(1) Sheeting is particularly suitable in coarse-grained material
with maximum sizes less than about 6 inches or in stratified subsoils with
alternating fine grained and pervious layers where horizontal permeability
greatly exceeds vertical.
(2) To be effective, sheeting must be carefully driven with
interlocks intact. Boulders or buried obstructions are almost certain to
damage sheeting and break interlock connections. Watertightness cannot be
assumed if obstructions are present.
(3) Loss of head across a straight wall of intact sheeting depends
on its watertightness relative to the permeability of the surrounding soil.
In homogeneous fine-grained soil, head loss created by sheeting may be
insignificant. In pervious sand and gravel, head loss may be substantial
depending on the extent to which the flow path is lengthened by sheeting.
In this case, the quantity of water passing through intact interlocks may be
as much as 0.1 gpm per foot of wall length for each 10 feet differential in
head across sheeting, unless special measures are taken to seal interlocks.
b. Penetration Required. This paragraph and Paragraph "c" below apply
equally to all impervious walls listed in Table 1. Seepage beneath sheeting
driven for partial cutoff may produce piping in dense sands or heave in
loose sands. Heave occurs if the uplift force at the sheeting toe exceeds
the submerged weight of the overlying soil column. To prevent piping or
heave of an excavation carried below groundwater, sheeting must penetrate a
sufficient depth below subgrade or supplementary drainage will be required
at subgrade. See Figure 2 (Reference 2, Model Experiments to Study the
Influence of Seepage on the Stability of a Sheeted Excavation in Sand, by
Marsland) for sheeting penetration required for various safety factors
against heave or piping in isotropic sands. For homogeneous but anisotropic
sands, reduce the horizontal cross-section dimensions by the transformation
factor of Figure 1 to obtain the equivalent cross section for isotropic
conditions. See Figure 3 (Reference 2) for sheeting penetration required in
layered subsoils. For clean sand, exit gradients between 0.5 and 0.75 will
cause unstable conditions for men and equipment operating on the subgrade.
To avoid this, provide sheeting penetration for a safety factor of 1.5 to 2
against piping or heave.
c. Supplementary Measures. If it is uneconomical or impractical to
provide required sheeting penetration, the seepage exit gradients may be
reduced as follows:
(1) For homogeneous materials or soils whose permeability decreases
with depth, place wellpoints, pumping wells, or sumps within the excavation.
Wellpoints and pumping wells outside the excavation are as effective in some
cases and do not interfere with bracing or excavation.
(2) For materials whose permeability increases with depth, ordinary
relief wells with collector pipes at subgrade may suffice.
(3) A pervious berm placed against the sheeting, or a filter blanket
at subgrade, will provide weight to balance uplift pressures. Material
placed directly on the subgrade should meet filter criteria of Section 4.