Inside Third & Bond: Week 29

Today, the Hudson Companies bloggers describe their efforts to get their foundation in before the 421-a deadline in June.

In building foundations, much like in people, there are two major categories: shallow and deep. Shallow foundations include slab on grade, spread footing, wall footing, and mat/raft foundations. Just imagine these as foundations that are poured concrete forming either a thick floor or thick walls and you’ll have the general idea. Deep foundations include piles and caissons. Here you can imagine a column of wood, concrete or steel that is driven into the ground and acts like stilts that go deep into the earth—that’s close enough for government blog work. Speaking of which (government), we have to get our piles going in order to meet the 421-a deadline of June 30, 2008 and we have a ways yet to go.

Deciding what kind of foundation to use requires you know:
• Soil strength and stiffness
• Loads from the building (how heavy the stuff is pushing down on the ground)
• Neighboring conditions
• Foundation type best for all the above

Soil strength and stiffness: Some find borings boring but those people are fools. Understanding your soil is numero uno in knowing what kind of structure you can put on a site. Even if zoning permits you to build something, the soil has final say on whether it can be done (and for how much money and headache). Borings are taken by machines like the one pictured here.
They bore 100’or so into the earth and pull out long cylinders from which samples are taken at 5′ intervals. The sample depth is carefully recorded for each. The samples are then taken to a lab to be characterized: sandy, fill, gravel, peat, glacial rock, medium-bodied with hint of cherry and leather, etc.

If the soil is wimpy, then deeper foundations are needed to keep the building tip-top and not top-tip. In the case of Third & Bond, we are working with some fairly wimpy soil. This is not too surprising since a couple hundred years ago the area was all marshland.

Loads from the building: Live and dead loads are the two major categories of load types the engineer will calculate and compare to the requirements of the Building Code and the Laws of Physics. Live loads are variable loads such as furniture, people, etc. Dead loads are the self-weight of the structure and everything permanently attached like sprinklers and hardwood floors. Some common building loads: ceramic tile—16 pounds per square foot, acoustic ceiling panels—3 pounds per square foot.

Neighboring conditions: When building at an in-fill site, understanding the conditions of your adjacent neighbors is also important. At Third & Bond, our neighbors are around 100 years old (the buildings, maybe some of the people). If we use a foundation system that requires driving piles into the ground, the vibrations from the driving could cause cracks, etc.

Foundation type: All foundations settle and that settlement is typically measured in millimeters. The goal is to get uniform settlement throughout the foundation and building. Uneven settlement is the most common foundation issue much more common than gross failure (i.e., the soil can’t support the building loads at all). The leaning tower of Pisa is an example of uneven settlement. Using the information from the soil type, building loads (as the building is designed), and neighboring conditions, the geotechnical engineer and/or structural engineer can recommend a foundation type. The engineer backing the design of the foundation puts her or his license on the line for every project.

At Third & Bond, we determined early on that piles are the way to go. Then we spent the next six months investigating what type of piles to use…and the clock keeps ticking. Our major concerns are cost, availability/ability of subcontractors, time, and neighboring structures. We’ve talked about so many different kinds of piles that just recapping those discussions constitutes a primer on pile types…which we’ll get to in Piles Part Deux.

For this week, let’s recap the high points: We don’t want our building to settle unevenly. We don’t want the neighbors to be unsettled by pile work. We want to make the 421-a deadline which requires us to be starting our foundation (e.g., driven first pile) by June 30. We want to go with a cost-effective option—we have to save some of our budget for marketing on Brownstoner (oh, wait). It’s a complicated and expensive decision. We have a team assembled with the equivalence of 90 years experience and yet months later we still haven’t concluded exactly which type is best for us. One step away from flipping a coin, we have now decided to drive test piles and see which one works best. We know you are on the edge of your seats…

Inside Third & Bond: Week 28 [Brownstoner]
Inside Third & Bond: Week 27 [Brownstoner]
Inside Third & Bond: Week 26 [Brownstoner]
Inside Third & Bond: Week 25 [Brownstoner]
Pisa photo by ccgd

From our lawyers: This is not an offering. No offering can be made until an offering plan is filed with the Department of Law of the State of New York.”