Table 1    The site subsoil categories

(a) Rock or very stiff soil sites

     Sites where the low amplitude natural period is less than 0.25 s, or sites with bedrock, including
     weathered rock, with unconfined compressive strength greater than or equal to 500 kPa, or
     with bedrock overlain by:

     (ⅰ) Less than 20 m of very stiff cohesive material with undrained shear strength exceeding 100
       kPa ; or

     (ⅱ) Less than 20 m of very dense sand, with N1 > 30, where N1 is the SPTN value corrected
       to an effective overburden pressure of 100 kPa ; or

     (ⅲ) Less than 25 m of dense sandy gravel with N1 > 30.

(b) Intermediate soil sites

     Sites not described as category (a) or (c) may be taken as intermediate soil sites.

(c) Flexible or deep soil sites

     Sites where the low amplitude natural period exceeds 0.6 s, or sites with depths of soils
     exceeding the following values:

Soil type and description Depth of soil (m)
Cohesive soil


Soft
Firm
Stiff
Very stiff

Cohesionless soil


Loose
Medium dense
Dense
Very dense

Gravels

 Representative undrained
shear strengths (kPa)

 12.5 - 25
 25 - 50
 50 - 100
100 - 200

Representative
SPT (N) values

 4 - 10
10 - 30
30 - 50
     >50
    
     >30




20
25
40
60




40
45
55
60

100