When it’s time to build a large structure, structural engineers calculate the load of the building and determine the type of foundation it requires. For the largest and heaviest buildings, that means designing a deep foundation system that places piers or piles deep into the ground within the footprint of the building. There are three main types of piles: displacement or driven piles, bored pilings, and auger cast piles, also known as continuous flight auger (CFA) piles.
The Importance of Quality Concrete in the Foundation
Many people recognize the importance of quality concrete when it comes to what everyone sees when the project is complete – the building itself. As important, perhaps even more so, is the quality of concrete used to provide a stable foundation on which the structure sits.
Large buildings, particularly skyscrapers, are massively heavy. For example, a single-family home puts a pressure on the subsurface of around 100 lbs. per square foot of building footprint. San Francisco’s Millennium Tower, its tallest residential building at 58 stories and 645 feet, has a load 100 times that, or 11,000 lbs. per square foot sitting on a driven-pile foundation.
To support such a massive load, engineers specify the number and placement of piles to spread the weight evenly across the footprint. Piles are placed or driven deep into the soil until they reach bedrock or a suitably stable layer of geology. The load is spread out vertically so more of the underlying soil supports it, and it is ultimately transferred to stronger soils or bedrock to ensure there are no issues caused by compressive or expansive soils.
Designing a pile foundation system comes with some uncertainty, however, and designing and constructing one that meets or exceeds specifications is critical. Millennium Tower sits on a 10-foot-thick concrete slab supported by 950 concrete friction piles driven 80 feet into the earth – and it’s failing.
Before construction finished, the building had sunk several times the amount engineers predicted it would sink over its lifetime – 4-6 inches expected compared to 16 inches of actual settlement in three years. Settlement was uneven, so the structure tilted several inches to the northwest corner by the time it was completed in 2016.
It isn’t the concrete that’s the problem. It’s the deep subsurface layer of old bay mud allowing the settling. But for a $350 million building with 419 pre-sold dwelling units at an average pre-sale value of $1.8 million (the penthouse sold for $13 million), it’s clear what’s at risk if your foundation doesn’t do its job.
Pick Your Pile
Engineers have a few options when it comes to placing piles, including:
- Displacement or driven piles, which are large, rigid elements made of steel, concrete, or wood that are literally driven into the ground inch by inch using a pile driver. Installation can take quite some time. The process is noisy and vibrates a large area around the construction site, risking damage to surrounding structures and annoying those in the area.
- Bored pilings, which involve using an auger to remove soil from a column, placing a casing to hold the hole open, dropping a rebar-reinforcing cage into the hole, and filling the hole with concrete.
- Auger cast piles or continuous flight auger (CFA) piles, which use a similar auger to displace a column of soil, but the central part of the auger is hollow. The equipment operator drives the auger to the desired depth. As the auger is backed out, concrete or grout is pumped under pressure down the hollow auger stem, filling the hole as the auger pulls the soil that filled the space to the surface.
Continuous Pours are Ideal
Given what’s at stake, both for construction timeline and building stability, executing either a bored or auger cast piling placement continuous-pour project is ideal for one or more volumetric mixer trucks. Volumetric concrete mixed on-site guarantees a steady flow of fresh, precisely mixed grout exactly when needed for each pile placed.
A heavy building with a large footprint, of course, requires many columns to properly support the building when it’s complete. To keep construction on schedule, placing piles to prepare for constructing the building is typically done around the clock to finish this critical part of a building project and move onto the building itself.
For cast-in-place bored and auger cast pilings, completing this vitally important part of a project requires a steady supply of fresh, properly mixed concrete or grout to fill the holes the auger creates. Particularly in soft ground, depending on how deep the piles go, driving the auger to depth is faster than filling the holes with concrete.
An adequate supply of concrete must be ready on demand to prevent pumping the holes full of grout becoming the limiting factor or choke point in the process. Using traditional barrel trucks, which are known to deliver their payload late, can disrupt a crew’s drill-and-fill routine and has the potential to set back the entire project.
If a delay happens in this case, the problem can compound quickly by requiring subsequent trucks to wait while the concrete in their barrels continues to age. That means adding water to try to salvage the load and keep it flowable, which can compromise the concrete’s integrity as it sets in place. When dozens or hundreds of piles support a structure that weighs tens, hundreds, even hundreds of thousands of tons, compromised concrete can become a crisis or even a disaster if the concrete used does not meet specifications. At very least, it can lead to costly delays to address the problem.
Benefits of Auger Cast Piles
Auger cast piles or CFA piles are a deep-foundation approach that saves money and is environmentally friendly compared to other options. With no need for steel casings, temporarily or permanently, and a simpler approach and work area, the speed and reduced cost of the process can make a significant impact on construction schedule and costs. If needed, crews can drill and pour larger diameter piles, which can reduce the total number of piles needed for some projects, making the process faster and reducing costs.
This method of foundation construction works well in most soil conditions and eliminates problematic levels of noise and vibration that come with driving displacement piles. If the geological strata between the surface and the lower end of the pile is rocky, driving an auger to depth also avoids challenges that arise when trying to drive piles through boulders and rocky upper layers of earth. Using an auger also gives the crew a chance to identify other subsurface layer conditions that may warrant adjustments. It may be necessary to drive the auger deeper than planned to reach a stable geological stratum, or to use a larger-diameter auger to make sure the building is properly supported when complete.
The Advantages of Using Volumetric Concrete
The timeline to bore a specific number of piles using a continuous-pour volumetric approach becomes more predictable, and efficiency and production rate rise with one or more volumetric mixer trucks on site to mix and place fresh concrete exactly when needed.
There is no cost for crew and equipment waiting around if barrel trucks are late with deliveries. Quality control of the concrete is improved when there is no opportunity for the product to age while on the road or waiting in line to be pumped into place. If the weather changes or the temperature fluctuates from day to night during an extended auger-and-pour period, the concrete mix can be adjusted as needed in real time.
Individual piles – and the field of piles as a whole – must be monolithic, with no room for compromise to ensure the completed building is properly supported for decades to come. Because of this, it’s critical to keep a close watch and have complete control of when and how the product is mixed and placed.
Depending on where material stockpiles are placed, a single volumetric mixer can be parked and loaded where it sits to make concrete that is pumped directly into the auger. When space on the site is limited, materials can be queued up in a different area. A pair of volumetric mixer trucks can then shuttle back and forth, one refilling while the other supplies the concrete pump.
Using volumetric concrete made and used as needed also helps lower costs and the environmental impact of placing piles by eliminating excess concrete waste that otherwise must then be properly disposed of.
If you want a better way to provide concrete to continuous pour, deep-foundation-setting projects, volumetric concrete may be your solution. Contact us to schedule a meeting to discuss how our Holcombe volumetric concrete mixer trucks ensure a stable footing for large buildings and your business.