Geotechnical Engineering

DHARM

ELEMENTS OF SOIL DYNAMICS AND MACHINE FOUNDATIONS 837

I.S. 5249-1977 “Method of Test for insitu determination of dynamic properties of soil”
includes several methods for the determination of insitu dynamic properties of soils. One of
these is the ‘Standard Block vibration Test’.
A plain cement concrete block (M 150) of size 1.5 m × 0.75 m × 0.7 m shall be cast at site
at the depth where the machine foundation is to be laid as shown in Fig. 20.23.

1.5 m

0.75 m 0.75 m

0.75 m

1.0 m

6m

2.5 m

Plan

0.7 m

0.6 m Depth of foundation

Bolt0.05 m

Model block

Section

100

mm

500 mm

100 mm

Detail of bolt

25

mm

15 mmf

Angle iron

welded to bolt

Fig. 20.23 Test pit with concrete block (block vibration test–IS:5249-1977)
A mechanical oscillator should be mounted on the block so that the block is subjected to
purely sinusoidal vertical vibration. The oscillator is set to work at a certain low frequency
value. Two geophones of identical characteristics, one connected to the vertical plates and the
other to the horizontal plates of an oscilloscope are so positioned along a line in the longitudi-
nal direction of the block that the Lissajous figure on the oscilloscope screen becomes a circle.
The nearest geophone may be at a distance of 300 mm from the block and the farther one
varied in position till this condition is achieved. The block diagram for the testing arrange-
ment is shown in Fig. 20.24.

Oscillator Motor

Dg

Speed

controller

Amplifier Recorder

Power

supply

Geophones Test

pit

Concrete block

Vibration

pick-up

Oscilloscope

Fig. 20.24 Typical block diagram for the experimental

set up for insitu dynamic soil test (IS:5249–1977)