1 00:00:00,040 --> 00:00:02,000 Welcome curious minds to the Deep Dive. 2 00:00:02,520 --> 00:00:04,600 I want you to picture something with me for just a moment. 3 00:00:05,120 --> 00:00:08,640 Think about a quiet, maybe tree lined St. somewhere in South 4 00:00:08,640 --> 00:00:10,600 London. You know the ones, Rows of those 5 00:00:10,600 --> 00:00:13,800 beautiful Victorian terrace houses, all red brick, those 6 00:00:13,800 --> 00:00:16,400 characteristic shallow foundations we hear about lovely 7 00:00:16,400 --> 00:00:18,920 tiled roofs. It's just, well, it's a classic 8 00:00:18,920 --> 00:00:20,600 picture of history, isn't it? Now let's zoom in. 9 00:00:21,120 --> 00:00:24,360 Imagine two of them standing right next door to each other. 10 00:00:24,640 --> 00:00:27,320 Been there for over 100 years. Looked pretty much identical. 11 00:00:27,600 --> 00:00:30,040 Same break, same period, same kind of front garden. 12 00:00:30,040 --> 00:00:32,880 Maybe one of these houses. It's standing perfectly 13 00:00:32,880 --> 00:00:35,960 straight, the brickwork flawless. 14 00:00:36,360 --> 00:00:38,560 A real testament to how well they built things back then, you 15 00:00:38,560 --> 00:00:42,280 know, Looks solid as a rock, but its neighbor just a few feet 16 00:00:42,280 --> 00:00:43,920 away tells a comletely different story. 17 00:00:44,200 --> 00:00:47,200 You can see it clear as day, this distinct jagged crack 18 00:00:47,200 --> 00:00:50,320 running right U above the back door, sort of diagonally across 19 00:00:50,320 --> 00:00:51,960 the bricks. It's like a scar, really. 20 00:00:52,440 --> 00:00:53,800 And here's the thing that gets you thinking. 21 00:00:54,320 --> 00:00:57,560 They're on the exact same soil, this London clay we hear so much 22 00:00:57,560 --> 00:00:58,760 about. They've had the same weather, 23 00:00:58,760 --> 00:01:02,400 same rain, same hot summers, year in, year out, built in the 24 00:01:02,400 --> 00:01:04,680 same era, probably by the same team even. 25 00:01:04,680 --> 00:01:08,280 So why? Why is 1 pristine looking, you 26 00:01:08,280 --> 00:01:11,280 know, perfectly resilient while the other one's got this obvious 27 00:01:11,280 --> 00:01:13,360 wound? What explains that difference? 28 00:01:13,360 --> 00:01:16,200 What are these hidden forces doing today? 29 00:01:16,200 --> 00:01:20,000 On The Deep Dive, we're plunging into exactly that, the often, 30 00:01:20,000 --> 00:01:24,000 let's say, misunderstood world of clay soils, specifically 31 00:01:24,000 --> 00:01:26,800 London clay, and the impact it has, sometimes quite 32 00:01:26,800 --> 00:01:30,320 dramatically, on buildings. Our mission really is to unpack 33 00:01:30,320 --> 00:01:33,200 the myths from the realities about these expanding soils, 34 00:01:33,480 --> 00:01:35,800 give you a kind of shortcut to understanding what's a really 35 00:01:35,800 --> 00:01:38,600 complex issue, but one that affects so many homes, not just 36 00:01:38,600 --> 00:01:41,200 in London, but anywhere you find these reactive soils. 37 00:01:41,480 --> 00:01:44,400 So the core questions we're asking are, can clay really lift 38 00:01:44,400 --> 00:01:46,160 a whole house? And what truly makes some 39 00:01:46,160 --> 00:01:48,720 buildings move so much more than others, even when they look so 40 00:01:48,720 --> 00:01:51,000 similar? And this really does matter to 41 00:01:51,000 --> 00:01:53,320 you, the listener, Even if you're not living in one of 42 00:01:53,320 --> 00:01:58,000 those lovely Victorian terraces. Think about it, if you own any 43 00:01:58,000 --> 00:02:00,720 property or you're thinking about building an extension, 44 00:02:01,120 --> 00:02:04,600 maybe even just buying a house, what's happening underground, 45 00:02:04,720 --> 00:02:08,479 often silently, is affecting its structure, its health, its long 46 00:02:08,479 --> 00:02:11,800 term value. Understanding how soil behaves 47 00:02:11,880 --> 00:02:14,920 isn't just for engineers. You know it's, well, it's pretty 48 00:02:14,920 --> 00:02:17,760 crucial for any homeowner. It could affect your garden 49 00:02:17,760 --> 00:02:20,360 choices, how you manage drainage, even if you spot 50 00:02:20,360 --> 00:02:22,960 potential issues when you view a property and if you're 51 00:02:22,960 --> 00:02:24,520 renovating. Or honestly, if you're just 52 00:02:24,520 --> 00:02:28,320 curious about these hidden forces beneath our feet, this 53 00:02:28,320 --> 00:02:30,840 should be fascinating. We're aiming to shed some light 54 00:02:30,840 --> 00:02:33,480 on those things that seem inexplicable, like why does that 55 00:02:33,480 --> 00:02:36,160 door stick only in summer? Or where did that crack come 56 00:02:36,160 --> 00:02:37,520 from? Hopefully you'll have a few aha 57 00:02:37,520 --> 00:02:40,440 moments because it's not just about cracks on the surface. 58 00:02:40,440 --> 00:02:43,920 It's about this really intricate dance between huge natural 59 00:02:43,920 --> 00:02:46,960 forces and the structures we build, and even tiny persistent 60 00:02:46,960 --> 00:02:48,680 movements, the kind you might just ignore. 61 00:02:48,960 --> 00:02:51,680 They can add up to have really significant long term effects. 