Overview:
The paper presents a critique of existing methods for determining the ultimate load on pile foundations, arguing that static methods offer advantages over traditional dynamic analyses. Author John H. Griffith posits that a comprehensive theory should encompass both static and dynamic aspects, incorporating all relevant variables and their effects on ultimate load. He emphasizes the need for experimental data to validate theoretical models, highlighting the potential for laboratory tests to study basal and lateral stress effects.
The paper then delves into the limitations of the Rankine theory, arguing that it neglects important factors like friction on the vertical projections of the pile and assumes a constant coefficient of friction. Despite these limitations, Griffith acknowledges the theory’s potential utility in analyzing multiple-pile systems, where the denser soil due to driving contributes to increased friction.
Griffith introduces an elastic theory for pile foundations, arguing that it provides a more determinate approach compared to traditional methods. He emphasizes the importance of strain in determining lateral earth pressure, citing Boussinesq’s research. He explores the potential for applying elastic principles to understand the behavior of soils under pressure, arguing that under significant pressure, soils can be treated as elastic bodies.
Key Findings:
- Existing dynamic methods for calculating pile capacity are insufficient, with several factors remaining vague and indeterminate.
- Static theories offer a more precise approach, but require comprehensive data and experimental validation.
- The Rankine theory, while flawed, provides a useful starting point for analyzing multiple-pile systems.
- An elastic theory, incorporating strain and soil properties, provides a more accurate representation of pile behavior.
- Dilatancy, the volume change in granular media under strain, plays a significant role in understanding the load-bearing capacity of piles.
Learning:
- Static vs. Dynamic Analyses: Readers will learn that static methods for analyzing pile foundations offer advantages over traditional dynamic methods, providing a more comprehensive understanding of the factors influencing ultimate load.
- Rankine Theory Limitations: The text clarifies the limitations of the Rankine theory, which neglects factors like friction on the pile’s vertical projections and assumes a constant coefficient of friction.
- Elastic Theory for Pile Foundations: Readers will gain insights into the application of elastic principles to pile foundations, understanding how strain, soil properties, and dilatancy contribute to pile behavior.
- Importance of Experimental Data: The paper emphasizes the crucial role of experimental data in validating theoretical models, highlighting the need for further research and testing.
- Dilatancy in Granular Media: The text explores the phenomenon of dilatancy, a unique property of granular materials, and its impact on pile performance.
Historical Context: This paper was published in 1910, during a period of significant growth in civil engineering, particularly in areas like bridge construction and foundation design. The paper reflects the ongoing debate and search for more accurate methods for calculating pile foundations, a crucial aspect of infrastructure development.
Facts:
- Static vs. Dynamic: Static analysis focuses on the forces at equilibrium, while dynamic analysis considers the forces involved in motion.
- Ultimate Load: The maximum load a pile foundation can sustain without failure is known as the ultimate load.
- Rankine Theory: This theory assumes incompressible particles and uses the angle of repose to calculate earth pressure.
- Dilatancy: The change in volume of a granular material due to a change in its shape.
- Elastic Theory: This theory assumes that materials deform proportionally to the applied force and return to their original shape after the force is removed.
- Coefficient of Friction: The ratio of the force required to initiate sliding between two surfaces to the force pressing them together.
- Angle of Internal Friction: The angle at which a material will start to slide down a slope due to its own weight.
- Buoyancy: The upward force exerted by a fluid on an object submerged in it.
- Hydrostatic Pressure: The pressure exerted by a fluid at rest due to its weight.
- Modulus of Elasticity: A measure of a material’s stiffness or resistance to deformation.
- Strain: The deformation of a material under stress.
- Stress: The force per unit area acting on a material.
- Conoid of Pressure: The shape of the pressure distribution around a pile, often likened to a cone.
- Arch Effect: The distribution of load in a material such as soil that forms arch-like structures to support the weight above.
Statistics:
- Annapolis Tests:
- Piles driven to depths of 60 ft. in mud and 6 ft. in sand.
- Piles driven to depths of 60 ft. in mud and 12 ft. in sand.
- Piles driven to depths of 61 ft. in mud with 4 ft. of sand.
- Ultimate test loads ranged from 34,000 to 168,700 lb.
- Calculated loads using Griffith’s formula ranged from 38,000 to 133,610 lb.
- Louisiana Pile:
- Pile driven to a depth of 29.5 ft.
- Sustained a load of 29.9 tons without settlement.
- Settled slowly under a load of 31.2 tons.
- Calculated load using Griffith’s formula was 67,120 lb.
- Islais Creek:
- Mud sample taken from a depth of 10-15 ft. weighed 105 lb. per cu. ft.
- Water content was 34%, leaving a solid density of 162 lb. per cu. ft.
- Chelsea Docks:
- Four groups of four piles each were driven to a depth of 50 ft.
- Loaded with concrete blocks, subsidence was measured over 51 days.
- Total subsidence ranged from 1 12to 3 in. under loads of 18 to 34.6 tons.
Terms:
- Pile Foundation: A foundation system that uses piles, long slender structural elements driven into the ground, to transfer loads from a structure to a deeper, more stable soil layer.
- Dynamic Analysis: A method of analyzing the motion and forces involved in a system, often used for impact loads.
- Static Analysis: A method of analyzing forces and their effects at equilibrium, when the system is at rest.
- Isotropic: A material that has the same properties in all directions.
- Homogeneous: A material that has the same composition throughout.
- Dilatancy: A property of granular materials that causes them to expand in volume when subjected to shear stress.
- Cohesion: The attractive forces between particles in a material, contributing to its resistance to deformation.
- Viscosity: A fluid’s resistance to flow.
- Modulus of Elasticity: A measure of a material’s stiffness or resistance to deformation.
Examples:
- Annapolis Tests: These tests, conducted at Annapolis, Maryland, provided data on the load-bearing capacity of piles driven in various soil conditions.
- Louisiana Pile: This well-documented case of a pile foundation in Louisiana demonstrated the load-bearing capacity of piles in specific soil conditions.
- Islais Creek: The analysis of mud samples from Islais Creek in San Francisco Bay illustrated the composition and properties of mud, highlighting its high water content and relative incompressibility.
- Chelsea Docks: Tests conducted at the Chelsea Docks in New York City provided insights into the time-dependent behavior of piles in mud, showing the gradual subsidence under load.
- Goodrich Experiment: The Goodrich experiment, using a box filled with sand and a model pile, helped study the formation of a pressure conoid around the pile.
- Leygue’s Experiments: Experiments conducted by Leygue using stratified soil samples to study the behavior of retaining walls, demonstrating the formation of curved surfaces in the soil.
- Packing of Balls: The example of packing balls in a box illustrates the concept of density and its influence on lateral pressure.
Conclusion: This paper, while focusing on a specific aspect of civil engineering, provides a valuable overview of the history, current theories, and limitations in determining the ultimate load on pile foundations. It highlights the need for a more comprehensive approach that integrates both static and dynamic analysis, considers soil properties, and incorporates experimental data. It also emphasizes the importance of recognizing dilatancy as a critical factor in granular soils, emphasizing the need for further research and understanding of this phenomenon.