foundation system of burj al arab

Steps to Classify Site for Earthquake Design (Part-2)

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 u 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 ofu for layer soil.

We have discussed about determination of Shear Wave Velocity,S for layered soil; and it is very rare to find a soil profile not having layers.

Step-3

In this step we will need S, andu of theseS, andu have discussed and determination of have not discussed yet. We know N stand for standard penetration resistance according to ASTM D 1586.

Determination of Standard penetration resistance

Where

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

Standard penetration resistance for cohesionless soil


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 S, N̅, N̅ch andu we can determine sites using three methods:

  1. S for upper 100ft useS method
  2. for upper 100ft usemethod
  3. ch for soil layer having cohesionless soil having PI<20 obviously in upper 100 ft and use average undrained shear strength,u for cohesive soil layer having PI>20 in upper 100 ft. Follow the table below for reference.
    Site Classification for Earthquake Design

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