Geoscience Reference
In-Depth Information
on the loading condition, depth of embedment, foundation geometry, etc. Con-
siderations of these factors can optimise the design and is required for detailed
design.
•
The use of presumed bearing pressure from the soil description is simple - but
not very accurate. Therefore use only for preliminary estimate of foundation
size.
•
The table is for natural material and assumes that an allowable settlement of
25mm.
•
When the material is placed as structural fill and compacted to 98% relative
compaction, the bearing value in the table should be halved.
Sands
by 5
◦
.
-
* For Clayey Sands reduce
by 5
◦
.
-
* For Gravelly Sands increase
-
* Water level assumed to be greater than B (width of footing) below bottom
of footing.
-
* For saturated or submerged conditions - half the value in the Table.
-
Based on a foundation width greater than 1m and settlement
25mm. Divide
by 1.2 for strip foundation. The bearing value in sands can be doubled, if
settlement
=
=
50mm is acceptable.
-
For B
1m, the bearing pressure is reduced by a ratio of B (Peck, Hanson
and Thornburn, 1974).
<
21.4 Bearing capacity
•
Terzaghi presented the general bearing capacity theory, with the ability of the soil
to accept this load dependent on:
-
The soil properties - cohesion (c), angle of friction (
φ
) and unit weight (
γ
).
-
The footing geometry - embedment (D
f
) and width (B).
-
D
f
.
- Modifications of the above relationship occurs for:
•
Surcharge (q) resisting movement
=
γ
Water table.
•
Shape, depth and inclination factors.
•
Soil layering.
•
Adjacent to slopes.
Table 21.4
Bearing capacity equation.
Consideration
Cohesion
Embedment
Unit weight
Comments
Bearing capacity
N
c
N
q
N
These factors are non dimensional
γ
factors
and depend on
. See next Table
φ
Ultimate bearing
c N
c
+
qN
q
+
0.5
γ
BN
Strip footing
γ
capacity (q
ult
)
1.3cN
c
qN
q
0.4
BN
γ
Square footing
+
+
γ
1.3cN
c
qN
q
0.3
BN
Circular footing
+
+
γ
γ
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