Mat foundation

Mat foundation

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1.1 Introduction and types of mat foundation

Mat foundation:

  • A large footing which covers the entire area below a structure and supports arrangement of all wall and column.
  • Resist the differential settlement.

Types of mat foundation:

1. Flat slab mat

  • Single slab of uniform thickness.
  • Suitable for closed spaced.

2. Flat slab thickedd under column

  • Column is subjected to very heavy load.

3. Rigid frame

  • Used to carry extremely heavy load.

4. Piled raft mat

  • If soil is highly compressive then this is used.

5. Flat slab with pedestals

  • Same as flat slab but pedestals are provided at base.

1.2 Bearing capacity and settlements

1. For cohesive soil

  • In cohesive soil net ultimate bearing capacity is determined by skempton’s equation.

qnu = CNc[1+0.2(Df/B)]*[1+0.2(B/L)]

∴  qnu = 5C[1+0.2(Df/B)]*[1+0.2(B/L)]

Let,

Q = Total superimposed load

  • Gross pressure (qg) = Q/A
  • In term of net ultimate bearing capacity;

∴ qg = qns + γ Df = Q/A – γ Df

2. For cohesion less soil

  • Bearing capacity of cohesionless soil depends upon width of footing.

Safe bearing capacity is determined as:

∴ qns = 0.22 N2BRw1 + 0.67 (100+N2)DfRw2

  • Safe settlement pressure for a settlement 25 mm is

∴ qns = 17.5 (N-5)Rw1

  • Qnp = 22 Rw1 (for 50 mm settlement)

∴ Above equation is applicable for 5 ≤ N ≤50.

1.3 Compensated foundation (Floating foundation)

  • When structure in soil is nearly equal to the total excavated soil from ground including weight of water in soil.

i.e Excavated weight = Structural weight

  • Settlement of sub soil is prevented.
  • Rigid raft foundation is provided when floating foundation is needed.

Diffulties during construction of floating foundation:

  • Excavation: Should be done carefully.
  • Dewatering: If depth of excavation is below water table dewatering is necessary.
  • Critical depth: If soil has low shear strength the limited depth is excavated i.e called as critical depth.
  • Bottom heave: Excavation of foundation reduce pressure in below of foundation results in heaving of bottom of excavation.

1.4 Conventional method of analysis

  • Determine the line of action of all loads acting on the raft.
  • Determine the contact pressure.

a. If e = 0 , q = Q/A

b. If resultant has an ecentricity ex & ey in x & y direction.

     Then,

      q = (Q/A) ± Q(ex/Iyy)x ± Q(ey/Ixx)y

  • Divide the mat into number of beam or strip to analyze each beam separately.
  • Draw shear force and bending moment diagram of each strip and calculate qav below the beam.
  • Determine modified column load.

Qav = ½ (downward load + upward load)

or, Qav = ½ (Q1 + Q2 + Q3 + qav *B1*x

Modified average soil pressure

qav = Qav/x1* B

Column load modified factor

F = Qav / (Q1+Q2+Q3)

All the column load are multiplied by ‘F’ for the strip.

For strip

FQ1, FQ2 & FQ3

  • Draw BMD and SFD for modified column load.
  • Find maximum bending moment and shear force.
  • Find thickness of slab and reinforcement using BMmax and SFmax.

References:

  • Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
  • Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution.

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