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�FOUNDATION DESIGN PRINCIPLES

Dr Adewale Abimbola, FHEA, GMICE.

www.edulibrary.co.uk

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Aim & Objectives

Aim: Foundation Design Principles

Objectives: At the end of the lesson, the student should be able to:

Explain the basic principles of foundation design used in low-rise domestic buildings.

Calculate the minimum foundation depths needed to reduce the impact of soil shrinkage, ground heave, and root-related soil changes.

Design strip foundations by following the guidance in the Building Regulations Approved Document Part A.

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Foundation Design Principles

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Design Principles

  • The stability of a structure depends on the balance between the loads transmitted from the building and the soil’s capacity to safely sustain those loads.
  • When soil bearing capacity is adequate, settlement remains within acceptable limits, minimising risks of cracking or structural distortion.
  • Imbalances may cause differential movement, leading to damage far beyond minor surface cracks—such as misaligned frames, distorted doorways, or compromised structural integrity.
  • Accurate assessment of soil properties ensures that foundation design aligns with the expected load, preventing overstressing and long-term instability.
  • Evenly distributing building loads through appropriate foundation type and layout minimises excessive pressure points and uneven settlement across the structure.

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Factors to Consider When Minimising Foundation Settlement

Building Load

Excessive or uneven loading from a structure can exceed the soil’s bearing capacity, leading to compression and progressive settlement. 

Balanced design and load distribution are essential for long-term stability

Comprehensive Site Investigation

Conduct detailed geotechnical and soil assessments to understand bearing capacity, moisture content, and ground variability before design and construction.

Uniform Soil Compaction

Ensure consistent and adequate compaction of fill materials during site preparation to prevent uneven compression under structural loads.

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Factors to Consider When Minimising Foundation Settlement

Appropriate Foundation Design

Select foundations suited to site-specific soil conditions—such as raft or piled foundations for weak or compressible strata—to distribute loads evenly and limit differential settlement.

Foundation Depth

Deeper foundations are generally less affected by surface moisture fluctuations.

Proper design ensures consistent bearing capacity and reduces risks from shrink–swell soil behaviour.

Groundwater Conditions

Fluctuating water tables can either soften or dry out supporting soils.

Maintaining stable groundwater through controlled drainage minimises both settlement and heave-related movement.

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Factors to Consider When Minimising Foundation Settlement

Differential Settlement

Takes place when parts of a structure settle unevenly due to variations in soil type, load distribution, or moisture content. It can result in visible distortion, cracking, or misalignment in buildings

Load Distribution Control

Avoid eccentric or uneven loading by maintaining balanced structural layouts and incorporating tie beams or rigid foundation systems to distribute stresses.

Monitoring and Maintenance

Implement post-construction monitoring through settlement markers or sensors to detect early ground movement, allowing for timely corrective action. 

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Factors to Consider When Minimising Foundation Settlement

Soil Shrinkage

Occurs mainly in clay-rich soils during dry periods as moisture is lost from the ground, causing the soil to contract and create voids. This can lead to uneven support and cracking in foundations. 

Ground Heave

The opposite of shrinkage—happens when soil absorbs water and expands, exerting upward pressure on foundations. 

Seasonal moisture variation can cause cyclical movement and structural stress.

Effects of Tree Growth and Removal

Tree roots near foundations can draw large quantities of water from the soil, causing shrinkage and settlement. 

Conversely, tree removal may lead to rehydration and soil expansion (heave) beneath the structure.

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Substructure Design Consideration: Tree Position

Figure 1. 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 1 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 Consideration: Tree Position

Figure 1. Minimum foundation depths for tree species/proximities (Chudley and Greeno, 2016)

 

Video: Root Barrier

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Group-assessment Task

Question 1

For the sycamore tree measuring 4 metres in height and situated 3 metres from the residential structure shown, calculate the minimum foundation depth to be designed to adequately account for potential soil movement caused by root activity and moisture variation.

Question 2

Considering the presence of a 5-metre high poplar tree located five metres from the building, determine the minimum foundation depths that will mitigate the effects of soil shrinkage, ground heave, and other root-induced soil changes. 

Do you think it is practical to build the structure at this distance from the tree, if not, why?

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Building Regulations�Approved Documents Part A – Structural Safety

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Introduction

  • The Building Act 1984 is the primary piece of legislation that formed the Building Regulations and their Approved Documents.
  • The Building Act 1984 is an overarching legislation that provides the legal framework for building and construction activities.
  • Building regulations are minimum standards for design, construction and alterations to virtually every building. The regulations are developed by the UK government and approved by Parliament.
  • Compliance with Building Regulations is mandatory, and failure to adhere to these standards can result in legal consequences.
  • Approved Documents set out detailed practical guidance on compliance with the regulations: (Set into a whole range of subsections: Parts A-S).
  • The Building Act covers some of the following provisions:
  • Power to make the Building Regulations and any subsequent revisions.
  • Production of Approved Documents
  • To allow the passing of submitted plans.
  • To give powers to inspectors.
  • Penalties for breaches to the regulations.

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Building Regulations Part A - �Worked Example

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

Figure 2. Minimum width of strip footings (The Welsh Government, 2010)

Figure 3. Strip foundation dimensions (The Welsh Government, 2010)

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

  • From fig. 2, the foundation width should be 600 mm.
  • From fig. 3, 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.
  • Strip foundations require a minimum of 750mm depth in clay soils or 450mm in others. Most local authorities will require 1m deep.

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https://www.youtube.com/watch?v=a5LfHmaH8O0

https://www.youtube.com/watch?v=QqHA_DkkTt8

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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.

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Scenario:

You are part of a design team deciding on the foundation type for a new community centre. The site has mixed soil profiles, nearby mature trees, varying groundwater levels, and is in a region prone to seasonal droughts and rains. Assess key factors and propose solutions to minimise risk of foundation movement.

Self-assessment Task

  • Research how any THREE of the factors below can affect foundation stability and identify practical mitigation strategies.
    1. Soil shrinkage
    2. Ground heave
    3. Differential settlement
    4. Effects of tree growth and tree removal
    5. Foundation depth
    6. Groundwater conditions
    7. Building load

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REFERENCES/BIBLIOGRAPHY

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