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TitleEarth and Rock Fill Dams.PDF
Tags Reservoir Dam Spillway
File Size2.4 MB
Total Pages119
Document Text Contents
Page 1

Design and Construction of Earth
and Rock-Fill Dams


Course No: G07-001


Credit: 7 PDH






Gilbert Gedeon, P.E.










Continuing Education and Development, Inc.
9 Greyridge Farm Court
Stony Point, NY 10980

P: (877) 322-5800
F: (877) 322-4774

[email protected]

Page 2

US Army Corps
of Engineers®


ENGINEERING AND DESIGN

EM 1110-2-2300
30 July 2004

General Design and Construction
Considerations for Earth and
Rock-Fill Dams














ENGINEER MANUAL

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EM 1110-2-2300
30 Jul 04




9-3

seepage considerations require an upstream impervious blanket on a cofferdam built of pervious soil, the blanket
should be removed later if it restricts drainage during drawdown.

c. Cofferdam design. Major cofferdams are those cellular or embankment cofferdams, which, upon failure,
would cause major damage downstream and/or considerable damage to the permanent work. Minor cofferdams
are those which would result in only minor flooding of the construction work. All major cofferdams should be
planned, designed, and constructed to the same level of engineering competency as for main dams. Design con-
siderations should include minimum required top elevation, hydrologic records, hydrographic and topographic
information, subsurface exploration, slope protection, seepage control, stability and settlement analyses, and
sources of construction materials. The rate of construction and fill placement must be such to prevent over-
topping during initial closure of the cofferdam. The cofferdam for Cerrillos Dam, Puerto Rico, was unique in
that it was designed to handle being overtopped. The overtopping protection consisted of anchoring welded steel
rebar/wire mesh to the downstream face. Crest protection was provided by gabions with asphalt paving (U.S.
Army Engineer District, Jacksonville 1983). Minor cofferdams can be the responsibility of the contractor.
Excavations for permanent structures should be made so as not to undermine the cofferdam foundation or other-
wise lead to instability. Adequate space should be provided between the cofferdam and structural excavation to
accommodate remedial work such as berms, toe buttresses, and foundation anchors should they be necessary.

d. Protection of embankment.

(1) Where hydrologic conditions require, emergency outlets should be provided to avoid possible
overtopping of the incomplete embankment by floods that exceed the capacity of the outlet works. As the dam is
raised, the probability of overtopping gradually decreases as a result of increased discharge capacity and
reservoir storage. Should overtopping occur, however, damage to the partially completed structure and to
downstream property increases with increased embankment heights. It is prudent to provide emergency outlets
by leaving gaps or low areas in the concrete spillway or gate structure, or in the embankment during wintering
over periods. Excavation of portions of the spillway approach and discharge channels, combined with
maintaining low concrete weir sections, may provide protection for the later phases of embankment construction
during which the potential damage is greatest.

(2) When a portion of the embankment is constructed before diversion of the river, temporary riprap or
other erosion protection may be required for the toe of the embankment adjacent to the channel. This temporary
protection must be removed before placement of fill for the closure section.

(3) In some cases the cost of providing sufficient flow capacity to avoid overtopping becomes excessive,
and it is more appropriate to provide protection for possible overflow during high water conditions, as was done
at Blakely Mountain Dam (U.S. Army Engineer Waterways Experiment Station 1956).

(4) Within the past 10 years innovative methods for providing overtopping protection of embankments have
been developed. These include roller-compacted concrete and articulated concrete blocks tied together by cables
and anchored in place (see Hansen 1992; Powledge, Rhone, and Clopper 1991; Wooten, Powledge, and
Whiteside 1992; and Powledge and Pravdivets 1992).

9-5. Closure Section

a. Introduction. Because closure sections of earth dams are usually short in length and are rapidly brought
to grade, two problems are inherent in their construction. First, the development of high excess porewater
pressures in the foundation and/or embankment is accentuated, and second, transverse cracks may develop at the
juncture of the closure section with the adjacent already constructed embankment as a result of differential
settlement. When the construction schedule permits, excess porewater pressures in the embankment may be
minimized by providing inclined drainage layers adjacent to the impervious core and by placing gently sloping

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9-4

drainage layers at vertical intervals within semi-impervious random zones. However, acceleration of foundation
consolidation by means of sand blankets and vertical wick drains or reduction of embankment pore pressures by
stage construction is generally impracticable in a closure section. A more suitable procedure is to use flatter
slopes or stabilizing berms. Cracking because of differential settlement may be minimized by making the end
slopes of previously completed embankment sections no steeper than 1 vertical on 4 horizontal. The soil on the
end slopes of previously completed embankment sections should be cut back to well-compacted material that has
not been affected by wetting, drying, or frost action. It may be desirable to place core material at higher water
contents than elsewhere to ensure a more plastic material which can adjust without cracking, but the closure
section design must then consider the effects of increased porewater pressures within the fill. The stability of
temporary end slopes of embankment sections should be checked.

b. Limit. If specifications limit the rate of fill placement, piezometers must be installed with tips in the
foundation and in the embankment to monitor porewater pressures. Conduits should not be built in closure sec-
tions or near enough to closure sections to be influenced by the induced loads.

c. Closure section. Closure sections, with foundation cutoff trenches if required, are generally constructed
in the dry, behind diversion cofferdams. In a few cases, the lower portions of rock-fill closure sections with
“impervious” zones of cohesionless sands and gravels have been successfully constructed under water (see Pope
1960). Hydraulic aspects of river diversion and closures are presented in EM 1110-2-1602.

9-6. Construction/Design Interface

It is essential that all of the construction personnel associated with an earth or rock-fill dam be familiar with the
design criteria, performance requirements, and any special details of the project. As discussed in paragraph 4-7,
coordination between design and construction is accomplished through the report on engineering considerations
and instructions to field personnel, preconstruction orientation for construction engineers by the designers, and
required visits to the site by the designers.

9-7. Visual Observations

Visual observations during all phases of construction provide one of the most useful means for controlling
construction and assessing validity of design assumptions. It is not practical, for economic reasons, to perform
enough field density control tests, to install enough instrumentation, and to obtain enough data from pre-
construction subsurface explorations to ensure that all troublesome conditions are detected and that satisfactory
construction is being achieved. While test data and instrument observations provide more detailed and quanti-
tative information than visual observations, they serve principally to strengthen and supplement visual observa-
tions of the embankment and foundation as the various construction activities are going on. Field forces should
be constantly on the alert for conditions not anticipated in the design, such as excessively soft areas in the
foundation; jointing, faulting, and fracturing in rock foundations; unusual seepage; bulging and slumping of
embankment slopes; excavation movements; cracks in slopes; and the like. It is particularly important to make
observations during the first filling of the reservoir as weaknesses in a completed dam often show up at this time.
For this reason, each reservoir project is required to have an “Initial Filling Plan” (discussed in paragraph 9-8).
Visual observations of possible distress such as cracking, the appearance of turbid water in downstream toe
drainage systems, erosion of riprap, soft wet spots downstream of the abutments or at the downstream toe or on
the downstream slope, and other observations are important. Observations of instrumentation also yield valuable
data in this respect.

9-8. Compaction Control

a. Principal compaction. Principal compaction control is achieved by enforcement of specifications relat-
ing to placement water content, lift thickness, compacting equipment, and number of passes for the various types

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