Guidelines for Diagnosing Heave Subsidence and Settlement – DIAGNOSES

Page Summary

• Heave is upward movement commonly tied to moisture gain in expansive clay, subsidence is downward movement tied to moisture loss, and settlement is downward movement tied to bearing capacity and load effects.
• Correct diagnosis depends on direction of movement plus evidence such as crack patterns, drainage and moisture conditions, vegetation influence, plumbing history, and measured elevation profiles.
• A Level C style investigation, including interviews, document review, detailed site observations, and targeted testing when needed, reduces misdiagnosis and supports repair choices that match the movement mechanism.

Foundation movement rarely comes down to one visible crack or one sticking door. Floors, walls, and slabs respond to what the supporting soil is doing over time, and in Arizona the combination of heat, irrigation patterns, monsoon weather, and variable soils can create conditions where movement shows up in more than one way. Diagnosing Heave Subsidence and Settlement diagnoses starts with a simple, practical question, which way is the foundation moving, then follows the evidence that explains why.

Heave, subsidence, and settlement are not interchangeable terms. Each describes a different mechanism, often with different triggers, different field signs, and different repair priorities. When the movement type is mislabeled, the repair approach can target the wrong problem, and the structure may continue to move even after work is completed. A structured, documentation driven diagnostic process improves accuracy, clarifies risk, and helps set realistic expectations for performance over time.

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

DIAGNOSES

Who can properly diagnose the type of movement that has occurred?

Only experienced forensic engineers who carry out Level C investigations have a reasonable chance of correctly diagnosing the movement type. Others who perform Level A or B investigations tend to misdiagnose the type of movement.

What is a Level C investigation?

Following Section 3.3 of 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 a Level C investigation, includes, but is not limited to, the following steps:

• Interview the occupant, owner, and/or client, if available, regarding the history of the property and the performance of the structure
• Document visual observations made during a physical walk-through
• Review pertinent documents, including geotechnical reports, construction drawings, field reports, and repair documents
• Observe factors influencing the performance of the foundation
• Determine relative foundation elevations to assess levelness and establish a baseline
• Document locations of large trees and other vegetation
• Determine whether site drainage issues exist
• Document the analysis process, data, and observations
• Site-specific soil sampling and testing, if applicable
• Hydrostatic leak test, with leak location and flow test, if applicable
• Material testing, if applicable
• Post-tensioning cable testing or steel reinforcing survey, if applicable
• Aerial photographs to determine prior land usage or construction issues
• Observations of cut and fill

If the client requests a report, it should contain the following as a minimum:

• Scope of services
• A list of the reviewed documents
• Description of factors that affect soil moisture
• Observations
• Scaled drawings
• Site photographs
• Survey elevation plan
• Detailed phenomena plan
• Results from any testing done as part of the investigation
• Discussion of factors identified as influencing the foundation’s performance and rationale in reaching opinions concerning the foundation
• Conclusions and recommendations for further investigation, remediation, or preventative measures

Why is monitoring used?

A single site visit provides only a snapshot of the foundation’s condition. Only by making more than one site visit to document distress and foundation elevation changes can an engineer determine whether a foundation continues to move. If monitoring is pursued, it should follow the procedures outlined in the Foundation Performance Association’s Document No. FPA-SC-12-0, Guidelines for Evaluating Foundation Performance by Monitoring, published 9 Jan 06 at: http://www.foundationperformance.org/.

By correlating time-change elevations with events such as climate, change in drainage patterns, distress observations, etc., it is possible to correctly ascertain the direction of movement even when an interior datum is used. When an external deep benchmark is available, it becomes easier to determine the direction of movement. Monitoring is also used to determine when the heave has ended so that the superstructure can be repaired.