The sites on which landscapes have been created have been modified
by grading and construction activity resulting in a new altered
soil surface layer of mixed, contaminated soil. The characteristics
of a healthy, functioning soil are significantly altered during
the site development process. "As soil is relocated on the
site, taken away, and added to, the natural horizons are mixed and
altered. Through this process a new layer is created that blends
existing textures creating a heterogeneous and structureless soil."(Harris
1999)
Today, compaction is the biggest problem in dealing with the urban
soil environment and has led to various impacts both on-site and
off-site. Compaction is caused by various factors such as machinery
and foot traffic. Compaction is not only a result of compression;
it happens more specifically when delicate aggregates are broken
down. The break down of these aggregates, which form the soil structure,
is most susceptible when it is wet. "Water causes aggregate
cementing agents to become more fluid so soil particles slip into
positions that take up less space. Once the soil is saturated it
cannot further be compacted, but localized disturbance, of tire
compression from heavy machinery, will shear aggregates apart and
rearrange the particles. These puddled surfaces that remain upon
drying, shrink and become compacted." (Harris 1999)
Within the construction process, many aspects contribute to soil
compaction, taking place within the grading limit. Phillip Craul,
Senior lecturer in Landscape Architecture, Graduate School of Design,
Harvard University says that "soil specifications for contractors
in the past have been superficial and incomplete at best."
He also states that "there is a greater sensitivity to the
requirements of sustainability" and that we as landscape architects
need to "Say no" when becoming a loser to construction
cost overruns that filter down, ultimately creating low quality
soil management practices. The soil is the foundation for our designs
and landscape architects are taking liability for the success of
these landscapes dependent on properly functioning soil systems,
yet steps aren't being taken to ensure quality.
The general purpose of grading soil within the construction site
is to reduce the voids within the soil structure. Whether it is
cut or fill, mechanical compaction is necessary to stabilize the
subgrade for building, but the effects of heavy equipment such as
trucks, backhoes, and grading equipment incur compaction at about
6 to 8 inches in depth, at which point the soil is so wet that it
flows from under the tires where compaction occurs at much greater
depths. Meanwhile, areas adjacent to construction activities are
losing their indigenous properties. Contractors are affecting the
whole site during construction. Existing trees can be severely impacted,
future plantings can be severely impacted, and future stormwater
management practices can be significantly altered as a result of
a non-functioning soil. This upper layer of soil is where most of
the processes take place. New standards need to be set and awareness
needs to spread throughout the landscaping profession about the
importance of natural process, especially in our urban environments.
A required understanding of natural soil profiles and the native
ecosystem are a good beginning place. Native soil profiles consist
of about 4 to 6 percent organic matter whereas imported topsoil
typically run 15% by weight and 33% by volume organic matter content
in the mix. Topsoils are designed soils, focused on maximizing drainage
and compaction prevention. With higher levels of organic matter
in the soil mixture the soil is more vulnerable to compaction. As
the organic material decomposes the soil pore space can be reduced
when the soil is compressed. So it is as important to know your
sources of topsoil as it is to know the correct ways to apply it.
"Additions to the soil profile are best installed by horizon
to make certain that the materials are in proper sequence and horizontally
continuous. Wide variations in horizon thickness and presence of
discontinuities are not acceptable for a designed soil. Standard
specifications as written do not ensure uniformity of soil structure
upon installation. Because of differentiation of deposition of material
and compaction in the spreading process, greater uniformity of the
material can be obtained by placing the soil in 4 to 6 inch lifts."
(Craul 1999)
Homogeneous topsoil mixes decrease development costs, but may not
provide optimum soil conditions and may compact over time. Fitting
horizons together and understanding soil interfaces is important
because sharp contrasts in bulk densities are the result of not
designing the right textural combination. Mixing interfaces not
based on ideal soil properties will result in a failure to the system.
The importance of careful placement of the soil materials and the
sequencing of operations cannot be overemphasized from a successful
installation and the sustainability of the design. Designed soil
profiles presents higher costs and more challenging applications
but carry a greater potential for success both for on-site and off-site
plant communities and water retention ability.
Another procedural element in the construction process is storage
of soil material. All soil material, whether it is topsoil from
the site or imported fill must be stored properly. General specifications
designate areas for topsoil storage but no steps are taken to ensure
that soil retains its functioning properties. Craul states that
only the amount of soil that can be use within a few days should
be brought to the site, furthermore no soil should be stockpiled
at heights over six feet because anaerobic conditions can occur.
Soil should never be handled during wet periods and should be kept
moist at times but not be exposed to rainsplash for purpose of protecting
from erosion and loss of aggregate stability.
The order at which the soil is placed on site and graded is important.
To prevent further compaction after grading install soil in a way
that prevents backtracking over already installed areas. Starting
in the center of the site is a good option if there is ample access
around the perimeter of the site. Also moving in a successional
pattern across the site from one end to the other can help to prevent
further compaction.
Further steps can be taken to install planting such as trees on
compacted pedestals that support root balls and bring in soil around
them while grading. This will prevent further compaction during
the construction process as well as provide a good porous structure
that will allow infiltration and aeration. When installing soil
set rough grade levels then irrigate for two weeks to allow for
the soil to naturally settle and reduced the major voids, then once
it has settled do the final grading.
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