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
SYMPTOMS
Following are some symptoms that can help provide the basis for a forensic engineer to opine on the type of foundation movement, but not all symptoms will be present or observable.
Heave Symptoms
Following are some symptoms of heave:
- Slab cracks resemble spider webs or chicken feet with at least two intersecting cracks.
- The damage began to occur during or soon after construction, accelerating within the first few years of construction.
- Nearby trees died or were felled within a year or two that the first distress was observed.
- The driveway concrete or other flatwork adjacent to the foundation translated upward and away, relative to the foundation.
- The soil plasticity index (PI) is greater than 25.
- The moisture content (MC) of the upper soil strata below the foundation has increased in recent years.
- There are gaps between grade beams and builder’s piers, or foundation repair shims are loose.
- There are no gaps between the perimeter grade beams and the adjacent soil at grade.
- There have been known leaks in the under slab plumbing or from nearby ponds or pools.
- There is a large amount of damage in the foundation and superstructure for a relatively small amount of level distortion in the foundation.
- Water meters or irrigation valve pits are submerged.
- A nearby in-ground pool is out of level.
- Site drainage around the foundation is flat or sloping toward the foundation.
- The soil just below the perimeter beam is easy to probe.
- Probing or excavating around the foundation reveals wet muddy soil in the upper several feet below the perimeter grade beam.
- Most of the damage is in the ground level flooring materials and elevations show that the slab is bowed up between grade beams when the grade beams are connected to builder’s piers that are founded well below the active zone.
- Time-change elevations show the high point of the foundation to move laterally while the foundation tends to not increase its overall level distortion.
Subsidence Symptoms
Following are some symptoms of subsidence:
- Matured or maturing trees and other large vegetation are growing near the lower area of the foundation.
- Cracks in the superstructure tend to open wider in the drier months and tend to close in the wetter months.
- The foundation is at least ten years old and there are matured or maturing trees nearby that are of similar age as the foundation.
- Large tree roots are visible at grade, extending towards the structure.
- The owner is unable to maintain carpet grass below the trees near the foundation.
- The soil plasticity index (PI) is greater than 25.
- The moisture content (MC) of the upper soil strata below the foundation has decreased in recent years.
- Pocket penetrometer readings of the soil near grade is high, on the order of 4 TSF.
- Slab cracks are single lines, i.e., without intersections.
- The concrete driveway is broken up or in poor condition.
- Exterior walls tend to lean outward at the top.
- The foundation was raised around the perimeter several years prior and it now has a bowl shape with the low point near its center.
- The soil just below the grade beam is difficult to probe more than an inch or two, and is dry when excavated.
- There are no gaps between piers and grade beams.
- There are gaps between the exterior foundation grade beam and the adjacent soil at grade, particularly during the drier months.
- Time-change elevations show the level distortion of the foundation to increase with time and seasonally, particularly in drier years.
Settlement Symptoms
Following are some symptoms of settlement:
- Pocket penetrometer readings for the clay supporting a slab-on-grade at the foundation grade beam bearing level are below 0.5 TSF.
- There is more relative downward movement in areas of the foundation with higher gravity loads.
- The heavier structures in the area appear to have more damage than the lighter ones.
- Nearby flatwork, pools or other lighter structures do not show signs of movement.
- The grade soil is loamy or silty material.
- The foundation is near a steep embankment without a proper retaining wall and there are signs of soil sloughing down the slope.
- Slab cracks are single lines, i.e., without intersections.
- The soil plasticity index (PI) is less than 15.
- There are no gaps between builder’s piers and grade beams.
- The soil just below the perimeter beam is easy to probe.
- The ground surface near the foundation appears to drain quickly with little slope, tends to vibrate when excited, and also tends to become soft after heavy rains.
