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Analysis and Interpretation

 

Data Analysis

In order to simplify the interpretation of the results, AquiferTest provides a set of diagnostic plots representing typical flow behaviours that can be encountered in fractured rock.  AquiferTest includes the typical Lugeon diagrams as proposed by Houlsby (1976), and also includes the additional typical curves for flow loss vs. pressure space, as described by Quiñones-Rozo (2010).

 

Pressure Diagram

The Gauge Pressure data are read from the grid and plotted on a simple Pressure vs. Step diagram as shown below

 

Lugeon Diagram

For each step, the Lugeon value is calculated using the equations described above and plotted on a simple bar chart as shown below.

The trends from the Lugeon Diagram can be compared to the diagnostic plots as described below to identify typical behaviour and choose a suitable Lugeon value.

 

Flow vs. Pressure Diagram

It is also possible 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). For each step, the Average Flow Rate is calculated from the defined readings and displayed in the table (in the column after the last flow reading). The Gauge Pressure and Average Flow Rate for each step are then plot on the "Flow vs Pressure" diagram as shown below.

Each orange point corresponds to one step, consisting of an average flow reading at a given pressure. A line is drawn starting at the origin and connecting each data point in sequence of the order of the steps (with the directional arrows corresponding to the sequence of the steps), thus forming the pressure loop. The slope of each line segment is indicative of the Lugeon value as the test proceeds. A shallow slope corresponds to a low Lugeon value, a steep slope corresponds to high Lugeon value. This interpretation technique makes it useful to do real-time monitoring and interpretation of the test data in the field.  The shape of these curves can be compared to the diagnostic plots as explained below.

 

Lugeon Test Interpretation

The following table summarizes the typical flow behaviours and corresponding diagnostic Lugeon Pattern and Representative Lugeon Value (based on Houlsby (1976) and Flow vs. Pressure Patterns based on Quiñones-Rozo, (2010).

 

Behaviour

Lugeon Pattern

Flow vs. Pressure Pattern

Representative Lugeon Value

Laminar Flow

Average of Lugeon values for all steps

Turbulent Flow

Lugeon value corresponding to the

highest water pressure (3rd step)

Dilation

Lowest Lugeon value recorded,

corresponding either to low or medium water pressures (1st, 2nd, 4th, 5th step)

Wash-out

Highest Lugeon value recorded

(5th step)

Void Filling

Final Lugeon value

(5th step)

 

 

Typical Lugeon Behaviours

The following typical Lugeon Test behaviors are described by Houlsby (1976):

 

Laminar Flow: The hydraulic conductivity of the rock mass is independent of the water pressure employed. This behavior is characteristic of rock masses with 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.

 

In AquiferTest, when you click on the icon that corresponds to the observed behavior, the program will determine which is the appropriate Representative Lugeon value from the calculated values, and place this in the "Interpretations" box.

The following table describes the conditions typically associated with different Lugeon Values, as well as the typical precision for reporting these values (Quiñones-Rozo, 2010):

 

Example Interpretation

The following is an example of a Lugeon Test interpretation with 5 pressure steps. The image below is from the "Lugeon Test Data & Analysis" tab in AquiferTest.

 

Once the data have been entered, AquiferTest will automatically calculate the Average Flow Rate, Hydraulic Conductivity, Lugeon value, and plot all of this data in the diagrams at the bottom of the window.  The interpretation involves assessing the trend of the bar charts in the Lugeon Diagram, and both the shape and direction of the pressure loop in the Flow vs. Pressure diagram.

In this example, the trend of data in the Lugeon Diagram indicates conditions of Wash-Out. The shape of the Flow vs. Pressure diagram also indicates Wash-Out behavior. The shape of the flow vs. pressure diagram for Wash Out is similar to Void Filling, however the directional arrows of the pressure loop are in opposite directions. If you click on the "Wash-Out" icon below the main diagrams (for either the Lugeon Diagram or the Flow vs. Pressure Diagram), AquiferTest will retrieve the Representative Lugeon value recommended in the summary table above, and place this into the "Test Result Interpretation" section. In the case for Wash-Out behavior, it is recommended to use the highest Lugeon value (5th step), which corresponds to a Lugeon value of 7.5, and you will see this value defined in the Interpretations text box.  Often the test may exhibit multiple behaviors.  For this reason, the "Test Results Interpretation" text box is fully-editable, where you can type in any other comments or Lugeon value that you wish to see appear in the final report.

 

 


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