Types of Failure in Soil - Ultimate Bearing Capacity of Soil

The soil or the ground when subjected to loads (foundation) more than its capacity, they undergo failure that is observed as certain failure surfaces or bulging of mass of soil.

Fig.1. Load-Settlement Curve for Shear Failure in Soils

By plotting several pressure-settlement curves for different soil types under a particular shallow foundation load, different failure mechanisms were determined. They are:

  1. General Shear Failure
  2. Local Shear Failure
  3. Punching Shear Failure

Shear failure of soil is defined as the failure mode where the soil divides into separate blocks or zones which move along slip surfaces.

1. General Shear Failure of Soil

General shear failure is the most common mode of shear failure in soils. In this failure mechanism, a continuous slip surface will be formed up to the ground level as shown in fig.2. This implies, that the shear resistance is fully developed along the entire failure surface that extends to the ground level.

Fig.2. General Shear Failure Mechanism in Soils

When the load coming from the foundation reaches the ultimate value (Qu), the state of plastic equilibrium is reached initially for the soil around the footing, which gradually spreads downwards and outwards soil that forms above the failure surface forms a heaving, and this will be in the state of plastic equilibrium. Later, the plastic equilibrium develops throughout the soil above the failure surface. This failure is reflected as heave on both sides of the footing. 

The load-settlement curve in a general shear failure mechanism is linear up to a substantial percentage of ultimate load, after which it shows rapid yielding till it reaches the ultimate value.
General shear failure in the soil is catastrophic and it results in tilting of the footing.

As per research and experimental investigations, a foundation on dense sand with a relative density greater than 70%, can undergo sudden failure, when the settlement reaches around 7% of the foundation width. At the failure point, the soil will have a peak resistance. 

General Shear failure is observed in:

  • Dense soil or stiff soil -low compressibility soils
  • Stiff clays
  • Firm grounds, where the failure load is well defined
  • Dense sand or stiff over-consolidated clays

2. Punching Shear Failure

Punching shear failure is the other extreme of general shear failure. This failure mechanism is observed in very loose soil, that the slip surfaces formed are almost vertical, with large vertical displacement. 

Fig.3. Punching Shear Failure Mechanism in Soil

There is no heaving, tilting, or catastrophic failure observed. With the compression from the foundation load, the soil density increases as shown in the curve in fig.3.

Punching shear failure is observed in soils with a relative density of less than 35%, like
  • Weak soil
  • Highly compressible soils
  • Loose sands
  • Partially saturated clays or peats

3. Local Shear Failure in Soils

Local shear failure shows significant compression under the footing that causes partial development of plastic equilibrium.
The failure surface created is not continuous.
Minor heaving is observed at the ground level, without catastrophic failure as shown in fig.4.

Fig.4. Local Shear Failure in Soils

Local shear failures are observed in soils with relative density between 30 to 70% like:
  • Moderately compressible soils
  • Medium dense or compact sands

Bearing Capacity Conditions

In practical conditions, the ground conditions are improved through compaction before constructing the footing. In shallow foundations in granular soils with a relative density greater than 70%, general shear failure is mainly observed. 
From the observations, the ultimate bearing capacity i.e the average contact pressure at the soil footing interface during the shear failure occurs is obtained. Considering factor of safety for the respective foundation, the allowable bearing capacity of soil used for the design of the foundation is given by:
qa = qu/FOS

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