Geotechnical Design of Shallow Foundations

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Geotechnical Design of Shallow Foundations

• Chapter 03

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GENERAL 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

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Case

• 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.

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Diagram 01
A
B
X
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Example

• 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.

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GENERAL 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.

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STRUCTURAL 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.

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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.

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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.

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SPECIFIC DESIGN OF FOUNDATION-
P

• Design means the determination of
• Df ?
• d ?
• B ?
• L ?
• As ?

G.S.L
Df
BXL
B
L
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Design 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.

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STRUCTURAL DESIGN-

• In design that takes into account the technical
  aspects related to concrete is called as
  Structural Design.

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GEOTECHNICAL 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.

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STEPS OF GEO-TECHNICAL DESIGN

1. Selection of the type of foundation system.
2. Fix the vertical location i.e. Df of the
   foundation.
3. Bearing Capacity and Settlement Analysis and from
   this a suitable value of qd i.e. the design
   pressure.
4. The dimensions in plane (B L)
5. Construction Specification.

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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
• Shallow Foundations.
• Deep Foundations.
• Floating Foundations.

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Step No. 2- DEPTH OF FOUNDAION

1. For depth of foundation, the following two
   considerations are kept in mind,
2. Mechanical Consideration.
3. Physical Consideration.
4. In mechanical consideration we check Bearing
   Capacity and Settlement.
5. For Bearing Capacity Terzaghis equation (for
   general share failure) is applied i.e.
6. qult ScCNc ? Df Nq S?0.5?BN?
7. 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.

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For 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)

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Physical 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

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Step 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.

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BEARING 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.

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Correction 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.

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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 ? then from Mayerhoffs Ø 28 N / 10
• And for the minimum density samd unit,
• Weight ? 115 120pcf.

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FOR 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.

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Settlement 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