W Sandy Lk Addn-CS051011· ,,'" Halff Associates, Inc.
ENGINEERS ARCHITECTS SCIENTISTS PLANNERS SURVEYORS
8616 Northwest Plaza Drive
Dallas, Texas 75225-4292
(214) 346-6200
Fax (214) 361-5573
LETTER OF TRANSMITTAL
TO:
FROM:
The City of Coppell
255 Parkway Boulevard
2"d Floor - Engineering Department
Coppell, TX 75019
Attn: Mr. Keith Marvin
B. David Littleton, P.E.
Email: dlittleton @ halff.com
DATE: October 11, 2005
AVO: ~
WE ARE SENDING YOU [] ATTACHED [] Under separate cover via__
[] Shop Drawings [] Prints [] Plans [] Drawings
[] Copyof letter [] Change order [] Other:
the following:
[] Specifications
THESE ARE TRANSMITTED as checked below:
[] For approval [] Approval as submitted
[] For your use [] Approved as noted
[] As requested [] Returned for corrections
[] For review/comment [] Other:
[] Resubmit
[] Submit
[] Return
copies for approval
__ copies for distribution
__ corrected prints
ITEMS SENT:
Section from Geotech Report
Development Agreement
Photographs of retaining wall construction
COMMENTS:
Keith, the slope stability analysis was completed by Reed Engineers and included in their report for project
number 11042. I thought the Development Agreement might have some information regarding maintenance of
the channel but it doe not seem to address this. The photos show the north retaining wall construction today.
Please let me know if you need additional information.
SIGNED: B. David Littleton, P.E.
COPIES: [] FILE
[] OWNER
[]CONTRACTOR
Below-Grade Walls and Retaining Wnlh
Below-grade walls will bc subject to lateral loeds associated with lateral earth pressures. The
magnitude of the ea.~ pressure will be a function off
· the type and compaction of backfill behind the walls within the *'active" zone; and
· the allowal?.le rotation of the top of the wall.
The active zone is the wedge of soil defined by the surface of the wall and a plane inclined 35'
from the vertical passing through the base of the wall.
Considering backfill using site-excavated materials compacted in lifts to the density and
moisture outlined in the Earthwork section, the lateral-loed soil pressures can be estimated
based on an equivalent fluid pressure of 52 pounds p~r cubic foot (pcf) "active" pressure or 74
pcf"at-rest" pressure.
Alt~malively, 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 per"at-rest" pressure.
These lateral pressures are applicable for horizontal surface grades and non-surcharged, drained
conditions. Design values do not incorporate specific factors of safety.
A drainage system should be installed behind the base of the below-gradc and retaining walls to
limit devclopment of excess hydrostatic pressures.
Project No. 11042 - 19 - August 11, 2004
The drainage system behind retaking walls should consist, as a minlm~m, of 12-inch by 12-
inch pocket drains spaced 15 feet on-center, installed near the base of the waLl.
Fill in the pocket dr'ain* should consist of durable crushed stone such as ASTM C-33, Size 67
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
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.
Baclcfill around the drainage pipe should consist of at least 2 feet of clean, free-draining,
durable crashed stone such as .SST/vi C-33, Size 67 or co .arser, wrapped in filter fabric CMirafi
140-N" or equivalent). Backfill around the gravel drain should consist of site-excavated soils
or select fill. 'A compacted clay cap is recomrn~aded wjthln thc upper two feet of the surface to
limit surface-water infillration behind the walls. All below-grade walls should be waterproofed
prior to plac~'ment of thc 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 (psi). Some
movement of the footings and walls should be anticipated.
Proj~t No. 11042 - 20 - August 11, 2004
Passive resistance to tateral movement can bc 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 sells 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.
Global Stability and Long-Term Slopas
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. 50(5 sloping on a one vertical to three horizontal (1V:3H) slope to Elev.
508.5. EIov. 508.5 is tho bottom wall elevation of a low-height vertical retaining wall with a
top ofwall 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 thc
channel will occur.
The global stability analysis was sided by the use of a ~omputer program, CLARA 2.31, to
allow for rapid analysis of a large number of potential failure surfaces. A search was performed
for a "minimum" factor of safety for the wall section analyzed. It should be noted that the
Project No. 11042 - 21 - August 11, 2004
factor of safety calculated may not be the absolute mlnlmlall 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.
The factor of safety represents the ratio of the forces tending to resist regional 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 gerterally considered
adequate depending upon the threat of'~njury and/or severity ofprop~'ty damage resulting from
such a failure.
The results of the global analyses indicate a rn~nlmum factor of safety of 1.54 for a slope of
1V:3H below an approximate 4-1/2-foot vertical wall. Thc specific section analyzed, and the
failure circle, arc provided as Plate 29.
Esrthwork
All 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 v,6th~,~ thc
building should be scarified to a depth of 6 inches and recompacted to a mi,~im,~n 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 fi'om +1 to +4 percentage points above
optimum.
Site-excavated soil.~ should be placed in maximum eight-inch horizontal loose lifts and
compacted to the moisture and density requirements outlined above.
Prcj~ct No. 11042 - 22 - August 11, 2004