Sachpazis: Foundation Analysis and Design: Single Piles

ρ. Κώστας Σαχπάζης: Foundation Analysis and Design: Single Piles

Welcome to this comprehensive presentation on "Foundation Analysis and Design," focusing on Single Piles—Static Capacity, Lateral Loads, and Pile/Pole Buckling. This presentation will explore the fundamental concepts, equations, and practical considerations for designing and analyzing pile foundations.
We'll examine different pile types, their characteristics, load transfer mechanisms, and the complex interactions between piles and surrounding soil. Throughout this presentation, we'll highlight key equations and methodologies for calculating pile capacities under various conditions.

1. Foundation Analysis and Design: Single Piles Welcome to this comprehensive presentation on "Foundation Analysis and Design," focusing on Single Piles—Static Capacity, Lateral Loads, and Pile/Pole Buckling. This presentation will explore the fundamental concepts, equations, and practical considerations for designing and analyzing pile foundations. We'll examine different pile types, their characteristics, load transfer mechanisms, and the complex interactions between piles and surrounding soil. Throughout this presentation, we'll highlight key equations and methodologies for calculating pile capacities under various conditions. by Dr. Costas Sachpazis

2. Introduction to Pile Foundations 1 Definition and Purpose Piles are structural elements made from timber, concrete, or steel designed to transmit surface loads to deeper soil layers. They provide foundation support when surface soils are inadequate for conventional spread footings or mat foundations. 2 Load Transfer Classification Piles are classified based on their load transfer mechanisms as either friction (floating) piles, end-bearing (point) piles, or combinations of both. This classification determines how loads are distributed through the pile into the surrounding soil. 3 Economic Considerations Pile foundations typically cost more than spread footings or mats, necessitating careful economic analysis and thorough soil property determination before selection. The higher initial investment must be justified by performance requirements.

3. Common Applications of Pile Foundations Superstructure Support Piles carry loads from buildings and other structures through weak surface soils to stronger, deeper strata. This is particularly important in areas with poor surface soil conditions or high water tables. Uplift Resistance Piles resist uplift or overturning forces in structures subjected to lateral loads, such as wind or seismic forces. This anchoring function is critical for tall structures or those in high-wind or seismic zones. Machine Foundations Piles stiffen soil beneath machine foundations to reduce vibration and provide stability. This application is essential for precision equipment and heavy machinery that requires minimal settlement. Bridge Support Piles provide additional safety beneath bridges, especially in areas where scour or erosion may occur. They transfer loads to stable soil layers below potential erosion zones.

4. Timber Piles: Characteristics and Applications Material Properties Timber piles are natural wood elements, typically treated with preservatives to prevent decay. They offer good flexibility and are relatively easy to handle during installation. Their natural taper provides increased bearing capacity at the tip. Dimensional Specifications Timber piles have typical dimensions and minimum requirements specified in building codes. The natural taper of trees results in a larger diameter at the butt end (top) and smaller at the tip, which must meet minimum diameter requirements. Common Challenges Key problems with timber piles include vulnerability to decay in fluctuating groundwater conditions and potential damage during driving. Fiber crushing (brooming) at the pile head during installation requires special attention and mitigation techniques.

5. Timber Pile Design Equation Allowable Design Load The allowable design load for timber piles is calculated using Equation 16-1: Pa = Apfa, where Pa is the allowable design load based on pile material, Ap is the average pile cross-sectional area at the pile cap, and fa is the allowable design stress for the type of timber. Material Considerations The allowable design stress (fa) varies by timber species and is specified in building codes. This value accounts for long-term loading effects and environmental factors that may affect the timber's strength over time. Safety Factors Design values incorporate safety factors to account for natural variations in timber properties, potential decay over time, and uncertainties in loading conditions. These factors ensure the pile performs reliably throughout its service life.

6. Precast and Prestressed Concrete Piles 1 Manufacturing Precast concrete piles are manufactured in controlled environments, allowing for consistent quality and strength. They are cast in forms, cured, and then transported to the construction site for installation. 2 Prestressing Process Prestressed concrete piles utilize tensioned steel strands or wires that compress the concrete when released, increasing the pile's ability to resist tensile stresses during handling and driving. This process significantly enhances the pile's structural performance. 3 Installation During installation, special attention must be paid to handling stresses, as precast piles can crack if improperly supported. Driving stresses must be carefully monitored to prevent damage to the pile structure. 4 Long-term Performance Over time, prestressed piles may experience stress loss due to concrete creep and steel relaxation. These factors must be accounted for in the design to ensure long-term structural integrity.

