Monday, April 4, 2016

How to Check Slab Foundations in Central Mississippi

Note this is a newly updated blog (Oct 28, 2017) A minimum foundation check should involve a complete floor elevation survey that should at least accurately describe the locations of the highest and lowest floor elevations. The survey plot should have enough detail so that anybody can locate those two points. The evaluation should include a calculation of apparent maximum differential movement which is the difference between the highest elevation and the lowest elevation corrected for any differences in floor thickness between those two points. A much more meaningful survey is one that has either the low point or the high point equal to zero. All of our surveys are done this way. The best way to accomplish this is to spend a few minutes scanning the floor elevations with a Zip level to find the low point. Once this has been found, zero the Zip level to the lowest point. When this is done it becomes much easier to draw elevation contours. But note that floor elevation contours only have meaning if you plot the elevations on a scaled floor plan. I should note that these things are not typically done by foundation contractors. Elevation surveys made by contractors typically do not have a datum at the low point or the high point and are not drawn to scale. Some elevations are negative and some are positive. This kind of survey appears to be cryptic to a novice and just hard to evaluate even by experienced engineers. If this kind of survey is your only guide, there is a good chance that you will miss subtle drainage problems that are connected with the movement.

The literature on slab foundations constructed on expansive soils recognizes two failure modes: one is edge lift and the other is center lift. All the failures we have seen in central Mississippi where expansive soils are present, fit into these two categories. It is important to recognize that edge lift failures are often related to poor drainage near the higher floor elevations. Center lift failures may be related to a perched water table or poor drainage several places around the house. Before investing thousands of dollars in a foundation repair, homeowners should have a better quality floor elevation survey made that more clearly shows the movement, identifies the failure mode and how that movement changes across the foundation.

The average annual movement can be computed when the age of the structure is known. This is a more important number than the apparent total differential movement. Experience has shown that the average annual movement rate tends to continue unless repairs are made that specifically address the cause of the movement. A cookie cutter foundation repair that just involves releveling will not reduce the movement rate if the movement is heaving (upward) from expansive soils. Many of the foundation releveling procedures will help to retard settlements.  But many foundations have a mixture of settlement and heave. Some in central Mississippi are all heave. My view is that when the movement rate is over 1/4 inch per year you have a potentially serious foundation problem that probably includes significant heave. The worst case scenario is when the movement rate is 3/4 inch per year or maybe a bit higher. The 1/4 inch per year limit is strictly an arbitrary number. I assume that no one wants to have foundation repairs done any more often than once every 12 to 15 years. So over this time frame, you can accumulate 3 to nearly 4 inches of differential movement with the 1/4 inch per year arbitrary limit. Total movements in this range is a limit that is often used in the real estate industry.

Experience has also shown that if a structure has an average movement rate of about 1/4 inch per year that damage will begin to show when it is about 3 to 5 years old and that damage may first look like minor cosmetic issues. By the 5th or 6th year, the damage begins to look more like it is related to foundation movements. So if you buy a house that is less than six years old it could have a fairly significant foundation issue that might only show up through a floor elevation survey. So for a rule of thumb in central Mississippi, I recommend that if you are buying a house that is newer than say 6 or 7 years and does not show any evidence of foundation damage, the floor elevations should be checked and the average annual movement rate computed. A residence that may not need a foundation check would be at least 8 years old and show no damage that might be attributed to foundation movement.

I recommend a detailed foundation evaluation for any residence with 1/4 inch or more annual movement rate. The detailed check should include the minimum check requirements and floor elevations and contours plotted on a scaled floor plan with a zero elevation datum. If the owner does not provide a scaled floor plan then the house has to be measured and a floor plan produced with those measurements. With a detailed check, I usually show the lowest elevation equal to zero. Elevations are adjusted for different floor thicknesses. I show on the plan the base elevation survey measurement and the adjusted elevations, I then draw floor elevation contours at ½  to 1-inch intervals. Using this plot as a guide I then look for contributing drainage issues. This may mean a second trip to the property. In addition to recommending the kind of foundation repair, I may recommend checking the plumbing lines for leaks or soil borings. Generally, I like to see these things done before I make recommendations for repair, but a soil boring can be made later as a check against your conclusions made with just the survey. If I suspect heaving from expansive soils, then I may request a 10-foot deep boring nearest the highest elevation to see the depth of the clay (CH soil) and possible contributions of a water table.

The benefit of the detailed foundation study is that it maps out the details of the movement. It shows what parts of the house have more slope than others and sometimes directs you to a serious problem like a broken plumbing line or an area with poor drainage. If an area of poor drainage is resulting in heave, the slope of the slab may continue to increase as you get closer to the source of the water. When you see this a classic edge lift condition has been identified that is well documented in the literature. It is also important to look at how nearby larger trees are related to the movements.

Following this procedure, it is possible that a newer house with minimum damage could be flagged because the movement rate is excessive. This does not mean that it has a foundation problem, but that it is at higher risk for future foundation problems. With a detailed study of the structure, maybe a related drainage problem can be found and corrected. Also, there are exceptions - not every structure with a high movement rate will continue to move at that rate and there is another issue in that not all structures with differential movement will result in wall cracks. It is rare but a structure can move so that it has an even slope across the whole structure (pure tilt). In this case, the only damage is floor slope. Whereas there are no cracks, it does make the structure less usable if the slope becomes excessive.