Mine Safety and Health Administration (MSHA)
Engineering and Design Manual for
Coal Refuse Disposal Facilities
Advance Draft for Industry Review and Comment, December 2007
Chapter 7, Seismic Design Stability and Deformation Analysis
A Designer’s Opinion – It’s déjà vu all over again
By: Barry Thacker, P.E.
barryt@geoe.com
--- FOR
REVIEW PURPOSES ONLY ---
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OVERVIEW
Designers of coal refuse disposal
facilities beware. Do not use the method of seismic stability analysis
proposed in the draft MSHA Design Manual for actual design purposes.
Instead, use a simplified method with a residual friction angle of 4 degrees
for the strength of fine refuse in post-earthquake static stability analyses
that provide factors of safety greater than 1.0. In my opinion, the methodology proposed
by MSHA in its draft manual appears academically sound for assessing the
stability of existing structures and should result in computed factors of
safety well in excess of 1.2 for facilities that are designed based on the
simplified method.
BACKGROUND
Reviewers from the Mine Safety and Health
Administration (MSHA) began scrutinizing the liquefaction potential and
seismic stability of coal refuse disposal facilities in 1982. At that time,
there were four generally recognized schools of thought on the topic located
in Boston, California, Canada, and Japan. Each time I submitted data and
analyses following the recommendations from one school of thought, I got
comments back from MSHA mirroring published criticism of that particular
method from the experts at the other schools of thought. In most cases, I
agreed with the MSHA reviewers because none of the schools of thought were
applicable to coal refuse disposal facilities.
MSHA worked with designers and operators, recommending
approval for individual stages while additional data and analyses could be
developed. In many cases, upstream construction was delayed by
concentrating on the downstream portion of construction still available.
MSHA also assigned Mr. Wade Cooper as its resident expert to resolve the
situation.
In 1992, I met with Mr. Cooper and others at MSHA to
propose a very conservative protocol to address MSHA’s concerns about
liquefaction potential and seismic stability. Rather than debate about
whether an earthquake record from California or Japan should be used to
design a dam in the Appalachian coal-fields where earthquakes are rare, I
proposed we just assume that the BFE (Big Freakin’ Earthquake) causes
liquefaction and that the hydraulically-placed fine refuse retains only its
residual strength.
The protocol recommends using an undrained steady-state
shear strength, Sus, equivalent to a residual friction angle
(phi) of 4 degrees based on results of previous testing of fine refuse at
the University of Louisville. The needed safety factor is incorporated into
the conservative strength parameters such that the minimum required factor
of safety computed in post-earthquake static analyses is 1.0. Where needed,
the conservative strength parameters can be verified during construction
using residual vane shear testing to estimate Sus.
MSHA personnel accepted the protocol, thus providing
owners with the confidence to proceed with upstream construction during the
early stages of a facility. Beginning upstream construction sooner, rather
than later, provides more time for dissipation of excess pore pressures and
results in a more consolidated and liquefaction-resistant material. Based
on experience gained with upstream construction during the early stages of
those projects, the modified upstream construction (MUSC) method was
developed that enables seismic stability to be assessed using the simplified
method over the life of a project.
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MODIFIED UPSTREAM CONSTRUCTION (MUSC) METHOD
MSHA’s early concerns over liquefaction and seismic
stability caused many slurry impoundments to be built initially by the
downstream method. Because MSHA requires that slurry impoundments be capped
upon abandonment, upstream construction must be performed eventually at all
facilities. Delaying upstream construction can result in the creation of a
zone of relatively low coarse refuse cover, called a “hinge” as shown in
Figure 1. In such an area, consolidation pressures in the fine refuse can
be low and the phreatic level may develop close to the outslope.
Furthermore, a hinge-zone can be susceptible to differential settlement,
both during construction and in the event of an earthquake. |
Click on
images to
enlarge
Figure 1.
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The MUSC method recommends starting upstream
construction early in a project as shown in Figure 2. Concerns over seismic
instability in subsequent stages are addressed by building a buttress as
shown in Figure 3. During later stages, the level of the buttress achieves
the level of the upstream structural embankment as shown in Figure 4.
Construction of a relatively wide crest can then continue until the start of
a reclamation stage to finish capping the impoundment as shown in Figure 5.
Voilà, no hinge, and concerns over seismic
stability can be addressed using the conservative simplified method (i.e.
phi = 4 degrees for fine refuse) as shown in Figure 6. |
Figure 2.
