How Underpinning Repair Faulty Foundation?

Underpinning is a repair technique that is used to increase the capacity of an existing foundation or correct a damaged foundation. Mostly, the principle of underpinning involves increasing the depth of foundation to increase the load carrying capacity.

Underpinning Procedure- Mass Concreting Underpinning
Underpinning Procedure- Mass Concreting Underpinning; Image Courtesy: Alleure Associates

Underpinning is an option selected when we wish to increase the number of stories of a building or we need to repair a building settlement or building footing damage. 

Building Symptoms that Require Underpinning

Serious cracks observed in the building is one sign to understand the need for foundation repair. Generally the cracks more than 1/4th inch is a sign of faulty foundation. 

Mostly they are diagonal cracks. Cracked or buckled walls, cracked concrete floors, heaved foundation etc are also a symptom of foundation failure.

How is Underpinning Performed?

General underpinning process involves extending the foundation either in depth or breadth so that the foundation lays over a more supportive soil stratum and distributes load into a more area.

There are different methods of underpinning namely:
  1. Pile and Beam Method
  2. Pile Raft Method
  3. Mass Pour Method
  4. Screw Piles and Brackets

Underpinning Tips

Underpinning procedures are conducted with strict guidance from a foundation or structural engineer for better results. Mentioned below are few tips to perform underpinning procedure efficiently:

  1. Underpinning procedure is conducted on load bearing walls
  2. Underpinning process must start from the corners and worked inward direction.
  3. Underpinning must not be performed below non-load bearing walls.
  4. Start the procedure below a strip footing, with minimum of length x width x depth as 3 x 2 x 2 feet dimension.
  5. After excavating the space around the foundation that is to be repaired, add the concrete to the cavity. The mix ratio used is 1: 3: 6.
  6. Employ formwork at the edges.
  7. The placed concrete must be allowed to set for 2 days.
  8. Check whether the cavity is completely filled by using a rod bar
  9. Provide proper curing before loading the structure.
  10. After gaining sufficient strength, the projecting footing must be removed.
  11. Back fill and compact.