7. Prestressed Concrete Pile Design Equation 0.33 Concrete Factor The factor 0.33 applied to concrete compressive strength (f'c) represents the allowable stress ratio for concrete under compression in the design equation. 0.27 Prestress Factor The factor 0.27 applied to effective prestress (fpe) accounts for the contribution of prestressing to the pile's load-carrying capacity. 5 Typical Prestress Loss (MPa) After accounting for losses due to creep, shrinkage, and steel relaxation, the effective prestress typically reduces by approximately 5 MPa from the initial value. The allowable design load for prestressed concrete piles is calculated using Equation 16-2: Pa = Ag(0.33f'c - 0.27fpe), where Ag is the gross concrete cross-sectional area, f'c is the concrete compressive strength, and fpe is the effective prestress after losses.

8. Cast-in-Place Concrete Piles Site Preparation The process begins with site preparation and layout of pile locations according to foundation plans. Proper site preparation ensures accurate pile positioning and alignment with the structural design. Installation Methods Cast-in-place piles can be installed through various methods: drilling, driven shells or casings, mandrel-driven casings, or auger-placed pressure-injected concrete. Each method has specific applications based on soil conditions and project requirements. Concrete Placement After creating the void or casing, concrete is placed either by free-fall (in dry conditions) or tremie methods (in wet conditions). Quality control during concrete placement is critical to ensure pile integrity and strength. Curing and Testing After placement, concrete must cure properly before loading. Integrity testing may be performed to verify the absence of defects such as necking, voids, or inclusions that could compromise structural performance.

9. Specialized Cast-in-Place Pile Systems Franki Piles Franki piles feature an enlarged base formed by dropping a concrete plug from the bottom of a driven tube, then forcing aggregate and concrete outward under pressure. This creates a bulb at the base that significantly increases end-bearing capacity. Continuous-Flight Auger Piles CFA piles are formed by drilling with a continuous auger while simultaneously pumping concrete through the hollow stem as the auger is withdrawn. This method minimizes soil disturbance and is ideal for sites with high groundwater or unstable soils. Pressure-Injected Concrete Piles These piles utilize high-pressure concrete injection to create an expanded base and densify surrounding soil. The pressure injection process increases both end-bearing capacity and skin friction along the pile shaft.

10. Non-Prestressed Concrete Pile Design Equation Combined Material Approach The allowable design load for non-prestressed concrete piles is calculated using Equation 16-3: Pa = Acfc + Asfs, which accounts for the contribution of both concrete and steel components to the pile's capacity. Concrete Contribution The term Acfc represents the load-carrying capacity of the concrete portion, where Ac is the concrete cross-sectional area and fc is the allowable concrete stress, typically a fraction of the concrete's compressive strength. Steel Contribution The term Asfs accounts for the load-carrying capacity of any steel reinforcement or shell, where As is the steel cross-sectional area and fs is the allowable steel stress, usually between 0.33 and 0.5 of the yield strength.

11. Steel Piles: Types and Characteristics HP Shapes HP (H-Pile) shapes are rolled steel sections with parallel flanges designed specifically for deep foundation applications. They offer high strength-to-weight ratios and excellent driving characteristics. HP piles primarily transfer loads through end bearing rather than displacement. Pipe Piles Steel pipe piles are hollow cylindrical sections that can be driven open-ended or closed with a plate at the bottom. Open-ended pipes may form soil plugs during driving, increasing their end-bearing capacity. They can be filled with concrete after driving for additional strength. Installation Considerations Steel piles can withstand high driving stresses, making them suitable for penetrating dense or hard strata. Special reinforcement at pile tips may be necessary when driving through soils containing boulders or into weathered rock.

12. Steel Pile Design Equation Basic Equation The allowable design load for steel piles is calculated using Equation 16-4: Pa = Apfs, where Pa is the allowable design load, Ap is the cross-sectional area of the pile at the cap, and fs is the allowable steel stress. Allowable Stress Determination The allowable steel stress (fs) typically ranges from 0.33 to 0.5 of the steel's yield strength (fy). This range accounts for safety factors and long-term loading conditions that the pile will experience throughout its service life. Application Considerations When applying this equation, engineers must consider potential reductions in cross-sectional area due to corrosion, especially in aggressive environments. Additional factors such as buckling potential and driving stresses may further limit the allowable load.

