3　Site

3.0.1　Project sites can be classified into favorable sites，general sites，unfavorable sites and hazardous sites in accordance with the current national standard Code for Seismic Design of Buildings GB 50011.

3.0.2　Project sites shall be classified by the equivalent shear wave velocity of soil and overburden depth of site.

3.0.3　The measurement of shear wave velocity of site soil layer shall comply with the current national standard Code for Seismic Design of Buildings GB 50011.

3.0.4　Determination of the overburden depth of site shall comply with the following requirements：

1　In general，it shall be the distance from the ground to the top surface of the soil layer which has a shear wave velocity of more than 500m/s and below which the shear wave velocity is no less than 500m/s；

2　When there is a soil layer at a depth of more than 5m below the ground which has a shear wave velocity more than 2.5 times that of each overlaying layer，and the shear wave velocity of the soil layer and of each underlying rock soil is no less than 400m/s，it can be the distance from the ground to the top surface of the soil layer；

3　Lone tones and lenticles with a shear wave velocity of more than 500m/s shall be regarded as soil layer；

4　The volcanic hard interlayer in the soil layer shall be regarded as rigid body，the thickness of which shall be deducted from the overburden depth of site.

3.0.5　The equivalent shear velocity of soil layer shall be calculated using the formulae below：

Where，vse——equivalent shear wave velocity of soil layer（m/s）；

d0——calculation depth（m），taken as the overburden depth or 20m，whichever is smaller；

t——duration of transmission of shear wave from the ground to the calculation depth（s）；

di——thickness of the ith soil layer within the calculation depth range（m）；

vsi——shear wave velocity of the ith soil layer within the calculation depth range（m/s）；

n——number of soil layers within the calculation depth range.

3.0.6　Project sites shall be classified into four types according to Table 3.0.6 by the equivalent shear wave velocity of soil layer and overburden depth of site，among which，typeⅠcan be further classified into subtypesⅠ0 andⅠ1.When reliable data of shear wave velocity and overburden depth of site is available，and the values are close to the boundary of site types listed in Table 3.0.6，it shall be allowed to use the interpolation method to determine the design characteristic period of ground motion.

3.0.7　In the event of seismogenic fault within the site，the impacts of the seismogenic fault on the project shall be evaluated and comply with the current national standard Code for Seismic Design of Buildings GB 50011.

3.0.8　In the case of construction in such unfavorable sites as protruding mountain spur，high-rise isolated mountain，steep slope of non-rocks and strongly weathered rocks，river banks and edges，in addition to ensuring the stability under the earthquake action，the possible impacts of such unfavorable sites on the design ground motion parameters shall be estimated，and the design ground motion parameters shall be corrected with the method specified in the current national standard Code for Seismic Design of Buildings GB 50011.

3.0.9　Geological survey of sites shall determine the type of the sites（favorable，general，unfavorable or hazardous），and provide the evaluation results of the overburden depth，shear wave velocity of soil layer，and seismic stability（landslide，collapse，etc.）of rock soil at the site of electrical installations，as well as such data as liquefaction criterion，liquefaction level and liquefaction depth in the case of a liquefied foundation.

3.0.10　The survey range and items for transmission lines may be determined according to applicable provisions.