When a drill rig can't get in, or you need to see the ground with your own eyes, a mechanical excavator cutting a clean trench tells you more in ten minutes than a dozen borehole logs. In Wellington, where we contend with greywacke at shallow depth, wind-blown loess over colluvium, and historic reclamation fill along Lambton Quay, nothing replaces direct observation. Our team runs these exploratory test pits with a hydraulic digger fitted with a toothless bucket to a typical depth of 3.5 to 4.5 metres, exposing the contact between the Port Hills loess and the underlying weathered bedrock. We log the profile to NZGS guidelines, photograph the face, and take undisturbed block samples right there in the trench. Often we combine a test pit with an in-situ permeability test to measure infiltration rates for stormwater design, giving the civil engineer a complete subsurface picture before a single footing is poured.
A clean trench wall exposes the contact between fill, natural soil, and greywacke bedrock in a way no core sample ever can.
Local ground factors
The difference between a hillside site in Wadestown and a flat section in Miramar is night and day. Wadestown sits on steep, loess-covered greywacke, and a test pit there often reveals a thin veneer of fill over natural colluvium that's stable enough to stand vertical for an hour. Miramar, by contrast, is built partly on Rongotai dune sand and compressible peat layers near the old lagoon. Dig a pit in Miramar and you might hit groundwater at a metre and watch the sandy walls start to ravel within minutes. This is where skipping an exploratory test pit becomes a costly mistake. Without seeing the actual stratigraphy, a designer might assume uniform bearing and miss the buried soft spot that leads to differential settlement and cracked floor slabs. We've pulled out old timber piles, buried demolition rubble, and even uncharted service trenches that no desktop study ever showed.
Common questions
What does an exploratory test pit cost in Wellington?
For a standard test pit to 3.5 metres depth in accessible ground, including a tracked excavator with operator, our engineer on site for logging and sampling, service location, and a factual report with photographs, you're generally looking at NZ$760 to NZ$1,240 per pit. The price varies depending on access constraints, the number of pits on the same site, and whether we're taking undisturbed samples or running in-situ permeability tests.
How deep can you go with a test pit in Wellington's hills?
Practical depth is usually 3.5 to 4.0 metres with a standard 5-8 tonne excavator on accessible slopes. On steeper terrain, or where greywacke rock is very shallow, we may only reach 1.5 to 2.0 metres before hitting refusal. For depths beyond 4.5 metres, or where groundwater is high, we move to a drill rig for an SPT borehole because trench wall stability becomes a safety issue.
Do you reinstate the ground after digging the test pit?
Yes, full reinstatement is part of the scope. We backfill the pit in compacted lifts using the excavated material, or with controlled fill if specified by the earthworks specification. The surface is graded to match the surrounding ground, and we can supply a compaction test report if the reinstated area will carry traffic or structural load.
