Vertical small diameter (< 800m) Wells - Underdrainage of perched acquifer

Category: MODIFYING THE GROUNDWATER REGIME – Deep drainage

Description

In a perched water table, water seeping downward is blocked by an impermeable layer of clay or silt, while groundwater saturates the area above the impermeable layer, as shown in Figure 2. An impervious stratum creates a basin that may hold groundwater that is perched above the main water table. A perched water table  is not frequent and is well recognizable by geologists and water engineers trough accurate investigations. Perched water is fed by surface water derived from precipitation and snow melt. When the area is urbanized, perched water is further fed by lawn watering, drain from leaking sewer lines, and other man-made sources. A perched water reservoir can be replenished by a water source as far as a mile away (depending also on the involved soils). The size of a perched water reservoir can vary considerably. A small reservoir can pose a seepage problem only after a prolonged wet season, while some perched water reservoirs do not dry up even during dry seasons. However, in the Rocky Mountain region where clay stone bedrock is near the ground surface, the extent of the perched water table can be very extensive.

Figure 1: Classification of wells
Figure 1: Classification of wells
Figure 2: Example of perched table
Figure 2:  Example of perched table

A small amount of perched water may be drained by drilling holes which cross the impervious basin, therefore small diameter well (<800 mm) without pumps but with the open bottom into the sand layer placed below the impervious soil layer can be used to under drainage the  perched table (Figure 3).

Figure 3: Example of well to underdrain the perched table
Figure 3: Example of well to underdrain the perched table

 


Design methods

The borehole should be designed to draw down a water flow enough to stabilize the area.


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 2 .
Topple 2
Slide 6
Spread 0
Flow 0

Material type

Descriptor Rating Notes
Earth 6 The perched water table usually develops into debris layer resting on clay
Debris 8
Rock 4

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 0 The effect of lowering of the water table by means of underdrainage is effective of course where the perched basin is placed, usually at 3-8 m deep from the groundsurface.
Shallow (0.5 to 3 m) 4
Medium (3 to 8 m) 6
Deep (8 to 15 m) 4
Very deep (> 15 m) 4

Rate of movement

Descriptor Rating Notes
Moderate to fast 0 The water must have time enough to reach the sand.
Slow 4
Very slow 8
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 0 This system is suitable only for high freatic level.
High 8
Low 0
Absent 0

Surface water

Descriptor Rating Notes
Rain 2 .
Snowmelt 2
Localized 0
Stream 0
Torrent 0
River 0

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 6 good working depends strongly on the maintenance.
Feasibility and Manageability 6 .

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

  • Chen F. (2000). Soil Engineering: Testing, Design, and Remediation. Chapter 13: Drainage. CRC Press LLC

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

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

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