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Substructure Design Principles

The main objective of foundation design is to:-

Guarantee the secure and cost-effective transmission of structural loads to the subsoil, with minimal acceptable movement throughout both the construction phase and the expected lifespan of the building or structure.

Figure 1. Types of substructure failures (Ground & Water Ltd, 2021)

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Substructure – Functional Characteristics

Enhanced Stability:

Foundations contribute to the lateral stability of the superstructure by securely anchoring it to the ground. This fortification against sliding and overturning becomes pivotal when facing horizontal forces like wind or earthquakes, enhancing overall structural stability.
Centralising loads on the foundation enhances stability by providing axial loading, emphasizing the importance of load placement for increased foundation stability.

Reducing Differential Settlement:

Foundations play a crucial role in achieving even distribution of superimposed loads on the subsoil, particularly when dealing with nonuniform loads. This can be effectively achieved through the construction of combined footings or raft foundations.

Protection Against Undermining:

Foundations offer protection against scouring or undermining caused by factors like floodwater or burrowing animals.

Mitigating Distress from Soil Movement:

Foundations, especially in clayey, prone to moisture-induced expansion or contraction, minimise distress and potential failures by incorporating specialised foundation types.

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Substructure – Functional Characteristics

Compatibility and Durability:

Ensuring concrete durability entails managing the water-to-cement ratio to prevent weaknesses. Excessive water may create voids, compromising the concrete's strength under load. Typically, water-to-cement ratios between 0.4 and 0.7 suffice for various applications.
Proper vibration during placement is crucial to eliminate trapped air pockets. In areas with sulphate-containing soil that could harm foundations, the use of sulphate-resistant cement is recommended.

Creating a Level Surface:

They ensure the creation of a level surface, providing a stable foundation for the construction of the superstructure.

Load Distribution:

Foundations play a crucial role in efficiently distributing superstructure loads across a broader soil area. This ensures that the load intensity at the foundation base remains within the safe bearing capacity of the soil.
In the case of deep foundations such as pile foundation, loads are transmitted through a combination of end bearing and side friction.
The efficacy of load transmission and foundation strength hinges on several factors, including concrete quality, foundation thickness, ground load-bearing capacity, building structure type, and foundation type.

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Types of Foundations

Raft foundation (Fig. 2).
Strip foundation (Fig. 3).
Trench fill foundation (Fig. 3).
Pad foundation (Fig. 4).
Pile foundation (Fig. 5).

Figure 3. Trench fill, and variations of strip foundation (University of the West of England, 2009)

Figure 4. Pad foundation (Heaton Manufacturing Ltd., 2023)

Figure 2. Raft foundation (LABC, 2023)

Figure 5. Pile foundation (Civil today, n.d.)

Raft, strip, and trench fill foundations will be considered today.
Pad, and pile foundations will be considered in our next lesson.

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Raft Foundations

The concrete raft foundation serves as a broad support for buildings, ideal for low load-bearing soil like loose sand or soft clays.
It is preferred on poor or unstable ground due to its lower bearing pressure compared to strip foundations.
Cost-effective when covering over half of the ground area, it minimises concrete usage and interference with subsurface water movement.
Particularly suitable for areas with expected differential movements or subsidence.
It requires design by a chartered civil or structural engineer.

Figure 6. Raft edge detail (Chudley and Greeno, 2016)

Figure 2. Raft foundation (LABC, 2023)

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Raft Foundation Design

Steel reinforcement is lapped at least 450mm and provided with a minimum of 40mm concrete cover (LABC, 2023).
To avoid damage from frost action or when building during cold weather, the minimum foundation depth should be 600 mm below the ground level.
Use of damp proof course to prevent the passage of moisture to the inside of the structure. Also, a drained cavity/cavity drainage system and damp proof membranes can be used.

Figure 7. The position of DPC to resist moisture in ground beam foundation (NHBC, 2023)

Figure 8. The position of DPC to resist moisture in raft foundation (NHBC, 2023)

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Trench Fill Foundation

Trench fill foundations offer a swift alternative to strip foundations, enhancing ground stability by minimising exposure.
The construction speed reduces the need for extensive support structures, resulting in cost savings and improved safety.
The concrete used stabilises the earth without risking collapses, making it a secure option for poor ground conditions.
Additionally, the concrete's strength allows it to bridge minor soft spots, and the minimum thickness (T) for a trench fill foundation is 500 mm.

