Shallow Foundation in Construction – Definition, Types, Uses and Diagrams

In the field of construction, the choice of a foundation is crucial as it directly impacts the stability, longevity, and safety of any structure. Among the types of foundations available, shallow foundations are commonly used in projects involving relatively light to moderate loads. In simple terms, a shallow foundation is one that transfers building loads to the earth very close to the surface, typically within 3 meters of the ground. This is often contrasted with deep foundations, which extend much deeper into the ground.

Shallow foundations are widely chosen in building houses, small buildings, and even some industrial structures due to their cost-effectiveness and the relatively straightforward construction process they offer.

Types of Shallow Foundations

1. Spread Footings
   Spread footings are one of the most commonly used shallow foundation types. Here, the load of the structure is “spread” across a larger area, providing stability. This type of footing is ideal for buildings with isolated columns and walls that bear loads evenly distributed across multiple points.
2. Combined Footings
   These are used when two or more columns are positioned close to each other, making individual footings impractical. Combined footings connect columns through a single foundation, ensuring that loads are balanced. These footings are often rectangular or trapezoidal to distribute the weight effectively.
3. Strap Footings
   When columns are close to property boundaries or other restrictions, strap footings help transfer the load by connecting individual footings with a strap beam. This type of footing is particularly useful for preventing unwanted bending and torsional forces.
4. Mat/Raft Foundations
   This foundation type consists of a single, continuous slab that spans the entire area beneath the structure. Mat foundations are suitable for buildings with poor soil conditions or those that need extensive load-bearing support.

Characteristics of Shallow Foundations

Shallow foundations are characterized by:

• Depth: These foundations are typically installed at a depth of around 1-3 meters.
• Load-Bearing Capacity: Ideal for light to medium loads, making them popular in residential and low-rise commercial buildings.
• Soil Suitability: Effective in stable soil types where bearing capacity is relatively high.

Factors Affecting Shallow Foundation Design

Several critical factors impact the choice and design of shallow foundations, including:

• Soil Properties: Understanding soil type, moisture content, and load-bearing capacity is vital for ensuring the foundation’s stability.
• Load Requirements: The foundation must handle both live (temporary) and dead (permanent) loads.
• Settlement Considerations: Ensuring that the foundation design accommodates minimal settlement to avoid structural instability.

Soil Testing for Shallow Foundations

Soil analysis is integral in determining the foundation’s type and depth. Common tests include:

• Standard Penetration Test (SPT): Measures soil resistance to penetration, providing insight into its load-bearing capability.
• Cone Penetration Test (CPT): Determines the consistency and compaction level of the soil.

Advantages of Shallow Foundations

Shallow foundations offer multiple benefits:

• Cost-Effectiveness: Materials, labor, and equipment are less expensive compared to deep foundations.
• Simplicity in Construction: The design and building process is generally more straightforward.
• Speed of Construction: Shallow foundations can be laid relatively quickly, saving time on the project timeline.

Limitations of Shallow Foundations

Despite their advantages, shallow foundations come with certain limitations:

• Depth Restrictions: These foundations are unsuitable for deep or unstable soil layers.
• Load Limitations: Not ideal for extremely heavy loads, such as those in high-rise or industrial buildings.
• Unstable Soil Limitations: Certain soils, like expansive clays, are challenging for shallow foundations without additional treatments.

Comparison with Deep Foundations

Shallow foundations differ significantly from deep foundations in both design and application. Deep foundations, like pile foundations, are better suited for large loads and poor soil conditions, making them a common choice in skyscrapers or soft, compressible soils. However, deep foundations require more resources and time compared to shallow options.

Materials Used in Shallow Foundations

Common materials include:

• Concrete: The primary material, often reinforced with steel.
• Steel Reinforcements: Provides additional tensile strength, crucial in areas of high load or seismic activity.

Steps in Constructing a Shallow Foundation

The process of constructing a shallow foundation typically involves:

1. Site Preparation: Clearing and leveling the site.
2. Excavation: Digging to the required depth.
3. Concrete Placement: Pouring and curing concrete to establish a stable foundation.

Maintenance of Shallow Foundations

Regular inspection and maintenance can extend the life of shallow foundations:

• Inspection: Routine checks for cracks or signs of settling.
• Repairs: Addressing small issues early on prevents significant damage.
• Preventive Measures: Mitigating erosion and soil movement helps maintain foundation stability.

