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The suction in soil mass can be measured mainly by three methods namely Tensiometer method, Suction Plate method and Centrifuge Method. Each method is briefly explained below.
1. Tensiometer Method
The figure-1 below shows the arrangement of tensiometer to determine the suction present in the soil sample.
The arrangement consists of a porous pot which is filled with water. A U-tube containing mercury is connected to this porous pot. The porous pot is placed inside the soil mass whose suction is to be determined.
As time passes, the soil starts to take water from the porous pot. This process continues until the suction inside the pot and outside the pot are equal. At this point, the system is said to achieve equilibrium.
The suction inside the porous medium can be measured by a u-tube manometer.
The pressure inside the porous medium Po is
Po = 0- 13.6 x 9.81 x h + (h+y) x 9.81
Po = -(12.6h +y)9.81
Where 'h' is the deflection of the mercury in meters, 'y' is the vertical intercept between the level of mercury in the right limb of the manometer and the centerline of the pot.
Soil suction values up to 800cm of the water column can be measured by this method.
2. Suction Plate Method
In this method, the soil sample is placed over a porous plate which is called as the suction plate. The porous plate is in contact with a reservoir as shown in figure-2.
A pipe is connected to the reservoir. At a particular point, a manometer is connected to this pipe. A vacuum pump is connected to the other end of the pipe.
The soil sample takes water from the reservoir through the porous plate. With the intake of water by the soil, the water meniscus in the pipe has a tendency to move left. The vacuum pump tries to maintain the water meniscus stationary.
At a particular point, a reduction in pressure is shown in the manometer as a deflection of mercury (h). This reduction in pressure is the suction of the soil.
The suction plate is best suitable for soil with a suction value of up to 10m height of the water column.
3. Centrifuge Method
In the centrifuge method, the soil sample is subjected to a centrifugal force. Initially, the soil sample is placed on to a porous pot which is, in turn, is enclosed inside a brass container. This brass container can rotate about its axis.
As the rotation proceeds, the water from the soil sample moves through the porous medium and joins the water reservoir present in the brass container. The brass container already possesses water which is filled till water level. Water more than this level goes out through an escape hole provided as shown in figure-3.
This migration of the water from the soil sample to the water table in the brass container continues until the suction of the water left in the soil is just as required for equilibrium.
Then the soil suction can be determined by the formula:
Where 'w' is the rotational speed in radians/second, 'r1' is the radial distance from the center of rotation to the water table and 'r2' is the radial distance from the center of rotation to the middle of the water sample respectively.
The centrifuge method is conducted at various speed in order to get a relationship between the water content and the soil suction. This method can be used to calculate a higher value of suction.
Read More: What is Soil Suction?