Engineering and Construction

For lightly loaded foundations supported by hard and sound rock, the total settlement ρmax and maximum differential settlement  ∆ may be essentially zero. In some cases, the rock may actually be stronger than the foundation concrete. When dealing with lightly loaded foundations to be supported by hard and sound rock, an extensive investigation is usually not justified and the allowable bearing capacity from the building code is often recommended (e.g., see Table 18.4, reproduced from the International Building Code, 2006). For hard and sound rock, usually the allowable bearing capacity from the building code will have an ample factor of safety. Exceptions include the following:

1. Weathered or fractured rock. Rock may become so weathered that its behavior is closer to a soil
than intact rock. The allowable bearing capacity from the building code, such as the values listed in Table 18.4, may also be too high for foliated, friable, weakly cemented, highly jointed, or other
conditions that result in weak rock.

2. Expansive rock. Some types of rock, such as claystone and shale, may be expansive. Instead of
settlement problems, these types of rock may expand upon moisture intrusion.

3. Secondary influences. Even for lightly loaded foundations on sound rock, they could still experience significant settlement due to secondary influences, such as the opening of limestone cavities or sinkholes or settlement related to the collapse of underground mines and tunnels.

4. Cut-fill transition. A cut-fill transition occurs when a building pad has some rock removed (the cut portion), with a level building pad being created by filling in with soil the remaining portion. If the cut side of the building pad contains non-expansive rock that is hard 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 and cause damage. For example, a slab crack will typically open at the location of the cut-fill transition as illustrated in Fig. 7.22. 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 is to use deepened footings or deep foundations to underpin that portion of the structure located on fill such that the entire foundation is anchored in rock. This option would be the preferred method, especially for heavily loaded foundations.

In summary, for lightly loaded foundations bearing on rock, an extensive investigation is usually not justified and the recommended allowable bearing capacity is often based on code values, such those values listed in Table 18.4.