Lugeon Value

Lugeon Test Interpretation 405

LUGEON TEST INTERPRETATION, REVISITED

Camilo Quiñones-Rozo, P.E.1

ABSTRACT The Lugeon test is widely used to estimate average hydraulic conductivity of rock masses. Interpretation methods currently available in the literature were developed at a time when measurements were made in an analogous fashion and data was subsequently recorded by hand at rather large intervals of time. Current technology allows measuring and digital recording of data in real time, thus granting us an opportunity to update the interpretation procedures for Lugeon tests. This paper provides an interpretation method that expands the current procedures to benefit from the recent advances in data acquisition equipment.

INTRODUCTION

The extents of grouting and cut-off depths required in a dam foundation are directly related to the hydraulic conductivity (permeability) of the rock masses involved. In contrast to other geotechnical parameters for which variations can usually be measured in percentage terms (e.g., shear strength, density, compressibility, etc.), variations in hydraulic conductivity are usually measured in terms of magnitudes (e.g., 10-2 to 10-3). Selecting a representative value of hydraulic conductivity becomes of the outmost importance during design; especially, since under such a wide variation range, averaging the measured values will not suffice. Unlike soils, where seepage takes place through a series of small, closely spaced, interconnected pore spaces, seepage through rock masses occurs mostly along discrete planar discontinuities (e.g., joints, foliations, shears, etc.). Thereby, whereas in soils hydraulic conductivity is mostly controlled by the size, shape and arrangement of its voids (Terzaghi et al., 1996), in rock masses the conductivity depends on the aperture, spacing and infilling characteristics of its discontinuities (Goodman, 1980). Discontinuity aperture plays a particularly important role in the hydraulic conductivity of a rock mass. Consequently changes in the stress condition of the rock mass can produce significant changes on its hydraulic conductivity. The existence of an interrelation between stress and hydraulic conductivity ultimately means that accurate estimates of the hydraulic conductivity of a rock mass can only be obtained using in-situ tests. The Lugeon Test The most commonly in-situ test used to estimate hydraulic conductivity of rock masses is the Lugeon test – also called the packer test. The test, which derives its name from Maurice Lugeon (1933), is a constant head type test that takes place in an isolated portion 1 Senior Civil/Geotechnical Engineer, URS Corporation, 1333 Broadway Suite 800, Oakland, CA 94612, [email protected]

Collaborative Management of Integrated Watersheds 406

of a borehole. Water at constant pressure is injected into the rock mass through a slotted pipe bounded by pneumatic packers (Figure 1). A pneumatic packer is an inflatable rubber sleeve that expands radially to seal the annulus space between the drill rods and the boring walls.

Figure 1. Lugeon test configuration

Prior to the beginning of the test a maximum test pressure (PMAX) is defined. PMAX is chosen such that it does not exceed the confinement stress (σ3) expected at the depth where the test is being conducted, thus avoiding the development of hydraulic fracturing or hydraulic jacking. As a rule of thumb, PMAX is usually established using Equation 1, where D is equal to the minimum ground coverage – depth in the case of a vertical boring in a flat site or minimum lateral coverage in the case of a test conducted in a hillside.

ftpsi1

×D=PMAX (1)


The test is conducted in five stages, with a particular water pressure magnitude associated with each stage. A single stage consists of keeping a constant water pressure at the test interval for 10 minutes by pumping as much water as required. The first stage is held at a low water pressure, increasing the pressure in each subsequent stage until reaching PMAX. Once PMAX is reached, pressures are decreased following the same pressure stages used on the way up, thus describing a “pressure loop”. Table 1 shows the pressure magnitudes customarily used during the five test stages.

