Last year I was commissioned to re-landscape a roundabout in a private development just north of Regents park. The project included reducing the size of the roundabout to give better access to emergency vehicles, regrading, and replanting. The groundworks were carried out in March in appalling weather conditions.
Eventually the weather dried up and the site was planted. All looked good for about six weeks, then the plants started to die. Investigation showed that the ground was completely waterlogged. A ring drain was put into the roundabout, out of which water poured continually. The weather was dry and for a while it seemed the problem had been solved. Then we had more rain and we were called back as the planting was looking worse. This time we found that except for small area alongside the land drain, the water table was within inches of the soil surface. Even in the centre of the roundabout, which was domed up to about 6ft above the surrounding road.
We considered all options, disrupted pipes, impervious subsoil. It was obvious that water was being forced up through the roundabout under some pressure. Eventually we deduced that the water was the result of building works further up the hill. Apparently double basements had been built to the north of the site, which appear to have diverted an underground water course. This has resulted in a neighbouring block of flats having to spend large sums on re-tanking a basement and basement flats on the site being rendered unusable.
Since then I have heard of three other cases where basements or semi- basements, which had long been dry were suddenly rendered unusable. In each case there has been recent work nearby with large new basements being dug out. This is all hearsay and conjecture. I cannot prove that the basement works some way from my project were the cause of the site problems, but there seems no other logical explanation.
In many areas in London, the water table is quite high, as Archimedes discovered, throwing more stuff into the bath tub will make it overflow. pipobrien [at] btinternet [dot] com