On October 11, 1998 Mr. William Stanton, P.E., Criterium-Stanton Engineers, performed a structural inspection of concrete retaining walls that had been constructed around a pool and patio area. Terry Taylor and Mr. Kohl Sturdevant, the pool builder, were present during the inspection. As a result of the inspection, it was decided that the walls should be further investigated for their construction and structural stability.

1.2 Method of Investigation

On April 23, we visited the site and visually inspected the walls. We excavated footings at several locations to determine the footing dimensions and construction. We hired L&T Engineering, a geotechnical engineering firm, to test each wall for the compressive strength of the concrete and determine the type and location of the steel reinforcement. We performed a stability analysis of the upper and lower walls to determine if the walls will fail by either rotating outward about their bases, or sliding. For purposes of the discussion below, all directions are in relation to a person standing at the lower wall and facing the driveway.

2.0 FIELD OBSERVATIONS

2.1 Inspection and Excavation

This 4'-5' retaining wall supports the pool deck and the walkway around the pool. The wall is approximately 60' long, with a 31' center section, and two 15' wings that angle back at 45° (see Photo 1). There is a diagonal crack in the right end of the center section of the wall. It is oriented at an angle of approximately 57° angle from horizontal. It begins as a hairline crack at the base of the wall and widens to approximately 1/8'' at the top (see Photos 2 & 3). There is a second diagonal crack in the left end of the center section of the wall, oriented at an angle of approximately 70° from horizontal. Starting as a hairline crack at the base of the wall, it widens to approximately 1/16'' at the top (see Photo 4). There are also several hairline shrinkage cracks in the wall.

We excavated the footing in the center wall section to determine the footing dimensions (see Photo 5). The toe projects out from the wall approximately 8". The footing is 10" thick. This is agrees with the dimensions given in the concrete contractor’s proposal (see Appendix B).

This 2' high wall supports terraced earth between the driveway and the pool. The wall is approximately 38' long, with a 14' center section, and two wings angled in at a 45° angle, each 12' long (see Photo 6). There are several hairline shrinkage cracks in the wall.

This 3'-5' high wall supports the driveway. The wall is approximately 35' long, extending 20' from the rear steps, parallel to the rear of the house, and then turning away from the pool area at a 45° angle for an additional 15' (see Photos 6-7). We excavated the footing at a location approximately 7' from the steps. The footing broke apart under the pressure of a hand shovel. Several vertical reinforcing rods were uncovered which were not embedded in the wall stem. We could not locate a footing, although we dug to a depth of 2' (see Photo 8).

We dug a second hole 3' left of the angle point in the wall. In this location the toe extends out approximately 18" from the stem, and the footing is 12" thick.

2.2 Concrete Testing and Location of Reinforcement

The strength of the concrete was tested at eight (8) locations. The test results indicate that the strength of the concrete in the walls varies from 2640 psi to 5690 psi. The low tests occurred on retaining wall #2. The contractor’s proposal indicated that 3000-psi concrete would be provided. The steel reinforcement in the walls was located by L&T Engineering at a spacing of 18" - 32" vertically and 16"- 22" horizontally. This compares with the 24" x 24" spacing specified in the contractor’s proposal. The reinforcement has a diameter of 3/8'' and is positioned in the center of the wall, as specified (see the Concrete Contractor's Proposal in Appendix B and the Geotechnical Report in Appendix C).

3.1 Retaining Wall #1

This wall was analyzed as a cantilever retaining wall to assess its ability to retain the soil mass between the wall and pool. A cantilever wall is stabilized against overturning by the weight of the soil on the heel. The heel projects back from the stem.

The type and density of soil behind the wall is important, as this will determine the pressure on the wall. Based on a telephone conversation with the pool contractor, the sequence of construction was as follows:

1. The site was graded level.

2. The retaining wall was constructed.

3. The pool walls were constructed.

4. The excavated material from the pool area was placed between the pool wall and the retaining wall in 6'' lifts and compacted.

Given this information, it can be assumed that the soil has a high density. The soil is a clay-silt mixture. The soil will expand when moist, further increasing the pressure on the wall.

