Building foundations are crucial for any construction project, despite their invisibility once the building or home is finished.
A building foundation serves as the lower part of a structure, intended to evenly distribute the weight of the new building and offer a stable base. Selecting the appropriate foundation type and concrete – tailored to the soil type and project requirements – is essential to prevent potential issues that could lead to severe consequences, possibly resulting in the demolition of a completed project. Be sure to refer to our guide from MLS Contractors on concrete types and consult with professionals such as building inspectors or structural engineers if you are still determining the best course of action.
Building Regulations and Foundations: Key Considerations
Ensuring the correct foundations for your building or extension from the outset is crucial for achieving a successful outcome. This involves selecting the appropriate foundation type and considering various factors, including proximity to boundaries, ground conditions, soil type, neighbouring structures, trees, drainage systems, and sewers.
To streamline the process and prevent any unforeseen complications later on, seek guidance and assistance from professionals at the early stages of your project. By adhering to all pertinent regulations and recommendations, you can minimise risks and avoid potential setbacks during the construction process.
Foundation Types in Building Construction
Soil samples are crucial before the construction of foundations for several reasons:
Load-Bearing Capacity: Soil samples help determine the load-bearing capacity of the ground. This information is essential for designing a foundation that can safely support the structure.
Soil Composition and Type: Different soil types (clay, sandy, silt, gravel) have varying properties, such as drainage capabilities and compaction characteristics. Understanding the soil composition helps in selecting appropriate foundation types and construction techniques.
Settlement Assessment: Soil samples allow engineers to assess potential settlement issues, which can lead to structural damage if the foundation sinks unevenly over time.
Water Table Location: Knowledge of the water table and its fluctuations can influence foundation design and drainage solutions, helping to prevent issues like flooding or moisture-related damage.
Soil Stability: Sampling helps identify potential risks like soil erosion, instability, or the presence of expansive soils, which can affect foundation integrity.
Environmental Concerns: Sampling can reveal contamination or pollutants in the soil, ensuring that proper remediation is conducted before construction begins.
Regulatory Compliance: Many local regulations require soil testing before construction activities commence, ensuring safety and adherence to building codes.
Overall, conducting soil samples is vital in the planning stages of a construction project to avoid costly mistakes and ensure the safety and longevity of the structure.
Foundations are typically categorised as either shallow or deep. Shallow foundations, commonly used for small buildings and residential projects, have a depth generally less than their width and are often chosen for house extensions.
Deep foundations are necessary for taller commercial or residential structures or buildings on unstable ground. These foundations transfer the structural load from weak soil to stronger soil or rock below. At MLS Contractors we provide concrete options suitable for all foundation types.
MLS Contractors – Overview of Shallow Foundation Types
Individual Footing (Isolated Footing)
This type of foundation, also known as spread footing or pad foundation, is typically utilised to support a single column in square, rectangular, or circular shapes. Characterised by a uniform thickness, individual footings are designed to distribute and bear concentrated loads based on the specific load and ground conditions.
An individual footing, also known as an isolated footing, is a type of foundation that is specifically designed to support a single column or load-bearing wall.
Characteristics of Individual Footings
Structure: Individual footings are typically square or rectangular in shape and are placed beneath each column or support point of a structure. They are designed to distribute the load from the column over a larger area of soil.
Load Distribution: The primary function of an isolated footing is to transfer the load from the structure to the underlying soil effectively. By increasing the area of contact with the soil, individual footings help reduce the pressure exerted on the ground, minimizing the risk of settlement.
Materials: Individual footings are usually constructed from reinforced concrete, which provides the necessary strength and stability to support the vertical load from the structure above. Reinforcement, typically in the form of steel bars (rebar), is used to enhance the tensile strength of the footing.
Depth and Size: The size and depth of an isolated footing depend on various factors, including the load it needs to support, the type of soil, and local building codes. A geotechnical engineer often determines the appropriate dimensions based on soil samples and load analysis.
Placement: Individual footings are placed directly beneath the columns they support and are usually located at the corners or at strategic points in a building’s layout.
Versatility: This type of footing is commonly used in both residential and commercial construction. They are particularly effective for structures with widely spaced columns or those that do not require continuous footings.
Advantages of Individual Footings
Simplicity: Their design and construction are relatively straightforward compared to more complex foundation systems.
Cost-Effective: They are usually more economical than slab or continuous footings when dealing with structures that have isolated loads.
Less Excavation: Individual footings typically require less excavation than other types of foundations, reducing overall site disturbance.
