When inquiring about below-ground waterproofing (also referred to as tanking), the first question to be answered is “what is your water control system after construction is complete?” If there is no water control in place, then depending on the water table, there will be a level of hydrostatic condition present (hydro — water, static — still). These conditions/pressure need to be detailed with great attention to prevent water ingress.
With the initial question answered, the design comes into question. Are the walls constructed of block, pre-cast panels, in-situ concrete or piles (secant or contiguous)? Are any of the neighbouring walls in close proximity, e.g. 100mm? The best methodology and best-suited membrane can be decided on answering these questions.
The perimeter walls may differ in construction, with one close to the boundary of the neighbouring building, the next having good access and not totally below ground. These variations and transitions also require special attention when detailing to ensure compatibility of products.
Many answers to these questions provoke additional questions. Due to the many and varied complexities of design, it is extremely important to gain as much information prior to design completion. Combined meetings with architects, engineers, geo-technicians, contractors, and in the case of waterproofing — a good technical advisor, provide the knowledge to prevent excessive design alterations after construction has commenced.
If there is a good water control system in place (for example waterproofed retained walls applied with drainage sheet, backed with a good healthy depth of drainage material falling to drain coils at the base of the walls), in conjunction with sub-floor drains (where necessary) all feeding or pumped to stormwater, after construction is completed no hydrostatic conditions will be present. All this in place means there is less concern of water reaching the retained wall.
One extremely important aspect to mention in this water control design is where the drain coil at the base of the retained wall is located. If the drain coil sits too close to the internal FFL (finished floor level), then in one-off rain events, this location could experience temporary hydrostatic conditions prior to draining away, causing water ingress. Locating the coils well below these FFLs will allow these vulnerable areas sufficient time for drainage to occur (see sketch below).
It's also important to revisit costings and in particular, below-ground waterproofing, prior to build commencement. During the time taken from initial build conception through to build commencement, including discussions with the client, architects, engineers and contractors, areas of the design may have been altered. Revisiting below-ground waterproofing at this stage can help ensure all parties are on the same page.