W Sandy Lk Addn-SY040811 I
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I Below-Grade Walls and Retaining Walls
Below-grade walls will be subject to lateral loads associated with lateral earth pressures. The
magnitude of the earth pressure will be a function of:
the type and compaction of backfill behind the walls within the"active" zone;and
• the allowable rotation of the top of the wall.
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The active zone is the wedge of soil defined by the surface of the wall and a plane inclined 35°
from the vertical assin the base of the wall.
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Considering backfill using site-excavated materials compacted in lifts to the density and
Imoisture outlined in the Earthwork section, the lateral-load soil pressures can be estimated
based on an equivalent fluid pressure of 52 pounds per cubic foot (pcf) "active" pressure or 74
pcf"at-rest"pressure.
Alternatively, imported "select" fill may be used as backfill in the "active zone". Considering
"select" fill, lateral load pressures can be estimated based on an equivalent fluid pressure of 35
pcf, "active"pressure or 60 pcf"at-rest"pressure.
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These lateral pressures are applicable for horizontal surface grades and non-surcharged, drained
1 conditions. Design values do not incorporate specific factors of safety.
dA drainage system should be installed behind the base of the below-grade and retaining walls to
Ilimit development of excess hydrostatic pressures.
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1 The drainage system behind retaining walls should consist, as a minimum, of 12-inch by 12-
inch pocket drains spaced 15 feet on-center,installed near the base of the wall.
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Fill in the pocket drains should consist of durable crushed stone such as ASTM C-33, Size 67
1 or coarser, wrapped in filter fabric (ADS 600•or equivalent). Backfill around the gravel drain
should consist of site-excavated soils or "select" fill. A compacted clay cap is recommended
within the upper two feet of the surface to limit surface-water infiltration behind the walls.
Two types of drainage systems may be considered for below-grade walls, a gravel drain or a
pre-manufactured drain such as "Miradrain 6000." Use of either system should reduce the
1 potential for development of hydrostatic pressure. The flow line of the pipe should be a
minimum of eight inches below the bottom of the wall and designed to drain by gravity flow.
Backfill around the drainage pipe should consist of at least 2 feet of clean, free-draining,
durable crushed stone such as ASTM C-33, Size 67 or coarser, wrapped in filter fabric (Mirafi
140-N" or equivalent). Backfill around the gravel drain should consist of site-excavated soils
or select fill. •A compacted clay cap is recommended within the upper two feet of the surface to
limit surface-water infiltration behind the walls. All below-grade walls should be waterproofed
prior to placement of the drainage medium and backfill.
Retaining walls may be founded on spread or continuous footings placed a minimum of 18
inches into undisturbed, on-site soils or compacted and tested fill. Footings should be
proportioned for a maximum bearing pressure of 3,000 pounds per square foot (psf). Some
movement of the footings and walls should be anticipated.
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Passive resistance to lateral movement can be estimated based on an equivalent fluid pressure
of 550 pcf for on-site materials. This value is applicable for footings founded on undisturbed,
on-site soils or compacted and tested fill. In addition to passive resistance, a coefficient of
friction between the base of the footing and the underlying soil equal to 0.45 may be used.
The lateral earth pressure values do not incorporate specific factors of safety. If applicable,
factors of safety should be integrated into the structural design of the wall.
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Global Stability and Long-Term Slopes
A proposed hydraulic structure is scheduled for construction south of the south building line
between the structure and Sandy Lake Road. The structure is a hydraulic open channel
designated for storm water drainage. Global stability was performed on slope sections provided
by Halff Associates, Inc. to this office on August 9, 2004. The analyzed section consisted of a
channel bottom at Elev. 506 sloping on a one vertical to three horizontal(1V:3H)slope to Elev.
508.5. Elev. 508.5 is the bottom wall elevation of a low-height vertical retaining wall with a
top of wall elevation at Elev. 513. Above the wall at Elev. 513 is a natural slope. Analysis was
based on an empty channel under the assumption short-term pooling of storm water within the
channel will occur.
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The global stability analysis was aided by the use of a computer program, CLARA 2.31, to
allow for rapid analysis of a large number of potential failure surfaces. A search was performed
1 for a"minimum"factor of safety for the wall section analyzed. It should be noted that the
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factor of safety calculated may not be the absolute minimum factor of safety for the retaining
wall section. It is possible that a lower factor of safety may exist which was not detected during
the search.
lThe factor of safety represents the ratio of the forces tending to resist rotational failure to the
forces tending to cause rotational failure. A factor of safety of one represents conditions of
incipient failure. A factor of safety of 1.5 against a global failure is generally considered
adequate depending upon the threat of injury and/or severity of property damage resulting from
such a failure.
lThe results of the global analyses indicate a minimum factor of safety of 1.54 for a slope of
1 1V:3H below an approximate 4-1/2-foot vertical wall. The specific section analyzed, and the
1 failure circle, are provided as Plate 29.
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Earthwork
lAll vegetation and topsoil containing organic material should be cleared and grubbed at the
beginning of earthwork construction. Areas of the site that will underlie fill or within the
building should be scarified to a depth of 6 inches and recompacted to a minimum of 92 percent
and a maximum of 98 percent of the maximum density, as determined by ASTM D-698,
"Standard Proctor". The moisture content should range from +1 to+4 percentage points above
optimum.
Site-excavated soils should be placed in maximum eight-inch horizontal loose lifts and
compacted to the moisture and density requirements outlined above.
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Project No. 11042 -22- August 11,2004