This article presents an overview of the key aspects of pile foundation design, outlining the principles, design methods, and specific requirements set out in Eurocode 7 and the UK National Annex.
Pile foundations are deep foundations driven or drilled into the ground to transfer structural loads to deeper, more stable soil or rock strata. The primary functions of piles include transferring vertical loads (compression), resisting lateral loads, and providing uplift resistance.
Like spread foundations, pile foundation has been designed using the permissible stress method. A situation where the ultimate load a foundation can sustain is derived and then a factor of safety is applied to obtain the safe working load the foundation can sustain. The factor of safety is a way of dealing with settlement considerations.
Eurocode 7 adopts a completely divergent design philosophy – the Limit State Design. Here partial factors are applied to both actions or their effects and material parameters. Eurocode 7 provides a design philosophy that allows designers to make reliable allowance for uncertainties by applying separate partial factors to actions, ground properties and material strength based on their individual degree of uncertainty, as opposed to the application of a global factor of safety. For a more detailed insights on how foundation design in the past differs from the Eurocode (See: Foundation Design – Does Eurocode 7 Justify Higher Bearing Capacity Values?). This article only presents an overview of the key aspects of pile foundation design, outlining the principles, design methods, and specific requirements set out in Eurocode 7 and the UK National Annex.
Eurocode 7: A Brief Overview
Eurocode 7 (EC7), formally known as EN 1997-1:2004, provides the European standards for geotechnical design. It covers a wide range of geotechnical issues, including pile foundation design. The UK National Annex offers additional guidelines specific to the UK, supplementing the Eurocode where necessary. Eurocode 7 is composed of two parts; Eurocode (part 1) also referred to as EN 1997-1 which sets out the principles and application rules for conducting geotechnical design and Eurocode (part 2) which provides the requirement for ground investigation and the derivation of various soil parameters from soil tests.
Within Eurocode 7, there are four main methods of conducting geotechnical designs, these include design by calculation, design by observation, design by testing and design by prescriptive measures. All four methods are equally valid and based on the assumption that no limit state is exceeded; however, some methods might produce more efficient and economical solution than others depending on the accuracy of the data available and the risk and complexity of the design. Talking about risk and complexity of design, Eurocode 7 recommends three Geotechnical categories that should be used in assessing risk and expected design method when undertaking a geotechnical design (see above referenced article).
Basic Principles of Pile Foundation Design
The design of pile foundations is contingent on the principle that the piles can safely transfer the imposed loads from the superstructure to the ground. The design process includes determining:
- Pile Capacity: The ultimate load-carrying capacity of a pile, which must be greater than the applied loads.
- Pile Group Behavior: The interaction between piles when used in groups.
- Settlement: The acceptable level of settlement under load.
- Structural Integrity: The pile must be strong enough to withstand the stresses imposed during installation and loading.
Design Approaches in Eurocode 7
Eurocode 7 specifies three different design approaches, known as DA-1, DA-2, and DA-3. These approaches differ in how they handle partial factors applied to actions (loads), resistances, and material properties. The UK National Annex predominantly endorses DA-1 and DA-2 for pile foundation design.
Design Approach 1 (DA-1)
In DA-1, two cases are considered:
- Case A: Partial factors are applied to the actions.
- Case B: Partial factors are applied to the resistances.
This approach ensures that the design is safe for both overestimation of loads and underestimation of resistances. For pile foundation design, this approach is commonly used when the ground conditions are well understood.
Design Approach 2 (DA-2)
In DA-2, partial factors are applied directly to the resistances (soil or rock). This approach is typically conservative and is preferred when there is significant uncertainty in the ground conditions. DA-2 is particularly useful in complex geotechnical scenarios where conservative estimates of resistance are crucial.
Design Approach 3 (DA-3)
In DA-3, partial factors are applied primarily to the materials rather than the actions or resistances. This approach is less commonly used for pile foundation design and is typically reserved for specific situations where material properties play a dominant role in the overall safety.
Determining Pile Capacity
Pile capacity is a critical aspect of pile foundation design. The ultimate capacity of a pile, Qu, is the sum of the shaft resistance Qs and the base resistance Qb. This above statement can be mathematically written as:
Q_u=Q_s+Q_b
Shaft Resistance
The shaft resistance, Qs is the frictional force generated between the pile surface and the surrounding soil. It can be calculated as:
Q_s =\sum(\alpha \cdot \sigma_{cv} \cdot A_s)where:
- α is the adhesion factor,
- σv′ is the effective vertical stress,
- As is the surface area of the pile in contact with the soil.
Base Resistance
The base resistance, Qb, is the bearing capacity of the soil or rock at the base of the pile. It can be calculated using:
Q_b= q_b \cdot A_b
where:
- qb is the unit base resistance,
- Ab is the cross-sectional area of the pile base.
