Caissons along a property line

[mijowe]

I have a foundation recommendation for caissons on a site that has buildings built up to the property line. Two questions:

1. How close can a caisson be drilled next to and adjacent building?

2. With my buildings perimeter columns going to need to be eccentric to the caisson, Would you build a haunch on the caisson, or build a strap beam to an interior caisson and cantilever it to the property line to pick up the column?

 

[JAE]

I would usually set the caisson back from the property line - probably about 3 diameters of the caisson.

Then I'd use a concrete cantilever beam (perpendicular to the property line) to extend across two caissons and over to the property line to pick up the column.

 

[mijowe]

I am more comfortable wit the cantilever beam, as you have described. Curious as to why 3 diameters?, i have a 3' caisson and that cantilever would get large. On a side note, the corner columns are going to be tricky as well.

 

[ToadJones]

I'd use a haunched caisson.

 

[jike]

Factors for caisson setback:

1) room needed from the edge of the rig to center of shaft for drilling

2) size of bell (usually 3 x shaft dia. max.)

3) drilling tolerance

 

[dik]

For belled caissons, I usually stipulate 6" with a 3" max tolerance...

Dik

 

[paddingtongreen]

It depends on the individual circumstances for me, but if i haunch the caisson, I tie it back back to the next line because I don't usually know enough about the backfill for the original structure.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

I would place the bored piles (I only call something a caisson if it is advanced within a perimeter casing), as close to the existing structure as allowed by drilling clearance and tolerance. Then I would use a cantilevered beam. I have never haunched a bored pile, so can't really comment about that method.

 

[mijowe]

The slab on grade is also required to be structural, so I am going to need grade beams anyhow. My problem is that i have a fairly irregular column grid so tieing back to the next line is going to get a little messy.

 

[mijowe]

Also, thanks for the input.

 

[Zambo]

Are the caissons really bored piles? How long (deep) are they?

I assume that yes the slab on grade is also supported by the piles. But that doesn't mean you need beams, maybe pilecaps and a slab.

Are these piles "bell ended"

I usually assume a spacing of 3 x diameter, so one question is whether the adjacent building is piled and whether the pile locations are known.

 

[TXStructural]

Depending on soil conditions, haunched tops may not be available (in expansive clay soil, or areas with frost heave, you cannot use these.) Also, unless you run an analysis for lateral loaded piles/piers and design for it, you need to remove the eccentricity with tie beams or cantilevers.

Your geotech will be able to tell you about setback and spacing, since there is no general rule applicable to all soils. In some cases, such as stable, cohesive soils drilled to shallow depths, it may acceptable to drill near the property line or existing structure.

You may want to consider auger-cast or micropiles if they fulfill your needs better (i.e., drilling closer to the existing structure.) If you need closely spaced piers, you may also need to drill and place them in stages, since part of the spacing requirement is based on disturbing an empty hole by drilling another too close to it. Some of the spacing requirements are also to avoid group action (reducing capacity like block shear in a steel connection.)

 

[mijowe]

The building is being built at grade and the top 10 feet of soil is fill. The caissons do require a casing and are relatively shallow, 20-25' until the get to a rock layer. The caissons are straight, not belled. The adjacent buildings vary, but most have basements that get past the poor soils and bear on a soil layer suitable for traditional shallow foundations. Eventually we will need to run a lateral pile analysis as some of the caissons are supporting shear walls anyway. I am assuming that the tops of the caisson will get little or no lateral support from the fill.