A reinforced concrete block wall, originally constructed 40 years ago for the Hiawassee Highlands, a single-family residential subdivision, showed rotation of instability that may impact the walking residents adjacent to a busy highway.
The existing concrete block wall is positioned above a lower tiered concrete gravity retaining wall, which was constructed around 1992 as a part of the Hiawassee Road roadway improvement.
A geotechnical evaluation of the failure was commissioned to assess the area’s ground conditions behind the unstable block wall as well as the structural conditions of the block wall and lower level gravity wall. The wall rotation was determined to be the result of lateral earth pressure from the growth of a tree root, which abuts the block wall. The soil borings behind the wall showed fine sands and none of the typical slope failure with a scarp crack or any other superficial manifestation of slope stability.
The localized area where the concrete block retaining wall is bulging out and on the verge of failure poised a potential hazard to pedestrians using the sidewalk and the motoring public using the roadway. The block wall also acts as a retaining wall for the higher elevation residential yards within the Hiawassee Highlands subdivision. The pronounced wall bulging areas correlate to the location of large trees, which are up against the wall. The affected resident 7005 Laurel Hill Drive has indicated that the distress has become progressively worse over time and it is now a significant concern for pedestrians, motorists and the residential properties.
The repair plan calls for:
- The removal of the tree and its root system.
- Demolition of the existing concrete block wall contiguous to the resident at 7005 Laurel Hill Drive.
- Building a replacement concrete block wall. The choice of reinforced masonry wall is also based on the horizontal and vertical working space limitations, especially the constraints from the overhead power lines. Moreover, the existing soil showed loose compaction and cohesion, and these properties extended to more than 16 feet in depth, making construction very challenging.
The construction of this wall had to be carefully planned and designed considering the proximity of the wall to the residential structure and the sidewalk on Hiawassee Road. The water table is deep and does not influence the design; however, the type of soil is a major factor in developing the repair plan since the light-colored sands are dry, of loose compactness and cohesion less, and extend to a depth of over 16 feet. Such sands can shift when, for example, a void is created by the removal of a large tree root, which is similar in effect to the movement toward a void created by a small sinkhole.
In addition, slope instability and sand sloughing are a major concern when the existing wall and its foot are removed for reconstruction. Removal of the wall and footing will leave a 6-foot-high unbraced vertical face of cohesion-less sand with the potential to collapse onto the sidewalk. A collapse at this location can propagate within the sand mass to causing shifting/settlement under the building foundation.
Based on the above concerns, the elements and sequence of the repair plan chosen to mitigate are:
- Possible adverse soil movement, which can cause settlement of the adjacent residential structure at 7005 Laurel Hill Drive, and/or
- Possible soil collapse onto the lower level pedestrian sidewalk — with collateral impact to residential structure due to sand shifting — when the existing wall is removed to make way for the new wall.
Due to all the constraints, the developed construction methodology compiled with the following sequential steps are:
- First the wooden fence in the residential yard was removed and stored for reinstallation.
- Secondly, the soil zone adjacent to the residential structure was chemically stabilized.
- Then the tree and its root system were removed and the removal hole was backfilled.
- The soil zone behind the existing block wall was also chemically stabilized.
- The existing wall/footer was demolished, and a replacement 8-inch-wide, cantilever reinforced masonry wall (minimum 2500 psi unit strength at 28 days), with continuous ladder reinforcing within every second horizontal joint, was constructed. The masonry wall will be anchored to a continuous reinforced cast-in-place footing (minimum 35 feet wide and 12 inch deep), using regular spacing of reinforcing steel vertical dowels. The finish is 5/8-foot stucco finish and exterior grade paint.
- Reinstated residential yard where disturbed by construction. The resident at 7005 Laurel Hill Drive was very elated that the risk was averted. The walking and motoring public were pleased that the wall failure was timely addressed and corrected.
The demolishing and construction of the wall commenced in August 2018 and was completed on October 19, 2018. The final cost for design chemical grouting and wall demolishing and reconstruction came in at $170,000.
Deodat Budhu, P.E., has more than 36 years of experience in consulting, contracting, teaching, research and public works in a variety of civil engineering disciplines. Budhu is the current manager of Orange County, Fla.’s, Roads and Drainage Division and is responsible for the daily administration and management of 235 employees in addition to an annual operation and capital budget totaling $98 million. He holds a Bachelor of Science in civil engineering and a Master of Science in water resources engineering. He is a registered professional engineer in both Florida and Virginia with memberships in several professional organizations and has authored more than 70 technical publications.