Big Cedar-SY 920221GEO -
TECHNIQUE
INC.
CONSULTING SPECIALISTS IN:
Environmental Assessments
Environmental Engineering
Soils & Foundation Engineering
Construction Materials Testing
Commercial & Industrial Inspection
"Excellence by Experience"
February 21, 1992
Legend Construction, Inc.
P.O. Box 606
Euless, Texas 76039-0606
Attn: Mr. David Chaney
Re:
Utility Trench Design
Water & Sewer Improvements
Big Cedar Addition
Coppell, Texas
92-547
Dear Mr. Chaney:
In accordance with your request, we dug three (3) test pits on the
above referenced project. The pit locations were dug in the areas
of deepest proposed excavation. Soils from these pits were
visually classified and logged for information purposes.
The purpose of this examination was to study the sub-surface soil
stratigraphy along the proposed utility lines in order to properly
design an open trench to be used in utility construction.
Examination of the test results and the boring logs revealed that
these sub-surface soils are typical of the Woodbine Geological
Formation and locally non-uniform. Generally, from the surface
down, we encountered varying thicknesses of loose to dense reddish
brown, tan and gray clayey sands, sandy clays or sandstone,
underlain by stiff to hard light brown to gray clays with sand and
sandstone seams. The sandstone stratum is not continuous and is
often overlain by tan and gray moderately active clay. Sandy shale
was also in evidence but not continuous.
The pits were allowed to remain open for water table measurements.
Groundwater was in evidence in one (1) pit only at about thirteen
(13) feet below ground surface.
--1--
1939 East Continental Boulevard · Southlake, Texas 76092
(817) 329-0281 · (800) 348-6308 · Fax (817) 488-1866
Considering the existence of this shallow water table, we expect
groundwater to influence the deeper excavations, particularly if
the trench is open for an extended period of time. Downhill sump
holes with pumps may be required in the deeper cuts, depending upon
the construction season.
In order to determine a safe open trench configuration, we referred
to a bulletin on "Excavating and Trenching Operations", published
by the U.S. Department of Labor Occupational Safety and Health
Administration, July 1975, revised December 1987 and Federal
Register, Part II, Department of Labor, 29 CFR Part 1926, dated
October 31, 1989. The angles of repose from our enclosed typical
trench cross-sections were taken from Table B-i, titled "Maximum
Allowable Slopes", referenced Federal Register, Vol. 54, No. 209,
Rules & Regulations 45965.
The project comprises approximately 1,577 feet of 8 inch PVC water
transmission main, 2,878 feet of 8 and 10 inch PVC sanitary sewer
main, and 744 feet of 18 inch to 36 inch R.C.P. storm sewer main.
A great majority of the excavation will require cuts between four
(4) and twelve (12) feet. A small segment will require cuts in
excess of twelve (12) feet, but in no case will the excavation be
deeper than fourteen (14) feet.
From the enclosed table B-i, "Maximum Allowable Slopes", it becomes
evident that OSHA allows trenches cut into stable rock to be cut
vertically (90 degrees) from the trench bottom to the top of the
stable rock. Where rock is not encountered, the maximum vertical
cut is limited to three and one-half (3.5) feet. Examination of
the boring logs reveals no stable rock is reached throughout the
depths investigated (15.0 feet). The sandstone may be hard in
places but the interbedded sand and clay seams must influence the
overall deep excavation.
At the top of vertical excavations, where necessary, the trench
walls should be sloped back on varying angles of repose up to the
existing ground surface, as shown on the enclosed typical trench
slope configuration diagrams.
Notes from Table B-1 allow steeper slopes for "short term"
excavation. Short term is generally defined as 24 hours.
On all vertical excavations adjacent to existing roadways or where
sloping is not practical, the contractor may elect to utilize a
sliding trench box in all cuts deeper than three and one-half (3.5)
feet in order to prevent caving or sloughing of the trench walls.
This method is acceptable by the Code.
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The foregoing recommendations are based on the analyses which
presume the condition of soil properties between the pits to
have a normally uniform variation of conditions revealed by the
pits. Should any unusual conditions be encountered during
construction, this office should be contacted immediately so that
further investigation and supplemental recommendations can be
given.
Further, since we are not actively engaged in any phase of trench
excavation, Geo-Technique, Inc., or any of its employees accepts
no responsibility for the safety of these operations.
I trust this is the information you desire; and if we can be of
further service, please call on us.
Respectfully submitted,
Geo-Tec~.que, Inc.
Louis L. Hargis, P.E.
President
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TABLE B-1 - MAXIMUM ALLOWABLE SLOPES
(REFERENCE: FEDERAL REGISTER, VOL. 54, NO. 209
RULES & REGULATIONS 45965)
SOIL OR ROCK TYPE
STABLE ROCK
Type A
(see Footnote 2)
Type B
Type C
MAXIMUM ALLOWABLE SLOPES (H:V){1}
SHORT-TERM
EXPOSURE
(DEGREES)
VERTICAL (90)
1/2 : 1 (63)
3/4 : 1 (53)
1 1/2 : 1 (34)
LONG-TERM
EXPOSURE
(DEGREES)
VERTICAL (90)
3/4 : 1 (53)
(45)
(27)
{1} Numbers shown in parentheses next to maximum allowable slopes
are angles expressed in degrees from the horizontal. Angles
have been rounded off.
{2} A short-term maximum allowable slope of 1/2 H:IV is allowed in
excavations that are 12 feet (3.67m) or less in depth. Short-
term maximum allowable slopes for excavations greater than
feet (3.67m) in depth shall be 3/4 H:IV (53 degrees).
Ii
Slope Configuration
Unsupported Vertically Sided Lower Portion
Maximum Depth Eight (8) Feet
Type A Soil
Slope Configuration
Unsupported Vertically Sided Lower Portion
Maximum Depth - Twelve (12) Feet
Type A Soil
~ ok-
Slope Configuration
Simple Slope - General
Maximum Depth - Twenty (20) Feet
Type A Soil
OSH" ,,'
.,-.,. SOi L CATEGOR1E~
S-r,. BiLE
ROCK ~'TYPE A" "TYPE B':
r,~'rr~;-'A~ i:-..'S ORDER OF STABILITY
"TYPE