Disadvantages of Individual Footings
Not Suitable for All Soil Types: In areas with weak or unstable soils, individual footings may not provide adequate support without additional measures (e.g., underpinning or using larger footings).
Limited Load Capacity: They may not be practical for structures with heavy, concentrated loads or where load distribution is uneven.
In summary, individual footings are a foundational element in construction, providing essential support for columns and walls while ensuring stability and load distribution. Their design must be carefully considered based on the specific structural and soil conditions present in a given area.
Combined Footing
Rectangular in shape, combined footings support two or more closely positioned columns, requiring overlapping individual footings if built separately.
A combined footing is a type of foundation that supports two or more columns or walls and is designed to distribute the load from those structures over a larger area of soil. It is commonly used in situations where the columns are close together, or when individual footings would overlap or when one column is significantly heavier than others. Here’s a detailed description:
Characteristics of Combined Footings
Structure: Combined footings are typically reinforced concrete slabs that extend beneath multiple columns. The shape can vary but it is often rectangular or trapezoidal, depending on the layout of the columns it supports.
Load Distribution: The primary purpose of a combined footing is to evenly distribute the load from multiple columns to the ground. This allows for better stability and reduces the risk of differential settlement, which occurs when adjacent footings settle unevenly.
Spacing of Columns: Combined footings are particularly useful when columns are placed close together, making it difficult or impractical to use individual footings. They can also be designed to support columns with differing loads or characteristics.
Materials: Similar to other footing types, combined footings are made of reinforced concrete. The reinforcement is designed to handle tensile stresses, ensuring the footing can safely bear the applied loads.
Prevention of Overlap: In cases where individual footings for separate columns may overlap, a combined footing provides a practical solution, simplifying construction and enhancing load distribution.
Advantages of Combined Footings
Cost-Effectiveness: They often require less concrete and materials than separate individual footings, which can lead to savings in construction costs.
Improved Stability: By distributing loads across a larger area, combined footings help reduce the risk of differential settlement between columns, enhancing the overall stability of the structure.
Minimal Site Disturbance: Since they consolidate the footing for multiple columns into one design, there is often less soil excavation and site disturbance compared to individual footings.
Simpler Design: Combined footings are straightforward to design for engineers since they can account for the interactions and loads between the closely spaced columns.
Disadvantages of Combined Footings
Design Complexity: The design of a combined footing must carefully consider the loads from all columns being supported, which can create complexity in the calculations and reinforcement placements.
Limitations on Column Arrangement: They are best suited for certain column arrangements. If columns are spaced too far apart or if they are of significantly different loads, combined footings may not be ideal.
Increased Foundation Depth: In some cases, to effectively support the loads, a combined footing may need greater depth than individual footings, depending on soil conditions.
Applications of Combined Footings
Combined footings are commonly used in various construction scenarios:
Close-Spaced Columns: Ideal for scenarios where columns are positioned closely together, such as in multi-story buildings or industrial structures.
Heavy Loads: Used where one column bears significantly more load than others, helping to balance the distribution of forces.
Retaining Walls: Sometimes combined with retaining walls, where the footing needs to support both the wall and the adjacent columns.
In summary, combined footings play a substantial role in ensuring the stability and integrity of structures with closely spaced or varying load columns. Their design aids in efficient load distribution, contributing to the overall safety and longevity of the structure.
Strip Foundation
Strip footings, used for load-bearing walls, extensions, conservatories, and house foundations, also accommodate closely spaced columns. With a broader base, this foundation disperses weight over a wider area for enhanced stability.
A strip foundation, also known as strip footing, is a type of shallow foundation that provides continuous support along the length of a wall or a series of columns. It is specifically designed to distribute the load of the structure evenly across a larger area of soil. Here’s a detailed description of strip foundations:
Characteristics of Strip Foundations
Structure: A strip foundation typically consists of a long, continuous strip of reinforced concrete that runs beneath the entire length of a load-bearing wall. The strip’s width can vary, depending on the load, soil conditions, and building codes. Strip foundations are often deeper than individual footings but shallower than other foundation types like pad foundations or deep piles.
Load Distribution: The primary purpose of a strip foundation is to distribute the weight of the structure over a larger area, which helps to prevent excessive settlement and ensures stability. The footing increases the area of contact with the soil, which reduces the pressure exerted on the ground.
Suitability: Strip foundations are particularly suitable for structures with walls carrying uniform loads and can be used for residential buildings, such as houses and low-rise commercial structures. They are less effective for heavily loaded structures or those with large, concentrated loads.