The values of qb and α depend on the type of soil or rock and the method of pile installation.
Factors Affecting Pile Capacity
Several factors influence pile capacity, including:
- Soil Type: The type of soil (e.g., clay, sand, rock) significantly affects the shaft and base resistance.
- Pile Material: The material of the pile (e.g., concrete, steel, timber) influences its interaction with the soil.
- Pile Installation Method: Driven piles, bored piles, and screw piles interact differently with the surrounding soil, affecting their capacity.
- Pile Length and Diameter: The dimensions of the pile determine the area available for shaft resistance and the bearing area at the base.
Limit State Design (LSD)
Eurocode 7 employs the Limit State Design (LSD) method, where partial factors are applied to both loads and resistances to ensure safety. The design equation for pile foundations is expressed as:
\gamma_F \cdot F_d \le \gamma_R \cdot R_d
where:
- γF is the partial factor for actions,
- Fd is the design action,
- γR is the partial factor for resistances
- Rd is the design resistance.
Serviceability Limit State (SLS)
In addition to the Ultimate Limit State (ULS), the Serviceability Limit State (SLS) must also be considered. The SLS addresses issues related to deformations, such as settlement, that may impair the functionality of the structure. The allowable settlement for a pile foundation is determined based on the structure’s tolerance to differential settlement.
Pile Group Design
When piles are used in groups, their behavior differs from that of single piles due to group interaction effects. The design of pile groups must account for:
- Group Efficiency: The interaction between piles in a group may reduce the overall efficiency compared to individual piles.
- Settlement Interaction: The settlement of a pile group can be greater than that of a single pile due to overlapping stress zones in the soil.
- Lateral Load Resistance: Pile groups must be designed to resist lateral loads, which can induce additional bending moments and shear forces.
Pile Testing and Verification
Pile testing is crucial in validating the assumptions made during design. Eurocode 7 outlines several methods for pile testing, including:
- Static Load Tests: These tests apply a controlled load to the pile and measure the resulting settlement. The load-settlement curve obtained from this test helps in assessing the pile’s load-bearing capacity.
- Dynamic Load Tests: These involve applying a dynamic load (such as a hammer blow) and measuring the response. This method is faster than static tests but may require interpretation using wave equation analysis.
- Integrity Testing: Integrity tests, such as sonic logging or low-strain impact testing, assess the pile’s structural soundness and detect any defects or anomalies.
Consideration of Pile Installation Effects
The installation process can significantly impact the performance of pile foundations. Eurocode 7 requires designers to consider:
- Soil Displacement: Driven piles displace soil, which can increase shaft resistance but may also cause soil heave and affect nearby structures.
- Pile Set-Up and Relaxation: Over time, the capacity of a pile may increase (set-up) or decrease (relaxation) due to changes in soil conditions after installation.
- Vibration Effects: The installation of driven piles generates vibrations, which can affect nearby structures and require mitigation measures.
Specific Guidance in the UK National Annex
The UK National Annex to Eurocode 7 provides additional guidance tailored to UK ground conditions and construction practices. Key points include:
- Partial Factors: The UK National Annex specifies the partial factors to be used in different design situations, which may differ slightly from those in the main Eurocode.
- Pile Load Tests: The UK National Annex emphasizes the importance of pile load tests and provides specific criteria for interpreting test results.
- Ground Investigation: The UK National Annex stresses the importance of thorough ground investigation to reduce uncertainty in pile design, particularly in complex or variable ground conditions.
Conclusion
Pile foundation design to Eurocode 7 and the UK National Annex is a systematic process that requires careful consideration of soil-structure interaction, load-resistance relationships, and installation effects.
Effective pile foundation design to Eurocode 7 hinges on understanding the unique properties of the ground, selecting the appropriate design approach, and validating the assumptions through testing. A combination of Eurocode 7 and the UK National Annex provides a robust framework for achieving these objectives, ultimately leading to safe, reliable, and cost-effective pile foundation solutions.
Also See: [Updated] A Background to Piling
Sources Citations
- Bond A and Harris A (2008) Decoding Eurocode 7. Taylor & Francis, London, UK.
- Institution of Structural Engineers (2013) Manual for the Geotechnical Engineers of Structures to EC7.
- Trevor L.L (2012) How Eurocode 7 has affected geotechnical design: a review, Proceedings of the Institution of Civil Engineers – Geotechnical Engineering.
- BSI (1986) BS 8004:1986: Code of practice for foundations. BSI, Milton Keynes, UK.
- BSI (2004b) BS EN 1997-1:2004: Eurocode 7: Geotechnical design – Part 1: General rules. BSI, Milton Keynes, UK.
- BSI (2007a) BS EN 1997-2:2007: Eurocode 7: Geotechnical Design – Part 2: Ground investigation and testing. BSI, Milton Keynes, UK