Our analysis shows that the wall fails the test of stability to resist overturning and horizontal sliding (see Appendix D for the Stability Analysis). The minimum recommended safety factor for overturning is 1.5. The safety factor is the ratio of the forces resisting overturning to those causing overturning. The forces resisting overturning include the weight of the wall and the weight of the soil on the footing. The soil pressure against the wall generates the overturning forces. The safety factor for retaining wall #1 against overturning is 0.93.

The minimum recommended safety factor for sliding is 1.5. The safety factor for sliding is the ratio of the forces resisting sliding to those causing sliding. The force resisting sliding is the weight of the wall on the soil, including the weight of the soil on the heel. The force causing sliding is the soil pressure on the wall. The sliding safety factor for retaining wall #1 is 0.45.

The reason for the low safety factor is that wall #1 is constructed as a building foundation wall rather than as a retaining wall. The base (footing) is not wide enough to provide the necessary resistance to overturning and sliding. The above diagram illustrates the difference in the two types of walls. A foundation wall is designed to handle vertical loads only. A retaining wall is designed to handle both lateral and vertical loads.

3.2 Retaining wall #2

The soil mass behind wall #2 is loosely compacted and is not exerting any significant pressure on the wall. Although the concrete strength tests in the wall are below 3000 psi construction specifications, this should not materially affect the structural capacity of the wall. In our opinion retaining wall #2 is strong enough to properly hold back the soil.

3.3 Retaining wall #3

This wall was analyzed as a gravity retaining wall. There is no evidence that the wall is attached to a footing. The pool contractor stated that the wall was formed on one side only due to a lack of space between this wall and the concrete block wall shown in photo 7. The wall is estimated to be 15" thick at the base. A gravity wall gains its stability against overturning and sliding from the weight of the wall.

This wall also failed the stability tests for resistance to overturning and sliding. The safety factor against overturning is 0.66, and sliding is 0.49. The primary cause of the low safety factors is that the wall is not heavy enough to resist the horizontal pressures generated by the soil mass behind the wall.

Retaining wall #1 is evidencing failure with the development of angular cracks at either end of the center wall section. The diagonal orientation of the cracks indicates that shear/torsion forces are acting on the wall. The origin of these forces may be differential settlement of the wall footing, overturning of the wall about its base, sliding of the wall, or a combination of all three. Although the wall is nearly vertical, gradual movement of the wall is likely to continue, causing the present cracks to widen, and possibly more cracks.

Retaining wall #3 does not show any indication of failure at this point. However, the stability analysis indicates that the wall will eventually fail when soil pressures fully develop on the wall. It should be noted that surcharge loads (additional loads generating pressure on the wall) were not included in the analysis. The proximity of the wall to the end of the driveway will result in surcharge loads from parked vehicles.

4.0 CONCLUSIONS & RECOMMENDATIONS

Retaining wall #1: The narrow footing width is the primary cause of the failure. Also, the steel reinforcement in the stem is not sufficient to resist the forces generated by the soil pressure. It is impractical to consider extending the heel behind the wall. Lateral stability against overturning and sliding can only be accomplished by extending the toe of the footing outward from the wall and constructing another stem in front of the existing wall.

Estimated Construction Cost: $10,000-$15,000.

Retaining wall #3: The lack of sufficient weight is the primary reason for an anticipated failure. The proximity of the lower masonry wall limits the solutions to the problem. One possible solution is to construct a footing and a second stem. This would effectively convert the wall from a gravity retaining wall to a cantilevered retaining wall.

Estimated Construction Cost: $3,000-$4,000.

The observations described in the report are valid on the date of the investigation. We prepared the report for the exclusive use of Mr. Terry Taylor and his 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 this 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 not to be considered a warranty of condition, and no warranty is implied.

The cost estimates are based on our general knowledge of building systems and the construction industry. When appropriate, we have relied on standard estimating sources, such as Means Building Construction Data. However, for items for which we have developed cost estimates (for example, structural repairs), no standard guide for developing such estimates exists. We have not obtained competitive quotations or estimates from contractors, as this also is outside the scope of the project. The actual cost to remedy deficiencies that we have identified may vary significantly from estimates and quotations received from contractors.

Respectfully submitted,

William A. Stanton, P.E.