Appendix A1 Backfill Materials

A1.1 Class A - Graded Granular Materials

1) Materials should be well-graded granular material with a uniformity coefficient greater than 10. Material shall, at the time of compaction, be at an appropriate moisture content between +1% and -2% of the optimum moisture content as determined by BS1377: Part 4; Vibrating Hammer Method, Method 3.7, or shall be acceptable when subjected to Field Identification Test No.3.

2) Materials shall show a Los Angeles Coefficient - maximum of LA₆₀ when tested in accordance with BS EN 1097-2. The principal materials that will be excluded are sandstones, weakly cemented gritstones, argillaceous (clay derived) materials, the softer magnesium limestone, oolitic limestone and the majority of chalks.

3) Manmade aggregates, e.g. slag, PFAs, clinkers and furnace bottom ash will need individual assessment; it is possible to demonstrate satisfactory performance with some of these materials, even when they fail to meet the 10% fines value requirement.

A1.2 Class B - Granular Materials

Material at the time of compaction shall be at an appropriate moisture content between +1% and -2% of the optimum moisture content as determined by BS1377: Part 4; Vibrating Hammer, Method 3.7, or shall be acceptable when subjected to Field Identification Test No.3.

A1.3 Class C - Cohesive/Granular Materials

1) Materials with less than 50% granular content by mass shall, at the time of compaction, be at appropriate moisture content between 0.8 and 1.2 times the plastic limit, or be acceptable when subjected to Field Identification Test No.2.

2) Materials with a minimum of 50% granular content by mass, shall at the time of compaction, be at an appropriate moisture content between +1% and -2% of the optimum moisture content as determined by BS1377: Part 4; Vibrating Hammer, Method 3.7, or shall be acceptable when subjected to Field Identification Test No.3.

A1.4 Class D - Cohesive Materials

1) Cohesive materials at the time of compaction shall be at appropriate moisture content between 0.8 and 1.2 times the plastic limit, or be acceptable when subjected to Field Identification Test No. 2.

2) Clays that contain insufficient moisture when excavated, or have dried excessively during site storage, as defined by Field Identification Test No. 2, may only be re-used provided that they are wetted to comply with Section A1.4 (1) and compacted in accordance with Appendix A8 for Class D Cohesive Materials.

3) It may be difficult to compact cohesive materials to uniformly achieve an adequate bearing capacity. Undertakers must select a lump size for clays within the limits specified in Appendix A2 and must ensure that all compaction equipment is operated within the requirements of Appendix A8. Failure to comply with Appendix A2 and/or Appendix A8 will result in unacceptable settlement and variable bearing capacity.

4) High silt content materials, as defined by Field Identification Test No. 1, shall be compacted in accordance with Appendix A8 requirements for Class D Cohesive Materials.

A1.5 Class E - Unacceptable Materials

The following materials, listed as unacceptable in SHW Clause 601 paragraphs 2(ii) and 3, shall not be used at any level within the permanent structure of any reinstatement:

1) Peat and materials from swamps, marshes or bogs.

2) Logs, stumps and perishable materials.

3) Materials in a frozen condition. (Such materials, if otherwise suitable, shall be classified as suitable when unfrozen.)

4) Clays having a liquid limit exceeding 90, determined in accordance with BS1377: Part 2 Method 4, or a Plasticity Index exceeding 65, determined in accordance with BS1377: Part 2, Method 5.4.

5) Materials susceptible to spontaneous combustion.

6) Materials having hazardous chemical or physical properties requiring special measures for excavation, handling, storage, transportation, deposition and disposal.

A1.6 Field Identification Tests

The following identification tests must be carried out immediately prior to the placement and compaction of the backfill material.

Field Identification Test No 1 - Silt Identification

High silt content materials can usually be identified by a simple hand test:

Preparation

Select a moist sample of the fine material only.

Test - Silt Identification

With clean dry hands, rub the sample between the palms, remove the excess material by striking the palms together and wait a few minutes for body heat to dry out any material adhering to the hands. Finally, rub hands together briskly.

Result

If no significant quantities of material remains adhering to the palms, i.e. the palms are relatively clean, then the sample tested is essentially a silt.

Note: The proportion of granular material discarded to produce the fine sample must to be taken into account when estimating the approximate silt content of the bulk material.

