Welcome curious minds to the Deep Dive.
I want you to picture something with me for just a moment.
Think about a quiet, maybe tree lined St. somewhere in South
London. You know the ones, Rows of those
beautiful Victorian terrace houses, all red brick, those
characteristic shallow foundations we hear about lovely
tiled roofs. It's just, well, it's a classic
picture of history, isn't it? Now let's zoom in.
Imagine two of them standing right next door to each other.
Been there for over 100 years. Looked pretty much identical.
Same break, same period, same kind of front garden.
Maybe one of these houses. It's standing perfectly
straight, the brickwork flawless.
A real testament to how well they built things back then, you
know, Looks solid as a rock, but its neighbor just a few feet
away tells a comletely different story.
You can see it clear as day, this distinct jagged crack
running right U above the back door, sort of diagonally across
the bricks. It's like a scar, really.
And here's the thing that gets you thinking.
They're on the exact same soil, this London clay we hear so much
about. They've had the same weather,
same rain, same hot summers, year in, year out, built in the
same era, probably by the same team even.
So why? Why is 1 pristine looking, you
know, perfectly resilient while the other one's got this obvious
wound? What explains that difference?
What are these hidden forces doing today?
On The Deep Dive, we're plunging into exactly that, the often,
let's say, misunderstood world of clay soils, specifically
London clay, and the impact it has, sometimes quite
dramatically, on buildings. Our mission really is to unpack
the myths from the realities about these expanding soils,
give you a kind of shortcut to understanding what's a really
complex issue, but one that affects so many homes, not just
in London, but anywhere you find these reactive soils.
So the core questions we're asking are, can clay really lift
a whole house? And what truly makes some
buildings move so much more than others, even when they look so
similar? And this really does matter to
you, the listener, Even if you're not living in one of
those lovely Victorian terraces. Think about it, if you own any
property or you're thinking about building an extension,
maybe even just buying a house, what's happening underground,
often silently, is affecting its structure, its health, its long
term value. Understanding how soil behaves
isn't just for engineers. You know it's, well, it's pretty
crucial for any homeowner. It could affect your garden
choices, how you manage drainage, even if you spot
potential issues when you view a property and if you're
renovating. Or honestly, if you're just
curious about these hidden forces beneath our feet, this
should be fascinating. We're aiming to shed some light
on those things that seem inexplicable, like why does that
door stick only in summer? Or where did that crack come
from? Hopefully you'll have a few aha
moments because it's not just about cracks on the surface.
It's about this really intricate dance between huge natural
forces and the structures we build, and even tiny persistent
movements, the kind you might just ignore.
They can add up to have really significant long term effects.
They tell a story. OK, let's get into it then.
First big question, and it seems simple, but maybe it's not.
How heavy is a house really? I think of most of us when we
picture a house, we imagine something incredibly heavy, like
hundreds of tons, maybe immovable.
It feels like this giant rooted thing, right?
And that idea, that perception of just immense weight, it
really shapes how we think about what could possibly move it.
You think no way can the ground lift that?
It's a totally natural thought. They feel so solid.
Dumb thing, permanent. But let's actually put some
numbers on it based on what the sources say.
We're focusing on a typical mid terrace built around 1900 in
London. It's all developed for the time.
Now, if we break it down component by component, the
weight adds up, sure, but maybe not quite to that colossal
figure you might have in mind. Think about just the brickwork,
those main walls, usually solid brick, 225mm.
Think this. Two bricks side by side,
basically. That alone account for a huge
chunk of the weight. We're talking somewhere in the
region of, say, 55 to 65 tons just for the brick.
Walls. Wow, just the walls.
That's already a lot. It is.
Think of the sheer volume of bricks, the density of fired
clay, layer upon layer, holding everything up.
They're the bones of the house, really.
OK, so 55 to 65 tons for brickwork.
What about the rest floors? Roof right?
Then you've got the floors, often suspended timber floors in
these houses, joists, floorboards, maybe plaster
underneath. Timber's lighter than brick,
obviously, but all together the floors might add another 5 to 7
tons. 5 to 7 tons. And the roof pitched timber
structure, rafters, beams, but often especially if re roofed,
covered with concrete tiles and concrete tiles are pretty heavy
compared to slate for example. So the roof structure and tiles
maybe another 3 to 4 tons. Right, so brick walls, floors,
roof. What else contributes?
