Site classification and reactive clay in Adelaide

What the NCC requires

Clause 4.2.2 of the ABCB Housing Provisions (NCC 2022) is one sentence: "The foundations where footings and slabs are to be located must be classified in accordance with AS 2870." South Australia’s variations to Part 4.2 of the Housing Provisions (Schedule 8) replace two other clauses in the Part and do not vary 4.2.2.

The explanatory information attached to the clause describes its own table as "a general description of foundation soil types that will assist in the classification of a site", and says that more detailed information, "including differentiation between classifications, can be found in AS 2870 or alternatively contact the appropriate authority". The table explains the classes; the classification itself is made under the Standard.

The classes, as the explanatory table describes them

Explanatory Table 4.2.2 gives these general definitions. Class A: "Most sand and rock sites with little or no ground movement from moisture changes". Class S: "Slightly reactive clay sites with only slight ground movement from moisture changes". Class M: "Moderately reactive clay or silt sites which can experience moderate ground movement from moisture changes". Class H: "Highly reactive clay sites which can experience high ground movement from moisture changes". Class E: "Extremely reactive clay sites which can experience extreme ground movement from moisture changes".

Two further entries sit outside that scale. Filled sites are listed as "A to P", with the note "see AS 2870". Class P covers "sites which include soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise."

"Reactive" in these definitions refers to the behaviour the table names: clay that moves as its moisture content changes. For Classes A to E, the letter describes how much ground movement a footing on that site has to accommodate.

Class M, and what M-D means

The notes to Table 4.2.2 add a subdivision that the single letters do not show. "For Class M, further division based on the depth of expected movement is required." The next note reads: "For deep-seated movement, characteristic of dry climates and corresponding to a design depth of suction change Hs, equal to or greater than 3 m, the classification must be M-D."

The consequence is stated in the third note: "If classification M-D is established due to further division, design of footings and slabs is beyond the scope of the ABCB Housing Provisions and reference must be made to AS 2870 for design and construction information." Whether a given site is M or M-D is part of its classification, not something that can be read from the letter M alone.

Where the Housing Provisions stop

Clause 4.2.1 states that Part 4.2 "is subject to the limitations set out in H1D4(2)". H1D4 sits in NCC Volume Two. Its subclause (1) lists the routes by which footings and slabs satisfy the structural Performance Requirement: AS 2870, AS 3600, or "subject to (2), Section 4 of the ABCB Housing Provisions". Subclause (2) then says that Section 4 "may only be used where" a list of conditions is met. The first is that "the footing is on a Class A, S or M site (classified in accordance with AS 2870) with a uniform bearing capacity".

The footing tables inside Part 4.2 follow the same line. Clause 4.2.14 covers "Stiffened rafts Class A, S and M sites" and clause 4.2.15 covers "Strip footings Class A, S and M sites". The explanatory information to 4.2.2 puts the other side plainly: "if a site is classified H, E or P then reference must be made to AS 2870 for design and construction information."

None of this says a house cannot be built on an H, E or P site. It says the Housing Provisions’ tables are not the design route for one, and points to AS 2870 instead.

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A block that has already carried a house

Under Volume Two clause H1D3(1), earthworks in accordance with Part 3.2 of the Housing Provisions satisfy Performance Requirement H1P1 "provided that the site is classified as A, S, M, H or E in accordance with 4.2.2 of the ABCB Housing Provisions and the work is undertaken in normal site conditions." Among the conditions in H1D4(2) is that "the site is considered to be normal as defined in Part 3.2 of the ABCB Housing Provisions".

The explanatory information to H1D3 lists parameters that "normal" site conditions relate to. Two of them are "the site conditions not being significantly modified by the removal of previous buildings or other structures", and "large trees have not been recently removed from the site in the area where the building is to be constructed". It adds that "further information regarding normal and abnormal site conditions can be found in AS 2870."

The NCC text does not say whether any particular demolition has "significantly modified" a site, and this note does not answer that for any block.

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