62 00:02:52,080 --> 00:02:54,360 They tell a story. OK, let's get into it then. 63 00:02:54,760 --> 00:02:58,760 First big question, and it seems simple, but maybe it's not. 64 00:02:59,160 --> 00:03:02,440 How heavy is a house really? I think of most of us when we 65 00:03:02,440 --> 00:03:05,240 picture a house, we imagine something incredibly heavy, like 66 00:03:05,680 --> 00:03:08,000 hundreds of tons, maybe immovable. 67 00:03:08,000 --> 00:03:10,320 It feels like this giant rooted thing, right? 68 00:03:10,680 --> 00:03:14,000 And that idea, that perception of just immense weight, it 69 00:03:14,000 --> 00:03:17,040 really shapes how we think about what could possibly move it. 70 00:03:17,040 --> 00:03:19,840 You think no way can the ground lift that? 71 00:03:20,120 --> 00:03:22,920 It's a totally natural thought. They feel so solid. 72 00:03:22,920 --> 00:03:25,880 Dumb thing, permanent. But let's actually put some 73 00:03:25,880 --> 00:03:27,840 numbers on it based on what the sources say. 74 00:03:28,160 --> 00:03:31,000 We're focusing on a typical mid terrace built around 1900 in 75 00:03:31,000 --> 00:03:32,680 London. It's all developed for the time. 76 00:03:33,160 --> 00:03:35,520 Now, if we break it down component by component, the 77 00:03:35,520 --> 00:03:39,360 weight adds up, sure, but maybe not quite to that colossal 78 00:03:39,360 --> 00:03:41,640 figure you might have in mind. Think about just the brickwork, 79 00:03:41,640 --> 00:03:45,240 those main walls, usually solid brick, 225mm. 80 00:03:45,240 --> 00:03:46,920 Think this. Two bricks side by side, 81 00:03:46,920 --> 00:03:48,840 basically. That alone account for a huge 82 00:03:48,840 --> 00:03:50,480 chunk of the weight. We're talking somewhere in the 83 00:03:50,480 --> 00:03:53,400 region of, say, 55 to 65 tons just for the brick. 84 00:03:53,400 --> 00:03:54,520 Walls. Wow, just the walls. 85 00:03:54,560 --> 00:03:56,000 That's already a lot. It is. 86 00:03:56,480 --> 00:03:59,040 Think of the sheer volume of bricks, the density of fired 87 00:03:59,040 --> 00:04:01,680 clay, layer upon layer, holding everything up. 88 00:04:01,920 --> 00:04:03,560 They're the bones of the house, really. 89 00:04:03,680 --> 00:04:06,440 OK, so 55 to 65 tons for brickwork. 90 00:04:06,440 --> 00:04:09,120 What about the rest floors? Roof right? 91 00:04:09,800 --> 00:04:12,760 Then you've got the floors, often suspended timber floors in 92 00:04:12,760 --> 00:04:15,920 these houses, joists, floorboards, maybe plaster 93 00:04:15,920 --> 00:04:17,560 underneath. Timber's lighter than brick, 94 00:04:17,560 --> 00:04:21,640 obviously, but all together the floors might add another 5 to 7 95 00:04:21,640 --> 00:04:24,800 tons. 5 to 7 tons. And the roof pitched timber 96 00:04:24,800 --> 00:04:28,720 structure, rafters, beams, but often especially if re roofed, 97 00:04:28,880 --> 00:04:32,240 covered with concrete tiles and concrete tiles are pretty heavy 98 00:04:32,240 --> 00:04:35,120 compared to slate for example. So the roof structure and tiles 99 00:04:35,480 --> 00:04:38,760 maybe another 3 to 4 tons. Right, so brick walls, floors, 100 00:04:38,840 --> 00:04:40,920 roof. What else contributes? 101 00:04:40,920 --> 00:04:42,400 We've got all the internal stuff too. 102 00:04:42,680 --> 00:04:46,000 Internal walls, maybe timber stud or thinner brick ones, all 103 00:04:46,000 --> 00:04:48,480 the plaster on the walls, ceilings, and don't forget the 104 00:04:48,480 --> 00:04:51,560 chimneys, often big solid brick structures running right up 105 00:04:51,560 --> 00:04:53,280 through the house. Yet all that up to 106 00:04:53,280 --> 00:04:55,720 superstructure, everything above the foundations, and the total 107 00:04:55,720 --> 00:04:58,720 comes out somewhere in the region of 80 to 100 tons. 80 to 108 00:04:58,760 --> 00:05:02,040 100 tons. OK, so that is heavy, 109 00:05:02,120 --> 00:05:04,040 definitely. I mean, what's that, like 15 110 00:05:04,040 --> 00:05:05,160 elephants? Something like that. 111 00:05:05,280 --> 00:05:07,560 It's substantial, no question. But here's the key point. 112 00:05:07,920 --> 00:05:11,360 It's not the 300 tons or more that some people might guess. 80 113 00:05:11,360 --> 00:05:15,680 to 100 tons, while a lot, isn't actually an insurmountable force 114 00:05:15,680 --> 00:05:18,800 when you consider persistent natural processes acting over a 115 00:05:18,800 --> 00:05:22,720 long time. OK, so it's heavy, but not so 116 00:05:22,720 --> 00:05:25,520 heavy that nothing could possibly move it, especially not 117 00:05:25,520 --> 00:05:27,120 over decades. Exactly. 118 00:05:27,240 --> 00:05:30,240 And understanding that weight, getting a realistic feel for it, 119 00:05:30,360 --> 00:05:32,760 is really important for debunking some of the other 120 00:05:32,760 --> 00:05:35,640 myths about how clay actually effects buildings. 121 00:05:36,080 --> 00:05:39,920 It's not an immovable object meeting an unstoppable force 122 00:05:39,920 --> 00:05:41,480 necessarily. Right, that makes sense. 123 00:05:41,480 --> 00:05:45,280 So if the weight isn't quite this mythical immovable number, 124 00:05:45,280 --> 00:05:47,720 then maybe the way the clay moves it isn't quite that 125 00:05:47,720 --> 00:05:49,920 dramatic hydraulic Jack image either. 