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Discussion by John K. McDonald,5 Member, ASCE
Four causes of small concentrated leaks in impervious sections of
embankment dams are listed in the paper and three of them are discussed.
The remaining one, construction deficiencies, would seem to be an obvious
problem with an obvious solution. However, some aspects of construction
that are in accordance with good modern practice may be the cause of
many cracks and leaks that have been ascribed to more esoteric origins,
such as hydraulic fracture.
About 15 years ago the technicians at a federal government dam in the
midsouth area were having trouble in securing monolithic block samples
for laboratory testing. No matter what care was taken, the blocks fell apart
because there were so many parting planes in the compacted soil. The
problem initially was blamed on the use of a new type of earth compaction
roller. The writer examined the site as a representative of the manufacturer
and observed the test fills that were made to try and isolate the real cause
of the problem.
The soil was a silty clay and it was being compacted at wet of optimum
moisture. In the borrow pit, numerous instances of "pancaking"—the
formation of thin layers of clay that could be peeled apart—could be seen
where the earth carriers had been operating. On the fill surface, especially
where the earth carrier traffic was concentrated, a little hand work with a
pick would allow huge sheets of clay to be separated and peeled back for
10-12 ft. The impression was that these cracks went on for an indefinite
distance. Perhaps, in line with the principal author's ideas, some form of
hydraulic fracturing was occurring that allowed the cracks to propagate
laterally. It was hoped at the time that these cracks did not extend through
the whole width of the impervious section and that they would heal with
time.
Two test fills were made, one for the older and one for the newer type of
compacter. The earth carriers delivered the soil but then immediately got
off the test fills. The result was that both test fills produced good block
samples. The newer compacter was allowed to stay and orders were given
to rip up the badly pancaked areas and recompact them. As far as is
known, no changes in procedure were made as a result of this test.
The compactive contributions of the tires of the earth carriers are well
recognized, and normal practice is to spread out the traffic to cover as
much of the fill surface as possible. However, in this instance (and
probably in many more instances as well), the tires caused more trouble
with cracks than the good they did in compaction. It is suggested that a
better procedure would be to confine the earth carriers to a defined haul
road that they could leave only to dump soil and then to return immediately. The haul road would have to be frequently ripped up, dozed to one
side, and then recompacted in layers by the compacters before additional
soil could be brought in to raise the level of the haul road.
The recommendations of the paper are very concise and helpful. A good
downstream filter seems to be a critical necessity at present, but with more
attention given to the prevention of pancakes, the impervious sections
'Consulting Engr., 10116 SE Stanley Ave., Portland, OR 97222.
226
J. Geotech. Engrg. 1988.114:226-227.
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could better perform their function and keep the downstream filters from
having to work so hard. It may also be that the hydraulic fracturing
phenomenon could be less of a problem than has been feared.
SEISMIC STABILITY OF GENTLE INFINITE SLOPES3
Discussion by D. R. Phatak3 and V. M. Karbhari4
The authors presented an interesting paper to evaluate the effects of
seismic accelerations on gentle infinite slopes, and it is a valuable contribution to the study of seismic stability. This discussion is related to the
effect of the component of acceleration normal to the slope, neglected in
the original paper.
The writers believe that this component cannot be neglected, even in the
case of gentle infinite slope in cases where the factor of safety approaches
1. Using the authors' convention, the factor of safety is computed as
FSt = ( 1 - — ) „ ~ n • cos p tan <f>'
r
y
7, J K + sin (3
(19)
Using the same nomenclature and representing K' as the acceleration in g
normal to the slope, the factor of safety as advocated by the writers is
_ /
mym
\
(1-/0(1-*')
(1 - K') sin p + K
cos (3 tan <>
j
(20)
So as to find out which factor of safety would be relevant in the case of a
gentle infinite slope under threat of failure (i.e., factor of safety very close
to 1), a computer program was run, and demonstrative results are given in
Table 4.
For the case considering accelerations normal to the slope, the table
clearly shows that there is a significant change in the factor of safety. This
is significant for a slope which is just on the point of failure (i.e., FS1 close
to unity), and for such slopes, it is felt that both accelerations, normal and
parallel to the slope, must be considered.
"June 1985, Vol. Ill, No. 6, by Tarik Hadj-Hamou and Edward Kavazanjian, Jr.
(Paper
19785).
3
Asst. Prof, of Civ. Engrg., Coll. of Engrg., Pune, Maharashtra State, India.
4
Postgrad. Stud., Coll. of Engrg., Pune, Maharashtra State, India.
227
J. Geotech. Engrg. 1988.114:226-227.