Geo-Technical Design of Shallow Foundation
Geo-Technical Design of Shallow Foundation
Geo-Technical Design of Shallow Foundation
Shallow Foundations
Chapter # 03
GENERAL REQUIREMENTS OF
FOUNDATION:
Case:
Diagram # 01
Example
STRUCTURAL ANALYSIS:
SIZING:
SIZING:
Step # 02 (Dimensioning)
Now using the data from analysis and the
material selected the dimension are
chosen (i.e. thickness, width, depth of
pad) and the design is completed.
Step # 03 (Documentation)
Now the design is represented in the form
of drawings and the construction
specifications (i.e. procedure, problems
and solutions) are also mentioned.
Df =?
d =?
B =?
L =?
As =?
G.S.L
Df
BXL
B
L
Design Components
STRUCTURAL DESIGN:
GEOTECHNICAL DESIGN OF
FOUNDATION:For the geotechnical deign of the foundation
the following steps are observed;
1.
2.
3.
4.
5.
STEPS OF GEO-TECHNICAL
DESIGN
1.
2.
3.
4.
5.
Shallow Foundations.
Deep Foundations.
Floating Foundations.
qu/2
Nc = 5.7
In F.P.S system;
For square footing,
qult =2N2BRw + 6(N2 + 100)DfRw
For very long footing,
qult =3N2BRw + 5(N2 + 100)DfRw
Where,
qult =Net ultimate bearing pressure,(PSF)
=Pressure at bottom of footing in excess of the pressure
at the
same level due to the weight of soil immediately
surrounding the
footing.
N
=Standard Penetration Test.
B
=Width of footing.
Df =Depth of footing.
If the ground levels on both sides of footing are not equal, D should be
measured from the lowest ground level.
If D > B, use D + B for computation.
1.
Rw & Rw
=correction factors for position of water levels.
If water table is at a depth B or greater from the bottom of footing then
Rw = Rw = 1
If water table is at base of footing
Rw = 1
Rw = 0.5
If water table is at top
Rw & Rw = 0.5
And in between the linear variation is made.
Is S.I system,
For square footing;
qs =
0.105N2BRw + 0.314(N2 + 100)DfRw
with F.O.S
=
3
Here B &Df are in m and q =
KPa.
1+D/B2
FOR SETTLEMENT
ANALYSIS:
=
[Cc / (1 + lo)] (H) log (o + ) / o
Where,
Cc =compression index, for normally consolidated clays.
Cc =0.009(L.L - 10)
=initial overburden pressure at that level.(above mid-height
of consolidating layer)
o =(Df + level).
=additional pressure at that level.
=P / (B + Z)
H =length of strata (layer). If the soil is drained on top and
bottom as in the consolidation test, half thickness should be
used
e0 =natural void ratio of the soil in place.
Cd
Kd
Cw
=
=
=
=
=
=
=
2 / N [B / (B + 0.3)]2 Cd Cw qa
Settlement in mm
Depth factor
=1 / Kd
1.033D / B 1.33
Water correction factor.
1 (always) It is reflected in N values.
qa
So,
=
2 / N [B / (B + 0.3)]2 Cd(o)
Here,
Cd =
1 / Kd & Kd
=
1 + Df / B (.33) 1.33
The Df used in the layers other than the first one is at the level of the layer under
consolidation.
Similarly in
=
P / (B + Z)2
Where,
Z is the distance up to the c/l of layer from the bottom of footing