lar stratum during underreaming of a bell, the base
pared with traditional strip footings, particularly in
may be placed beneath swelling soil near the top of a
open construction areas and with shaft lengths less
granular stratum.
than 10 to 13 feet, or if the active zone is deep, such as
(3) Underreams. Underreams are often used to in-
within areas influenced by tree roots.
crease anchorage to resist uplift forces (fig. 6-10). The
c. General considerations.
belled diameter is usually 2 to 2.5 times the shaft
(1) Causes of distress. The design and construc-
tion of drilled shaft foundations must be closely con-
45- or 60-degree bells may be used, but the 45-degree
trolled to avoid distress and damage. Most problems
bell is often preferred because concrete and construc-
have been caused by defects in construction and by in-
tion time requirements are less. Although the 45-de-
adequate design considerations for effects of swelling
gree bell may be slightly weaker than the 60-degree
soil (table 6-5). The defects attributed to construction
bell, no difference has been observed in practice. The
techniques are discontinuities in the shaft, which may
following considerations are necessary in comparing
occur from the segregation of concrete, failure to com-
underreamed shafts with straight shafts.
plete concreting before the concrete sets, and early set
(a) Straight shafts may be more economical
of concrete, caving of soils, and distortion of the steel
than underreams if the bearing stratum is hard or if
reinforcement. The distress resulting from inadequate
subsoils are fissured and friable. Soil above the under-
design considerations are usually caused by wetting of
ream may be loose and increase the upward movement
subsoil beneath the base, uplift forces, lack of an air
needed to develop the bell resistance.
gap beneath grade beams, and lateral movement from
(b) The shaft can often be lengthened to elimi-
downhill creep of expansive clay.
nate the need for an underream, particularly in soils
where underreams are very difficult to construct. Fric-
(2) Location of base. The base of shafts should be
tion resistance increases rapidly in comparison with
located below the depth of the active zone, such as be-
end bearing resistance as a function of the relative
low the groundwater level and within nonexpansive
shaft-soil vertical movement.
soil. The base should not normally be located within
(c) Underreams reduce the contact bearing pres-
three base diameters of an underlying unstable stra-
sure on potentially expansive soil and restrict the min-
tum.
imum diameter that may be used.
(a) Slabs-on-grade isolated from grade beams
(4) Uplift forces. If bells or underreams are not
and walls are often used in light structures, such as
residences and warehouses, rather than the more cost-
feasible, uplift forces (table 6-5) may be controlled by
the following methods:
ly structural slabs supported by grade beams and
shafts. These slabs-on-grade will move with the expan-
(a) The shaft diameter should be the minimum
sive soil and should be expected to crack.
required for downloads and construction procedures
(b) To avoid "fall-in" of material from the granu-
and control.
6-11
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