Environmental Impact of Shallow Foundations

Environmental considerations are essential in foundation planning. Some potential impacts include:

• Soil and Water Interaction: Groundwater movement around the foundation.
• Erosion Control: Ensuring that soil erosion does not destabilize the foundation over time.

Conclusion

Shallow foundations play an integral role in modern construction for light to medium loads. Their cost efficiency, ease of installation, and adaptability make them an excellent choice for residential and small to mid-sized commercial buildings. However, they require careful planning to accommodate soil conditions, load requirements, and environmental impacts. With proper maintenance and occasional innovations in materials and techniques, shallow foundations can support buildings reliably for decades.

A shallow foundation is a type of building foundation that transfers building loads to the earth very near to the surface, rather than to a subsurface layer or a range of depths as does a deep foundation.

Shallow foundations include spread footing foundations, mat-slab foundations, slab-on-grade foundations, pad foundations, rubble trench foundations and earthbag foundations.

Shallow foundations are also called spread footings or open footings. The ‘open’ refers to the fact that the foundations are made by first excavating all the earth till the bottom of the footing, and then constructing the footing.

Define Shallow Foundation

What is Shallow Foundation in civil engineering? A shallow foundation generally is defined as a foundation that bears at a depth less than about two times its width. There is a wide variety of shallow foundations. The most commonly used ones are isolated spread footings, continuous strip footings, and mat foundations.

Many shallow foundations are placed on reinforced concrete pads or mats, with the bottom of the foundation only a few feet below the ground surface.

The engineer will select the relatively inexpensive shallow foundation for support of the applied loads if analyses show that the near-surface soils can sustain the loads with an appropriate factory of safety and with acceptable short-term and long-term movement.

A shallow excavation can be made by earth-moving equipment, and many soils allow vertical cuts so that formwork is unnecessary.

Shallow Foundation Design

While designing a shallow foundation for a given loading system, the foundation must meet certain design requirements.

The three basic requirements or shallow foundation design criteria are as follows:

1. Placement of the foundation, which involves the depth and location of foundation.
2. Safety against bearing capacity is a requirement that includes suitable proportioning of the footing to avoid a catastrophic failure of the soil beneath the foundation.
3. Every soil settle due to load. Such settlement should be uniform and permissible. Tolerable foundation settlement includes keeping a check on the excessive settlement of a structure.

Shallow Foundation Pros and Cons

We use shallow foundation to distribute the structural loads over a wide horizontal area at shallow depth below the ground level. Shallow foundation is favorable for foundations having depth as equal as foundation width or where depth is less than the width.

Advantages of Shallow Foundation

• It requires less excavation hence reduce labour cost of excavation work.
• Construction of shallow foundations is simple as the depth involved in placing the foundation is less.
• Equipment required for construction of shallow foundation are simple and also less costly.
• Shallow foundations can be constructed in a short time, which also helps in reducing the cost of hiring equipment and labor.
• Construction of shallow foundations would cause lesser disturbance to geo-surface and hence good for ecology and environment.
• It helps to reduce settlement, if soil is compressive.
• No piling is required, which reduce the cost.
• There is less uncertainty in the prediction of behavior of shallow foundations and supporting soil. For deep foundations as the depth of soil involved is more, the uncertainty will be also more.

Disadvantages of Shallow Foundation

• There is a chance of scouring if the structure is near river or sea. Shallow foundation cannot be used at such places.
• If sub-soil water level is high and it is uneconomical to pump out the water from the pit or canal then the shallow foundation cannot be used.
• It cannot be used where the bearing capacity of top surface soil is less.
• It cannot be used when weight of structure is high and load of the structure is distributed unequally.

During the early stages of work, the entire footing is visible to the eye, and is therefore called an open foundation.

The idea is that each footing takes the concentrated load of the column and spreads it out over a large area, so that the actual weight on the soil does not exceed the safe bearing capacity of the soil.

There are several kinds of shallow footings: individual footings, strip footings and raft foundations.

In cold climates, shallow foundations must be protected from freezing. This is because water in the soil around the foundation can freeze and expand, thereby damaging the foundation.

These foundations should be built below the frost line, which is the level in the ground above which freezing occurs.

If they cannot be built below the frost line, they should be protected by insulation: normally a little heat from the building will permeate into the soil and prevent freezing.

Shallow Foundation Types

The different types of shallow foundations are:

1. Wall Footing ;
2. Spread or Isolated Footing ;
3. Strip Foundation ;
4. Mat or Raft Foundation ;
5. Combined Foundation.

These are the different type of shallow foundation listed above.