A geotechnical report from the Todos Santos Plaza area often paints a different subsurface picture than one from the newer developments near the Concord Naval Weapons Station. The former might intersect dense alluvial clays in the valley floor, while the latter frequently encounters sandy lenses and gravelly paleochannels closer to the Sacramento-San Joaquin Delta interface. That variability has a direct impact on groundwater flow, making generalized seepage assumptions risky. When infiltration rates or dewatering volumes need to be pinned down, the field permeability test (Lefranc/Lugeon) becomes the go-to tool. It provides direct measurements of hydraulic conductivity in the specific soil or rock mass, rather than relying on correlations from grain-size curves alone. For projects involving deep excavations in Concord's mixed alluvium, having an accurate k-value from a Lefranc test can prevent costly pumping system undersizing. The data also feeds directly into slope stability models where transient groundwater conditions govern the factor of safety.
A Lugeon value from fractured rock tells you more about the excavation's water behavior than a dozen lab perm tests on intact core samples.
Local ground factors
Concord sits at an elevation where the transition from the coastal hills to the alluvial plain creates a perched groundwater table in many east-side neighborhoods. A 2023 USGS groundwater monitoring report noted seasonal fluctuations exceeding 12 feet in the lower Willow Pass area. Ignoring this shallow, transient water when designing a basement or a stormwater infiltration trench leads to chronic wetness, uplift pressure on slabs, or complete exfiltration system failure. The Lefranc test quantifies the actual permeability of the silty clay lenses that often cap these perched zones, while the Lugeon test assesses whether the underlying weathered bedrock can accept injected stormwater without backing up. For retaining walls built into the hillside along Clayton Road, undetected fracture flow can build hydrostatic pressure behind the wall, eventually causing tilting or cracking. Field permeability data mitigates that risk by giving the designer a real number for the drain spacing calculation, rather than a textbook range that might be two orders of magnitude off.
Common questions
What is a Lugeon value and how is it interpreted?
One Lugeon unit equals a water take of 1 liter per minute per meter of test interval at 10 bars of injection pressure. A value below 1 Lu indicates very tight rock, often requiring no grouting. Between 1 and 5 Lu, the rock has moderate permeability and may need localized grouting. Above 5 Lu, the fracture network is open enough to require systematic grouting or significant dewatering. The pattern across the five pressure stages matters just as much as the absolute value.
How long does a field permeability test take on site?
A single Lefranc test in soil usually takes 30 to 90 minutes depending on the hydraulic conductivity—finer soils take longer for the water level to stabilize. A complete 5-stage Lugeon test in rock typically takes 60 to 120 minutes per interval. Multiple intervals down a single borehole can be tested in a day, though we allow extra time for packer seating and system saturation in dry formations.
What is the typical cost range for Lefranc/Lugeon testing in Concord?
Field permeability testing in the Concord area generally runs from US$540 to US$1,130 per test interval, depending on depth, access conditions, and whether it is a single Lefranc test or a multi-stage Lugeon sequence. Mobilization and borehole preparation are priced separately. A site-specific quote will account for the number of intervals and the expected rock hardness.
When do I need a Lugeon test instead of a lab permeability test?
Lab tests on intact core samples measure the matrix permeability of the rock, but they miss the fractures, joints, and fissures that dominate in-situ flow. A Lugeon test is necessary when the rock mass is fractured, when designing a grout curtain, or when evaluating tunnel inflow. In Concord's Martinez Formation sandstones, fracture permeability can be 100 to 1,000 times higher than the intact rock matrix permeability.
