Foundation, Concrete and Earthquake Engineering

Different Types of Earth Pressure Cells, Their Applications and Construction of Isobar Diagram Using Them-1

Stresses are induced in a soil mass due to weight of overlying soil and due to the applied loads. These stresses are required for the stability analysis of the soil mass, the settlement analysis of foundations and the determination of the earth pressures.

The stresses due to self weight of soils are generally large in comparison with those induced due to imposed loads. This is unlike many other civil engineering structures, such as steel bridges, wherein the stresses due to self weight are significant. In many cases the stresses due to self weight are a large proportion of the total stresses and may govern the design.

The stresses induced in soil due to applied loads depend upon its stress-strain characteristic. The stress-strain behaviour of soils is extremely complex and it depends upon a large number of factors, such as drainage conditions, water content, void ratio, rate of loading, the load level, and the stress path. However, simplifying assumptions are generally made in the analysis to obtain stresses. It is generally assumed that the soil mass is homogeneous and isotropic. The stress-strain relationship is assumed to be linear. The theory of elasticity is used to determine the stresses in the soil mass. It involves considerable simplification of real soil behaviour and the stresses computed are approximate ones. Fortunately, the results are good enough for soil problems usually encountered in practice. For more accurate results, realistic stress-strain characteristics should be used. However, the procedure becomes complex and numerical techniques and high speed computers are required.


The specific objectives of the study are:
  1. To determine the soil stress at different depth of layers by earth pressure cells.
  2. To assess for a particular soil layer with low bearing capacity, how much load can be applied at the top surface of soil.
  3. To construct the curve (Pressure Bulb) showing the intensity of load at varying depth.
  4. T o determine the depth of replaced materials by Isobar diagram for soil with low bearing capacity.

The methods discussed in the preceding sections are relatively more accurate, but are time consuming. Sometimes, the engineer is interested to estimate the vertical stresses approximately for preliminary designs.

The following methods can be used:-
(1) Equivalent Point-Load Method:- The vertical stress at a point under a loaded area of any shape can be determined by dividing the loaded area into small areas and replacing the distributed load on each small area by an equivalent point load acting at the centroid of the area. E.g. in Fig.1.1, Q = qa2 for each area. The total load is thus converted into a number of point loads. The vertical stress at any point below or outside the loaded area is equal to the sum of the vertical stresses due to these equivalent point loads. Using Boussinesq’s equation 

σz = IB . Q /z 2

we get

Equivalent Point Loads
Fig.1.1. Equivalent Point Loads.
Eq. 1.1 gives fairly accurate results if the side a of the small area unit is equal to or less than one-third of the depth z of point P at which the vertical stress is required.

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