Guest Post Written by James L. Gordon, P. Eng. (Ret'd)
Marine
clay, commonly known as “quick clay” and as “sensitive clay” by geotechnical
engineers is clay deposited through salt water where the particles pick up
salt, which alters the properties of the clay. The clay becomes “sensitive”
since it has a propensity to liquefy when disturbed or saturated. It has been
avoided by dam engineers due to its sensitivity.
Wikipedia
defines marine clays as – “Marine clay is a type of clay found in coastal
regions around the world. In the northern, de-glaciated regions, it can
sometimes be quick clay which is notorious for being involved in landslides.
Clay particles can self-assemble into various configurations, each with totally
different properties. When clay is deposited in the ocean the presence of
excess ions in seawater causes a loose, open structure of the clay particles to
form, a process known as flocculation. Once stranded and dried by ancient
changing ocean levels, this open framework means that such clay is open to
water infiltration. Construction in marine clays thus presents a geotechnical
engineering challenge.”
This
explains the controversy surrounding the North Spur dam at Muskrat Falls since
it will be the first hydro dam built on a quick clay foundation.
However, I
have come to the reluctant conclusion that the North Spur dam is not safe and
there is no easy way to make the dam safe. This conclusion has been arrived at
by applying some logic to the situation as outlined in the following.
Until
recently, dam design has been based entirely on precedent, since there was no
mathematical procedure available to determine the safety factor.
Dam design
began in 1857 with a paper by Professor Rankine titled “On the stability of
loose earth”. Since then hundreds of scientists working at many universities
have investigated the properties of soils and rocks in an effort to arrive at a
methodology for calculating a dam safety factor. A breakthrough did not happen
until about 1955, when the concept of a slip circle on the downstream face was
conceived by A. W. Bishop and a methodology developed to determine the
stability of the slip. It became known as the “limit equilibrium method”.
However, it took some time before the methodology became generally accepted.
 |
| Jim Gordon, P. Eng (Retired) |
During this
time, in 1968, I attended a short dam design course at Berkeley, where a
procedure for determining dam safety was developed by Professor Seed. The
process was to build a model of the dam on a shaking table, and gently shake it
if there was no earthquake, and more violently if there was. If the dam slopes
remained intact, then the dam was safe. However, the geotechnical community did
not follow this development since there were issues associated with calculating
the reduced size of rocks, gravel, sand and clay used in the model.
It was the
use of computers that helped solve the problem. Calculating the stability of a
slip circle was a laborious and tedious process, since many slip circles had to
be investigated to arrive at the circle with the lowest factor of safety. The
computer solved this problem, and with the use of more sophisticated programs
such as FLAC designed to calculate the stability of many slip circles, the
minimum factor of safety of 1.5 could be easily determined.
The safety
factor is simply the ratio of the failure force divided by actual force.
Perhaps best explained is in the case of a pipe, where the bursting pressure is
divided by the operating pressure to obtain the safety factor.
But how was
a factor of safety of 1.5 arrived at. It was the spectacular failure of the
Teton Dam in 1976, which energised the geotechnical community to determine an
acceptable safety factor. The Canadian Dam Association was founded in 1986,
with the prime purpose of developing safety standards. This was accomplished in
1995 after many years of work by a dedicated group of geotechnical engineers. A
safety factor of 1.5 under normal loading was considered to be acceptable, and
1.3 under an unusual loading such as an earthquake.
But why
1.5, why not a higher factor of safety. At 1.5 the dam factor of safety is the
lowest of any other component in a hydro development. For example, the pipe
carrying the water from the intake to the powerhouse has a factor of safety of
3.0 to the bursting pressure - the allowable maximum working stress being 1/3
of the ultimate stress. The wire rope that lifts the generator has a factor of
safety ranging between 4 and 8. For iron castings it is 8.
The
relatively low factor of safety in a dam reflects the geotechnical community’s
confidence in the calculation methodology and determination of soil/rock
properties based on over 170 years of research work. However, all this research
has been undertaken on non-marine clay and other materials, hence the same
safety factors cannot be applied to dams founded on marine clays.
Research
into the properties of marine clays has only recently been undertaken by a few
scientists, with the sole objective of determining the safety of quick clay
deposits – are they liable to liquefy, and hence only be suitable for farming,
or are they stable, allowing the construction of permanent buildings such as
housing. Absolutely no research has been undertaken on the safety of dams built
on quick clay deposits.
