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Which Way Is It Moving?
Guidelines for Diagnosing Heave, Subsidence and Settlement
Ron Kelm, P.E. | Nicole Wylie, P.E. | Forensic Engineers Inc. | Houston TX | www.forensicengineersinc.com
Most foundation repairs made in the Houston area consist of segmented concrete pilings driven against the weight and stiffness of the foundation and superstructure. This repair type has performed well in active soils, largely because of its ability to penetrate deeper through the active zone than the drilled and cast-in-place piers it has replaced. The design and installation of segmented concrete piles should follow the Foundation Performance Association’s Document No. FPA-SC-08-0, Design, Manufacture, and Installation Guidelines of Precast Concrete Segmented Piles For Foundation Underpinning, published 17 Jul 05 at: .
Because segmented concrete pilings are the norm for underpinning in the Houston area, the remedies discussed below will assume this type of repair is being done. As with other underpinning concepts, segmented concrete piles are used to lift the lower parts of concrete foundations toward a more level condition. The most current standard for determining whether a concrete foundation has sufficient level distortion to be considered functionally damaged, thereby requiring this repair, is the Foundation Performance Association’s Document No. FPA-SC-13-0, Guidelines for the Evaluation of Foundation Movement for Residential and Other Low-Rise Buildings, published 15 Jul 07 at: http://www.foundationperformance.org/. However, some owners may prefer their foundation to be more level than the above document provides.
Whatever the reason for repairing level distortion of a foundation, it is imperative to accurately understand and diagnose the movement types because their remedies may be different, as discussed below:
There are three common remedies for heave: 1) remove the source of moisture such that no heave can occur, 2) wait for the heave to run its course or, 3) lift the entire foundation so that continued heaving soil will not contact the underside of the foundation.
The first option, removing the water source, depends on the water source. It may be costly to find the source and sometimes it is not feasible to stop it. For a sewer leak, the obvious fix is to repair the leak. For poor site drainage, the fix is to repair the grade to provide proper drainage, perhaps including some underground piping for downspouts and area drains. For general wetting of soil due to rainfall or underlying soil phenomena, a moisture retarder may help.
As heave in the Houston area is most often caused by the rewetting of previously subsided soils, the second remedy, and often the most economical “fix”, is to just let the heave happen; heave will continue until the soil reaches moisture equilibrium, assuming that poor drainage or leaks are not exacerbating factors. Periodic monitoring of the foundation’s elevations and the superstructure’s distress will allow the engineer to determine when the movement due to heave has ended. Then, if the level distortion is not within acceptable limits after the foundation has stopped moving, the foundation can be underpinned much the same as outlined for subsidence below. Typically, for heave cases it will be easier to achieve a deeper penetration (and get beyond the active zone) than if subsidence had occurred, because in a heave condition the clays tend to lose much of their shear strength and therefore they also lose some of their resistance to driving.
The third remedy, lifting the entire foundation sufficiently so that continued heaving soil will not contact the underside of the foundation is offered by some local repair contractors. It is considerably more expensive but allows these contractors to offer a warranty, which they otherwise may not offer for heave conditions. Lifting an entire foundation adds engineering challenges because the typical foundation that was originally engineered as a slab-on-grade must now be designed as a suspended structural slab foundation. In addition, there are plumbing and other interfaces to consider as well as new vertical steps at porches and at the attached garage apron/driveway junction.
The remedy for repairing a concrete foundation where the level distortion is due to subsidence involves removing or heavily pruning trees and other large vegetation. It is almost certain that mature vegetation is involved if the movement type is subsidence, even if on a neighbor’s property. In subsidence repairs the soil is often hard, desiccated clay with a deep active zone, making it difficult to achieve sufficient penetration below the active zone to stop future subsidence particularly when the structure used for driving the piles does not weigh much.
Repair contractors normally accept these problems and offer lifetime warranties to adjust their pilings should movement continue. When subsidence is known to be the movement type, the repair contractor can reduce his call-back rate by educating the owner on the cause of subsidence and encouraging the owner to heavily prune or in some cases remove the offending vegetation, either of which can raise the active zone and help keep the piling more stationary.
The remedy for repairing a concrete foundation where the level distortion is due to settlement is perhaps the safest of the three movement types when a lifetime warranty is offered. Knowing where the active zone ends is not of as much concern as with subsidence and heave, only that sufficient capacity is achieved during driving. The goal should be to support the foundation at a deeper stratum to achieve more bearing capacity than is available near grade.