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The degree of conservatism of the simplified seismic
stability method has been documented at 20 sites using in-situ residual vane
shear testing as shown in Figure 7. Moreover, researchers from the
University of Kentucky are conducting a study of the seismic stability of
coal tailings dams, which is being funded by the National Science Foundation
and mine owners. The two facilities selected for the study are both being built
by the MUSC method and residual vane shear data generated during that study are included in Figure 7. |
Figure 7. |
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In a few cases, fine refuse strength slightly higher
than phi = 4 degrees has been used on the sites referenced in Figure 7, when
justified by field and laboratory data. Also, a few MSHA reviewers have
required that deformation analyses be performed, but as would be expected in
a dam built by the MUSC method with relatively wide coarse refuse zones and
no abrupt hinges, predicted total and differential settlements have been
small.
For construction of a slurry impoundment to be
successful, all stakeholders must sing off the same song sheet. The
simplified seismic assessment method and the MUSC technique are simple to
understand, thus making training of stakeholders more effective.
For example, when I explain procedures to the pushout
technicians (dozer operators) on a project, they can understand the
importance of beginning upstream construction early in the project. If the
pushout starts to crack, they know that coarse refuse disposal can move to
the buttress zone to enable time for pore pressures in the fine refuse
beneath the pushout to dissipate. They also can appreciate why we need to
stage the construction to keep a consistent, relatively thick zone of coarse
refuse downstream of the fine refuse to avoid creating a hinge, or potential
weak zone near the outslope, decades down the road.
When I explain that we stick a vane in the fine refuse,
swirl it around 10 times to simulate the BFE disturbing the ground, and then
measure the remaining residual strength to verify parameters used in design,
they pay attention when the drill rig shows up. When they ask, “Did we
pass?”, vertical effective stress and residual shear strength can be
calculated in the field to show them where the point plots on Figure 7. For
99.5% of the tests, I can say, “Yes, you passed and keep up the good work”.
REVIEW OF CHAPTER 7 OF MSHA’S DRAFT MANUAL
General
MSHA is proposing to step back 20 years in time and
return to the good old days of forcing a square peg (i.e. conflicting
techniques developed for other applications) into a round hole (i.e. coal
refuse disposal facilities). I remember the words to that song, dazed
and confused. |
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It looks like the folks from the Boston school of
thought are the big winners. The draft MSHA manual references a technical
paper that describes where the Boston method was applied to five coal refuse
disposal facilities. The referenced technical paper also includes a
down-valley cross-section for one of those sites, which is shown as Figure 8
for illustration purposes. |
Figure 8. |
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I’m not a graduate of the Boston school, but unless I’m
mistaken, I see a hinge in the referenced example. If the new draft MSHA
manual is now saying that hinges are acceptable, then times sure have
changed over the past 20 years. |
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Results of Independent Analyses
I applied the simplified method of
seismic analysis to the example referenced by MSHA. As shown by the results
in Figure 9, maybe times haven’t changed and hinges are still not advisable,
as indicated by a computed factor of safety of 0.49. |
Figure 9. |
| If the MUSC
method had been used at the site shown in Figure 8, modifications would have
been considered to reduce the impact of the hinge. Figure 10 shows how
upstream construction might have been started earlier and more coarse refuse
could have been used to build a steeper buttress zone rather than being
placed in the upper pushout construction. Using phi = 4 degrees for the
fine refuse in the referenced example as revised using the MUSC method,
Figure 11 shows a computed factor of safety of 1.02 based on the simplified
method. |
Figure 10. |
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Figure 11. |
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As a final analysis, I used the strength parameters
determined using the Boston method, as published for the example shown in
Figure 8, and applied them to the example dam revised by the MUSC method.
As shown by the results in Figure 12, a factor of safety of 1.67 is
predicted. Voilà, use of the MUSC technique,
coupled with the simplified seismic stability method during design, removes
the hinge and provides a factor of safety well in excess of 1.2 for the
expected as-built facility. |
Figure 12. |
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Exclusion of the Simplified Seismic Stability
Method
At first, I intended to challenge MSHA
personnel for advocating use of the Boston method which justifies the
creation of hinges, and for excluding the more conservative simplified
method which magically precludes hinges from being built. I even developed
considerations for MSHA personnel to review as follows: |
 | One of the justifications MSHA personnel used for approving the simplified method in 1992 was a reference published by a
graduate of the Boston school of thought that shows the relationship between Sus as determined for clean sand in the laboratory and
Sus as determined for clayey silt using in-situ vane shear
testing. I have taken the liberty to include that relationship as Figure
13, which shows essentially the same value of Sus for both
clean sand measured in the laboratory and plastic clayey silt measured
in-situ by residual vane shear testing. Unfortunately, fine coal refuse
is neither and judgment is required to assess which methodology to use.
