What is Differential Settlement?

The stability of any building structure is based on the stability of the foundation underneath the structure. Any variation in the strength or bearing capacity of the soil over which the foundation is constructed can result in sinking of the whole structure. This sinking is called a settlement of the structure.

The settlement occurred can be uniform throughout the plan of the building. This uniform process is called as a uniform settlement.This settlement is an expected process during the service life of any building structure. This can occur after 2 to 3 years from the completion of the building. Uniform settlement does not bring any severe issue to the building.

If the settlement caused is non-uniform in nature, we call the settlement as the differential settlement. And this settlement is an undesirable process for a building which will require study, analysis, and rehabilitation.

Fig.1.Differential Settlement of the Building 

Define Differential Settlement of Structures? 

The difference of elevation/settlement of two adjacent columns in a structure will give the value of the differential settlement. The difference of elevation across the boundary of the structure can also give a value for differential settlement if there exists any non-uniformity.

Suppose, we have a plan to add a structure to an existing one. The differential settlement here can

 be calculated as

 = Settlement after addition of structure - Settlement of the existing structure

The accepted levels of the settlement are:

• For Institutional or commercial building = 1inch
• Industrial Buildings = 1.5inches
• Ware Houses = 2incg
• Machine Foundations < 0.002 inch 

What Causes Differential Settlement?

The main cause of differential settlement is the expansion and contraction of the soil beneath the foundation. The main reasons for this movement are:

• Frost Actions
• Vibrations from Nearby Construction work
• Improper compaction of the soil
• Floods
• Root penetration of the matured tree
• Hydraulic pressure exerted from groundwater or broken water lines

Fig.2.Causes of Differential Settlement

Note: A well-known example of differential settlement is the Lean Tower of Pisa. This leaning was due to the insufficient bearing capacity of the soil below the structure. 

Signs of Differential Settlement

The following signs observed in the building reflects differential settlement:

1. Leaking of the roof and the basement
2. Cracking of bricks or the walls
3. Drywall Separation
4. Windows and Door Sticking
5. Leaning of Garage
6. Wall bulging or leaning
7. Tilting of Chimney
8. Cracks in steps - masonry mortar joints

Prevention of Differential Settlement

Following preventive measures will help in avoiding the chances of differential settlement:

1. Careful Inspection of the site prior to the construction process
2. Check for the closeness of the site to earthquake-prone faults
3. Its not necessary that the differential settlement will leave behind a symptom. So its necessary to evaluate the site with utmost accuracy and care.
4. If the site is made of clayey or expansive or contracting soils, a resistance or helical pier system will be the best choice.