Rock strength parameters from classification and testing - Geotechnical Investigation and Design Tables - page 71

Table 6.8 Field evaluation of rock weathering.

Properties Weathering

XW DW SW FR

Field description Total Discolouration & Strength No

discolouration. strength loss, but seems evidence of

Readily not enough to similar to chemical

disintegrates allow small dry fresh rock, weathering

when gently pieces to be but more

shaken in broken across discoloured

water the fabric - MW

Broken and
crumbled by
hand - HW

Struck by hammer Dull thud Rings Rings

Allowable bearing

≤

1 MPa HW: 1-2 MPa 5-6 MPa 8 MPa

capacity Q all , other MW: 2-4 MPa

than rocks below
Allowable bearing 0.75 MPa HW: 0.75-1.0 MPa 2-3 MPa 4 MPa ≤

capacity Q all of MW: 1.0-1.5 MPa

argillaceous, organic
and chemically formed
sedimentary and
foliated metamorphic
rocks

•

Use of presumed bearing pressure from weathering only is simple - but not very
accurate - use only for preliminary estimate of foundation size.

•

Weathered shales, sandstones and siltstones can deteriorate rapidly upon exposure
or slake and soften when in contact with water. Final excavation in such materials
should be deferred until just before construction of the retaining wall/foundation
is ready to commence.

•

Alternatively the exposed surface should be protected with a blinding layer
immediately after excavation, provided water build up behind a wall is not a
concern.

•

A weathered rock can have a higher intact rock strength than the less weathered
grade of the same rock type, as a result of secondary cementation.

6.9 Rock classification
•

The likely bearing capacity can be made based on the rock classification.

•

There is approximately a ten fold increase in allowable bearing capacity from an
extremely weathered to a fresh rock.

•

The table is for shallow footings.