HOW TO CALCULATE SAFE BEARING CAPACITY OF SOIL ON SITE?

How to calculate safe bearing capacity of soil on site

This simple and quick field test will give you a rough idea of bearing capacity of soil.

Procedure

  1. Excavate a pit of required depth. (preferably equal to the depth of foundation)
  2. Take a solid ball or square cube of known weight and dimension.
  3. Drop the ball or square cube several times, from a known height on to the bottom surface of excavated pit.
  4. Calculate the average depth of impression made several times on the bottom surface of the excavated pit. Let “d” is the average depth of impression.

Calculation

Calculate the ultimate resistance of soil ( R ) using the formula given below.

R = (w * h) / d

Where,

R = Ultimate resistance of soil (in kg)

d = Average depth of impression (in cm)

w = Weight of the solid ball or square cube (in kg)

h = Height of fall of solid ball or cube (in cm)

If “A” is the cross-sectional area of the solid steel ball or cube, then resistance of soil per unit area is calculated using following formula.

Resistance of soil per unit area (in kg/cm2) = R / A

Safe bearing capacity (in kg/cm2) = R / (A * F.O.S)

Where,

F.O.S = Factor of safety

Notes to Remember

  1. F.O.S varies from 2 to 3 depending upon the type of structure and site condition.
  2. In order to get reliable test result, perform this test on different types of soil and then use your judgment to reach at any conclusion.
  3. Instead of using solid steel ball or square cube, we can also use CBR test plunger, or Cone (used for cone penetration test)

Presumptive Bearing Capacity

The table given below shows the presumptive bearing capacity values for different types of soils. This table will guide you to reach at any conclusion after conducting the test.

Type of Soil / Rock Safe / Allowable Bearing Capacity (kg/cm2)
Rock 32.40
Soft rock 4.40
Coarse sand 4.40
Medium sand 2.45
Fine sand 4.40
Soft shell / Stiff clay 1.00
Soft clay 1.00
Very soft clay 0.50

Notes to Remember

  1. For non-cohesive soils the values should be reduced by 50% if the water table is above or near base of footing.
  2. These values should be used only for preliminary design. The actual bearing capacity of soil should be calculated as per the standard codes.

Also Read: How to Calculate Bearing Capacity of Soil On Site Using N-Value

Also Read: How to Calculate Bearing Capacity of Soil From Plate Load Test

40 thoughts on “HOW TO CALCULATE SAFE BEARING CAPACITY OF SOIL ON SITE?”

  1. Can you help me to understand this:

    In Miami, the bedrock is limestone, but it’s highly porous. According to a chart on another table, (limestone in general) has a safe load capacity of 16.5 kg/cm2. So if I dig down to the bedrock to build a skyscraper, assuming a footprint of about 50m x 60m, that multiplies out to a maximum weight of the building to be about 495m kg. That should be about a 50-60 stories tall, else the bedrock won’t support the weight.

    Do I understand this correctly, and is this the reason there are no super-tall skyscrapers in Miami?

    Is there a way to amend the footing to increase the load capacity?

    Reply
  2. a p brick pier 450ms exerts a force of 200k/ns including the mass of the pier. calculate the pressure per metre square on brick pier and the minimum area foundation required if the oil has a bearing capacity of 150k/N per square metre

    Reply
  3. Good day Sir. I need your help in solving the bearing capacity of soil given LL,PI,moisture content, number of blows,N, from a geotech exploration because it is not stated in the result. Please. Thank you so much. I have a full result if you wanted to see. God bless.

    Reply
  4. What’s unclear is if you drop your object several times in the same spot or multiple times in one spot. One hint that it might be several separate drops in separate places is that you say, “Calculate the average depth of impression made several times on the bottom surface.” But is that the average of several separate impressions, or the average depth of one impression, if it is not perfectly level?

    Seems like dropping a cube would be less accurate, if you can’t ensure that it would land perfectly flat each time. A corner would go in deeper than the whole surface.

    Reply
  5. Dear Suryakanta how are your .?
    i am build a residential Apartment , The plot size is 30’x30′ so how i calculate the bearing capacity .Excavation depth 12 foot

    Reply
  6. I am James, when I do this I found that I drop weight 2 times and 3 times are different depth of impact, how to use your formular to calculate the R?

    Reply
  7. Sir, I have a question related to IS 1888 for safe bearing capacity and settlement when the size of the plate is maximum 750 mm?

    Reply
  8. My name is kasiraj and my profession is chartered engineer , I want to know The design of footing ,gradebeam and column by using excell spread sheet,can I get that above software

    Reply

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