Guidelines for Diagnosing Heave Subsidence and Settlement – DEFINITIONS

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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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DEFINITIONS

The following definitions are taken from 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/:

Heave is upward movement of an underlying supporting soil stratum usually due to the addition of water to an unsaturated expansive soil in the active zone. When moisture is added to a soil with clay content, expansion occurs within the structure of the soil, and the corresponding area of the foundation and superstructure is moved upward. Heave normally only occurs within clayey soils that have a high suction potential and an available moisture source.

Subsidence is downward movement of an underlying supporting soil stratum due to the withdrawal of moisture. When moisture is extracted from the soil, shrinkage occurs within the structure of the soil, and the corresponding area of the foundation and superstructure move downward. Subsidence normally occurs within clayey soils and is often the result of soil desiccation that is caused by trees or other large vegetation.

Settlement is downward movement of an underlying supporting soil stratum due to loading in excess of the bearing capacity of the soil below. When the vertical loads from above are in excess of the bearing capacity of the soil strata directly below the foundation, the foundation and superstructure move downward. Encompassed in settlement are a) the immediate elastic consolidation and distortion of granular or clay soil particles, b) slope instability, and c) the long-term consolidation resulting from gradual expulsion of pore water from voids between saturated clay soil particles. Settlement may occur in all types of soils.

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