The bearing capacity of the soil determines the type of foundation selection. The maximum capacity of the soil that supports the structural load without any shear failure or settlement is called the bearing capacity of the soil. It is calculated in load per unit area.
One of the tests that is performed to determine this value is known as Plate Load Test.
The plate load test is used to determine the ultimate bearing capacity of the soil and the settlement of foundations of cohesionless soil. It is also known as the Plate bearing test.
There are other types of test used to determine the same such as
A square plate of 300mm x 300mm or a circular plate of 300mm diameter should be placed in the foundation at a certain depth. Then gradual load will be applied on the plate. Due to the pressure, the foundation settles down at each load increment.
Based on that settlement, the bearing capacity of the soil will be calculated.
The plate load test or plate bearing test should be performed as per the IS code 1888 either by gravity load test or reaction truss method.
In gravity load tests, sandbags or concrete blocks are used as a dead load which is placed over the platform made by steel joists. A hydraulic jack is provided at an appropriate height to initiate the pressure between the vertical column and the steel plate.
In here, the reaction truss will be installed replacing the concrete blocks or sandbags. The procedures are the same. With the help of hammers, the truss is controlled by soil anchors and nails fixed into the soil. The truss is made of mild steel sections.
The hydraulic jack bears the truss which is monitored by the dial gauge. The reaction generated on truss will be recorded on the gauge. To avoid the later movement, the truss is fastened by Guy-wire.
The equipment used for both tests is varied but the methodology and calculations to determine the ultimate bearing capacity is the same.
In reaction truss loading method the load will be 2Kn/Sqm for every interval and the settlement has been recorded until the settlement rate reaches 0.2mm per hour.
Soil bearing capacity of clay soil
The equation to determine soil bearing capacity for clay
Ultimate bearing capacity = Ultimate load on Plate
Safe Bearing Capacity = Ultimate bearing capacity / Factor of Safety
The factor of safety = 2 or 3
Soil bearing capacity of Sandy Soil
The equation to determine soil bearing capacity for sandy soil
Settlement calculation of Clay Soil
The following formula can be used to determine the clayey soil from the plate load test.
Settlement calculation of Sandy Soil
The following formula can be used to determine the sandy soil from the plate load test.
The general formula for calculating the settlement of granular soil is,
Sf = Sp[Bf (Bp + 0.3)/Bp(Bf + 0.3)]²
For Clayey Soil is,
Sf = Sp*(B/Bp)
Sf = Settlement of footing in m
Sp = Settlement of plate in m
Bp = width of test plate in m
B = width of the footing in m
Happy Learning 🙂
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Thank you
Good.please send future articles related to caculation of structural stability soundnes
For the buildings
Thanking you
Er.R.Jayachandran