A sample method for deriving presumed bearing resistance for spread foundations on rock
(1) For weak and broken rocks with tight joints, including chalk with porosity less than 35 %, the presumed bearing resistance may be derived from figure G.1. This is based on the grouping given in table G.1 with the assumption that the structure can tolerate settlements equal to 0,5 % of the foundation width. Values of presumed bearing resistance for other settlements may be derived by direct proportion. For weak and broken rocks with open or infilled joints, reduced values of presumed bearing pressure should be used.
|Group||Type of rock|
|1||Pure limestones and dolomites
Carbonate sandstones of low porosity
Oolitic and marly limestones
Well cemented sandstones
Indurated carbonate mudstones
Metamorphic rocks, including slates and schist
|3||Very marly limestones
Poorly cemented sandstones
Slates and schists (steep cleavage/foliation)
|4||Uncemented mudstones and shales|
1 Group 1 rocks, 2 Group 2 rocks, 3 Group 3 rocks, 4 Group 4 rocks, 5 Allowable bearing pressure not to exceed uniaxial compressive strength of rock if joints are tight or 50 % of this value if joints are open, 6 Allowable bearing pressures
a) very weak rock, b) weak rock c) moderately weak rock d) moderately strong rock e) strong rock f) closely spaced discontinuities g) medium spaced discontinuities h) widely spaced dicontinuities
For types of rock in each of four groups, see table G.1. Presumed bearing resistance in hatched areas to be assessed after inspection and/or making tests on rock.