A plate load test is a field test that is used to calculate the bearing capacity of the soil, allowable bearing pressure of soil, and settlement of the foundation.
Table of Contents
Apparatus Of Plate Load Test
- Excavating tools.
- Hydraulic jack.
- Mild steel plate.
- Dial gauges.
- Reaction beam and reaction truss with soil nails.
- Plum Bob.
Test Procedure of Plate Load Test
The pit is excavated over the site of the test with the size of 1.5×1.5 meters and to the depth of the proposed foundations. One requirement is to find the center of the excavated pit and a portion to the size of the plate is eroded to the depth of approx 1-2 centimeters.
Make sure that the foundation area should be 5 times the area of the plate and the seating load of 0.07 Kg/cm² is applied to stop undulations under the plate.
The eroded part is filled with rock dust in order to tackle undulations and that of plate installed is completely horizontal which later is checked by tube level.
Spacers are installed on the top of the hydraulic plate on which the hydraulic jack is installed. The hydraulic jack in order to counter the applied stress which in result leads to the transmission of pressure over the hydraulic plate.
Setting load of 7kN/m² is applied for some time and released. This is followed by the application of safe load with an increment of 30%. In the case of loading through the truss, both free sides are anchored with soil nails with uniformly loading at the ends.
Determination Of Bearing Capacity Of Soil
There are four steps to determine the bearing capacity of the soil.
- Plotting a Load-Settlement Curve
- Finding Ultimate Bearing Capacity
- Calculating Safe Bearing Capacity
- Calculating Settlement of Proposed Footing
1. Load Settlement Curve
After completion of the plate load test, plot a load settlement curve on the arithmetic scale, with applied pressure in kn/m2 on the x-axis and corresponding settlement (in mm) on the y-axis.
Based on the type of soil below the test plate, the load settlement curve can be of 4 different types as shown in the below figure.
Curve A: This kind of curve is obtained in the case of loose to medium cohesionless soil (i.e. 15 < N < 30). This sort of curve shows no clear point of failure.
Curve B: This kind of curve is acquired in the case of cohesive soil. As the load increases, the curve leans toward the settlement curve.
Curve C: This kind of curve is acquired in the case of partially cohesive soil (i.e. C-phi soil). This type of curve does not indicate a clear point of failure.
Curve D: This type of curve is acquired in the case of dense cohesionless soil (i.e. N > 30). This type of curve is common in the case of dense sand or gravel deposits.
2. Finding Ultimate Bearing Capacity
After plotting the load settlement curve, the next step is to recognize the failure point on the load settlement curve.
In the case of curves B & D, the point of failure can be easily recognized by the sudden drop down of the curve. Or draw two tangent lines from the beginning straight portion of the curve and the end straight portion of the curve. The point where these two tangents cross each other is called the point of failure. The pressure corresponding to this point is called the ultimate bearing capacity of the soil.
But in the case of curves A & C, it is difficult to find out a clear point of failure on the load settlement curve. To overcome this issue scientist Abbet introduced another method of drawing the load settlement curve. According to this method, a log-log chart is plotted with settlements are plotted as abscissa against corresponding pressure intensities as ordinate. Such plotting will give two straight lines, the intersection of which will be considered as the failure point. Find the corresponding value of pressure to the failure point. This is considered the ultimate bearing capacity of the soil.
3. Calculating Safe Bearing Capacity
In order to calculate the safe bearing capacity of the soil, divide the ultimate bearing capacity of soil by a suitable factor of safety.
Safe bearing capacity of soil = Ultimate bearing capacity / Factor of safety
The values of the factor of safety generally used are 2, 2.5, or 3 based upon the site condition and the importance of the structure.
4. Calculating Settlement of Proposed Footing
The following formula is given by Terzaghi and Peck is utilized to calculate the settlement of footing for granular soil.
Sf = Sp {[B*(Bp+0.3)] / [Bp*(B+0.3)]}2
For clayey soil, the following equation can be used.
Sf = Sp*(B/Bp)
Where,
Sp = Settlement of plate, millimeter
Sf = Settlement of footing, millimeter
Bp = Width or dia of plate, meter
B = Width of footing, meter
For the calculating settlement of a given size of foundation subjecting to a given load, first, determine the corresponding intensity of loading by dividing the area of footing by the load, i.e.
Intensity of Loading = Load (kN) / Area of footing (m2)
Then from the load settlement curve drawn using plate load test data, calculate the plate settlement corresponding to the calculated intensity of loading and use this value of a settlement in the formula to find out the settlement of footing.