How do I underpin a cement wall foundation?

Written by allen douglas
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How do I underpin a cement wall foundation?
Underpinning a cement wall foundation can repair differential settlement. (Concrete mixer image by zalisa from Fotolia.com)

One method of repairing differential settlement of a cement wall foundation is underpinning. Although expensive, underpinning provides a new, permanent foundation for a structure. A careful evaluation by an engineer experienced in underpinning methods is recommended before spending the money to underpin a cement wall foundation. Underpinning has been used on projects in the past where the underlying soils were near primary consolidation. Consolidation testing, in these cases, would have determined that underpinning was unnecessary.

Skill level:
Easy

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Things you need

  • Hammer
  • Utility chisel
  • Reciprocating saw with concrete blade
  • Needle beam
  • Solid concrete blocks (4)
  • Adjustable jacks (3)
  • Shovel
  • Deep foundation (screw anchor or micropile), if needed
  • Backhoe with screw anchor or micropile installation attachment (if needed)
  • Steel plate, 1 cm (3/8 inch) thick x 1.2 m x 15 cm (4 feet x 6 inches)
  • Masonry wedges
  • Concrete and concrete mixer

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Instructions

  1. 1

    Shore a small section of wall using a needle beam. Cut a hole through the lower section of the cement wall. Use a hammer and chisel to start the hole in the wall. Cut the hole large enough for the needle beam using a reciprocating saw. Insert a needle beam horizontally through the wall. Support both beam ends with temporary footings and jacks.

  2. 2

    Excavate the soil under the wall section supported by the needle beam. Limit the depth of the excavation to the size of the base of the new foundation. Do not risk undermining the two jacks supporting the needle beam.

  3. 3

    Install a new foundation. Base the type of new permanent foundation on known geotechnical data. Select a new foundation type that will support the structure at a depth where the soils have adequate bearing capacity. Deepened shallow foundations, micropiles or screw-type anchors are all good foundation options for installation in a compact work area.

  4. 4

    Prestress the new foundation. Place a jack between the new foundation and a flat steel plate under the bottom of the cement wall, and slowly transfer the wall load to the new foundation. Monitor the wall section for any distress. Prestressing the new foundation will help avoid cracking the foundation.

  5. 5

    Load the new foundation for at least 24 hours. If the foundation settles, shifting the load back to the needle beam, apply more load to the foundation using the jack. Repeat this process until there is no movement over a 24-hour period.

  6. 6

    Place masonry wedges between the new foundation and the existing wall, transferring the load of the cement wall away from the jack. Remove the jack.

  7. 7

    Fill the space between the new foundation and the cement wall, around the masonry wedges, with concrete.

  8. 8

    Repeat the entire process in an adjacent wall section that requires underpinning. Continue this operation until new foundations are provided for all the areas required under the existing cement wall foundation.

Tips and warnings

  • Allow concrete strength to develop before moving the work to an adjacent section of wall. Normal concrete requires 28 days to gain full strength. Fast-setting concrete will reach full strength in as little as seven days.

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