Field Identification Test No 2 - Clay Condition

Clays suitable for compaction with pedestrian controlled compaction plant can usually be identified by a simple roll test:

Preparation

Select a sample of small lumps of the fine material only, at a moisture content representative of the bulk material.

Test - Clay Condition

With clean dry hands, take the sample and squeeze together in one hand and release. If the sample crumbles away and mostly fails to hold together into a 'ball' then the sample is too dry for compaction. If not, break off part of the ball and roll between the palms or between one palm and any convenient clean dry flat surface, for example the back of a spade. Roll out the sample into a long thin cylinder until it fractures or begins to show significant transverse cracks.

Result

If the strand can be rolled into intact or uncracked lengths that are thinner or longer than a standard pencil, i.e. less than 7 mm diameter or more than 175 mm length then the sample is too wet or too plastic for compaction. Any result between the ball and the pencil is acceptable for use provided the bulk of the material consists of lumps less than 75 mm in size.

Field Identification Test No 3 - Granular Condition

All granular materials including Type 1 granular sub-base to clause 803 of the SHW must be compacted near to their optimum moisture content. The optimum moisture content can vary considerably depending on the average particle size and to a much smaller extent, on the type of mineral or rock involved. However, a laboratory compaction test is invariably carried out on a sample of material from which the larger particles have been removed. The sample is always compacted in a small smooth sided steel cylinder and the standard methods of compaction bear little similarity with current compaction plant. Experience has shown that the most commonly specified laboratory compaction test i.e. BS1377: Part 4; Vibrating Hammer, Method 3.7 will produce an optimum moisture content result that is, typically, significantly wetter than the field optimum for a granular material that is to be compacted within a trench using a vibrotamper.

Granular materials suitable for compaction by pedestrian controlled plant can usually be identified by a simple visual examination. Typically, the test will identify materials within 1% to 1.5% of the field optimum moisture content depending on the mineral type. Experience has shown that compaction within this visual moisture range will not normally show any significant reduction in compaction performance. The test cannot indicate the actual moisture content of any material but this is rarely of any relevance as far as an operator is concerned.

Preparation

Depending on the size of the stockpile, dig out representative samples from beneath the outer surface, at several positions around the outside in a conical shape.

Test - Coarse Aggregate

Examine several of the medium- and larger-sized particles from each sample extracted.

Result

Material within the target moisture content range will show a dull sheen when viewed obliquely against the light, with all fines adhering to the larger particles, and no free water will be visible. Material at the dry limit will not show the characteristic sheen, fines will not be strongly adherent and many of the fines will be free. Material at the wet limit will begin to show free moisture collecting in surface grooves or amongst the fines, fines will not be strongly adherent and many of the fines will amalgamate as soggy clusters. Any result between the wet and dry limits is acceptable provided the bulk of the sample is reasonably well graded.

Sands used as finefill or as a regulating layer also need to be used near to the optimum moisture content and can be identified by a simple squeeze test.

Test - Fine Aggregate

Take a small representative sample , squeeze in one hand and release.

Result

If the sample crumbles away and mostly fails to adhere together into a 'ball' then the sample is too dry. Any reasonable degree of adherence is acceptable provided no free water is squeezed out.

Field Identification Test No 4 - Granular Grading

All unbound granular materials must be reasonably well graded; i.e. must contain a range of particle sizes, from fine to coarse, with an adequate proportion of particles of intermediate sizes. A well-graded material can be compacted to give a dense and stable structure of interlocking particles with a low proportion of air voids within the structure.

Preparation

Depending on the size of the stockpile, dig out representative samples from beneath the outer surface at several positions around the outside.

Test - Granular Grading

Spread out each sample and examine under good light.

Result

Class A Graded Granular Materials - should not contain any particles greater than 75 mm nominal size and, in general, should be 50 mm or smaller. Smaller particles down to less than 5 mm nominal size should be present in gradually increasing numbers as the size decreases. Finer particles, from sand size down to dust, should be present and will usually be adhering to the larger particles. Fine particles should be visible adhering to around 30 per cent or more of the surface of the majority of the larger particles.

Class B Granular Materials - should show the same general features as described above but will usually be less well graded overall compared with Class A Graded Granular Materials.

Class C Cohesive/Granular Materials - will usually contain a much larger proportion of fine material. The granular content should still be less than 75 mm nominal size, down to less than 5 mm nominal size and should not be single sized.