PLATE LOAD TEST – its 7 [ Procedure steps, Equations & advantages]

Plate Load Test is used for selection and design of shallow foundation plate load test is very popular, this test is used to determine the ultimate bearing capacity of the soil and probable settlement under given load the Plate Load Test is performed for bearing capacity of the soil. A suitable factor of safety is applied to calculate bearing capacity.

Plate load test was conducted on subgrade, base and surface course layer and due to this test and for performing the plate load test, the plate is placed at the desired depth and then the load is applied and settlement under load is recorded.

To determine the ultimate bearing capacity of soil at that depth the total load up to that point is calculated and divided by the area of the plate and then to determine the safe bearing capacity the ultimate bearing capacity is divided by a safety factor.

For the selection and designing the foundation plate load test is helpful to determine the ultimate bearing capacity of the soil and also helps in the shallow foundation.

Equipments Of Plate Load Test:

There are following equipments used in the plate load test as given below;

1. Test plate
2. Hydraulic jack and pump
3. Reaction truss or reaction beam
4. Dial gauges
5. Pressure gauge
6. Loading columns
7. Necessary equipment for loading platform
8. Spirit level, plumb bob, tripod, etc.

Procedure Of Plate Load Test:

There are following steps in the procedure of plate load test such as;

Step 1:

To desired depth excavate test pit up and pit size should be at least five times the size of the test plate.

Step 2:

A small hole is created at the center of the pit and the size of the hole must be the same as the size of the steel plate and the level of the actual foundation should correspond to the level of the bottom of the hole. The hole is created that the ratio of depth to width of the hole is equal to the actual foundation depth and width.

Step 3:

As a load-bearing plate a mild steel plate is used and the size of this plate may vary from 300mm to 750mm and the thickness of this plate should be at least 25mm. the plate can be circular or square, for circular footing circular plate is used and for square footing square plates are used.

Step 4:

At the center of the plate a column is placed and through the centrally placed column the load is transferred to the plate.

Step 5:

By gravity loading method or by truss method the load can be transferred to the column. A platform is constructed over the column for gravity loading method and load is applied to the platform. For the application of gradual loading hydraulic jack is placed in between the column and loading platform.

Step 6:

To record the settlement at least two dial gauges should be placed at diagonal corners if the plate and apply seating load and note the initial readings. Then the load is applied through the hydraulic jack and from pressure gauge the applied load is noted.

Step 7:

From dial gauge the settlement is observed for each increment and load should be observed after 1, 10, 20 and 60 minutes after increasing load settlement, in tabular form readings are noted. Then under new load readings are noted and until the maximum load is applied increment and data collection is repeated.

Calculation Of Bearing Capacity From Plate Load Test:

The load settlement curve is drawn after collection of the field data and in this where the load applied is plotted on x-axis and settlement in y-axis so the ultimate load for plate is obtained and is the corresponding load for the settlement.

On the graph points are plotted and at one point curve are broken and the corresponding load at that breakup point is considered to be the ultimate load. From the ultimate load ultimate bearing capacity can be calculated and then to determine the safe bearing capacity of soil the ultimate bearing capacity is divided by a suitable factor of safety.

Equation For Calculation of Bearing Capacity Of Clayey Soil:

The equation for the calculation of bearing capacity of the clayey soil is;

qu(f) = qu(p) 

As qu(f) is the ultimate bearing capacity and

qu(p) is the ultimate load for a plate.

Equation For Calculation Of Bearing Capacity Of Sandy Soil:

The equation for the calculation of bearing capacity of the sandy soil is;

qu(f) = qu(p) x Bf / Bp

Where

qu(f) is the ultimate bearing capacity

qu(p) is the ultimate load for plate

Bf is the width of the pit and Bp is the size of the plate.

So,

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

Equations For Foundation Settlement Calculation From Plate Load Test:

For foundation settlement calculation following equations can be used;

For Clayey Soil:

The equation for calculating the foundation settlement of clayey soil is;

sf = sp x Bf/Bp

Where

sf is settlement of foundation

sp is settlement of plate

Bf is the width of the pit and Bp is the size of the plate.

For Sandy Soil:

The equation for calculating foundation settlement of sandy soil is;

(sf) = sp [{Bf(Bp + 0.3)}/{Bp(Bf + 0.3)}]2

Where

Sf is settlement of foundation

sp is settlement of plate

Bf and Bp are widths of plate and foundation.

Advantages Of Plate Load Test:

There are following advantages of plate load test such as;

1. To understand the foundation behavior under loading the condition, the plate test helps in it.
2. For a certain load, evaluation of bearing capacity of soil at a certain depth and prediction of settlement.
3. Shallow foundation can be calculated.
4. Efficient cost and time.
5. Reliable and easy to perform.

Disadvantages Of Plate Load Test:

There are following disadvantages of plate load test such as;

1. Depth of influence is limited and it could only give the bearing capacity of soil.
2. For long term consolidation of foundation soil it may not provide information on the potential.
3. The size of the test plate is smaller than the actual foundation that’s why there is a scale effect.
4. Significant ground disturbance occurs when excavation is carried out.