Appreciation of the limitations in the condition of the sub-grade

It has been well documented that it is essential to create adequate macro-pore space and consequent permeability within the soil profile to sustain quality plant growth (Buckman, H.O. and Brady, N.C.1994).  However, a sports turf sub-grade has to be graded or shaped to achieve the desired final levels and grade.  To achieve this, heavy earthworks equipment is utilised.  In the cut and fill operation there can be considerable mixing of the layers in the soil profile.  These layers could include made-up ground and unweathered fractions.  The resulting compaction after grading, besides dramatically increasing the bulk density, can produce a very hard and virtually impermeable surface.  This restricts vertical drainage from the surface downwards increasing the water retained near the surface (Lacey, 2008).  Drainage down through the base, even slow, is vital if water-logging is to be prevented.  In essence this potential is considered as important as drainage through the topsoil and the installation of drains (McIntyre, 2004).  However, the compacted surface layer is generally restricted to within the top 300mm – though ripping can be of benefit to depths of 450mm to 600mm (Lacey, 2008).    If nothing is done before the return of the topsoil, the ability of the grass cover to develop a good root system is severely restricted and water penetration is inadequate.  In the earthwork operation timing is an important factor; equipment moves large volumes of soil in a short period of time and unless procedures are adopted to protect the underlying layers at the time of the operation, the consequences can be far reaching.

Fracturing

There are two treatments necessary once the desired sub-grade is obtained and before the topsoil is returned (Lacey, 2008).  Deep ripping is needed in order to restore the soil pore space and create vertical channels within the sub-grade profile to facilitate downward drainage and air access.  There is often a need to allow for two or more passes to gain adequate penetration.  Where a ripper tines on a doser or grader are employed instead of under-powered tractors, special care is needed to ensure the minimum depth is attained.  Secondly, there needs to be lateral fracturing of the compacted sub-grade.  Return passes to gain penetration should ensure fracturing over a width of 300mm to 400mm.  With the heavy equipment used in the formation, the severe compaction makes it necessary to restore the water penetration and attain a suitable bulk density.  The additional need for sub-soiling or fracturing the compacted sub grade layers becomes evident in sub-soils with the slightest plasticity – ripping has only created vertical channels often smearing the passage of the tine while the compacted nature of the sub grade material remains (Batey, 1988; Lacey, 2008).

It becomes vital that ripping and fracturing of the sub-grade is undertaken in suitable moisture conditions.  Take a sample of the subsoil at the depth of ripping and hand-roll into a thread about 4mm in diameter.  If the thread crumbles in segments greater than 10mm in length the moisture content is too high for deep ripping and decompaction (Lacey, 2008; McIntyre, 2004).  The only solution is to suspend operations until suitable moisture conditions are restored – the soil particles crumble and can no longer be retained in fragments greater than 10mm in length.

Final preparation

In sub soils of high clay content the application of gypsum at the time of ripping and fracturing can significantly improve the structure by flocculating the fine clay particles to form larger soil aggregates.  This application hence improves the porosity and water distribution in these soils.  Applications of 5t/ha prior to ripping and sub soiling has been found to create significant changes in the clay complex (Bolan and Gregg, 1994; McIntyre, 2004).  This occurrence is readily evident where calcium-rich line marking has been continually applied over a season to grassed tennis courts constructed with clay loam soils.

The deep-ripped and fractured sub-grade is too unstable and out of level to enable an even depth of topsoil to be spread satisfactorily.  This sub grade needs to be reconsolidated to re-create an even surface prior to replacing the topsoil.  In order to achieve this with the minimum compaction soil moisture conditions must be optimum.  The use of wide-tracked equipment or light ridge rollers is the only means whereby a suitable and firm sub grade surface can be created without re-compacting.

The replacement of indigenous topsoil generally to a depth of in the region of 200mm becomes a critical exercise.  Importing topsoil with scrapers and even wide tyred dumpers would naturally recompact the sub grade as well as the topsoil.  From the moment topsoil replacement commences every effort should be made to prevent compaction of the sub grade and negation of the efforts already made.     Generally the structure of the topsoil has been substantially destroyed with the stripping, stockpiling and reloading in order to return the topsoil to the graded subsoil.  This loss of structure cannot be restored in the short term and as such is very vulnerable to compaction.  To avoid this, special care needs to be taken in returning the topsoil.  By creating deep ramps of topsoil to allow access to dumpers, while not travelling on the sub grade, topsoil can be spread using only the dozer with wide tracks.  After spreading the topsoil only with dozers all further tractors employed should be of a compact nature with wide turf tyres.  All heavy equipment should have already been removed from site.

Provided the operation of returning topsoil is carried out so as to reduce compaction to a minimum as discussed, further ripping of the graded topsoil surface has been found to be unnecessary.  With the variable nature of the sub grade material there remains the fear of bringing undesirable aggregates to the topsoiled surface.  However, when the graded subsoil is of similar texture to the topsoil and relatively free of stone there would be benefit from sub-soiling through the topsoil into the subsoil below.  In this instance penetration would need to be at least 600mm through the topsoil.

Conclusion

The construction process of cut to fill to create the desired levels and grade results in severe compaction of the sub-grade. The bulk density is significantly increased, vertical drainage potential is prevented and porosity is reduced well below limits for satisfactory root growth.  Within this virtually impermeable crusted surface, severe breaking of the affected layers must be carried out prior to the return of the topsoil.  Drainage through the base, however slow, is vital in sustaining a playable surface.  The objective must be to create a gradual transition between the topsoil and the sub grade material below.  This can only be achieved by deep ripping together with subsoiling to distinctly fracture the entire affected layers.  The application of gypsum can further assist in improving the structure of heavy impermeable clay subsoils.  Special care thereafter is necessary to reconsolidate the levels, avoiding any undue compaction before and during the return of the topsoil and finally preparing the surface for seeding.

Literature cited

Batey,T.1988. Soil Husbandry.  Soil and land use consultants, Aberdeen. p.84.

Bolan, N. and Gregg, P. 1994. The use of gypsum in turf soils. Fertiliser and Lime Research Centre, Massey University, Palmerston North.

Buckman,H.O. and Brady, N.C. 1972. The nature and properties of soils. The Macmillan Company, London. p.6, 53-59.

Lacey, J.E. 2008. Deep ripping and decompaction.  New York State Department of Environmental Conservation. p. 1-2.

McIntyre, K. 2004.  Problem solving for golf courses, the landscape, sports grounds and race courses.

Horticulture Engineering Consultancy, Kambah ACT, Australia. p.109.