New House Constructed on Unstable Soil Settles GENERAL INFORMATION Date of Site Inspection: March 29, 2000 @ 10:00 A.M. Weather Conditions: Sunny, 80° Name of Owner(s): Mr. and Mrs. John Miller People Present During the Field Inspection Mr. John Miller and William A. Stanton of Criterium-Stanton Engineers, Mr. Joe Miller, Dave Miller, and Mrs. Miller. Source(s) of Information Field observations. Interview with John Miller. A.B. Chance Company website. Literature about underpinning residential foundations. Telephone consultations with Eric Jones, P.E. of Soil Pressure Grouting, Inc., Joseph Roberts of Roberts Constructions Company, and Ron Smith, P.E. of J&S Associates, Geotechnical Engineers. Purpose of the Investigation Determine the extent of the damage to the house. Determine the cause of settlement. Make recommendations on how to restore the foundation to new condition. Orientation Unless otherwise stated, all directions (left, right, rear, etc.) are in relation to an observer facing the front of the house. Description of the Property The house is a new two-story wood-framed structure with an attached garage. It has a floor area of approximately 1500 square feet. It has vinyl siding and a slab-on-grade foundation. It is situated on a lot that slopes from front to back at about a 10% grade. There is a 20' deep ravine in the back of the lot. The foundation wall appears to be 8"" thick (4"" brick veneer with 4"" concrete backer blocks). It is about 3' high in the rear of the house and 2' high on the sides (Photo 3). The footing is poured concrete. It appears to be about 8"" thick. It probably is not reinforced. The floor slab is not supported by or integral with the foundation wall. There is about 3' of fill under the slab. Background Mr. and Mrs. Miller purchased the house from Nice Homes in August 1999. In April 2000, they discovered that the foundation under the right rear section of the house was settling. They immediately reported the problem to Nice Homes. Nice Homes contacted American Concrete who sent a representative out to the site. American Concrete recommended that the foundation wall be lifted back into position with A. B. Chance Co. helical piers. The representative told Mr. Miller that he could not guarantee that the wall would go back into its original position, nor could he guarantee the future performance of the wall. The performance of the helical piers is the only thing he would guarantee. In addition to having American Concrete look at the problem, Nice Homes had Earth Technology take soil samples near where the cracks were located. At this point, Mr. and Mrs. Miller decided to seek the advice of a structural engineer. They retained us on May 17. Method of Investigation Upon arriving at the site, I interviewed Mr. Miller about the history of the problem. I then looked at the two cracks in the foundation and the holes that Mr. Miller had dug at the crack locations. I removed a clay chunk from one of the holes and rolled it between the palms of my hands to get an idea of its moisture content and plasticity. I photographed the foundation and exterior of the house, including the ravine in the rear of the property. I went into the kitchen area and inspected the condition of the floor slab, walls, and rear door. After returning to my office, I researched information about underpinning residential foundation walls. I also consulted by telephone with Ron Smith, Joe Roberts, and Eric Jones about the problem, the needed information, possible methods of repair, and the best way to proceed. | Top | Table of Contents | OBSERVATIONS The house is located on the lower part of a 300' long side hill. There is a deep ravine at the bottom of the hill. The site gives the appearance that marginal soils and springs may be present. There is a v-shaped crack in the right foundation wall about 10' in from the right rear corner. The crack is about 5/8"" wide at the top of the wall and tapers to about 1/16"" wide at the footing (Photos 2 & 3). There is another v-shaped crack in the rear foundation wall, about 20' from the right rear corner. The crack is about º"" wide at the top of the wall and tapers to hairline width at the footing (Photo 1). There is a 1"" space between the top of the foundation wall and underneath side of the rear doorsill. At this location, the foundation wall leans outward slightly. The footing projects out from the foundation wall about 2' at both hole locations (Photo 3). We dug next to the footing under the rear wall and found it to be about 8"" thick. The soil in the hole that was dug beside the right foundation wall contains much moisture and large chunks of clay. The clay is highly plastic and can be rolled into a small diameter thread before crumbling (Photo 4). The rear door has started to bind, and the rear steps had to be removed to prevent them from being damaged due to the downward movement of the foundation. I did not observe any evidence that the floor slab has settled. In addition, I did not observe any settlement cracks in the kitchen walls. The ground slopes from the street toward the house. The site is graded so that most of the runoff flows around the sides of the house. Water from a roof downspout collects in a low spot in the concrete driveway. The water can seep into the joint between the concrete pavement and house foundation. | Top | Table of Contents | FINDINGS AND CONCLUSIONS There is a good chance that the house is in an area where there are problem soils and springs. The Builder may have had soils borings performed at the site during the planning stages of construction of the subdivision. The extra wide footing that we observed may be standard practice for the homes in the subdivision. It is possible that a soils engineer recommended wide footings as a way of overcoming the compressible soils. It is also possible that the wide footings are the result of a field decision made by the construction supervisor when soft soils were unexpectedly encountered. The v-shaped cracks in the foundation indicate that the right corner of the house is settling and rotating about axes at the bases of the cracks. The widths of the tops of the cracks indicate that the corner of the foundation has dropped about an inch. The gap under the rear doorsill confirms this conclusion. The settlement is categorized as serious since a large amount of movement has occurred in a short time. If nothing were done, the foundation would probably settle a lot more. In addition, it is probably only a matter of time before other sections of the foundation and the floor slab are similarly affected. When