Geotechnical Engineering

DHARM

748 GEOTECHNICAL ENGINEERING

In terms of the critical distance, dc, and the velocities V 1 and V 2 in the upper soft layer
of thickness H 1 and the lower hard layer respectively, the thickness of the upper layer may be
written as follows:

H 1 =

dVV

VV

c

2

21

21

()

()

−

• ...(Eq. 18.10)
  The method can be extended to any situation with greater number of strata, provided
  each is successively harder than the one above. Typical wave velocities are given in Table 18.6.

Table 18.6 Typical Wave Velocities for Different Materials

(IS: 1892-1979 Appendix B)

Material Velocity (m/s) Material Velocity (m/s)

Sand and top soil 180 to 365 Water in

loose materials 1400 to 1830

Sandy clay 365 to 580 Shale 790 to 3350

Gravel 490 to 790 Sandstone 915 to 2740

Glacial till 550 to 2135 Granite 3050 to 6100

Rock talus 400 to 760 Limestone 1830 to 6100

There are certain significant limitations to the use of the seismic refraction method for
determining the subsurface conditions. These are:

1. The method cannot be used where a hard-layer overlies a soft layer, because there
   will be no measurable refraction from a deeper soft layer. Test data from such an
   area would tend to give a single-slope line on the travel-time graph, indicating a
   deep layer of uniform material.


2. The method cannot be used in an area covered by concrete or asphalt pavement,
   since these materials represent a condition of hard surface over a softer stratum.


3. A frozen surface layer also may give results similar to the situation of a hard layer
   over a soft layer.


4. Discontinuities such as rock faults or earth cuts, dipping or irregular underground
   rock surface and the existence of thin layers of varying materials may also cause
   misinterpretation of test data.

18.6.2Electrical Resistivity

Resistivity is a property possessed by all materials. The electrical resistivity method is based
on the fact that in soil and rock materials the resistivity values differ sufficiently to permit
that property to be used for purposes of identification.

Resistivity is usually defined as the resistance between opposite faces of a unit cube of
the material. Each soil has its own resistivity depending upon the water content, compaction
and composition; for example, the resistivity is high for loose dry gravel or solid rock and is low
for saturated silt.

To determine the resistivity at a site, electrical currents are induced into the ground
through the use of electrodes. Soil resistivity can then be measured by determining the change