Cambridge Phase 2-SY 961216GEO -
TECHNIQUE
INc.
CONSULTING SPECIALISTS IN:
Environmental Assessments
Environmental Engineering
Soils & Foundation Engineering
Construction Materials Testing
Commercial & Industrial Inspection
"Excellence by Experience"
December 16, 1996
Rodman Underground, Inc.
P.O. Box 1152
Frisco, Texas 75034
Attn: Mr. Bill Thornton
Re:
Utility Trench Design
Water & Sewer Improvements
The Estates of Cambridge Manor, Phase II
Coppell, Texas
96-1829
Dear Mr. Thornton:
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 .;eological
Formation and locally non-uniform. Generally, from tire surface
down, we encountered varying thicknesses of stiff to ha:d reddish
brown, brown, tan and gray clays, sandy clays or cla~'ey sands,
underlain by very dense tan fine to medium cemented ~and. The
underlying primary (rock) formation, dark gray Slale, 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 acc[rate 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 · Southlake, Texas 76092
(817) 329-0281 · (800) 348-6308 · Fax (817) 488-1866
Page Two
Water & Sewer Improvements
The Estates of Cambridge Manor, Phase II
Coppell, Texas
96-1829
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 770 feet of 8 inch PVC water
transmission main, 615 feet of 10 inch PVC sanitary sewer main, and
282 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
six (6) feet. A small segment will require cuts in excess of six
(6) feet, but in no case will the excavation be deeper than ten
(10) feet.
From the enclosed table B-l, "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
Water & Sewer Improvements
The Estates of Cambridge Manor, Phase II
Coppell, Texas
96-1829
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-Teohnique, Ino.
Louis L. Hargis, P.E.
President
TABLE B-1 - MAXIMUM ALLOWABLE SLOPES
(REFERENCE: FEDERAL REGISTER, VOL. 54, NO. 209
RULES &'REGULATIONS 45965)
MAXIMUM ALLOWABLE SLOPES (H=V) {1}
SOIL 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 S 3/4 : 1 (53) 1 : 1 (45)
Type C 1 1/2 : 1 (34) 2 : 1 (27)
{1}
. {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 C~TEGORIES
Type "A" Type "B"
decreasing order 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
S ~_~e ConFiguration
Unsupported Vertically Sided Lower Portion
Maximum Del)th - El§hr (8) Feel
Type A So i I
Slope Config. uration
Unsupported Vertically Sided Lower Portion
Maximum Depth - Twelve (12) Feel
Type A Soil