ETL 1110-2-355
31 Dec 93
the density of the soil by the volume of soil present.
foundation, and the backfill. For guidance regarding
For soil above the water table, a moist density is
downdrag, reference should be made to the results
normally used. For soil below the water table, a
discussed in the report on the Port Allen and Old
submerged (buoyant) density is used.
River Locks (Clough and Duncan 1969) as well as
the report on Red River Lock and Dam No. 1
(Ebeling et al. 1993). These studies show the
b. Lateral. The value of lateral soil pressure at
extremes with respect to downdrag which have been
any point is determined by multiplying the vertical
computed to date. Given the state of practice, a
weight of soil above the point (with the appropriate
complete soil-structure interaction (SSI) analysis is
reduction in soil weight for when submerged) by the
the most reliable procedure available for estimating
lateral pressure coefficient k. Observations from
downdrag forces on soil-founded U-frame locks
measured earth pressures on U-frame locks have
because the more compressible the foundation is, the
shown that the coefficient of lateral earth pressure
greater the need for SSI analyses to determine the
varies over a wide range along the height of the lock
values to be assigned for downdrag forces. The finite
wall and also varies with time. Because of this varia-
element method of analysis is used in this type of
tion, it is common practice to bracket the earth pres-
analysis to compute the stresses and displacements
sure by using an upper and a lower bound for k.
for both the structure and the backfill. The finite
These maximum and minimum values are then com-
element program SOILSTRUCT has the capabilities
bined with the appropriate load cases for analysis in a
for performing a complete SSI analysis to obtain
conservative manner. Selection of k values for design
downdrag forces and has been used successfully on
should include knowledge gained from past instru-
numerous projects, including those cited in this sec-
mentation programs as well as results of classical
tion (Ebeling, Peters, and Clough 1992).
solutions. Factors affecting k are flexibility of the
wall, soil types, loading case, seismic condition,
4-6. Foundation pressure. Foundation pressure
density of the backfill, and shear strength of the back-
is the response of the soil to the force of the structure
fill. Examples of k values used vary from 0.2 to 1.0.
placed upon the soil. To determine the foundation
pressure, see paragraph 6, Foundation Analysis.
c. Silt. Since the load that the structure reacts
to from earth pressures depends upon the depth of
4-7. Impact. Impact loads from barges striking the
backfill, the possibility that silt could be deposited
above the backfill must be taken into account. The
lock walls should not be considered in the overall
amount of silt that may be deposited upon a structure
design of the monolith, but should be considered in
is dependent upon the silt load the waterway is carry-
localized areas. These loads are covered in
ing, the water velocity at the location siltation is
ETL 1110-2-338. The impact loads on the lock walls
expected to occur, and the geometry of the structure.
are generally less severe than those on more exposed
These factors vary for each project and must be con-
components of a navigation project since the angle of
sidered when deciding upon the amount of deposited
impact is limited in the lock, and speeds are generally
silt. For example, the lower Red River in Louisiana
much slower in the lock. The magnitude of these
sees silt deposits of between 5 and 20 ft every time
forces is dependent upon the size of the tow and
the river rises and falls. Silt loads are incorporated
barge that the lock can accommodate and therefore is
into the vertical and horizontal earth pressures by
project-dependent. Loads transferred through the lock
considering the silt as an additional soil layer above
gates due to impacts on the gates should be consid-
other existing soil layers. Normally silt deposited in
ered. Guidance on values to be applied to gate
the lock chamber is not considered as a load on the
impacts is included in EM 1110-2-2703 and
structure.
EM 1110-2-2105.
4-5. Drag. The downdrag force is a shear force
4-8. Hawser. Hawser loads should not be consid-
acting downward along a vertical plane adjacent to or
ered in the overall design of the monolith, but should
near a structure-to-soil interface (Ebeling, Duncan,
be considered in the design of the upper part of the
and Clough 1990; Ebeling et al. 1992). Downdrag
lock wall. Hawser loads are the forces generated
forces acting on the stem and culvert walls of
resisting the inertial forces of moving barges.
U-frame locks are more difficult to characterize than
Hawser loads act in the opposite direction to impact
are those for gravity walls founded on rock because
loads. These loads are covered in other guidance.
of the interactions among the structure, the
For additional information, see ETL 1110-2-247.
A-7
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