West Cary's newer subdivisions sit on heavily reworked Piedmont residuum, while the older lots near downtown hold onto deep bands of partially weathered rock that drain completely differently. On one project off Kildaire Farm Road we saw infiltration rates change by an order of magnitude within 40 horizontal feet. That kind of variability makes a desk-study permeability estimate useless. You need the actual value measured in the ground. The test pits crew can expose the soil profile, but a field permeability test, whether Lefranc in soil or Lugeon in fractured bedrock, gives you the K value that a stormwater reviewer in Wake County will actually accept. Without it, retention basin sizing is guesswork and dewatering plans for deep excavations become a liability.
A Lugeon value above 5 in Piedmont bedrock usually means open joints and a dewatering plan that cannot be an afterthought.
How we work
Local ground factors
The wireline packer assembly is deceptively simple: a reinforced rubber sleeve inflated with nitrogen, a pressure transducer at the test interval, and a flow meter at the surface that reads to tenths of a gallon. But in Cary's weathered transition zone, where saprolite grades into foliated gneiss, a poorly seated packer will leak around the seal and give you a K value that looks like open gravel when it is actually tight rock. The crew runs an inflation test against the casing before every stage, checks for bypass flow with a telltale line, and will not sign off on a Lugeon reading unless the pressure-step curve follows Houlsby's laminar-to-dilatant pattern. A fake high-permeability number here could let a contractor skip dewatering on a Walnut Creek tributary site, and that gets expensive when the excavation floods.
Regulatory framework
ASTM D6391-11 provides the standard test method for field measurement of hydraulic conductivity using borehole infiltration; USBR 6510 and Houlsby (1976) govern Lugeon test interpretation for rock mass permeability; and the NCDEQ Stormwater BMP Manual (2023) specifies infiltration rate requirements for Cary watersheds.
Complementary services
Lefranc Permeability Test in Soil
Run in a cased borehole with a screened interval at the target depth, typically in residual silty sands or saprolite. We use both falling-head and constant-head protocols depending on expected K range. Data is corrected for borehole geometry using Hvorslev's shape factor and reported as cm/s for direct input into stormwater models and basement drain designs.
Lugeon Packer Test in Rock
Performed in rotary-drilled NX or HQ holes in the partially weathered to fresh rock zone. The test isolates a 1.5 to 3-meter interval between inflatable packers. Five pressure steps (low-moderate-high-moderate-low) reveal flow regime behavior: laminar, turbulent, dilation, or washing-out. The Lugeon value directly informs grouting decisions and shaft dewatering plans.
Typical parameters
Questions and answers
How much does a field permeability test cost in Cary?
When does Wake County require a Lugeon test instead of a Lefranc test?
Lugeon testing applies when the proposed infiltration surface or excavation bottom is within fractured bedrock. If the boring log shows less than 2 feet of soil over competent rock, or if the design places a stormwater BMP in the transition zone, the county reviewer will ask for rock mass permeability, not soil conductivity. Lefranc tests are accepted for soil and highly weathered saprolite above the rock head.
How long does a permeability test take on site?
A single Lefranc falling-head test in soil typically wraps in 45 to 90 minutes once the borehole is at depth. A five-stage Lugeon test in rock runs about 90 to 120 minutes per interval, including packer seating verification and pressure stabilization. The main variable is the time required to drill and case the hole to the test depth, which depends on the rock hardness at the Cary site.
Can the test be run in an existing monitoring well?
It can, but with important caveats. A standard 2-inch monitoring well with a short screen can be used for a falling-head slug test, which gives a ballpark K value. For a design-grade Lefranc test we prefer a freshly drilled open borehole with a known diameter and a clean screen zone. Existing wells often have silted-in screens or filter packs of unknown condition that bias the result low.
What Lugeon value triggers a grouting recommendation in Piedmont rock?
For dam foundations and cutoff walls, values above 3 Lugeon units typically trigger a grouting program. For building excavations and retention basins in Cary, we flag values above 5 to 7 Lugeons as requiring a re-evaluation of the dewatering plan, especially if the rock mass shows a washing-out behavior in the high-pressure steps. The threshold depends on the hydraulic gradient and the tolerance for inflow on the project.