Table 1. Pressure magnitudes typically used for each test stage Test Stage Description Pressure Step

1st Low 0.50·PMAX 2nd Medium 0.75·PMAX 3rd Maximum (peak) PMAX 4th Medium 0.75·PMAX 5th Low 0.50·PMAX

During the execution of each stage, both water pressure (P) and flow rate (q) values are recorded every minute. Subsequently, average values for P and q are then used to compute the hydraulic conductivity for each stage. The hydraulic conductivity is expressed in terms of the Lugeon value, which is empirically defined as the hydraulic conductivity required to achieve a flow rate of 1 liter/minute per meter of test interval under a reference water pressure equal to 1 MPa (Equation 2).

PP

×Lq

×α=ValueLugeon 0 (2)

Since the Lugeon value is defined in SI units, it is required to introduce a dimensionless factor α in Equation 2 to accommodate the use of different systems of units. This factor takes a value of 1 when the SI units system is used (q [lt/min], L[m], and P [MPa]) and a value of 12.42 when the English units system is used (q [gal/min], L[ft], and P [psi]). The term P0 corresponds to a reference pressure equal to 1MPa or 145 psi. Under ideal conditions (i.e., homogeneous and isotropic) one Lugeon is equivalent to 1.3 x 10-5 cm/sec (Fell et al., 2005). Table 2 describes the conditions typically associated with different Lugeon values, as well as the typical precision used to report these values. Table 2. Condition of rock mass discontinuities associated with different Lugeon values

Lugeon Range Classification

Hydraulic Conductivity

Range (cm/sec)

Condition of Rock Mass Discontinuities

Reporting Precision (Lugeons)

<1 Very Low < 1 x 10-5 Very tight <1 1-5 Low 1 x 10-5 - 6 x 10-5 Tight ± 0

5-15 Moderate 6 x 10-5 - 2 x 10-4 Few partly open ± 1 15-50 Medium 2 x 10-4 - 6 x 10-4 Some open ± 5 50-100 High 6 x 10-4 - 1 x 10-3 Many open ± 10 >100 Very High > 1 x 10-3 Open closely spaced or voids >100


Once a Lugeon value has been computed for each of the five test stages, a representative value of hydraulic conductivity is selected based on the trend observed throughout the test, as explained in the next two sections.

CURRENT LUGEON INTERPRETATION PRACTICE The current Lugeon interpretation practice is mainly derived from the work performed by Houlsby (1976). On his work, geared towards establishing grouting requirements, Houlsby proposed that representative hydraulic conductivity values should be selected based on the behavior observed in the Lugeon values computed for the different pressure stages. Houlsby (1976) classified the typical behaviors observed in practice into five different groups, as follows: - Laminar Flow: The hydraulic conductivity of the rock mass is independent of the

water pressure employed. This behavior is characteristic of rock masses observing low hydraulic conductivities, where seepage velocities are relatively small (i.e., less than four Lugeons).

- Turbulent Flow: The hydraulic conductivity of the rock mass decreases as the water pressure increases. This behavior is characteristic of rock masses exhibiting partly open to moderately wide cracks.

- Dilation: Similar hydraulic conductivities are observed at low and medium pressures; however, a much greater value is recorded at the maximum pressure. This behavior – which is sometimes also observed at medium pressures – occurs when the water pressure applied is greater than the minimum principal stress of the rock mass, thus causing a temporary dilatancy (hydro-jacking) of the fissures within the rock mass. Dilatancy causes an increase in the cross sectional area available for water to flow, and thereby increases the hydraulic conductivity.

- Wash-Out: Hydraulic conductivities increase as the test proceeds, regardless of the changes observed in water pressure. This behavior indicates that seepage induces permanent and irrecoverable damage on the rock mass, usually due to infillings wash out and/or permanent rock movements.

- Void Filling: Hydraulic conductivities decrease as the test proceeds, regardless of the changes observed in water pressure. This behavior indicates that either: (1) water progressively fills isolated/non-persistent discontinuities, (2) swelling occurs in the discontinuities, or (3) fines flow slowly into the discontinuities building up a cake layer that clogs them.

Table 3 presents a graphic summary of the five behavior groups defined by Houlsby (1976), as well as the representative Lugeon value that should be reported for each group.