126 00:05:49,920 --> 00:05:53,200 Is that the next misconception? Picturing the clay just pushing 127 00:05:53,200 --> 00:05:55,280 the whole house up. That's precisely it. 128 00:05:55,640 --> 00:05:57,960 That hydraulic Jack idea is powerful. 129 00:05:57,960 --> 00:06:00,920 It creates a strong visual. But it's what It's fundamentally 130 00:06:00,920 --> 00:06:02,680 misleading about the physics involved. 131 00:06:03,400 --> 00:06:08,160 The reality of how clay soil, especially London clay, works is 132 00:06:08,160 --> 00:06:11,280 much more subtle and in some ways more persistent. 133 00:06:11,280 --> 00:06:14,200 So how does it really work then, at a sort of microscopic level? 134 00:06:14,200 --> 00:06:16,120 OK, this is where it gets really interesting. 135 00:06:16,520 --> 00:06:19,680 Clay isn't like sand, you know, with gritty particles. 136 00:06:19,880 --> 00:06:23,840 Clay is made of incredibly tiny, flat plate like particles. 137 00:06:24,240 --> 00:06:26,800 Imagine microscopic flakes. Millions of them. 138 00:06:26,800 --> 00:06:29,320 Like tiny dinner plates. Sort of, yeah. 139 00:06:29,760 --> 00:06:33,160 And these plates, especially in London clay, have a negative 140 00:06:33,160 --> 00:06:34,680 electrical charge on their surface. 141 00:06:35,360 --> 00:06:38,400 Water molecules, which are slightly polar positive at one 142 00:06:38,400 --> 00:06:41,000 end, negative at the other, are strongly attracted to these 143 00:06:41,000 --> 00:06:43,960 negative surfaces. So when the clay gets wet, water 144 00:06:43,960 --> 00:06:46,800 molecules are drawn in and formed thin films around each 145 00:06:46,800 --> 00:06:49,080 tiny plate. These water films act like 146 00:06:49,080 --> 00:06:51,920 microscopic cushions or wedges, pushing the plates apart. 147 00:06:52,480 --> 00:06:55,240 So the individual particles swell, or rather the gaps 148 00:06:55,240 --> 00:06:57,240 between them increase. Exactly. 149 00:06:57,920 --> 00:07:02,680 The whole soil mass expands in volume, and conversely, when the 150 00:07:02,680 --> 00:07:06,480 clay dries out, those water films shrink or disappear and 151 00:07:06,480 --> 00:07:09,560 the plates pull closer together. The soil contracts. 152 00:07:09,840 --> 00:07:11,960 It's a physical property of the material itself. 153 00:07:12,040 --> 00:07:14,840 Right, but said it's subtle, so it doesn't just lift the whole 154 00:07:14,840 --> 00:07:17,480 house evenly. No, and that's the absolute key. 155 00:07:17,800 --> 00:07:20,920 This swelling and shrinking doesn't happen uniformly under 156 00:07:20,920 --> 00:07:22,760 the entire house. It's uneven. 157 00:07:23,280 --> 00:07:26,760 Maybe one corner gets wetter, perhaps near leaky drain, or 158 00:07:26,760 --> 00:07:28,840 maybe gets drier near big trees roots. 159 00:07:28,880 --> 00:07:32,080 So one part might swell up a few millimeters while another part 160 00:07:32,080 --> 00:07:34,000 stays the same or even shrinks a bit. 161 00:07:34,280 --> 00:07:36,160 Precisely. It's differential movement. 162 00:07:36,600 --> 00:07:38,600 The ground isn't a flat piston pushing up. 163 00:07:38,680 --> 00:07:42,240 It's more like a lumpy mattress, inflating and deflating unevenly 164 00:07:42,480 --> 00:07:44,280 underneath the building. OK, I'm getting the picture now 165 00:07:44,280 --> 00:07:46,640 so the house isn't being jacked up like a car. 166 00:07:46,800 --> 00:07:50,800 No, it's being gently, persistently bent and flexed. 167 00:07:52,000 --> 00:07:54,840 The house structure tries to conform to these uneven ground 168 00:07:54,840 --> 00:07:57,840 movements. It's this bending, this flexing 169 00:07:57,840 --> 00:08:02,040 over time that puts stress on the building fabric, the bricks, 170 00:08:02,040 --> 00:08:04,240 the mortar, and eventually leads to cracking. 171 00:08:04,600 --> 00:08:07,640 It's trying to adapt to a constantly shifting base. 172 00:08:07,640 --> 00:08:10,920 So the cracks are a sign of that stress, that bending. 173 00:08:10,920 --> 00:08:13,960 Exactly. Now we do have a term for actual 174 00:08:14,040 --> 00:08:17,120 upward movement. Engineers call it heave, and it 175 00:08:17,120 --> 00:08:19,560 can happen significantly, just not usually like that instant 176 00:08:19,560 --> 00:08:21,480 Jack. A really compelling example is 177 00:08:21,480 --> 00:08:23,640 what happens when you remove a large tree that's been growing 178 00:08:23,640 --> 00:08:25,960 near a building on clay soil for a long time. 179 00:08:26,480 --> 00:08:28,360 Yes, you mentioned trees before. Right. 180 00:08:28,360 --> 00:08:30,800 That tree has been sucking huge amounts of water out of the 181 00:08:30,800 --> 00:08:33,760 ground, keeping the clay relatively shrunk in that area. 182 00:08:34,000 --> 00:08:35,880 You remove the tree. You remove that massive water 183 00:08:35,880 --> 00:08:37,720 pump. So the ground starts to get wet 184 00:08:37,720 --> 00:08:38,400 again. Correct. 185 00:08:38,400 --> 00:08:41,320 The clay slowly rehydrates, it swells back up, and it can 186 00:08:41,320 --> 00:08:42,960 actually lift the nearby foundations. 187 00:08:43,480 --> 00:08:45,880 There are documented cases where buildings have been lifted by 188 00:08:45,880 --> 00:08:49,080 several centimeters, you know, an inch or two over a few years 189 00:08:49,080 --> 00:08:51,480 after a large tree was felled. Centimeters. 