Villages of CC 6-LR 950112GEO CONSULTING SPECIALISTS IN:
Environmental Assessments
T Environmental Engineering
ECHNIQUE Soil., & Foundation Engineering
Construction Materials Testing
INc. Commercial & Industrial
Inspection
"Excellence by Experience"
January 12, 1995
Legend Construction
P.O. Box 606
Euless, Texas 76039-0606
Attn: Mr. David Chaney
Re: Utility Trench Design
Water & Sewer Improvements
Villages at Cottonwood Creek, Section VI
Coppell, Texas
95-1311
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 uery dense tan fine to medium cemented sand. The
underlying primary (rock) formation, dark gray shale, was
encountered in many of the borings, often shallower 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
three (3) 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 · Southlak¢, Texas 76092
(817) 329-0281 · (800) 348-6308 · Fax (817) 488-1866
Page Two
Utility Trench Design
Villages at Cottonwood Creek, Section VI
Coppell, Texas
95-1311
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-l, titled "Maximum
Allowable Slopes", referenced Federal Register, Vol. 54, No. 209,
Rules & Regulations 45965.
The project comprises approximately 9,250 feet of 6 and 8 inch PVC
water transmission main, 9,450 feet of 8 and 12 inch PVC sanitary
sewer main, and 3,430 feet of 18 to 90 inch R.C.P. storm sewer
main. A great majority of the excavation will require cuts between
four (4) and ten (10) feet. A small segment will require cuts in
excess of ten (10) 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 revealed solid shale as shallow as five (5) 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
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 current OSHA specifications.
Page Three
Utility Trench Design
Villages at Cottonwood Creek, Section VI
Coppell, Texas
95-1311
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.
...t:'. ......... ....... ":
~ ~.'. ..............
Louis L. }{argis, P.E.
President
TABLE B-X - MAXIMUM ALLOWABLE SLOPES
(REFERENCE: FEDERAL REGISTER, VOL. 54, NO. 209
RULES &'REGULATIONS 45965)
MAXIMUM ALLOWABLE SLOPES (HzV) {1}
BOIL OR ROCK TYPE
SHORT-TERM EXPOSURE LONG-TERM EXPOSURE
(DEGREES) (DEGREES}
STABLE ROCK VERTICAL (90) VERTICAL (90)
Type A* 1/2 : 1 (63) 3/4 : 1 (53)
Type B 3/4 : 1 (53) 1 : 1 (45)
Type C 1 1/2 : 1 (34) 2 : i (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 12 feet (3.67m) in depth shall
be 3/4 H:IV (53 degrees).
OSHA SOl LCATEGORIES
STABLE
ROCK "TYPE A" "TYPE B" "TYPE C" ~
DECR£ASIN6 ORDER OF STABILITY
Rock
Slope £o~ig~ration
Unsupported Vertically Sided Lower Portion
Maximum Depth - Varies
Stable Rock to Type A Soil
~nsupported Vertically Si(led Lower Portinn
r,laximum Depl:h - light (!!,: Feel,
Type A So i!
Slope Configuratio~
Unsupported Vertically Sided Lower Portion
~qaximum 9ept~ - Twelve (12) Feet
Type ^ Soil
~1~9.~ Confiquration
Simple Slope - General
Maximum Depth - Twenty (20) Feet
Type A Soil