Specification Part 3 - Road & Bridge Works - Urban Administration ...

dimensional stability relative to each other, including selvedges. These materials shall conform to the
physical requirements of Table 700-5
Table 700-5 Erosion Control Geotextile Requirements
Percent
in-situ soil
passing
0.075 mm
sieve
Permitivity,
per sec ASTM
D 4491 Min ARV
Geotextile drainage property
Requirement
Apparent opening
size, mm ASTM
D 4751Max ARV
Recommended
geotextile *
< 15
15 to 50
> 50
0.7
0.2
0.1
0.43
0.25
0.22
Type I from
Table 700-1
704.2.2 Geotextile rolls shall be furnished with suitable wrapping for protection against
moisture, and extended ultraviolet exposure prior to placement. Each roll shall be labeled or tagged to
provide product identification sufficient for inventory and quality control purposes. Rolls shall be stored
in a manner which protects them from the elements. If stored outdoors, they shall be elevated and
protected with a waterproof cover.
704.3 Construction Requirements
704.3.1 Geotextile Exposure Following Placement – The geotextile shall be placed and anchored
on a smooth graded surface approved by the Engineer. The geotextile shall be placed in such a
manner that placement of the overlying materials will not excessively stretch or tear the fabric.
Anchoring of the terminal ends of the geotextiles shall be accomplished through the use of key trenches
or aprons at the crest and toe of slope.
(In certain applications to expedite construction, about 40 cm long anchoring pins placed on 1-3 m
centers depending on the slope of the covered area have been used successfully).
704.3.2 Slope protection placement : Successive geotextile sheets shall be overlapped in
such a manner that the upstream sheet is placed over the downstream sheet and/or upslope over down
slope. In underwater application, the geotextile and required thickness of backfill material shall be
placed the same day. The backfill placement shall begin at the toe and proceed up the slope. Riprap
and heavy stone shall not be dropped onto the geotextile from the height of more than 30cm. Slope
protection and smaller sizes of stone filling shall not be dropped onto the geotextile from the height
exceeding 1m. Any geotextile damaged during placement shall be replaced as directed by the Engineer
at Contractor’s cost.
704.3.3 Seams: The geotextile shall be joined by either sewing or overlapping. All seams shall
be subject to the approval of the Engineer. Overlapped seams shall be a minimum overlap of 30cm.
except where placed under water where the overlap shall be a minimum of 1m.
704.3.4 Repair: A geotextile patch shall be placed over the damaged area and extend 1m
beyond the perimeter of the tear or damage.
704.4 Measurement for Payments
The geotextile shall be measured by the number of square metres computed from the payment lines
shown on the plans with no allowance for overlapping at transverse and longitudinal joints or from
payment lines established in writing by the Engineer. This excludes seam overlaps, but shall include
geotextiles used in crest and toe of slope treatments, slope preparation, excavation and backfill,
bedding, and cover material which would be covered as per relevant clauses of these Specifications.