clay can be rolled into a small diameter thread, it is highly plastic and compressible. Natives of the area often refer to this type of clay as ìbull tallowî. Experienced builders know that special precautions must be taken when it is present, because of the potential for settlement problems. Apparently, the floor slab has not settled yet. Eventually, it probably will, since subsidence of the slab usually accompanies foundation movement of this magnitude. The reason the floor slab may not have settled is that it may be bridging over the voids that have formed under it. Since the soil under the edge of the slab has been disturbed, it will probably be necessary to pressure grout under the slab once the foundation wall has been repaired. The proposal by American Concrete to simply lift the damaged section back into position with helical piers is not likely to produce the desired results. First, there is the risk that the unreinforced footing will not be able to withstand the high concentrated forces that will develop during the lifting operation, and the footing and wall will crack more. Secondly, a differential settlement condition is apt to be created where the section of the wall that is underpinned stays put while adjacent wall sections settle. It is not a good idea to have very solid support under one section of the foundation and marginal support under other sections. Differential settlement is less likely to occur if the foundation walls are uniformly supported. It will be important to take soil borings in the soil on either side of the damaged wall section to determine how solid the bearing is. If the bearing capacity is low, and the underpinning option is chosen, we will probably recommend underpinning the entire rear wall and maybe parts of the sidewalls to prevent the possibility of differential settlement. In addition, we will probably recommend that a reinforced concrete grade beam be installed under the existing foundation walls and footings to ensure that they are not overstressed. A. B. Chance Co. recommends this procedure when helical piers are used to support new foundations (see Appendix B). If it can be determined that the only place the soil is bad is where the wall has settled, and if it is practical to remove all the bad soil, probably the best way to restore the wall to new condition is to demolish the damaged section and replace it with a new wall. This would entail shoring up the house superstructure. The advantage of this method of repair over underpinning the wall is that the risk of differential settlement is reduced. | Top | Table of Contents | RECOMMENDATIONS It is important to be certain about the bearing capacity of the soil under the rear and side walls. This information will be the main determinant in deciding how to best repair the foundation and prevent a recurrence of the problem. The information can only be obtained by making several borings around the perimeter of the foundation. In addition, a soil boring should be made through the floor slab in the kitchen. It is important to know if there are any voids under the slab, the type of fill material that was used, and its density. This information will be needed in order to determine if the slab needs to be pressure grouted, and if so, how to do it. J&S Associates indicated to us that it would cost about $2000 to perform the borings, run laboratory tests on the soil, and write a report. The results of the soil tests that Earth Technology performed and any other information about the soils that the Builder has in his possession could be very useful to us. In addition, it could reduce the cost of getting new soils information. We would also like to talk to the person or persons who supervised the construction of the foundation. They should be able to provide information about the soil conditions that were encountered and why the wide footing was constructed. As mentioned earlier in the report, one way to repair the foundation wall is to replace the damaged section with a new wall. If this option is chosen, we recommend that you have us prepare a repair plan and specifications for the contractor to follow. This will ensure that there is no misunderstanding about the repair procedure to follow. Our fee for preparing the plan and inspecting the construction work will be $1500. A second repair option is to install a grade beam and helical piers under the existing foundation and lift it back into position. If this option is chosen, we recommend that you have the underpinning contractor prepare a plan and specifications for our approval showing how the work is to be done. Our fee for reviewing the repair plan and inspecting the work will be $1000. Even if the repair procedure is carefully planned and executed, the foundation may still settle and crack. In addition, when you sell the house, you will have to disclose that extensive repairs were performed on the foundation. As I am sure you are aware, these factors could potentially diminish the value of your property. | Top | Table of Contents | LIMITATIONS The observations described in the report are valid on the date of the investigation and were made under the conditions noted in the report. We prepared the report for the exclusive use of Mr. and Mrs. Brad Miller and their successors and assignees. Criterium-Stanton Engineers does not intend any other individual or party to rely upon the report without our express written consent. If another individual or party relies on the report, they shall indemnify and hold Criterium-Stanton Engineers harmless for any damages, losses, or expenses they may incur as a result of its use. The report is limited to the visual observations we made during our inspection. We did not remove surface materials, furnishings or equipment, undertake any digging or excavation, or perform any destructive or invasive testing. Accordingly, we cannot comment on the condition of systems and components that we could not see. In addition, we are not responsible for conditions that were not within the scope of our services at the time of inspection. We did not assess the stability of the building or the underlying foundation soil since this effort would require excavation and destructive testing. We did not perform any computations or engineering analysis as part of this evaluation, nor did we conduct a comprehensive code review. This report should not be considered a warranty of condition, and no warranty is implied. | Top | Table of Contents | William A. Stanton, P.E. | Top | Table of Contents | | Structural Inspections | Residential & Commercial Services |