�SUBSTRUCTURE

Dr Adewale Abimbola, FHEA, GMICE.

www.edulibrary.co.uk

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Aim & Objectives

Aim: Substructure

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

• Explain the functional characteristics for substructure.
• Explain the design criteria for substructure.
• Analyse how site conditions impact on the design of foundations.

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Learning Outcomes and Assessment Criteria

(Part of) P4 – Explain the functional characteristics and design criteria for substructure.

M4 – Analyse how site conditions impact on the design of foundations.

Substructure Design Principles

• The main objectives of foundation design are to:-
• ensure that the structural loads are transmitted to the subsoil safely, economically and without any unacceptable movement during the construction period and throughout the anticipated life of the building or structure

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

Types of Foundations

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

Pad Foundation

• The fundamental foundation for columns in steel or reinforced concrete frame structures is the 'pad foundation’
• Comprising a typically square excavation filled with in-situ concrete.
• Embedded within the concrete are starter bars for reinforced concrete columns or holding down bolts for steel columns.

https://www.youtube.com/watch?v=ADhbGfbd43k

Mass Concrete Bases:

• Preferable Usage: Recommended whenever feasible.
• Scenario Suitability:
• Unstable excavations.
• Requirements for rapid and cost-effective construction.

Reinforced Bases:

• Appropriate in Conditions of:
• High water table.
• Adverse alterations in soil strata.
• Presence of buried services/structures.
• Thin soil strata.
• Existence of underground obstructions.

Figure 6. Types of pad foundation

Pad Foundation

• Pad foundations are a type of shallow foundation, with depth below finished ground level less than 3m.
• Ground Condition Considerations:
• Risk Factors:
• Concrete vulnerability to:
• Ground or groundwater contamination.
• Effects of freeze/thaw cycles.
• Damage from vegetation.
• Soil Types Prone to Expansion:
• Silts, chalks, fine sands, and certain clays.
• Expansion risk during freeze. Soil freezes to a depth of 450mm.
• Pads on susceptible soils placed at 500mm below ground level.

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Potential solutions to mitigate against pad deformation include:

• Increased Thickness (D > B/2):
• Ensure pad thickness (D) is greater than half the column base dimension (B/2).
• Minimum thickness per BS 8004: 150 mm, but consider a minimum of 300 mm.
• Steel Reinforcement (Bottom of Foundation):
• Incorporate steel reinforcement at the foundation's bottom.
• Enhances structural integrity and load-bearing capacity.
• Sulphate Salts Presence:
• Use sulphate-resisting cement in pad concrete to mitigate potential deterioration.

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Soil Type and Content

Table 1. Typical ground bearing capacities of rock and soil.

Table 2. Typical ground bearing capacities of cohesive soils

Soil Type and Content

Table 3. Typical ground bearing capacities of non-cohesive soils.

Pad Foundation – Calculate the size of the pad

Pad Foundation – Calculate the size of the pad

Pad Foundation – Calculate the size of the pad

Pad Foundation – Calculate the size of the pad

Self-assessment Task

Pad Foundation Sizing

Calculate the plan area of a pad foundation to axial loaded columns, where:

• The total load from the column is 195 kN and the soil is a sandy clay.
• The superstructure dead load is 500 kN and the variable dead load is 300 kN is on a hard clay.

Refer to tables 1, 2 and 3 for the allowable bearing capacity values.

Pile Foundation

• Piles are vertical foundation elements designed to transfer loads from superstructures.
• Functions:
• Load Transfer:
• Transmit loads through weak, compressible strata or water to stable bedrock.
• Channel loads onto stiffer, less compressible soils or rock.
• Uplift Load Support:
• Handle uplift loads for tall structures facing overturning forces (e.g., winds, waves).
• Marine Structure Considerations:
• Endure lateral loads from berthing ships and wave impacts.
• Versatile Applications:
• Support retaining walls, bridge piers, abutments, and similar structures.

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Figure 6. Pile foundation (Jamal, 2017)

Pile Foundation - Classification

Load Bearing Piles

• The "soil mechanics approach" calculates a pile's capacity by considering two primary components: skin friction and end resistance.
• Piles are categorized based on the predominant component: friction piles emphasize skin friction, while end-bearing piles rely on support from hard substrates.
• Components:
• Skin Friction:
• Predominant in friction piles.
• Arises from frictional forces along the pile's surface with the surrounding soil.
• End Resistance:
• Predominant in end-bearing piles.
• Occurs when the pile rests on hard, incompressible material like rock.

