Dulwich inhabitants cannot fail to be aware of Dawson's Heights, the battleship-like block of flats that dominates the eastern sky-line from the Park or as one descends Dog Kennel Hill. I happened to see a reference to the engineering works that preceded its construction in some local publication and thought that readers of the Dulwich Society Newsletter might be interested in knowing why and how they came about.
As a child, I lived in Hillcourt Road, just at the foot of Donkey Lane, the muddy track that led up to the hill. The hill was a playground for my pals and me and many were the dug-outs we created from time to time. From there, I watched with my father Crystal Palace burning, and later, in 1940, I saw great formations of German bombers, harassed by RAF fighters, bombing London Docks.
Did my sins, ie. my excavations, come back to haunt me? I don't think so, but later, in my career as a Civil Engineer, I became responsible for running the contract for stabilising the hill. By this time, the houses that crowned the top of the hill, mainly built in the 20s and 30s had been compulsorily purchased. Indeed, many had become uninhabitable in any case due to the gross instability of the ground on which they stood. The whole hillside was on the move and houses at its foot in Dunstan's Road, now demolished, were literally being pushed over.
The hill was formed of London Clay, which, if waterlogged and over a certain slope, is very unstable. The instability had been exacerbated by the tipping of waste material at the top of the slope. It was obvious that something had to be done to stop the situation getting worse, especially as the crown of the hill was due to be developed. The firm of consulting engineers that I worked for were engaged to seek a solution.
To assist with the preliminary investigation we obtained the services of an expert from Imperial College who knew a lot about London Clay. We had a couple of trial trenches dug, heavily propped because the clay was so unstable, so that he could examine the sides to form a picture of how the clay had behaved as it shifted down the slope. He got very excited about what he observed and went back and told his boss about it. This was the great Professor Alec Skempton, the leading geotechnical expert at the time, and certainly the man who knew the most about the behaviour of clay soils.
Following an investigation carried out by Skempton, it was decided to excavate what are called buttress drains, about 10 metres apart, and some 6m deep, running down the slope. These would be filled with granular material, free-draining, in contrast to the surrounding clay. Here we have to get a bit technical, because the water in the clay generated something called "pore-water pressure" which had to be reduced if the clay was to be stabilised. The buttress drains, filled with the granular material, would serve to reduce the pressure by gradually draining water from the clay. This water would flow to the lower end of the slope along the edge of Dunstan's Road and would be collected by a drain running parallel to the road into an intercepting chamber and then discharged into the sewer in the road. Installation of this drain, which was at some depth, caused me some worry. Digging a trench across the line of this grossly unstable slope was asking for trouble.
So, in the end, I specified that the drain should be installed by a process called "thrust- boring" which involved pushing a "mole", pulling a perforated pipe behind it, from a pit at the upper end of the drain to the interceptor pit , intersecting with the lower ends of the buttress drains as it did. It came out at more or less where we hoped. Thrust boring, especially in this sort of material, is not an exact science!
However, I've got ahead of myself. We had to get contractors willing to tender for the work. I got a few firms interested but I wanted them to be under no illusions as to how difficult the work would be due to the condition of the clay. So I decided to stage a demonstration on the site for tenderers to attend. For this purpose we had to bring an excavator to the site capable of digging the required depth, quite a large machine, especially in 1967. I tried a number of plant-hire firms but I could not persuade any of them to bring a suitable machine to site just for one day, certainly not at a reasonable price. However, thumbing through a paper devoted to the construction industry I chanced upon an advertisement by a major French firm of plant manufacturers for a new machine that they were just putting onto the UK market. They offered to bring this splendid machine to any site to demonstrate it at no cost! Manna from heaven! I contacted them and they were delighted to come. On the appointed day, they turned up with not one machine but two, one being a bit smaller. Just as well they did, because no sooner had the little machine driven off the low-loader than it got stuck and had to be rescued by the bigger one, which then proceeded to demonstrate its capabilities.
So the contractors went away, suitably impressed with the difficulties of the site which were reflected in their prices! However, the cheapest contractor got the job and bought one of the larger machines! Satisfaction all round!
Every time I drive down Dunstan's Road I take a sidelong glance up the slope to see if there is any sign of movement but, nearly 40 years later, it still seems OK. Incidentally, if any reader is concerned over the stability of Dawson's Heights on the crown of the hill, they are on very deep foundations well below any likelihood of instability. I specified those as well, just in case!