Concrete has advantages over other less dense materials for use in earth-contact structures because of its superior strength and its low water permeability. Because this writer has had excellent continued experiences with poured reinforced concrete earth-contact structures, this article will be primarily concerned with tested satisfactory use of this material. Topics covered include requirements for concrete and steel; post-tensioning; precast roof slabs; uniform foundation support needed; adequate waterproofing and drainage needed; placing and protecting the steel; structurally adequate retaining walls; thorough curing; stripping requirements; and not backfilling too soon.
Because of the unusual sustained load that the roof of an earth-sheltered dwelling is subjected to, it is advisable to allow the concrete to achieve its full design strength. This strength should be reached in 28 days; therefore, no soil should be placed on the roof until after this lapse of time. In order not to foul the foundation drain tile, it will be necessary to compact 1 foot or more of fill material on top of the gravel or crushed stone that is over the tile. After this is done, the fill material can be placed ready for natural settlement. When backfilling is done by natural settlement, a certain amount of dressing up is required from time to time. If this important function is neglected water will begin to pond. Ponding will cause a greater than normal amount of water to flow into the foundation drain tile, possibly causing early fouling of this vital part of the building.
Only select fill materials should be used for backfilling. If the fill contains roots and sizable rocks it tends not to achieve acceptable density by natural settlement and leaves the roof cover susceptible to water penetration.