Vertical small diameter (< 800m) Wells - relief of artesian pressure

Category: MODIFYING THE GROUNDWATER REGIME – Deep drainage

Description

Relief wells, characterized by a small diameter (<800 mm) (Fig.1), may be used to reduce piezometric head in a confined aquifer. No pumping is necessary, relief wells can only discharge water when the piezometric level in the aquifer is above the level of their outlet (Fig. 2a). Therefore a relief well is able only to reduce the piezometric level to the level of the well’s outlet. At worst, the level of the outlet may be that of the ground surface, but discharge may also be at lower level, through a pipe installed in a trench (Fig. 2b).

Figure 1: Classification of wells
Figure 2: Typical relief well in a confined acquifer with the piezometric level higher a) and lower b) than the groundsurface
Figure 2: Typical relief well in a confined acquifer with the piezometric level higher a) and lower b) than the groundsurface  

These drainages are mostly appropriate in not very steep slopes where there is not sufficient fall for a gravity drain. Their most frequent application is therefore related to areas downstream of an earth dam or at the toe of a riverbank levee. Therefore in a slope, relief wells may be used to relieve the artesian pressure in a confined aquifer under the toe area, where the ground surface is usually on a flatter gradient (Forrester, 2001).

The technology used to construct the well is discussed in fact sheet 4.5.1. The only thing to add is that the length of the filter might be equal to the thickness of the aquifer.

Figure 3: Example of artesian well
Figure 3: Example of artesian well

 


Design methods

Once the decrease of the piezometric level is known according to the design, the corresponding discharged flow is calculated. A pipe being able to discharge this flow should be designed.


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 0 A relief well is only able to reduce the piezometric level to the level of the well’s outlet. Therefore, relief wells can only discharge water when the piezometric level in the aquifer is above the level of their outlet and only in this case they are used.
Topple 0
Slide 4
Spread 2
Flow 4

Material type

Descriptor Rating Notes
Earth 8 .
Debris 4
Rock 4

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 0 This system can lower the pore water pressure in a confined acquifer and it is usually placed 3-4 m deep from the ground surface.
Shallow (0.5 to 3 m) 6
Medium (3 to 8 m) 8
Deep (8 to 15 m) 8
Very deep (> 15 m) 6

Rate of movement

Descriptor Rating Notes
Moderate to fast 0 The steady-state condition is attained when the hydraulic equilibrium is reached and it is a function of the aquifer properties.
Slow 4
Very slow 8
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 2 This system is suitable only for artesian groundwater.
High 0
Low 0
Absent 0

Surface water

Descriptor Rating Notes
Rain 2 Relief wells modify the piezometric level of the confined aquifer and they are completely separated from the ground surface by means of a grouting cap.
Snowmelt 2
Localized 0
Stream 0
Torrent 0
River 0

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 7 good working depends strongly on the maintenance.
Feasibility and Manageability 8 Technique and design processes are well established and widely used in suitable conditions.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 7 .
Environmental suitability 4 will be updated
Economic suitability (cost) 6 The cost of these drainages is more expensive than the other drainage systems.

References

  • Charles I. Mansur, Fellow, ASCE, George Postol,  Member, ASCE, and J. Ronald Salley, Fellow, ASCE. Performance of relief well system along Mississippi river levees . Journal of geotechnical  and enviromental engineering (ASCE) August 2000 pp 727-738.

  • Forrester Kevin (2001). Subsurface drainage for slope stabilization. ASCE Press.

  • Milano V.(2005). Acquedotti. Guida alla Progettazione. Ed. Hoepli Milano pp 643.

  • Salama R,  Ali R., Pollock D., Rutherford J. and Baker V. (2003). Review of Relief Wells and Siphons to Reduce Groundwater Pressures and Water Levels in Discharge Areas to Manage Salinity. Report to Water & Rivers Commission, WA March 2003.

  • US Army Corps of Engineers (1993). Design, Construction, and Maintenance of Relief Wells. New York: ASCE Press, 90 pp.

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