Materials: Like other foundations, strip foundations are typically constructed from reinforced concrete. The reinforcement with steel bars (rebar) enhances the tensile strength and resists any bending moments due to the loads from the structure above.
Depth and Width: The depth and width of the strip foundation are determined by factors such as the load carried by the wall, the type of soil, and local building regulations. Generally, strip foundations are wider and deeper for heavier constructions or weaker soil.
Advantages of Strip Foundations
Cost-Effective: They are generally less expensive to construct compared to more complex foundation systems, making them suitable for many low-rise buildings.
Simplicity in Design and Construction: Strip foundations have a relatively simple design process and are easier to construct than other foundation types. The continuous nature of the footing simplifies site work.
Good Load Distribution: They provide excellent load distribution along the lengths of the wall, minimising the risk of differential settlement.
Suitable for Various Soil Types: Depending on design specifications, strip foundations can be effective in a variety of soil conditions, particularly in stable, cohesive soils.
Disadvantages of Strip Foundations
Limited to Shallow Loads: Strip foundations are typically only suitable for lighter structures and may not support heavy buildings with concentrated loads without additional structural measures.
Depth Considerations: If the soil is not sufficiently strong, strip foundations may require deeper excavation or additional support, which can increase construction costs.
Potential for Differential Settlement: If not designed properly, strip foundations can lead to differential settlement, especially if the wall is not uniform in load or if the foundation is not constructed with appropriate drainage.
Applications of Strip Foundations
Strip foundations are commonly used in various applications, including:
Residential Buildings: Used to support load-bearing walls in single and multi-story houses.
Low-Rise Commercial Structures: Suitable for light commercial buildings such as retail stores and offices.
Boundary Walls and Fences: Used to support retaining walls or boundary walls that require even load distribution.
In summary, strip foundations are a practical solution for providing continuous support to load-bearing walls, ensuring stability and effective load distribution. Their simple design and cost-effectiveness make them a popular choice for residential and low-rise commercial construction.
Raft or Mat Foundation
A large slab supporting multiple columns and walls and a raft or mat foundation span the entire building footprint. This type of foundation offers uniform support across the structure, ideal for sites with low soil pressure or where close columns and walls necessitate cost-effective solutions.
A raft or mat foundation is a type of deep foundation that consists of a large, thick slab of reinforced concrete extending over a substantial area of a structure. This foundation type is designed to support multiple columns and walls and is particularly useful in situations where the soil has a low load-bearing capacity or where the loads need to be spread out over a large area to prevent excessive settlement. Here’s a detailed description of raft foundations:
Characteristics of Raft or Mat Foundations
Structure: A raft foundation is essentially a solid slab of concrete that rests directly on the ground, covering the entire area of the building. It can be flat or have varying thicknesses, depending on the load requirements, soil conditions, and structural design. The raft typically incorporates reinforcement to enhance strength and durability.
Load Distribution: Raft foundations distribute the building’s load over a wide area, which minimizes the pressure on the underlying soil. This can be critical in weak or unstable soil conditions, as it helps to reduce the risk of differential settlement and provides stability to the entire structure.
Thickness and Reinforcement: The thickness of a raft foundation may vary across its area to meet the structural demands of the building. Reinforcement, usually in the form of steel rebar, is placed within the slab to manage tensile forces and bending moments that occur due to loads from the superstructure.
Soil Conditions: Raft foundations are particularly beneficial in scenarios where soil conditions are poor or variable. They can be used when the bearing capacity of the soil is insufficient for traditional footings, providing a larger bearing area to evenly distribute loads.
Advantages of Raft or Mat Foundations
Reduced Settlement Risk: By distributing loads over a larger area, raft foundations minimize the risk of differential settlement, making them ideal for buildings on weak or compressible soils.
Cost-Effectiveness: In some cases, a raft foundation can reduce the need for multiple individual footings, leading to savings in concrete and construction costs compared to more extensive foundation systems.
Simplified Construction: The continuous slab design simplifies the construction process, reducing the complexity of creating separate footings for each column or bearing wall.
Supports Heavy Loads: Raft foundations are suitable for large structures, including high-rise buildings, industrial plants, and bridges, allowing for the support of heavy and concentrated loads.
Disadvantages of Raft or Mat Foundations
Higher Material Costs: Because of the larger volume of concrete required, raft foundations can be more expensive to construct in terms of materials than some other foundation types.
Potential for Cracking: If not designed and constructed properly, raft foundations can be susceptible to cracking due to shrinkage or settlement, which may affect the performance of the building.
