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Originally Posted by GrowinUp
My new friend told me this -- Both project WILL break ground by the end of the year. 621 will be pile-driving, but Aura is using a new method that employs a large auger of some sort that I guess kinda screws the pilons deep into the ground. This should be interesting, I thought -- I bet it's much quieter than pile-driving. Perhaps Construction Guy could shed a little light on this subject, hmmmm?
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There are four types of foundation systems that come to mind which use an auger that I can think of off the top of my head. They are (in order of my belief of the likely candidate)
1. Auger Cast piles
2. Drilled caissons
3. Soil Cement columns
4. Geo-Piers
1. Auger Cast piles are constructed by drilling a hole in the ground using a “hollow flight auger”. A hollow flight auger is a big drill bit whose center is hollow (think of a drinking straw with spiral blades on the outside. As the auger is advanced (or drilled) into the ground, the soil it is drilling is transported up the outside of the auger to the top of the hole, where the soil is removed, or piled up for later removal. Once the desired depth is achieved, concrete is then pumped down through the center of the auger to the bottom of the hole. The auger is slowly withdrawn as the hole is filled up with concrete. Once the hole is completely filled and the auger completely removed, a steel reinforcing cage is then pushed and vibrated down through the fresh concrete. These piles are commonly drilled to 70 feet although some have gone as deep as 100 feet.
2. Drilled caissons are constructed by drilling a hole in the ground using an auger (not hollow). All the dirt is removed, then a steel reinforcing cage is installed into the hole, and then the concrete is placed.
3. Soil Cement columns are a relatively new development, and I believe the design is patented by a Japanese company. In this process, a hollow flight auger with a mixing paddle is bored into the ground. Very little of the soil is removed. Once the desired depth is achieved, grout is pumped down the center of the auger, and the auger is spun as it is slowly removed. The design intent is to mix cementitious material with the existing soil to give it strength. This process is continued all the way back up to ground surface.
4. Geo-Piers are also a relatively new development which have been patented as well. In this process, a hole is drilled (same as a drilled caisson) but rather than filling the hole with concrete, rock or stone is dumped into the hole in layers (2 or 3 feet). The rock is then mechanically compacted from above. As the rock is compacted, it exerts pressure on the sidewalls of the hole, thereby compressing the surrounding soil. The process is repeated all the way to the top of the hole. No steel reinforcing cage is used with GeoPiers, but anchors can be installed at the top of the pier to resist uplift forces.
Two advantages of all of these methods over traditional pile driving:
a. Less noise – The machinery itself is noisy (diesel engines) but not the impact noise of a pile hammer.
b. Far less vibration – A pile hammer pounding steel or concrete piling into the ground imparts shock waves through the surrounding ground, which can cause damage to nearby structures, as well as discomfort to occupants of nearby buildings.
With each of these methods, the bottom of the hole is typically “belled” to improve the bearing capacity of the pile/caisson.
I believe auger cast piles are the most likely candidate that your friend was talking about. They can be used in areas with a high groundwater table (such as downtown) and you don’t have to worry about the sides of the hole caving in while you are drilling as you do in drilled caissons or Geo Piers. The structural capacity of soil cement columns and geopiers are not as high as auger cast piles and drilled caissons.
This concludes your first lesson on foundations.
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