Why Bridges Don't Sink

Summary

This video from Practical Engineering explores the complexities of bridge foundations, particularly deep foundations like piles. It explains how piles, as structural members, transfer forces from the bridge to the ground, overcoming challenges in soil conditions. The video delves into the history, types, and installation methods of piles, including driven and drilled shafts, and touches on their limitations and the importance of geotechnical engineering in ensuring their stability and the safety of structures they support.

Takeaways

• 🌉 The fundamental purpose of a bridge is to provide a clear span for traffic below, whether it's a river, railway, or road.
• 🏗️ Bridge substructures are among the strongest engineered systems due to the need to support concentrated loads from the span.
• 🌊 Foundations of bridges are often built in challenging environments like rivers and oceans with soft, unstable soils.
• 🔨 Overloading soil can lead to bearing failure, where the foundation sinks into the ground, similar to stepping in mud.
• 📍 The invention of the foundation pile was a response to the need for a structural member that can support heavy loads in poor soil conditions.
• 📚 Pile design and construction is complex, with manuals from the Federal Highway Administration detailing the process in extensive detail.
• 🪵 Timber was the first material used for piles, but it has limitations in terms of strength, durability, and length.
• 🧱 Concrete piles offer advantages over timber in strength and durability but require significant force to be driven into the ground.
• 💎 Steel piles are more slender and easier to drive than concrete, making them suitable for deep foundation work.
• 🔧 Piles are installed using a hammer that converts potential energy into kinetic energy to drive the pile into the ground.
• 🔄 Piles resist vertical loads through end-bearing and skin friction, which increase as the pile is driven deeper into the ground.
• 🏗️ Driven piles are tested during installation, verifying their ability to withstand the forces they will later support in the structure.
• 💨 Structures like buildings and bridges experience not only vertical but also horizontal and uplift forces, which piles must resist.
• 🌪 Scour, the erosion of soil around a bridge, is a concern that engineers must anticipate and design foundations to accommodate.
• 🛠 Inclined or battered piles are used to strengthen foundations against lateral loads, taking advantage of shaft resistance.
• 🚫 Driven piles have limitations, including noise, disruption, and difficulty penetrating hard geological layers.
• 🕳️ Drilled shafts offer a solution to many of the issues with driven piles, allowing for more precise installation and larger sizes.
• 🌊 Underwater installation of drilled shafts requires special techniques to prevent water from mixing with the concrete.
• 📈 The use of a tremie pipe ensures that concrete can be placed underwater without dilution, maintaining its strength.
• 🔩 There are many variations of deep foundation systems, each designed to solve specific engineering challenges.
• 🛑 Pile foundations can fail if not properly designed and installed, as evidenced by case studies of the Millennium Tower and a bridge in Tampa, Florida.

Q & A

What is the fundamental purpose of a bridge?

The fundamental purpose of a bridge is not just to span over something, but to provide clear space underneath for a river, railway, or road to pass through.

Why are bridge substructures considered among the strongest engineered systems?

Bridge substructures are considered among the strongest engineered systems because they must handle and concentrate all the forces from the span into the piers or abutments on either side, which is a unique structural challenge.

What is a bearing failure in the context of engineering?

In engineering, a bearing failure occurs when soil is overloaded with a weight it can't handle, causing the foundation to sink into the ground, similar to stepping in mud.

What is a foundation pile and how does it work?

A foundation pile is a structural member, typically a long pole, that is driven or drilled into the ground. It works by either end-bearing into a layer of strong soil or hard rock, or by skin friction along its walls as it is driven into the soil, creating friction that holds it in place.

Why are timber piles still used around the world despite their limitations?

Timber piles are still used because they are cheap, resilient to driving forces, and easy to install. However, they have limitations such as rotting, limited length due to tree size, and being less strong compared to alternatives like concrete or steel piles.

What are the advantages of concrete piles over timber piles?

Concrete piles solve problems like rotting and limited length of timber piles. They come in various sizes and shapes, are strong, and are widely used for deep foundations, although they require being larger to withstand the force needed to drive them into the ground.

Why are steel piles preferred over concrete piles in certain situations?

Steel piles are preferred when slenderness is required because they can be more slender than concrete piles, making it easier to drive them into the ground without the need for as much force.

How do pile drivers create the necessary force to drive piles into the ground?

Pile drivers use a large hammer or vibratory head to create the necessary force. They convert the potential energy of a comparatively small weight dropped from a height into kinetic energy, which is then transferred into the pile as it is driven into the ground.

What is the significance of a pile reaching 'refusal' during installation?

Reaching 'refusal' means that the pile has been driven to a depth where it barely moves with each successive hammer blow. This indicates that the pile has developed enough resistance to vertical loads to be considered stable and secure for supporting the structure.

How do piles resist loads from different directions, such as horizontal or uplift forces?

Piles can resist loads from different directions through their design and installation. For horizontal loads, piles can be inclined or battered to better utilize shaft resistance. For uplift forces, the design and installation of piles ensure they can withstand buoyancy or strong winds counteracting gravity.

What are some limitations of driven piles during installation?

Driven piles have limitations such as being noisy and disruptive, causing vibrations that can affect surrounding infrastructure. They also cannot easily penetrate hard geological layers like cobbles or boulders, and they can cause ground heave due to the forceful installation without soil removal.

How does the process of installing a drilled shaft differ from driven piles?

Drilled shafts involve drilling a long hole into the ground, placing reinforcing steel, and then filling the hole with concrete. This process is more controlled and can be adapted for underwater installations using a casing to support the hole and a tremie to pour concrete without mixing with water.

Why are drilled shafts advantageous for underwater installations?

Drilled shafts are advantageous for underwater installations because they can be installed using a casing that keeps the hole open for rebar and concrete placement. The use of a tremie pipe ensures that concrete is placed without mixing with water, maintaining its strength and performance.

What is a mono-pile foundation and why is it used?

A mono-pile foundation is a single drilled shaft deep in the ground, used to support structures like wind turbines or highway signs. It eliminates the need for a pile cap and multiple piles, simplifying the foundation system.

How can the bottom of a drilled shaft be modified to increase its effectiveness?

The bottom of a drilled shaft can be reamed out to create an enlarged base, which increases the surface area at the toe of the pile. This modification helps reduce the pile's tendency to sink and improves its resistance to uplift forces.

What are some variations of deep foundation systems mentioned in the script?

Some variations of deep foundation systems include continuous flight auger piles, stone columns or Geopier, helical or screw piles, micropiles, and sheet piles. Each variation is designed to address specific challenges in foundation construction.

What are some real-world examples of pile foundation failures mentioned in the script?

The script mentions the Millennium Tower in San Francisco, which sank into the ground causing the building to tilt, and a bridge pier on the Lee Roy Selmon Expressway in Tampa, Florida, which sank 11 feet during construction due to complicated geology.