All below ground structures should be considered liable to water pressure. Even if ground water is anticipated below the excavation level, capillary action will allow ground water created by post construction changes in adjacent ground conditions. Flood or adverse weather conditions may cause water to rise. Structures are not completely rigid and by consequence even small movements can lead to serious cracking which will allow water ingress!
New structures can be made water resistant by correct design to the requirements prescribed within BS8102:2009 by providing correctly placed hydrophilic water stops at all concrete construction / day work joints and kickers and by incorporating a continuous, passive waterproof membrane which fully encapsulates the structure by linking with the DPC / cavity tray detailing. Concrete is naturally porous and cannot be considered on its own to be water-tight as water will seep through pores within the concrete. This problem is exacerbated when the concrete is not vibrated adequately and when the water becomes under pressure.
Structural waterproofing or "tanking" is the procedure for providing a water and vapour proof structure. Waterproofing to the external face - which ensures all ground water actually forces the membrane onto the structure - with a passive, non-reactive membrane ensures the entire structure remains watertight for its anticipated life. Waterproofing on internal faces, other than multi layer applications of proprietary renders and internal pumped drained cavity designs, should be avoided as they could allow any water under pressure to force off the membrane.
It is important to consider that ALL joints between concrete blocks or mortar in brickwork/blockwork are weak points in a structure and therefore more likely to leak. The use of concrete infill between two skins of block or brick is often employed, however, it is critical that adequate attention is given to concrete mix design and vibration as cavitation occurs. The detailing of all penetrations such as service and pipe ducting is essential as is adequate site management of all following trades.