# Steps to Classify Site for Earthquake Design

Dear reader, throughout last

few posts we were discussing about site classification for design of structure for

some criteria upon which we can determine them say

standard penetration resistance, undrained shear strength of soil at site.

few posts we were discussing about site classification for design of structure for

*. Of these site class A, class B, class C, class D and class E have***earthquake**some criteria upon which we can determine them say

*,***shear wave velocity**standard penetration resistance, undrained shear strength of soil at site.

**Step-1**

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.

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

of

**s̄**_{u}**.** Where

S_{ui} = Undrained

shear strength (psf), ≤5000 psf

In kpa this value is

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

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

d

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

_{c}= Totalthickness of soil layers within top 100ft (cohesive soil is considered)

K = Number of the cohesive

layers within top 100ft

layers within top 100ft

d

of soil layer at any depth within upper 100ft

_{i}= Thicknessof 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

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.

found positive then examine undrained

*shear strength*if s̄_{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 of s̄

have discussed about determination of s̄

_{u}for layer soil.We have discussed about

determination of Shear Wave Velocity, v̅

layered soil; and it is very rare to find a soil profile not having layers.

determination of Shear Wave Velocity, v̅

_{S }forlayered soil; and it is very rare to find a soil profile not having layers.

Step-3

In this step we will need v̅

and determination of N̅ have not discussed yet. We know N stand for standard

penetration resistance according to ASTM D 1586.

_{S, }N̅ and s̄_{u}of these v̅_{S, }and s̄_{u }have discussedand determination of N̅ have not discussed yet. We know N stand for standard

penetration resistance according to ASTM D 1586.

Where

N

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, N

per foot is taken

_{i}=standard penetrationresistance ≤ 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, N

_{i}=100 blowper foot is taken

d

of soil layer at any depth within upper 100ft.

_{i}=thicknessof soil layer at any depth within upper 100ft.

For cohesionless soil we

can write the formula as

can write the formula as

N̅

respective values as in equation (2) but for cohesionless soil.

_{ch}and d_{s}represents forrespective 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.

cohesionless soil layers respectively within upper 100 ft.

After knowing v̅

N̅

_{S, }N̅,N̅

_{ch }and s̄_{u }we can determine sites using three methods:- v̅
_{S}for upper 100ft use v̅_{S }method - N̅ for upper 100ft use N̅ method
- N̅
_{ch}for soil layer having cohesionless soil having

PI<20 obviously in upper 100 ft and use average undrained shear

strength, s̄_{u}for cohesive soil layer having PI>20 in upper

100 ft. Follow the table below for reference.