The Foundation of a Home is Underpinned and Piers are Added, But House Continues to Settle GENERAL INFORMATION Date of Site Inspections: November 17, 1999 Prepared for: Robert Hill, Wallace Builders | Top | Table of Contents | BACKGROUND Sometime before 1996 (probably many years ago) cracks developed in the interior and exterior walls. In early 1996, the Jones decided to undertake major interior renovations and wanted the foundation stabilized so the cracks would not occur again. They hired your firm to do the work. Your firm consulted with Soils Engineering Inc. to determine the cause of the cracks and devise a foundation repair plan. Soils Engineering Inc. attributed the interior cracking to overspanned floor joists and recommended that drop beams and additional piers be installed under the joists and girders. To stop the cracking in the brickwork on the front elevation, they recommended that the exterior wall footing be underpinned with caissons. About this time, Subsurface Engineering made hand auger soil borings at three of the house corners (Appendix A). The borings indicated that the bearing capacity of the soil was good. Based on this information, Soils Engineering Inc. concluded that, in addition to overspanned joists, the cause of the cracking could be due to undersized footings. They indicated that the additional piers should take care of the problem, since they would help distribute the load. New piers, footings, drop beams, and caissons were installed in late summer 1996. | Top | Table of Contents | DESCRIPTION The house is a 1½-story ranch-style structure with a floor area of approximately 2500 sq. ft. The original house was constructed in the 1950’s. A sunroom was added later. Within the past year, your firm constructed a new ell and patio and renovated the interior. The building is a wood-framed structure with brick veneer cladding. The house has a pier and curtain wall foundation. The sunroom is supported on a suspended slab. The house has a gable roof with rafters. The roof is partly supported by interior walls. The lot is relatively flat and is located in the Briar Creek floodplain. | Top | Table of Contents | METHOD OF INVESTIGATION You and Jason Smith met with me in my office on October 19 at 1:30 PM. During this meeting, you described the history of the problems, the investigative and design work performed by Soils Engineering Inc., and the construction work that your firm did. You provided me with copies of documents prepared by Soils Engineering Inc. and Subsurface Engineering. The documents included their diagnosis, repair designs, and soil borings. I reviewed this material and met you at the site the next day at 10:00 AM. In your company, I inspected the interior walls, floors, and ceilings of the house, the exterior walls and foundation, the crawlspace under where the cracks were occurring, and the lot. We talked briefly with Mrs. Jones during this visit. After visiting the site, I researched the Mecklenburg County Soil Survey Maps. In addition, I reviewed information about expansive soils, and consulted with a geotechnical engineer and an engineer with a foundation underpinning company. | Top | Table of Contents | OBSERVATIONS There are several diagonal cracks in the bedroom walls. Mostly, the cracks are less than 1/8"" wide and occur over the corners of doorframes. The most significant crack that I observed was in the office on the second floor. This crack is about 3/16"" wide. The only other cracks that I observed were a few hairline cracks in the ceilings and walls that I attributed to thermal movement. I walked around the outside of the house and inspected the exterior brick walls. In general, I thought the walls looked good. I did observe two or three 1/8"" wide step cracks in the brick veneer on the left and front elevations. According to you, these are new and have developed since the walls were repaired. I inspected the crawlspace under the left side of the house. My impression was that the crawlspace looked good. There was no evidence of excessive moisture. The old piers and girders were in good condition, and the new piers and drop beams were well constructed. I was surprised, however, to find that in almost all cases the new piers were not in contact with the girders. Although there were no discernable gaps between the piers and girders, the shims were loose. I observed that the neighborhood is adjacent to Briar Creek, quite level, and subject to flooding. | Top | Table of Contents | FINDINGS AND CONCLUSIONS The cracks in the bedroom walls are the types of cracks produced by vertical displacement (up or down movement). They do not have any of the characteristics of thermal induced cracks. Because the cracks are generally located above girders and piers, it is probable that they were caused by the displacement of the piers and not by deflection of the floor joists. The cracks in the brick veneer are typical of cracks that are caused by vertical displacement of the perimeter wall footings. Cracks such as these can be the result of downward or upward movement of the footings. The interior and exterior cracks seem to be mostly located in the left front of the house. It may be because the nature of the soils under the house varies somewhat from one end to the other. The looseness in the shims can be attributed to four possible causes. First, the gaps could be caused by the new piers settling more than the old piers. It is possible that the new piers are settling because the soil under these piers is not as compact as the soil under the old piers. Secondly, the gaps could be caused by the old piers rising more than the new piers. If the soil swells, it will tend to push up the old piers more than the new piers because the soil under the new piers is more compressible. Third, the gaps could be caused by shrinkage of the mortar in the new piers or shrinkage of the wood drop beams. Finally, the gaps could be caused by differential settlement due to the existence of pockets of poor soil under the house. It is difficult to explain