Geoscience Reference
In-Depth Information
•
The rock weathering affects the rock strength. This table uses this consideration
to provide the likely bearing capacity based on the weathering description, and
rock type.
•
The design values are a combination of both rock strength and defects.
Table 6.14
Estimate of allowable bearing capacity in rock.
Presumed allowable bearing capacity (kPa)
XW
DW
SW
FR
Igneous
Tuff
500
1,000
3,000
5,000
Rhyolite,Andesite, Basalt
800
2,000
4,000
8,000
Granite, Diorite
1,000
3,000
7,000
10,000
Metamorphic
Schist, Phyllite, Slate
400
1,000
2,500
4,000
Gneiss, Migmatite
800
2,500
5,000
8,000
Marble, Hornfels, Quartzite
1,200
4,000
8,000
12,000
Sedimentary
Shale, Mudstone, Siltstone
400
800
1,500
3,000
Limestone, Coral
600
1,000
2,000
4,000
Sandstone, Greywacke,Argillite
800
1,500
3,000
6,000
Conglomerate, Breccia
1,000
2,000
4,000
8,000
-
The Igneous rocks which cooled rapidly with deep shrinkage cracks, such as the
Basalts, tend to have a deep weathering profile.
-
The foliated metamorphic rocks such as Phyllites can degrade when exposed with
a resulting softening and loss of strength.
6.15 Rock durability
•
Rock durability is important when the rock is exposed for a considerable
time (in a cutting) or when to be used in earthworks (breakwater, or
compaction).
•
Sedimentary rocks are the main types of rocks which can degrade to a soil when
exposed, examples:
-
shales, claystone.
-
but also foliated metamorphic rock such as phyllites.
-
and igneous rocks with deep weathering profiles such as basalts.
Table 6.15
Rock degradation (Walkinshaw and Santi, 1996).
Test
Strong and durable
Weak and non durable
Point load index ( MPa)
6 MPa
2 MPa
>
<
Free swell (%)
4%
4%
≤
>
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