Page 82 - Buried Pipe Design

58 Chapter Two

Loads due to Expansive Soils
Expansive soils were mentioned briefly in the Longitudinal Loading
section concerning possible ground movement. Certain soils, primarily
bentonite clays, expand and contract severely as a function of moisture
content. Soil expansion can cause an increase in soil pressure just as
frost can cause an increase in soil pressure. This rise in pressure is
directly due to expansion and is a function of confinement. Tremendously
high pressures can result if such soils are confined between nonyielding
surfaces. However, data are lacking concerning such forces which may
be induced on buried conduits. This lack of data can probably be attrib-
uted to design practices that do not allow such soils to be placed directly
around the pipe. Also, in the case of gravity sewers, designs usually
require such material to be removed for certain depths below the pipe if
moisture content is variable at such depths. The primary reason for this
requirement is to ensure that the grade is maintained. The design engi-
neer should be cognizant that expansive soils do pose certain potential
problems. He or she should seek advice from a component soils (geot-
echnical) engineer and then take appropriate steps in the installation
design to mitigate adverse effects of expanding soils.

Flotation

Buried pipes and tanks are often placed below the water table. High
soil cover can prevent flotation, but in shallow cover, holddowns,
weights, etc., may be required to prevent flotation. Reinforced concrete
pavement over a pipe helps to resist flotation. Holddowns require
anchors—a concrete slab or deadmen. When the water table is a prob-
lem, soil at the bottom of the excavation is so wet that a concrete slab
is used as a platform on which to work. In some cases, two longitudinal
footings (deadmen) may be adequate anchors. Straps are sometimes
used to tie the pipe or tank to the anchors.

Soil wedge
If the embedment is granular and compacted, a floating pipe must lift
a soil wedge. See Fig. 2.31. If the buoyant force of the pipe exceeds the
pipe weight and the effective weight of the soil wedge, anchors must
restrain the difference.

Example Problem 2.9 Suppose that a large-diameter steel pipe is buried
under 2 ft of soil cover. Is any anchorage required if the pipe is empty when
the water table rises to or above the ground surface?
Steel pipe: D 105 in

Pipe Weight 580 lb/ft