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Are Engineers Created Equal? Part II

Last week, the blog discussed an example of poor engineering judgment. Another example of bad engineering advise was at a home built on the side of a steep hill which ran down to a creek. It was estimated that the builder brought in over 15 foot of fill material to level the lot prior to installing the foundation. As is usually the case, the fill material was not properly compacted so, over time, the fill material consolidated causing the slab foundation to settle several inches. (Even if the dirt the builder imported – the fill material – had been properly compacted, the foundation would have still settled because gravity, over time, would have pushed the dirt downhill. Only a solid retaining wall would correct this problem.)

Within a few years of moving into his brand new home, the homeowner started noticing indications of severe  foundation settling. This resulted in numerous slab cracks, out of level floors, sheetrock cracks, brick mortar cracks and out of level doors.

He retained an engineer to design a repair for the foundation. The engineer specified the installation of about 50 drilled concrete piers that only went to the 12 foot depth. In other words, the piers on half of the home were installed completely within the fill material. Needless to say, 1/2 of the foundation continued to be unstable for several years, requiring continual “adjustment” of the piers. The only remedy for this foundation is to replace the existing piers with piers that are founded in a stable bearing stratum. However, because of the expense, the homeowner has not been able to do so.

It should be said that a post construction pier will typically only prevent vertical movement (downward or settlement movement) and not horizontal movement. Some extreme topographies require a stable retaining wall to stop horizontal movement of the soil.

So, a foundation situated on the side of an extreme slope needs special attention from an experienced engineer who understands the forces of gravity.

Both of these homeowners had retained structural engineers but unfortunately, the engineers did not understand the basics of soil behavior.

Jim McNeme, P.E.

Structural Engineer

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

 
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Posted by on April 10, 2011 in foundation repair

 

Foundation Repairs That Did Not Work Out

Part I

There is a neighborhood in North Dallas that is well known for its highly expansive clay soils and its poor quality foundations. Most of the foundations of the homes in this neighborhood have had foundation repairs. Several years ago, a homeowner asked me to investigate his continuing foundation movement. Several years prior to calling me, he had the foundation repaired by a well known foundation repair company in Dallas. That company recommended the installation of 40 or so drilled concrete piers. The piers installed were double shaft concrete piers (8” diameter) that were to extend “to a depth of 9 feet or to rock whichever came first”. Well, rock in that neighborhood was over 40 feet down, i.e., the piers did not extend to a stable stratum and the piers were so unstable so the home was continuing to experience significant differential foundation movement to the point that doors were jammed shut, sheetrock was always cracking, etc. The severity of the problem varied with the seasons (wet springs and dry summers).

In North Texas, the soils tend to dry out every summer. It is generally accepted that they dry to depths of 12 to 15 feet. They may not get bone dry, but they get drier than during the rainy seasons. As expansive clays dry out, they shrink and anything within that zone of moisture change will move. This includes piers. Every summer, some of the piers would settle an inch or so but every winter/spring, they would rebound back upward. He said the foundation used to recover better but lately, the settlement is getting worse and staying bad. The only remedy was to install new piers of a known quality and disconnect the old piers from the foundation. This cost him about $35k. The new piers I specified were steel pipe piers that went to depths of about 40 feet. The foundation has been performing properly since.

Jim McNeme, P.E.

Structural Engineer

http://www.GeoDFW.com

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

 
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Posted by on April 2, 2011 in foundation repair

 

Foundation Movement IV

Upheaval – revisited again

I recently inspected a slab foundation of a 10 year old house in an unincorporated area in North Texas (no local building codes). The residence had been experiencing foundation movement to the point that the front door was jammed shut, numerous large sheetrock cracks existed throughout the house and much of the foundation was obviously out of level. The homeowner said that sometimes the front door would work properly (during the dry summer months) but other times, it could not be used (typically during the winter/spring or after a few days of rain).

The homeowner also said that when the home was being built, the builder installed numerous concrete piers around the outer perimeter of the foundation and that they were 6 to 8 feet deep, however, no piers were installed under the middle of the slab. I asked the owner if the builder had obtained a site-specific soil report for the foundation design engineer to use. He said no, that the builder had over 20 years experience in construction and said he did not need an engineer or soil report.

