Thin unreinforced concrete house foundation walls

Abstract

The objective of this study was to determine the feasibility of building residential basement foundation walls of unreinforced concrete thinner than the conventional 200 mm thick wall. The thin-wall foundation should require no special engineering attention, and should be acceptable as a standard to the industry.The study comprised a literature search, building code review, and a feasibility analysis which included technical and cost considerations. Additionally, an industry perspective was obtained through interviews with house builders and contractors specializing in residential basement work.Key issues were determined to be strength, quality control, construction methods and tolerances, cost savings, and market acceptance. As thickness decreases, strength is sacrificed, and the negative effect of axial eccentricity increases, particularly for foundation walls supporting brick veneer. Quality control, particularly in respect of concrete strength and water-to-cement ratios, is important from the perspective of maintaining bending capacities. Some construction methods, for example, cast-in-place joist or the use of nailing ladders, make pouring concrete more difficult in thinner walls. Cost savings and market acceptance are linked, and though costs can be reduced, it may be difficult to influence buyers to break from the traditional 200 mm wall in favour of a thinner one.Consideration of the issues led to the determination of 150 mm as the optimum reduced thickness for an unreinforced 2400 mm high foundation wall. A basement wall of 150 mm thickness and 20 MPa concrete strength supporting a house with brick veneer would be capable of retaining 1500 mm of backfill material. The same wall supporting a house without brick veneer would be capable of retaining 1800 mm of backfill. The cost of the 150 mm thick wall would be in the order of $600 less than that of the conventional 200 mm wall, and those savings could be passed on to the consumer.Technical analysis was based on equivalent fluid pressures and indicated that the 150 mm foundation wall would be suitable for use with clean sand and gravel backfill material. For walls backfilled with soils other than clean sand and gravel, it was found that the theoretical maximum backfill heights for both 150 and 200 mm walls are substantially less than those presently specified in Table 9.15.4.A of the National Building Code. The practice of backfilling with stable native soils (which are characterized by significantly higher equivalent fluid pressures) is common and house foundations have performed well under those conditions. The study therefore concluded that either the accepted values for equivalent fluid pressures or the assumed triangular distribution of lateral soil pressures are in question. Key words: foundations, concrete, residential buildings, houses, soil pressures, lateral pressures, unreinforced concrete walls.