ETL 1110-2-536
31 Dec 94
dry streambed. The structure will impound the
Because of the magnitude of the cracks at the
100-year flood for no more than 20 days. A self-
foundation/RCC interface, it was necessary to deter-
regulating outlet provides reservoir drawdown at a
mine the suitability of a longitudinal model of a
controlled rate. The structure is designed to require
vertical plane for use in predicting crack potential.
no manned operations in the event of a flood.
As a result, two 2-dimensional models were analyzed.
One model computed stresses resulting from a trans-
(3) After final excavation of the foundation, the
verse model and the other computed stresses from a
rock surfaces were cleaned and covered with a wet-
longitudinal model in a vertical plane. While these
mix shotcrete and foundation concrete. RCC was
two-dimensional models provide less accurate solu-
placed in 12-in. thick horizontal layers on and against
tions than a 3-dimensional model, the accuracy is
these foundation surfaces. Interfaces of the RCC and
appropriate considering the size of the project and
the foundation concrete were bonded with a bedding
available funding for the study. The limitation of the
mortar. Similarly, the RCC lift joints were fully
two-dimensional, longitudinal model, is that it is
bonded with the same bedding mortar. RCC place-
assumed that symmetry exists on either side of the
ment began on 6 July 1992 and was completed on
longitudinal plane, which is not the case. This
15 October 1992. The 126 RCC lifts were placed in
assumption may cause a shifting of the thermal gradi-
approximately 75 placing days during a 100-day
ent from its actual location. In the case of the trans-
period. In general, the process was to deliver the
verse model, the full section is modeled which more
RCC to the dam on a conveyor. Front-end loaders
accurately models the thermal gradient. Appendix A,
received the RCC and transported it to the desired
Figure A-1, shows the location of the assumed two-
location. The RCC was spread with a small dozer
dimensional plane through the dam. Thermal con-
and compacted with a vibratory roller.
tours of both models, shown in Appendix A,
demonstrate a good correlation of peak temperatures
between the two models.
c. ABAQUS model and data.
(1) After developing the mesh for a two-dimen-
(2) Because RCC exhibits similar material char-
sional transverse model through the spillway section,
acteristics as conventionally placed mass concrete, the
it became apparent that a three-dimensional model
analytical process for determining thermal gradients
would become extremely large. Although the CRAY
and stresses in RCC is practically the same. The
could handle the computational analysis during this
analytical procedures have been well documented by
study, the time required to execute such a model
previous analyses and authors and should follow the
would have been extensive, due to the system work-
general guidance established in ETL 1110-2-324.
load. In addition, the geometry of the dam does not
However, RCC construction generally occurs over a
lend itself to easy input generation for ABAQUS and
relatively short time frame with numerous lift joints
as a result, extensive efforts to generate the model
(usually 1 to 2 ft in height). Conventionally placed
would be required. A three-dimensional model would
mass concrete usually involves placements with lift
increase the cost of the study significantly beyond
heights of 4 to 7 ft with 5- to 7-day restrictions
initial estimates. Because of these factors, the study
placed on form removal. The exposure and depth of
was limited to two two-dimensional models. Thermal
each lift in conventionally placed concrete will gener-
analyses of this nature have most often been done
ally define the limits of the number of steps neces-
using two-dimensional models. A two-dimensional
sary to perform the incremental analysis. In contrast,
transverse model usually gives good analytical results
continuous placement of RCC on some projects has
that can be used to predict cracks that propagate
achieved four 1-ft lift heights in 24 hr. In a 7-day
inward from the surface and cracks originating from
period, RCC can achieve changes in elevation of 25
the foundation. However, recent projects, most nota-
to 30 ft, depending on the project specifics and resul-
bly Upper Stillwater Dam, experienced significant
tant production rates. Therefore, before a NISA can
cracking. These cracks propagate vertically from the
be performed for RCC, a comprehensive study of
foundation and are oriented perpendicular to the
production rates must be completed in order to select
longitudinal axis. The Bureau of Reclamation per-
time steps, the number of steps, and element mesh
formed a thermal analysis to predict the crack
size. The results of the production rates, in combina-
potential. They utilized a transverse, two-dimensional
tion with the element size, and parametric studies
model and a longitudinal model of a horizontal plane.
A-3
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