Slab-On-Grade Sinks 2.5 Inches Due to Poor Compaction Under the Slab GENERAL INFORMATION Date of Site Inspections: September 18, 1998 Location: 3526 Hanford Court, Charlotte, NC Prepared for: Dave and Mary Matthis | Top | Table of Contents | BACKGROUND At your request, I performed a limited structural inspection at the above referenced address on October 14. You and your wife were present during the inspection. The purpose of the inspection was to: Inspect the unevenness in the floor slab, cracks in the walls, and gaps under kitchen cabinets and between walls and floors. Determine the cause(s) of the above defects. Make recommendations on how to correct the problems. During my field inspection, I observed the floors on the first and second floors, interior walls on both levels, interior doors and wall openings on both levels, the roof framing system, the foundation walls, exterior walls, and topography around the house. For purposes of the discussion below, all directions are in relation to a person facing the front of the house. | Top | Table of Contents | DESCRIPTION The house is a two-story wood-framed structure, approximately 2000 sq. ft in size. There is no garage or carport. It was constructed 13 years ago. The present owners have lived in it for the past 7 years. The house has a slab-on-grade foundation and brick veneer foundation wall. The slab is independent of the perimeter foundation wall. The house has a gable roof and vinyl siding. The rafters are supported at midspans with struts that bear on the interior walls. The house is located on a lot that slopes at about a 10% grade from front to back. The floor height on the right front corner of the house is about 0.5' above the outside grade, while the left rear corner is about 3.0' above the outside grade. | Top | Table of Contents | OBSERVATIONS There are noticeable slopes in the concrete slab-on-grade floor on the left rear of the house. Using a 4' carpenters level, I determined that there was a 1"" slope from the center of the kitchen floor to the cabinets and a æ"" slope in the left corner of the dining room. There are also other measurable slopes in the floor slab in this part of the house. I observed that several of the interior door frames in the left rear of the house are out-of-square. Mr. Matthis told me that he had to plane at least one of the doors in order to get it to close. There are angular cracks in the walls at the upper corners of several of the door frames. Space has opened up between the kitchen cabinets and kitchen floor and between the bases of some of the walls and adjacent floors. Similar problems are present on the second floor, but they are not as severe as on the first floor. The majority of the defects are located on the left rear part of the house, although I did observe cracks on the second floor over doors in the master bedroom in the right rear part of the house. In general, I did not observe these problems on the front part or the right side of the house. I did not observe and cracks or other signs of settlement on the interior surfaces of the exterior walls or the perimeter foundation wall. The front door binds slightly, but it probably has more to do with the steel door fitting tightly in the frame than settlement of the foundation. The exterior wall lines and roof eave are straight and plumb. This is another indication that the perimeter foundation walls have not subsided. In my opinion, the perimeter foundation walls and exterior walls have remained stationery, or nearly so, since the house was built. | Top | Table of Contents | FINDINGS AND CONCLUSIONS Since there is little or no evidence that the exterior walls have moved, volumetric changes in the virgin soil (swelling, shrinking, and consolidation) can probably be ruled out as a cause of the problems. The likely cause is settlement of the fill under the slab-on-grade. Because of the sloping site, the depth of fill under the slab varies from no fill in the front right corner of the house to 3' of fill in the left rear corner. Settlement can easily occur if the soil under the slab is not compacted properly. Usually, the amount of settlement will be proportional to the depth of fill. This explains why most of the slope and crack problems are in the left rear part of the house. The sewer line under the slab may also be contributing to the settlement. The trench may not have been backfilled very well and is therefore settling, or possibly, a leak has developed in the sewer line. Running water can gradually undermine the floor slab by carrying off soil particles. In my opinion, a break in the sewer line is a possible cause of the settlement, but not a likely cause. You can rule out a sewer line leak by having the pipe pressure-tested by a plumber. The exact cause of the settlement can be determined by boring through the concrete slab in several places and testing the density of the underlying soil. This would mean hiring a geotechnical engineering firm at considerable expense. In residential structures, the cost of acquiring this information is usually not justified. | Top | Table of Contents | RECOMMENDATIONS The floor slab can be raised back into a level position by pressure grouting. With this procedure, 2"" diameter holes are usually made in the slab at 3' x 3' or 6' x 6' grid points. Next, a cementitous grout is pumped into the holes at a pressure of about 5 -10 psi. The pressure lifts or ""floats"" the slab back into its original position. The uplift force is high enough to also lift the walls, door frames, and kitchen cabinets at the same time. The cementitous grout hardens and holds the slab in position. There are several companies in Charlotte that specialize in this type of work. The cost to pressure grout a floor is about $10 per sq. ft. After the pressure grouting work is complete, it is usually necessary to patch cracks in the walls, adjust doors and cabinets, and repaint and repaper the walls. One disadvantage of pressure grouting is that there is the chance of future settlement due to further consolidation of the fill under the slab. For this reason, the companies that do this work will not provide a warranty against future settlement, unless augered piers are installed along with the grout. This greatly increases the cost of repairs, as piles are usually about $600 - $1000 apiece. Usually, they are not installed. You may want to consider having a topographic survey done on the slab-on-grade to accurately determine which areas of the slab have settled and how much. The grouting company can use this information to plan the pressure grouting work. In addition, this information will give you better control of the work and expense. The cost to have us to do a laser survey and provide a topography map of the first floor would be $500. If you do decide to pressure grout the floor, you may want to consider rerouting the sewer line around the outside of the house before doing this work. If left in place, there is a risk that the pressure grouting will damage the pipe. In addition, because of the potential for leaks or stoppages in the pipeline, it is better if the sewer line is located where it can be easily accessed. Before making any decisions concerning the repairs or having our firm do a topographic survey, I suggest that you have one or two pressure grout companies look at the problems. Their advice can be valuable because they deal with similar problems all the time. | Top | Table of Contents | William A. Stanton, P.E. | Top | Table of Contents | | Structural Inspections | Residential & Commercial Services |