Field Permeability Testing in Sunnyvale: Lefranc and Lugeon Methods

The rig arrives on a Sunnyvale site, and the first thing the drill crew sets up is the double-packer assembly for a Lugeon test or the slotted casing for a Lefranc test. This isn't lab work with remolded samples; it's direct measurement of how water flows through the formation beneath the surface. In a city built on the alluvial fan of Stevens Creek, where layers of silty clay interbed with sandy lenses, knowing the actual field permeability can make or break a dewatering plan. We run these tests at specific intervals during drilling, isolating the zone with packers and recording pressure versus flow. For projects near the Baylands where groundwater is barely five feet down, the data from a CPT test gives us the stratigraphy, but the permeability test tells us exactly how fast the water comes back when we pump.

A falling-head test in a Sunnyvale silty sand can show permeability ten times higher than a constant-head test in the adjacent clay lens—the formation controls the method, not the other way around.

Methodology and scope

Sunnyvale's transformation from orchards to tech campuses happened fast, and the geotechnical legacy is a patchwork of engineered fill over natural lowland deposits. The historic downtown grid sits on soils that can alternate from dense clay to loose sand within a few vertical feet. When a design calls for a basement excavation or a stormwater infiltration gallery, the standard approach of estimating permeability from grain size distributions—like those from our grain size analysis—needs field validation. A Lefranc test in a borehole measures the hydraulic conductivity of the soil by adding or removing water and tracking the recovery head over time. For weathered bedrock zones, the Lugeon method uses pressure stages to check if fractures open up under higher head, which is critical data for grouting decisions. Our setup runs off the same drill rigs we use for geotechnical exploration, so we can test multiple horizons in a single mobilization.

Site-specific factors

A 5-story mixed-use project on Mathilda Avenue ran into trouble when the dewatering system, designed from lab permeameter tests, couldn't keep the excavation dry. The lab samples were remolded and didn't capture the interconnected sand seams in the native alluvium. We mobilized within 48 hours and ran Lefranc tests at three depths across the site. The field data showed the sand lenses had hydraulic conductivity an order of magnitude higher than assumed. That changed the well-point spacing from 15 feet to 8 feet and added a deep sump at the low corner. The excavation stayed dry, but the lesson was clear: Sunnyvale's heterogeneous deposits demand in-situ hydraulic characterization. Relying solely on correlations from particle size can underestimate inflow rates and lead to slope instability at the excavation face. Groundwater in the Santa Clara Valley doesn't follow textbook assumptions.

Technical parameters

Parameter	Typical value
Test Standard	ASTM D6391 (Lefranc), USBR procedures (Lugeon)
Soil Types Tested	Sands, gravels, silts, weathered rock, fractured bedrock
Measurement Range	10^-5 to 10^-2 cm/s (Lefranc); 1 to 100 Lugeon units
Test Interval Length	18 to 40 inches isolated with pneumatic packers
Pressure Stages (Lugeon)	Typically 5 stages at increasing/decreasing pressure
Drilling Method	Hollow-stem auger, rotary wash, or diamond coring
Data Output	Hydraulic conductivity (k) in cm/s or Lugeon values

Complementary services

Lefranc Variable-Head Testing

Falling-head and rising-head tests in soil boreholes to determine hydraulic conductivity. We use standpipe and pressure transducer setups for accuracy across low to moderate permeability formations.

Lugeon Pressure Testing in Rock

Multi-stage pressure tests in fractured rock using double packers. We evaluate the hydraulic behavior of discontinuities and the need for curtain grouting beneath dams, shafts, or deep foundations.

Pumping Tests and Slug Tests

Larger-scale aquifer characterization using observation wells. We design the test layout, monitor drawdown with transducers, and interpret the data using Theis and Cooper-Jacob solutions for transmissivity and storativity.

Questions and answers

What does a field permeability test cost in Sunnyvale?

A single Lefranc or Lugeon test in Sunnyvale typically ranges from US$700 to US$1.090 per test interval, depending on depth, access, and whether it can be combined with an existing geotechnical drilling program. Mobilization costs are separate and depend on the rig type and site logistics.

When is a Lugeon test required instead of a Lefranc test?

The Lugeon test is specifically for fractured rock masses. If the Sunnyvale site has weathered sandstone or shale bedrock at depth, the Lugeon method applies. It measures how fractures open or close under pressure, which tells us if grouting is needed. Lefranc tests are for soils above the bedrock—sands, silts, and gravels.

How long does a field permeability test take on site?

A single Lefranc test in a soil borehole takes about 45 to 90 minutes, including stabilization time. A full Lugeon test with five pressure stages in rock can take two to three hours per interval. For a typical Sunnyvale project with three test depths, plan on a full day of testing integrated with the drilling program.

Field Permeability Testing in Sunnyvale: Lefranc and Lugeon Methods

Methodology and scope

Site-specific factors

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