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ABSTRACT
Wood chip fill material from a highway shoulder-widening in northern Minnesota was evaluated by a variety of physical tests after 19 years of burial. Chips discolored when excavated (typical tannate oxidation reaction) and remained gray on air-drying. Chips from the shallow layer burial sites were essentially sound if collected just beneath the clay cap and approximated sound chips in moisture level, bulk density, and specific gravity, but were slightly less acidic than fresh aspen/balsam poplar. Those chips collected near the sphagnum peat base (50 cm down from clay) were somewhat soft on the surface and broke easily on bending. These chips had notably higher moisture contents at collection and lower specific gravities than those from the clay cap zone. Microscopic examination of chips noted extensive pitting and erosion of wood fiber walls and destruction of wood parenchyma (storage) cells consistent with attack by bacteria known to develop in waterlogged wood with limited oxygen available. Chips from a thicker fill layer design followed this pattern for one collection site, (i.e., sound chips at upper level under clay cap; degraded chips near peat, 150 cm from clay), but chips at upper and lower levels were basically sound at the second collection location. Though chips did have different moisture-holding capacities, their overall bulk density was essentially the same for locations within each site. Absence of fungal degradation is attributed to lack of oxygen rather than limitations of pH, water, or nutrients. These data support use of wood chips for lightweight fill of roadways in swamps.
In 1976, aspen (Populus tremuloides Michx.) and balsam poplar (P balsamifera L.) whole tree chips were mixed and used as experimental fill to widen shoulders along two sections of a heavy-use trunk highway south of International Falls, Minn. This approach is considered a superior alternative to common practices of excavation and granular fill when unstable foundation conditions exist, as in peat swamps (10). Although wood chips have seen increasing use in road construction (2,7), few data on physical properties and deterioration of such hardwood chips in swamp burial are available to help assess the service life of such light and inexpensive fill material. This study determined a variety of physical and chemical properties of chips after 19 years of burial in...