The beauty of residual vane shear testing is that a material must have
significant plasticity or it is physically impossible to rotate the
apparatus 10 times to set up the test. Because vane shear testing is
common, many professionals possess the expertise to perform the test in
accordance with ASTM procedures, and the laws of nature inhibit
misapplying the test to anything but plastic material, vane shear testing
was selected for use in the conservative simplified method in 1992. Figure 7 is offered as validation of the previous MSHA decision to
approve that protocol. |
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Click on
images to
enlarge
Figure 13.
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| They present laboratory test results in Boston
differently than the way we normally recognize them in the coal-fields. I
have taken the liberty of including the site-specific data from
the technical paper referenced in the draft MSHA manual as Figure 14a.
If you rotate it 90
degrees and re-label the results, it will be more recognizable as shown
in Figure 14b. Voilà, the results range from phi = 3.4 degrees to phi = 15.1 degrees for samples of fine refuse
tested in the Boston laboratory. Considering the disturbance that is likely to occur in transporting high quality undisturbed fine refuse samples from the
coal-fields to Boston, and the potential for error in interpreting CPT data so field conditions can be accurately simulated in the laboratory, I
believe it is prudent to assess that Figure 14b provides further justification for using a design friction angle of 4 degrees as a
conservative estimate of Sus for fine refuse, regardless of its plasticity. |
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Figure 14a.
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Figure 14b.
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| Aha, but should the residual friction angle used in
the simplified method be lowered to 3.4 degrees? No, the technical paper
referenced in the draft MSHA manual reports that when that particular fine
refuse sample was remolded, reconstituted, and consolidated at known
confining pressures to reduce the potential for sample disturbance and
error from misinterpretation of CPT data, a residual friction angle of
10.2 degrees was measured as shown in Figure 15. |
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Figure 15. |
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But after presenting my findings to the former chief engineer who was in charge of a coal refuse disposal facility where the
Boston method was used to assess seismic stability, he told me that his former employer spent nearly a million dollars for that study. Hmmm, you
know, I’m just a poor old dumb engineer from the coal-fields, and who am I to question the experts on the need to evaluate
all existing coal refuse disposal facilities using the Boston method. After further consideration, I have decided not to challenge MSHA’s decision to
exclude the use of the simplified method even though a blind man can see that factors of safety calculated using the Boston method will exceed 1.2
for existing facilities where the MUSC and simplified seismic stability methods were used for design and construction.
CONCLUSIONS AND
RECOMMENDATIONS
Conclusions and recommendations from my review of
Chapter 7 are presented as follows:
1. Designers of coal refuse disposal facilities beware. Do not use the
method of seismic stability analysis proposed in the draft MSHA Design
Manual for actual design purposes. Instead, use a simplified method with a
residual friction angle of 4 degrees for the strength of fine refuse in
post-earthquake static stability analyses that provide factors of safety
greater than 1.0. In my opinion, the methodology proposed by MSHA in
its draft manual appears
academically sound for assessing the stability of existing structures and
should result in computed factors of safety well in excess of 1.2 for
facilities that are designed based on the simplified method. Please
remember that when you get your lab data from Boston for the purpose of
verifying the obvious,
turn it 90 degrees counter-clockwise, convert the relationship to a phi
angle as shown in Figure 14b, and you’ll know how to proceed from there.
Also, don't forget that the method is pronounced "Basston", or something like
that, and we certainly want to be politically correct. Breathe in, breathe out, start upstream construction as early in the life of
a project as feasible, and everything will be OK. Follow my
recommendations, don’t complain about Chapter 7 of MSHA’s draft manual, and
there will be plenty of work for all of us. Trust me, it will be just like
Christmas, every day for decades.
2. Other
stakeholders may have a different perspective and might want to take
exception to the methodology proposed in Chapter 7.
3. MSHA’s
attempt at improving the safety of dams built by the upstream method by
prescribing a stringent protocol for seismic analysis could have the opposite
effect. Examples are presented that document how beginning upstream
construction early in a project is the most important seismic design
consideration for maintaining dam safety during later stages. Based on past
experience, some designers and operators will delay upstream construction to
avoid the confusion of meeting these new requirements, resulting in the
development of undesirable hinge conditions during later stages as
illustrated by Figures 8 and 9... It’ll be déja vu all over again.
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