GARAGE FLOOR SETTLEMENT

Description of the Property

The house is a two-story wood frame structure with a gable roof and crawl space (photo 1). It has a two-car attached garage. The house was built 1993. The lot slopes moderately from the street to the house (about 50 feet), then it pitches downhill sharply. The sill-to-footing height of the house foundation wall varies from about three feet high in the front of the house to over eight feet high in the rear. The sill-to-footing height of the garage foundation wall varies from about two feet in the front of the garage to six feet in the rear.

The house and garage foundation walls are constructed of concrete blocks and brick. The thickness varies with the wall height. The house foundation walls have concrete block pilasters that are spaced about six feet apart. The pilasters support spandrel girders. The top of the garage foundation wall is four inches thick. Mr. Waterhouse told us that he observed the foundation wall during construction. He thinks that the lower part of the wall is thicker than four inches. Without drilling into the wall, we cannot tell where the wall transitions to a thicker wall. The garage floor is a slab-on-grade of unknown thickness.

Purpose of the Investigation

There are several problems with the garage, drainage around the house, and the driveway. The purpose of this investigation is to:

1. DESCRIBE THE PROBLEMS.

2. DETERMINE THEIR CAUSES.

3. OUTLINE POSSIBLE SOLUTIONS.

4. ESTIMATE REPAIR COSTS.

Method of Investigation

The evaluation was conducted by William A. Stanton, P.E. Mr. Stanton is a professional engineer with knowledge in civil and structural engineering, building inspection, and construction practices. Mr. Stanton was assisted in the investigation by Mr. and Mrs. Waterhouse.

The evaluation consisted of observing the house site, adjacent contours, exterior of the house, crawl space under the house, the garage and driveway. Photos were taken of the deficiencies and other salient features. Measurements were made on the depression in the garage floor and the crack in the garage foundation wall. The visual evidence was supplemented with information that was obtained from Mr. Waterhouse about the history of the defects and what he remembered about the way the house was constructed.

Defects in the Garage Floor Slab

The middle of the garage floor slab has dropped approximately 1-1/2 inches (photos 4 and 5). A crack runs down the middle of the floor from the front of the garage to the back of the garage (photo 6). The extent of the depression is evident from the shape of the light under the garage door (photo 4). The depth of the depression is illustrated in photos 5 and 6. A taut string line is being pulled across the floor. The three 1/2 -inch blocks beside the string indicate the depth of the depression.

Mr. Waterhouse first noticed the crack and depression in the slab about 18 months ago. The suddenness with which the slab caved in indicates that the fill under the slab receded, creating a void under the slab. The support under the slab diminished to a point where the slab could no longer support the weight of a car. Mr. Waterhouse reported that the condition seems to be getting worse. The slab caved in about six months after Mr. Waterhouse noticed that there was a void under the driveway slab by the entrance to the garage (photo 2).

Defects in the Garage Foundation Wall

The top eight courses of brick at the outside corner of the rear garage foundation wall have cracked and moved outward. Mr. Waterhouse discovered the condition about 18 months ago. The crack goes through three of the bricks. This is an indication that much force has acted on the corner of the foundation wall. Other than right at the crack, the foundation walls are plumb.

Defects in the Drainage System

Because the lawn and driveway slope toward the house, surface water is trapped in the corner between the house and garage (photo 1). Water from the house and garage roofs also runs into this area. Mr. Waterhouse explained that he was aware the water could not drain out of the corner. He assumed that all the water was dissipating into the soil next to the foundation, because a lot of water came into the crawl space at this corner of the house whenever it rained. The water mostly entered around the hole where the foundation drain penetrated the wall. Also, water leaked out of the perforations in the bottom of the drain pipe. Mr. Waterhouse also noted after a heavy rain that the slab in the garage seemed to darken as though it were taking on moisture. He also observed moisture on the bottom three feet of the rear garage foundation wall after it rained.

About two years ago, Mr. Waterhouse decided he should do something about the water that was going into the ground at the corner of the house and garage. He became concerned that the water might eventually cause harm to the house and garage foundations. To correct the problem, Mr. Waterhouse dug up the foundation drain at the corner of the house and garage and installed a vertical drain to carry off the surface water. (He actually dug up the drain twice because his first efforts at correcting the drainage problems failed to completely fix them.) He connected the drain to the existing foundation drain. In the process of uncovering the foundation drain, he noted that it was pinched. Mr. Waterhouse repaired this obstruction, grouted around the pipe where it penetrated the foundation, and turned the pipe 180 degrees so that water would not leak out of the perforations. After these repairs were made, the ground in the crawl space dried up and the signs of moisture on the garage slab and foundation wall went away.