NALCOR’s
engineers have developed a design for the North Spur dam using the FLAC program
which has been shown to give incorrect results both by Dr. Bernander and Dr.
Locat when used on marine clays.
In Dr.
Bernander’s thesis he states (Page XXII) “Landslide hazards in long natural
slopes of soft sensitive clays may – on a strict structure-mechanical basis –
only be reliably dealt with in terms of progressive failure analysis. There
exist, for instance, no fixed relationships between safety factors based on the
conventional limit equilibrium concept and those defining risk of progressive
failure formation. In consequence, the safety criteria have to be redefined for
landslides in soft sensitive clays”. In other words - conventional safety
factors are not applicable to sensitive clays.
And from
Page XX in the Bernander thesis - Considering deformations and strain-softening
in the assessment of slope stability normally results in a higher computed risk
of slope failure than that emerging from the conventional ideal-plastic
approach, depending in particular on the nature and the location of the applied
additional load. In other words the FLAC approach currently used in
conventional dam safety analysis is not correct.
In the
abstract for the lecture titled “Spreads in Eastern Canadian Sensitive Clays”
by Dr. Locat recently presented at
Memorial University, Dr. Locat states “Based on witnesses, spreads (landslides)
generally occur rapidly, without any apparent warning sign, and cover large
areas (> 1 ha). In addition, conventional stability analyses give too large
safety factors when applied to this landslide type. Spreads are therefore
serious threats to population and infrastructures on sensitive clays and the
need for tools enabling their prediction and mitigation is quite necessary”.
Again, regular stability analysis in not applicable to marine clays.
So, if the
FLAC stability analysis cannot be used, can the old method of precedent be
invoked, neglecting for the moment that there is no precedent for a dam founded
on marine clays, and assuming the North Spur is founded on a soft non-marine
clay.
Why soft
clay? Over a month ago, I had the opportunity to discuss drilling on the North
Spur undertaken in 2013 with the mechanic operating a vibrating drill rig. What
he told me is not at all reassuring. He mentioned that on several occasions,
the casing would slowly descend under its own weight. On other occasions it
would drop suddenly by about 4 ft. On one memorable occasion, when the casing
was left protruding some 20ft above the earth at the end of the shift, on
returning the next morning, it had disappeared and was found some 20ft below
ground. It had descended 40ft under its own weight overnight. All this
indicates a soft to very soft foundation. Samples obtained from the drilling
were placed in core boxes, now stored on site. The logs are also stored in
NALCOR site files.
Figure 1 – North Spur modified downstream slope.
SNC and MWH
have developed a design for the downstream slope of the North Spur with a 1:8
slope from El. 25m to water level at about El. 2m, which requires a horizontal
slope length of about 184m. Above El 25.0m to berm at El. 40.0m, the slope is
at 1:3, requiring a horizontal slope length of 45.0m. Above the berm at El.
40.0m and on to top of Spur, the slope is 1:2.5. With top of spur at about El.
64m, the horizontal distance from berm to top will be about 60m. Allowing for
three berm thickness at 12m each, the total horizontal distance from the shore
up to the crest is then about 326m. Source – Poster presentation by “Lower
Churchill Project Geotechnical delivery team” October 28-30, 2013. All as
illustrated in Figure 1.
At a total
horizontal distance from shore to crest of 326m, the downstream work will cover
over half of the Spur’s thickness of 570m at the narrowest section. Current
work on the downstream face is shown in Figure 2. Normally, the upstream face
of a dam has a flatter slope than the downstream face due to the lower friction
from water lubrication of the particles. With over half of the Spur thickness
taken up by the downstream slope, there is insufficient room for the flatter
upstream slope.
Figure 2 – re-shaping work on the
downstream face. August 2016.
A Google
Earth view of the North Spur is shown in Figure 3, marked up to show the
location of landslides. It is interesting to note that the large 1978 landslide
extended upstream to the middle of the Spur, not a very reassuring development,
since it could happen again.
Figure 3 - Source – North Spur
Stabilization Works Progressive Failure Study Figure 1-1.
Aerial photo of the North Spur. SNC-Lavalin 21 Dec. 2015.
