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Technical Notes.
<br />Table 107.4: Reported Land Use or Activities That Increase Soil Bulk Density
<br />Grazing
<br />Crops
<br />ncrease in "Bulk Density
<br />(gms /cc)
<br />0.12 to 0.20
<br />Smith, 1999 (Table 2)
<br />Construction, mass grading
<br />Construction, mast gradin
<br />Constru
<br />ion, -no gradin
<br />Construction. traffic
<br />Construction traffi
<br />Athletic field
<br />25 to 0.35
<br />Smith,: 1999 (Table 2)
<br />Randrup, 1998
<br />Lichter and - Lindsey, 1994
<br />Lichter and ;Lindsey, 1994
<br />.25 to 0.40
<br />Lichter and - 'Lindsey, ,1994
<br />Smith, ,1999, Friedman, ~ 1
<br />`Urban lawn and turf',;
<br />38 to 0.54'
<br />Smith, 1999
<br />Various Sources
<br />struction process. First, grading equipment works over
<br />the site to cut and fill and achieve the desired elevations
<br />for building. As a consequence, existing top soil is
<br />stripped, stockpiled or even removed from the site, and
<br />compacted subsoils are exposed at the surface. Second,
<br />as construction equipment and vehicles work the site,
<br />their tracks and tires compress the remaining soils several
<br />feet below the new surface.
<br />Lastly, certain portions of the site are intentionally
<br />compacted with vibrators or rollers to meet soil engineer-
<br />ing standards for bearing structures or traffic loads. This
<br />intentional compaction usually occurs along the right of
<br />ways for roads, a ten foot envelope around building pads,
<br />and around stormwater ponds. Other areas of the site are
<br />also frequently compacted as the equipment moves from
<br />lot to lot. Local development standards typically require
<br />that soils be compacted to within 90 or 95% of their
<br />maximum bulk density within these zones.
<br />Taken together, construction increases the bulk
<br />density of surface soils on the order of 0.35 gm /cc over
<br />the pre - development land use, whether
<br />it is forest, pasture or crops (Table
<br />107.4). The compaction can extend up
<br />to two feet down in the soil profile,
<br />according to Smith (1999). One of the
<br />best studies on the impact of construc-
<br />tion on soil compaction was performed
<br />by Randrup (1998), who examined 47
<br />Danish construction sites and adjacent undeveloped
<br />soils. He reported an average increase in bulk density
<br />from 1.60 gms /cc to 1.94 gms /cc, with the greatest
<br />compaction found more than a foot below the soil.
<br />Lichter and Lindsay (1994) found a similar increase in
<br />soil bulk density at several California construction sites.
<br />They also noted that bulk density increased by 0.2
<br />gms /cc at a construction site whose soil was not mass
<br />The compaction of urban soils
<br />has many implications for the
<br />watershed manager.
<br />' ri•. e:!: 'i'.i`1iic.�i'•:+:53'�'.".L�r,Y o:ti ic.J��,�!"'•c' ^•?dl.
<br />graded nor compacted to meet engineering standards.
<br />Clearly, mass grading and the passage of construction
<br />equipment are both important factors leading to soil
<br />compaction on most construction sites (see Table
<br />107.4).
<br />According to recent research, soil compaction
<br />continues after turf and landscaping are established at
<br />the site, at least for the first few years. Bulk density
<br />values typically remain about 0.30 to 0.40 gms /cc
<br />above pre - development levels after development
<br />(Table 107.4). A few urban areas continue to become
<br />more compacted. Most notable are athletic fields, park
<br />areas, pathways and unpaved parking lots that con-
<br />tinue to experience extensive foot and/or vehicular
<br />traffic after development. Surface bulk densities for
<br />these compacted soils often range from 1.9 to 2.1
<br />gms /cc, which is almost equivalent to the bulk density
<br />for impermeable concrete surfaces.
<br />Implications of Soil Compaction for the Watershed
<br />Manager
<br />The compaction of urban soils has many implica-
<br />tions for the watershed manager. As soil compaction
<br />appears to be virtually unavoidable once clearing
<br />begins and the site experiences construction traffic and
<br />activity, site planners must physically exclude any
<br />construction equipment from portions of the site where
<br />undisturbed soils are required or desired. Many storm -
<br />water practices utilize the soil to treat or infiltrate
<br />stormwater runoff, and are designed under the assump-
<br />tion that the underlying soil is uncompacted and
<br />relatively undisturbed (infiltration, filter strips, grass
<br />swales, disconnection of rooftop runoff, some forms of
<br />bioretention and even septic systems). As aresult, these
<br />practices should be located outside the limits of con-
<br />struction disturbance. Otherwise, they may require
<br />664
<br />atershea Protection ::Techniques Vol , 3;. No. 2- " < `January ' 2000`
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