We've got all the internal stuff too.
Internal walls, maybe timber stud or thinner brick ones, all
the plaster on the walls, ceilings, and don't forget the
chimneys, often big solid brick structures running right up
through the house. Yet all that up to
superstructure, everything above the foundations, and the total
comes out somewhere in the region of 80 to 100 tons. 80 to
100 tons. OK, so that is heavy,
definitely. I mean, what's that, like 15
elephants? Something like that.
It's substantial, no question. But here's the key point.
It's not the 300 tons or more that some people might guess. 80
to 100 tons, while a lot, isn't actually an insurmountable force
when you consider persistent natural processes acting over a
long time. OK, so it's heavy, but not so
heavy that nothing could possibly move it, especially not
over decades. Exactly.
And understanding that weight, getting a realistic feel for it,
is really important for debunking some of the other
myths about how clay actually effects buildings.
It's not an immovable object meeting an unstoppable force
necessarily. Right, that makes sense.
So if the weight isn't quite this mythical immovable number,
then maybe the way the clay moves it isn't quite that
dramatic hydraulic Jack image either.
Is that the next misconception? Picturing the clay just pushing
the whole house up. That's precisely it.
That hydraulic Jack idea is powerful.
It creates a strong visual. But it's what It's fundamentally
misleading about the physics involved.
The reality of how clay soil, especially London clay, works is
much more subtle and in some ways more persistent.
So how does it really work then, at a sort of microscopic level?
OK, this is where it gets really interesting.
Clay isn't like sand, you know, with gritty particles.
Clay is made of incredibly tiny, flat plate like particles.
Imagine microscopic flakes. Millions of them.
Like tiny dinner plates. Sort of, yeah.
And these plates, especially in London clay, have a negative
electrical charge on their surface.
Water molecules, which are slightly polar positive at one
end, negative at the other, are strongly attracted to these
negative surfaces. So when the clay gets wet, water
molecules are drawn in and formed thin films around each
tiny plate. These water films act like
microscopic cushions or wedges, pushing the plates apart.
So the individual particles swell, or rather the gaps
between them increase. Exactly.
The whole soil mass expands in volume, and conversely, when the
clay dries out, those water films shrink or disappear and
the plates pull closer together. The soil contracts.
It's a physical property of the material itself.
Right, but said it's subtle, so it doesn't just lift the whole
house evenly. No, and that's the absolute key.
This swelling and shrinking doesn't happen uniformly under
the entire house. It's uneven.
Maybe one corner gets wetter, perhaps near leaky drain, or
maybe gets drier near big trees roots.
So one part might swell up a few millimeters while another part
stays the same or even shrinks a bit.
Precisely. It's differential movement.
The ground isn't a flat piston pushing up.
It's more like a lumpy mattress, inflating and deflating unevenly
underneath the building. OK, I'm getting the picture now
so the house isn't being jacked up like a car.
No, it's being gently, persistently bent and flexed.
The house structure tries to conform to these uneven ground
movements. It's this bending, this flexing
over time that puts stress on the building fabric, the bricks,
the mortar, and eventually leads to cracking.
It's trying to adapt to a constantly shifting base.
So the cracks are a sign of that stress, that bending.
Exactly. Now we do have a term for actual
upward movement. Engineers call it heave, and it
can happen significantly, just not usually like that instant
Jack. A really compelling example is
what happens when you remove a large tree that's been growing
near a building on clay soil for a long time.
Yes, you mentioned trees before. Right.
That tree has been sucking huge amounts of water out of the
ground, keeping the clay relatively shrunk in that area.
You remove the tree. You remove that massive water
pump. So the ground starts to get wet
again. Correct.
The clay slowly rehydrates, it swells back up, and it can
actually lift the nearby foundations.
There are documented cases where buildings have been lifted by
several centimeters, you know, an inch or two over a few years
after a large tree was felled. Centimeters.
That's quite a. Lot it is.
It shows the power is an instant brute force, but it's this slow,
persistent, significant pressure over time, as the sources say.
Not dramatic, not overnight, but very real.
It really demonstrates the cumulative effect.
That tree example makes it so clear.
