until construction work below existing water levels is complete. Submit
performance records weekly.] [Measure and record performance of dewatering
system at same time each day by use of observation wells or piezometers
installed in conjunction with the dewatering system.] [Relieve hydrostatic
head in previous zones below subgrade elevation in layered soils to prevent
uplift.]
3.2.5
Trench Excavation Requirements
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NOTE: The width of the trench below the top of the
pipe will depend on the type of pipe used and soil
conditions. The pipe manufacturer's installation
manual should provide this information, and if so,
it will be followed. In general, the width of
trench will be 300 mm (12 inches) to 600 mm (24
inches), plus pipe O.D. for smaller pipe sizes, and
600 mm (24 inches) to 900 mm (36 inches) plus pipe
O.D. for larger pipe sizes. Sloping walls below the
top of the pipe are allowed for certain types of
pipe in special ground conditions.
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The trench shall be excavated as recommended by the manufacturer of the
pipe to be installed. Trench walls below the top of the pipe shall be
sloped, or made vertical, and of such width as recommended in the
manufacturer's installation manual. Where no manufacturer's installation
manual is available, trench walls shall be made vertical. Trench walls
more than [_____] meters feet high shall be shored, cut back to a stable
slope, or provided with equivalent means of protection for employees who
may be exposed to moving ground or cave in. Vertical trench walls more
than [_____] meters feet high shall be shored. Trench walls which are cut
back shall be excavated to at least the angle of repose of the soil.
Special attention shall be given to slopes which may be adversely affected
by weather or moisture content. The trench width below the top of pipe
shall not exceed 600 mm (24 inches) 24 inches plus pipe outside diameter
(O.D.) for pipes of less than 600 mm (24 inches) 24 inches inside diameter
and shall not exceed 900 mm (36 inches) 36 inches plus pipe outside
diameter for sizes larger than 600 mm (24 inches) 24 inches inside
diameter. Where recommended trench widths are exceeded, redesign, stronger
pipe, or special installation procedures shall be utilized by the
Contractor. The cost of redesign, stronger pipe, or special installation
procedures shall be borne by the Contractor without any additional cost to
the Government.
3.2.5.1
Bottom Preparation
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NOTE: Stones 75 mm (3 inches) or greater should be
removed. However, pipe manufacturer's criteria, if
any, should be used.
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The bottoms of trenches shall be accurately graded to provide uniform
bearing and support for the bottom quadrant of each section of the pipe.
Bell holes shall be excavated to the necessary size at each joint or
coupling to eliminate point bearing. Stones of [_____] millimeters inches
or greater in any dimension, or as recommended by the pipe manufacturer,
whichever is smaller, shall be removed to avoid point bearing.
SECTION 02300
Page 25
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