Foundation underpinning typically refers to transferring the foundation load of an existing structure deeper into the ground to a depth below the elevation of adjacent new construction.
Foundation underpinning techniques include:
- Micropiles (or other deep foundation element)
- Soil nails, tiebacks and shotcrete (or other anchor and earth retention systems)
- Hand-dug underpinning piers
- Needle beams (used in conjunction with micropiles)
- Sister walls (used in conjunction with micropiles)
The underpinning technique(s) employed will depend upon access, clearance between existing structure and planned structure, overhead clearance, existing foundation estimated load, construction material type and condition, soil conditions anticipated, and the maximum allowable movement of the structure to be underpinned.
Foundation Underpinning Techniques
Micropiles are used to extend the load of an existing foundation below the elevation of planned construction. Micropiles are drilled adjacent to the existing foundation to a depth below the new construction and are attached to the existing foundation. If the existing foundation is a strip footing or spread footing that is wide enough, micropiles can be drilled through core holes that penetrate the existing foundation. Micropiles can be ‘pre-compressed’ once attached to the foundation. Pre-compression can significantly reduce the amount of anticipated settlement by mobilizing inelastic strain due to load transfer of the system and to mobilize elastic strain of the pile and is performed prior to excavating below the footing.
Soil nails and shotcrete (or other anchor and earth retention systems) retain the soil beneath the existing foundation once the micropiles (or other deep foundation system) are in place.
Hand-dug underpinning piers are as large as 5 ft long by 3 ft wide. Hand dug pits beneath the foundation are dug and lined with wood lagging to temporarily retain the soil. Once the underpinning pit is dug below the planned structure concrete is then poured into the pit. Load transfer between the existing foundation and underpinning pier is achieved by placing dry packing between the bottom of existing foundation and the top of the concrete pier once poured and cured.
Needle beams are used when eccentric loads cannot be resolved with micropile only on the exterior foundation wall. Needle beams are structural steel elements oriented perpendicular to and placed beneath the existing foundation so that the ends of the needle beam extend beyond the interior and exterior wall face The needle beam straddles and is attached to the micropiles at each end. Small access drilling equipment is used to drill micropiles on the interior of the existing structure.
Sister walls are foundation walls that are constructed with rebar doweled into the existing foundation wall and covered with shotcrete. Sister walls are installed as a means to augment masonry or stone walls that lack the shear and bending strength of reinforced concrete walls. Micropiles are then attached to the sister wall.
A True Craft and Expertise by Underpinning Contractors
GeoCraft has underpinned structures dating from the 19th century built on cobble stone foundations and century old masonry foundations. Underpinning structures of stone and masonry footings require strict adherence to construction sequencing and attentiveness to ground conditions and ground behavior. GeoCraft installs movement monitoring systems using instruments such as tiltmeters, electronic water levels, inclinometers, load cells and/or strain gauges. Movement is monitored and recorded via dataloggers at a set frequency 24 hrs/day, 7 days a week. All project parties (Owner, engineer, etc.) can remotely access dataloggers and download data.
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Our team has the experience, expertise and technical knowledge to successfully and safely construct foundation underpinning systems on all foundation types and in the most challenging ground conditions. We are here to help answer any questions you may have. Engage our pre-construction services today to mitigate risk, improve constructability and safety and maximize economy. Let’s discuss your needs, give us a call today!