The Fleet, the Tyburn, the Lea, the Peck, the Westbourne, the Walbrook and the Effra are probably most well known. Some have intriguing names like the Cock and Pie Ditch, the Ching and the Quaggy, while others are just plain uninviting like Carbuncle Ditch and the Earl’s Sluice, but they’re all still there – somewhere.
The Great Stink
London has long had a habit of polluting its streams, and people began paving some of them over as long ago as 1460 to subdue the smell. But rivers are not easily subdued. The Fleet was an important tidal basin several hundred feet wide when it reached the Thames at Blackfriars Bridge. Its pollution consisted of “the sweepings from butchers’ stalls, dung, guts and blood” from Smithfield market and yet a plaque at St. Pancras Old Church shows people still bathing on its banks in 1827. In 1838 Charles Dickens placed Fagin’s Den there in Oliver Twist.
The routes of some are well known, but smaller streams that once drained into them were left to find their own way after the rivers were enclosed, and today they are still largely uncharted except when they attract attention by flooding someone’s cellar. Occasionally they overflow and try to re-establish their old routes: the banks around the Oval cricket ground are made of mud excavated when sinking the River Effra that surrounds it, but they didn’t stop it taking a short-cut across the pitch in the 1950s.
Constructing a basement in London therefore often depends upon advanced waterproofing methods, including membranes, seals and gaskets, low-maintenance pumps and a range of concrete additives.
Cavity draining membranes are often applied internally or externally or both but are usually used in combination with other techniques.
Waterproof barrier protections are applied externally mostly on new-builds, but it is possible to excavate and apply them to existing buildings. Polymer-modified bitumen coatings, polymer-modified cement slurries and resin-based coatings are all used in different circumstances.
Structural integral protection is possible in new-builds or newly built extensions. The walls themselves are constructed from waterproof materials. They have extra density and incorporate sophisticated waterproofing additives like water-repellent stearates, colloidal silicate densifiers and hydrophilic chemicals that crystallize into calcium silicate hydrate inside concrete pores. Different admixtures are chosen depending upon the anticipated hydrostatic pressure.
Joints and Services
Extra attention is always given to construction and expansion joints and the points where external pipes and services penetrate the walls and their protective coatings. They are dealt with by incorporating hydrophilic gaskets, sealants, internal and external waterstops, waterbars and injection hoses. Extruded flexible profiles of PVC or elastomer are cast into the concrete on both sides of any joint, and hydrophilic materials swell on contact with water to seal leakage points.
Many connection pieces can be pre-assembled off-site for a fast build with minimum disturbance to residents.
So there is much more to converting a basement in London than slabbing and toshing bitumen on top. That’s why the services of a basement construction specialist who will recognise the appropriate methods to use are indispensable.