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What is Plate Load Test?

Plate load test is done at site to determine the ultimate bearing capacity of soil and settlement of foundation under the loads for clayey and sandy soils. So, plate load test is helpful for the selection and design the foundation. To calculate safe bearing capacity suitable factor of safety is applied.

Apparatus for Plate Load Test on Soil

  • Mild Steel plate
  • Hydraulic jack
  • Reaction beam or reaction truss
  • Dial gauges
  • Excavating tools

Procedure of Plate Load Test on Soil

A Pit is excavated in the ground at which foundation is to be laid. The size of pit is generally 5 times the size of the plate. The depth excavated should be equal to proposed foundation depth.

The plate used is made of mild steel. It may be square (0.3m x 0.3m) or circular (0.3m diameter) with 25mm thickness.

After excavation of pit, at center of excavated pit steel plate sized hole with “Dp” thickness is excavated and arrange the plate in it.

The Thickness of plate Dp is calculated by below formula

Plate Thickness Calculation for Plate Load Test

After arranging the plate in central hole hydraulic jack is arranged on top of plate to apply load. Reaction beam or reaction trusses is provided for the hydraulic jack to take up the reaction. Otherwise a loaded platform is created (using sand bags etc.) on the top of hydraulic jack and provided the reaction.

After that seating load of 7kN/m2 is applied to set the plate and released after some time. Now load is applied with an increment of 20% of safe load.

Dial gauges are arranged at bottom to record the settlement values. At 1min, 5min, 10min, 20min, 40min, and 60min and after that for every one-hour interval the settlement is observed and noted. The observations are made until the total settlement of 25mm has occurred.

Procedure for Plate Load Test

In case of reaction truss loading, a truss is arranged on jack and both sides of truss are anchored to the ground with strong support. Two ends of truss are loaded uniformly, then truss transforms the load into the plate and settlement occurs.

Load is applied with an increment of 2kN at every interval. Settlement is observed at different intervals as said in above method. For clayey soils, the observations are made until the rate of settlement is to be 0.2mm per hour.

Procedure for Plate Load Test

Calculation of Bearing Capacity from Plate Load Test

From the results obtained in plate load test, a logarithmic graph is drawn between loads applied to the corresponding settlement. Load is taken on x-axis and settlement is on y-axis.

From the graph, we can find out the value of ultimate load for the plate, which is equal to the corresponding settlement of 1/5th of plate width.

Calculation of Bearing Capacity from Plate Load Test

The curve breaks at one point, the load corresponding to that break point is considered as ultimate load for plate. From this ultimate load for plate we can determine the value of ultimate bearing capacity and safe bearing capacity of soil for foundation.

Bearing Capacity Calculation for Clayey Soils

Ultimate bearing capacity = ultimate load for plate

I.e. qu(f) = qu(p)

Bearing Capacity Calculation for Sandy Soils

Ultimate bearing capacity = ultimate load for plate x {Width of pit (Bf) / Size of Plate (Bp)}

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

Finally, safe bearing capacity = ultimate bearing capacity / factor of safety

The factor of safety ranges from 2 to 3.

Calculation of Foundation Settlement from Plate Load Test

We can also calculate settlement for given load from plate load test as follows

Foundation Settlement Calculation on Clayey Soils

Settlement of foundation (sf) = sp x Bf/Bp

Foundation Settlement Calculation on Sandy Soils

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

Where Bf and Bp are widths of foundation and plate.

Read More on Various Tests on Soil