13. Pile Corrosion: Factors and Protection Natural Soil Conditions Studies indicate that undisturbed natural soils have limited corrosive impact on piles. The corrosion rate in these environments is typically slow enough that it doesn't significantly affect the structural integrity during the design life of most structures. 1 Disturbed and Fill Soils Disturbed or fill soils present a much greater corrosion potential than undisturbed soils. These materials often contain various contaminants, oxygen, and moisture that accelerate the corrosion process, particularly for steel piles. 2 Aggressive Environments Seawater and soils with extreme pH conditions (highly acidic or alkaline) create particularly aggressive corrosion environments. In these conditions, corrosion rates can be significantly higher, necessitating special protective measures. 3 Protective Measures Protective strategies include painting, cathodic protection, concrete encasement, and sacrificial thickness allowances. The selection of protection method depends on the environment, pile material, and design life requirements. 4

14. Soil Properties for Static Pile Capacity 1 Challenges in Parameter Determination Obtaining reliable soil parameters for pile analysis presents significant challenges due to the disturbance and remolding effects of pile installation. The driving process alters the original soil properties, making laboratory test results on undisturbed samples potentially misleading. 2 In Situ Testing Preference In situ testing methods such as Standard Penetration Tests (SPT), Cone Penetration Tests (CPT), and Pressuremeter Tests (PMT) are generally preferred over laboratory tests for pile design. These tests provide more accurate estimations of soil behavior under actual field conditions. 3 Parameter Variability Soil properties can vary significantly across a site, requiring comprehensive site investigation to capture spatial variations. Statistical approaches may be necessary to account for this variability in design parameters.

15. General Static Pile Capacity Equations Compression Capacity The ultimate pile capacity in compression is given by Equation 16-5a: Pu = Ppu + ∑Psi,u or Pu = Pp + ∑Psi,u, where Ppu is the ultimate pile tip capacity, and ∑Psi,u is the ultimate skin resistance developing simultaneously with the tip capacity. Tension Capacity The ultimate tension (pullout) capacity is calculated using Equation 16-5b: Tu = ∑Psi,u + Wp, where ∑Psi,u is the ultimate skin resistance and Wp is the weight of the pile being pulled. Allowable Capacity The allowable pile capacity based on soil resistance can be determined by either applying separate safety factors to tip and skin components (Equation 16-5c): Pa = Pp/SFp + ∑Psi/SFs, or more commonly, by applying a single safety factor: Pa = Pu/SF.

16. Safety Factors in Pile Design 2.0 Minimum Safety Factor The minimum recommended safety factor for pile design, typically used when soil conditions are well-understood and pile load tests have been conducted to verify capacity predictions. 3.0 Typical Safety Factor The most commonly applied safety factor in practice, providing a balance between economic design and sufficient protection against uncertainties in soil conditions and loading. 4.0 Conservative Safety Factor The upper range of safety factors, applied when soil conditions are highly variable or poorly characterized, or when the consequences of failure would be particularly severe. Safety factors in pile design generally exceed those used for spread foundations due to the complexities and uncertainties involved in pile-soil interactions. The selection of an appropriate safety factor depends on the quality of site investigation data, the variability of soil conditions, the type of structure, and whether pile load tests will be performed.

17. Load Transfer Mechanisms in Piles Initial Loading During initial loading, most of the applied load is carried by skin friction along the upper portion of the pile. The load transfer is primarily through shear stresses at the pile-soil interface, with minimal tip resistance mobilization. Intermediate Loading As the load increases, skin friction is progressively mobilized along greater lengths of the pile. The load transfer zone extends deeper, and the tip begins to develop more significant resistance as the pile settlement increases. Approaching Failure Near the ultimate capacity, skin friction may reach its maximum value along most of the pile length. At this stage, any additional load is primarily carried by the pile tip, resulting in accelerated settlement rates. Post-Failure Behavior After exceeding the ultimate capacity, the pile experiences continued settlement with potential reduction in skin friction due to soil structure breakdown. The tip resistance may continue to increase with settlement, but at a rate insufficient to maintain stability.

18. Soil-Pile Slip Behavior Pile Movement (mm) Skin Friction Mobilization (%) Tip Resistance Mobilization... The chart illustrates the typical relationship between pile movement and the mobilization of skin friction and tip resistance. Maximum skin resistance typically requires relatively small displacements (about 5-10mm for most soils), while full tip resistance mobilization may require much larger movements (10-25mm or more). This differential mobilization rate explains why skin friction typically dominates in working load conditions. The eventual reduction in skin friction at larger displacements (slip values) represents the breakdown of the soil structure at the pile-soil interface, a phenomenon particularly pronounced in sensitive clays and dense sands.

