construct100
Civil/Environmental
Hello. In DM 7_03, it says the following:
"In saturated loose to medium compact granular soils
seismic shocks may produce unacceptable shear strains. These
strains may be the result of shear stresses exceeding the
strength of a soil that softens beyond peak shear strength. The
high shear deformations and decreased shear strength may also be
a consequence of the progressive buildup of high pore pressures
generated by seismic shaking. With no or limited drainage,
cyclic shear stresses can produce a progressive buildup of pore
water pressures that significantly reduce the effective stress,
which controls the strength. For practical purposes, the
effective stress after several cycles of shear straining may
ultimately be reduced to zero, leading to liquefaction. The
progressive weakening before total liquefaction is called cyclic
mobility."
But again in DM 7_03, Page 9, it says the following:
"It is also important to know whether the
dynamic loading is a transient phenomenon, such as a blast
loading or earthquake, or is a long term phenomenon, like a
vibratory loading from rotating machinery. The distinction is
important because a transient dynamic phenomenon occurs so
rapidly that excess pore pressure does not have time to dissipate
except in the case of very coarse, clean gravels. In this
context the length of the drainage path is also important; even a
clean, granular material may retain large excess pore pressure if
the drainage path is so long that the pressures cannot dissipate
during the dynamic loading."
My question: Don't these two information conflict? One says it is a cyclic thing, other says it is sudden thing. I mean, I know I am wrong but what am I missing?
"In saturated loose to medium compact granular soils
seismic shocks may produce unacceptable shear strains. These
strains may be the result of shear stresses exceeding the
strength of a soil that softens beyond peak shear strength. The
high shear deformations and decreased shear strength may also be
a consequence of the progressive buildup of high pore pressures
generated by seismic shaking. With no or limited drainage,
cyclic shear stresses can produce a progressive buildup of pore
water pressures that significantly reduce the effective stress,
which controls the strength. For practical purposes, the
effective stress after several cycles of shear straining may
ultimately be reduced to zero, leading to liquefaction. The
progressive weakening before total liquefaction is called cyclic
mobility."
But again in DM 7_03, Page 9, it says the following:
"It is also important to know whether the
dynamic loading is a transient phenomenon, such as a blast
loading or earthquake, or is a long term phenomenon, like a
vibratory loading from rotating machinery. The distinction is
important because a transient dynamic phenomenon occurs so
rapidly that excess pore pressure does not have time to dissipate
except in the case of very coarse, clean gravels. In this
context the length of the drainage path is also important; even a
clean, granular material may retain large excess pore pressure if
the drainage path is so long that the pressures cannot dissipate
during the dynamic loading."
My question: Don't these two information conflict? One says it is a cyclic thing, other says it is sudden thing. I mean, I know I am wrong but what am I missing?
