In most cities and districts the infrastructure requirements for subdivision are usually different from the NZ Building Code. This is because subdivision infrastructure is often shared and must last longer than the buildings it serves. It serves the land, which will be around a very long time indeed.
There are some helpful rules of thumb that can be used when considering how to service a proposed subdivision. Hopefully these can be used by developers and designers to avoid common issues associated with land development proposals.
Underground Utilities
Shared pipes should have a minimum cover of 600mm. This is sometimes known as the depth below which ‘mechanical protection’ is not required. The purpose being to prevent damage from vehicle loads and shallow excavation.
For the same reason services should also be separated from each other vertically and horizontally. The clearances vary depending on the service (always water above sewer)! It’s a good idea to design a ‘trench spec’ for shared trench situations.
Shared pipes should also be laid parallel to boundaries and structures at a minimum offset of 1.5m. Deeper pipes should be offset at least their invert depth to allow easier access. Easements are required to protect shared services and the width will depend on the depth or at least three metres.
Drainage
Gravity wastewater systems should be ‘self-cleaning’. This usually means a minimum grade of 0.55% for 150 diameter mains. This is 1 in 182 or a 6cm fall over 10 metres. Larger pipes and stormwater drains can be laid flatter.
Maintenance structures are required at bends and specified intervals on long straight stretches. The standard manhole size has a 1050mm internal diameter. Deep manholes, those with large pipes or which have an internal drop structure are usually required to be 1200mm or larger.
Stormwater collection and disposal systems are usually required to achieve ‘hydraulic neutrality’. This essentially means that runoff is attenuated on-site before being discharged downstream. Slimline under eave water tanks or underground soak-pits are common ways of achieving this.
Roading
Roads are still built the Roman way. The topsoil is excavated to a firm subgrade. Hardfill is used to create a pavement comprising a firm sub-base over which the basecourse is compacted. The pavement is often around 300mm thick depending on the ground conditions. Typical finished surfaces used are chipseal, asphalt (both flexible pavements) or concrete. Kerbs are only required where a flexible pavement is used.
The minimum width for two vehicles to pass safely is 5.5m. A long shared driveway should have passing bays of this width every 50 metres.
Road surfaces must be graded to allow stormwater to run off. This is usually a minimum longitudinal grade of 0.4% (1 in 250) with a 3% crossfall. Maximum longitudinal grades are ideally 10% (1 in 10) for roads, 12.5% (1 in 8) for vehicle crossings and 20% (1 in 5) for driveways.
Sight lines for intersections and vehicle crossings can be complicated. Usually the speed limit is a good indicator of how far a driver should be able to see when approaching any turning vehicle. The drivers eye height is usually measured at around 1.2m.
Summary
These are just some of the considerations when contemplating how to access and service a proposed subdivision. Having a good concept engineering design early in the process is key to saving money during consenting and construction.
