Rock mass strength. The allowable bearing value q
all
could be based on the rock mass
strength. The approach would be to determine the unconfined compressive strength q
c
and then apply
a factor or safety, or q
all
=q
c
/F. The goal is to take into account the effect of both intact material
behavior and the behavior of discontinuities contained within the specimen block. The rock mass
strength could be determined by performing in situ unconfined compressive tests as described in
ASTM D 4555-01 (2004), “Standard Test Method for Determining Deformability and Strength of
Weak Rock by an In Situ Uniaxial Compressive Test.” The procedure is to perform in situ compres-sion tests on specimens of rock that are large enough so that the rock mass unconfined compressive
strength q
c
is obtained, such as illustrated in Fig. 7.23. Similar to the load tests described above, this
approach is very expensive and time consuming.