Chapter 3.4 Underpinning

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    UNDERPINNING

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    Underpinning is the process of strengthening and stabilizing the

    foundation of an existing building. It may be required for any of a

    number of reasons:i) The existing foundations may never have been adequate to carry loads,

    leading to excessive settlement of the building over time.

    ii) A change in building use or additions to the building may overload the

    existing foundations.

    iii)New construction near a building may disturb the soil around itsfoundations or required that its foundations be carried deeper.

    iv)The construction of nearby structures necessitates the excavation of

    soil supporting existing foundations

    v) It is more economical, due to land price or otherwise, to work on the

    present structure's foundation than to build a new one.

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    Three different alternatives are available when foundation capacity needs

    to be increased:

    i) The foundations may be enlarged by extending the foundation indepth or in breadth so it either rests on more supportive soil stratum

    or distributes its load across a greater area

    ii) New, deep foundations can be inserted under shallow ones to carry

    the load to a deeper, stronger stratum of soil such as using

    micropiles and jet grouting

    iii) The soil itself can be strengthened by the introduction of a grout

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    Underpinning both piles

    outside

    - Cantilever Beam

    Underpinning with one

    pile inside and on outside

    - Bridging Beam

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    Piles cut through existing foundations

    Conventional Underpinning

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    Compaction Grouting

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    Two methods of supporting a building while carrying out

    underpinning work beneath its foundation

    1) Trenches are dug beneath the existing foundation at intervals, leaving

    the majority of the foundation supported by the soil. When theportions of the new foundations have been completed in the trenches,

    another set of trenches is dug between them and the remainder of the

    foundations completed

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    ELEVATION SECTION

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    2) The foundations of an entire wall can be exposed at once by

    needling, in which the wall is supported temporarily on needle beams

    threaded through holes cut in the wall. After underpinning has been

    accomplished, the jacks and needle beams are removed ant the

    trench is backfilled

    ELEVATION SECTION

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    THREE TYPES OF UNDERPINNING

    1) A new foundation wall and footing are constructed beneath the existing

    foundation

    ELEVATION SECTION

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    2) New piles or caissons are constructed on either side of the

    existing foundation

    ELEVATION SECTION

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    3) Concrete mini piles are cast into holes drilled diagonally

    through the existing foundation. Mini piles do not generally

    required excavation or temporary support of the building.

    SECTION

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    UNDERPINNING WITH PIPE PILES

    Pipe piles are used to gain vertical support in poor soils. Pilings are driven

    through the weak soil to reach competent soil for proper support. Piles are

    often used in areas with limited access for big equipment.

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    UNDERPINNING WITH CAISSONS

    Caissons are steel reinforced columns of concrete placed

    into pre drilled holes to gain vertical and horizontal

    support.

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    UNDERPINNING WITH GRADE BEAMS

    Underpinning using a grade beam is intended to support from

    below and significantly stiffen the weak area.

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    Foundation UnderpinningWhen a home's foundation has structural damage, it needs to be fixed to

    ensure no further damage and the possible loss of the home. A foundation

    Underpinning System is used to permanently correct this problem. The

    foundation is stabilized by using galvanized pipe that is hydraulically driven

    into the hard bedrock beneath the house. These pipes, known as piers are

    underneath and attached the foundation, correcting the damage. This methodis proven to be very effective and economical compared to the expense of

    replacing the existing foundation.

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    The A.B. Chance Helical pier is perfectly suited for area soils and the extreme

    depths they must reach to find load bearing strata.

    The Helix of the helical pier (or the load bearing plate) turns and twists into the

    soil like a screw driver by the hydraulics with precise measurement of the

    hydraulic pressure which then correlates to the load that the pier can support.

    Once the helical Pier is driven to a sufficient depth pressure or load bearingstrata is reached the weight of the structure is then transferred to the stable

    soil through the helix (load plate).

    The Foundation footing is notched and patented bracket is bolted to the

    foundation, this bracket secures to the helical pier transferring the load.

    The structure can then be raised to its original level position by hydraulic force.

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    Concrete Underpinning

    Conventional underpinning is the process of making small excavations under

    existing footings then filling the excavated area with concrete. Underpinning

    typically occurs:i) When you are protecting the foundation from an adjacent excavation.

    ii) When the existing footing is deteriorated and failing.

    ii) When the bearing (soil) beneath the footing has eroded or sunk.

    If it is necessary to reinforce or support the existing building during construction, no

    problem, a support system using steel, piers, or tieback anchors.

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    Push Pier Installation Steps:

    Step 1: Footing is exposed and prepared for the bracket.

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    Step 2: Foundation Bracket is secured to the footing.

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    Step 3: Steel pier sections are hydraulically driven through the bracket to

    competent soil or bedrock

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    Step 4: The weight of the home is transferred through the piers to

    load bearing strata. Home is lifted back to

    level if possible.

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    Helical Pier Installation Steps:

    Step 1: Footing is exposed and prepared for the bracket.

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    Step 2: Round-shaft helical piers are mechanically advanced into the soil

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    Step 3: Foundation Bracket is secured to the footing

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    Step 4: The weight of the home is transferred through the piers

    to load bearing strata. Home is lifted back to

    level if possible.