Deep Foundations - Caissons

A caisson, or drilled pier (Figure 2.37), is similar to a column footing in that it spreads the load from a column over a large enough area of soil that the allowable stress in the soil is not exceeded. It differs from a column footing in that it extends through strata of unsatisfactory soil beneath the substructure of a building until it reaches a more suitable stratum. A caisson is constructed by drilling or hand-digging a hole, belling (flaring) the hole out at the bottom as neces- sary to achieve the required bearing area, and fi lling the hole with concrete. Large auger drills (Figures 2.38 and 2.39) are used for drilling caissons; hand excavation is used only if the soil is too full of boulders for the drill. A temporary cylindrical steel casing is usually lowered around the drill as it progresses to support the soil around the hole. When a fi  rm bearing stratum is reached, the bell, if required, is created at the bottom of the shaft either by hand excavation  or by a special  belling bucket on the drill (Figure 2.40). The bearing sur- face of the soil at the bottom of the  hole is then inspected to be sure it  is of the anticipated quality, and the  hole is fi lled with concrete, withdrawing the casing as the concrete rises.

Reinforcing is seldom used in the  concrete except near the top of the  caisson, where it joins the columns of  the superstructure.
Figure 2.37 Deep foundations. Caissons are concrete  cylinders poured into drilled holes. They
reach through weaker soil (light shading)  to bear on competent soil beneath. The
end bearing caisson at the left is belled as  shown when additional bearing capacity is
required. The socketed caisson is drilled  into a hard stratum and transfers its load
primarily by friction between the soil or  rock and the sides of the caisson. Piles
are driven into the earth. End bearing  piles act in the same way as caissons.
The friction pile derives its load-carrying  capacity from friction between the soil
and the sides of the pile.
A 6-foot- (1828-mm)-diameter auger on a telescoping 70-foot (21-m) bar brings up a load of soil from a caisson hole. The auger will be rotated rapidly to spin off the soil before being reinserted in the  hole.
Figure 2.38 A 6-foot- (1828-mm)-diameter auger on a
telescoping 70-foot (21-m) bar brings up
a load of soil from a caisson hole. The
auger will be rotated rapidly to spin off
the soil before being reinserted in the  hole.
For cutting through hard material, the caisson drill is equipped with a carbide-toothed coring bucket.
Figure 2.39
For cutting through hard material,
the caisson drill is equipped with a
carbide-toothed coring bucket.

The bell is formed at the bottom of the  caisson shaft by a belling bucket with retractable cutters. The example shown  here is for an 8-foot- (2.44-m)-diameter shaft and makes a bell 21 feet (6.40 m) in  diameter.
Figure 2.40 The bell is formed at the bottom of the  caisson shaft by a belling bucket with
retractable cutters. The example shown  here is for an 8-foot- (2.44-m)-diameter
shaft and makes a bell 21 feet (6.40 m) in  diameter.
Caissons are large, heavy-duty foundation components. Their shaft diameters range from 18 inches  (460 mm) up to 8 feet (2.4 m) or  more.  Belled caissons are practical  only where the bell can be excavated  in a cohesive soil (such as clay) that  can retain its shape until concrete is  poured. Where groundwater is present, the temporary steel casing can  prevent flooding of the caisson hole  during its construction. But where  the bearing stratum is permeable, water may be able to fi ll the hole from  below and caisson construction may  not be practical.

socketed caisson (Figure 2.37) is  drilled into rock at the bottom rather than belled. Its bearing capacity
comes not only from its end bearing, but from the frictional forces between  the sides of the caisson and the rock  as well. Figure 2.41 shows the installation of a rock caisson or drilled-in caisson, a special type of socketed  caisson with a steel H-section core.

Installing a rock caisson. (a) The shaft of the caisson has been drilled through softer soil to the rock beneath and cased with a steel pipe. A churn drill is being lowered into the casing to begin advancing the hole into the rock. (b) When the hole has penetrated the required distance into the rock stratum, a heavy steel H-section is lowered into the hole and suspended on steel channels across the mouth of the casing. The space between the casing and the H-section is then fi lled with concrete, producing a caisson with a very high load-carrying capacity because of the composite structural action of the steel and the concrete.
Figure 2.41
Installing a rock caisson. (a) The shaft of the caisson has been drilled through softer soil to the rock beneath and cased with a steel
pipe. A churn drill is being lowered into the casing to begin advancing the hole into the rock. (b) When the hole has penetrated the
required distance into the rock stratum, a heavy steel H-section is lowered into the hole and suspended on steel channels across the
mouth of the casing. The space between the casing and the H-section is then fi lled with concrete, producing a caisson with a very high
load-carrying capacity because of the composite structural action of the steel and the concrete.

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