5.2. RUBBLE-MOUND STRUCTURES
209
because the strength was properly scaled. Therefore, we should expect
breakage of the strength-simulated model armor units to be in similitude
with the prototype with a stress scale ratio of NOp = N^.
Timco dynamically tested model material designed for a length scale
of Nl = 20. He estimated the elastic modulus scale ratio to be about
Ne = 3.2 for impact loads. Based on Burcharth, et al.’s (1991) scaling
for impact stresses (Eqn. 5.45), this would mean that impact stresses in
Timco’s strength-simulated units at Nl = 20 would scale as
Na. = VNeNl = \/(3.2)(20) = 8
rather than the required stress scaling of Na = 20. If these strength-scaled
model units were to be used in a hydraulic model study, and breakage
of units occurred, then it would be impossible to scale the known model
unit fracture stress to prototype values because the ratio of impact and
nonimpact stresses that occurred during breakage in the model is unknown
(Burcharth, et al. 1991). However, strength-scaled units can be used in
model tests if the model engineer can reasonably assume that breakage will
be due to either static/pulsating stresses or to impact stresses, but not a
combination of the two types of loading.
Timco and Mansard (1Q82) described model tests where the strengthscaled armor units were used in a 1:25 scale model of a failed breakwater.
Conventional hydraulic stability tests indicated that the structure was stable despite rocking of some units. However, when the strength-scaled units
were used, breakage occurred and the structure was damaged in a manner
similar to what occurred in the prototype. Once armor unit breakage was
induced “the stability and usefulness of the breakwater was substantially
reduced” (Timco and Mansard 1982).
Additional tests at two different scales using the strength-simulated
model armor units are described in Timco and Mansard (1983). A test
conducted at 1:25 scale representing 4600-kg (4.5-ton) dolosse had some
movement and breakage, but the structure survived intact. The same test
was repeated using dolosse that had been made to simulate the strength of
19 000-kg (18.5-ton) dolosse at a 1:40 scale. In this test many of the armor
units broke, and the structure suffered general collapse. These tests demonstrated the necessity of including armor unit strength as a critical factor
in designing armor layers composed of slender concrete armor units. Conventional tests would have indicated that the structure was hydraulically
stable.
Strength-scaled model armor units are a viable method for use in examining potential structure instability due to broken concrete armor units.
However, the design engineer must be reasonably sure that either impact
stresses or static/pulsating stresses are primarily responsible for breakage.
209
because the strength was properly scaled. Therefore, we should expect
breakage of the strength-simulated model armor units to be in similitude
with the prototype with a stress scale ratio of NOp = N^.
Timco dynamically tested model material designed for a length scale
of Nl = 20. He estimated the elastic modulus scale ratio to be about
Ne = 3.2 for impact loads. Based on Burcharth, et al.’s (1991) scaling
for impact stresses (Eqn. 5.45), this would mean that impact stresses in
Timco’s strength-simulated units at Nl = 20 would scale as
Na. = VNeNl = \/(3.2)(20) = 8
rather than the required stress scaling of Na = 20. If these strength-scaled
model units were to be used in a hydraulic model study, and breakage
of units occurred, then it would be impossible to scale the known model
unit fracture stress to prototype values because the ratio of impact and
nonimpact stresses that occurred during breakage in the model is unknown
(Burcharth, et al. 1991). However, strength-scaled units can be used in
model tests if the model engineer can reasonably assume that breakage will
be due to either static/pulsating stresses or to impact stresses, but not a
combination of the two types of loading.
Timco and Mansard (1Q82) described model tests where the strengthscaled armor units were used in a 1:25 scale model of a failed breakwater.
Conventional hydraulic stability tests indicated that the structure was stable despite rocking of some units. However, when the strength-scaled units
were used, breakage occurred and the structure was damaged in a manner
similar to what occurred in the prototype. Once armor unit breakage was
induced “the stability and usefulness of the breakwater was substantially
reduced” (Timco and Mansard 1982).
Additional tests at two different scales using the strength-simulated
model armor units are described in Timco and Mansard (1983). A test
conducted at 1:25 scale representing 4600-kg (4.5-ton) dolosse had some
movement and breakage, but the structure survived intact. The same test
was repeated using dolosse that had been made to simulate the strength of
19 000-kg (18.5-ton) dolosse at a 1:40 scale. In this test many of the armor
units broke, and the structure suffered general collapse. These tests demonstrated the necessity of including armor unit strength as a critical factor
in designing armor layers composed of slender concrete armor units. Conventional tests would have indicated that the structure was hydraulically
stable.
Strength-scaled model armor units are a viable method for use in examining potential structure instability due to broken concrete armor units.
However, the design engineer must be reasonably sure that either impact
stresses or static/pulsating stresses are primarily responsible for breakage.