Table 3. Summary of current Lugeon interpretation practice (as proposed by Houlsby, 1976)

BEH

AVI

OR

PRESSURE STAGES LUGEON PATTERN DESCRIPTION REPRESENTATIVE LUGEON VALUE

LAM

INA

R

0.50PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageWater Pressure, P

0.75PMAX 1.00PMAX0.50PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage


0.75PMAX 1.00PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

All Lugeon values about equal regardless of the

water pressure

Average of Lugeon values for all stages

TUR

BU

LEN

T

0.50PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage



1st Stage

2nd Stage

3rd Stage

4th Stage


0.75PMAX 1.00PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

Lugeon values decrease as the water pressures increase. The minimum

Lugeon value is observed at the stage with the

maximum water pressure

Lugeon value corresponding to the

highest water pressure (3rd stage)

DIL

ATI

ON

0.50PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage



1st Stage

2nd Stage

3rd Stage

4th Stage


0.75PMAX 1.00PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

Lugeon values vary proportionally to the water pressures. The maximum Lugeon value is observed

at the stage with the maximum water pressure

Lowest Lugeon value recorded,

corresponding either to low or medium

water pressures (1st, 2nd, 4th, 5th stage)

WA

SH-O

UT

0.50PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage



1st Stage

2nd Stage

3rd Stage

4th Stage


0.75PMAX 1.00PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

Lugeon values increase as the test proceeds.

Discontinuities’ infillings are progressively washed-

out by the water

Highest Lugeon value recorded

(5th stage)

VOID

FI

LLIN

G

0.50PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage



1st Stage

2nd Stage

3rd Stage

4th Stage


0.75PMAX 1.00PMAX

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

1st Stage

2nd Stage

3rd Stage

4th Stage

5th StageLugeons

Lugeon values decrease as the test proceeds. Either non-persistent

discontinuities are progressively being filled or swelling is taking place

Final Lugeon value (5th stage)

PROPOSED MODIFICATION TO LUGEON INTERPRETATION PROCEDURE

Despite its inherent simplicity the interpretation procedure proposed by Houlsby (1976) correctly captures the interaction between the different variables involved in the phenomena of seepage through rocks. However, the procedure was devised at a time when discrete readings were made using dial gages at rather large intervals of time. The procedure proposed below, aims to update the Lugeon interpretation process to incorporate the use of current technology. Furthermore, this procedure will not only contribute to streamline the Lugeon interpretation process, but will also facilitate interpretation in those occasions when the test does not proceed according to plan. Use of Automated Data Acquisition Systems Automated data acquisition systems capable of measuring, displaying and recording Lugeon test and grouting data in real time have become available over the last years. This equipment measures flow rate and pressure at regular intervals of time and displays the information on an LCD display (Figure 2).


Figure 2. Data acquisition equipment for real time monitoring of Lugeon tests and grouting (Photo by Atlas Copco)

Since this equipment is able to measure both pressure and flow rate in real time it is possible to monitor the behavior of the Lugeon value as the test proceeds. In order to take advantage of this possibility, it is proposed to analyze the Lugeon test results using the flow loss vs. pressure space, with flow loss defined as the flow rate divided by the length of the test interval (q/L). Lugeon interpretation using the flow loss vs. pressure space The terms in the equation defining the Lugeon value (Equation 2) can be rearranged such that the flow loss (q/L) is expressed as shown below.

0PP

×α1

×ValueLugeon=Lq

(3)

If the product of the last two factors in Equation 3 is defined as a dimensionless pressure factor (ψ), then the flow loss could be ultimately expressed as shown in Equation 5.

0PP

×α1

=ψ (4)

ψ×ValueLugeon=Lq

(5)

In other words, the flow loss could be interpreted as the product of the Lugeon value and the dimensionless pressure factor ψ. According to this interpretation, if the results of the


Lugeon test are plotted in a flow loss vs. pressure space (q/L vs. ψ), sets of data having the same Lugeon value will plot over a straight line (Points and in Figure 3). Furthermore, this line – which will start at the origin – will have a slope equal to the Lugeon value.

