The fractured shale and diabase bedrock that sits just a few feet under most of downtown Trenton doesn't behave like a uniform sponge. Water moves through joints and fissures at wildly different rates depending on where you drill. The Delaware River floodplain adds another layer of complication — silty overbank deposits that trap groundwater and create artesian conditions near Route 29. A standard lab permeability test on a remolded sample won't capture this. That's why we run field permeability tests directly in the borehole. The Lefranc method gives us reliable K values in the overburden soils near the river, while Lugeon testing tells us exactly how fractured the bedrock is at depth. For deep excavation projects near the State House or along the Assunpink Creek corridor, combining these results with data from a CPT test helps us map the transition zone between alluvium and weathered rock without missing thin, water-bearing layers.
One Lugeon test in fractured Passaic Formation tells you more about actual water flow than ten lab permeability tests on intact rock cores.
Process and scope
We keep seeing the same mistake on projects along the Route 1 corridor: engineers assume one K value for the entire site based on a single boring. Trenton's geology doesn't work that way. The Passaic Formation underneath Chambersburg can yield 10 times more water in a fractured zone than in intact rock twenty feet away. When we run a Lugeon test sequence at multiple pressure stages — 1, 3, 5, 7, and 10 bar — the pressure-versus-flow curve tells us whether joints are dilating, washing out, or staying tight. That pattern matters. A dilating fracture under high pressure means grouting will behave differently than the contractor expects. We typically pair this with
grouting recommendations based on Lugeon values, and if the project involves retaining structures along the canal, we reference
retaining walls design parameters derived from the measured permeability to calculate realistic hydrostatic loads. The Lefranc test in the overburden uses a constant or falling-head procedure depending on whether we're in the sandy layers of the Trenton gravel or the low-permeability varved clays left by glacial Lake Mercer.
Common questions
How long does a typical Lugeon test take per interval in Trenton bedrock?
A complete five-stage Lugeon test on one 3-meter interval takes about 90 to 120 minutes from packer inflation to final depressurization. The actual time depends on how quickly flow stabilizes at each pressure stage. In highly fractured zones with high Lugeon values — say above 30 — stabilization happens fast, often within 2 or 3 minutes per stage. Tight rock with values below 1 Lugeon can require 10 to 15 minutes per stage to confirm steady-state flow. We typically complete two to three intervals per drilling shift when alternating between coring and testing.
What's the cost range for field permeability testing in the Trenton area?
Field permeability testing in Trenton typically falls between US$690 and US$940 per test interval, depending on whether it's a Lefranc in overburden or a multi-stage Lugeon in rock. The Lugeon test is at the upper end of that range because it requires a packer system, pressure transducer instrumentation, and longer field time for the full pressure sequence. Mobilization to your site within Mercer County is included in day-rate drilling costs, not charged separately.
Do I need both Lefranc and Lugeon tests for my Trenton project?
If your project involves excavation or foundations extending through both overburden and bedrock — which is almost always the case in Trenton due to shallow rock depth — then yes, you need both. The Lefranc test characterizes the overburden permeability for dewatering design, while the Lugeon test evaluates fracture flow in the underlying Passaic or Lockatong bedrock. They measure fundamentally different flow regimes: porous media flow versus fracture flow. Using only one method leaves a critical data gap that can lead to unexpected water inflows during construction.
