more than 70% saturated (Smith 1979). At higher
PROPOSED NEW METHOD FOR
saturation levels, the moisture within the MESL
MESL CONSTRUCTION
can migrate to the top during freezing and cause
A proposed new method for MESL construc-
localized thaw weakening.
tion would eliminate the use of emulsified asphalt
If encapsulated soils are too wet to be placed
and use plastic membranes and tapes instead.
directly in the MESL, the soil is dried or modified
This would allow construction at all tempera-
by one of several methods: air drying, adding drier
tures and would eliminate the need for an asphalt
material to it, or using chemicals to dry or modify
distributor. The membranes are not affected by
the soil.
temperature and are easily transported and
Wet soil is commonly air-dried. This method is
stored.
very weather-dependent and time-consuming.
Lime (quick or hydrated) is an excellent drying
Plastic upper membrane
chemical for clay and clayey soils. Lime modifies
The emulsified asphalt will be replaced with a
the soil and can act as a stabilizing agent if used in
plastic membrane. Polyethylene plastic mem-
sufficient quantity. However, it is relatively ineffec-
branes are flexible at 0C (32F) and below (Raven
tive on silts and silty sands. Fly ash effectively
Industries undated bd). They also require a min-
modifies clayey soils and dries soils because it is
imal amount of special equipment to install. The
typically dry when added.
selection of the upper membrane is critical to the
success of the MESL. There must be a balance
Current problems in cold regions
between cost, performance, and ease of installa-
tion. It is estimated that a 0.31-mm- (12-mil-) thick
Emulsified asphalt
membrane could be used for short design lives
Emulsified asphalt is used to form the top mem-
(up to one year) and a 0.51-mm (20-mil) mem-
brane, as part of the lower membrane, and for seal-
ing the membranes. It is best used when the air and
brane for longer design lives (one to five years).
surface temperature is above 10C (50F) (Asphalt
As the membrane gets thicker, however, it be-
Institute undated). Thus, for cold regions, the con-
comes more difficult to handle and to lay flat.
struction season could be limited. The liquid emul-
sified asphalt also needs to be kept from freezing
Membrane protection
because freezing will prematurely break the emul-
The upper membrane must be protected from
sion (Asphalt Institute undated). A potential prob-
traffic. It is likely that a CSE company would use
lem is that military Combat Support Equipment
gravel as a wearing surface in a theater of opera-
(CSE) companies do not have the asphalt distribu-
tions. For a gravel wearing surface, the amount of
membrane protection required will depend on
tors required to spray the asphalt.
the type of gravel used, thickness of the gravel
Membrane damage
layer, type and quantity of traffic, and service life.
Geotextiles are commonly used to protect
The current membrane allows only wheeled ve-
membranes from puncture. They are inexpen-
hicles to travel on it. If a gravel wearing surface
sive, easy to install, transport, and store. The total
must carry tracked-vehicle traffic, a cushion layer
mass per unit area of the geotextile should be at
of clean sand is necessary. Clean sand will not have
least 813 g/m2 (24 oz/yd2) (Koerner et al. 1996,
sharp, larger particles that could puncture the
Narejo et al. 1996, Richardson 1996, Wilson-Fahmy
membrane, and will retain its strength when wet.
and Koerner 1996), such as three layers of a 271
This requires a clean sand source. The problem is
g/m2 (8 oz/yd2) fabric, to provide sufficient pro-
compounded by the difficulty of placing a 51-mm-
tection for most applications.
(2-in.-) thick sand layer without damaging the
membrane.
Membrane sealing
Heat and tape are the two basic methods used
Drying encapsulated soil
Borrow soil used for a MESL commonly has an
to seal the membranes. The heat methods, such as
above-optimum water content. Lime is commonly
iron or hot air welding, generally give higher-
used to dry soil and is very effective in clayey soils;
quality seams. However, they require skilled
however, it has little effect in other soils. Therefore,
technicians to run the specialized equipment, and
a quick method to dry soils such as dirty sands is
it is very easy to melt holes in thin membranes.
required.
Tapes provide good seals with limited train-
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