ETL 1110-2-344
31 Dec 93
b. Uplift pressure models.
(1) Modeling of uplift pressures at the base of
a gravity structure (monolith) requires knowledge of
the actual foundation conditions and requires that
various assumptions be made as to the magnitude
and distribution of pressure. There are many
approaches which may be used to model uplift pres-
sures at the base of a gravity structure. Three basic
approaches are described by the following cases. In
each case the uplift pressure along the cracked
portion at the base of the structure is assumed to be
constant at a magnitude equal to the hydrostatic
pressure at the mouth of the crack.
(a) Case 1. The monolith (concrete) and
foundation (rock) are considered impervious and
elastic, while the monolith/foundation interface is
considered pervious. Uplift pressures are assumed
to vary linearly from the crack tip to the toe of the
Figure A-2. Critical load combination for
monolith. This corresponds to the pipe flow anal-
monolith 7E
ogy shown in Figure A-3.
b. Results. The classical analysis procedure
(b) Case 2. The monolith and foundation are
considered impervious and elastic. Uplift pressures
was performed for a variety of combinations of
are computed from piezometer readings taken at
backfill saturation elevations and lock water eleva-
several locations along the uncracked portion of the
tions (ETL 1110-8-16(FR)). For the current study,
monolith.
results for an actual observed condition with a lock
water elevation of 340.0 ft (lock dewatered) and a
(c) Case 3. The monolith is considered
backfill saturation elevation of 396.0 ft (piezometer
impervious and elastic and the foundation is con-
reading) are used. Based on these conditions, the
sidered impervious and infinitely rigid. Uplift pres-
traditional analysis results in a calculated percentage
sures are assumed to vary linearly from the crack
of base in compression of 48.6% (51.4% of the
tip to the toe of the monolith.
base in tension translates to a crack of length
23.13 ft assuming no ability to transmit tensile
(2) The approach used in this study (described
forces). This does not meet the requirement of at
in paragraph 3c) corresponds to Case 1; the same
least 75% of base in compression as specified by
modeling techniques would also be valid for
ETL 1110-2-22 "Design of Navigation Lock
Case 2. Case 3 corresponds to the approach most
Gravity Walls." When the lock was dewatered, no
commonly assumed with the traditional method of
signs of distress were detected, and recent instru-
analysis.
mentation has indicated no significant movement of
the lock wall.
(3) Using the uplift models defined by Cases
1 and 2 within the context of a finite element analy-
3. Finite Element/Fracture Mechanics Based
sis requires special considerations. Pressures acting
Analysis
along the interface between the monolith and foun-
dation result in hydrostatic forces of equal magni-
tude in all directions. In a finite element analysis,
if uplift pressures along the uncracked portion of
mechanics based analysis consisting of a finite
the base are applied to the elements adjacent to the
element structural analysis supplemented with frac-
interface between the monolith and the foundation
ture mechanics is described. A discussion on the
as vertical pressures in equal and opposite
modeling of uplift pressures at the base of a gravity
structure is included.
A-2
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