19. Ultimate Static Pile Point Capacity General Equation The ultimate static pile point capacity is calculated using Equation 16-6: Ppu = Ap(cN'cdcsc + qN'qdqsq + ½γ'BpNγsγ), where Ap is the effective area of the pile point, and the other terms represent soil properties and bearing capacity factors. Simplified Form For practical applications, a simplified form is often used (Equation 16-6a): Ppu = Ap[cN'cdc + q(N'q-1)dq], which neglects the Nγ term as its contribution is typically small for deep foundations. Cohesive Soils For pure cohesive soils (φ = 0), the equation simplifies further to Equation 16-6b: Ppu = Ap(9su), where su is the undrained shear strength of the soil beneath the pile tip.

20. Bearing Capacity Factors: Vesic Method Cohesive Soils For cohesive soils with internal friction angle φ > 0, Vesic provides Equation 16-7a: N'c = (Nq-1)cot φ to calculate the bearing capacity factor N'c. 1 Undrained Conditions When undrained conditions apply (φ = 0), Vesic suggests Equation 16-7b: N'c = [1+⅔ln Irr]+1+⅔(π/2) for calculating the bearing capacity factor. 2 Rigidity Index The reduced rigidity index Irr is calculated using Equation 16-7c: Irr = Ir/(1+evIr), where Ir is the rigidity index and ev is the volumetric strain. 3 Shear Modulus Relation The rigidity index Ir is determined using Equation 16-7d: Ir = G'/(c+q tan φ) = G'/s, where G' is the shear modulus and s is the shear strength of the soil. 4

21. Using Penetration Test Data for Pile Design Standard Penetration Test (SPT) Meyerhof suggested using SPT data to estimate pile capacity with Equation 16-8: Ppu = Ap(40N)Lb/B ≤ Ap[38N(Lb/B)], where N is the SPT blow count, Lb is the pile embedment depth, and B is the pile width. Cone Penetration Test (CPT) CPT data provides more continuous soil profile information and can be directly correlated to pile capacity. The cone resistance is particularly useful for estimating both tip resistance and skin friction along the pile shaft. Japanese Method Shioi and Fukui (1982) proposed Equation 16-9 for Japanese practice: Ppu = quitAp, where quit is the ultimate bearing pressure determined from either Dutch or Electric CPT cones or SPT tests.

22. Plug Formation in HP-Piles 1 Plug Mechanism When driven in cohesionless soils, H-piles may develop a soil plug between the flanges, effectively increasing their end-bearing area and capacity. The plug forms due to soil arching and friction between the soil and pile flanges. 2 Prediction Equation The depth of plug formation can be estimated using the equation: xp = (bf/2)(tan δ/tan φ - 1), where bf is the flange width, δ is the pile-soil friction angle, and φ is the soil internal friction angle. 3 Plug Assessment If xp > bf/2, a full plug forms; if smaller, only a partial plug develops. The extent of plugging significantly affects the pile's end-bearing capacity and should be carefully evaluated in design.

23. Skin Friction Development in Piles Skin friction develops along the pile shaft as relative movement occurs between the pile and surrounding soil. The magnitude of skin friction depends on soil type, installation method, pile material, and surface roughness. In cohesive soils, skin friction is primarily related to the undrained shear strength, while in cohesionless soils, it depends on the effective stress and soil friction angle. The distribution of skin friction is typically non-uniform along the pile length, with values generally increasing with depth in homogeneous soils due to increasing confining pressure. However, installation effects can significantly alter this distribution, particularly for displacement piles.

24. Pile Group Effects 1 Group Efficiency The capacity of a pile group may differ from the sum of individual pile capacities 2 Spacing Considerations Minimum spacing requirements prevent adverse interaction effects 3 Block Failure Groups may fail as a single unit rather than as individual piles 4 Settlement Behavior Group settlement typically exceeds that of isolated piles 5 Load Distribution Uneven load distribution occurs among piles in a group While Joseph E. Bowles focuses primarily on single pile behavior, the interaction effects in pile groups are crucial for practical design. When piles are placed in groups, their zones of influence overlap, potentially reducing the capacity per pile compared to isolated piles. This reduction is quantified through group efficiency factors, which depend on pile spacing, arrangement, soil type, and installation method. Block failure occurs when the entire group of piles and the soil contained between them move as a unit. This failure mechanism often governs design in cohesive soils when pile spacing is relatively close.

