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2 WALL DESIGN
The wall is about 175 m long and typically ranges from about 4 m to 8 m high. A
loading dock and access drive for a large retail building are located at the top of
the wall. The wall was designed using the computer program and methodology
developed by the National Concrete Masonry Association (NCMA) for external
stability (sliding, overturning, bearing capacity) and internal stability (geogrid
overtensioning and pullout). The computer program PCSTABL6 was used to
analyze for global and compound failures. In addition, the geogrid and block
manufacturer performed independent analyses using similar methodologies to
confirm the design.
The project is located in the Piedmont Physiographic Region of the Eastern
United States. A geotechnical engineering study conducted by a geotechnical
consultant indicated that the on-site soils generally consisted of sandy silts and
silty sands derived from weathering of the underlying phyllitic limestone. The
wall design was based on using the on-site soils for backfill and specifically
required silty sand material with a unit weight of 18.8 kN/m
3
, a minimum
effective friction angle of 32
8
and effective cohesion of 0 kPa. A PVC coated
polyester geogrid with a long-term allowable design strength of 12.9 kN/m was
selected. A coefficient of interaction value of 0.9 was used for the grid on backfill
soil. The segmental blocks consisted of dry stack, pinless, concrete masonry units
about 0.2m high with a design offset that achieved about a 6
batter at the wall
face. Drainage was provided by a perforated pipe and weep hole system with a
blanket drain, and a 1.2-m-thick crushed stone layer behind the block facing.
A variable geogrid reinforcement layout was used with a spacing of about
0.37m in the lower sections, about 0.55m in the middle sections, and a maximum
spacing of about 0.73m at the top of the wall. Geogrid lengths typically ranged
from 4.3 to 4.9m, resulting in a height-to-length ratio of at least 0.6. A surface
live load surcharge of 12.0 kPa was used to model traffic loading at the top of the
wall. The design reinforcement layout with the above parameters resulted in a
minimum factor of safety of 1.5 for internal and external stability and 1.4 for
global or compound stability. A typical wall section is shown in
Fig. 1
.
8
3 FIRST WALL FAILURE
About 5 months after construction of the wall, but prior to opening of the retail
store, a section of the wall collapsed. Immediately after the failure, the facing
blocks and drainage gravel were piled up at the base of the wall and the reinforced
soil mass was standing vertically with lengths of geogrid hanging from the soil.
The block facing for the portions of the wall adjacent to the failed section curled
outward, away from the wall. A precast concrete stormwater drop inlet was
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