190 00:08:51,480 --> 00:08:53,080 That's quite a. Lot it is. 191 00:08:53,400 --> 00:08:57,560 It shows the power is an instant brute force, but it's this slow, 192 00:08:57,560 --> 00:09:01,440 persistent, significant pressure over time, as the sources say. 193 00:09:01,440 --> 00:09:04,080 Not dramatic, not overnight, but very real. 194 00:09:04,440 --> 00:09:06,520 It really demonstrates the cumulative effect. 195 00:09:06,520 --> 00:09:08,640 That tree example makes it so clear. 196 00:09:08,680 --> 00:09:12,160 It's not a sudden shove, it's the slow relentless process. 197 00:09:12,600 --> 00:09:15,680 So if it's not just a one off lift or settlement, but this 198 00:09:15,680 --> 00:09:20,000 ongoing uneven movement, does that mean the real problem is 199 00:09:20,000 --> 00:09:22,640 the cycle itself, this constant back and forth? 200 00:09:22,720 --> 00:09:25,000 You've absolutely nailed it. It's not just that clay can 201 00:09:25,000 --> 00:09:28,120 swell or shrink. The real damage comes from the 202 00:09:28,120 --> 00:09:31,040 repeated cycling of that movement year after year. 203 00:09:31,040 --> 00:09:34,480 Like the ground is. That's a perfect analogy really. 204 00:09:34,480 --> 00:09:37,720 Is like the ground is breathing, inhaling water and expanding in 205 00:09:37,720 --> 00:09:40,040 the wetter months, exhaling water and contracting in the 206 00:09:40,040 --> 00:09:42,280 drier months. It's driven by the seasons. 207 00:09:42,280 --> 00:09:44,360 OK, explain that seasonal cycle a bit more. 208 00:09:44,360 --> 00:09:46,320 How does it work? We'll think about summer. 209 00:09:46,440 --> 00:09:50,200 It's warmer, often drier, and critically, vegetation is 210 00:09:50,200 --> 00:09:52,880 active, especially those big thirsty trees. 211 00:09:52,880 --> 00:09:55,000 We talked about oaks, poplars, willows. 212 00:09:55,160 --> 00:09:58,280 They act like massive pumps, drawing huge amounts of water 213 00:09:58,280 --> 00:09:59,840 out of the clay through their roots. 214 00:10:00,280 --> 00:10:03,640 This process transpiration really dries out the soil, 215 00:10:03,880 --> 00:10:07,120 sometimes quite deep down, causing significant shrinkage. 216 00:10:07,120 --> 00:10:09,000 Right, the trees are wringing the soil out. 217 00:10:09,400 --> 00:10:12,240 Retty much. Then come autumn and winter, it 218 00:10:12,240 --> 00:10:15,320 gets wetter, cooler. The trees become dormant or 219 00:10:15,320 --> 00:10:18,320 deciduous ones lose their leaves so they stop drawing so much 220 00:10:18,320 --> 00:10:20,520 water. Rainfall starts to soak back 221 00:10:20,520 --> 00:10:23,920 into the ground, rehydrating the clay and causing it to swell 222 00:10:23,920 --> 00:10:26,760 back up again. So it's this annual shrink, 223 00:10:26,760 --> 00:10:28,720 swell, shrink, swell. Exactly. 224 00:10:28,920 --> 00:10:32,400 And this breathing happens most intensely in the upper layers of 225 00:10:32,400 --> 00:10:35,040 the soil. Engineers call this the active 226 00:10:35,040 --> 00:10:38,000 zone, maybe the top meter or two, sometimes more, depending 227 00:10:38,000 --> 00:10:41,080 on the location and vegetation. This is where the moisture 228 00:10:41,080 --> 00:10:43,440 content fluctuates the most dramatically through the 229 00:10:43,440 --> 00:10:45,320 seasons. And how does this connect back 230 00:10:45,320 --> 00:10:47,240 to those Victorian houses specifically? 231 00:10:47,840 --> 00:10:49,760 Well, that's where their construction comes in. 232 00:10:50,480 --> 00:10:54,040 As we touched on, they often had relatively shallow foundations 233 00:10:54,040 --> 00:10:58,400 or footings, maybe only going down, say, 600 to 900 234 00:10:58,400 --> 00:11:01,440 millimeters, two or three feet. Which puts them right in that 235 00:11:01,440 --> 00:11:04,200 active zone. Precisely unlike modern 236 00:11:04,200 --> 00:11:07,280 foundations, which often go deeper specifically to get below 237 00:11:07,280 --> 00:11:10,960 that zone of major seasonal movement, these Victorian 238 00:11:10,960 --> 00:11:13,240 footings are sitting right in the middle of the action. 239 00:11:13,760 --> 00:11:16,680 They experienced the full force of that annual shrinks wall 240 00:11:16,680 --> 00:11:18,800 cycle year after year after year. 241 00:11:18,800 --> 00:11:21,400 OK, I can see how that would be a problem long term. 242 00:11:21,400 --> 00:11:23,840 Definitely think about the structure, the brickwork. 243 00:11:24,200 --> 00:11:26,520 It's strong, but it's also relatively brittle. 244 00:11:26,840 --> 00:11:29,960 The mortar joints give it a tiny bit of flexibility, but they 245 00:11:29,960 --> 00:11:33,040 weren't designed for constant cyclical movement of this scale. 246 00:11:33,040 --> 00:11:35,160 Like bending a paper clip back and forth. 247 00:11:35,360 --> 00:11:37,800 That's a great analogy. Each individual movement might 248 00:11:37,800 --> 00:11:40,120 be small, maybe just a few millimeters. 249 00:11:40,560 --> 00:11:43,680 But repeating that cycle thousands of times over decades 250 00:11:43,880 --> 00:11:46,600 causes fatigue. The materials get stressed. 251 00:11:46,840 --> 00:11:48,680 The mortar starts to crack and weaken. 