Figure 9. Foundation dimensions. (NHBC, 2023)

Figure 10. Trench fill foundation.

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Strip Foundation Design

The design of foundations is covered by BS 8004:2015+A1:2020, Code of practice for Foundations.
Strip foundations for example require a minimum of 750mm depth in clay soils or 450mm in others. Most local authorities will require 1m deep.
Other factors include ‘dead’ and imposed loads such as wind, snow, floors etc.
When designing a foundation, engineers concentrate on the unit of force (Kilo-newton) rather than mass in kg. [1 tonne is equal to 10 kilo-newton].
An average building load to a house is 120 tonnes which exerts a force of 1200 kN to the ground
This number is then divided into the perimeter length of the building e.g. 30 m which is then divided into 1200kN. This formula will show how much each metre run is carrying e.g. 40kN/m. [Examples will be considered next week].

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Strip Foundation Design

A strip foundation, situated under load-bearing walls, varies in width and depth based on the building load and ground characteristics.
Stability and good bearing capacity of the underlying soil are crucial due to foundation design.
This economical choice allows for adaptation to sloping ground by 'stepping up' the strip foundations
To avoid damage from frost action or when building during cold weather, the minimum foundation depth should be 450 mm below the ground level.
Shrinkable soils are classified as containing more than 35% fine particles (clay and silt) and have a Modified Plasticity Index of 10% or greater.

Table 1. Minimum foundation depths in shrinkable soil (NHBC, 2023)

Figure 11. The position of DPC to resist moisture in ground beam foundation (NHBC, 2023)

Figure 12. The position of DPC to resist moisture in raft foundation (NHBC, 2023)

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Worked Example

Using the information given below, determine the strip foundation sizes required for a stiff sandy clay, 250mm wall supporting 60kN/m.

Figure 13. Minimum width of strip footings (Welsh Government, 2019)

Figure 14. Strip foundation dimensions (Welsh Government, 2019)

Solution: Design considerations: Legal consideration, economic consideration, & plant requirements.

From fig. 13, the foundation width should be 600 mm.
From fig. 2, the thickness can either be P or 150 mm, whichever is greater.
Since W + 2P = 600, therefore P = 175 mm.
Thickness, T = Max [P: 150 mm] = 175 mm.
Since the standard widths of excavator buckets are in multiples of 150 mm, the minimum width is acceptable.

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Group-assessment Task

Using the information given below, determine the strip foundation sizes required for the following cases:

I. Firm clay, 300mm wall supporting 30kN/m

II. Medium dense gravelly sand, 225mm wall supporting 60kN/m

III. Sandy soil that can be excavated with a spade, 450mm wall supporting 50kN/m.

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Other Substructure Design Considerations

Soil is broadly classified into five categories:
Granular soil (e.g. sands or gravels)
Cohesive soil (e.g. clays)
Organic soil (e.g. peats)
Fill or made ground
Rock

Soil Type and Content

In the British Soil Classification System, soils are classified into named Basic Soil Type groups according to size, and the groups further divided into coarse, medium and fine sub-groups.

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Other Substructure Design Considerations

Soil Type and Content

Cohesive soils	Maximum safe bearing capacity (kN/m^2)
Hard clays	430 – 650
Stiff clays and sandy clays	220 - 430
Firm clays and sandy clays	110 – 220
Soft clays and silts	55 – 110
Very soft clays and silts Peat and made ground	55 - 0

Rocks	Maximum safe bearing capacity (kN/m^2)
Igneous and gneissic rocks in sound condition	10,700
Limestones and sandstones	4,300
Schists and slated	3,200
Hard shales, mudstones and soft sandstones	2,200
Clay shales	1,100
Hard solid chalk	650

Table 2. Typical ground bearing capacities of rock and soil.