One of the
simplest measures of the dam stability using the precedent analysis, is the
ratio of base thickness to dam height. For a dam founded on rock, this ratio
can be as low as 3.5, but the ratio increases rapidly as the foundation
material becomes softer, and more so as the dam height increases. For the North
Spur, with crest at El. 64.0m, and a base thickness of 570m, the ratio is
570/64 = 8.9. The question of a precedent then becomes – what is the ratio for
a dam of similar height founded on a soft clay foundation.
The base
thickness is the actual thickness of the dam at the contact with the foundation
from upstream to downstream, as shown in Figure 4. For example, a 10m high dam
on bedrock could have an upstream slope of 2:1 for a horizontal length of 20m.
The downstream slope could be 1.5:1 for a downstream horizontal length of 15m.
Neglecting the crest thickness, the base thickness is then 20+15 = 35m, and the
thickness/height ratio is 35/10 = 3.5. For dams on softer materials, such as
deep deposits of clay overlying bedrock, the side slopes are much flatter,
resulting in higher thickness/height ratios. Also, as the height of the dam
increases, the weight of the dam increases, requiring the slopes to become even
flatter, again increasing the thickness/height ratio.
At Muskrat,
the problem is even more complex due to the layers of quick clay within the
body of the natural dam. Theoretically, this will require some further
flattening of the slopes, but the effect has been neglected in the precedent analysis.
Figure 4. Thickness to height ratio.
For
Muskrat, fortunately there is a precedent in the Gardiner Dam on the South
Saskatchewan River in Saskatchewan. It is also 64m high and founded on soft
clay. There the base thickness is 1,500m, for a base-height ratio of 1,500/64 =
23.4, or 2.6 times the North Spur ratio. Thus precedent indicates that the
North Spur dam cannot be stable when founded on soft clay.
Figure 5 - Gardiner Dam.
64m high, base thickness 1,500m. Thickness/height ratio = 23.4
A
photograph of the Gardiner Dam is shown in Figure 5, where the flat downstream
slope is clearly evident. In fact, the slope is so flat, that it is rented to a
local farmer as a hay field!
This
analysis can be criticised as being incorrect, since the North Spur dam crest
could be cut down to El 45.0m, requiring a much shorter base thickness, since
the thickness ratio is also a function of the height for a dam on the same soft
foundation. Fortunately, there is precedent for this in the Rafferty Dam, also
in Saskatchewan, which has a height of
only 20m, and a base thickness of 278m, for a thickness/height ratio = 13.9. If
it is assumed that the thickness/height ratio is a linear function of the
height for dams on the same type of foundation, a 45m high dam on a soft clay
foundation would require a thickness/height ratio = 19.3, for a base thickness
of 869m. This is considerably wider than the Spur thickness, hence there is
insufficient room for a 45m high dam with sufficiently flat side slopes to be
stable.
This
analysis has indicated that –
1. Dam stability analysis using conventional liquid equilibrium methods cannot be
applied to dams on marine clays.
2. Safety factors developed for dams on non-marine clays cannot be
applied to
dams on marine clays.
3. There has been no research into the stability of dams founded on
marine
clays.
4.
The North Spur foundation consists of soft to very soft marine clay.
5. There is no precedent for a dam founded on
a soft clay foundation with
the steep slopes shown for the
North Spur, where the thickness to
height ratio is only 8.9.
6.
Based on precedent, the thickness to height ratio for a 45m high dam
on the North Spur has to be at
least 19.3 for a base thickness of about
870m.
7.
Based on precedent, the North
Spur with a 570m thickness, has is
insufficient thickness to construct a 45m
high dam with safe side
slopes.
Conclusion
– the dam design developed for the North Spur is just not acceptable, and I
hope that this analysis will be proved to be incorrect by a panel of
international experts which should be convened immediately to resolve this
issue.
- Jim Gordon, P. Eng. (Retired)
_________________________________________________________________
Editor's Note:
Jim Gordon has authored or co-authored 90 papers and 44 articles on a large variety of subjects ranging from submergence at intakes to powerhouse concrete volume, cavitation in turbines, generator inertia and costing of hydropower projects. He has worked on 113 hydro projects, six of which received awards "for excellence in design" by the Association of Consulting Engineers of Canada. He was also awarded the Rickey Gold Medal (1989) by the American Society of Civil Engineers "for outstanding contributions to the advancement of hydroelectric engineering...". As an independent consultant, his work assignments have ranged from investigating turbine foundation micro-movements to acting on review boards for major Canadian utilities. He has also developed software for RETScreen and HydroHelp.