Title: Geotechnical Design of Shallow Foundations
1Geotechnical Design of Shallow Foundations
2GENERAL REQUIREMENTS OF FOUNDATION-
- 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
3Case
- 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.
4Diagram 01
A
B
X
5Example
- 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.
6GENERAL REQUIERMENT FOR ANY DESIGN-
- 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.
7STRUCTURAL ANALYSIS-
- 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.
8SIZING-
- 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.
9SIZING-
- 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.
10SPECIFIC DESIGN OF FOUNDATION-
P
- Design means the determination of
- Df ?
- d ?
- B ?
- L ?
- As ?
G.S.L
Df
BXL
B
L
11Design Components
- 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.
12STRUCTURAL DESIGN-
- In design that takes into account the technical
aspects related to concrete is called as
Structural Design.
13GEOTECHNICAL DESIGN OF FOUNDATION-
- For the geotechnical deign of the foundation the
following steps are observed - 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.
14STEPS OF GEO-TECHNICAL DESIGN
- 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.
15Step 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
- Shallow Foundations.
- Deep Foundations.
- Floating Foundations.
16Step No. 2- DEPTH OF FOUNDAION
- 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 Terzaghis equation (for
general share failure) is applied i.e. - qult ScCNc ? Df Nq S?0.5?BN?
- 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.
17For pure clay
- (qult )Net ScCNc ?Df (Nq-1) S? 0.5?BN?
- 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 (s0 ? s)/(s0)
18Physical Requirements-
- Following are the different physical
requirements - Footing should be below
- Top organic soil.
- Susceptible zone.
- Surface erosion zone.
- Frost line.
- Scour depth.
- There should be the specified edge distance i.e.
- Level Difference
19Step No. 3- BEARING CAPACITY ANALYSIS-
- FOR BEARING CAPACITY-
- qult CNc ?DfNq 0.5?BN?
- qult / F.O.S safe gross B.C or safe B.C
- (qult )Net CNc ?Df (Nq-1) 0.5?BN?
- (qult )Net / F.O.S safe net B.C
- For square footing and circular footing
- qult 1.3CNc ?DfNq 0.5?BN?
- For pure clay
- Nc 5.7, Nq1 N?0
- B.C calculated by these equations is called B.C
w.r.t. shear.
20BEARING CAPACITY WITH S.P.T N VALUES-
- 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 gt B, use D B for computation.
21Correction factors for position of water levels
- 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.
22- 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 ? then from Mayerhoffs Ø 28 N / 10 - And for the minimum density samd unit,
- Weight ? 115 120pcf.
23FOR SETTLEMENT ANALYSIS-
- d 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.
24Settlement from S.P.T N value-
- d 2 / N B / (B 0.3)2 Cd Cw qa
- d Settlement in mm
- Cd Depth factor 1 / Kd
- Kd 1.033D / B 1.33
- Cw Water correction factor.
- 1 (always) It is reflected in N values.
- qa ? ?
- So,
- d 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