GEOGUIDE 1 GUIDE TO RETAINING WALL ... - HKU Libraries

142Table 4 - Grading and Plasticity Requirements for Retaining Wall BackfillBackfill TypeRequirementCrushed Rock ProductsSoilMaximum Size (mm)% Passing 63 micronsBS Sieve SizeCoefficient of UniformityLiquid Limit (%)Plasticity Index (%)2000> 5Not applicableNot applicable75 (2)0-45> 50 (4)< 45 (5)< 20 (5)Notes :(1) Relevant test methods for grading and plasticity of fill materials arespecified in Clauses 5.1 and 5.2 of Geospec 2 (GCO, 1989a).(2) The backfill may contain up to 5% of rock fragments not exceeding 200mm in size, provided that these neither interfere with the compactionrequirements nor cause any damage to the retaining wall.(3) In addition to the above requirements, the maximum particle sizeshould not exceed two-thirds of the thickness of the compacted layer ofbackfill in order to ensure good compaction.(4) This applies to soils derived from insitu rock weathering only. Forsands and gravels of alluvial origin, the coefficient of uniformity shouldbe not less than 5 and the material should not be gap-graded (i.e.having two or more distinct sections of the grading curve separated bysub-horizontal portions).(5) There is no need to check the liquid limit and plasticity index of thesoil if the backfill contains less than about 30% by weight of particlesless than 63 jum.(6) The determination of the particle size distribution of the backfill shouldbe carried out without using dispersants.