I determined that the piers installed by the builder were so shallow that the expansion of the clay soil (during the wet weather) caused the clay to grab the sides of the piers and push the piers up. During the dry weather, the soils would shrink and allow some subsidence of the piers. Unfortunately, the upheaval each year was much more severe than the settlement so that, each year, the outer edge of the foundation moved a little higher than the previous year. When I inspected the foundation the outer perimeter of the slab was over 4” higher than the middle.

There were two possible remedies to this, neither of them delightful. One remedy would be to install piers both around the outer perimeter of the foundation AND throughout the interior. The interior piers would then be used to raise up the interior to match (as close as possible) the slab elevations around the outer perimeter of the house. Then the unstable builder installed piers would be cut loose from the foundation and a void would be left under the slab to allow some movement of the soils. This technique, though used somewhat commonly (usually when the builder is threatened with a lawsuit) has some limitations to its success and requires the homeowner to move out of the home for a few weeks and it also devastates the below slab plumbing (and other things).

The other possible remedy may have been less invasive and less costly but it involved a two step phased approach to the repair. IF a void remained under the outer portions of the slab (because the piers pushed the foundation up), it may have been possible to expose all the piers and cut them free of the foundation and attempt to lower the slab back down to its original position. This is a rather risky technique because sometimes the soil under the slab expands up and leaves no void space between the slab and the dirt, so the slab could not be lowered. So before this technique is attempted, further investigation would be required. Another potential problem with this approach was that when the soil moved again, so would the foundation.

In the end, the homeowner decided that he was ok with the foundation as it was and was not interested in spending any further money on it.

It would not have taken much very much additional money for the builder to install the piers properly (to a stable bearing stratum) but apparently the builder did not understand all that was involved in building a slab-on-pier foundation in expansive clay soils (even though he claimed that he had been building homes for over 20 years and never had a problem with one of his foundations).

The primary lesson to be learned here is when constructing a foundation on problem soils, the piers must be designed by an experienced structural engineer and installed by a knowledgeable builder. Also, piers must be installed into a stable bearing stratum (among other things). This requires conducting soil borings, testing and an analysis by a geotechnical engineer.

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

Jim McNeme, P.E.
GeoDynamics

 
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Posted by on February 26, 2011 in foundation repair

 

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

 
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Posted by on February 5, 2011 in foundation repair

 

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Foundation Movement – Part II

As discussed in the last blog post, foundation movement is usually related to soil movement. The highly plastic clay soils in portions of Texas have caused 100’s of millions of dollars of damage to foundations. In non technical terms, a highly plastic clay is one which expands a lot with an increase in the soil moisture content and then shrinks a lot when it dries. It has been my experience that the worst foundation movement occurs during dry weather, which occurs every summer in north Texas. This dry weather causes the soils to shrink and allows many slab foundations to settle. Fortunately, settlement of a foundation can usually be remedied by installing piers (piers are expensive but are usually very effective in stopping further settlement in the area of the piers).

However, depending on the soil conditions, the opposite can occur when the soils become re-hydrated (either from Mother Nature or homeowner irrigation) and the foundations are heaved upward by the swelling clays. Upheaval can be very difficult to remedy. I have seen some residential slab foundations in Irving and Carrollton (Texas) heave upward 8 to 10 inches and a commercial building slab heave over 12”. Needless to say, this can be catastrophic to the structure. This type of movement creates significant sheetrock cracks, slab cracks, brick cracks, out of level doors and floors, etc. and obviously lowers the value (and livability) of a home and usability of an office building. Foundation upheaval can be a very difficult and expensive problem to remedy.

 My next post will discuss an actual case file where a custom home with a well designed slab foundation experienced about 5” of upheaval during the first year.

For more information on foundation movement, visit www.GeoDFW.com

 
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Posted by on January 29, 2011 in foundation repair

 

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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

 
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Posted by on January 2, 2011 in foundation repair

 

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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

 
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Posted by on January 2, 2011 in foundation repair

 

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