NG5.1 Backfill Material Classification
The assumed limiting performance of the five classes of backfill material defined in Appendix A1 is shown in Table NG5.1.
Table NG 5.1 - Backfill Material Performance
|Backfill Material Class
|Material Performance % CBR
| Over 15
| 7 to 15
| 4 to 7
| 2 to 4
| Less than 2
NG5.2 There are no Notes for Guidance
NG5.3 Additional Requirements
1) Frost Heave Susceptibility
The frost heave test described in BS 812: Part 124 (as amended by SHW Clause 801.8) is costly and time consuming and is not suitable for routine on-site control checks. The test is primarily intended as a method to establish whether an aggregate from a particular source is likely to be frost susceptible when used in road pavement construction. Material for the frost heave test must be representative of the source or sub-grade encountered. Authorities usually maintain a list of "Approved Suppliers of Non-frost Susceptible Materials" and should have knowledge of frost susceptible sub-grades in their locality.
The following notes on identification of potentially frost heave susceptible material are for guidance but are not, in themselves, exhaustive:
a) Clay materials can be regarded as non-frost susceptible, particularly when the plasticity index is greater than 15%. Clay/silt mixtures are more difficult to assess and are likely to be of marginal frost susceptibility.
b) Silts, particularly those with more than 10% passing a 75 micron (µm) BS sieve size, are likely to be frost susceptible.
c) Cohesive/granular materials will often be frost susceptible; the quantity and type of granular aggregate and, to a lesser degree the silt fraction are the controlling factors. If the aggregate is a frost susceptible type, then it is very probable that the mixture will also be frost susceptible.
d) Granular materials with more than 10% passing a 75 micron (µm) BS sieve size have a high potential for frost susceptibility and granular materials with more than 12% passing 75 micron (µm) are likely to be frost susceptible.
e) All crushed chalks are frost susceptible and the magnitude of the frost heave will increase with the saturation moisture content of the chalk.
f) Oolitic and magnesium limestones are likely to be frost susceptible, particularly those where the aggregate saturation moisture content exceeds 3.5%.
g) Hard carboniferous limestones are unlikely to be frost susceptible unless they have been contaminated with clay or have more than 12% passing 75 micron (µm).
h) Crushed granites will only be frost susceptible if the percentage passing 75 micron (µm) exceeds 12% and is partially plastic.
i) 'As dug' sands and gravels are frequently frost susceptible especially if the percentage passing 75 micron (µm) BS sieve size is greater than 12% or if it is plastic. Sands and gravels won by "wet working" techniques are unlikely to be frost susceptible unless contaminated by a clay or a high silt fraction.
j) Burnt colliery shales, slags, PFAs, etc. are secondary materials, and it is not possible to give general guidance to their potential for frost heave resistance. Each source is different and will need to be assessed by the frost heave test. The exception to this is graded bottom furnace ash produced by modern power stations, which has been found to be non-frost susceptible.
k) Foamed concretes can generally be regarded as non-frost susceptible.