On occasion we are asked to provide remedial works to existing basements. These might be required because of people changing a storage cellar to habitable use or to overcome design/construction faults. When building a basement from scratch it is normally possible to seal the basement so that water cannot enter (Tanking).
However it should be noted that the relevant UK building standard warns about the effect of less than perfect workmanship! (BS 8102:1990 Code of practice for protection of structures against water from the ground) So you should never assume that a contractor is skilled enough to fully tank a basement.
Once the basement has been built it is more difficult to enhance the tanking and you need to consider ways of managing the water instead. To manager water you need to divert water into drains, channel it where you want it to go and finally pump it out.
In these pictures you can see the 52 square meter basement of a house in Drumcondra, 
on the north side of Dublin. We were asked to see if we could provide a secondary waterproofing system for an existing basement. We designed and implemented a solution based around "Internal Cavity Drainage Mats" from Delta Membranes and a sump with twin (active and standby) pumps.
Internal cavity drainage mats are made from high density polyethylene that have a dimple patterned side in contact with the floor and wall. These very hard mats trap any water than enters the basement and guides the water to a defined point called a sump. 
The mats are attached to the existing basement structure using mechanical fixtures so there is no reliance on glues in what could be a very damp environment. Because only the dimples on the mats are in contact with walls, water which crosses the wall is free to move about under the mat. In this way any damp pressure is taken by the entire mat and weak points do not occur.
Naturally any water that crosses the walls will fall under gravity to the floor. By sloping the floor towards a sump, we can ensure that any water either from a leak in the tanking or from an internal source like burst radiator will be gathered at a single point. This type of design is commonly called water management, it is about using the natural properties of water and not simply trying to block water from entering (Tanking).
The sump itself is simply a depression in the floor where water will naturally gather. The sump is drained by a pump out of the basement and into a suitable drain. There is a very wide selection of pumps available on the market that have various degrees of redundancy built in. In this case the pump is a DELTA 302SE/HP. This is duel automatic pump in a single sealed fitting and alarmed. The active and redundant pump are both capable of handling the design load. The purpose of the redundant pump is to kick in if the active pump fails. 
Naturally these pumps are designed to work automatically so they have sensors to detect when the water level in the sump reaches a predetermined point. So if there is no water in the sump the pumps do not burn themselves out. The entire sump pump is monitored by an alarm system so that if either pump fails to work correctly an alarm is raised.
This solution is designed to handle up to 13 litres per second for each square meter of surface protected, automatically. The entire project from start to finish took just a few weeks and the client is delighted having quickly converted poor quality storage space into very useful habitable space.