Civil Engineering Reference
In-Depth Information
FIGURE 11.12
Cut-fill transition lot.
and sound, then very little settlement would be expected for that part of the building on
cut. But the fill portion could settle under its own weight or it could settle during an earth-
quake. A slab crack will often open at the location of the cut-fill transition as illustrated in
Fig. 11.12. Damage is caused by the vertical foundation movement (settlement of fill) and
the horizontal movement that manifests itself as a slab crack and drag effect.
One option to prevent this type of damage is to use deepened footings or deep founda-
tions to underpin that portion of the structure located on fill such that the entire foundation
is anchored in rock. For sites with significant earthquake shaking, this option would be the
preferred method. Another option is to overexcavate the cut portion of the building pad and
replace it with fill to eliminate the abrupt change in bearing resistance at a cut-fill transition.
In summary, for lightly loaded foundations bearing on rock, an extensive investigation
is usually not justified and the recommended allowable bearing pressure is often based on
code values, such those values listed in Tables 8.2 and 14.6. A one-third increase in the rock
bearing pressure is usually recommended for earthquake loading provided the rock will not
be weakened or fractured apart during the earthquake.
11.6.2
Heavily Loaded Foundations on Rock
For heavily loaded foundations, using the bearing pressure values listed in Tables 8.2
and 14.6 may be too conservative for hard and sound rock, which will result in an
uneconomical foundation. For example, Table 8.2 specifies a maximum allowable bearing
pressure of 12,000 psf (570 kPa) for massive crystalline bedrock. But other sources have
recommended much higher allowable bearing values, such as 160,000 psf (7.7 MPa) for
massive crystalline bedrock (NAVFAC DM-7.2, 1982). Local experience may even dictate
higher allowable bearing values, but the allowable bearing pressure should never exceed
the compressive strength of the foundation concrete.
It has been stated that there is no reliable method of predicting the overall strength and
deformation behavior of a rock mass from the results of laboratory tests, such as the uncon-
fined compression test on small rock specimens. This is because the settlement behavior of
rock is strongly influenced by large-scale in situ properties, such as joints, fractures, faults,
inhomogeneities, weakness planes, and other factors. Hence using small-core specimens to
