Overview:
This 1910 paper, presented to the American Society of Civil Engineers, dives into the complex subject of earth pressures, resistance, and stability. Author J.C. Meem, an experienced engineer, uses experiments and practical observations to challenge conventional theories. The paper examines various aspects of pressure distribution and its impact on structures:
Meem starts by investigating the arching properties of sand in a controlled experiment. This leads to the concept of pressure transmission along lines of repose in dry earth. He then applies this knowledge to tunnel design, suggesting that arching action can relieve structures of load. He presents case studies from tunnel construction to support his claims.
The paper then moves to subaqueous materials, classifying them into firm, semi-aqueous, and aqueous. Meem conducts several experiments to demonstrate that water and earth pressures act independently in firm materials like sand. He shows how water pressure is transmitted through voids and not directly against sand grains. He also investigates the buoyancy of structures submerged in sand and finds that buoyancy is proportional to the voids present.
The paper concludes by exploring the bearing power of soil, particularly emphasizing the importance of lateral earth pressure. Meem critiques traditional theories and highlights the need for further research. He proposes using confined foundations instead of spreading them, advocating for a deeper understanding of soil behavior.
Key Findings:
- Sand exhibits arching properties: Sand particles in confined spaces can form self-supporting arches, relieving structures beneath of significant load.
- Water and earth pressures act independently: In firm, subaqueous materials, water pressure is transmitted through the voids and not directly against the sand grains, minimizing the impact on structures.
- Buoyancy in sand is proportional to voids: Structures submerged in sand experience buoyancy only proportional to the voids present, not the full volume of displaced water.
- Lateral earth pressure is a significant factor: Lateral pressure is a crucial consideration in foundation design and stability, particularly when dealing with confined foundations.
- Bearing value is more important than skin friction: For piles, the bearing value at the pile tip is the primary factor in determining load-carrying capacity.
Learning:
- Understanding Arching Action: Learn about the self-supporting arching behavior of sand particles and its implications for stability and load distribution in structures.
- Detail: Meem’s experiments highlight how sand, even under pressure, can form arches, thus relieving structures of load. He suggests this phenomenon is key to understanding the pressure behavior of underground structures like tunnels.
- Subaqueous Pressures: Learn how water and earth pressures interact in subaqueous environments and how the presence of voids affects buoyancy and pressure distribution.
- Detail: Meem’s experiments demonstrate that water pressure is transmitted through the voids in sand and not directly against the grains. This understanding is crucial for designing structures that can withstand the combined forces of water and soil pressure.
- The Importance of Lateral Pressure: Gain a deeper understanding of the significance of lateral pressure in earth structures and its role in both retaining walls and confined foundations.
- Detail: Meem argues that lateral pressure, often overlooked, is a critical factor in foundation design. He suggests that confined foundations, which utilize lateral pressure for stability, can be more efficient than spreading foundations.
- Evaluating Pile Design: Learn how to prioritize bearing value over skin friction in pile design, recognizing the limitations of skin friction in certain soil types.
- Detail: Meem’s observations show that piles in soft, displaceable materials like clay or peat may rely heavily on tip bearing, while skin friction may be less significant.
Historical Context:
The paper is written in 1910, a time of rapid growth in infrastructure projects, particularly tunneling and building in urban areas. This paper addresses the practical challenges engineers faced during those times, when the behavior of soil under different conditions was not fully understood.
Facts:
- Sand can act as a self-supporting arch: This is due to the friction between sand grains, which prevents them from collapsing under load.
- Sand arching is influenced by the angle of repose: The steeper the angle of repose, the stronger the arching effect.
- Earth and water pressures act independently in firm materials: This is because water pressure is transmitted through voids, not directly against the earth.
- Buoyancy in sand is less than in water: This is due to the presence of solid particles that reduce the volume of water displaced.
- Lateral earth pressure is significant in retaining walls: This pressure is greatest at the top of the wall and decreases towards the bottom.
- The bearing value of piles is critical: The strength of the material at the pile tip is more important than the length of the pile or the friction along the sides.
- Soil behavior varies widely: The same type of soil can exhibit different properties depending on factors like moisture content, compaction, and the presence of fines.
- Dry sand exhibits greater arching strength than moist sand: The increased friction between dry grains contributes to stronger arching.
- Sand does not flow readily under ordinary conditions: It only flows when subjected to high-velocity water or when disturbed.
- Sand and water pressures are independent: Water pressure acts independently on sand grains, not as a unified liquid mass.
- Semi-aqueous materials act as mixtures of solids and liquids: The aqueous portion, like quicksand, contributes to pressure like water, while the solids contribute like dry material.
- Pure quicksand acts like a liquid: It does not contribute to pressure beyond its specific gravity as a liquid.
- Soil properties can change under erosion: Previously firm soil can become “soup-like” under erosion, significantly impacting stability.
- Water in voids can be cut off from pressure sources: This is possible in undisturbed soil, reducing the pressure on structures.
- Lateral pressure acts along or parallel to lines of repose: The pressure is transmitted throughout the earth mass along these lines, influencing stability and load distribution.
- Confined foundations can be more efficient than spreading foundations: Confined foundations utilize lateral pressure to transfer loads to deeper, more stable soil layers.
- Skin friction on piles is generally low: It can be significant in large caissons but is typically less than 100 pounds per square foot for smaller piles.
- Piles driven in silt can offer insufficient resistance: This is because the pile tip may not reach a solid bearing layer, relying instead on limited skin friction.
- Distortion at the pile tip can indicate solid material: Piles that distort under heavy hammering or impact firm material at their tip.