Defects in the Driveway Slab

There is a transverse crack (right angles to the centerline) in the concrete driveway slab near where the driveway meets the street. The crack is about 1/4 to 3/8 inches wide and irregular. Mr. Waterhouse said that the crack has been there for some time.

Garage Floor Slab

The probable cause of the slab cave-in is consolidation of the fill under the slab due to poor compaction and the effects of water getting under the slab.

The degree of consolidation in soil depends on four factors:

• The nature of the soil.

• How well it has been compacted.

• The depth of the fill.

• The weight on the fill (poorly compacted soil will consolidate under its own weight).

Mr. Waterhouse indicated that the soil used in the fill was obtained from the site, therefore it is likely that it contained high amounts of silt and clay. Clay and silty soils can be difficult to compact well, especially if the moisture content is high. Mr. Waterhouse indicated that the fill contained pieces of wood and other debris from the site. Fill that contains deleterious material cannot be compacted well, and the organic material may rot and cause voids in the soil. The depth of the fill under the garage floor ranges from approximately one foot to six feet. Several inches of consolidation can occur in a fill with these depths when the fill is lightly compacted. Now that the slab has cracked, the weight of the car is also acting on the fill. Additional weight on a fill tends to accelerate the consolidation process.

There is evidence that surface water got under the garage floor through the hole at the entry way or by some other means. If the fill under the garage slab in fact settled, a void would have been created between the underneath side of the slab and the top of the fill. This would give water a place to go. Running water could then wash out the soil under the slab, depositing it in voids in the uncompacted fill. The water could have also saturated the fill and caused it to further consolidate by lubricating the soil particles so that they moved closer together.

Garage Foundation Wall

The crack in the foundation wall was probably caused by torsion forces acting on the foundation corner. This is where such forces tend to be relatively high. Recognizing that foundation walls must be able to withstand torsion forces as well as axial loads, building codes officials created strict specifications for the thickness of foundation walls. The North Carolina State Building Code requires that unreinforced solid masonry walls have a thickness of eight inches when the depth of unbalanced fill is five feet. Unreinforced hollow masonry walls must be ten inches thick. The foundation wall where the crack developed is approximately five feet high. Evidence suggests that the top portion of the wall is only four inches wide. If so, the wall does not meet the strength requirements of the Building Code. In our opinion, inadequate wall thickness is the reason why the wall cracked.

Drainage System

When the house was constructed, there were no provisions to drain off the surface water that landed in the corner between the house and garage. The builder should have either graded the area so that surface water would drain out of this area or installed a drain system to lead it away. The owner had to correct the problem by connecting a surface drain to the foundation drain.

The foundation drain had a kink in the line so that it did not function well. Also, water that accumulated at the base of the foundation came into the crawl space through the hole in the foundation wall that had been made for the drain pipe. Peculiarly, the foundation drain was installed so that it came through the crawl space rather then going around the outside of the foundation wall. Since the bottom of the drain pipe has perforations, the water in the drain emptied out into the crawl space.

Driveway Slab

The crack in the driveway slab is a shrinkage crack. Shrinkage cracks occur in driveway slabs when freshly poured concrete dries out and proper control joints have not been installed to accommodate change in the concrete volume. Unsightly shrinkage cracks such as this one are avoided when proper construction practices are followed.

1. The garage floor slab should be demolished and the soil under the garage slab should be excavated. New fill and a concrete slab should be installed in accordance with the requirements of the North Carolina State Building Code.

2. The garage foundation wall should be rebuilt so that it complies with the North Carolina State Building Code. The corrective work can most easily be accomplished after the soil under the floor slab is removed and the foundation wall is exposed.

3. Work that Mr. Waterhouse performed on the drain system has temporarily taken care of the water problem at the corner of the house and garage. However, directing surface water into a foundation drain system is not a good long term solution. Therefore, we recommend that a separate drain system be installed to take care of the surface water.

4. The section of the driveway slab that is cracked should be removed and replaced with a new concrete slab. Proper joints should be installed to allow for expansion and contraction of the concrete.

The estimated total cost to carry out the above recommendations is $11,000. A cost breakdown can be found in Appendix B.

The observations described in the report are valid on the date of the investigation. The opinions are based on visual evidences of readily accessible areas. No invasive or destructive testing was performed. The investigation was not for the purpose of determining compliance with the North Carolina State Building Code. The report was prepared for the exclusive use of Mr. and Mrs. Waterhouse 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.

Respectfully submitted,

William A. Stanton, P.E.