require a strong and watertight base.

In short, raft foundations are essential for ensuring stability, safety, and durability when other foundation options would fail.

When to Use a Raft Foundation in Nigeria

In Nigeria, soil conditions vary widely, from clayey, water-logged soils in the southern regions to sandy or lateritic soils in the north. Raft foundations are especially suited to low-bearing-capacity soils where pad or strip foundations would sink or settle unevenly.

Some specific conditions where raft foundations are recommended in Nigeria include:

• Low Bearing Capacity Soils: In areas where soil cannot carry high loads, raft foundations distribute weight evenly and prevent differential settlement that could cause cracks.
• High Water Table Areas: In regions prone to flooding or where the groundwater is close to the surface, raft foundations reduce the risk of seepage and provide a stable base for structures.
• Large Building Footprints: For duplexes, schools, factories, and commercial complexes, raft foundations are more cost-effective than using multiple isolated or strip footings.
• Economic Suitability: Though more expensive initially than shallow foundations, raft foundations reduce long-term risks like structural failure, collapse, or costly repairs. This makes them a practical investment in Nigeria’s construction industry, where building collapses due to poor foundations are unfortunately common.

In essence, raft foundations are not only a technical solution but also an economic safeguard against Nigeria’s soil and environmental challenges.

Step-by-Step Construction Process

Constructing a raft foundation requires precision at every stage to ensure safety, stability, and durability. Below is the standard process:

1. Site Investigation and Soil Testing

• A geotechnical engineer conducts borehole drilling, sampling, and laboratory analysis.
• Soil bearing capacity and groundwater table are determined.
• The results guide the choice of raft type, slab thickness, and reinforcement details.

2. Setting Out and Excavation

• Surveyors set out the foundation boundaries using pegs and strings.
• Excavation is carried out to the required depth, typically 600-1500 mm, depending on design and soil conditions.
• Excavated soil is stockpiled for later backfilling if suitable.

3. Blinding Concrete (Base Layer)

• A 50-75 mm layer of lean concrete (1:4:8 mix) is poured on the excavation bed.
• This creates a smooth working surface, prevents contamination of structural concrete, and helps with alignment.

4. Formwork Installation

• Formwork (usually timber or steel) is placed around the excavation perimeter.
• It must be well-braced and checked for accuracy in level and dimensions.

5. Placing Reinforcement

• Steel bars are cut, bent, and tied according to structural drawings.
• Spacers are used to maintain the correct concrete cover.
• Mesh reinforcement may be placed at the bottom and top layers of the slab.

6. Concrete Pouring and Compaction

• Ready-mix or site-mixed concrete is poured into the formwork.
• Pouring should be continuous to avoid cold joints.
• Vibrators are used to compact the concrete and remove air pockets.

7. Curing Procedures

• After pouring, concrete must be cured for 7-14 days.
• Methods include water ponding, covering with wet hessian cloth, or applying curing compounds.
• Proper curing improves strength and prevents cracks.

8. Waterproofing and Drainage Integration

• Waterproofing membranes or coatings are applied after curing.
• Drainage channels or sumps may be installed around the raft to divert groundwater.

9. Backfilling and Compaction

• Excavated soil (or imported lateritic soil) is used to backfill around the foundation.
• Layers are compacted mechanically to reduce settlement.

10. Final Inspection and Quality Control

• Engineers inspect dimensions, levels, reinforcement placement, and concrete quality.
• Non-destructive tests (NDT) such as rebound hammer or cube tests may be conducted.
• Only after passing quality checks should superstructure work (walls, columns) commence.

Types of Raft Foundations

Raft foundations come in different forms, depending on the building’s load requirements, soil strength, and design considerations. Below are the main types:

1. Flat Plate Mat: A simple, uniform-thickness concrete slab without beams. Used where loads are light and soil conditions are favorable.
2. Plate Thickened Under Columns: Similar to the flat plate mat but with additional thickness or reinforcement under columns to handle concentrated loads.
3. Two-Way Beam and Slab Raft: A slab stiffened with beams running in both directions. Common in high-load buildings like commercial centers and residential towers.
4. Plate Raft with Pedestals: Includes raised pedestals beneath columns to provide extra support and reduce punching shear around column locations.
5. Piled Raft: Combines raft foundations with piles driven into the ground. Ideal for very poor soils or heavy structures such as bridges and skyscrapers.
6. Rigid Frame or Cellular Raft: Constructed as a grid of thick slabs and beams forming rigid boxes. Extremely strong and suitable for heavy industrial plants or nuclear facilities.

