Zero Azania has successfully completed numerous high profile Bulk Excavation & Platform contracts across Gauteng and has gained extensive insight and experience on the various Gravel types encountered and most commonly required in fill operations. The objective of this information sheet is to provide clarity regarding gravel material classification.

Materials Classification

The classification of construction materials is a complicated field. In this article the classification of natural materials as used in earthworks will be discussed. The general classification is done in terms of a "G" notification, like G5, G6, G7, G8, G9 and G10. Generally a G5 and G6 type material is classified as natural gravel and the G7 to G10 type of material is a gravel-soil. The lower the number (5 for example) the better quality material it is.

Types of classification used in South Africa:

  1. TRH 14 (1985) Road Construction Materials

Of importance are the following test methods and specifications:

  • Grading Analysis (Test Method TMH1; A1(a) and A1(b))

Natural Gravel (G5 and G6) should have a maximum aggregate size of 63mm or two-thirds of the compacted layer thickness, whichever is smaller. A minimum grading modulus of 1.5 (G5) or 1.2 (G6) should be obtained.

Gravel-soil (G7) should have a maximum size, in place, after compaction, not greater than two-thirds of the compacted thickness of the layer. A minimum grading modulus of 0.75 should be obtained.

Gravel-soil (G8, G9 and G10): no grading requirements

The Grading Modulus or GM is given by

GM = (P2,00 mm + P0,425 mm + P0,075 mm) / 100 where P2,00 mm, etc., denote the percentage retained on the indicated sieve size.

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Sieves utilised in grading analysys

  • Bearing Strength and Swell (Test Method TMH1; A8)

Material G5 should have a CBR after soaking of not less than 45 percent at 95 percent of Mod AASHTO density and a maximum swell of 0.5 percent at 100 percent Mod AASHTO density.

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Soaking of samples from the Modified AASHTO test in order to execute the CBR test

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Execution of Modified AASHTO density test in order to determine Maximum dry density (MDD) and Optimum moisture content (OMC)

  • Atterberg Limits (Test Method TMH1; A2 and A3)
  • The Atterberg limits apply to the soil fines (<0.425mm)
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Apparatus utilised to determine the Atterberg limits

2. Colto Standard Specifications for Road and Bridgework

The following is a shortened version of the Colto Specification. Properties like additional fines, durability and soluble salts are not discussed and can be found in the specification.

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Conclusion

In general fill material only have a CBR specification of at least 3% at 93% of Modified AASHTO density. The higher quality the material used the easier it will compact, the higher the bearing strength will be and there will be less swell and it will be less prone to cracking. If one has to choose a material utilised in earthworks and there is a small price difference in choosing a better quality material it is well worth the additional outlay.