20 NHBRC Test Questions, with Answers and Explanations

Answer B: A Geotechnical Engineer. The standard reserves this work for a “competent person (geotechnical)”. Definition 3.10 asks for more than registration: suitable experience in geotechnical site investigations or foundation design. A general civil engineer does not automatically hold that experience, and an inspector checks work rather than designs it.

SANS10400-H Foundation: Page 5

Answer A: A sink hole. Definition 3.40 states that a sinkhole is a feature that occurs suddenly and manifests itself as a hole in the ground. The suddenness is the point: subsidence is the slower settlement that happens when sinkhole formation stops early or the overburden consolidates.

SANS10400-H Foundation: Page 9, Par 3.40

Answer B: To document the parameters upon which the design of the foundation is to be based. Clause 4.2.1 sets out the purpose of the investigation: to document and form an opinion on the parameters the foundation design will rest on, and to classify the site against table 1. It is the input to a design decision, not paperwork produced for its own sake.

SANS10400-H Foundation: Page 11, par 4.2.1

Answer A: Clay soil site classification 'H". Table 1 groups fine-grained soils of moderate to very high plasticity (clays, silty clays, clayey silts and sandy clays) as expansive soils, designated H. They take up water and swell, then shrink as they dry, and that cycle is what cracks the masonry above them. Rock is class R and dolomite land falls under P.

SANS10400-H Foundation: Page 11, Table 1

Answer D: Contaminated soil, fill & marshes. Class P is table 1’s designation for ground whose behaviour the usual descriptors cannot predict: contaminated soils, controlled and uncontrolled fill, dolomite land, landslip, landfill, marshy areas, mine waste and mining subsidence. Rock, clay and sand each have classes of their own.

SANS10400-H Foundation: Page 11, Table 1

Answer A: Rock soil. Table 1 ranks founding materials by how much they move. Rock is the only entry listed as stable with negligible movement, site class R. Clays are expansive, and sands and gravels are compressible or potentially collapsible, so every other option brings movement the design has to absorb.

SANS10400-H Foundation: Page 11, Table 1

Answer C: 300mm. Figures 3 and 4 dimension the slab-on-the-ground option at 300 mm minimum, against 400 mm for a strip footing in the same drawings. A slab spreads load across its whole area instead of concentrating it in a trench, so it does not have to reach as deep.

SANS10400-H Foundation: Page 16, Fig.a

Answer B: 400mm. Figure 5 marks the strip footing “400 min.”, with rock called out as the exception. Rock is excepted because it is already a stable founding material; everything else has to get below the zone where seasonal moisture moves the soil.

SANS10400-H Foundation: Page 16 Fig.b or Page 17 Table 3

Answer C: 500mm for the external wall & 400mm for the internal wall. Table 3 splits the width by wall and by roof. For soil under a tiled or sheeted roof it asks 500 mm on the external wall and 400 mm internally. The external wall takes the roof load down one side, so it needs the wider spread. On rock both drop to 400 mm.

SANS10400-H Foundation: Page 17, Table 3

Answer B: 200mm. Clause 4.3.2.2.1(c) sets the thickness at not less than 200 mm, with one exception: bearing straight onto solid rock, where it need only be thick enough to give a level surface. Width comes from table 3. Thickness is specified separately.

SANS10400-H Foundation: Page 17, Par 4.3.2.2.1ac

Answer B: 400mm. Table 3’s note allows 400 mm for internal walls carrying no bearing from a reinforced concrete roof, the same width the table already gives internal walls under a tiled or sheeted roof. The wider 600 mm and 750 mm figures apply only once a concrete roof bears on the wall.

SANS10400-H Foundation: Page 17, Table 3

Answer B: 750mm & 600mm. Table 3’s reinforced concrete roof columns read 750 mm internal and 600 mm external on soil. The internal figure being the larger one catches people out: an internal wall under a concrete roof can pick up load from both sides, while an external wall picks it up from one.

SANS10400-H Foundation: Page 17, Table 3

Answer D: Sink hole. What makes dolomite land hazardous in this standard is not the mineral but the sinkhole. Definition 3.40 describes a feature that occurs suddenly and manifests as a hole in the ground. Annex A.7.3 is built around keeping a building standing, and its occupants able to escape, if one opens beneath it.

SANS10400-H Foundation: Page 31, Par A.7.3.1

Answer B: A raft, mat or slab on the ground foundation. Clause A.7.3.1(d) requires a soil mattress or a reinforced concrete foundation on D3 dolomite sites, and A.7.3.4 sets what it has to survive: a loss of support 2 m or 5 m across, anywhere under the building. A strip footing cannot bridge a void of that size; a raft, mattress or slab can.

SANS10400-H Foundation: Page 33, Par A.7.3.4

Answer A: 50mm. Definition 3.7 fixes a flooring board’s face side width between 50 mm and 140 mm, so 50 mm is the floor and 140 mm the ceiling. Both limits come from that one definition, which is why the pair of questions has the same source.

SANS10400-J Flooring: Page 5, Par 3.7

Answer B: 140mm. Definition 3.7 caps a flooring board’s face side width at 140 mm, with 50 mm as the lower bound. Capping the width matters because timber moves most across the grain: the wider the board, the more it cups and shrinks in service.

SANS10400-J Flooring: Page 5, Par 3.7

Answer D: 450mm. Clause 4.3.3 requires at least 450 mm between the under-surface of the ground floor joists and the ground below. That gap is what lets the subfloor ventilate and be inspected; the same clause calls for access and for all debris to be cleared from the void.

SANS10400-J Flooring: Page 10, Par 4.3.3

Answer A: 100 mm. Clause 4.4.6 sets two layer thicknesses and the tool decides which applies: 100 mm for hand compaction, 150 mm by mechanical means. Hand compaction gets the thinner layer because it drives less energy into the fill.

SANS10400-J Flooring: Page 11, par 4.4.6

Answer B: 150 mm. Clause 4.4.6 allows 150 mm uncompacted layers where compaction is mechanical, against 100 mm by hand, and each layer must be well compacted before more fill goes on. The limit is about getting compaction right through the depth, not about how fast the fill is placed.

SANS10400-J Flooring: Page 11, par 4.4.6

Answer B: 400 mm. Clause 4.4.7 hands the job to a competent person (civil engineering) once fill beneath a floor exceeds 400 mm at any point. Deep fill settles under its own weight as well as the building’s, so past that depth it stops being a standard detail and becomes a design.

SANS10400-J Flooring: Page 11, Par 4.4.7