Caisson (> 5-6m), with gravity drainage (and secondary sub-horizontal drains)

Category: MODIFYING THE GROUNDWATER REGIME – Deep drainage

Description

Large diameter caissons excavated as described in fact-sheet 6.4 may be left with an open shaft and equipped with arrays of sub-horizontal microdrains as described in fact sheets 4.3 and 4.4 to supplement their structural role with drainage. Typical vertical and horizontal sections are shown in Figures 2 and 3. Figures 4 to 7 illustrate significant details. A typical application is shown in figures 8 and 9. In theory, such caissons could be constructed ourely for their drainage function, but this is unlikely to be appropriate and economic in practice.

Figure 1: Classification of wells
Figure 1: Classification of wells
Figure 2: Typical large diameter caisson with drainage function: vertical Section (source: SGI-MI project files)
Figure 2: Typical large diameter caisson with drainage function: vertical Section (source: SGI-MI project files)
Figure 3: Typical large diameter caisson with drainage function: Section 1-1 on horizontal plane (source: SGI-MI project files)
Figure 3: Typical large diameter caisson with drainage function: Section 1-1 on horizontal plane (source: SGI-MI project files)
Figure 4: Well excavation; support structure consists of micropiles and steel ribs (source (SGI-MI project file
Figure 4: Well excavation; support structure consists of micropiles and steel ribs (source (SGI-MI project file
Figure 5: detail of microdrain heads (source (SGI-MI project file
Figure 5: detail of microdrain heads (source (SGI-MI project file
Figure 6: Well with secondary drainage; structure consists of discrete piles and concrete ribs (source (SGI-MI project file
Figure 6: Well with secondary drainage; structure consists of discrete piles and concrete ribs (source (SGI-MI project file
Figure 7: Vertical drain mats in adherence with the ground between piles (source (SGI-MI project file
Figure 7: Vertical drain mats in adherence with the ground between piles (source (SGI-MI project file

Additional drainage may occur along the shaft wall, if this consists of discrete columnar elements (piles, miicropiles) with a gap between them and vertical draining mats are installed adhering with the ground between adjacent piles around the perimeter of the shaft.

The minimum diameter of the caisson is dictated by the space required for the installation of the microdrains. Indicatively, the minimum diameter is 5 m for microdraind 20 to 30 m long and 8 to 10 m for microdrains 50 to 60 m long.

The water intercepted may be discharged connecting the wells at the base by one or two small diameter collectors, allowing the water to flow away at the base of the slope to be stabilized.Wells with diameter of 8 to 10 m, equipped with a large number of drains need large diameter discharge collectors. In this case, collectors up to 1000 mm diameter are carried out using microtunnelling technology.This type of shaft may also be used as the starting or arrival point of drainage tunnels or as otfall for deep drainage trenches.

Figure 8: Typical application of caissons with combined structural and drainage function, provided by arrays of sub-horizontal microdrains and gravity discharge from base of hollow shaft. Plan and Longitudinal Section along basal discharge conduit (source: SGI-MI project files)
Figure 8: Typical application of caissons with combined structural and drainage function, provided by arrays of sub-horizontal microdrains and gravity discharge from base of hollow shaft. Plan and Longitudinal Section along basal discharge conduit (source: SGI-MI project files)
Figure 9: Telescopic caisson with combined structural and drainage function, provided by arrays of sub-horizontal microdrains and gravity discharge from base of hollow shaft (source: SGI-MI project files)
Figure 9: Telescopic caisson with combined structural and drainage function, provided by arrays of sub-horizontal microdrains and gravity discharge from base of hollow shaft (source: SGI-MI project files)

Design methods

For the structural design of these caissons, reference may be made to fact sheets 6.4. Where the main drainage function is provided by sub-horizontal drains, the design must define the number, elevation, orientation and length of subhorizontal drains pipes. In this case reference may be made to fact sheet 4.3 for guidance on the design of the sub-horizontal drains.

The minimum section of the base conductor are determined by conventional hydraulic calculations based on the required drawdown and the associated flow. Spare capacity should be provided, to minimize maintenance requirements.


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 0 This system is usually adopted to stabilize landslides with deep slip surface.
Topple 0
Slide 6
Spread 6
Flow 4

Material type

Descriptor Rating Notes
Earth 8 Deep well systems are effective in a range of soil from gravel to silty fine sands.
Debris 6
Rock 2

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 0 This system can reach typical depths of 10 - 15 m.
Shallow (0.5 to 3 m) 0
Medium (3 to 8 m) 0
Deep (8 to 15 m) 6
Very deep (> 15 m) 8

Rate of movement

Descriptor Rating Notes
Moderate to fast 0 The steady-state condition is attained when the cone of depression reaches the equilibrium; this time is a function of the aquifer properties.
Slow 2
Very slow 8
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 4 This system is suitable for high freatic level.
High 8
Low 6
Absent 0

Surface water

Descriptor Rating Notes
Rain 2 This system is not suitable to drainage shallow water.
Snowmelt 2
Localized 0
Stream 0
Torrent 0
River 0

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 7 Good performance depends strongly on the maintenance of the discharge pipe and sub horizontal drains
Feasibility and Manageability 7 Technique and design processes are well established and widely used in suitable conditions.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 7 Large spaces need for shafts 6 to 10 m in diameter, plus additional working space.
Environmental suitability 4 will be updated
Economic suitability (cost) 2 The range of costs is very large and depends on many factors as the well dimensions, the soil nature, the number and the length of drains, etc.

References

.

back to top