HOW TO CALCULATE BEARING CAPACITY OF SOIL FROM PLATE LOAD TEST?

Plate load test is a field test commonly adopted to determine the bearing capacity and settlement of soil under a given loading condition.

In this test a square or circular rigid plate of standard dimension (typically 300 mm2 for square or 300 mm diameter for circular) is placed at the foundation level and the load is applied in increments. Then the settling of the plate corresponding to each load increase is recorded to calculate the bearing capacity of the soil.


Click on the link given below to know the test procedure in detail.

How to do plate load test on site?

The entire process of calculating the bearing capacity of a soil using plate load test data involves the following 4 primary steps.

  • Step-1 (Plotting the load-disposal curve)
  • Step-2 (Finding Ultimate Bearing Capacity)
  • Step-3 (Calculation of Safe Bearing Capacity)
  • Step-4 (Calculation of Settlement of Proposed Aadhar)

1. Step-1 (Plotting the load-settlement curve)

After performing the plate load test, plot the arithmetic scale by applying a load settlement curve (see fig-1) Pressure kN/m. In2 in x-axis and compatible a settlement (in mm) in y-axis,

Depending on the type of soil under the test plate, the load settlement curve can be of 4 different types as shown below.

Figure-1 Types of load handling curves
Figure-1 Types of load handling curves

Curve A: This type of curve is loose to . is received in the case of medium cohesion low soil (In other words 15 < N < 30) This type of curve shows no obvious point of failure.

Curve B: This type of curve is obtained in the case of bonding soil, As the load increases, the curve tilts towards the settlement curve.

Curve C: This type of curve is obtained in the case of Partially Adhesive Soil (ie Si-Fi Soil), This type of curve also does not show a clear point of failure.

Curve D: This type of curve is obtained in the case of Soil with dense consistency (ie N > 30), This type of curve is common in the case of dense sand or gravel deposits.

2. Step-2 (finding the ultimate bearing capacity)

After creating the load settlement curve, the next step is to identify the failure point on the load settlement curve.

In the case of Curves B and D, the point of failure can be easily identified by a sudden downturn of the curve. or draw two tangent lines The straight part of the curve from the beginning and the straight part of the curve ends. The point where these two tangents intersect each other is called point of failure, The pressure corresponding to this point is called final bearing capacity of soil.

but in the case of curve a and cIt is difficult to locate the obvious point of failure on the load settlement curve. Scientists to solve this problem abate Introduced another way of drawing the load disposal curve. According to his method, a log-log chart Plotted with settlement, plotted as abscissa against the respective pressure intensity as coordinate. Such plotting would give two straight lines, the intersection of which would be considered the failure point. Find the corresponding value of pressure at the failure point. it is considered final bearing capacity Of clay.

log-log scale
log-log scale

3. Step-3 (Calculation of safe bearing capacity)

To calculate the safe bearing capacity of the soil, divide the final bearing capacity of the soil by the appropriate factor of safety.

Safe bearing capacity of soil = final bearing capacity / factor of safety

The values ​​of the factor of safety commonly used are 2, 2.5 or 3 Depending on the site condition and the importance of the structure.

4. Step-4 (Calculation of settlement of proposed footing)

The following formula as suggested by Terzaghi and Peck To calculate the settlement of footing is used for granular soil,

sF = sP ,[B*(Bp+0.3)] , [Bp*(B+0.3)],2

for clay The following equation can be used.

sF = sP*(b/bP,

Where,

sP = settling plate, mm

sF = footing settling, mm

bP = plate width or diameter, m

b = footing width, m

For Calculation of settlement of a given size of foundation under a given load, first calculate the corresponding intensity of loading by dividing the area of ​​the footing by the load, i.e.

Loading Intensity = Weight (kN) / Base Area (m.)2,

Then from the load settlement curve drawn using PLT data, find the plate settlement corresponding to the calculated intensity of loading and use this value of settlement in the formula to calculate the settlement of the footing.

A plate load test was carried out using a plate of size 0.75m*0.75m on a uniform deposit of sand and the following data was obtained:

Pressure (kN/m)2, Settlement (mm)
0 0
50 1.5
100 2
200 4
300 7.5
400 12.5
500 20
600 40.6

Calculate the safe bearing capacity of the soil taking factor of protection as 2.5.

Calculate square foot settlement of 2mx2m size at safe bearing capacity

Answer

Calculation of safe bearing capacity

From the load settlement curve (see figure below) the ultimate bearing capacity of the soil is 500 kN/m. is found2 And the corresponding settlement is 12mm.

load handling curve
load handling curve

therefore Safe Bearing Capacity (QSafe, , Final Bearing Capacity (QCorrect) / factor of protection = 500 / 2.5 = 200 kN/m2

Calculation of footing settlement

Width of base (B) = 2m

Width of plate (BP) = 750mm = 0.75m


Plate Settlement (S)P) = 12 mm


So the settlement of the footing at the safe load can be calculated using the following formula:

sF = sP ,[B*(Bp+0.3)] , [Bp*(B+0.3)],2

sF = 12 {[2*(0.75+0.3)] , [0.75*(2+0.3)],2

sF = 17.78 = 18 mm

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.