It's not a sudden shove, it's the slow relentless process.
So if it's not just a one off lift or settlement, but this
ongoing uneven movement, does that mean the real problem is
the cycle itself, this constant back and forth?
You've absolutely nailed it. It's not just that clay can
swell or shrink. The real damage comes from the
repeated cycling of that movement year after year.
Like the ground is. That's a perfect analogy really.
Is like the ground is breathing, inhaling water and expanding in
the wetter months, exhaling water and contracting in the
drier months. It's driven by the seasons.
OK, explain that seasonal cycle a bit more.
How does it work? We'll think about summer.
It's warmer, often drier, and critically, vegetation is
active, especially those big thirsty trees.
We talked about oaks, poplars, willows.
They act like massive pumps, drawing huge amounts of water
out of the clay through their roots.
This process transpiration really dries out the soil,
sometimes quite deep down, causing significant shrinkage.
Right, the trees are wringing the soil out.
Retty much. Then come autumn and winter, it
gets wetter, cooler. The trees become dormant or
deciduous ones lose their leaves so they stop drawing so much
water. Rainfall starts to soak back
into the ground, rehydrating the clay and causing it to swell
back up again. So it's this annual shrink,
swell, shrink, swell. Exactly.
And this breathing happens most intensely in the upper layers of
the soil. Engineers call this the active
zone, maybe the top meter or two, sometimes more, depending
on the location and vegetation. This is where the moisture
content fluctuates the most dramatically through the
seasons. And how does this connect back
to those Victorian houses specifically?
Well, that's where their construction comes in.
As we touched on, they often had relatively shallow foundations
or footings, maybe only going down, say, 600 to 900
millimeters, two or three feet. Which puts them right in that
active zone. Precisely unlike modern
foundations, which often go deeper specifically to get below
that zone of major seasonal movement, these Victorian
footings are sitting right in the middle of the action.
They experienced the full force of that annual shrinks wall
cycle year after year after year.
OK, I can see how that would be a problem long term.
Definitely think about the structure, the brickwork.
It's strong, but it's also relatively brittle.
The mortar joints give it a tiny bit of flexibility, but they
weren't designed for constant cyclical movement of this scale.
Like bending a paper clip back and forth.
That's a great analogy. Each individual movement might
be small, maybe just a few millimeters.
But repeating that cycle thousands of times over decades
causes fatigue. The materials get stressed.
The mortar starts to crack and weaken.
Eventually, even the bricks can crack.
It's this cumulative damage from the relentless cycling that
leads to the problems we see. And that connects to something
interesting. The sources mentioned rendered
walls often showing cracks first.
Why is that? Yeah, that's a good observation
render. The plaster or cement coating on
the outside is generally much more brittle than the brickwork
underneath it. Brick and mortar have lots of
small joints, giving them a little bit of inherent wiggle
room if you like. But render is just one solid
sheet basically. Exactly.
It's a less forgiving. So when the underlying structure
starts to move even slightly, that brittle render cracks first
because it just can't accommodate the movement.
It acts like an early warning sign, showing stress before the
main brick structure might show visible failure, like the skin
cracking before the bone breaks almost.
Right, a sensitive indicator. So we're talking about
millimeters of movement repeated over and over causing this
fatigue. But millimeter sounds so tiny.
How much movement does it actually take before it becomes,
you know, a real problem you'd notice day-to-day, not just a
cosmetic line? That's a really important
question because you're right, millimeters sounds negligible.
But in building terms they add up fast and can have very real
consequences. To make sense of it, engineers
use a standard scale, the BRE Building Research Establishment
crack classification. It helps with those millimeters
into context. OK.
So what are the levels on the scale?
Well, at the very bottom you have what are called negligible
cracks. These are hairline less than .1
millimeters wide, thinner than a hair.
You barely notice them generally.
No structural concern at all. Might just be plaster shrinkage.
OK, harmless hairlines. What's next?
Then you get fine cracks, maybe 1 to 2mm wide.
You can see these easily enough, but typically they're still
considered mostly cosmetic. You can often fill them and
paint over them. Redecoration level damage.
Essentially, they indicate minor movement, but usually don't
affect how the building works. Right, fill and paint, but when
does it get serious? It starts getting significant
when cracks reach the 5 to 15mm range.
