Civil Insight: A Technical Magazine Volume 2 | Page 20

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TEACHERS’ SECTION
CIVIL INSIGHT 2018
Seismic evaluation of underground structures is so different from structures on the ground because of their
interaction with surrounding soil. A tunnel lining undergoes three primary modes of deformation during
seismic shaking, ovaling/racking, axial and curvature deformation (FHWA, 2009). The racking deformation
is the argument of this paper, which is caused by seismic waves propagating perpendicular to the tunnel
longitudinal axis, causing deformation in the plane of the tunnel cross section (Owen and Scholl 1981).
Moreover, among variety of these waves such as Rayleigh, the vertically propagating shear wave (SV-wave) is
generally considered as the most critical one for this mode of deformation (Wang 1993). There are two main
types of seismic design approaches for assessing a tunnel lining’s ovaling/racking deformation induced by S-V
waves:a) Free-fi eld racking deformation methods and b) Soil-structure interaction approaches (Hashash 2001).
ANALYTICAL METHOD TO FIND RACKING DEFORMATION
Here the method developed by Wang et al., (1993) and by Hashash et al., (2001)has been used to fi nd the
racking deformation of the structure. Basic data required in this analytical method are stated as below.
Earthquake and soil parameters (25 th April 2015 Nepal earthquake)
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Peak Magnitude of previous earthquakes at study site (Mw) = 7.8
Peak ground particle acceleration at surface (a max ) = 0.49g
Apparent velocity of s-wave propagation in soil (Cm) = 750 m/sec.
Density of soil (Stiff soil) ( ɏ m )= 2000 kg /m 3
Structural parameters
Figure 2: Tunnel section dimensions
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Width of the tunnel (W) = 22.1 m.
Height of the tunnel (H) = 6.9 m.
Depth of soil layer to the top of tunnel (D) = 4 m.
Per unit Length for the rectangular cross section was considered.