Geology Reference
In-Depth Information
assessment, are a selection of more extensive recommendations given in Tomlinson
(2001) and BS 8004 (BSI, 1986).
Examples of rock type
(indicative only)
Presumed bearing value
(MPa)
Bearing on surface of rock
Strip footings < 3m wide.
Length not more than ten
times width
Strong. Discontinuity spacing more than 200mm
Strong. Discontinuity spacing 60
10
-
12.5
200mm
Moderately strong. Discontinuity spacing 60
-
5
-
10
-
200mm
1
-
5
ROCK
Notes:
Figures given are for igneous rocks, well-cemented sandstone, mudstone and schist/slate with
flat-lying cleavage/foliation. For other rock types see references quoted. Strength de
nitions are
from BS 5930:1999.
Strong rock (
100 MPa) requires more than one hammer blow to break.
Moderately strong rock (
σ
c
=50
-
σ
c
= 12.5
-
50 MPa)
-
intact core cannot be broken by hand.
Examples of soil type
(indicative only)
Presumed bearing value
(MPa)
Foundation width
Sand and gravel: foundations at least
0.75m below ground level
SPT N-value
<1m
<2m
Very dense
Dense
Medium dense
Loose
>50
30
-
50
10
-
30
5
0.8
0.5
-
0.8
0.15
-
0.5
0.05
0.6
0.4
-
0.6
0.1
-
0.4
0.05
-
10
-
0.15
-
0.1
SOIL
Foundation width
Clay: foundations at least 1m below
ground level
Undrained shear
strength (MPa)
<1m
<2m
0.6
0.3
Hard
Very stiff
Stiff
Firm
Soft
> 0.30
0.15
0.8
0.4
-
0.30
-
0.8
-
0.5
0.075
0.15
0.04
-
0.075
0.02
-
0.04
-
0.2
0.4
0.1
-
0.2
0.05
-
0.1
-
0.15
0.25
0.075
-
0.1
0.025
-
0.05
-
in BS 8004 and similar standards worldwide. For rock such as sandstone
or granite with an intact compressive strength of 12.5MPa (just break by
hand), the allowable bearing pressure would also be 12.5MPa, provided
discontinuities are widely spaced apart, reducing to about 10 MPa as
discontinuity spacing is about 0.5m and reducing to 2.5 MPa when
discontinuity spacing is 150mm. If the fracturing is particularly adverse
or includes discontinuities with low shear strength that could combine to
form a failing wedge, then this needs speci
c consideration and analysis,
as dealt with by Goodman (1980) and Wyllie (1999).
Variability across the foundation footprint may also be an issue. If
there are soft or weathered pockets, these may need to be excavated
