STRUCTURAL MODELS 685
16.2.2 Modelling concrete damsConcrete dam models are almost invariably of arch or cupola structures.
Simulation of the external hydrostatic load in compliance with the appro-
priate scaling laws requires a liquid of very high density. In practice a
mercury-filled flexible bag is most commonly employed; the implication of
a predetermined density ratio of 13.6 with respect to other scale ratios will
be noted (see Table 16.1).
Desirable characteristics of the model material are a correct
representation of stress–strain response and a relatively low modulus of
elasticity, E, to enhance model deformations and permit the use of
mercury loading on a model of manageable dimensions. The material
should have appreciable tensile and compressive strength to reduce the
risk of cracking, and must be readily cast or machined to the correct
profile for the dam. Materials which have been successfully employed
include microconcrete, plastics and plaster-based compounds. Mixtures of
high-grade plaster of Paris with a chemically inert filler (such as diatomite
or sodium montmorillonite) and water have proved particularly suitable.
Such mixtures have the following additional desirable characteristics:
- isotropy and homogeneity;
- they are uniformly elastic within model stress range;
- the Poisson ratio, v, is approximately equal to that of concrete and
rock (0.15–0.22); - elastic properties are not time dependent;
- modulus of elasticity, E, and strength characteristics can be accu-
rately controlled.
The purpose of the filler is to permit a high water:plaster ratio and
hence a low Evalue. The plaster–filler mixtures require careful curing
over a period, the final product having the texture of a soft chalk and
being readily machined to profile. It will be noted that appropriately
scaled representation of Eand+, rather than strength, is the most import-
ant parameter in relation to correct simulation of structural response.
Representative values of the principal engineering characteristics of
plaster–filler mixtures include Eranging from 0.7 103 to 5.0 103 MN m^2
andv0.20. Compressive strengths lie in the range 2.0–20.0 MN m^2 , with
tensile strengths typically 12% to 25% as great. Linear scales for concrete
dam models are commonly in the range 1:50 to 1:200, care being taken to
model an appreciable portion of the abutments and underlying founda-
tions to correctly simulate interaction.
Major structural features of the prototype abutment and/or founda-
tion geology, e.g. faults or crush zones etc., must be reproduced on the
model. Plaster–filler mixes with appropriate stiffness characteristics may