Guidelines for Diagnosing Heave Subsidence and Settlement – OTHER

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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

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OTHER TYPES OF MOVEMENT

There are other less common types of foundation movement occurring in the Houston area of which the forensic engineer should be aware. Though they are outside the scope of this paper, we will briefly cover active fault slippage and root heave, two movement types we have found in our work.

There are more than 350 known active faults in the Houston area, although only about one-third of these are well documented. Hundreds of structures have been built across fault lines, often inadvertently. The following photo is an example of a road and house in the Spring Branch area built across a fault line.

Fault slippage rates in the Houston area vary, with the faster ones moving about 1 inch per year. It has been found that soil and foundations near the active fault line will translate at a rate of 3 vertical to 1 horizontal, meaning that there will be walking in addition to vertical movement.

Root heave is another type of movement that can be seen in the flatwork in many areas with mature trees nearby. As the following photo shows, the tree roots can grow under the flatwork, and as the root diameter increases, the flatwork is raised. Reinforcing the flatwork can slow the movement, though a maturing tree is usually able to break up the flatwork.

Root heave in a foundation is less common in the Houston area, because roots require oxygen, and oxygen is not usually plentiful beneath a foundation. One exception occurs when the perimeter grade beam is raised for leveling purposes, as tree roots (and roots of other large vegetation) will grow under the foundation simply because they are able to. If an interior plumbing leak is present, or if the grade slopes towards the house causing water to pond near/under the foundation, then the roots will thrive and extend further under the foundation. This may lead to localized distress.

A second exception is when there are sandy soils with high bearing capacity and good oxygen transmission. In this case the grade beams and walls are typically unaffected, while the slab between the stiffening grade beams exhibits heave-like cracks. The slab movement due to root heave can occur over a short period of time. The following photos depict the diagnosis and remedy of one such case.

A pine tree was growing too close to this house, in an area with sandy soils with high bearing capacity. Due to the high bearing capacity, as the roots grew under the foundation they were unable to fail the soil to expand downward so instead they caused cracks in the slab as they expanded upward.

Heave-like slab cracks were visible in the slab. No distress could be seen in the grade beams or superstructure as a result of the tree. A coring was made through the an intersection of the heave-like slab cracks

The concrete coring showed a crack that was wider at the top and did not penetrate the entire slab thickness. Were this a crack due to heave, we would expect the crack to penetrate the entire slab and into grade beams.

Further coring revealed a portion of tree root. Based on odor and texture, it was determined to be a pine tree root, correlating with the too-close tree seen growing outside.

The slab and rebar were removed to expose the root system. The offending tree was just behind the white wall near the top of the photo.

The tree roots were removed.

Select fill was brought in and properly compacted, rebar was replaced, and the slab section was placed.

Finally, the offending tree was removed to prevent a repeat performance.