Page 34 - Build 153

BASEMENT WALLS UP TO 1 STOREY BELOW GROUND WITH NO HYDROSTATIC PRESSURE
DESIGN
Down and dirty
Basements can be a handy part of any home. Ensuring they will be dry and warm throughout their life can be a signi cant challenge for both the designer and the builder.
RIGHT
BY TREVOR PRINGLE,
ANZIA, BRANZ PRINCIPAL WRITER
RESIDENTIAL BASEMENTS are common in houses built on sloping ground and can be valuable and useful spaces if they are dry and warm. In many cases, the use of a basement varies over time as household size, lifestyle and interests change.
In this article, we look at basement walls of residential buildings on sloping sites that are no more than 1 storey in height below natural ground and not subject to hydrostatic pressure.
Factors a ecting basements
When designing or retro tting basements, consider the:
● groundwater, which may:
● be a free- owing liquid
● move by wicking (capillary ow)
● move by di usion through the soil or an
unprotected or leaking basement wall
● slope
● ground conditions – rock, sand, loam
● proximity to adjacent structures, roadways or
banks
● access for machinery
● removal of wastewater.
Signi cant challenges to address
Developing spaces below ground signi cantly increase the challenges for both the designer and the builder. It adds the need to ensure:
● groundwater is excluded
● excavated earth or sand banks are temporarily retained
● the work below natural ground can be carried out safely without risk to workers from collapse
● su cient space is provided behind the wall to carry out essential tasks such as wall construction and damp-proo ng
structural concrete or concrete masonry wall
framed wall (at least 70 mm wide) with bulk insulation
25 mm rebate (recommended) DPC
top of wall sealed
footpath or ground to fall away from building
surface water channel
base course
continuous waterproof membrane
membrane protection drainage metal back ll
lter fabric
slotted drain between oor and footing level (must be below oor level) surrounded in drainage metal to fall to two separate drainage points
concrete oor slab
Figure 1
tidy slab
Constructing an insulated framed wall inside the basement wall.
● any hydrostatic pressure is managed – a wall subject to hydrostatic pressure requires a speci cally designed tanking solution
● the complete wall has su cient thermal insulation
● any surcharge applied to the wall (from adjacent foundations, driveways etc) is allowed for
● any services installed are well thought out, for
example, a pumped system is typically required to lift waste to the council sewer lines.
Critical to get right rst time
If we don’t get it right at design and during construction, any leaks through basement walls are likely to be extremely difficult and expensive to repair.
32 — April/May 2016 — Build 153