Description
This technique consists of forming a deep drainage screen in low permeability soils by installing alignments of wells at 6 to 8 m spacing, connected at the base by drainage pipes to allow the gravity discharge of the water collected in the wells (Figure 2, Leoni et al. 2003). A typical plan and longitudinal section is shown in Figure 3.
The diameter of the wells is typically 1200 to 1500 mm. They can reach typical depths of 20 to 30 m and in particular cases more than 50 m (Beer et al, 1992, Manassero, 2001). They are excavated using the same equipment and techniques used for bored piles without bentionite mud (Figure 4).
The wells are typically of two types:
Standard wells are filled with drainage material, simultaneously extracting the casing used for temporary support of the hole during drilling. The top of the well is sealed with say minimum 1.0m of impervious fill and topsoil to prevent infiltration of surface runoff.
Inspection wells are formed by installing in the well a permanent 1200 mm diameter corrugated hot galvanized steel casing perforated near the base (Figure 5), filling the annular space between the casing and the borehole with drainage material while extracting the temporary casing as above. These wells are placed at suitable distance along the array, typically one every three wells (Figure 3). Besides being used to drill the base conductor, inspection wells are used to monitor the correct performance of the system and in particular to measure and, if necessary, to regulate the flow rate.
The base conductor, which allows the wells to discharge by gravity, is the main feature of this technology. It typically consists of twin pipes (to guarantee adequate redundancy), installed by drilling through the casings from one inspection well to the other by means of mini-probes (Figures 9 and 10) and installing the pipe in short (450 mm) sections. Inspectionable wells are completed with access ladders, head and bottom sealing and the installation of manhole covers in reinforced concrete.The typical detail of inspection wells is shown in Figure 6. Typical applications are shown in Figures 7 and 8.
Increasingly, the focus on safety of construction and ever greater restrictions on working pracices tend to make the traditional method of forming the base conductor impractical, since it requires man entry to the base of the well. This may be obviated in whole or in part by the use of directional drilling.
Design methods
The depth of the wells and 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 usually is 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 20 - 30 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 | 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 to allow gravity drainage |
| 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 and good access required for construction of well at 6 – 8 m spacing |
| Environmental suitability | 4 | will be updated |
| Economic suitability (cost) | 4 | Costs are very high, depending on number of wells along an array; also costs for maintenance of the discharge pipes at the bottom could be high |
References
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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.
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Beer P., Hegg U., Manassero V. (1992). “landslide stabilization at Ancona, Italy, dy deep drainage wells”. In 6th International Symposium on Landslides, Christchurch, New Zealand, 10-14 February, 663-670.
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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.
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Leoni F., Manassero V. (2003). Consolidamento e rinforzo dei pendii in terra Atti del XIX Ciclo di Conferenze di Geotecnica di Torino (CGT 2003).
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Manassero V. (2001). ”Il consolidamento dei pendii mediante drenaggio profondo”. Convegno su rilevamento e tutela del territorio. Hydrogeo, Rimini 9-11 May, Maggioli Ed., 483-496.