WHICH METHODS ARE USED TO DETERMINE THE BEARING CAPACITY OF SOIL

Methods for determining the bearing capacity of soil

The bearing power of soil can be determined by any one of the following two methods:
1. Method of loading
2. Method of dropping a weight

1. Method of loading:
The material required for this test are given below.
1. A square steel plate
2. Wooden table
3. Wooden pegs
4. Dumpy level
5. Rolled steel section
6. Sand bags

The procedure to carry out the test is as follows:
1. A square pit of required size is excavated upto required depth. The side of the pit should be at least five times the side of steel plate. At the centre of the pit,a square hole is dug of size equal to steel plate. The bottom of the hole should correspond to the foundation level and the ratio of depth to breadth of pit should be made equal to the ratio of depth to break of hole.
2. The bottom of the hole is made level by scraping only and it is well protected against disturbing forces before and during the test.
3. The steel plate is put up in the hole and then the plateform is prepared as shown in figure.
4. The amount of initial load is decided according to the type of soil to be tested. The weights of steel plate, wooden table should be carefully worked out.
5. A level is planted to note the setting of the steel plate with refference to a permanent bench mark.
6. The load is to be kept on the platform till the settlement of the ground stops.
7. Then the load is increased by a suitable amount, usually 0.5 tonne and the procedure is continued.
8. The recording of the results is carried out.
9. The settlement of the ground will be fairly in proportion to the load upto a certain limit. When the bearing power of the soil is exceeded, the settlement will be out of proportion.
10. The bearing capacity and safe bearing capacity of the soil are calculated as follows:
Bearing capacity of soil in tonnes/sqm
         =    Maximum load
              Area of steel plate
Safe bearing capacity of soil in tonnes/sqm
         = Bearing capacity of soil
                  Factor of safety

2. Method of dropping a weight:
In this method, a substance of known weight is dropped from a known height as shown in figure. The depth of impression made by the weight on the soil is noted. Then the bearing capacity of the soil is worked out as follows:

Let, R= Resistance of soil
        h= Height
        d= Depth of impression
        W= weight of the substance
Then,
           Total energy= R X d = W X h
            R= W X h   =  Ultimate resistance of soil
                      d

If, A = cross-sectional area of the substance
    R = resistance of the soil per unit area
    A
Safe bearing capacity of the soil per unit area=
                            R         
                         A X F
Where f = factor of safety.
The results obtained by this method are approximate and hence, this method is used for minor engineering structures or at places where first method would be impracticable.

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