PLAIN AND CIVIL: SOIL PROFILE TYPES

Soil shall be classified into one of the categories shown in the code. With the soil profile type, the appropriate site-dependent design spectrum can be defined.

Site categorization schemes of the seismic codes use different descriptions of geological and geotechnical parameters to define the soil classes. The most commonly used parameter is the $V_{s,30}$ which is the average shear wave velocity of the top 30m of the soil profile.This was introduced in post 1994 in US seismic codes (the 1994 and 1997 edicitons of NEHR and also the 2000 IBC) as the main categorization parameter.

$V_{s,30}=\frac{30}{\sum\frac{h_{i}}{v_{i}}}$
where:

di = thickness of layer i in meters
Vsi = shear wave velocity in layer i in m/s

Soil Penetration Test (SPT) is one of the procedures to determine the soil bearing capacity. This test is economical, used to identify surface information on land and offshore. Standard penetration blow count $N_{SPT}$ is used.

$N_{SPT}=\frac{\sum_{i=1}^{n}d_{i}}{\sum_{i=1}^{n}\frac{d_{i}}{N_{i}}}$
where:
di = thickness of layer i in mm
ds = the total thickness of cohesionless soil layers in the top 30m
Ni = the standard penetration resistance of soil layer from standards

And lastly, the undrained shear strength $S_{u}$ may also be used to characterize the top 30m of the soil.

$S_{u}=\frac{d_{c}}{\sum_{i=1}^{n}\frac{d_{i}}{S_{ui}}}$

where:
dc = the total thickness of cohesive layers in the top 30m.
Sui = the undrained shear strength in the standards, not to exceed 250 kPa.

SOIL PROFILE TYPES

SOIL PROFILE TYPE	GENERIC DESCRIPTION	SHEAR WAVE VELOCITY VS,30	SPT, N (BLOWS/300mm)	UNDRAINED SHEAR STRENGTH SU (kPa)
Sa	Hard rock	> 1500
Sb	Rock	760 to 1500
Sc	Very dense soil and soft rock	360 to 760	> 50	> 100
Sd	Stiff soil profile	180 to 360	15 to 50	50 to 100
Se	Soft soil profile	< 180	< 15	< 50
Sf

NOTE:

Soil Profile Sf is noted as soil requiring site-specific evaluation.
Soil Profile Type Se also includes any soil profile with more than 3.0m of soft clay defined as a soil with plasticity index > 20, and Su<24kPa.

NSCP reserves the exception:

"When the soil properties are not known or with insufficient detail , Type Sd shall be used." Soil profile type Se or Sf need not be assumed unless the building official determines that type Se or Sf may be present at the site or in the event that type Se or Sf is established by geotechnical data."

PLAIN AND CIVIL

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Wednesday 31 October 2018

SOIL PROFILE TYPES

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