Plate Load Test – Procedure, Safe Bearing Capacity Calculation


What Is Plate Load Test?

Plate load test is a test conducted on the construction site itself, to figure out the ultimate bearing capacity of soil and the anticipated settlement under a given load. This test is usually carried out on field to calculate the ultimate bearing capacity of sandy soils and clayey soils, at desired depth. Plate load test helps in determining the type of foundation and its design, for the structure. Plate load test is popularly used for shallow foundation.

Plate load test is performed using a square shaped plate made up of steel. The size of the plate usually varies from 30 cm square to 75 cm square, respectively. The plate should be considerably thick so that it can withstand the applied load on it without bending or deforming. The thickness of the plate should not be less than 25 mm.

A test pit is dug out of size 5 times greater than the size of the plate. A small square hole is excavated in the centre of the pit which is equivalent to the size of the steel plate. The plate is then placed at some depth. The load is applied on the plate gradually and settlement for each increment of load is recorded.

There comes a point where the settlement occurs at a faster rate. So at that point, the load applied on the plate is stopped and the total load counts up to that particular point are calculated and divided by the total area of the plate. This calculation gives the ultimate bearing capacity of soil at that particular point of depth.

Once we get the ultimate bearing capacity of soil, it is divided by a safety factor, which is usually taken as 2 or 2.5, which gives us the safe bearing capacity of soil.

Apparatus

  1. Steel test plate
  2. Hydraulic jack and pump
  3. Reaction beam or truss
  4. Dial gauge
  5. Pressure gauge
  6.  Excavation tools
  7. Equipment for loading
  8. Plumb bob, tripod, spirit level

Different Methods Of Plate Load Test

There are two different methods known for plate load tests.

  1. Gravity loading platform method, and
  2. Reaction truss method.

Let us learn these methods one by one in detail below.

1. Gravity Loading Platform Method

In this method, a platform is built on the top of a vertical post, resting on a platform. The load is applied on the platform with the help of sandbags, stones, or concrete blocks.

When the load is applied on the platform, the test plate starts to settle down. That settlement is precisely measured using two sensitive dial gauges, which are mounted on the independently supported datum bar. The total load is calculated and ultimate bearing capacity is measured using the calculations and formulas.

2. Reaction Truss Method

In the reaction truss method, the reaction of the jack is supported by a reaction truss. The truss is clutched to the ground using soil anchors. Using hammer, the anchors are forcefully driven into the ground. The reaction truss is made out of mild steel sections.

The guy ropes are utilized for lateral stability of the truss. The reaction truss method is popularly used because it is a quick and simple method.

Procedure Of Plate Load Test

A test plate made out of steel is firmly seated in the hole, present in the excavated pit. If the ground is uneven, a thin layer of sand is spread beneath the test plate.

According to the Indian standards (1888 – 1982) a seating load of 70 g/cm^2 or 0.7 t/m^2 is applied before the actual load is applied. A hydraulic jack is used for applying load in convenient increments, say about one-fifth of the expected safe bearing capacity or one-tenth of the ultimate bearing capacity of soil.

Two dial gauges are fixed at the opposite ends and supported on a datum bar, which is used to record the load applied on plate.

The settlement of soil is recorded on the two dial gauges attached to the test setup, at an interval of 1, 4, 10, 20, 40, and 60 minutes respectively, and then at hourly intervals until the rate of settlement comes down to less than 0.02mm per hour. The readings acquired, are noted systematically in a table.

After this, the next increment in load is applied similarly. The maximum amount of load applied is 1.5 times the expected ultimate load or 3 times the expected allowable bearing pressure.

Calculation Of Bearing Capacity

Once the plate load test is over, a graph is plotted based on the observations acquired from the test conducted. The load is plotted from the x-axis and the settlement is plotted on the y-axis respectively.

The amount of ultimate load can be obtained through a graph corresponding to the settlement load of 1/5th of the width of the plate. A factor of safety of value 2 or 2.5 on ultimate bearing capacity is used to determine the safe bearing capacity. 

Some Equations Used To Calculate Bearing Capacity & Settlement Of Soil

Equation for calculating the soil bearing capacity of clayey soil

Ultimate bearing capacity = ultimate load for the plate

Equation for calculating the soil bearing capacity of sandy soil

Ultimate bearing capacity = ultimate load of plate x ( width of pit/size of the plate ) Safe bearing capacity of soil = ultimate bearing capacity/factor of safety

Where, the factor of safety varies from 2 to 3.

Equation for calculating the foundation settlement of clayey soil 

Settlement of foundation = settlement of plate x ( width of pit / size of plate ) 

Equation for calculating the foundation settlement of sandy soil

Settlement of foundation =

Settlement of plate x width of pit x ( size of plate + 0.3) / size of plate x (width of pit + 0.3)^2

Advantages Of Plate Load Test

i) Plate load test proves out to be an economical method for determining the ultimate bearing capacity of soil and the expected settlement of the soil under a given load.

ii) Plate load test is a simple method with a simple procedure throughout.

iii) Plate load test provides us an understanding of soil sustainability against a heavy load.

iv) The plate load test also helps in selecting the type and design of foundation for the structure, based on the outcomes and results of the plate load test.

Disadvantages Of Plate Load Test

i) Plate load test does not prove to be feasible for cohesive soils as the plate load test is a time-consuming process and so it fails to give the ultimate settlement for cohesive soils.

ii) If the layer of soil is not uniform, some errors can occur in the results.

iii) The groundwater may affect the test results. So before the commencement of the test, all the groundwater must be pumped out to obtain an accurate result.

Factors Affecting Plate Load Test

There are some factors which affects the bearing capacity of soil. They are as follows:-

  1. Groundwater level
  2. Shear strength of soil
  3. Permeability of soil
  4. Soil density
  5. The angle of internal friction
  6. Void ratio of soil

The above stated factors may also affect the results of plate load test. Hence it is better to check for all the factors affecting the test before the commencement of the plate load test. Variations or errors in results may lead to wrong selection and design of the foundation of structure, which may sometimes be the cause of the destruction of the structure.

Also Read –

Slump Test Of Concrete

CBR Test Of Concrete

FDT Test OF Soil

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