Guidelines for Diagnosing Heave Subsidence and Settlement – INTRODUCTION

Page Summary

• Subsidence and heave cause most light foundation movement, while true settlement occurs less often under typical residential conditions.
• Settlement tends to appear with specific site conditions that reduce bearing support, including unstable fills, silts with perched water, and changes in soil strength under load.
• Heave is more common and harder to mitigate than settlement, so accurate movement direction and soil moisture drivers guide both design choices and repair planning.

Foundation movement can look the same from inside a home or a commercial suite, a diagonal drywall crack, a sticky door, a gap at base trim, or tile that sounds hollow. The cause is not always the same. Correct diagnosis starts with one practical question, which way is the foundation moving. Upward movement points toward heave, downward movement can point toward subsidence or settlement, and each pathway has different triggers, risks, and repair priorities. In Arizona, where long dry stretches, irrigation patterns, and intense storm events can swing soil moisture conditions quickly, separating these movement types early helps property owners make smarter decisions about evaluation, repairs, and long term performance.

This introduction focuses on the core concepts that guide early diagnosis: what heave, subsidence, and settlement mean in plain terms, why true settlement is less common in many light foundation scenarios, why heave is often harder to mitigate, and how design and investigation choices connect to the movement mechanism rather than the visible symptoms alone.

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

INTRODUCTION

In Houston, the common movement types plaguing light foundations are subsidence and heave. Settlement occurs rarely and primarily in cases that involve embankment instability or where slab-on-grade foundations are founded on silts underlain by relatively impermeable clays that cause perched water tables and subsequent loss in the bearing capacity of the silts.

While settlement is rare and easy to prevent, heave is much more common and difficult to mitigate. Foundation design engineers sometimes specify “Isolated Structural Systems with Deep Foundations” when they know that the site has a damaging heave potential at the design stage. These systems are described in Section 4.1.1.1 of the Foundation Performance Association’s Document No. FPA-SC-01-0, Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils, published 30 Jun 04 at: .

The foundation design engineer bases his design on the soil data from the geotechnical report. Unfortunately, some geotechnical reports produced for the Houston area have underestimated soils’ swell (heave) potential, leading to unacceptably flexible foundation designs that are susceptible to damaging movement.

Foundation repair contractors, many of whom sell repairs without the benefit of proper forensic engineering evaluations, often will not warrant their work if heave is later proven to be the cause of continued movement and distress. Some of the larger foundation repair contractors may provide a warranty for heave if they are allowed to sell an entire foundation lift, i.e., raising the entire foundation a certain elevation above the soil to ensure that heaving soils cannot apply uplift forces to the underside.

Heave can be a difficult movement to predict, diagnose, and mitigate. Local geotechnical engineers sometimes do not provide the testing or engineering needed to predict heave accurately. This promotes selling full remedial lifting after unplanned heave occurs, which can be expensive. We have seen both remedial lifting and isolated foundation designs fail to perform as planned when the depth of the piers or piles is not sufficiently anchored below the active zone. The common reason this occurs is the lack of proper geotechnical testing that identifies the depth of the active zone.

The proper depth to prevent heave or subsidence of a pier or pile should be computed based on suction testing and is normally 1.5 to 2.0 times the depth of the active zone. In Houston, we have measured active zones as deep as 20 ft, meaning the builder’s piers or the repair piers or piles may need to be found as deep as 30 to 40 ft penetration, depending on the site characteristics.