The Problem with Screw Piers
Screw piers are an easy way to found a building at a deeper level, but there are a number of problems with screw piers that could contribute to slab heave and house damage. The problems are severe enough that I rarely use screw piers in new designs.
Look for these problems and find correlations to slab heave in your investigation.
Screw Piers Inject Moisture into Deep Soils
This is my number one concern with using screw piers on reactive clay sites. As screw piers are installed into the ground, they leave a hole of loose soil above the plane of the pier. This soil is loose and extremely porous. Stormwater, irrigation water and flood water will seep into the hole very easily and fill it up. There a re a number of issues when this occurs.
- The Hs (depth of ground movement) is dramatically increased at each screw pier. The positive effect of founding below the seasonal moisture depth is demolished once water enters these holes.
- The soil under the screw pier blades increases in moisture content well beyond its liquid limit. The ability of the soil to support vertical forces is eliminated.
- If aeration is available, the inevitable corrosion of the steel pier, particularly on the driving edge of the bottom blade. Loss of steel will result in loss of bearing capacity and loss of moment capacity of the pier.
Screw Piers Not Founded Deep Enough
Piers are screwed until an acceptable installation torque is achieved. Considered in relation to soil testing that estabklished a required founding depth, a screw pier may still be founded at an inadequate depth if uneven sub-surface conditions are mis-interpreted by simply looking for achievement of an installation torque.
For example, a large floater in a filled platform may have prevented adequate compaction of the fill. A screw pier achieving an installation torque when it comes close to the boulder results in the completion of that pier. However the vertical capacity of the screw pier is dimished by the proximity of solft soil under the boulder or under the blades of the screw pier. Either one of the these effects could result in uncontrolled settlement. Be careful not to confuse this with uncontrolled heave in the remainder of the building.
Screw Piers Pulled Out of Bottom of Footing
Screw pier posts are connected to concrete footings by cutting the steel of the pier off within the footing depth. If the screw pier steelwork is terminated too low in the concrete, footing heave may result in the pier shearing out of the bottom of the footing. Once a screw pier stump is pulled clear of the bootom of a footing it will never re-seat back into the footing no matter how well you improve soil moisture conditions. Where slab heave has occurred on a screw pier footing, excavate and check the connection between EVERY screw pier and the concrete footing. If you are designing screw piers for reactive sites, please re-consider the connection between the pier and the strip footings. Site conditions are never ideal and the cut-off height becomes super critical. Consider the likelihood that a screw pier stump terminates near the bottom of the footing. Do you think a contractor is going to want to extend the pier to achieve 150mm embedment? Even if the steel stump is extended, this could happen:
Screw Pier Extension Pieces Have Failed
Screw pier columns are extended by linkages that rely on the shear capacity of a steel pin. If the steel corrodes or the heae forces shears the pin, then the linkage will not reseat. Look for areas of slab that are not reversing when the remainder of the slab is. Excavate and check that the screw piers are still linked and have not failed. Consider backfilling these excavations with concrete to limit future moisture inflow into the excavation.
Screw Piers under Steel Stumps
When screw piers are used to support houses on steel stumps, consider whether the pier design and installation has met the requirements for the basement level transfer of shear forces to the ground. Simply install9ing cross bracing under a house does not guarantee adequate shear transfer to the soil. Screw piers supporting steel stumps need to be capped with concrete to provide a good bearing trabnsfer point to the ground, if a concrete cap has not been provided, consider whether sideways buckling of the steel stuimp has contributed to the observed damage,
Also consider whether 2nd order buckling might have occurred. Has the steel been braced at ground level by a properly installed concrete plug only to buckle at a stump joiner located below ground level? Look for rotation of ht estump at the concrete plug and possible cracking in the plug.