Question

Why are voids filled with lightweight infilling material in raft base of pumping stations?

Answer

To decrease dead load and therefore to decrease settlement of pumping stations, voids inside raft foundations are overflowing with light material. If in its place concrete is placed in these voids, it poses harsh thermal cracking difficulty and drastically increases loads to pumping stations. Use of general fill is also not desirable since its self-weight is comparable to that of concrete. On contrary, if these voids are left vacant, water might penetrate into these voids during future operation and increases dead load of pumping stations during its standard operation. So, lightweight infill material that is non-water-absorbing and non-biodegradable is intended inside these voids to stay away from ingress of water and to reduce dead load of structure.