The Lugeon test is a type of geotechnical investigation carried out within a borehole (in situ test), which defines the value of permeability (K) in variably fractured rocks (rock masses), where water is introduced at constant pressure in isolated sections and semi isolated.
The Lugeon test is normally carried out in 5 meter sections isolated by pneumatic seals, in five 10-minute intervals with a constant water pressure in each interval, the test is carried out at increasing and decreasing pressure between zero and maximum pressure.
Lugeon Test Procedure
As a general rule, the Lugeon test is carried out by introducing water at constant pressure in successive loading and unloading steps of 0, 1, 2, 5 and 10 Kp/cm2 in sections isolated by shutters, however this can cause that in certain types of lithology fracturing occurs because the pressure is too high.
For this reason, it is advisable to define a maximum test pressure (pmax) that does not exceed the minimum stress in situ, thus avoiding hydraulic fracturing and the test pressures in successive steps will be based on the maximum pressure as indicated. shows in the following table:
| Lugeon Test Intervals | Pressure |
|---|---|
| first interval | 0.5 maximum pressure |
| second interval | 0.75 maximum pressure |
| third interval | Maximum pressure |
| fourth interval | 0.75 maximum pressure |
| fifth interval | 0.5 |
Using the average values of water pressure and flow measured at each stage, the average hydraulic conductivity of the rock mass can be determined, where the hydraulic conductivity is expressed in terms of the Lugeon unit.
Interpretation
The lugeon essay is defined as the water absorption of 1 liter per meter of drilling and per minute, carrying out the test at 10 atmospheres of pressure for 10 minutes and one Lugeon unit is equivalent to a coefficient of permeability of 10-5 cm/s (UL = 1 1 /mx min = 10-5 cm/s).
Classification of the types of rock masses according to their permeability.
| solid type | Lugeon Units | Pressure (kp/cm2) |
|---|---|---|
| very waterproof | 0-1 | 10 |
| little permeable | 1-3 | 10 |
| Permeable | 3 | 10 |
| 1.5-6 | 5 | |
| very permeable | 3 | 10 |
| extremely permeable | 6 | 5 |
Therefore, the Lugeon value for each test is calculated as follows and then an average representative value for the rock mass tested is selected.
Lugeon value = (q / L) x (P0 / P)
where,
q – flow rate (lit / min)
L – Well test interval length (m)
P0 – 1 MPa reference pressure (MPa)
P – Test pressure (MPa)
Permeability relationship and Lugeon value with discontinuities
Considering a homogeneous and isotropic condition, a Lugeon will be equal to 1.3e-7m/s. Contrary to continuum media, the hydraulic conductivity of the rock mass is greatly influenced by discontinuities in the rock, the typical range of Lugeon values. and the condition of the corresponding rock is indicated in the following table:
| lugeon value | conductivity rating | rock discontinuity condition |
| 1 | very low | Very tight |
| 1-5 | Bass | Tight |
| 5-15 | Moderate | Few and partially open |
| 15-50 | Medium | Some and moderately open |
| 50-100 | Tall | many and open |
| 100 | Very high | Open discontinuities, several empty spaces |