There is a particular sound a double-packer assembly makes when it seats properly at depth inside a NMLC borehole. In Mildura, where the near-surface geology can shift from stiff Shepparton Formation clays into clean Parilla Sand aquifers within a few metres, getting that seal right determines whether your Lugeon test yields reliable data or just a few hours of wasted rig time. We run these tests across the Sunraysia district with equipment calibrated for the low-transmissivity lenses common under Red Cliffs and Merbein. The Lefranc method handles the shallow clayey zones that dominate the floodplain near the Murray River, while the Lugeon setup works best when you punch into the semi-consolidated sand layers that feed the regional groundwater system. Both procedures follow the constant-head and falling-head protocols detailed in AS 1726, and we record pressure-step responses at 1-minute intervals to catch any fracture dilation or clogging before it skews the mean permeability value. Before mobilising the packer rig, many contractors combine the programme with SPT drilling to log refusal depths and identify the target test zones precisely.
A single Lugeon pressure stage run for 10 minutes instead of 20 can overestimate the rock mass permeability by 40% — short cycles are the most common error we correct on Sunraysia dam audits.
Site-specific factors
Mildura's annual evaporation rate exceeds 2,200 mm while rainfall barely tops 290 mm, creating a perched-water-table scenario across much of the irrigation district that can mislead a quick interpretation of a Lefranc test. A lens of sandy clay sitting above a dense clay aquitard will drain rapidly when a test pocket is opened, mimicking a permeable layer, when in reality it is simply releasing stored capillary water. The Lugeon test deeper in the sequence avoids this artefact but introduces its own risk: if the injection pressure accidentally exceeds the overburden stress in the Parilla Sand, you hydrofracture the formation and record a permeability that does not exist under natural gradient conditions. We keep net injection pressure below 25 kPa per metre of depth in uncemented sands, which is conservative relative to the AS 1726 guideline but has prevented more than one unnecessary grouting programme. For sites near the Murray where artesian conditions can develop seasonally, the rising-head Lefranc variant combined with in-situ permeability monitoring during excavation gives a far more representative operating k-value than any single pump-in test.
Quick answers
What is the difference between a Lefranc test and a Lugeon test, and which one does my Mildura site need?
The Lefranc test measures permeability in soil by injecting or withdrawing water from an open pocket in a borehole, typically in clays, silts, or sandy soils where the hole stays open without support. The Lugeon test uses an inflatable double-packer to isolate a section of borehole in rock or semi-consolidated material — in Mildura, that is usually the Parilla Sand or the underlying Renmark Group — and injects water under controlled pressure to measure hydraulic conductivity and assess fracture behaviour. If your project involves dewatering an excavation in the floodplain clays, a Lefranc test is the right starting point. If you are designing a dam curtain or assessing leakage beneath a levee, the Lugeon test gives the pressure-dependent data you need.
How long does a field permeability test programme take in the Sunraysia region?
A single Lugeon test at one 5-metre interval typically requires 2 to 3 hours of rig time once the hole is drilled and cased through the overburden, because each of the five pressure stages runs for a minimum of 10 minutes and we need time to equalise the packer and stabilise flow between stages. A Lefranc test is quicker — usually 30 to 60 minutes per pocket — but borehole development in the Blanchetown Clay can add an extra hour if smearing is significant. A full programme of four to six test intervals across one deep investigation borehole in Mildura generally takes a full working day.
What does an in-situ permeability test cost in Mildura?
Can Lugeon testing help design a grouting programme for a dam cut-off wall?
That is exactly the primary application of the Lugeon test in dam engineering. By running the full five-stage pressure cycle, you can distinguish between laminar flow through fine pores (which grout will penetrate poorly) and turbulent flow through open fractures (which are groutable). The ANCOLD guidelines use a threshold of about 5 Lugeons to decide whether a rock mass requires curtain grouting. In the fractured sandstone of the Parilla Formation under Mildura, we often see Lugeon values above 10 Lu in the upper 15 metres, which reduces to below 3 Lu once the fractures close with depth — that transition zone is where most of the grout take occurs and where your curtain design needs to focus.
