RSS

Tag Archives: foundation settlement

Foundation Movement – Part III

Foundation Upheaval

In previous blogs, we discussed that foundation movement can be categorized into two types: foundation settlement (the most common) and foundation upheaval (the most difficult to remedy by foundation repairs). Today, I will discuss the latter: upheaval.

As a Structural Engineer, I recently represented a homeowner of a new custom home (costing about $1mm) where the foundation heaved upwards over 5” – within the first year! The slab was constructed over drilled concrete piers; which is the type of foundation I would design for myself if I ever built a new home (shoot me dead first).

The homeowner called me to check out his foundation after the builder (and his engineer) had inspected the home and told the homeowner that “it was just normal settlement” (however, it did not settle, it heaved upward). As part of my inspection, I reviewed the foundation design and determined it to be a good design. So, if it was a good design, why did the foundation move and cause numerous sheetrock cracks, slab cracks, etc.?

The answer to this very good question is that it moved primarily for 4 reasons:

  • The foundation was constructed on a lot with highly plastic clay soils (soils that expand and lot when the soil moisture content increases and shrinks a lot when the soils dry).
  • The builder did not follow the engineer’s specifications for site preparation.
  • The builder did not provide adequate drainage conditions around the foundation.
  • The builder built the foundation in 2006, during a record breaking drought in North Texas.

I will focus my brief discussion on the last reason, which I consider being the primary problem.

This particular residence is built in a suburb of Dallas that is well known for its highly expansive clay soils. Prior to designing the foundation, the builder obtained a geotechnical report that indicated that the soils at the site were such that 6 inches of PVR were possible. PVR is “Potential for Vertical Rise”; this means that under the “right” conditions, the soils could expand upward over 6 inches. The geotechnical engineer recommended several measures the builder should take to minimize any foundation movement. However, the builder did not follow the recommendations and the foundation heaved upwards about 5” in the first year.

In 2006, when the slab was built, the highly plastic clay soils, because of a record breaking drought, were in an extremely “shrunken” state. Unfortunately, when the drought ended and the rains returned, the soils expanded significantly and lifted the slab up off the piers. Because of the large size and shape of the house, a satisfactory remedy was not economically feasible.

It should be said that even if the drainage conditions around the foundation had of been perfect (and they were not), the slab would have still been lifted up off the piers by the expanding clay. When the rains returned, the soils were going to be re-hydrated no matter what the drainage conditions.

As a side note, the builder also built the slab too close to the ground surface, plus it was a flat lot so the surface water had no place to run. I guess the builder was not aware that water runs down hill.

Fortunately, the homeowner retained an experienced attorney who was able to get the builder to buy the house back. The builder told me that he now pre-swells the soils prior to installing a foundation (which was one of the recommendations his geotechnical engineer had made originally). After the builder bought the house back, he fixed the cosmetic defects in the house and then resold it in an “as is” condition at a discounted price.

For more information on foundation movement and foundation repairs, go to www.GeoDFW.com

Advertisements
 
4 Comments

Posted by on February 5, 2011 in foundation repair

 

Tags: , , , , , , , , ,

Slab Cracks

Slab cracks in slab foundations can cause great anxiety to homeowners. However, many slab cracks do not pose a structural problem to the foundation. For example, many times, slab cracks appear soon after the concrete is installed and are usually caused by poor quality control by the concrete contractor. Most of the time, these types of cracks are hairline in width and are only superficial (do not penetrate deep into the slab) – however, there are exceptions. Generally speaking, these types of cracks are related to the excessive loss of moisture in the concrete and are called plastic shrinkage cracking. Plastic cracking is related to many factors during the slab installation process, including: wind, low humidity, excessive water content in the concrete mix, exposure to the sun, etc. These types of cracks will not usually have a negative impact on the structural ability of the slab.

The cracks that concern us the most are the cracks caused by foundation movement. Most of these types of cracks are hairline to 1/16” wide or so, however, I have seen slab cracks that are ¼” and wider. Many times, an extensive network of slab cracks is an indication of poor rebar placement and/or significant foundation movement.

Unless a slab crack is covered with floor tile (which will also crack) or the slab crack allows excessive moisture penetration from the subgrade soils, most slab cracks cannot be easily detected under carpet or wood flooring.

The usual remedy for a slab crack caused by foundation movement is twofold:

  1. Stabilize the slab so it cannot move. This is typically done by installing piers.
  2. Epoxy inject cracks that are 1/32” wide or wider.  This type of work needs to be done by an experienced technician.

There are other reasons concrete slabs crack but that is another topic for another day.

Jim McNeme, P.E.

Foundation Engineer

www.GeoDFW.com

 
Leave a comment

Posted by on January 2, 2011 in foundation repair

 

Tags: , , , , , , , , , ,

Foundation Movement

It can be disturbing for a homeowner to realize that his residence has experienced foundation movement. Many homeowners have heard some of the horror stories about super expensive foundation repairs.

Foundation movement, almost all the time, is related to soil movement. Slab foundation stability depends on the support of the soil, so when the soil moves, most foundations (unfortunately) move differentially.  “Differential Foundation Movement” means that the foundation moves more in one area than in another. The principle reason for this kind of movement is that the foundation is not rigid enough to overcome the movement of the soils. This type of movement creates sheetrock cracks, slab cracks, brick cracks, out of level doors and floors, etc. Unfortunately, this is the most common type of foundation movement in expansive clay soils.

The other kind of foundation movement (as opposed to differential movement) is the type where the foundation tilts (like when you pick up the edge of a card table). Tilting usually indicates that the foundation is well designed and constructed (it is rigid) but since the supporting soils below moved, the foundation moves as a unit. In these instances, a foundation that experiences tilt many times does not have a lot of cosmetic distress (sheetrock cracks, brick mortar cracks, etc.) or structural distress (slab cracks).

Neither type of foundation movement is pleasant to remedy. Both types typically require underpinning with piers. There are two reasons to install post construction piers:

1.   To stabilize that area of the foundation from experiencing additional settlement, and/or

2.   To attempt to make the foundation more level

Both of the above goals require using an experienced engineer, the right pier for the soil conditions and a reputable foundation repair contractor.

For more information on foundation movement, go to www.GeoDFW.com

 
Leave a comment

Posted by on January 2, 2011 in foundation repair

 

Tags: , , , ,