Now we're talking about cracks you could easily stick a pencil
into. This indicates more substantial
movement and starts to have real practical and structural
impacts. Like what kind of impacts?
Well, functionally this is where you start noticing things like
doors and windows sticking or jamming.
They just don't fit their frames properly anymore because the
frame has distorted. OK, that makes sense.
The frame twists slightly. Exactly.
You might also see gaps appearing around window frames
or skirting boards pulling away from the wall.
Wallpaper might tear, floors might feel a bit sloped.
These are things that directly affect how you live in the
house. And structurally, what does a
515mm crack mean for the building itself?
Structurally, cracks this wide mean the masonry needs
attention. It needs patching or repair.
If you leave them, they become pathways for water to get into
the wall. That can lead to damp frost
damage in winter, expanding the cracks further and generally
accelerating the decay of the kick and mortar.
It's a clear sign the building's envelope is compromised.
And that pencil line analogy the sources use about 5mm.
That really brings it home. Just a crack the width of a
pencil can mean your door doesn't shut properly.
Precisely. It makes that seemingly small
measurement relatable to a real world inconvenience.
It shows how quickly those millimeters translate from a
visual issue into a functional problem affecting the usability
and comfort of your home. So even though the movements are
small, they're persistence, and the way they add up or happen
unevenly creates these noticeable, significant
problems. It affects how you live the
building's health, and presumably its value too.
Absolutely. Those millimeters tell a story.
They're visible record of the stresses the building is under.
Far from being insignificant, they're key indicators of the
building's structural narrative. OK, this is making so much more
sense now. We know houses aren't quite as
heavy as we thought. Clay doesn't act like a sudden
Jack and it's this cyclical millimeter level breathing and
stressing that's the real issue. Which brings us right back to
where we started, doesn't it? Those two houses in South
London, side by side, same soil, same weather, 1 cracked one.
Perfect. If the general principles apply
to both, why the different outcome?
What are the specific hidden variables that make one crack
and the other stand firm? That really is the $1,000,000
question, isn't it? And the answer is usually it's
complicated. There's rarely just one single
smoking gun. It's almost always a combination
of factors, often very specific to that exact spot, that exact
house. That's why you get these stark
contrast right next door to each other.
So what are some of these key interacting factors?
Well, arguably the biggest player, as the sources
emphasize, is often trees it We've touched on their thirst,
but the proximity and type of tree matters hugely.
A single large mature oak or Poplar within say 10 or 20
meters of one house, but not the other.
Creates a massive difference and how much the soil dries out
locally. Exactly.
That tree is pulling thousands of liters of water out each
year, creating a localized zone of intense seasonal shrinkage
right under or near 1 houses foundations.
The neighbor further away or with just small shrubs nearby
experiences much less soil desiccation.
That difference alone can be huge.
OK, so trees are a big one. What else?
Another factor is subtle variations in foundation depth.
We said Victorian foundations were generally shallow, but not
every Victorian builder dug to the same level.
Maybe 1 crew dug down 600 millimeter.
The next door team went to 800mm.
And that extra 200mm, maybe 8 inches could make a real
difference it. Absolutely could over a century.
It might mean one foundation sits slightly deeper, maybe just
below the most intensely fluctuating layer of the active
zone, while his neighbor is right in the thick of it.
That small difference in construction practice 120 years
ago can dictate whether cracks appear today.
History matters. Wow.
OK, trees, foundation depth, What else?
Then there are local geological nuances.
We talked about London clay, but it's not perfectly uniform.
Like any geological deposit, it can vary even across a single
St. You might get patches where the
clay has a slightly higher clay content, making it more plastic
or reactive, more prone to swell and shrink.
So one house might just happen to be sitting on a slightly more
volatile patch of clay than its neighbor.
Potentially yes, or a patch with slightly different drainage
characteristics. Maybe a thin layer of gravel
running through it underneath one house but not the other.
These micro variations in the ground itself can play a role.
Geology isn't always smooth and consistent.
Fascinating. Are there any other factors?
Human factors, maybe? Definitely the legacy of past
water management is crucial. Think about drainage.
Maybe one house had a leaky drain pipe near his back corner
for decades, constantly saturating the ground there.
That creates A localized wet spot, potentially leading to
swelling or even washing away fine soil particles over time.
