- [Narrator] Big Ben (energetic electronic music)
is tilting to the north
and keeps leaning about a millimeter a year.
The 58 story Millennium Tower in San Francisco has sunk
over a foot since it opened in 2009 according to a lawsuit.
And the former construction manager for this 57 story,
77 million building in Manhattan
said it's leaning about three inches.
Its developer says it's structurally stable.
Still, the building sits unfinished.
The Millennium Tower has undergone a 100 million
construction project to fix the unanticipated
movement according to its owner.
These issues all have one main cause, the ground.
So to build taller buildings on imperfect soil,
developers are using an old technology for a new purpose.
It's called deep soil mixing.
- I think it has been a game changer,
completely game changer.
- [Narrator] But there are risks.
Here's how it works.
(energetic electronic music)
The incredible weight of skyscrapers
requires a strong foundation to stand on.
In Miami, Andres Baquerizo
is helping to build the city's first super tall skyscraper,
the Waldorf Astoria Hotel
and Residences Miami on ground that looks like this.
Layers of porous limestone and sand.
Not ideal for a 100 story tower.
So Baquerizo and the team are literally
changing the earth using deep soil mixing.
- At the end,
what you end up doing is improving the capacity
of the soil to withstand better loads
and support buildings or different conditions.
- [Narrator] To do this, workers bring
in a high torque or really powerful drill rig.
This big blade carves out wells in the ground
that can be up to 110 feet deep.
Here, they'll be around 40.
- [Andres] Imagine your kitchen blender, but in this case,
it's a nine to 10 foot diameter blender.
This blender has some blades that
are going around the ground
and is shooting cement through those blades.
- [Narrator] Then cement mixes with the soil
to form a super strong slurry called soilcrete,
giving the soil a higher load capacity.
- So what you end up taking
is a soil that would only have
a certain amount of capacity,
and you'll increase the compression capacity,
the lateral capacity on it
and you're gonna make it really stiff,
making it stiff and almost impermeable.
- [Instructor] These wells are arranged
in an overlapping grid,
so that all of the soil is properly mixed.
- Every one of these cells, right?
That little circle there that seems little
is really between nine to 10 foot diameter.
- [Narrator] It also makes the ground
almost impervious to water,
which at waterside properties like the Waldorf Astoria,
means you don't have to fight the tide.
- Before, when you were having the water being run,
it's a race against the water to when you can do the work
if you're gonna have a high tide, low tide.
- [Narrator] Deep soil mixing
can also help buildings withstand natural disasters
like earthquakes and the hurricanes
seen in Miami when wind can cause problems
for super tall buildings.
- [Narrator] What happens typically with winds
is that it will push the building to try to tilt
and when it's tilting,
it's literally push the building sideways.
But more importantly, in the foundation,
if you realize if it's going this way,
one side is pushing and the other one is lifting.
So now you have a tension condition
and a compression condition.
If you increase the capacity
of the piles to not be bent laterally
and increase the compression capacity,
and a little bit of the tension,
it's gonna end up helping the building to be more stable.
- [Narrator] Deep soil mixing's
ground-hardening abilities not only make a building
like this possible, they make it more profitable.
For example, developers have been able
to add more high floor condos
than they would have if the building was shorter.
They'll start at $10 million each
and will look like these renderings.
- More people are willing to say, you know what?
It's interesting, let's go for it.
So you're beginning to see this increase
of the size of the city
and the height of the buildings.
- [Narrator] The construction company building
the Waldorf Astoria Tower is now using deep soil mixing
for shorter residential projects
across the country and around the world.
They've used it here, here,
and here to prevent soil shifting under skyscrapers,
and that's just one company's projects.
Other companies also have projects underway
across the globe,
but the method comes with some risks.
- So if a building was demolished to be able
to put a new building and they left anything made by man,
the big blender is coming into the ground
and shooting cement will hit this obstruction
and may shift out of location or damage the tool.
If you get shift out of location
and you don't realize that it happened,
then during the excavation process,
you wind up getting some water intrusion.
- [Narrator] That's when water flows into the excavation.
A small amount is normal, but any flow larger
than a garden hose can be hard to fix.
Take a nearby Miami project, UNA Residences,
as an example.
In March, 2022, water entered the unfinished foundation
during deep soil mixing,
creating what looked like a lagoon
and concerning residents.
The developers said that construction is back underway
and some land like beach sand is just too soft
for deep soil mixing.
If the technique is used on soil
that's not quite right,
the layer of soilcrete can crack.
- When it begins to break,
the difficult thing is, in most cases,
you almost have to start from scratch,
because if you already found you have a weakness,
it's not something you can reinforce
as the water will always want to come in.
- [Narrator] Previously, deep soil mixing was mainly used
for industrial sites like oil refineries
where the land needed to hold heavy equipment.
- Miami was the first city, where at some point,
the industry realized that this condition
could help us improve the capacity of onsite soils.
And from Miami, there has been developing
is now beginning to go into other places of the world.
- [Narrator] But due to the unique engineering challenges
that Miami's sandy soil presents,
Andres's team of engineers
are paying close attention to the construction.
The progress the team makes here will help
inform the company's engineers around the globe
on how to make more stable ground for their buildings.
- As we use it more,
we have been able to understand better the system
to keep using their benefits
into the benefit of the buildings.
So for me, has been a game changer.