CROSS Safety Report
Underpinning using screw piles
Overview
A reporter is concerned that inclined screw piles used for underpinning may be subject to significant bending moments that compromise their capacity.
Key Learning Outcomes
For civil and structural engineers:
- Avoid, if possible, the use of inclined screw piles where loads are eccentric to the pile, and generate bending moments and shear forces within the pile
- Where inclined screw piles are considered, ensure there is a geotechnical report that covers this situation, and that the sub-soil conditions and soil-structure interaction are fully understood
- Ensure that the response of screw pile foundations to seasonal changes in water level is similar to that of other foundations in the same structure
For contractors:
- Review the stability provisions advised by the designer, and contact the designer as a matter of urgency if there are any concerns or ambiguities
- Adhere to all hold points for inspection, monitoring requirements, or limitations specified by the designer
- Keep meticulous records of the construction works and all communications
- Stop work immediately if untoward movement or new cracking is detected
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The reporter was involved in a review of works involving the stabilisation of a brickwork wall forming the gable-end of a terraced house. The wall had settled, rotated outwards and presented with several major cracks. The review revealed the wall had been underpinned using a combination of mass concrete and inclined screw piles.
The reporter's concern is that the use of inclined screw piles resulted in significant eccentricity of loading to the piles, requiring them to resist a combination of axial force and bending moment. The reporter notes the situation was further compounded by the geotechnical reports for the site, which identified the top 1400mm of soil as poor quality, low-strength fill which was highly vulnerable to further loss of strength once it became wet. There were no pile load test results confirming the screw piles would be capable of resisting the loads in these ground conditions and the reporter concludes the underpinning was unlikely to prevent further settlement, which could lead to possible collapse of the wall.
The reporter previously encountered similar uses of screw piles in underpinning works, which required the screw piles to resist substantial bending moments induced due to eccentricities between the screw pile and the centre-line of the underpinned wall. While the reporter does not have an issue using screw piles in lightly loaded systems for resisting vertical tension and compression loads, they do have serious concerns with their effectiveness when required to resist substantial bending moments or lateral forces.
It is the reporter's opinion that, although underpinning with inclined screw piles may appear to be an easier alternative to traditional underpinning with mass concrete and/or bored piers, it is not an appropriate design unless it can be shown (by suitable calculation and testing) that it will achieve the desired outcome.
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Expert Panel Comments
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Once again, this report illustrates the importance of understanding what is being designed and the actions that must be resisted.
Screw piles are typically used for load transfer in their axial direction, not for transverse (shear) loading or for bending moment transfer. The arrangement usually considers screw pile heads to be nominally pinned with reinforced pile caps or crossheads. However, the application described by the reporter involves eccentrically loaded screw piles, requiring a transfer of bending moment between both foundation and screw pile elements. As noted by the reporter, the adoption of a screw pile solution in this type of situation therefore requires design of the screw pile and of its head connection to accommodate such bending moments, and the checking of the foundation for its capacity to accommodate a similar moment.
The tops of screw piles are normally fitted with large steel angle sections which are often bolted to the foundations. In situations where movement caused by the eccentricity of the pile can be rectified using cross beams (such as waffle slabs), the problem of bending of the pile may not be present. In many other situations, the eccentricity will be real and bending in the pile shaft will be resisted to some extent by the soil. This interaction is clearly complex, and made more so as the soil providing resistance is disturbed by the screw itself.
As a note of caution, while the practice of underpinning is generally appropriate where uncontrolled fill or other forms of inadequate bearing capacity are present, there have been cases of localised underpinning carried out on reactive clay sites where the underpinning process caused more cracking than that which it intended to address. If foundations are all moving with moisture variation, founding a portion of the building on rock is unlikely to provide a permanent solution to masonry cracking.
Further, attention should be given to the life cycle differential between steel screw piles and the underpinned masonry structure, with adequate provision of durability protection for the former in order to provide some level of compatibility between the elements. In earthquake environments, consideration should also be given to the possibility and potential consequences of decoupling of the piles from the main structure.
There has been some research into the use of inclined screw piles, such as the Post-installation performance of eccentric screw pile underpinning systems for residential foundations by Konstantin G. Ashkinadze, Consulting Engineer, Edmonton, AB, Canada.
For further information on the use of screw piles in general, we suggest reviewing IPENZ's Practice Note 28 - Screw Piles: Guidelines for Design, Construction & Installation.
If you have any experience on the use of, or know of any research into, inclined screw piles for underpinning, please get in touch via our Contact CROSS-AUS form on our website or the Feedback Form below.
Our general recommendations below can serve as useful guidance of some of the potential risk areas associated with underpinning and combined foundation arrangements.
Designers should:
Assess the stability at every stage of construction. For example, additional temporary shoring of the wall may be necessary
Consider the recommendations in the geotechnical report or, if one is not available, undertake sufficient testing to ensure the soil conditions are well understood and the combined cast in-situ reinforced concrete and screw pile combination will be effective. Often the use of shrinkage-corrected grouts or expansive grouts, jacking, or other means of pre-load are required. Cracks in walls are usually repaired only after the foundations have stabilised
Evaluate the settlement potential of the completed integrated solution
Specify any additional controls that may be necessary to mitigate risk including, but not limited to, movement and monitoring settlement and cracks in walls. Note that this is often extended to two years after construction to account for seasonal fluctuation of groundwater levels
Contractors should:
Review the stability provisions advised by the designer, and contact the designer as a matter of urgency if there are any concerns or ambiguities
Adhere to all hold points for inspection, monitoring requirements, or limitations specified by the designer
Keep meticulous records of the construction works and all communications
Stop work immediately if untoward movement or new cracking is detected.