252 00:11:48,880 --> 00:11:50,720 Eventually, even the bricks can crack. 253 00:11:51,480 --> 00:11:54,760 It's this cumulative damage from the relentless cycling that 254 00:11:54,760 --> 00:11:57,640 leads to the problems we see. And that connects to something 255 00:11:57,640 --> 00:11:59,880 interesting. The sources mentioned rendered 256 00:11:59,880 --> 00:12:01,800 walls often showing cracks first. 257 00:12:01,800 --> 00:12:04,000 Why is that? Yeah, that's a good observation 258 00:12:04,360 --> 00:12:06,360 render. The plaster or cement coating on 259 00:12:06,360 --> 00:12:09,280 the outside is generally much more brittle than the brickwork 260 00:12:09,280 --> 00:12:11,440 underneath it. Brick and mortar have lots of 261 00:12:11,440 --> 00:12:14,200 small joints, giving them a little bit of inherent wiggle 262 00:12:14,200 --> 00:12:17,680 room if you like. But render is just one solid 263 00:12:17,680 --> 00:12:19,400 sheet basically. Exactly. 264 00:12:19,400 --> 00:12:22,360 It's a less forgiving. So when the underlying structure 265 00:12:22,360 --> 00:12:25,760 starts to move even slightly, that brittle render cracks first 266 00:12:25,760 --> 00:12:27,560 because it just can't accommodate the movement. 267 00:12:27,800 --> 00:12:30,720 It acts like an early warning sign, showing stress before the 268 00:12:30,720 --> 00:12:33,920 main brick structure might show visible failure, like the skin 269 00:12:33,920 --> 00:12:35,880 cracking before the bone breaks almost. 270 00:12:36,120 --> 00:12:38,160 Right, a sensitive indicator. So we're talking about 271 00:12:38,160 --> 00:12:41,360 millimeters of movement repeated over and over causing this 272 00:12:41,360 --> 00:12:44,360 fatigue. But millimeter sounds so tiny. 273 00:12:44,400 --> 00:12:47,920 How much movement does it actually take before it becomes, 274 00:12:48,000 --> 00:12:50,800 you know, a real problem you'd notice day-to-day, not just a 275 00:12:50,800 --> 00:12:52,960 cosmetic line? That's a really important 276 00:12:52,960 --> 00:12:56,240 question because you're right, millimeters sounds negligible. 277 00:12:57,000 --> 00:13:00,600 But in building terms they add up fast and can have very real 278 00:13:00,600 --> 00:13:03,480 consequences. To make sense of it, engineers 279 00:13:03,480 --> 00:13:07,080 use a standard scale, the BRE Building Research Establishment 280 00:13:07,080 --> 00:13:09,800 crack classification. It helps with those millimeters 281 00:13:09,800 --> 00:13:10,760 into context. OK. 282 00:13:10,800 --> 00:13:12,600 So what are the levels on the scale? 283 00:13:12,680 --> 00:13:14,920 Well, at the very bottom you have what are called negligible 284 00:13:14,920 --> 00:13:17,440 cracks. These are hairline less than .1 285 00:13:17,440 --> 00:13:19,720 millimeters wide, thinner than a hair. 286 00:13:20,000 --> 00:13:21,760 You barely notice them generally. 287 00:13:21,760 --> 00:13:24,840 No structural concern at all. Might just be plaster shrinkage. 288 00:13:25,080 --> 00:13:27,040 OK, harmless hairlines. What's next? 289 00:13:27,280 --> 00:13:30,520 Then you get fine cracks, maybe 1 to 2mm wide. 290 00:13:30,640 --> 00:13:33,520 You can see these easily enough, but typically they're still 291 00:13:33,520 --> 00:13:36,600 considered mostly cosmetic. You can often fill them and 292 00:13:36,600 --> 00:13:39,040 paint over them. Redecoration level damage. 293 00:13:39,040 --> 00:13:42,360 Essentially, they indicate minor movement, but usually don't 294 00:13:42,360 --> 00:13:45,680 affect how the building works. Right, fill and paint, but when 295 00:13:45,680 --> 00:13:48,400 does it get serious? It starts getting significant 296 00:13:48,400 --> 00:13:51,360 when cracks reach the 5 to 15mm range. 297 00:13:51,640 --> 00:13:53,960 Now we're talking about cracks you could easily stick a pencil 298 00:13:53,960 --> 00:13:56,720 into. This indicates more substantial 299 00:13:56,720 --> 00:13:59,640 movement and starts to have real practical and structural 300 00:13:59,640 --> 00:14:01,600 impacts. Like what kind of impacts? 301 00:14:02,040 --> 00:14:05,160 Well, functionally this is where you start noticing things like 302 00:14:05,160 --> 00:14:07,360 doors and windows sticking or jamming. 303 00:14:07,560 --> 00:14:10,280 They just don't fit their frames properly anymore because the 304 00:14:10,280 --> 00:14:13,040 frame has distorted. OK, that makes sense. 305 00:14:13,040 --> 00:14:15,120 The frame twists slightly. Exactly. 306 00:14:15,720 --> 00:14:18,200 You might also see gaps appearing around window frames 307 00:14:18,200 --> 00:14:20,400 or skirting boards pulling away from the wall. 308 00:14:20,720 --> 00:14:24,240 Wallpaper might tear, floors might feel a bit sloped. 309 00:14:24,600 --> 00:14:27,120 These are things that directly affect how you live in the 310 00:14:27,120 --> 00:14:29,400 house. And structurally, what does a 311 00:14:29,400 --> 00:14:32,440 515mm crack mean for the building itself? 312 00:14:32,760 --> 00:14:36,080 Structurally, cracks this wide mean the masonry needs 313 00:14:36,080 --> 00:14:38,760 attention. It needs patching or repair. 