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

|# PLT (Plate Load Test)

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

In this test a square or circular rigid plate of standard dimension (generally 300 mm2 for square or 300 mm dia for circular) is placed at foundation level and load is applied in increments. Then settlement of the plate corresponding to each load increment is recorded for calculating bearing capacity of soil.

To know the detail test procedure click the link given below.

**How to perform Plate Load Test On site?**

# Calculation of Bearing Capacity of Soil

The whole process of calculating bearing capacity of soil using plate load test data consists of the following 4 primary steps.

**Step-1 (Plotting a Load-Settlement Curve)****Step-2 (Finding Ultimate Bearing Capacity)****Step-3 (Calculating Safe Bearing Capacity)****Step-4 (Calculating Settlement of Proposed Footing)**

## 1. Step-1 (Plotting a Load-Settlement Curve)

After performing the plate load test, plot a load settlement curve (see fig-1) on arithmetic scale, with applied **pressure** in kN/m^{2} **in x-axis** and corresponding **settlement** (in mm)** in y-axis**.

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

**Curve A:** This type curve is obtained in case of loose to **medium cohesion less soil** (i.e. **15 < N < 30**). This type of curve shows no clear point of failure.

**Curve B:** This type curve is obtained in case of **cohesive soil**. As the load increases the curve leans toward settlement curve.

**Curve C:** This type curve is obtained in case of **partially cohesive soil (i.e. C-phi soil)**. This type of curve also does not show a clear point of failure.

**Curve D:** This type curve is obtained in case of **dense cohesionless soil (i.e. N > 30)**. This type of curve is common in case of dense sand or gravel deposits.

## 2. Step-2 (Finding Ultimate Bearing Capacity)

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

In case of **curve B & D**, the point of failure can be easily identified by the sudden drop down of the curve. Or draw two **tangent lines** from the beginning straight portion of the curve and 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 **ultimate bearing capacity** of the soil.

But in case of **curve A & C**, it is difficult to find out a clear point of failure on the load settlement curve. To overcome this problem scientist **Abbet** introduced another method of drawing load settlement curve. According to his method, a **log-log chart** is plotted with settlement are plotted as abscissa against corresponding pressure intensities as ordinate. Such a 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 as the **ultimate bearing capacity** of soil.

## 3. Step-3 (Calculating Safe Bearing Capacity)

In order to calculate 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 factor of safety normally used are **2**, **2.5** or **3** depending upon the site condition and importance of the structure.

## 4. Step-4 (Calculating Settlement of Proposed Footing)

The following formula as suggested by **Terzaghi and Peck** is used to calculate the settlement of footing **for granular soil**.

**S _{f} = S_{p} {[B*(B_{p}+0.3)] / [B_{p}*(B+0.3)]}^{2}**

**For clayey soil** the following equation can be used.

**S _{f} = S_{p}*(B/B_{p})**

Where,

**S _{p}** = Settlement of plate, mm

**S _{f}** = Settlemnt of footing, mm

**B _{p}** = Width or dia of plate, m

**B** = Width of footing, m

For **calculating settlement of a given size of foundation subjecting to a given load**, first calculate the corresponding intensity of loading by dividing the area of footing by the load, i.e.

**Intensity of Loading = Load (kN) / Area of footing (m ^{2})**

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

# Example Calculation

A plate load test was conducted using a plate of 0.75m*0.75m size, on a uniform deposit of sand and the following data were 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 soil taking factor of safety as 2.5.

Calculate settlement of a square footing of size 2mx2m at safe bearing capacity

## Answer

__Calculation of Safe Bearing Capacity__

From the load settlement curve (see fig below) the ultimate bearing capacity of soil is found to be 500 kN/m^{2} and corresponding settlement is 12 mm.

So **Safe bearing capacity (q _{safe})** =

**Ultimate bearing capacity (q**

_{ult}) / Factor of safety = 500 / 2.5 = 200 kN/m^{2}__Calculation of Footing Settlement__

Width of footing (B) = 2m

Width of plate (B_{p}) = 750 mm = 0.75m

Plate settlement (S_{p}) = 12mm

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

**S _{f} = S_{p} {[B*(B_{p}+0.3)] / [B_{p}*(B+0.3)]}^{2}**

**S _{f} = 12 {[2*(0.75+0.3)] / [0.75*(2+0.3)]}^{2}**

**S _{f} = 17.78 = 18 mm**

for instant decision of bearing capacity of soil can we use dynamic falling weight deflectometer

can we decide ultimate bearing capacity of soil by dynamic falling weight deflectometer instantly.

