Drainage board is made of HDPE with impact resistance and pressure resistance which can resist long-term high pressure,and will not generate deformation. The long-term pressure resistance not only can resist weight of concrete bottom plate and its load, but also can bear unavoidable outside pressure and impact force during construction, for example, vehicle, worker stepping, concrete pouring impact force,etc. It can keep bottom plate undeformed for long term, so original quantity can be kept.

Application of filter layer earthwork fabric on bottom plate is to ensure drainage channel is not blocked by foreign object, for example, backfilling particle or concrete.