Pit Underpinning | Step by Step Procedure

The pit method of underpinning is a construction technique used for the repair of foundations. The method is an extended type of mass concreting method of underpinning, where the former is used for repairing concrete at a deeper soil depth.

This article explains in detail the basic methodology and the construction steps of the pit method of underpinning.

What is the Pit Method of Underpinning?

Pit method involves creating vertical pits at intervals under the existing footing and then pouring concrete, often reinforced, to increase support depth. This technique is ideal for buildings with narrow foundations or where spot improvements are needed.

During foundation repair, some buildings may need a temporary foundation before starting the repair. So, we need to provide an external temporary support and then perform the underpinning.

Let's discuss the construction steps involved in the pit method of underpinning with and without a temporary foundation support.

Pit Method of Underpinning – Step-by-Step Construction (Without Temporary Support)

1. Site Preparation and Marking

• Identify the wall sections to be underpinned.
• Mark out small rectangular segments (called "bays" or "pins") typically 1 to 1.2 meters in length along the wall.
• Ensure alternate pit sequencing (not all sections at once) for safety and stability.

2. Excavation of First Pit (Alternate Bays)

• Start excavating one bay (leaving the adjacent bays untouched).
• Dig a vertical pit beneath the foundation to the required depth (until firm strata is reached).
• The wall remains intact as only a small portion is disturbed, so temporary supports are not usually needed for shallow foundations in good soil.

Pits are made under the foundation at intervals for mass concrete

3. Trimming and Cleaning

• Clean the sides and bottom of the pit.
• Undermine the foundation carefully by removing soil below the old footing in the bay area.

4. Casting the New Foundation

• Lay PCC (Plain Cement Concrete) if required as a leveling course.
• Construct the new concrete footing or pier to the desired depth.
• Ensure proper compaction and curing of the concrete.

5. Curing and Load Transfer

• Allow the new concrete to cure adequately (usually 3 to 7 days, depending on strength requirement).
• The wall load is gradually transferred to the new foundation naturally, without jacking.

6. Filling and Finishing

• Backfill the pit with suitable material and compact it.
• Seal joints between new and old masonry if needed.

7. Repeat for Adjacent Bays

• After the first set of bays has fully cured and settled, start underpinning the alternate untouched bays in the same manner.
• This ensures uniform load transfer and maintains wall stability throughout the process.

When Can Temporary Supports Be Avoided?

• The wall is not heavily loaded
• Soil is firm and cohesive
• Foundation is shallow (not more than 1.2 m)
• Work is done in small alternate sections

Pit Method of Underpinning – Step-by-Step Construction Using Needle Beam Temporary Support

1. Site Assessment and Planning

• Conduct structural and soil assessment to confirm the need for temporary support.
• Identify foundation depth, wall load, and soil type.
• Prepare a support plan including needle beams, props, or shoring.

 

2. Marking and Division

• Divide the wall into small bays (typically 1 to 1.2 meters wide).
• Follow the alternate bay sequence to maintain wall balance during the process.
• Mark the insertion points for needle beams.

 

3. Installation of Temporary Supports

• Drill holes through the wall (just above the foundation level) at marked bay sections.
• Insert needle beams (usually steel I-beams or channels) through these holes.
• Support each needle beam with adjustable props (acrow props, screw jacks) at both ends.
• Tighten props gradually to transfer part of the wall load to the temporary system.

 

Pit Method of Underpinning – Step-by-Step Construction Using Needle Beam Temporary Support

4. Excavation of First Pit (Below Needle Beam)

• Begin excavation of the first set of alternate pits directly beneath the needle beams.
• Excavate carefully to the desired depth, usually up to hard bearing strata.
• The wall is now temporarily supported from above by the needle beam and props, allowing safe excavation.

 

5. Preparation and Trimming

• Clean and level the bottom of the pit.
• Undermine the old footing slightly within the bay to make room for new concrete.

 

6. Casting the New Concrete Foundation

• If needed, lay a PCC leveling layer.
• Place and compact the mass concrete to form the new underpin footing or block.
• The concrete is brought up just below the old foundation base (not touching it yet).
• Allow concrete to set and cure adequately (3 to 7 days).

 

7. Load Transfer ("Pinning Up")

• After curing, fill the gap between the existing foundation and the new concrete block using:
• Dry pack mortar or Non-shrink grout
• This step transfers the load gradually from the temporary supports back to the new underpin.

 

8. Remove Temporary Supports

• After the load is fully transferred, gradually release the props and remove needle beams.
• Fill the holes in the wall where beams were inserted with suitable mortar or concrete.

 

9. Backfilling and Finishing

• Backfill around the new underpin block with compacted soil or granular material.
• Restore surface finishes or floors if disturbed during the process.

 

10. Repeat for Remaining Bays

• Proceed to excavate and underpin the next set of alternate bays following the same procedure.
• Ensure each completed bay is fully cured and stable before moving on.

 
When Temporary Supports Are Essential

• Load-bearing walls of multi-storey buildings
• Weak, loose, or water-logged soils
• Deep foundations or footings below 1.2 meters
• Sites near vibrating equipment, roads, or adjacent structures
• Buildings showing signs of distress (cracks, settlements)

FAQ

 1. What is the pit method of underpinning?

The pit method (also known as mass concrete underpinning) is a traditional technique that strengthens an existing foundation by excavating small sections ("pits") beneath it (typically 1.2–1.5 m long), then filling each pit with concrete to reach a stable soil layer. Needles support the load during curing.

2. What are its main advantages?

Simple and cost-effective due to minimal equipment needed.
Ideal for shallow foundations and smaller structural loads.
Often allows the building to remain occupied during underpinning

  

3. What are the limitations?

Generally suitable only for shallow depths (up to around 6 m or 50 ft).
Labour-intensive and time-consuming excavation.
Less efficient if access is limited to one side of the foundation

 

4. When is it the best choice?

When existing foundation soil is weak but there is stable soil at shallow depths.
When the structure can be supported in stages.
When budget and simplicity are priorities for modest foundation repairs 

5. How does it differ from cantilever needle beam underpinning?

Cantilever method is a variation where underpinning is performed from one side only, using a strong interior column and needle beams. It's faster for heavy loads but requires single-side access and is less economical for deeper foundations.

6. What safety measures are critical?

• Work in alternate bays to preserve structural stability.
• Begin in the middle of walls and move outward.
• Provide proper shoring or timbering during excavation.
• Only remove needles once the concrete is fully cured.
• Clean and fill gaps between old and new foundation thoroughly

  

7. Can it handle deep or heavy-load foundations?

No—this method is not suitable for deep underpinning or for substantial loads on unstable or waterlogged soil. In such cases, consider alternative techniques like mini-piles or pile underpinning 

8. What are common pitfalls to avoid?

• Excavating too much in one go or skipping support stages.
• Failing to properly cure concrete before load transfer.
• Not sealing beneath existing footings, risking load bypass.
• Inadequate shoring leading to collapse or uneven settlement.

9. Is this safe for occupied buildings?

Yes—if executed correctly. Proper sequencing, shoring and full curing ensure that the existing structure stays stable throughout underpinning .