why only the new piers are affected, however. The first set of soil borings indicated that the soil had good bearing capacity, at least down to a depth of 5'. Normally, very little settlement occurs in house foundations that are founded on soil this good. The second set of borings, while not as good, still indicated suitable bearing capacity. I asked James Monroe, Head Engineer of FAS Associates, a local geotechnical-engineering firm, to evaluate the boring logs. Appendix B contains information that I obtained from soil survey maps of Mecklenburg County. The maps were published by the United States Department of Agriculture, Soil Conservation Service (SCS). They indicate that the soils in this area are Monacan soils with Arents. These soils are in flood plains and contain loam, clay, and silt. The soils are alluvial (stream deposited) and residual (created in place through weathering). These types of soils also contain 30% - 40% fill materials (Arents) that have been deposited by man. According to the SCS information, the fill depths vary from 20"""" to 8'. The fill could contain anything - organic material, stumps, trash, or bull tallow clay - depending on the source. It is likely that the soil under and around this house is partly fill. If so, pockets of organic material or bull tallow clay may be present in the soil. Because only a few shallow borings were made, the pockets of poor soil may have been missed, and the soil borings may not accurately represent the soil. If organic material is present in the soil under the house, the soil will be highly susceptible to consolidation and settlement. If bull tallow clay is present, the soil will be subject to periodic swelling and shrinking with changes in the moisture content. It is possible that removing the large tree caused or contributed to the foundation movement. Large trees extract a lot of water from the surrounding soil, causing it to dry out and even desiccate during periods of drought. If the soil is highly plastic (bull tallow clay, for example), it will shrink. If the tree is removed, the moisture gradually returns, causing the clay soil to swell. I have included a booklet published by the American Society of Civil Engineers with my report. The booklet is titled, ""So Your Home Is Built On Expansive Soils."" The purpose of the booklet is to help homeowners understand why expansive soils shrink and heave and how to mitigate this problem. It is hard to understand how the excavation that was made for the new patio could have caused the settlement. For one thing, the excavated soil was replaced immediately with material of similar weight. For another, the theory does not fit well with the manner in which stresses from foundation loads are transferred into earth. Subsurface stresses generally spread out uniformly with depth, the stressed area increasing at a slope of 1' horizontally for each 2' of depth as measured from the edges of the footing. It is possible that cause of the cracks is settlement due to the excessive weight of the house. If the soil is uniform and reasonably good, adding additional piers and footings until the settlement stops may be the best way to address the problem. On the other hand, if pockets of deleterious material in the soil caused the settlement, this solution may not stop long-term differential settlement. If the expansion and contraction of the soil caused the cracks, adding more piers and footings will not help. In this case, ways must be found to control the moisture content of the soil. The solution may be as simple as waiting for the soil to stabilize now that the patio has been constructed and the tree is gone. | Top | Table of Contents | RECOMMENDATIONS In my opinion, there is uncertainty as to what is causing the foundation movement. For this reason, I recommend that no more work be done on the foundation until more is learned about the nature of the movement and the soil conditions. I do not think that the risk and cost of further damage to the interior and exterior walls warrants spending thousands of dollars on foundation modifications that are unlikely to correct the problem. It is possible that the problem will fix itself when the soil under the house has had a chance to stabilize. Cutting the tree, constructing the patio, and installing subsurface drainage drastically altered the moisture content of the soil. In time, the moisture content of the soil should stabilize at a new level. When it does, the foundation movement may also stabilize. It would be very useful to know the magnitude and direction of the foundation movement. This information can be obtained by periodically measuring the elevation of floor and foundation with a laser-leveling instrument. A year of measurements should suffice. If the elevations increase or fluctuate, we will know that something other than settlement is occurring. The five soil borings that were done should have provided enough information about the soil conditions to correctly diagnose the cause of the foundation movement. In this case, however, the soil is more unpredictable than usual. For this reason, I think more soils consulting work is warranted. I recommend that FAS Associates be retained to make additional borings and perform more laboratory tests on the soil. Of critical importance will be whether we can rule out expansive soil as a causative factor. We should also be able to determine if the house can be successfully supported on a shallow foundation system or if other measures such as piles or chemical grouting will be required. Until more is learned about the foundation movement and soil, I am reluctant to make corrective recommendations. After all is said and done, it may turn out that the Soils Engineering Inc. solution is the most practical and cost effective way to proceed. Alternatively, I may recommend other measures based on new information and the advice of James Monroe. | Top | Table of Contents | William A. Stanton, P.E. | Top | Table of Contents | | Structural Inspections | Residential & Commercial Services |