314 00:14:39,160 --> 00:14:41,760 If you leave them, they become pathways for water to get into 315 00:14:41,760 --> 00:14:44,360 the wall. That can lead to damp frost 316 00:14:44,360 --> 00:14:47,880 damage in winter, expanding the cracks further and generally 317 00:14:47,880 --> 00:14:50,200 accelerating the decay of the kick and mortar. 318 00:14:50,520 --> 00:14:53,440 It's a clear sign the building's envelope is compromised. 319 00:14:53,560 --> 00:14:56,760 And that pencil line analogy the sources use about 5mm. 320 00:14:56,760 --> 00:14:59,520 That really brings it home. Just a crack the width of a 321 00:14:59,520 --> 00:15:02,000 pencil can mean your door doesn't shut properly. 322 00:15:02,120 --> 00:15:04,520 Precisely. It makes that seemingly small 323 00:15:04,520 --> 00:15:07,520 measurement relatable to a real world inconvenience. 324 00:15:08,000 --> 00:15:10,320 It shows how quickly those millimeters translate from a 325 00:15:10,320 --> 00:15:14,360 visual issue into a functional problem affecting the usability 326 00:15:14,360 --> 00:15:16,760 and comfort of your home. So even though the movements are 327 00:15:16,760 --> 00:15:19,760 small, they're persistence, and the way they add up or happen 328 00:15:19,760 --> 00:15:22,440 unevenly creates these noticeable, significant 329 00:15:22,440 --> 00:15:24,520 problems. It affects how you live the 330 00:15:24,520 --> 00:15:27,080 building's health, and presumably its value too. 331 00:15:27,120 --> 00:15:29,440 Absolutely. Those millimeters tell a story. 332 00:15:30,080 --> 00:15:32,880 They're visible record of the stresses the building is under. 333 00:15:33,480 --> 00:15:35,920 Far from being insignificant, they're key indicators of the 334 00:15:35,920 --> 00:15:38,960 building's structural narrative. OK, this is making so much more 335 00:15:38,960 --> 00:15:41,160 sense now. We know houses aren't quite as 336 00:15:41,160 --> 00:15:43,360 heavy as we thought. Clay doesn't act like a sudden 337 00:15:43,360 --> 00:15:47,640 Jack and it's this cyclical millimeter level breathing and 338 00:15:47,640 --> 00:15:50,840 stressing that's the real issue. Which brings us right back to 339 00:15:50,840 --> 00:15:53,440 where we started, doesn't it? Those two houses in South 340 00:15:53,440 --> 00:15:56,720 London, side by side, same soil, same weather, 1 cracked one. 341 00:15:56,720 --> 00:15:59,240 Perfect. If the general principles apply 342 00:15:59,240 --> 00:16:01,560 to both, why the different outcome? 343 00:16:01,760 --> 00:16:04,840 What are the specific hidden variables that make one crack 344 00:16:04,840 --> 00:16:07,960 and the other stand firm? That really is the $1,000,000 345 00:16:07,960 --> 00:16:11,000 question, isn't it? And the answer is usually it's 346 00:16:11,000 --> 00:16:13,400 complicated. There's rarely just one single 347 00:16:13,400 --> 00:16:15,560 smoking gun. It's almost always a combination 348 00:16:15,560 --> 00:16:19,680 of factors, often very specific to that exact spot, that exact 349 00:16:19,680 --> 00:16:21,120 house. That's why you get these stark 350 00:16:21,120 --> 00:16:22,680 contrast right next door to each other. 351 00:16:23,240 --> 00:16:25,920 So what are some of these key interacting factors? 352 00:16:26,120 --> 00:16:28,720 Well, arguably the biggest player, as the sources 353 00:16:28,720 --> 00:16:31,560 emphasize, is often trees it We've touched on their thirst, 354 00:16:31,800 --> 00:16:34,120 but the proximity and type of tree matters hugely. 355 00:16:34,360 --> 00:16:37,880 A single large mature oak or Poplar within say 10 or 20 356 00:16:37,880 --> 00:16:39,400 meters of one house, but not the other. 357 00:16:39,520 --> 00:16:42,440 Creates a massive difference and how much the soil dries out 358 00:16:42,440 --> 00:16:43,560 locally. Exactly. 359 00:16:43,880 --> 00:16:46,720 That tree is pulling thousands of liters of water out each 360 00:16:46,720 --> 00:16:50,320 year, creating a localized zone of intense seasonal shrinkage 361 00:16:50,640 --> 00:16:53,120 right under or near 1 houses foundations. 362 00:16:53,720 --> 00:16:56,880 The neighbor further away or with just small shrubs nearby 363 00:16:57,240 --> 00:16:59,400 experiences much less soil desiccation. 364 00:17:00,200 --> 00:17:02,120 That difference alone can be huge. 365 00:17:02,120 --> 00:17:04,160 OK, so trees are a big one. What else? 366 00:17:04,440 --> 00:17:07,359 Another factor is subtle variations in foundation depth. 367 00:17:07,680 --> 00:17:11,079 We said Victorian foundations were generally shallow, but not 368 00:17:11,079 --> 00:17:13,200 every Victorian builder dug to the same level. 369 00:17:13,319 --> 00:17:15,440 Maybe 1 crew dug down 600 millimeter. 370 00:17:15,640 --> 00:17:17,520 The next door team went to 800mm. 371 00:17:17,520 --> 00:17:20,720 And that extra 200mm, maybe 8 inches could make a real 372 00:17:20,720 --> 00:17:22,920 difference it. Absolutely could over a century. 373 00:17:23,280 --> 00:17:26,280 It might mean one foundation sits slightly deeper, maybe just 374 00:17:26,280 --> 00:17:28,800 below the most intensely fluctuating layer of the active 375 00:17:28,800 --> 00:17:30,960 zone, while his neighbor is right in the thick of it. 376 00:17:31,680 --> 00:17:34,640 That small difference in construction practice 120 years 377 00:17:34,640 --> 00:17:37,080 ago can dictate whether cracks appear today. 