Updating Bearing Capacity – SPT Graphs - Maryland State Highway ...

2.3 Friction Angle of Granular SoilsThe SPT can be used to estimate the in-situ angle of internal friction φ for granular soils.The SPT test is commonly used to estimate the properties of cohesionless soils due to thedifficulty in obtaining undisturbed samples. It should be stated that the SPT number may bemisleading if large-size gravel is wedged into the split spoon sampler, resulting in apparentlyhigh N-values.The angle of friction of granular soils, ϕ, has been correlated to the standard penetrationnumber. Peck, Hanson, and Thornburn (1953) gave a correlation between N and ϕ in a graphicalform, Fig 2.1, which can be approximated as (Wolff, 1989)2ϕ ° = 27.1+0.3N − 0.00054N(2.1)In Japan the “Road Bridge Specifications” (Shioi and Fukui 1982) suggests for N > 5,( 15 ) 1 2 + 15ϕ = N(2.2)and the “Design Standards for Structures” (Shioi and Fukui, 1982):ϕ = 0.3N+ 27°(2.3)Table 2.1 shows the values of ϕ as a function of the SPT N-value. The values from equation 2.3,are very close to the values of equation 2.1. Thus, equation 2.3 is recommended in this study asa good linear approximation of the relationship between the angle ϕ and the blow count N.2-2