If more than one of the support systems
described earlier is installed at the same location, their combined effect can
be determined by adding the stiffnesses (i.e., the slope of the load vs. deformation
curve) for each of the individual, supports.
This has the effect of increasing
the slope of the elastic part of the SCC--
the segment KR in Figure 3.
Consider, for example, the case in
which two supports --characterized by
maximum pressures p ]n~, and p m~ and
elastic stiffnesses K 1 and Ke, respectively,
are installed in a section of tunnel.
The stiffness K for the two systems
acting together can be computed as
K = K 1 + K 2. This value is assumed to
remain valid until one of the two sup-
10 ports achieves its maximum possible
elastic deformation u maX--computed as
U rm a1x max rm~X = _ ma x /K ---- P, 1 /KI and u v, ~ ,e,
respectively (see equation 9). The combined
support system is assumed to fail
at that point. The support with the lowest
value of u r~ ax determines the maximum
support pressure available for the
two supports acting together.