Most landscape construction projects begin with some form of earthworks, ranging from simple small-scale levelling to major excavation or cutting and filling using light or heavy machinery. This insight from BRANZ will help you go about it the right way and avoid the ground falling away under your feet
The question of whether it really is necessary to disturb the earth should be considered for each construction project. Stripping of topsoil and excavation involves destruction of biological systems and can result in habitat destruction, erosion and increased sediments entering waterways.
Strategies for design and construction:
Minimise the scope of earthworks by careful design of grading in the overall project.
The use of plant materials or erosion control matting (which can also be planted) along slopes may be suitable alternatives to excavation for retaining walls to stabilise slopes.
Stockpile topsoil for reuse on site.
Reuse other surplus excavated material preferably on site or at a site in close proximity.
Select recycled materials, materials with recycled content or materials with low environmental impact for construction of retaining walls and paving.
Consider providing drainage swales and rain gardens (garden areas of high permeability) to minimise run-off. Use construction materials that help to minimise run-off to waterways, for example, crib-type retaining walls that can be planted.
Use locally sourced materials such as stone and timber for retaining structures.
Further considerations for earthworks should include:
Full investigation of site conditions, which may include engineering and geotechnical reports for larger works.
Compliance with the requirements of the Resource Management Act (excavation work not requiring a resource consent may be very limited) or the appropriate district plan.
Compliance with the Historic Places Act for sites that were in use before 1900.
Access for machinery.
Removal and conservation of topsoil (including providing space for storage).
Slope of any batters or banks created – the final angle should be less than the natural angle of repose (steepest slope at which loose material will lie without rolling down the slope) for the soil type and the cut height – this angle applies equally to the edges of platforms created with excavated material (see Table on opposite page).
The bearing capacity of soil and any fill.
Grades or falls to facilitate surface water and groundwater dispersal specifying finished levels for the contractor to work to.
Preventing contamination of waterways and stormwater drains with silt by providing for temporary surface water drainage, for example, with silt ponds or straw bales.
Rehabilitation of work areas on completion.
Specific requirements apply when excavating, particularly for trenches to ensure that the sides of the trench do not collapse. Where the excavation depth is less than 1.5 m, it is up to the contractor to determine if special precautions or work methods are necessary. Where the excavation is 1.5 m or deeper, the face must be cut to a safe slope, shored (with an approved shoring system) or certified by a chartered professional engineer that adequate safety precautions are being taken. Safe slopes are defined in the OSH publication Approved code of practice for safety in excavation and shafts for foundations.
This article was first published in BRANZ’s Landscape Construction book, Second Edition and is reproduced with permission.
The book covers the design, building and planning rules as they apply to landscape construction and provides guidance on design and construction to ensure good practices are followed. It can be purchased at www.branz.co.nz.