378 00:17:37,360 --> 00:17:38,880 History matters. Wow. 379 00:17:39,040 --> 00:17:41,440 OK, trees, foundation depth, What else? 380 00:17:41,720 --> 00:17:43,960 Then there are local geological nuances. 381 00:17:43,960 --> 00:17:47,280 We talked about London clay, but it's not perfectly uniform. 382 00:17:47,560 --> 00:17:51,080 Like any geological deposit, it can vary even across a single 383 00:17:51,080 --> 00:17:52,680 St. You might get patches where the 384 00:17:52,680 --> 00:17:56,160 clay has a slightly higher clay content, making it more plastic 385 00:17:56,160 --> 00:17:58,320 or reactive, more prone to swell and shrink. 386 00:17:58,520 --> 00:18:01,440 So one house might just happen to be sitting on a slightly more 387 00:18:01,680 --> 00:18:03,800 volatile patch of clay than its neighbor. 388 00:18:04,040 --> 00:18:07,200 Potentially yes, or a patch with slightly different drainage 389 00:18:07,200 --> 00:18:09,680 characteristics. Maybe a thin layer of gravel 390 00:18:09,680 --> 00:18:12,120 running through it underneath one house but not the other. 391 00:18:12,600 --> 00:18:15,640 These micro variations in the ground itself can play a role. 392 00:18:15,880 --> 00:18:18,000 Geology isn't always smooth and consistent. 393 00:18:18,240 --> 00:18:20,560 Fascinating. Are there any other factors? 394 00:18:20,560 --> 00:18:24,200 Human factors, maybe? Definitely the legacy of past 395 00:18:24,200 --> 00:18:26,880 water management is crucial. Think about drainage. 396 00:18:27,200 --> 00:18:30,080 Maybe one house had a leaky drain pipe near his back corner 397 00:18:30,080 --> 00:18:32,680 for decades, constantly saturating the ground there. 398 00:18:33,040 --> 00:18:35,920 That creates A localized wet spot, potentially leading to 399 00:18:35,920 --> 00:18:39,600 swelling or even washing away fine soil particles over time. 400 00:18:39,600 --> 00:18:42,800 Or maybe a blocked soak away causing water to pool. 401 00:18:43,000 --> 00:18:46,520 Exactly Or conversely, maybe excellent drainage on one 402 00:18:46,520 --> 00:18:49,240 property has kept the ground consistently drier. 403 00:18:50,120 --> 00:18:53,880 These historical or ongoing differences in how water behaves 404 00:18:53,880 --> 00:18:57,040 around the foundations, whether it's managed well, pools up, or 405 00:18:57,040 --> 00:19:00,360 leaks persistently, can significantly alter the local 406 00:19:00,360 --> 00:19:02,840 moisture conditions and thus the ground movement experienced by 407 00:19:02,840 --> 00:19:04,520 one house compared to its neighbor. 408 00:19:04,560 --> 00:19:08,320 So it's really a cocktail of potential causes, the tree, the 409 00:19:08,320 --> 00:19:11,840 exact footing depth, the specific patch of clay, how the 410 00:19:11,840 --> 00:19:13,640 drains have worked for the last century. 411 00:19:13,680 --> 00:19:15,760 Precisely, it's rarely just one thing. 412 00:19:15,760 --> 00:19:19,200 It's the unique combination of these factors, nature, history, 413 00:19:19,320 --> 00:19:21,960 construction, maintenance that comes together to write the 414 00:19:21,960 --> 00:19:24,480 specific structural story of each individual house. 415 00:19:24,920 --> 00:19:27,240 That's why you see that divergent, that crack on one and 416 00:19:27,240 --> 00:19:29,640 not the other. It's the result of a complex 417 00:19:29,640 --> 00:19:31,800 interplay playing out over a very long time. 418 00:19:32,040 --> 00:19:33,800 OK, let's try and pull this all together then. 419 00:19:34,160 --> 00:19:36,000 This has been incredibly insightful. 420 00:19:36,040 --> 00:19:38,240 We've shifted from thinking about a massive house being 421 00:19:38,240 --> 00:19:42,160 shoved upwards by powerful clay to understanding it's more like 422 00:19:42,160 --> 00:19:46,720 the ground breathing beneath it. This slow, persistent cycle of 423 00:19:46,720 --> 00:19:50,800 shrinking and swelling driven by water and roots and seasons. 424 00:19:51,200 --> 00:19:53,360 And this happens unevenly, stressing the building over 425 00:19:53,360 --> 00:19:55,800 decades. That's the core picture, yes, 426 00:19:56,720 --> 00:19:58,360 and it's important to keep that balance. 427 00:19:58,360 --> 00:20:01,920 As you said earlier, most houses on clay, even London clay, 428 00:20:02,160 --> 00:20:05,440 actually cope reasonably well. They move a little, sure, but 429 00:20:05,440 --> 00:20:08,080 many stand for over a century without serious issues. 430 00:20:08,360 --> 00:20:10,240 London wouldn't exist as it does otherwise. 431 00:20:10,800 --> 00:20:13,600 But as the sources point out, you get this perfect recipe for 432 00:20:13,600 --> 00:20:16,560 problems when you combine that breathing clay with thirsty 433 00:20:16,560 --> 00:20:19,440 trees nearby, those shallow footing sitting right in the 434 00:20:19,440 --> 00:20:22,640 active zone, and the relentless annual cycles of drying and 435 00:20:22,640 --> 00:20:24,880 wetting. That combination makes a house 436 00:20:25,080 --> 00:20:27,760 particularly vulnerable. And this leads to that really 437 00:20:27,760 --> 00:20:29,280 crucial insight from the sources. 438 00:20:29,280 --> 00:20:31,240 It's not about weight versus strength. 439 00:20:31,240 --> 00:20:34,480 It's not just a simple battle of house mass against clay force. 