Turf-grass Establishment - Fundamentals of Tropical Turf Management - page 54

Finished grade
Subgrade
Fig. 4.2. The subgrade should conform to the finished grade. (Drawing by
R. Castro.)
(Fig. 4.2). If a 2% slope is planned for the finished area, the same 2%
should be established in the subgrade. If this rule is not followed, the
water that infiltrates through the topsoil may accumulate at the surface of
the subsoil, forming puddles below the soil surface. If it accumulates
close to the building foundation, it can cause serious damage. Another
important rule is that the subgrade should be lowered to allow for topsoil
placement. Because topsoil should be at least 10 cm thick, the subgrade
should be at least 10 cm below the intended final grade. Steep slopes
should also be avoided because of their susceptibility to water erosion
and the difficulties of establishing and mowing turf. If the original con-
tour includes a rapid change in elevation, the use of a retaining wall may
be the only solution (Fig. 4.3). If the slope is moderately steep, a hedge-
row of densely planted shrubs or of erosion-control grasses such as
vetiver grass ( Vetiveria spp.) should be considered.
Surface and Subsurface Drainage
Effective surface and sometimes subsurface drainage can eliminate many
potential problems in turf-grass culture. The contours of a turf-grass area
should be both aesthetic and functional; they should rapidly remove
excess surface water. Home lawns should slope by at least 1% away
from the buildings (i.e. should descend by at least 1 m per 100 m of
lateral distance). Athletic turf such as football fields should slope about
2% from the centre. Depressions - areas lower than all surrounding turf -
should be avoided. Sometimes, despite proper slope, surface drainage is
not sufficient. On soils containing large amounts of clay, rainwater may
not be able to drain quickly enough by infiltration, and water will pool on
the surface. If the soil is poorly drained and impermeable, and the water
table is frequently less than 1.5 m from the soil surface, the area is likely