Freeport NIP(4)-SP 960311GEO -
T :¢I-I IQtJ
INC.
CONSULTING SPECIALISTS IN:
Environmental Assessments
Environmental Engineering
Soils & Foundation Engineering
Construction Materials Testing
Commercial & Industrial Inspection
"Excellence by Experience"
March 11, 1996
Legend Construction
P.O. Box 606
Euless, Texas 76039-0606
Attn: Mr. David Chaney
Re:
Utility Trench Design
Water & Sewer Improvements
Freeport North; Creek View Drive
Coppell, Texas
96-1606
Dear Mr. Chaney:
In accordance with your request, we have reviewed our soil core
borings and summary of tests, which are incorporated in a
previously submitted subsurface soils investigation. These borings
and test results are adequate for this report. The boring logs and
test summaries are a part of project specifications.
The purpose of this examination was to study the subsurface 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 subsurface soils are typical of the Woodbine Geological
Formation and locally non-uniform. Generally, from the surface
down, we encountered varying thicknesses of stiff to hard reddish
brown, brown, tan and gray clays, sandy clays or clayey sands,
underlain by very dense tan fine to medium cemented sand. The
underlying primary (rock) formation, dark gray shale, was
encountered in a few of the borings, but deeper than ten (10) feet
below ground surface.
The borings were advanced dry to allow for extremely accurate water
table measurements. After a twenty-four (24) hour period,
groundwater was in evidence in a few of the borings, as shallow as
twelve (12) feet.
Considering the non-existence of a consistent wate table, we do not
expect groundwater to significantly influence the' deeper
excavations on this project.
P.O. Box 92548 · Southlake, Texas 76092
(817) 329-0281 · (800) 348-6308 · Fax (817) 488-1866
Page Two
Utility Trench Design
Freeport North; Creek View Drive
Coppell, Texas
96-1606
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 771 feet of 12 CL 150,DR-18 PVC
water transmission main, 825 feet of 6 and 8 inch SDR 35 PVC
sanitary sewer main, and 600 feet of 18 to 24 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 fifteen (15) 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 revealed no solid shale as shallow as ten (10) feet
in isolated areas, while in others no stable rock was reached
throughout the depths investigated (20 feet).
At the top of vertical excavations, where necessary, the trench
wails 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 current OSHA specifications.
Page Three
Utility Trench Design
Freeport North; Creek View Drive
Coppell, Texas
96-1606
The foregoing recommendations are based on the analyses which
presume the condition of soil properties between the borings to
have a normally uniform variation of conditions revealed by the
borings. 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-Teohni~ue, Ino.
Louis L. Hargis, P.E.
President
TABLE B-X - MAXIMUM ALLOMABLE SLOPES
(REFERENCE: FEDERAL REGISTER, VOL. 54, NO. 209
RULES &'REGULATIONS 45965)
MAXlMUM ALLOFABLE SLOPES (H~V)
SOIL OR ROCK TYPE
SHORT-TERM EXPOSURE LONG-TERM EXPOSURE
(DEGREES) (DEGREES)
STABLE ROCK VERTICAL (90) VERTICAL (90)
Type A* 1/2 : i (63) 3/4~: i (53)
Type B 3/4 : i (53) 1 : 1 ,(45)
Type C i 1/2 : 1 (34) 2 : i (27)
, {2}
Numbers shown in parentheses next to maximum allowable
slopes are angles expressed in degrees from the
horizontal. Angles have been rounded off.
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 12 feet (3.67m) in depth shall
be 3/4 H:IV (53 degrees).
Stable
Rock
OSHA SOIL CATEGORIES
Type "A" Type "B"
decreasing or'der of stability
Type "C" ~
Type A
Rock
Type A
Slope Configuration
Unsupported Vertically Sided Lower Portion
Maximum Depth - Varies
Stable Rock to Type A Soil
II
Slope Conf!guration
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
Slope Configuration
Simple Slope - General
Maximum Depth - Twenty (20) Feet
Type A Soil