25. Lateral Load Considerations 1 Lateral Loading Sources Piles must often resist lateral loads from wind, earthquakes, earth pressure, water currents, or eccentric vertical loads. These lateral forces create bending moments in the pile that must be considered in design. 2 Pile-Soil Interaction The response of piles to lateral loads depends on complex pile-soil interaction. The soil provides lateral support that varies with depth and deflection magnitude, creating a nonlinear response system. 3 Analysis Methods Several methods exist for analyzing laterally loaded piles, including p-y curve methods, elastic continuum approaches, and finite element analysis. Each method has specific applications and limitations based on soil conditions and pile characteristics. 4 Design Considerations Lateral load design must consider both pile structural capacity (bending strength) and soil resistance. Excessive lateral deflection can cause serviceability issues even if structural failure doesn't occur.

26. Pile Buckling Considerations Slenderness Ratio The slenderness ratio (length/radius of gyration) is a critical parameter for assessing buckling potential. Piles with high slenderness ratios are more susceptible to buckling failure, particularly when they extend through weak soils or water. Lateral Support The surrounding soil provides lateral support that helps prevent buckling. The degree of support depends on soil stiffness, with stiffer soils providing greater resistance to lateral deformation and thus higher critical buckling loads. Risk Conditions Buckling risk is highest for long, slender piles in very soft soils, liquefiable soils during earthquakes, or piles extending through water or air. These conditions reduce lateral support and increase effective length for buckling calculations. Analysis Methods Buckling analysis typically involves modeling the pile as a beam-column with elastic support. The critical buckling load depends on pile stiffness, soil support characteristics, and boundary conditions at the pile ends.

27. Pile Load Testing Static Load Tests Static load tests involve applying incremental loads to a pile and measuring the resulting displacement. These tests provide the most direct measurement of pile capacity but are time-consuming and expensive. Results are typically presented as load-settlement curves that reveal both ultimate capacity and load-settlement behavior. Dynamic Load Tests Dynamic load tests measure the response of a pile to impact loading, typically during driving. These tests are faster and less expensive than static tests but require sophisticated signal processing and analysis. Methods like Case and CAPWAP analysis convert dynamic measurements to static capacity estimates. Integrity Tests Integrity tests assess the structural condition of installed piles, detecting defects such as necking, cracking, or voids. Common methods include low-strain impact tests (pulse echo), sonic logging, and thermal integrity profiling, each with specific applications and limitations.

28. Reliability in Pile Design The chart illustrates the typical coefficient of variation (measure of uncertainty) associated with different pile capacity prediction methods. Static analysis methods have the highest uncertainty, particularly in sands, while direct measurement through static load testing provides the most reliable results. This inherent uncertainty in pile capacity prediction explains why relatively high safety factors (2.0-4.0) are commonly used in traditional deterministic design approaches. Modern reliability-based design methods explicitly account for these uncertainties by targeting a specific probability of failure or reliability index rather than applying a single safety factor.

29. Practical Design Considerations 1 Site Investigation Thorough site investigation is essential for reliable pile design. The investigation should characterize soil stratigraphy, engineering properties, groundwater conditions, and potential obstructions. The extent and detail of investigation should be proportional to the project complexity and soil variability. 2 Pile Selection Pile type selection should consider soil conditions, loading requirements, construction constraints, environmental factors, and economic considerations. Different pile types perform optimally in different situations, and the selection process should evaluate multiple viable options. 3 Installation Effects Installation methods significantly affect pile performance. Displacement piles densify surrounding granular soils but may cause remolding and temporary strength reduction in clays. Non-displacement piles minimize soil disturbance but may have reduced skin friction in some conditions. 4 Quality Control Rigorous quality control during installation is critical for ensuring that piles achieve their design capacity. This includes monitoring driving resistance, maintaining installation tolerances, and verifying pile integrity after installation through appropriate testing methods.

30. Summary and Key Takeaways Pile Types and Materials We've explored various pile types including timber, concrete (precast, prestressed, and cast-in-place), and steel piles. Each material has specific design equations, advantages, limitations, and applications based on soil conditions and structural requirements. Capacity Calculation Methods Multiple approaches exist for calculating pile capacity, from theoretical bearing capacity equations to empirical methods based on in-situ tests. The selection of appropriate methods depends on soil type, available data, and project requirements. Load Transfer Mechanisms Understanding the complex load transfer mechanisms between piles and soil is fundamental to proper design. The relative contributions of tip resistance and skin friction vary with pile type, soil conditions, and loading magnitude. Uncertainty Management Pile design inherently involves significant uncertainties due to soil variability and installation effects. These uncertainties are managed through appropriate safety factors, reliability-based design approaches, and field verification through pile load testing.

31. Thank You for Your Attention Dr. Costas Sachpazis Civil & Geotechnical Engineer Specialized in foundation engineering with expertise in pile design and analysis. Available for consultation on your next geotechnical engineering project.