DHARM836 GEOTECHNICAL ENGINEERINGwhere Cuf = Cu-value for the machine foundation,
Cup = Cu-value from the repeated plate bearing test,
Ap = area of the plate,
and Af = base area of the machine foundation.
Experiments have shown this relationship to be valid only upto a certain area limit of about
10 m^2.
Barkan (1962) recommends certain values of Cu for different soils to be used in case soil investi-
gations are not possible. These are given in Table 20.2.
Since it has been established that Cu is a function of the foundation size, it would be erroneous
and misleading to use these values for-design. Further, Barkan found a large discrepancy between
experimental and computed values of Cu from Eq. 20.68 and attributed it to the possibility of the soil
not behaving like an ideal material.
But Subrahmanyam (1971) has shown that Eq. 20.68 is valid for different types of soils such as
silty clay, uniform fine sand, loess loam, and silty sand, provided Cu is evaluated from a field vibration
test and not from repeated plate bearing test.
Table 20.2 Recommended design values of Cu (after Barkan, 1962)Category Soil group qa Cu
(kg/cm^2 ) (kN/m^2 ) (kg/cm^2 ) (kN/m^3 )I Weak soils (clays and silty clays with sand, clayey upto 1.5 (150) upto 3 (3 × 10^4 )
and silty sands; also soils of Categories II & III with
laminae of organic silt and of peat).
II Soils of medium strength (clays and silty clays with 1.5–3.5 (150–350) 3–5 (3 to 5
sand, close to the plastic limit; sand) × 10^4 )
III Strong soils (clays and silty clays with sand, of 3.5–5 (350–500) 5–10 (5 to 10
hard consistency; gravels and gravelly sands; × 10^4 )
loess and loessial soils)
IV Rocks > 5 (> 500) > 10 (> 10
× 10^4 )qa: permissible static pressure on soil
He concludes that the repeated plate bearing test, being a static test, cannot effectively simulate
the highly dynamic nature of loading that occurs in the soil under a machine foundation, and hence is
not considered to be satisfactory to evaluate the coefficient of elastic compression, and there from the
spring constant for the machine foundation-soil system.
Determination of Damping Ratio will be also taken up along with that of shear modulus
since the same test data may be used to evaluate both.
The major items of equipment that are involved in any dynamic test fall into two catego-
ries—one required for inducing a known pattern of vibration (for example, sinusoidal wave
form) and the other required for measuring the vibration response. The principle unit of the
first group of equipment is the ‘vibrator’ or the ‘oscillator’, which may work on mechanical,
electro-magnetic, or hydraulic principle. The mechanical type is commonly used for applica-
tion to machine foundation. The equipment of the second group includes essentially a trans-
ducer (or a vibration pick-up), an amplifier and a recorder.