Site Assessment, Ground Conditions and Earthworks
Before the first spade
A drainlayer reads the site as carefully as the drawings. Establish datum and levels (confirm whether the plan uses NZVD2016, an older national datum or a local authority datum), then check ground conditions: soil type and classification, bearing capacity, groundwater level and its seasonal variation, rock, fill, expansive clays, liquefaction-prone or contaminated ground (HAIL sites), and slope stability.
Services must be located before excavating - a service-location request through the national before-you-dig service, plus council plans and a cable locator. Consent conditions usually carry erosion and sediment control requirements (silt fences, decanting earth bunds, stabilised entry/exit), and unstable ground may trigger specific foundation or drainage details.
Trenching is earthworks plus a safety system. Trench width, bedding depth and backfill material depend on pipe type: rigid pipes tolerate different bedding from flexible pipes, and trench spoil is rarely suitable as embedment. Compaction must meet the specified density without point-loading the pipe. Where cover is shallow or the drain passes under a driveway, concrete encasement or a load-bearing slab is typically required.
Traps
Confusing depth of cover with depth of invert. Assuming the excavated spoil is acceptable backfill. Ignoring a high water table until the trench floods. Forgetting that a consent may require a sediment control plan before works start. Mixing datums between site plan and as-built is a classic source of a drain that will not connect.
Worked example: a 40 m trench, 0.6 m wide and 1.2 m deep, is 40 × 0.6 × 1.2 = 28.8 m³ in situ. Allow bulking of roughly 20–30% for cartage, so plan for about 35–37 m³ of spoil - plus bedding import and disposal costs.