Limited Flexibility: Once the raft is constructed, modifying the foundation is more complex and challenging than modifying individual footing systems.
Applications of Raft or Mat Foundations
Raft or mat foundations are commonly employed in various situations, including:
High-Rise Buildings: Ideal for skyscrapers or multi-story buildings that exert significant loads on the foundation.
Heavy Industrial Structures: Often used in factories, warehouses, and plants that house large machinery or heavy equipment.
Structures on Weak or Variable Soil: Suitable for locations with poor soil conditions, such as silty or clayey soils, where traditional foundations may not perform adequately.
Bridges and Large Overhangs: Employed where significant load distribution is necessary to ensure structural integrity.
In summary, raft or mat foundations provide a robust solution for supporting heavy loads, especially in challenging soil conditions. The continuous slab design enhances stability, load distribution, and resistance to settlement, making it a preferred choice for many large-scale and complex constructions.
Overview of Deep Foundation Types
Deep foundations are structural elements that transfer loads from a building to deeper, more stable soil or bedrock layers. They are used when surface soil conditions are inadequate to support the structural loads or when the loads are particularly heavy. There are several types of deep foundations, each with distinct characteristics and applications. Here’s an overview of the main types of deep foundations:
Piles
Piles are long, slender columns made of wood, concrete, or steel that are driven deep into the ground to reach stable soil or bedrock.
Types of Piles:
End-Bearing Piles: Transfer loads directly to the bedrock or a strong soil layer at their tip.
Friction Piles: Rely on friction between the pile surface and the surrounding soil to support the load. These piles are effective in soils that lack a strong bearing layer.
Composite Piles: Made from different materials, combining the properties of each to improve load-bearing capacity and flexibility.
Applications: Commonly used in bridge construction, high-rise buildings, and areas with poor soil conditions.
Drilled Shafts (Bored Piles)
Drilled shafts are large-diameter concrete columns that are excavated and then filled with concrete, often reinforced with steel.
Characteristics:
Can reach substantial depths and sizes, allowing for significant load capacity.
The excavation procedure minimises soil disturbance.
Applications: Suitable for heavy structures like bridges and buildings, especially in rocky or mixed soil strata.
Caissons (Foundation Piers)
Caissons, or foundation piers, are large-diameter concrete structures that are installed by excavating the ground, then either filling the cavity with concrete or sinking the caisson into the soil.
Types of Caissons:
Open Caissons: Used when the soil is soft and can be dug out, allowing for direct placement of concrete.
Closed Caissons: Used underwater or in deep excavations, featuring a watertight structure that is filled with concrete.
Applications: Common in deep foundations for bridges, marine structures, and high-rise buildings.
Foundation Piers
Foundation piers are similar to drilled shafts but are sometimes used to support heavy loads with a focus on smaller diameters and depths depending on conditions.
Characteristics:
Built with concrete or steel and can be embedded or seated into the ground.
Used to support loads in challenging soil conditions.
Applications: Often found in residential and commercial buildings, particularly where soil issues are present.
Micro Piles
Micro piles are small-diameter, high-capacity piles that are drilled and grouted into the ground. They are often used for retrofitting existing structures and in confined spaces.
Characteristics:
Flexible and can be installed with minimal disturbance to surrounding soil and structures.
Have a high load-bearing capacity relative to their size.
Applications: Useful for foundation repair, seismic retrofitting, and new construction in urban environments.
Summary of Applications and Considerations
Load Capacity: Deep foundations are designed based on the anticipated loads from the superstructure and soil conditions.
Soil Conditions: The choice of deep foundation type largely depends on soil properties, load requirements, and environmental factors such as seismic activity or groundwater.
Construction Methods: Different types of deep foundations require various construction methods, machinery, and site restrictions.
In summary, deep foundations are essential for ensuring the stability and safety of structures in challenging soil conditions or when dealing with significant loads. The choice of deep foundation type depends on specific project requirements, including soil characteristics, load capacities, and construction feasibility.
Pile Foundations
Pile foundations are employed when surface ground conditions are unsuitable for supporting heavy loads. Specialised equipment drives piles into the ground, which is then filled with concrete before a ground beam is incorporated to create a stable surface for construction.
Drilled Shafts or Caissons
Also referred to as caissons, drilled shafts are cast-in-place foundations. A column is bored to the desired depth, reinforced steel is inserted into the cavity, and concrete is subsequently filled to form a solid foundation.