Each type has its strengths, and the choice depends on soil test results, load calculations, and cost considerations. In Nigeria, two-way beam and slab rafts and piled rafts are the most common for large buildings, while flat mats are often used for smaller residential structures.

Materials & Design Specifications

The performance of a raft foundation depends heavily on the quality of materials and adherence to engineering design standards. Below are the key materials and design considerations:

Key Materials

• Concrete: Raft foundations require high-strength concrete, typically between grade 25 to 40 (C25/30 – C40/50), depending on load requirements. Concrete should have good workability, low shrinkage, and resistance to water penetration, especially in areas with high water tables.

• Reinforcement (Steel Bars): High-yield reinforcement steel bars (usually T12, T16, T20, or T25 mm in Nigeria) are used to resist tensile stresses. Mesh reinforcement may also be added for slab uniformity. Adequate spacing and cover (usually 40 – 75 mm) must be maintained to prevent corrosion.

• Formwork: Timber, steel, or plywood formwork is used to hold fresh concrete in shape until it sets. For raft foundations, formwork must be strong enough to withstand concrete pressure and aligned accurately to the design layout.

• Waterproofing Membranes: Waterproofing sheets or liquid-applied membranes are essential, especially in waterlogged areas. These prevent seepage and ensure durability.

Factors Influencing Design

• Slab Thickness: Raft thickness typically ranges from 150 mm to 300 mm for residential buildings, and 400 mm or more for industrial or high-rise structures. The thickness depends on soil bearing capacity, load magnitude, and settlement analysis.

• Beam Dimensions: In beam-and-slab rafts, beams are usually 300-600 mm wide and 450-900 mm deep, designed to stiffen the slab and transfer loads efficiently.

• Soil Test Results: A geotechnical soil investigation is mandatory before design. Parameters such as bearing capacity, settlement potential, groundwater level, and soil type (clay, sand, laterite) guide the design thickness, reinforcement, and whether piling is needed.

In Nigeria, many structural failures occur due to skipping soil tests or under-designing slab thickness. Engineers must strictly follow BS 8110, Eurocode 2, or Nigerian Building Codes when designing raft foundations.

Dutum’s Approach to Raft Foundation Construction

As one of Nigeria’s leading construction companies, Dutum Construction has developed a refined methodology for executing raft foundations that ensures durability, cost efficiency, and compliance with international best practices.

Before construction begins, we emphasize comprehensive geotechnical surveys to understand soil composition, groundwater levels, and load-bearing capacity. This allows engineers to determine whether a simple raft or a more complex piled raft is required.

We also ensure that all structural designs follow Eurocode 2, BS 8110, and Nigerian Building Code (NBC) specifications. This alignment guarantees that raft thickness, reinforcement detailing, and concrete grades are appropriate for both the soil and building loads.

Through this disciplined approach, we consistently deliver raft foundations that stand the test of time, even under Nigeria’s challenging geotechnical and climatic conditions.

Contact Us

Visit our website at Dutum Group

Email: info@dutumgroup.com

We are proudly located in:

Abuja:
10 Danube Close, Off Danube Crescent, Maitama
Phone: +234 805 831 7200

Lagos:
2 Abisogun Road, Off Palace Road, Oniru Estate, Maroko
Phone: +234 805 831 7200

Ibadan:
Isaac’s Place, Opp. TOTAL Petrol Station, Onireke, G.R.A Dugbe
Phone: +234 706 967 1901

Conclusion

Raft foundations play a critical role in ensuring the safety, stability, and longevity of buildings, particularly in areas with weak soils and high load demands.  In Nigeria, where poor soil conditions, high water tables, and unpredictable weather often complicate projects, raft foundations provide a reliable solution. However, success depends on professional planning, quality materials, skilled supervision, and compliance with codes of practice.

By adopting best practices and engaging experienced construction companies like Dutum, you can secure durable foundations that minimize settlement risks, extend building lifespans, and ultimately safeguard investments.