- Excavation below piles can impact skin friction: Removing material below piles can reduce the skin friction, impacting load-bearing capacity.
Statistics:
- 4,000 ft. of arch timber bracing used in the Bay Ridge tunnel sewer: This demonstrates the reliance on bracing in tunnel construction at the time.
- Ultimate strength of dry North Carolina pine: Around 1,000 pounds per square inch in compression.
- Checks in water-soaked yellow pine appear at 388 to 581 pounds per square inch: This shows the impact of water on timber strength.
- Water pumped at 300 gallons per minute during “bottoming” operations: This highlights the challenges faced during tunneling in a water-saturated environment.
- Sand contains 40% voids: This is a typical value used in the paper’s experiments and analysis.
- Weight required to lift a bucket in sand: More than double the weight needed to lift the bucket in water, demonstrating the impact of sand on buoyancy.
- Friction on a piston in sand: Negligible, at less than 25 pounds per square foot.
- Pressure required to lift the piston in sand: Approximately 22 pounds, compared to 8.5 pounds in clear water, indicating the effect of sand on pressure transmission.
- Pressure on piston in clay and peat: As high as 40 pounds, demonstrating the greater resistance of these materials.
- Sand containing 40% voids acts like a solid: It exhibits resistance to pressure and can form arches, even under water.
- Soft clay arch can withstand significant loads: This demonstrates the arching properties of even softer materials like clay.
- Pressure on tunnel roof: Can be calculated as the weight of the earth prism plus water in the voids plus water pressure above the prism.
- Water pressure on a sheeted trench face: Can be calculated as the sum of water pressure through the voids and the thrust of the sand, after correcting for buoyancy.
- Skin friction on caissons: Can range from 200 to 600 pounds per square foot.
- Load on a 14-inch pipe sunk in sand: Approximates 28 tons per square foot without significant settlement.
- Load on a 14-inch hollow cylindrical pile: Approximates 60 tons after settlement of 9 to 13 inches.
- Load on a 10-inch pipe in gravel and sand: Approximates 50 tons at a depth of 20 feet.
- Load on a cylindrical pile with a bearing ring: Approximates 60 tons without settlement at a depth of 20 to 30 feet.
- Skin friction on a 14-inch hollow cylindrical pile: Less than 78 pounds per square foot, including bearing value.
- Skin friction on a 12-inch California stove-pipe pile: Less than 75 pounds per square foot after sinking to 850 feet.
Terms:
- Angle of Repose: The steepest angle at which a material can stand without collapsing.
- Arching Action: The ability of granular materials, like sand, to form self-supporting arches, distributing load and relieving pressure on structures below.
- Buoyancy: The upward force exerted on an object submerged in a fluid.
- Cohesion: The force that holds particles together in a material.
- Friction: The force that resists motion between two surfaces in contact.
- Hydrostatic Pressure: The pressure exerted by a fluid at rest.
- Lateral Pressure: The pressure exerted by soil or other materials against the sides of a structure.
- Lines of Repose: The lines along which pressure is transmitted in a soil mass, often parallel to the angle of repose.
- Quicksand: A type of soil, typically a fine sand, that becomes unstable and fluid under water pressure.
- Skin Friction: The frictional resistance between a pile or caisson and the surrounding soil.
Examples:
- Bay Ridge Tunnel Sewer: The paper describes the use of arch timber bracing for this sewer, demonstrating how arching action is applied in real-world construction.
- Joralemon Street Battery Tubes: The paper describes how the roof of these tubes was successfully raised using hydraulic jacks and “bleeding” sand through holes in the roof, illustrating the controlled manipulation of soil pressure.
- California Stove-Pipe Wells: The paper mentions the driving of these wells to a depth of 850 feet, demonstrating the limitations of skin friction in certain soil conditions.
- Chenoweth Pile: The paper discusses an experimental pile that shattered upon encountering hard material, emphasizing the importance of bearing value.
- Toronto Water Conduit: The paper highlights the collapse of this conduit due to the buildup of pressure when water was drawn out, emphasizing the importance of addressing buoyancy in structures.
- Coffer-dam in Maryland: The paper describes the failure of a coffer-dam in silty material, demonstrating the high pressures that can be exerted by certain types of soil.
- Canadian Pacific Railroad Tunnel: The paper recounts the collapse of this tunnel, possibly due to swelling of the clayey material, highlighting the need to address swelling potential.
- Municipal Building Caissons: The paper cites observations made during the sinking of these caissons, showing how air pressure was used to overcome hydrostatic pressure.
- Reinforced Concrete Retaining Wall at St. George’s Ferry: The paper mentions experiments on this wall, where the author claims to have demonstrated that lateral pressure increases with depth.
- Hudson and North River Tunnels: The paper references these tunnels to illustrate the potential of soil to solidify and reduce pressure over time after construction.
Conclusion:
This 1910 paper offers a valuable glimpse into the evolving understanding of soil behavior and its influence on structural design. J.C. Meem challenges traditional theories by emphasizing the importance of arching action in dry and subaqueous materials. He argues that soil pressure is not solely determined by weight and depth but is also affected by factors like lateral pressure, angle of repose, and the presence of voids. Meem’s experiments and observations emphasize the need for a more nuanced approach to soil mechanics and demonstrate the potential for confined foundations and a greater focus on bearing value in pile design. While the paper’s conclusions and methods have been subject to debate and critique, it remains a significant contribution to the field of geotechnical engineering, highlighting the complexities of soil behavior and the ongoing need for research in this critical area.