30 Sep 01
(e) Sewn Seams. Both sewn and overlapped seams are used for landfill cover geotextiles. A
flat seam using a two-thread chain stitch is often used for filtration geotextiles. Heat bonding and
overlapping are other potential methods of joining geotextiles. The contract documents should indicate
which seams must be sewn. Field seam testing is rarely performed for geotextiles used for filtration and
(f) Overlapped Seams. For overlapped seams, a 300-mm (12-in.) overlap should be the
minimum requirement in all cases. Table 3 of AASHTO M 288-96 provides additional guidance on
e. Filter Layer--Granular Materials.
(1) General. Design criteria for granular filters are largely empirical and are given in terms of
characteristic particle sizes (D15, D85, etc.). Engineer Manual (EM) 1110-2-1913 specifies that a
granular filter must meet three requirements: piping or stability, permeability, and sufficient discharge
capacity if the filter is also used as a drain. In some cases, it may be necessary to design a multiple
layered or graded filter to meet these requirements. For severely gap-graded soils, the design of a filter
using standard empirical relationships may not be appropriate. In these cases, it may be necessary to do
(2) Design Criteria.
(a) Piping or Stability. Filters allow seepage to move out of a protected soil more quickly
than the seepage moves within the protected soil. Thus, the filter material must be more open and have
a larger grain size than the protected soil. To prevent the movement of erodible soils and rocks through
filters, the pore spaces between filter particles should be small enough to hold some of the larger
particles of the protected material in place. These larger sized particles will eventually block finer soil
particles from migrating through the filters if the soils are well graded. Appendix D of EM 1110-2-1913
provides tables and equations for designing filters to prevent piping.
(b) Permeability. The design of the filter must assure that the filter material is much more
permeable than the material being drained. Appendix D of EM 1110-2-1913 provides the following
guidance for the permeability criterion. The requirement that seepage move more quickly through the
filter than through the protected soil is met by comparing the D15 size of the filter to the D15 size of the
protected soil. If the D15 of the filter is at least 5 times greater than the D15 of the soil, the filter will be
approximately 25 times more permeable than the protected soil. The criteria listed above define a wide
range or band of gradations that will satisfy the design requirements. The gradation that is specified
should fall within this band, be approximately parallel to the gradation curve or curves of the protected
soil, and reflect materials that are locally available.