At a city's sewer construction site, a quicksand condition developed,
pulling a worker into the ground.
This is what happened.
A new lift station was being built.
Lift stations facilitate the flow of wastewater by moving it from a lower to a higher elevation.
The lift station was to be installed about seven metres below grade.
The original engineering drawings for the project proposed an open cut excavation
to a depth of just below the final construction.
The city commissioned a geotechnical report — Report 1 — for the project,
but then put the project on hold.
Years later, when the project was restarted, the city commissioned Report 2 —
an updated geotechnical report — that took into account Report 1.
To determine the soil layering, three boreholes had been drilled to 6.1 metres for Report 1,
and one borehole to 9.1 metres for Report 2.
The reports gave the impression that down at the base of the proposed lift station
there would be a substantial layer of silt, and just above it, a layer of coarse sand.
Groundwater was found at about 2.3 metres, making removal of water from the site,
or what is known as "dewatering", necessary.
The city provided project information — including Report 2 — to Firm A, the prime contractor.
The planning and construction decisions were based on the understanding of the
subsurface conditions provided by Reports 1 and 2.
However, the boreholes had not gone deep enough.
The data from them was inadequate for creating an effective dewatering plan.
According to "Groundwater Lowering in Construction: A Practical Guide", boreholes should penetrate
to a depth of 1.5 to 2 times the depth of the excavation.
Based on the reports provided, Firm A conceived a dewatering and shoring concept
using an open steel caisson, instead of going with an open cut to full depth.
An open cut was made to about two metres — just above the groundwater.
Then a caisson, made up of three ring-shaped sections, was driven into the soil.
Its walls created a barrier to prevent groundwater and soil from entering the sides of the caisson.
The silt layer, being semi-impervious, would reduce groundwater penetration from below.
In addition to the caisson, the dewatering system had three other elements:
- Dozens of dewatering well-points for pre-drainage, which is the lowering of groundwater levels before excavating.
- Two slotted steel pipes that were installed six metres deep for additional pre-drainage
and to allow monitoring of the groundwater level.
- And two submersion pumps placed in the caisson to remove groundwater that leaked into it.
The dewatering system pumped ground water from the site continuously, at high volume.
On the day of the incident, most of the soil had already been removed from inside the caisson.
The submersion pumps were being run to lower the water level inside it.
Crushed stone around the caisson provided a good walking surface.
Worker 1 had been adjusting the pipes for the submersion pumps.
Afterward, he saw that the water level had gone down to almost the bottom of the caisson.
He took a step,
his foot sank, and then he was pulled into the ground.
Workers 2 and 3 saw the water in the caisson start to "boil" and that the level of the water was rising quickly.
They grabbed Worker 1's hands and arms.
Worker 3 began to be pulled down.
He released his hold on Worker 1, got out of the collapsing area, and then tried shovelling material away,
but it kept collapsing into the hole.
He then called 911 and went to shut down the pumps.
Worker 4 arrived.
Minutes later, the top of Worker 1's head was about a metre below the surface.
A lifting strap was looped under Worker 1's arms and tied to the dewatering manifold.
Completely immobile, he was stretched between the strap and the weight of the material pulling him down.
Eventually, he was able to wiggle his feet out of his boots and was freed with the help of the other workers.
What led to this incident?
An aquifer is a naturally occurring underground layer of rock through which water can easily flow.
Underneath the project area, there was not just one, but two aquifers — an upper aquifer and a lower aquifer.
Unlike the upper aquifer, the lower aquifer was under pressure.
The boreholes from Reports 1 and 2 had detected the upper aquifer, but they hadn't gone deep enough to detect the lower one.
As a result, the dewatering system was too shallow to reduce the water from the lower aquifer.
Both aquifers had been identified in a third report — Report 3 —
but the city had failed to provide this report to Firm A.
As the soil was removed from inside the caisson, a proportional amount of holding weight
was removed from the soil at the bottom of the caisson.
As this process continued, groundwater infiltrated the caisson, replacing the volume of removed soil.
The water temporarily provided replacement holding weight for the lower-layer soils
and also equalized the hydrostatic pressure between the caisson interior and the groundwater.
Right before the incident, the water was pumped out of the caisson.
This removed the holding weight and increased the hydrostatic pressure differential
between the outside and the inside of the caisson.
The pressure differential contributed to what happened next.
Several potential mechanisms, or a combination of mechanisms, may have led to the quicksand condition.
One potential mechanism is that the caisson never fully penetrated the silt layer on the east side,
possibly because the layer was higher on the west side than the east, and that the coarse sand layer
bore the full force of the pressure differential deep below where the worker was standing and it rapidly let go downwards.
Alternatively, the silt layer may have heaved up, releasing pressurized water from the lower aquifer.
Finally, the whole ground system may have been destabilized from the washing out
of fine soil grains during the aggressive dewatering activities.
Although the exact mechanism that resulted in the quicksand condition couldn't be determined,
the differential between the hydrostatic pressure inside and outside the caisson was present in each case
and was likely a factor.
Without the help of his co-workers, Worker 1's situation could have turned out much worse.
His boots remain underground to this day.
Whether your construction project is a lift station, a high rise, or other structure
that depends on stable soil conditions, know that the movement of groundwater can create potentially deadly hazards.