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Geotechnical Design of Shallow Foundations

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In foundation design these stresses are called as bearing capacity and strains as settlements. ... in case of pile ... Design of Shallow Foundations ... – PowerPoint PPT presentation

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Title: Geotechnical Design of Shallow Foundations


1
Geotechnical Design of Shallow Foundations
  • Chapter 03

2
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

3
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.

4
Diagram 01
A
B
X
5
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.

6
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.

7
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.

8
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.

9
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.

10
SPECIFIC DESIGN OF FOUNDATION-
P
  • Design means the determination of
  • Df ?
  • d ?
  • B ?
  • L ?
  • As ?

G.S.L
Df
BXL
B
L
11
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.

12
STRUCTURAL DESIGN-
  • In design that takes into account the technical
    aspects related to concrete is called as
    Structural Design.

13
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.

14
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.

15
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.

16
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.

17
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)

18
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

19
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.

20
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.

21
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.

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.

23
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.

24
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
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