Foundation, Concrete and Earthquake Engineering

Steps to Classify Site for Earthquake Design

Dear reader, throughout last few posts we were discussing about site classification for design of structure for earthquake. Of these site class A, class B, class C, class D and class E have some criteria upon which we can determine them say shear wave velocity, standard penetration resistance, undrained shear strength of soil at site.


The site class F requires site-specific tests; these are collapse behavior, organic content say peat or other highly organic clays, plasticity and thickness of clay layer of soft or medium stiff. When a site represent these classes, complying any of these; we can classify it as site class F. That is from boring data and in-situ tests, we can primarily suspect a site as class F and then more elaborate site-specific evaluations are conducted. Field identification of different types of soils have discussed in our previous posts. 

Organic soil


Here we can ensure a site as class E. In determining this we have to know about plasticity index, water content, undrained shear strength of soil; the thickness of layers of soil layer is also important. Dear reader at first we will learn about determination of u.
Determination undrained shear strength

Sui =  Undrained shear strength (psf), ≤5000 psf

In kpa this value is ≤240kpa; use ASTM D2166 or D2850 to determine this parameter.

dc= Total thickness of soil layers within top 100ft (cohesive soil is considered)

K = Number of the cohesive layers within top 100ft

di = Thickness of soil layer at any depth within upper 100ft

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 ifu 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.


In this step we will needS, 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


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

Site Classification for Earthquake Design
  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.

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