Seismic Design of Tunnels - Parsons Brinckerhoff

In each pair of analyses, the parameters characterizing the soil/structure system
are identical except for the input ground motions (i.e., the northeastern versus the
western earthquakes). The seismically induced racking distortions of the structures are
much greater under the assumed western design earthquake than the northeastern
design earthquake. However, for the three comparisons made in this study, the
normalized racking response with respect to the free-field, R, is very little affected by
the type of ground motions used in the analysis. For instance, the calculated racking
response coefficients show negligible difference (R=0.445 vs.R=0.448) between cases
22 and 23.
Effect of Embedment Depth. To determine the effect of shallow embedment depth on
the normalized racking response, finite-element analyses were performed using Type 2
structure as an example. Here, the burial depths of the structure were varied. Table 5
presents the cases that were analyzed for this purpose. Note that flexibility ratio, F,
remained the same for all cases. The normalized racking distortions from these analyses
versus the dimensionless depth of burial, h/H, are presented in Figure 36.
Based on the results, it appears that:
• The normalized racking distortion, R, is relatively independent of the depth of burial for
h/H>1.5 (i.e., soil cover thickness equal to structure height). At this burial depth the
structure can be considered to respond as a deeply buried structure.
• For cases where the depth of embedment is less than 1.5, the normalized racking
distortion decreases as the depth of burial decreases, implying that design based on
data presented in Figures 34 and 35 is on the safe side for tunnels with little to no soil
cover.
Effect of Stiffer Foundation. The results of analyses discussed thus far are primarily for
cases involving structures entirely surrounded by relatively homogeneous soil medium,
including soil profiles with linearly increasing stiffness. A frequently encountered situation
for cut-and-cover tunnels is when structures are built directly on the top of geological
strata (e.g., rock) that are much stiffer than the overlying soft soils.
To investigate the effect of stiffer foundation, seven analyses were performed with
varying foundation material properties as well as varying overlying soil properties. Table 6
lists the various parameters used in each of these analyses. The flexibility ratios shown in
Table 6 are based on the overlying soil modulus only. The stiffness of the more competent
foundation material is not taken into account. The calculated racking distortions, as
normalized by the free-field shear deformations, are presented as a function of the
flexibility ratio in Figure 37.
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