Civil Engineering Reference
In-Depth Information
Height of clay left above gravel after excavation
=
8
−
2
=
6 m
Upward pressure from water on base of clay
=
10
×
9.81
=
98.1 kPa
Downward pressure of clay
=
6
×
19
=
114 kPa
It is clear that the downward pressure exceeds the upward pressure and thus, on the
face of it, the foundation will not be lifted by the buoyant effect of the upward-acting
water pressure, i.e. it is
safe
. We can quantify how 'safe' the foundation is against
buoyancy by introducing the term
factor of safet
y, F:
Downward pressure
Upward pressure
=
114
98 1
Factor of safety F
,
=
=
.
1 16
.
(b) (ii) Downward pressure after construction
=
114
+
100
=
214 kPa
214
98 1
Factor of safety F
,
=
=
2 18
.
.
i.e. the factor of safety against buoyant uplift is higher after construction.
We can also assess the safety against buoyancy using the limit state design
approach defined in Eurocode 7 (see Chapter
5)
. The solution to Example 2.6 when
assessed in accordance with Eurocode 7 is available for download.
Units of pressure
The pascal is the stress value of one newton per square metre, 1.0 N/m
2
, and is given the symbol Pa. In
the example above, pressure has been expressed in kilopascals, kPa. Pressure could have equally been
expressed in kN/m
2
as the two terms are synonymous.
1 0
.
kN m
/
2
=
1 0
.
kPa
2
1 0
.
MN m
/
=
1 0
.
MPa
Example 2.7:
Filter material limits
Determine the approximate limits for a filter material suitable for the material shown in
Fig.
2.13.
Solution:
From the particle size distribution curve:
D
=
0 01
.
mm D
;
=
0 2
.
mm
15
85
Using Terzaghi's method:
Maximum size of D for filter
= ×
5
D of base
= ×
5 0 2
.
=
1 0
.
mm
15
85
Minimum size of D for filter
= ×
5
D of base
mm
= ×
5 0 01
.
15
15
=
0 05
.
This method gives two points on the 15% summation line. Two lines can be drawn
through these points roughly parallel to the grading curve of the soil, and the space
between them is the range of material suitable as a filter (Fig.
2.13)
.
