FLOOD06430

• County: Statewide
• Basin: Statewide
• Title: Conduits, Culverts and Pipes
• Date: 3/3/1969
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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EM 1110-2-2902
3 Mar 1969

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mum of 20 feet on earth foundations and 30 feet on rock. Concrete collars are used to provide
for differential movement on yielding foundations and to control seepage between the outer
surface of the conduit and the fill. A minimum of three cutoff collars should be located in the
impervious zone of the dam if founded on rock where little differential settlement will occur.
For all earth foundations and weak rock foundations where considerable settlement or out-
ward fill movement.is expected, all joints should be provided with collars. For large settlement,
reduced spacing of joints should be used. The collars must be designed either to resist or accomo-
date differential movement without losing watertight integrity. A compressible filler about %
inch thick should be applied between the conduit surface and the collar to permit rotation caused
by differential settlement and to avoid concentrated stresses in the conduit walls resulting from
differential movement. However, where the cond uit rests on rock having insignificant differential
settlement, the collar need not completely encircle the conduit, in which case there is little chance
of shrinkage of the collar inducing stress in the conduit. Therefore, it is sufficient to separate these
collars from the conduit by two coats of mastic.
For conduits placed on competent rock, seep age collars may be located between the joints
and designed for the fill load only. For pressure discharge conduits (sewers) not under levees,
collars are not usually needed except for cast-in-place conduits.
The position of the longitudinal construction joints shown on plate 9 can be varied slightly
to suit construction methods and equipment but, for circular and oblong conduits, the concrete
in the invert section should be top-formed above the point where the tangent to the invert
is steeper than 1 vertical on 1%, horizontal.
To insure a watertight bond between a cut-and-cover conduit and the walls of rock cuts, a
concrete plug should be placed within the core zones. A plug 50 feet in length of lean concrete and
extending up to the original rock surface is considered to be the minimum useful size. Where the
origi.nal rock surface is exceptionally high however, judgment should be used in establishing
the height of the concrete plug. Hand-tamped earth fill is not considered to be an acceptable sub-
stitute for the lean concrete. Cutoff collars are no~ necessary when the top of the plug is above the
top of the conduit. Where the top of the plug is below the top of the conduit, however, collal's
are needed at the top.
d. Waterstops. Waters tops of a flexible type should be used in all transverse contraction
joints. Guidance on the selection of waterstop materials is given in EM 1110-2-2102, Water-
stops. Where large differential movement is expected, a center-bulb type waterstop should be
used and a joint separation of about one-half inch provided. Where the conduit rests on a rather
firm foundation, a two-bulb or other equivalent type waterstop should be used with a joint sep-
aration of about one-quarter inch. Where the conduit rests on rock with little deformation, the
joint should be provided with two coats of mastic and an approved type waterstop.
e. Camber. Where considerable foundation settlement is likely to occur, camber should be
employed to assure positive drainage.
6. Special Considerations for Culverts.

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a. Determination of required capacity. Refer to EM 1110-2-1410, Interior Drainage of Lev-
eed Urban Areas: Hydrology, for the method of determining the required culvert capacity for a
gi ven set of field conditions.
b. Features affecting structure capacity. Having determined the required capacity, the de-
signer must select the size and shape of the' culvert to provide this capacity. The following sut>.
paragraphs include a discussion of features which affect the determination of culvert size.
(1) Location. Ideally, the axis of a culvert should coincide with that of the natural stream
bed and the structure should be as straight and as short as practicable. This may require some

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