Transcript Geotechnical Design of Shallow Foundations

Geotechnical Design of
Shallow Foundations
Chapter # 03
GENERAL REQUIREMENTS OF
FOUNDATION:
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The only requirement is that it should not fail
i.e. it should work efficiently under all
conditions of working.
Failures are of two types;
Bearing capacity failure.
Excessive settlement.
So the foundation must be safe against both
the above failures and also it must be properly
located. This proper orientation of a foundation
can be well explained with the following
example;
Case:
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Consider a footing ‘A’ say at depth D,
and width B. the effective stress envelope
is shown in Figure 01. The stresses
within the zone are within permissible
limits. Latter on a foundation ‘B’ is
constructed such that stress envelopes
intersect. Now the stress at point ‘X’ will
be the sum of the affects of ‘A’ &’B’. This
should not increase than the bearing
capacity of the soil.
Diagram # 01
A
B
X
Example
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In Badami Bagh area, many tall buildings
collapse. Actually close to these, excavation
was carried out and as a result stress pattern
under the old buildings changes and ultimately
collapses. That is if latter on these changes are
to be made, these must be designed already
and temporary supports must be given to
existing buildings, called as underpinning.
So for design of a footing, both settlement and
bearing capacity are checked.
GENERAL REQUIERMENT FOR ANY
DESIGN:
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By design we always mean that what are the
stresses acting on a particular member and the
corresponding size of the members that theses
stresses can be carried out efficiently. The
general principle of the design is to design the
structure into different elements. We are going
to discus only one element, i.e. foundation.
Designing is done in two stages;
Analysis
Sizing.
STRUCTURAL ANALYSIS:
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In first step of every design, we analyze the
state of stress and see the strain due to these
stresses. In analysis we see the type of
loading, type of strain and the modes of
failure. In foundation design these stresses
are called as bearing capacity and strains as
settlements.
So in foundation design, analysis means the
determination of bearing capacity and
settlement. We have various methods both
field tests and empirical methods for finding
bearing capacity and settlements.
SIZING:
Step # 01(Material Selection)
Before going to sizing, we decide about the
material to be used in the construction of the
footing e.g. wood, concrete, steel etc. it depends
upon the availability of the material and
economy. The cost of project mainly depends on
it.
Since the foundation system is a very complex
system, the construction material is not
homogeneous. It consists of soil and other
materials (wood, concrete etc.). here we will
take concrete only.
SIZING:
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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.
SPECIFIC DESIGN OF FOUNDATION:
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P
Design means the determination of;
Df =?
d =?
B =?
L =?
As =?
G.S.L
Df
BXL
B
L
Design Components
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Design is divided into two parts;
GEOTECHNICAL DESIGN:The design that takes into account only the properties of
soil is called as Geotechnical Design.
SCOPE OF DESIGN:The scope of geotechnical design is;
a) Df
=?
b) B
=?
c) L
=?
GOAL:The goal of geotechnical design is;
Bearing capacity.
Settlement should be within permissible limits.
STRUCTURAL DESIGN:
In design that takes into account the
technical aspects related to concrete is
called as Structural Design.
GEOTECHNICAL DESIGN OF
FOUNDATION:For the geotechnical deign of the foundation
the following steps are observed;
1.
2.
3.
4.
5.
The choice of foundation system between deep and
shallow foundation.
Fix the vertical location of the foundation i.e. Df in
case of shallow and ‘L’ in case of pile foundation.
Bearing capacity and settlement analysis and choose
an appropriate value of the design pressure “qd”,
bearing capacity equation and settlement equation.
Using the information of (3) fix the dimensions in the
plane i.e. ‘B’ & ‘L’.
Evaluate the construction problems such as the
problem of excavation, dewatering, water proofing and
water tightening, deterioration of concrete and
suggest their remedial measures.
STEPS OF GEO-TECHNICAL
DESIGN
1.
2.
3.
4.
5.
Selection of the type of foundation system.
Fix the vertical location i.e. Df of the
foundation.
Bearing Capacity and Settlement Analysis
and from this a suitable value of “qd” i.e. the
design pressure.
The dimensions in plane (B & L)
Construction Specification.
Step No. 1:-Selection of the foundation type:For this the following steps are kept in mind
Type of structure and its requirements
Sub soil profile at the site.
Overall impact on the environment.
Relative cost and construction facilities.
Broadly speaking the types of foundation are;
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Shallow Foundations.
Deep Foundations.
Floating Foundations.
Step No. 2:- DEPTH OF FOUNDAION;
1.
2.
3.
4.
5.
6.
7.
For depth of foundation, the following two
considerations are kept in mind,
Mechanical Consideration.
Physical Consideration.
In mechanical consideration we check Bearing
Capacity and Settlement.
For Bearing Capacity Terzaghi’s equation (for
general share failure) is applied i.e.
qult
=
ScCNc + γ Df Nq + Sγ0.5γBNγ
From this equation it is clear that with the same
soil, the properties remain the same and if ‘b’ is
kept constant, then the Bearing Capacity goes on
increasing by increasing the depth, but economy
is also given due regards.
For pure clay;
(qult )Net
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=ScCNc + γDf (Nq-1) + Sγ 0.5γBNγ
C=
qu/2
Nc = 5.7
i.e. for clays, the depth effect is zero. But for
sands there is effect of depth.
Similarly in the case of Settlements, it goes
on decreasing by increasing the depth.
S
=
Cc/ (1 + eo) (H) log (σ0 +Δ σ)/(σ0)
Physical Requirements:Following are the different physical requirements;
Footing should be below
Top organic soil.
Susceptible zone.
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Surface erosion zone.
Frost line.
Scour depth.
There should be the specified edge distance i.e.
Level Difference
Step No. 3:- BEARING CAPACITY
ANALYSIS:
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FOR BEARING CAPACITY:qult
=
CNc + γDfNq + 0.5γBNγ
qult / F.O.S
=
safe gross B.C or safe B.C
(qult )Net
=
CNc+ γDf (Nq-1) + 0.5γBNγ
(qult )Net / F.O.S =
safe net B.C
For square footing and circular footing;
qult =
1.3CNc + γDfNq + 0.5γBNγ
For pure clay
Nc =5.7, Nq=1 &
Nγ=0
B.C calculated by these equations is called B.C w.r.t. shear.
BEARING CAPACITY WITH S.P.T
‘N’ VALUES:
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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.
Correction factors for position of
water levels
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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.
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Net allowable bearing pressure in psf for maximum
settlement of 1” is
qa =
720(N-3) (B = 1 / 2B)2 Rw Kd
Where,
Kd =
1+D/B≤2
If N value is given and there is no information about
Ø & γ then from Mayerhoff’s Ø
=28 + N / 10
And for the minimum density samd unit,
Weight γ
=
115 – 120pcf.
FOR SETTLEMENT ANALYSIS:
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δ
=
[Cc / (1 + lo)] (H) log (бo +Δ б) / бo
Where,
Cc =compression index, for normally consolidated clays.
Cc =0.009(L.L - 10)
Δ =initial overburden pressure at that level.(above mid-height
of consolidating layer)
бo =γ(Df + level).
Δ б =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.
Settlement from S.P.T ‘N’ value:
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δ
δ
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.
∆ б’
qa
So,
δ
=
2 / N [B / (B + 0.3)]2 Cd(∆бo)
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
∆б
=
P / (B + Z)2
Where,
Z is the distance up to the c/l of layer from the bottom of footing