#21 Road Cave-ins

Japan has a long history of natural disasters.

Now, power of science is helping to over come them.

BOSAI: Science that Can Save Your Life.

In November 2021, the surface of a road in Mikasa, Hokkaido suddenly caved in.

A car traveling above fell into the hole, seriously injuring the three occupants.

No underground construction work had been going on there, and the accident occurred without warning.

Road cave-ins like this have also occurred one after another in Tokyo.

In June 2022, a cave-in occurred in Tokyo's Chiyoda City, forming a hole nearly 1.5 meters in length and width and 1.2 meters in depth.

According to the Ministry of Land, Infrastructure, Transport and Tourism, there are as many as 9,000 such road cave-ins a year across Japan.

Road cave-ins or sinkholes happen elsewhere, too.

This sinkhole is in New York City.

In Latvia, a car got stuck in a sudden cave-in.

In Malaysia, a huge sinkhole caused power to be cut off to the surrounding area.

What would happen if holes suddenly appeared on the roads we use every day?

Sinkholes can form without warning, and are a frequent cause of road cave-ins not only in Japan but also in cities around the world.

We'll learn why they occur so suddenly and examine the latest countermeasures to prevent them.

Dr. Reiko KUWANO of the University of Tokyo's Institute of Industrial Science is studying the mechanism of sinkholes.

Road cave-ins may look like a pit suddenly formed,
but in fact there's already a void in the ground.

The cave-in occurs when
the top of the void collapses.

Are there different causes?

Some voids form naturally.

Others are due to aging infrastructure, such as water or drainage pipes
that have become damaged,

resulting in spaces that
draw in sediment from above.

One of the most common causes of sinkholes is the drainage pipes that run underneath urban areas.

Cracks form in aging pipes, causing them to cave in.

There are about 3,000 cases a year
of road cave-ins caused by drainage pipes.

3,000 cases?
That's almost 10 cases a day!

How does a crack in a buried drainage pipe cause the road surface to cave in?

Kuwano gave us a demonstration.

What is this setup here?

It's a simulation of the ground.

We're doing an experiment by creating layers of soil
and a cavity inside these layers.

Here is video of the experiment.

These layers of soil contain groundwater, and we'll suppose a drainage pipe runs below them.

A plug is pulled out below, simulating a slight crack that has formed in the drainage pipe.

Then, sand containing water is sucked into the crack, and a small cavity forms.

At this point, the width is so narrow that the arching effect acts on the cavity's upper part

and the outflow is stopped.

When a cavity is created, the top is subjected to an arching effect.

The soil pushes against itself, supporting itself against the pressure from above.

However, as groundwater enters the cavity and the hole widens, the arching effect weakens.

The top of the cavity collapses, and the resulting void spreads upward.

As the void continues to expand towards the ground surface...

It suddenly caves in all at once.

We're viewing it from the side,
so it looks like a big hole, but from the surface,
you can't see it at all.

So you wouldn't know it's there
until it caved in.

It's terrifying.

Underground infrastructure built during periods of rapid urbanization starts to age after 20 to 30 years, and the number of road cave-ins is expected to increase.

We now know the risk factors that spur the formation of voids in the ground and trigger road cave-ins.

This graph shows the number
of road cave-ins by month.

They are remarkably high in June, July, and August.

Why do you think this is?

June, July, and August are the hottest months.
Does the heat have an effect?

Heat and rain are the two factors.

During the warmer months, road asphalt heats up and softens,
making it easier for sinkholes to appear.

In the accident in Tokyo we mentioned earlier, it's thought that the asphalt softened and caved in due to intense 35-degree heat over several days.

Rainfall, which often occurs at this time of year, is also a major factor.

Groundwater increases as rain accumulates underground.

Soil then flows more easily and the top of the void is more likely to collapse, triggering a sinkhole.

Earthquakes further increase the risk of sinkholes.

Many roads caved in during the Great East Japan Earthquake in 2011.