Our minimum requirement of Bearing Capacity is 100kN/m2, how to do PLT and how to report on this ?please explain me .Thanks!

I HAVE STRIP FOOTING WITH SHORT DIMENSION 1 M, IN THIS CASE WHAT I WILL CONSIDER B ? IS B ALWAYS THE SHORTEST DIRECTION OF THE FOOTING OR THIS FORMULA FOR SQUARE FOOTINGS ONLY

HI

I HAVE STRIP FOOTING WITH SHORT DIMENSION 1 M, IN THIS CASE WHAT I WILL CONSIDER B ? IS B ALWAYS THE SHORTEST DIRECTION OF THE FOOTING OR THIS FORMULA FOR SQUARE FOOTINGS ONLY

Hi Mr Suryakanta can you send me the references of the figures published (Fig-1 Types of load settlement curves and figure for log-log scale)?? I also need the bibliographic reference, some work published by Abbet which cites in his article. thank you very much

Diego

thank you very much

Why the load is in kg/cm2..?

Why did you only plot it on a normal scale and not a Log-Log Scale as per Abbott’s method?

Very educative and thoughtful.

Hi you mentioned in your entry above in example calculation to calculate actual settlement at safe bearing pressure.

“From the load settlement curve (see fig below) the ultimate bearing capacity of soil is found to be 500 kN/m2 and corresponding settlement is 12 mm”

so when to calculate the settlement at safe bearing pressure of 200kN/m2, do we need to recheck the corresponding plate settlement at 200kN/m2 to use in the settlement formula or just use the settlement of 500kN/m2 (12mm) just like what you did in your example?

Would appreciate if you could answer me. Thank you

No need to check for settlement at 200kN/m2

Hi,

Mr.Suryakanta however, your question is

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

Calculate settlement of a square footing of size 2mx2m at safe bearing capacity”

So, we already know the safe bearing capacity is 200kN/m2 from the ultimate bearing capacity of 500kN/m2.

And if we directly use the 12mm for calculating the settlement, it will represent the settlement at ultimate bearing capacity (from graph) and not safe bearing capacity. So if we recheck the graph, the approximate settlement at 200 kN/m2 is 4mm, then we calculate the actual footing 2mx2m settlement using this plate settlement of 4mm instead of 4mm.

Can you kindly explain to me. This make me a bit confuse. Is this the same approach for clay soil as well.Thank you

Hi Mr Suryakanta,

Hopefully you can help me here to clear the my confusion. why we dont need to check settlement at 200kN/m2 as you said above?

your question is

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

Calculate settlement of a square footing of size 2mx2m at safe bearing capacity”

So, we already know the safe bearing capacity is 200kN/m2 from the ultimate bearing capacity of 500kN/m2.

And if we directly use the 12mm for calculating the settlement, it will represent the settlement at ultimate bearing capacity (from graph) and not safe bearing capacity.

So if we recheck the graph, the approximate settlement at 200 kN/m2 is 4mm, then we calculate the actual footing 2mx2m settlement using this plate settlement of 4mm instead of 12mm.

Can you kindly explain to me. This make me a bit confuse. Is this the same approach for clay soil as well.Thank you

Hi, please outside PLT what other test method can be adopted for determining the load bearing capacity of soil for a building.

thanks

Dear sir, yesterday I performed plate load test for building I used showel of 21ton weight. Consultant did objection and force me to minimum 50 ton load. Kindly tell me the min load and maximum load that we can apply during this test? Thank you very much

The plate bearing test is a simple test and quite suitable for buildings. In case of a road, we are dealing with dynamic loads, which are better simulated with the Falling Weight Deflectometer. I have problem, whereby the road is heavily trafficked, but short. The FWD needs to be mobilised from 500 km, which is not economical for a 1 km road. In your opinion, what is the correlation between the Ultimate Bearing Capacity using the Plate Bearing Test and ” Equivalent half space long term stiffness” of composite foundation as stipulated in the British Road Design Manual?Eng Peter Nduati

Thank you very much for your informaton

hi..we have did plate load test using square plate…but our consultant is asking it to be done by circular plate..so is there any supporting document is available to justify that both will give same results…

Dear Vikram, plz refer

clause 4.4 of IS: 1888. According to this clause; except in case of road problems and circular footings, square plates may be adopted.I think this is a rational method of determining the bearing capacity of the ground. I also believe that making alot of plate bearing tests on the soil type and making correlations of the values ( the applied pressure and settlements values) using log-log graph could give a more approximate values of ultimate and safe bearing capacities formulae respectively.Thanks.

Can I ask questions about any subject on civil engg?