We were in step-2 for determination of earthquake site classification. Here we check the total thickness of any layer having soft clay exceeding 100ft i.e. 30480 mm when found positive then examine undrained shear strength if s̄u is less than 500 psf i.e. 24 kpa; simultaneously the water content is equal or greater than 40 percent and plasticity index is more than 20. We can classify this site as class E.
In our previous part we have discussed about determination of s̄u for layer soil.
We have discussed about determination of Shear Wave Velocity, v̅S for layered soil; and it is very rare to find a soil profile not having layers.
In this step we will need v̅S, N̅ and s̄u of these v̅S, and s̄u have discussed and determination of N̅ have not discussed yet. We know N stand for standard penetration resistance according to ASTM D 1586.
Ni =standard penetration resistance ≤ 100 blows per foot i.e. 328 blows per meter. In this regard we can include that no correction is required. When we determine SPT at rock, i.e. refusal reach to such that 100 blow per foot is exceed, Ni=100 blow per foot is taken
di=thickness of soil layer at any depth within upper 100ft.
For cohesionless soil we can write the formula as
N̅ ch and ds represents for respective values as in equation (2) but for cohesionless soil.
N and m stand for number of cohesive and cohesionless soil layers respectively within upper 100 ft.
After knowing v̅S, N̅, N̅ch and s̄u we can determine sites using three methods:
- v̅S for upper 100ft use v̅S method
- N̅ for upper 100ft use N̅ method
- N̅ch for soil layer having cohesionless soil having PI<20 obviously in upper 100 ft and use average undrained shear strength, s̄u for cohesive soil layer having PI>20 in upper 100 ft. Follow the table below for reference.