ABSTRACT
Soil type USC Description Drainage CBR % symbol (standard)
Competent broken rock, GW, GP eg Sandstone, granite, All 20 Gravel sizes greywacke
Well graded, poorly graded
Competent broken rock – some GM, GC eg Phyllites, siltstones Silty, All 15 fines formed during construction SW, SP Clayey, well graded, Gravel sizes, sands Poorly graded
Weathered Rock likely to weather ALL eg Shales, mudstones All Treat as or degrade during construction soil below Sands SM, SC Silty, clayey Good 10 Sands SM, SC Silty, clayey Poor 7 Inorganic silts ML Low plasticity Good
Inorganic silts ML Low plasticity Poor 5 Inorganic clays CL Low plasticity Good Inorganic clays CH High plasticity Good
Inorganic silts MH High plasticity Good 3 Inorganic clays CL Low plasticity Poor
Inorganic silts MH High plasticity Poor <3 Inorganic clays CH High plasticity Poor
• The issues with converting CBR to modulus values are discussed in later sections. • Underlying support is also required to obtain the above CBR values (Chapter 11). • At the edge of an embankment (lack of edge support), CBR value is not applicable.
Figure 13.2 Laboratory CBR model versus field condition.
