The Transcona foundation failure baffled engineers for more than 40 years.
Prior to construction, the soil had been successfully tested with nearly twice the
weight under which it collapsed. Even after the failure, the investigation confirmed
that the design assumptions were correct. Everything indicated that the underlain
soil should be able to comfortably bear the structure at 100% capacity. Furthermore,
similar structures in the area had existed for many years with no foundation problems at all.
What could have possibly gone wrong here? The failure remained a mystery for another
40 years [1]. In meanwhile, new soil mechanics theories were developed and finally in 1951 a new
investigation was launched which explained what had gone wrong. In this video, we will dive deeper
into foundation design principles and explain them intuitively with a simple experimental model. By
the end of the video you will be able to explain the critical design considerations of foundations
and why this grain elevator failed. Something that the engineers could not do only a century ago.
Let us first start with some basics. Shallow foundations transmit the applied structural
loads to the near-surface soils. The main goal of a foundation is to safely distribute the loads
from the structure into the ground. The word distribute is key because in almost all cases
the gravity loads coming down the columns would punch right through the soil unless
they are properly distributed over a larger area. This is exactly what the foundation
does. A foundation spreads an otherwise concentrated force onto a larger area.
In doing so, the foundation induces compressive and shear stresses in the underlain soils.
During the late 19th and early 20th century, engineers realized that foundation design could
be based on the contact pressure between the footing and the underlain soil. This is exactly
what was done at the Transcona site. When the excavation reached its design depth of 3.7m,
field-bearing capacity tests were carried out. A field bearing test consists of loading a plate of
predetermined area until the soil under it starts to excessively settle or heave around the plate.
Several tests were performed at the foundation level and all of them carried a pressure of at
least 400 kPa (58 psi) [2]. This means that a plate of 1 m2 could carry more than 40
000 kg (40 tons) before the soil begins to fail. With this knowledge, the bin house was designed
to occupy an area of 23.5 by 59.5 meters where a total of 65 grain bins were placed on top [2]. At
full capacity, the structure together with the grain was designed to weigh around 42 500 tons
which meant the distributed pressure to the soil would be around 300 kPa (43.5 psi) well
below the 400 kPa failure load repeatedly obtained from the plate loadings. In reality,
the structure failed at 87.5% grain capacity with an estimated bearing failure pressure
of 296 kPa an error of more than 25% [3]. To be able to understand what went wrong we need
to understand what happens under the plate as its being loaded. The vertical stress radiates under
the foundation with the load decreasing in value as it goes deeper. This makes sense since as the
load travels deeper into the ground it is carried by a larger area. This vertical stress pushes the
soil grains down which want to relief the pressure by displacing to the side. This mechanism creates
shearing stresses, which as you know from the previous videos in this series, are the reason
why soils fail. This theory was developed decades before the failure at Transcona. What
was not known was the bearing failure mechanism which was first explained by Terzaghi in 1943,
30 years after the failure at Transcona [4]. Terzaghi realized that when the soil slips it
does so at an angle which depends on the friction angle of the soil.
The soil near the corners of the foundation is subjected to the highest shearing stress and
fails first. The displaced soil from under the foundation has to find itself an exit,
causing a ground heave in the vicinity of the structure. The movement of the soil causes a
slip plane to occur whose geometry depends on the properties of the soil and size of the foundation.
This failure mechanism is well illustrated with a simple model.
The straws in this case represent soil particles that get interlocked to form a block of soil.
Representing the soil particles as long tubes helps us achieve a plane strain condition which
in simple terms means we can look at the problem in 2D. In reality, all failures are 3 dimensional
but engineers often analyze them in simplified configurations that allows 2D simplifications.
Notice how as the bottle is being filled, or the structure being constructed ,the added
weight causes the particles to start rearranging. At a particular point, when the weight of the
structure exceeds the friction force between the particles, or in this case the straws,
a slip plane forms starting from the corner of the foundation, exactly like Terzaghi predicted.
Another important observation, in line with Terzaghi’s theory, is the triangular soil block
that is formed under the foundation and displaces the rest of the soil causing soil bulging on the
surface of the terrain. When it comes to soils, there is always a factor of uncertainty. This
means that it is nearly impossible to replicate the same failure twice. The slipping always
happens at the line of least resistance which is different for every experiment and doesn’t
follow a nice curved trajectory. In fact, the failure plane predicted by Terzaghi is very
theoretical and rarely achieved but it is a good approximation of the overall failure mechanism.
Now with the slipping plane defined, Terzaghi calculated the forces needed to overcome the
resistance of the soil at the slipping plane which give the bearing strength of the soil.
These forces are the interlocking of the particles and the stickiness or cohesion of the soil.
Notice that the length of the slip plane depends on the size of the foundation. In other words,
larger foundations scoop out more soil underneath them resulting in a longer failure plane and
therefore higher resistance of the soil. Burying the foundation further extends the slip plane and
provides confining pressure to the soil under and near the foundation that makes heaving
out more difficult. This is the main reason why foundations are often built below surface level.
Another failure mechanism that often governs foundation design is settlement. Settlement
is arguably a much more problematic issue which is often the reason for using deep foundations
such as piles. Let us know in the comments if you know of any popular settlement failures
and we might do a similar video on that case With this new knowledge of bearing capacity,
let’s have a second look at the Transcona failure. When the engineers performed the bearing capacity
tests they used plates of around 30 to 60cm in diameter. Based on the Terzhagi failure model,
this meant that they were only testing the soil in the near vicinity of the plate. The
actual foundation was 23.5 m long which meant the failure line would go much, much, deeper. To make
this scenario even more unfortunate, a separate boring investigation was launched in 1951 which
found that at around 7.5m below the foundation a weaker clay layer existed with half of the shear
strength of the upper layer. In terms of its colour, grain distribution and mineralogical
content the weaker clay looked almost indistinguishable from the stronger upper layer.
The wash-borings in 1913 provided only a visual assessment which mislead the designers into
thinking that the underlain soil is homogeneous. Based on the stress contours we saw earlier,
the induced stress from the full-scale foundation would have induced significant stresses in the
weaker clay. These stresses manifested in consolidation of the lower clay layer which
resulted in an initial settlement of the structure and a full bearing failure shortly after.
Looking at the bearing failure plane, it is clear that a significant length of this
plane is within the weaker layer which had poor shear resistance. In comparison,
the plate loading did not come even close to loading this layer which gave a false sense
of reliability and strength. It may be unfair to blame this failure on lack of knowledge because
if the site investigation had been properly conducted and the weak layer identified. The
engineers may have included an additional safety factor that would have prevented this failure
Within days from the failure, the bins were holed at the bottom and the thousands
of tons of grain were safely extracted out. Within 12 months from the failure,
the bin house was back into an upright position.
The righting of the bin house itself is an amazing engineering feat which we will have to leave for
another video. The Transcona Grain Elevator continued to provided relief for the Winnipeg
Yards during the months of peak grain-shipment for more than 100 years until it was decommissioned
in 2021 [5]. Thanks for watching, subscribe if you enjoyed the video and see you next time.