Plate Load Test – Its Procedure, Limitations, Advantages & Precautions

Plate Load Test

The plate load test is performed in the field to find out the bearing capacity of the soil or how it can be settled at each increment under the given load. This test is really useful for designing the foundation, especially the shallow foundation.

Easy to determine the bearing capacity of the soil, so we can design a suitable foundation according to the bearing capacity of the area or place.

To perform this test, we need to place the plate at desired depth then apply the load gradually so the settlement of the soil under the given load is recorded.

A situation arrives where the settlement occurs at a rapid rate, then the total load of that point is calculated and it is divided by the area of the plate to find out the ultimate bearing capacity of soil at that depth. And the ultimate bearing capacity is divided by the safe load to find out the safe bearing capacity.

The bearing plate is square and its dimensions

• Minimum size = 30cm²
• Maximum size = 75cm²
• Thickness of plate = >25mm.

Also Read: 8 Major types of soils

Method of Plate Load Test

There are two different methods to perform this test:

1. Gravity loading platform method
2. Reaction truss method

1. Gravity loading platform method

In this method of plate load test, a column is constructed over a column on the test plate and then the link ad is applied by the sandbags and the sandbags are considered as a dead load.

The hydraulic jack is in between the rigid plate and the top surface of the column to transfer the load properly. The dial gauge is used to record the settlement of the plate.

2. Reaction truss method

In this method, a steel truss is placed of the ideal size to resist the reaction of the hydraulic jack. The instability of the truss is controlled or prevented by the nails which are fixed in the soil with the help of a hammer. Generally, the truss is made up of a mild steel section.

The settlement is recorded by the dial gauge. This method is more popular nowadays because it is quite easy and simple to implement.

Also Read: Unit Weight of Soil 

Calculation of Bearing Capacity

The plate load test is performed so we used the date which we have taken from the test to determine the bearing capacity. By the data a curve is drawn which is known as load settlement curve and this is a logarithmic graph where the applied load is drawn or plotted on the X-axis and the settlement of plates on the Y-axis.

The ultimate load of the plate is the corresponding load for the settlement of one-fifth of the width of the plate.

The point where the graph curve breaks, can be used for the calculation of the ultimate load of the plate. And we can calculate the ultimate bearing and safe bearing capacity of the soil from the ultimate load.

The safe bearing capacity of the soil can be determined by dividing the ultimate bearing capacity with the factor of safety 2 or 3.

Equation for Calculating Bearing Capacity of Soil

1. Bearing capacity for clayey soil

Ultimate bearing capacity = ultimate load for the plate

2. Bearing capacity for sandy soil

Ultimate bearing capacity =  (size of pit/size of the plate) x ultimate load for the plate

Safe bearing capacity = ultimate bearing capacity/factor of safety.

Factor of safety varies from 2 to 3.

Equation for Foundation Settlement

For clayey soil: Settlement of the foundation= settlement of plate x (width of pit/ size of the plate)

For sandy soil: Settlement of the foundation:- settlement of plate x (width of plate x (size of plate +3))/(size of the plate x (width of pit +3)²).

Equipment for Plate Load Test

The following apparatus used in Plate load test:

1. Plumb bob, tripod, spirit level (to determine the centre)
2. Hydraulic jack (ball socket type with the 50T capacity)
3. pump
4. Mild steel plate of thickness 25 mm and dimension 30X30cm(test plate)
5. Dial gauges (minimum 3)
6. Reaction beam and reaction trusses with nails
7. Pressure gauges
8. Loading column

Procedure of Plate Load Test

1. We need to excavate the test pit up to your desired depth or the foundation we have to build (size 1.5m X1.5m), and the size should be at least 5 times of the size of the plate(B_p).
2. Then find the centre of the pit and the size of the hole is similar to the size of the plate.
3. The steel plate is used as the load-bearing member and the square plate is used for square footing and circular plate is used for circular footing.
4. Thereafter a column is placed at the centre so the load is transferred through the column to the plate.
5. As we know that there are two methods for the transfer of load which are by gravity loading method and truss method and already discussed above.
6. There are two dial gauges placed on the corners of the plate to record the settlement of the plate at each increment of load.
7. Then the seating load of .7T/m² and then release before the actual loading starts.
8. All the initial readings are noted.
9. The hydraulic jack is used to apply the load and the load is increased gradually. The increase in load is generally one-fifth of the safe bearing capacity or one-tenth of the ultimate load or any smaller values. The pressure gauge is used to note the reading.
10. The settlement of the plate at each increment is taken or recorded from the dial gauges. By the increasing settlement, the load settlement maybe 1, 4, 10, 20, 40, and 60 minutes or specific hourly intervals is repeated until it reached .2mm per hour, and the reading is noted down in the tabular form.
11. Collect all the data from the dial gauge or pressure gauge for a particular loading.

Limitations of Plate Load Test

• This test is really good for selecting the foundation depending on the bearing capacity but has some limitations also.
• There is some size scale effect issue because of the smaller size of the plate.
• The test is for a short period of time so can’t predict the settlement of the plate for the longer period of the settlement.
• The settlement for the loose sandy soil is greater than the settlement as indicated by the bearing test.
• It has only been performed for the shorter depth, it only gives the bearing capacity of the plate with depth up to 2 times of the diameter of the plate.

Advantages of  Plate Load Test

• From this method, we can predict the assumption of designing the foundation for a certain depth and specific load.
• It is cost and time-efficient.
• We can design a shallow foundation for this test.
• It is quite easy to perform and very reliable.

Precautions 

It is very necessary to maintain the alignment of the hydraulic jack. It should be in a straight line so the load is transferred vertically. The minimum duration must be 60min.

Plate Load Test IS Code

IS 1888-1982 can be used to calculate the bearing capacity of Soils using the plate load test method

Also check out: IS Codes List in Civil Engineering