Building Foundations: Step-by-Step Instructions for Crafting Concrete Footings
Crafting concrete footings is a crucial step in building foundations, providing the necessary support for structures. Here’s a detailed step-by-step guide on how to construct concrete footings:
Step 1: Planning and Design
Determine Load Requirements: Understand the intended load of the structure and consult with a structural engineer if necessary.
Check Local Building Codes: Ensure compliance with local building codes and regulations regarding footing depth, width, and material specifications.
Site Assessment: Conduct a soil test to determine soil type and load-bearing capacity, which will inform the design and dimensions of the footings.
Step 2: Gather Materials and Tools
Materials:
Concrete mix (typically a mix of cement, sand, gravel, and water)
Rebar (reinforcing steel bars)
Formwork materials (plywood or 2x4s)
Stakes and braces (for securing the formwork)
Tools:
Shovel and spade
Excavator (if necessary, depending on the size of the footings)
Level
String line and measuring tape
Trowel and float
Concrete mixer (or wheelbarrow if mixing by hand)
Safety gear (gloves, goggles, and a dust mask)
Step 3: Mark the Footing Location
Site Layout: Use a measuring tape to mark the outline of the footings on the ground. Ensure the dimensions match the approved plans.
Set Up Stakes: Drive wooden stakes at the corners and along the outline to define the perimeter of the footings.
Use String: Tie a string around the stakes to create a clear, visible line for excavation.
Step 4: Excavate the Footing Trenches
Excavation: Using a shovel or an excavator, dig trenches for the footings to the specified depth and width, following the layout marked by the string.
Check Depth and Width: Ensure the trench dimensions are consistent and level along the entire length. The depth will depend on local frost lines and soil conditions.
Step 5: Prepare the Footing Base
Compact the Soil: Use a hand tamper or mechanical compactor to compact the soil at the bottom of the trenches, ensuring a stable base.
Add Gravel Base (Optional): In some cases, adding a layer of gravel can improve drainage and further stabilize the footings.
Step 6: Set Up Formwork
Build the Forms: Cut plywood or use 2x4s to create rectangular forms that will shape the concrete footings. Ensure they are sturdy and rigid.
Secure the Forms: Place the forms in the trenches and use stakes and braces to hold them in place. Ensure that the tops of the forms are level and at the correct elevation.
Check Alignment: Use a level to confirm that the forms are straight and level.
Step 7: Install Reinforcement
Cut and Place Rebar: Cut rebar to the lengths required, and place it within the forms to reinforce the concrete. Use rebar chairs or spacers to ensure the rebar is elevated in the concrete.
Tie the Rebar: Use wire to tie the rebar together and ensure it is securely placed to resist bending under load.
Step 8: Mix and Pour Concrete
Mix Concrete: Follow the instructions on the concrete mix bag or use a concrete mixer to prepare the concrete mix with the appropriate water-to-cement ratio.
Pour Concrete: Start pouring concrete into the forms, beginning at one end and working to the other. Ensure that the concrete is evenly distributed and fills the forms completely.
Use a Vibrator (Optional): If available, use a concrete vibrator to remove air bubbles and ensure compactness.
Step 9: Finish the Surface
Screeding: Use a screed board to level the top surface of the concrete with the top of the forms.
Float the Surface: After the concrete has started to set, use a float to smooth the surface and finish it according to the desired texture.
Step 10: Cure the Concrete
Curing Process: Cover the concrete with damp burlap, plastic sheeting, or curing compound to retain moisture while it cures.
Curing Time: Allow the concrete to cure for at least 7 days (some sources recommend up to 28 days) to achieve its full strength. Keep it moist during this time.
Step 11: Remove Formwork
Stripping Forms: Once the concrete has cured sufficiently, carefully remove the formwork. Be cautious not to damage the freshly poured concrete.
Check for Defects: Inspect the concrete surface for any defects or inconsistencies.
Step 12: Backfill and Finish the Site
Backfill: Replace any excavated soil around the footings and compact it to ensure stability.
Final Grading: Grade the surrounding area to ensure proper drainage away from the foundation.
Conclusion
Constructing concrete footings involves careful planning, precise execution, and thorough knowledge of local building codes. Following these steps will ensure that the foundation is strong and stable, providing robust support for the structure above. Always consult with a professional engineer or qualified contractor if unsure about any part of the process.
Are you a facilities or projects manager seeking an established external contractor to support your current project? Please get in touch with Max Stevens on 01293 225 853 or e-mail your enquiry – max@mlscontractors.co.uk