Table 3. Typical ground bearing capacities of cohesive soils

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Other Substructure Design Considerations

Soil Type and Content

Non-cohesive soils	Maximum safe bearing capacity (Dry) (kN/m^2)	Maximum safe bearing capacity (Submerged) (kN/m^2)
Compact, well graded sands and gravel	430 – 650	220 - 320
Loose, well graded sands gravel	220 – 430	110 – 220
Compact uniform sands	220 – 430	110 - 220

Table 4. Typical ground bearing capacities of non-cohesive soils.

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Other Substructure Design Consideration

Position of trees

Figure 15. Minimum foundation depths for tree species/proximities (Chudley and Greeno, 2016)

In areas with trees, their root which may reach down to 5 m are capable of desiccating soil.
These can have effect on the building foundations.
Tree Preservation Order (TPO) may be in place. Check with local authorities. https://www.gov.uk/search-local-land-charges
These can easily affect shallow foundations less than 3 metres deep, as against deep foundations with depths greater than 3 metres.
Figure 15 shows the minimum depths in shrinkable subsoils.
Alternatively, use the free NHBC Foundation Depth Calculator on App Store, Google Play Store, or download the Windows OS version.

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Substructure Design Procedure - Summary

1) Assess the site conditions following the predesign studies; desk study, site reconnaissance, and soil exploration

2) Calculate the structural design loads: consider all the dead loads, imposed loads, etc., allowances by using appropriate partial safety factors.

3) Decide the foundation types based on:

- Soil type and content

- Position of trees

- Economic considerations

- Legal considerations

- Plant requirements

4) Design the dimensions of the chosen foundation, making references to the Building Regulations requirements, site’s soil properties such as the bearing capacity, consolidation, settlement values, etc.

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Substructure Choice

For soils with adequate bearing capacity, the types of foundations in common use have usually been:
strip for loadbearing walls
pads for the columns of framed structures
Where soil conditions are poor, or if the building loads are high, then trench fill or piled foundations may be required
Foundations for most lightly loaded structures can be designed on the basis of Approved Document A to the Building Regulations
More heavily loaded foundations need to be designed in accordance with accepted practice (BS 8004, BS 8103 & Eurocode 7)

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Self-assessment Task

Explain at least Any THREE functional characteristics for substructure.
Explain at least Any THREE design criteria for substructure.

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Reference/Bibliography

Carter, M. and Bentley, S. P. (2016) Soil properties and their correlations. 2^nd edn. John Wiley and Sons.

Chudley, R. and Greeno, R. (2016) Building construction handbook. 11^th edn. London: Routledge.

Ground & Water Ltd (2001) The g&w guide to… foundation design. Available at: https://groundandwater.co.uk/blogs/the-gw-guide-to-foundation-design/ (Accessed: 08 November 2023)

Heaton Manufacturing Ltd. (2023) Pad foundations: types and uses. Available at: https://heatonmanufacturing.co.uk/pad-foundations/ (Accessed: 08 November 2023)

Less, A. (2021) Bearing capacity of soil - types and calculations. Available at: https://www.tensar.co.uk/resources/articles/what-is-the-bearing-capacity-of-soil(Accessed: 11 November 2023)

LABC (2023) Raft foundation basics. Available at: https://www.labc.co.uk/news/raft-foundation-basics (Accessed: 08 November 2023)

NHBC (2023) NHBC standards. Available at: https://www.nhbc.co.uk/binaries/content/assets/nhbc/tech-zone/nhbc-standards/nhbc-standards-2023-complete-compressed.pdf (Accessed: 09 November 2023)

University of the West of England (n.d.) Types of foundation. Available at: https://environment.uwe.ac.uk/geocal/foundations/fountype.htm#:~:text=Raft%20foundations%20have%20the%20advantage,loads%20over%20a%20larger%20area. (Accessed: 09 November 2023)

University of the West of England (2009) Evolution of building elements. Available at: https://fet.uwe.ac.uk/conweb/house_ages/elements/section1.htm (Accessed: 08 November 2023)

Welsh Government (2019) Approved document a: Structure. Available at: https://www.gov.wales/sites/default/files/publications/2019-04/170403building-regs-approved-document-a-structure-en.pdf (Accessed: 09 November 2023)