Pressure Factor, ψ

Flow

Los

s, q

/L

1 lugeon

5 lugeons

15 lu

geon

s

50 lu

geon

s

100

luge

ons

1

2

34

5

Figure 3. Interpretation of Lugeon test data in the flow loss v pressure space

If a set of Lugeon values corresponding to the five stages of a test are plotted in the q/L vs. ψ space, a “pressure loop” will be observed. The shape of this loop describes the behavior of the Lugeon value as the test proceeds, and thereby can be used for interpretation purposes. For example, if all the points lie atop of a line crossing through the origin it is known that the Lugeon value remained constant throughout the test, implying that a laminar behavior was observed. The same type of analysis can be performed for each of the behavior categories proposed by Houlsby, as summarized in Table 4. The proposed Lugeon interpretation procedure conserves the same behavior categories proposed by Houlsby (1976), while using an approach that renders it compatible with the use of automated data acquisition systems. It is expected that the use of this interpretation procedure will allow real time monitoring and interpretation of test data. The choice of the representative Lugeon value for each behavior category remains essentially unchanged. However, in those cases where turbulent or dilation behaviors are observed, it is recommended that the Lugeon value selected corresponds to those values observed at the range of pressures expected during operation (e.g., after dam filling).


Table 4. Proposed Lugeon interpretation procedure using the flow loss vs. pressure space

BEHAVIOR WATER LOSS VS PRESSURE PATTERN DESCRIPTION REPRESENTATIVE

LUGEON VALUE LA

MIN

AR

Water Pressure, P

Flow

Los

s, q

/L

1

2

3

4

5

Water Pressure, P

Flow

Los

s, q

/L

1

2

3

4

5

All Lugeon values about equal regardless of the water

pressure

Average of Lugeon values for all stages

TUR

BU

LEN

T

12 3

4

5

Water Pressure, P

Flow

Los

s, q

/L

12 3

4

5

Water Pressure, P

Flow

Los

s, q

/L Lugeon values decrease as the water pressures increase. The minimum Lugeon value is observed at the stage with the

maximum water pressure

Range of Lugeon values observed at water pressures expected during operation. If

water pressure expected during operation is unknown use the value corresponding

to the medium water pressure (2nd or 4th stage)

DIL

ATI

ON

Water Pressure, P

Flow

Los

s, q

/L

1

2

3

4

5

Water Pressure, P

Flow

Los

s, q

/L

1

2

3

4

5

Lugeon values vary proportionally to the water pressures. The maximum

Lugeon value is observed at the stage with the maximum

water pressure

Range of Lugeon values observed at water pressures expected during operation. If

water pressure expected during operation is unknown use the value corresponding

to either low or medium water pressures (1st, 2nd, 4th,

or 5th stage)

WA

SH-O

UT

1

2

345

Water Pressure, P

Flow

Los

s, q

/L

1

2

345

Water Pressure, P

1

2

345

Water Pressure, P

Flow

Los

s, q

/L Lugeon values increase as the test proceeds.

Discontinuities’ infillings are progressively washed-out by

the water

Highest Lugeon value recorded (5th stage)

VOID

FIL

LIN

G

1

23

4

5

Water Pressure, P

Flow

Los

s, q

/L

1

23

4

5

Water Pressure, P

Flow

Los

s, q

/L Lugeon values decrease as the test proceeds. Either non-persistent discontinuities are progressively being filled or

swelling is taking place

Use final Lugeon value (5th stage), provided that

presence of non-persistent discontinuities and/or

occurrence of swelling is confirmed by observation of

rock core.


Interpretation of Lugeon data when test does not proceed according to plan In practice it is common to encounter situations where the five pressure stages required to complete a “pressure loop” can not be completed (e.g., pump used was not able to achieve the intended pressure at the maximum flow capacity, the drilling rods could not be filled, etc). Although, it would be advisable to ignore these data points, there are occasions where the amount of information at hand is so limited that disregarding data is not an option. In such cases, it is advisable to interpret the Lugeon data as follows:

- If results from the test stages available describe a convex curve in the q/L vs. ψ space (i.e., slope decreases as ψ increases), the maximum Lugeon value obtained should be reported as an upper bound value (i.e., less than).