Figure 7. Pile foundation (Tomlinson and Woodward, 2008)

Pile Foundation - classification

Non-Load bearing piles

• They primarily serve as separating members below ground and are not designed to bear vertical loads.
• Commonly referred to as sheet piles, emphasizing their role in creating partitions or barriers.
• Materials Used:
• Timber Sheet Piles:
• Utilized for their natural resilience and environmental friendliness.
• Steel Sheet Piles:
• Chosen for their durability, strength, and suitability in various soil conditions.
• Concrete Sheet Piles:
• Employed due to the robustness of concrete and its adaptability to different applications.

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Figure 8. Sheet piles (Designing Buildings Ltd., 2023)

Pile Foundation - classification

Applications of Non-Load Bearing Piles

• 1. Foundation Isolation:
• Non-load bearing piles serve to isolate foundations from adjacent soils, preventing soil escape and minimising the transmission of shocks and vibrations to nearby structures.
• 2. Water Movement Control:
• Used in cofferdams to create a watertight enclosure during underwater foundation construction, preventing the underground movement of water.
• 3. Vibration Prevention:
• Aids in preventing the transfer of machine vibrations to surrounding structures, contributing to a stable environment.

Applications of Non-Load Bearing Piles

• 4. Retaining Wall Construction:
• Employed in the construction of retaining walls, providing structural support and stability.
• 5. River Bank Protection:
• Used to protect river banks, offering stability against erosion and soil displacement.
• 6. Trench Side Retention:
• Utilised for retaining the sides of foundation trenches, ensuring safety and preventing collapses.

Pile Classification According to BS 8004

• Large Displacement Piles:
• Definition: Solid-section or hollow-section piles with a closed end, either driven or cast-in-place, displacing the soil (Cranston, 2021).
• Examples:
• Timber (round or square section, jointed or continuous).
• Precast concrete (solid or tubular section in continuous or jointed units).
• Prestressed concrete (solid or tubular section).
• Steel tube (driven with a closed end).
• Steel tube driven and withdrawn after placing concrete.

Figure 9. Displacement piles (Engineers Today, 2012)

Watch driven piles

Pile Classification According to BS 8004

• 2. Small-Displacement Piles:
• Definition: Driven or jacked into the ground with a relatively small cross-sectional area (Cranston, 2021).
• Examples:
• Rolled steel H- or I-sections.
• Pipe or box sections driven with an open end (soil enters the hollow section).
• Precast concrete (tubular section driven with an open end).
• Prestressed concrete (tubular section driven with an open end).
• Steel H-section.
• Steel tube section (driven with an open end, soil removed as required).
• Steel box section (driven with an open end, soil removed as required).

Figure 10. Displacement piles (Superior Foundations, 2023)

Pile Classification According to BS 8004

• Replacement piles are created through various drilling methods, where soil is removed before inserting a substitute material. Techniques include:
• Bored and Cast-in-Place:
• Concrete is placed in a hole drilled using rotary auger, baling, grabbing, airlift, or reverse-circulation methods.
• Tubular Insertion:
• Tubes are placed in a hole drilled as described above, filled with concrete as necessary.
• Precast Concrete Units:
• Preformed concrete elements are placed in a drilled hole.
• Injection Techniques:
• Cement mortar or concrete is injected into a drilled hole.
• Steel Sections:
• Steel sections are placed in a drilled hole.

Figure 11. Replacement piles. (Ougan Technology, 2020)

Watch CFA Piles

Watch cast-in-place piles

Pile Classification According to BS 8004

• Composite piles leverage the strengths of different materials and categories, providing tailored solutions for specific foundation requirements.
• Composite piles result from combining units of different categories:
• Combining Different Types:
• Formation: Units from various categories (large-displacement, small-displacement, or replacement) are combined.
• Timber and Precast Concrete Joint:
• Process: Jointing a timber section to a precast concrete section creates a composite pile.
• H-Section Attachment:
• Process: A precast concrete pile may have an H-section jointed to its lower extremity for added strength and support.

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Figure 12. Composite piles. (CTech-LLC, n.d.)

Factors Influencing Pile Selection

• Choosing the suitable pile type involves consideration of the following key factors:
• Structure Location and Type:
• The nature and positioning of the structure significantly influence the pile selection process.
• Ground Conditions:
• The composition and characteristics of the ground play a crucial role in determining the most effective pile type.
• Durability Requirements:
•  The anticipated lifespan and durability expectations guide the choice of pile materials and construction methods.

Factors Influencing Pile Selection

• Structure Location and Type:
• The nature and position of the structure heavily influence pile selection.
• Displacement Piles for Marine Structures:
• Displacement piles are often the primary choice for marine structures due to their effectiveness.
• Pile Selection in Shallow and Deep Water:
• Solid precast or prestressed concrete piles are suitable for shallow waters, while in deep waters, the weight concern favours steel tubular or tubular precast concrete piles.
• Preference for Steel Tubular Piles:
• In exposed marine conditions, steel tubular piles are preferred over H-sections due to lower drag forces from waves and currents.
• Economic Solution with Large-Diameter Steel Tubes:
• Large-diameter steel tubes present an economical solution for addressing impact forces from waves and berthing ships.

