Seismic Design of Tunnels - Parsons Brinckerhoff

• The effects of overburden depths on damage are shown in Figure 2A for 132 of the
192 cases. Apparently, the reported damage decreases with increasing overburden
depth.
• Figure 2B shows the damage distribution as a function of material type surrounding
the underground opening. In this figure, the data labeled “Rock (?)” were used for all
deep mines where details about the surrounding medium were not known. The data
indicate more damage for underground facilities constructed in soil than in competent
rock.
• The relationship between peak ground acceleration (PGA) and the number of
damaged cases are shown in Figure 2C.
- For PGA values less than 0.15g, only 20 out of 80 cases reported damage.
- For PGA values greater than 0.15g, there were 65 cases of reported damage out
of a total of 94 cases.
• Figure 2D summarizes the data for damage associated with earthquake magnitude.
The figure shows that more than half of the damage reports were for events that
exceeded magnitude M=7.
• The damage distribution according to the epicentral distance is presented in Figure
2E. As indicated, damage increases with decreasing epicentral distance, and tunnels
are most vulnerable when they are located within 25 to 50 km from the epicenter.
• Among the 192 cases, unlined openings account for 106 cases. Figure 2F shows the
statistical damage data for each type of support. There were only 33 cases of
concrete-lined openings including 24 openings lined with plain concrete and 9 cases
with reinforced concrete linings. Of the 33 cases, 7 were undamaged, 12 were
slightly damaged, 3 were moderately damaged, and 11 were heavily damaged.
It is interesting to note that, according to the statistical data shown in Figure 2F, the
proportion of damaged cases for the concrete and reinforced concrete lined tunnels
appears to be greater than that for the unlined cases. Sharma and Judd attributed
this phenomenon to the poor ground conditions that originally required the openings
to be lined. Richardson and Blejwas (1992) offered two other possible explanations:
-Damage in the form of cracking or spalling is easier to identify in lined openings
than in unlined cases.
-Lined openings are more likely to be classified as damaged because of their
high cost and importance.
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