(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

monolith. This corresponds to the pipe flow anal-

ogy shown in Figure A-3.

(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 3*c*) 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-

sis requires special considerations. Pressures acting

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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