The September 2018 earthquake in Hokkaido, caused a massive cave-in spanning 4 kilometers in the city of Sapporo.

This is an experiment to see how an earthquake affects a cavity.

Soil slides down into the cavity, causing it to widen and the surface to cave in.

Past studies have shown a clear increase
in the number of cavities and cave-ins after tremors with a seismic intensity of 5 or more.

Japan experiences heavy rains and earthquakes all over the country.

Urban areas that are full of underground infrastructure are particularly at risk of major sinkholes under roads.

The only way to protect against them is to quickly detect and deal with underground voids.

This is why efforts are underway to prevent sinkholes by utilizing the latest technology.

One Japanese company is using the latest technology to prevent sinkholes.

Surveys are conducted using specialized vehicles equipped with microwave radar.

How do you detect cavities?

This white and red striped unit
contains sensors that emit microwaves.

Microwaves are directed underground
and we collect the ones that bounce back.

How deep can this detect cavities?

3 meters, since it's state-of-the-art.

That's quite deep.

I rode along to observe a survey in action.

While in motion, the vehicle collects both microwave and GPS data, and also captures images of its surroundings and the road.

The data collected is then later analyzed to find cavities or voids that are at high risk of becoming sinkholes.

How do you know when a cavity is found?

Here's an example of a typical cavity signal.

It's the wedge or mountain-shaped part.

This data shows that there is a void just 20 centimeters below the surface.

This area was actually excavated.
Here's what it looked like.

It's quite extensive.

Thanks to the analysis system that accurately found the void, a sinkhole was prevented.

This company has surveyed 250,000 kilometers of roads and found more than 110,000 cavities.

The depth and extent of the cavities are collected into data and prioritized according to the immediate risk of a sinkhole forming.

This is, so to speak, road triage.

Shallow and large voids in particular
require immediate repair because they're really dangerous.

Smaller or deeper voids
will be checked on another survey to see if they have become larger or shallower.

Where the risk is deemed high, a small hole is made in the ground and a compact camera is placed inside the void to check its condition.

In some cases, the damaged drainage pipe is also examined and monitored on a regular basis.

The company has also developed technology to create a 3D map of infrastructure underneath roads.

This cutting-edge ground-penetrating radar is used to collect the data.

It collects data at a higher resolution
than the vehicle.

This allows us to visualize in 3D
the various lifelines underground, such as pipes for water, electricity, and gas.

Equipped with multiple microwave-transmitting antennas, the system scans the ground to collect detailed data at intervals of 1 centimeter in depth,

5 centimeters in cross-section, and 8 centimeters in longitudinal section.

Using this data, the bends and overlaps of buried pipes can be identified.

This enables the creation of detailed 3D maps of underground infrastructure.

If we find a void with this data, we can guess which pipe is causing it and why,
depending on its type and underground location.

When a void is found, workers dig it up, repair any damage to the pipe, and then backfill the area with soil.

And now with this 3D map, they can accurately plan that repair work well in advance.

The company is also developing technology to assist when filling a void: Augmented reality glasses that allow you to see the 3D map in real time.

- Do you want to try them on?
- May I?

Oh! These lines here?

Someone wearing these smart glasses
can see what it's like underground in 3D.

There's a blue pipe here.

Blue is for water pipes.
Green is for gas pipes.

I can see the water and gas pipes, and way over there,
some pipes crossing each other.

What is that yellow pipe?

That's an unknown pipe.

It isn't recorded in any ledger or data,
so we don't know who manages it.

Knowing this before construction
allows us to make various plans, so such information is also input here.

This technology is used to check what is located where before proceeding, so work can be done safely and efficiently.

The company says it's gradually starting to be used.

It's clear that to prevent sinkholes from destroying roads, we need to identify voids early and address those that are most at risk of collapsing.

I'm really grateful that there are professionals working to prevent sinkholes, and hopefully technology will continue to develop.