Or maybe a blocked soak away causing water to pool.
Exactly Or conversely, maybe excellent drainage on one
property has kept the ground consistently drier.
These historical or ongoing differences in how water behaves
around the foundations, whether it's managed well, pools up, or
leaks persistently, can significantly alter the local
moisture conditions and thus the ground movement experienced by
one house compared to its neighbor.
So it's really a cocktail of potential causes, the tree, the
exact footing depth, the specific patch of clay, how the
drains have worked for the last century.
Precisely, it's rarely just one thing.
It's the unique combination of these factors, nature, history,
construction, maintenance that comes together to write the
specific structural story of each individual house.
That's why you see that divergent, that crack on one and
not the other. It's the result of a complex
interplay playing out over a very long time.
OK, let's try and pull this all together then.
This has been incredibly insightful.
We've shifted from thinking about a massive house being
shoved upwards by powerful clay to understanding it's more like
the ground breathing beneath it. This slow, persistent cycle of
shrinking and swelling driven by water and roots and seasons.
And this happens unevenly, stressing the building over
decades. That's the core picture, yes,
and it's important to keep that balance.
As you said earlier, most houses on clay, even London clay,
actually cope reasonably well. They move a little, sure, but
many stand for over a century without serious issues.
London wouldn't exist as it does otherwise.
But as the sources point out, you get this perfect recipe for
problems when you combine that breathing clay with thirsty
trees nearby, those shallow footing sitting right in the
active zone, and the relentless annual cycles of drying and
wetting. That combination makes a house
particularly vulnerable. And this leads to that really
crucial insight from the sources.
It's not about weight versus strength.
It's not just a simple battle of house mass against clay force.
Exactly. That's the wrong way to frame
it. It's much more about achieving
balance. It's about managing the
interaction between the building and its environment.
Can you maintain a relatively stable equilibrium, or is the
house constantly fighting against extreme fluctuations?
It's about stewardship, not just brute resistance.
So thinking about that balance, what are the practical takeaways
for listeners? What can people actually do with
this knowledge? Well, the concept of balance
points directly to a few key areas.
First, as we discussed, is water management.
Really focus on keeping water where it belongs.
Good drainage is essential. Make sure gutters aren't
blocked. Downpipes discharge away from
the foundations. Fix any leaks promptly.
Avoid letting large amounts of water soak into the ground right
next to the house, but also avoid excessive drying.
Consistency is key. Right control the extremes of
wet and dry near the foundations.
Exactly. Second is vegetation management.
Be mindful of large thirsty trees close to the house.
This doesn't automatically mean chop them down.
Trees have many benefits and removal can sometimes cause
heave as we noted, but understand their impact.
Maybe consider pruning, manage their size, or in some cases
installing root barriers if they're causing demonstrable
problems. It's about informed choices with
your planting and tree care. OK, water and trees.
Anything else? The third take away is really an
attitude shift, remembering that in surveying, millimeters
matter. Don't dismiss small cracks or
slight movements out of hand, especially if they seem to be
changing or getting worse over time.
Those tiny increments are the language the building uses to
tell you something might be happening underneath.
Pay attention to the small stuff, it can prevent bigger
problems later. So be observant, manage water,
manage vegetation. It's about proactive care based
on understanding these subtle forces.
Precisely, it encourages you to look at buildings differently
with a more informed eye, understanding that constant,
subtle dialogue happening between the structure and the
ground beneath it. Absolutely.
So the next time any of us walk past a cracked building,
especially on clay, we won't just see a crack.
We'll see a story. Not a monster clay myth, but a
complex history written by water, roots and thyme, slowly
shaping the story of that house. It's a much richer, more nuanced
understanding. It really is.
And maybe a final thought to leave you with if these tiny
millimeter scale movements accumulating over decades can
tell us so much about a building's resilience and its
environment, what other slow, gradual, almost invisible
processes are shaping our world around us right now?
Think about micro changes in climate and ecosystems, maybe
even in society. Things that seem insignificant
day-to-day, but whose cumulative effects over time might be
setting the stage for major impacts tomorrow.
Our homes, our cities, our planet.
This deep dive really highlights that sometimes to understand the
big picture, you first have to learn to appreciate the dance of
the small.