Backfill material classification (S5.1)

General

S5.1.1: Backfill materials, whether imported or sourced from excavated materials, are classified as follows:

S5.1.2: Materials with a maximum of 10% by mass passing a 63 µm BS sieve, and with all material passing a 425 µm BS sieve showing a plasticity index of 6 or less, determined in accordance with BS 1377-2: Method 5.4, are classified as Class A graded granular materials.
S5.1.3: Class A Graded Granular Materials may include Type 2 Unbound Mixtures to MCHW Clause 804 (excluding natural sands and gravels) and Type 1 Unbound Mixtures to MCHW Clause 803.
S5.1.4: Class A Graded Granular Materials may also include the Modified Type 1F Unbound Mixture specified in A10.1. This is the preferred unbound material. This type of unbound mixture cannot be used if direct trafficking of the backfill could occur during construction.

S5.1.6: Mixtures of granular, silt and clay materials with between 10% and 80% by mass passing a 63 µm BS sieve.

S5.1.7: Clay, silt or mixtures of clay and silt with at least 80% by mass passing a 63 µm BS sieve.

S5.1.8: Materials listed as unacceptable in paragraphs 2 ii) and 3 of MCHW Clause 601 cannot be used at any level in the permanent structure of any reinstatement. Materials classified as unacceptable are listed in A1.5.

S5.1.9: FCRs and HBMs, complying with A2.5.3 to A2.5.18 and A10.2 respectively, are permitted within the backfill as the entire layer or combined with any other permitted backfill materials, in any proportion, within any reinstatement. These materials are classified as Class A for the purposes of determining the requirements for sub-base reinstatement in accordance with S6.2.
S5.1.10: The requirements of A1 apply to unbound backfill materials.
S5.1.11: Backfill materials Classes A to D must be compacted in accordance with A8. HBMs must be compacted in accordance with A10.2. and FCR as per A2.5.16.