Vertical large diameter (>2000mm) Wells – gravity drainage through base conductor

Category: MODIFYING THE GROUNDWATER REGIME – Deep drainage

Description

This technique consists of installing large diameter (≥ 2000 mm) wells similar in every respect to the inspection wells described in fact sheet 4.5.2, except that the 1200 mm diameter casing of the standard inspection well is installed inside a separate, 1800 mm diameter,  permanent corrugated hot galvanized steel casing, which is installed first, filling the annular space between the external casing and the soil with drainage material, simultaneously extracting the temporary casing used to support the hole during drilling. (Fig. 2).

Figure 1: Classification of wells
Figure 1: Classification of wells
Figure 2) Typical structural well section and machinery to construct the borehole for PVC pipes (Leoni et al., 2003)
Figure 2: Typical structural well section and machinery to construct the borehole for PVC pipes (Leoni et al., 2003)

Once installation of the external casing is complete, it is possible to install a reinforcment cage and the inner casing, filling the annular space between the two casings with concrete. This makes these wells resistant to bending and shear, such that they double up as structural elements transferring loads from the landslide mass to more competent strata below. Accordingly, these wells are often referred to as “structural wells”.

The hydraulic connection between the external filter and the inner cavity of the well is provided by one or more short PVC pipe(s) placed in short horizontal drillholes across the two steel linings and the concrete in between. The well is then completed with the discharge pipe at the bottom, sub-horizontal drains, stairs, sealing of head and base and manhole cover.

Prior to the introduction of directional drilling, this type of well was used to install longer sub-horizontal drains than would have been possible otherwise.

Depending on the specific requirements of the project, these wells can be used in isolation as described above, as arrays of structural wells or in combination with the “hydraulic wells” described in fact sheet 4.5.2.

Figure 3) Beacon Hill landslides, Herne Bay,UK (Bromhead,1978). Wells deep 14 m, with a diameter of 4 m.
Figure 3: Beacon Hill landslides, Herne Bay,UK (Bromhead,1978). Wells deep 14 m, with a diameter of 4 m.

Design methods

For the structural design of these wells, where they intersect the shear plane and toe into competent material, reference may be made to fact sheets 6.2 and 6.3. For their hydraulic design when used in arrays, reference may be made as far as applicable to fact sheet 4.5.2. Where additional 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.


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 4
Flow 2

Material type

Descriptor Rating Notes
Earth 8 Deep well systems are effective in a range of soil from gravel to salty 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) 2
Deep (8 to 15 m) 8
Very deep (> 15 m) 4

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 9

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 8 Technique and design processes are well established and widely used in suitable conditions.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 6 Large spaces need for wells spaced 6 m
Environmental suitability 4 will be updated
Economic suitability (cost) 4 Costs are very high, due to the number of wells along an array; also costs for maintenance of the discharge pipes at the bottom and of the subhorizontal drains could be high

References

  • Barile A., Leonetti F., Silvestri F., Troncone A. TASK 2 – Progetto VIA  “Riduzione della Vulnerabilità Sismica  dei Sistemi Infrastrutturali ed Ambiente Fisico” Vulnerabilità dell’Ambiente Fisico:  INTERVENTI DI RIDUZIONE DEL  RISCHIO DI INSTABILITÀ DEI PENDII:  TIPOLOGIE E METODI DI DIMENSIONAMENTO. Unical.

  • Bromhead EN (1978) Large landslides in London Clay at Herne Bay, Kent. Q J Eng Geol 11:291–304

  • Gullà G., Antropico L., Ferrari E., Sorriso-Valvo M., Tansi C., Terranova O., Aceto L., Nice-foro D., (2003). “Linee guida per interventi di stabilizzazione di pendii in aree urbane da riqualificare”, POP 1994/99 (Regione Calabria – Fondi UE), Misura 4.4 “Ricerca Scienti-fica e Tecnologica”. Rubettino Editore Soveria Manelli.

  • Leoni F., Manassero V. (2003). Consolidamento e rinforzo dei pendii in terra Atti del XIX Ciclo di Conferenze di Geotecnica di Torino (CGT 2003).

  • Ziccarelli M.(2010). Stabilizzazione dei pendii mediante drenaggi. Rende, 26 Maggio 2010.

back to top