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

Note 1) In subsurface drainage, installation conditions are generally not very severe and hence
geotextile Type II from Table 700-1 is recommended. However, the Engineer may specify
Type III geotextile for trench drain applications on one or more of the following conditions:
a) The Engineer has found Type III geotextiles to have sufficient survivability on field
experience.
b) Subsurface drain depth is less than 2 m; and drain aggregatediameter is less than 30
mm and compaction requirement is less than 95 per cent of Proctor density as per IS
2720: Part – 7.
2) Site specific geotextile design should be performed especially if one or more of the following
problematic soil environments are encountered :
Unstable or highly erodible soils such as non cohesive silts, gap graded soils, dispersive
clays, etc.
3) A nominal coefficient of permeability may be determined by multiplying permittivity value by
a nominal thickness. The K value of fabric should be greater than the K value of soil.
Recommended Permittivity values for different soils are given below in Table 700-4.
Table 700-4 Recommended Permittivity for Different Soils
Soil type
Sandy Gravel
Clean Sand
Silty Sand
Silt
Clay
Typical Range of Soil
Permeability, Ks (cm/sec)
1 X 10 -1 to 1 X 10 -2
Minimum
Recommended
Geotextile Filter
Permittivity, Y
(per second)
0.5 to 0.1
1X10 -2 to 1 X 10 -3
0.1 to 0.01
1X10 -3 to 1 X 10 -4
0.01
1X10 -4 to 1X10 -5
0.01
1X10 5 0.01
702.3 Construction Requirements
Geotextile exposure following placement – Exposure of geotextiles to the elements between lay
down and cover shall be a maximum of 14 days to minimize damage potential.
Geotextle Placement – In trenches, after placing the backfill material, the geotextile shall be folded
over the top of the filter material to produce a minimum overlap of 30cm for trenches greater than 30cm
wide. In trenches less than 30 cm wide, the overlap shall be equal to the width of the trench. The
geotextile shall then be covered with the subsequent course.
Successive sheets of geotextiles shall be overlapped a minimum of 30 cm in the direction of low.
Seams – Where seams are required in the longitudinal trench direction, they shall be joined by either
sewing or overlapping. All seams shall be subject to the approval of the Engineer. Over seams shall
have a minimum overlap equal to the width of the trench.
Repair – A geotextile patch shall be placed over the damaged area and extend 1m beyond the
perimeter of the tear or damage.