Factors Influencing Pile Selection

• Structure Location and Type:
• Timber Piles in Shallow Water:
• Timber piles find application in temporary works, particularly in relatively shallow waters.
• Augered Piles for Urban Areas:
• Augered piles are suitable for urban areas, offering advantages such as avoiding ground heave, noise, and vibration, making them compliant with stringent noise regulations.
• Economical Use of Driven and Cast-in-Place Piles:
• Driven and cast-in-place piles prove cost-effective for land structures, especially when dealing with light to moderate loads.
• Timber Piles for Light to Moderate Loadings:
• Timber piles are well-suited for applications with light to moderate loads, particularly in regions where timber is readily available.

Factors Influencing Pile Selection

• The ground condition
• The choice of pile material and installation method is significantly influenced by soil conditions.
• Preference for Augered Bored Pile in Firm to Stiff Cohesive Soils:
• Augered bored piles are favoured in firm to stiff cohesive soils. However, this method is not suitable for very soft clays or loose, water-bearing granular soils.
• Limitations of Driven Piles in Certain Soil Conditions:
• Driven and driven-and-cast-in-place piles are not suitable for ground containing boulders, massive obstructions, or soils prone to ground heave.
• Suitability of Steel Piles in Hard Driving Conditions:
• In challenging driving conditions like boulder clays or gravelly soils, thick-walled steel tubular piles or steel H-sections demonstrate better resistance to heavy driving compared to precast concrete piles.

Factors Influencing Pile Selection

• Durability
• The environmental context significantly influences the choice of material for constructing piles.
• Challenges with Timber Piles:
• Cost vs. Decay Concerns: Timber piles, while cost-effective in certain regions, are susceptible to decay above the groundwater level. In marine structures, they face damage from destructive mollusc-type organisms.
• Advantages of Precast Concrete Piles:
• Corrosion Resistance: Precast concrete piles exhibit resistance to corrosion in saline water. Well-compacted concrete can withstand attacks from high concentrations of sulphates in soils and groundwater.
• Considerations for Cast-in-Place Concrete Piles:
• Compaction Challenges: Cast-in-place concrete piles may be less resistant to aggressive substances due to compaction challenges. Protection against corrosion can be achieved by placing the concrete in permanent linings made of coated light-gauge metal or plastics.

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)

Self-assessment Task

Explain at least Any THREE design criteria each for a pad and pile 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.

Cranston, P. (2021) Onshore process facilities civil engineering demystified. Available at: https://www.linkedin.com/pulse/onshore-process-facilities-civil-engineering-peter-cranston/ (Accessed: 17 November 2023)

CTech-LLC (n.d.) Retrofitting of marine structures. Available at: https://ctech-llc.com/en/applications/marine-structures/ (Accessed: 17 November 2023)

Curtin, W. G., Shaw, G., Parkinson, G. I., Golding, J. M. and Seward, N. J. (2006) Structural foundation designer’s manual. 2^nd edn. Oxford: Blackwell.

Designing Buildings Ltd. (2023) Sheet piles. Available at: https://www.designingbuildings.co.uk/wiki/Sheet_piles (Accessed: 17 November 2023)

Engineers Today (2012) How to install pile to foundation. Available at: https://engineerstoday.blogspot.com/2012/04/how-to-install-pile-to-foundation.html (Accessed: 17November 2023)

Geotechnique.info (2004) Chapter 3: The desk study and walk-over survey. Available at: http://www.geotechnique.info/SI/SI%20Book%20Chapter%203.pdf (Accessed: 31 October 2023)

Jamal, H. (2017) Types of piles based on load transfer, installation methods and materials. Available at: https://www.aboutcivil.org/types-classification-of-piles.html (Accessed: 31 October 2023)

Ougan Technology (2020) Fully cased drilling with oscillator. Available at: https://www.ougangroup.com/en/news/detail_675.html (Accessed: 17November 2023)

Southern Testing Ltd (2023) Geological and geotechnical hazards in south east england: part 2 solution features in chalk. Available at: https://www.southerntesting.co.uk/blog/geological-and-geotechnical-hazards-in-south-east-england-part-2-solution-features-in-chalk/#:~:text=Solution%20features%20(or%20dissolution%20features,%2C%20if%20inundated%2C%20will%20subside. (Accessed: 31 October 2023)

Superior Foundations (2023) Driven piles. Available at: https://www.superiorfoundationsllc.com/our-services/driven-piles/ (Accessed: 17November 2023)

Tomlinson, M. and Woodward, J. (2008) Pile design and construction practice. 5^th edn. Oxon: Taylor & Francis.

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