- If results from the test stages available describe a concave curve in the q/L vs. ψ space (i.e., slope increases as ψ increases), the maximum Lugeon value obtained should be reported as a lower bound value (i.e., greater than).

The procedure above allows using the limited information available to gain a better understanding of the rock mass permeability. However, by reporting lower and higher bound values –rather than representative values –, it assigns a lower level of reliability to these results.

LIMITATIONS OF THE LUGEON TEST One of the main drawbacks of the Lugeon test is that only a limited volume of rock around the hole is actually affected by the test. It has been estimated that the effect of the Lugeon tests – with a test interval length of 10 feet - is restricted to an approximate radius of 30 feet around the bore hole (Bliss and Rushton, 1984). This suggests that the hydraulic conductivity value estimated from this test is only representative for a cylinder of rock delimited by the length of the test interval and the radius given above. Although the use of well-pumping tests with observation wells can overcome this limitation (Cedergren, 1989), such tests are seldom conducted since they involve drilling several holes which increases the exploration cost considerably. Due to the spatial limitation of the Lugeon test it is not recommended to estimate the hydraulic conductivity using closed-form analytical solutions that rely on the assumption that a large portion of the rock mass is engaged during the test. Furthermore, such analytical solutions usually require an adequate knowledge of the location of the ground water table elevation. However, it is usually observed that ground water elevation measurements while drilling can be artificially high due to the large amounts of water pumped into the hole to circulate the cuttings. As observed by Hoek and Bray (1974) many of the mathematical theories available in the literature have gone beyond the bounds of practical application. In most practical cases, the assumptions used by the analytical methods do not correspond to the actual conditions of the rock mass to be studied (i.e., laminar flow through homogeneous, isotropic, continuous media) or the parameters required in these equations can not be


readily estimated or quantified. Due to these limitations it is recommended to avoid over reliance on such analytical methods and limit their use to perform sensitivity analysis that can be used to assess the validity of the results obtained from Equation 2.

SUMMARY

This article presents a modification to the Lugeon interpretation procedure proposed by Houlsby (1976). Under this updated procedure data corresponding to different stages of the test are plotted in the flow loss vs. pressure space and interpreted based on the shape of the resulting “pressure loop”. Equations that can be used to automate this procedure are provided to facilitate its use with automated data acquisition systems. It is expected that the use of this method can contribute to focus the interpretation of hydraulic conductivity exclusively on data collected in the field. This will avoid the use of elaborate closed-form analytical solutions that rely on assumptions that seldom correspond to the conditions observed in practice.

REFERENCES

Bliss, J., Rushton, K. (1984). The reliability of packer tests for estimating the hydraulic conductivity of aquifers. Q. J. Eng. Geol. Vol. 17, pp. 81-91. Cedergren, H. (1989). Seepage, Drainage, and Flow Nets. Third Edition. J. Wiley & Sons. New York, N.Y. Fell, R., MacGregor, P., Stapledon. D., Bell, G. (2005). Geotechnical Engineering of Dams. Taylor & Francis. London. UK. Goodman, R. (1980). Introduction to Rock Mechanics. First Edition. J. Wiley & Sons. New York, N.Y. pp. 32-34. Hoek, E., Bray, J. (1974). Rock Slope Engineering. Institute of Mining and Metallurgy, London. UK. Houlsby, A. (1976). Routine Interpretation of the Lugeon Water-Test. Q. J. Eng. Geol. Vol. 9, pp. 303-313. Lugeon, M. (1933). Barrage et Géologie. Dunod. Paris Terzaghi, K., Peck, R., Mesri, G. (1996). Soil Mechanics in Engineering Practice. Third Edition. J. Wiley & Sons. New York, N.Y. pp. 72-73.

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