Geotechnical Engineering

DHARM

ELEMENTS OF SOIL DYNAMICS AND MACHINE FOUNDATIONS 869

Pressure on Foundation Soil

Total weight coming on the soil = (200 + 432) kN = 632 kN

Area of base = 6 × 3 = 18 m^2

∴ Stress on soil =

632

18

= 36.8 kN/m^2

Safe bearing capacity of the soil under static loading conditions = 100 kN/m^2

Assuming a reduction factor of 0.5,

dynamic bearing capacity = 50 kN/m^2

Since this is more than the stress on the soil the size is satisfactory.

Amplitudes of motion

ωn^2 =

CA

M

u =45 000 18×

632 9 81

,

(/.)

= 12,573/sec^2

ω^2 =

()600 2

60

×π^2

= 3.948/sec^2

Amplitude of vibration =

P

M n

0

()ωω^22 − , neglecting damping of soil.

=

P

M

z

n

max

()ωω^22 −

=

75 124

632 9 81 12573 3948

.

(/.)( − )

m

= 0.135 mm
Since the permissible amplitude is 0.15 mm, this is safe.
Note:- Moments due to unbalanced forces may be determined from appropriate data
regarding positioning of the compressor and the motor on the foundation block; also, analysis
for other modes of vibration such as sliding and rocking may also be performed and the corre-
sponding amplitudes may be obtained and verified.

Example 20.11: Design a suitable foundation for a double-acting steam hammer for the fol-
lowing data:

Weight of tup = 50 kN

Height of fall = 1 m

Area of piston = 0.2 m^2

Steam pressure on piston = 900 kN/m^2

Weight of anvil and frame = 1000 kN

Safe bearing capacity under static loading conditions = 200 kN/m^2

Coefficient of elastic uniform compression of soil = 5 × 10^4 kN/m^3

Base area of anvil (base area of elastic pad also) = 5.5 m^2

Thickness of elastic pad = 0.60 m

Modulus of elasticity of the material of the pad = 5 × 10^5 kN/m^2

Coefficient of restitution = 0.5