440 00:20:34,920 --> 00:20:36,440 Exactly. That's the wrong way to frame 441 00:20:36,440 --> 00:20:38,320 it. It's much more about achieving 442 00:20:38,320 --> 00:20:40,200 balance. It's about managing the 443 00:20:40,200 --> 00:20:42,440 interaction between the building and its environment. 444 00:20:42,960 --> 00:20:46,600 Can you maintain a relatively stable equilibrium, or is the 445 00:20:46,600 --> 00:20:49,640 house constantly fighting against extreme fluctuations? 446 00:20:50,160 --> 00:20:53,000 It's about stewardship, not just brute resistance. 447 00:20:53,680 --> 00:20:56,960 So thinking about that balance, what are the practical takeaways 448 00:20:56,960 --> 00:20:59,360 for listeners? What can people actually do with 449 00:20:59,360 --> 00:21:01,720 this knowledge? Well, the concept of balance 450 00:21:01,720 --> 00:21:03,640 points directly to a few key areas. 451 00:21:04,120 --> 00:21:06,280 First, as we discussed, is water management. 452 00:21:06,680 --> 00:21:09,000 Really focus on keeping water where it belongs. 453 00:21:09,320 --> 00:21:11,840 Good drainage is essential. Make sure gutters aren't 454 00:21:11,840 --> 00:21:13,920 blocked. Downpipes discharge away from 455 00:21:13,920 --> 00:21:16,280 the foundations. Fix any leaks promptly. 456 00:21:16,760 --> 00:21:19,360 Avoid letting large amounts of water soak into the ground right 457 00:21:19,360 --> 00:21:22,120 next to the house, but also avoid excessive drying. 458 00:21:22,400 --> 00:21:25,480 Consistency is key. Right control the extremes of 459 00:21:25,480 --> 00:21:27,480 wet and dry near the foundations. 460 00:21:27,760 --> 00:21:30,480 Exactly. Second is vegetation management. 461 00:21:31,440 --> 00:21:34,480 Be mindful of large thirsty trees close to the house. 462 00:21:34,880 --> 00:21:37,200 This doesn't automatically mean chop them down. 463 00:21:37,400 --> 00:21:40,480 Trees have many benefits and removal can sometimes cause 464 00:21:40,480 --> 00:21:44,040 heave as we noted, but understand their impact. 465 00:21:44,400 --> 00:21:47,840 Maybe consider pruning, manage their size, or in some cases 466 00:21:48,000 --> 00:21:50,600 installing root barriers if they're causing demonstrable 467 00:21:50,600 --> 00:21:52,920 problems. It's about informed choices with 468 00:21:52,920 --> 00:21:56,080 your planting and tree care. OK, water and trees. 469 00:21:56,440 --> 00:21:58,720 Anything else? The third take away is really an 470 00:21:58,720 --> 00:22:02,840 attitude shift, remembering that in surveying, millimeters 471 00:22:02,840 --> 00:22:05,520 matter. Don't dismiss small cracks or 472 00:22:05,520 --> 00:22:08,040 slight movements out of hand, especially if they seem to be 473 00:22:08,040 --> 00:22:09,960 changing or getting worse over time. 474 00:22:10,280 --> 00:22:12,800 Those tiny increments are the language the building uses to 475 00:22:12,800 --> 00:22:14,520 tell you something might be happening underneath. 476 00:22:14,920 --> 00:22:16,960 Pay attention to the small stuff, it can prevent bigger 477 00:22:16,960 --> 00:22:20,240 problems later. So be observant, manage water, 478 00:22:20,240 --> 00:22:23,240 manage vegetation. It's about proactive care based 479 00:22:23,240 --> 00:22:25,080 on understanding these subtle forces. 480 00:22:25,080 --> 00:22:28,040 Precisely, it encourages you to look at buildings differently 481 00:22:28,160 --> 00:22:30,760 with a more informed eye, understanding that constant, 482 00:22:30,760 --> 00:22:32,880 subtle dialogue happening between the structure and the 483 00:22:32,880 --> 00:22:34,560 ground beneath it. Absolutely. 484 00:22:34,800 --> 00:22:37,680 So the next time any of us walk past a cracked building, 485 00:22:37,680 --> 00:22:39,640 especially on clay, we won't just see a crack. 486 00:22:39,640 --> 00:22:42,400 We'll see a story. Not a monster clay myth, but a 487 00:22:42,400 --> 00:22:47,040 complex history written by water, roots and thyme, slowly 488 00:22:47,040 --> 00:22:50,560 shaping the story of that house. It's a much richer, more nuanced 489 00:22:50,560 --> 00:22:52,000 understanding. It really is. 490 00:22:52,440 --> 00:22:55,760 And maybe a final thought to leave you with if these tiny 491 00:22:55,760 --> 00:22:59,640 millimeter scale movements accumulating over decades can 492 00:22:59,640 --> 00:23:02,240 tell us so much about a building's resilience and its 493 00:23:02,240 --> 00:23:06,400 environment, what other slow, gradual, almost invisible 494 00:23:06,400 --> 00:23:08,880 processes are shaping our world around us right now? 495 00:23:09,520 --> 00:23:12,800 Think about micro changes in climate and ecosystems, maybe 496 00:23:12,800 --> 00:23:15,760 even in society. Things that seem insignificant 497 00:23:15,760 --> 00:23:18,720 day-to-day, but whose cumulative effects over time might be 498 00:23:18,720 --> 00:23:20,640 setting the stage for major impacts tomorrow. 499 00:23:20,640 --> 00:23:22,280 Our homes, our cities, our planet. 500 00:23:22,360 --> 00:23:25,120 This deep dive really highlights that sometimes to understand the 501 00:23:25,120 --> 00:23:28,120 big picture, you first have to learn to appreciate the dance of 502 00:23:28,120 --> 00:23:28,600 the small.