Pumps

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Description

The role of pumped wells as a mean of slope stabilization is mostly limited to dewatering excavations for structural foundation, where their work is purely temporary. They are not often used as a permanent mean of slope stabilization in fact this technology has been developed mainly in relation to the extraction of groundwater as a resource, but also in relation to structural dewatering problems.

Their main advantages and disadvantages are:

advantages: all types of wells can extract groundwater from locations where gravity methods are impractical. Their drainage capacity can be increased at any time by placing more wells. In the case of pumped wells, drainage performance may also be adjusted by altering the on/off switching levels, by increasing or decreasing the pumping rate, or hutting some pumps down (Forrester, 2000);

disadvantages: pumped wells require an on-going commitment for maintenance and intermittent or continuous operations. They are therefore only used if stabilization by drainage is essential, but no method of gravity drainage is feasible. Therefore the maintenance costs very much and influences the service-life and the good working of the wells.

Pumping system

The selection of the pumping plant will be influenced by the quantity of water to be extracted and the height to which it must be pumped to ground level. Typical details of three pumping systems that are most commonly used for dewatering, and consequently for slope stabilization, are:

Wellpoint (Fig.2a, 3a, 4a): this consists of a well screen set on the end of a 38 mm diasteel pipe. Several wellpoints are connected to a common pump through a header pipe at ground level. A wellpoint may be driven into the ground or  placed in a borehole, but it is usually jetted into place to the required level. This is done by applying water pressure to the tip through a temporary jetting pipe, with a rubber ball valve that allows a jet of water to be directed downwards. The valve closes when the jetting pipe is removed and the direction of flow is reversed for groundwater extraction. The wellpoint’s biggest disadvantage is that it works by suction and is therefore unable to raise water more than about 7.5 m. The maximum limit of drawdown: 3-4 m in silty fine sands, 5-5.5 m generally. It’s common practice to use two pumps initially and then continue using one pump at a time with the second one available as a stand-by.

Ejector (Fig.2b, 3b, 4b): also known as an eductor, this is placed in a cased borehole with a well screen as part of the casing. Its essential futures are a jet-the supply water-directed upwards through a venture. The venture is also open to the surrounding groundwater that has passed through the screen and into the casing. This water is carried with the supply water through the venture to the ground level. There, discharge in excess of the supply water flow is wasted. There are two different pipe arrangements. The first uses two pipes - one to lead the supply water to the ejector and the other to carry up the combined supply water and the groundwater. The other arrangement requires only a single pipe. The disvantages of an ejector are its high power consumption, since the same flow of supply water must be pumped continuously out of well  for as long as pumping continues. Economically, it is not worth using it to raise water more than about 40 m. The advantage of the eductor system is that the water table can be lowered in one stage from depths of 10-45 m. However the efficiency of such system is lower than that of other pumping system. They become economically competitive in soils of relativity low permeability.  Well diameter: 50 mm minimum-Well depth: up to 30 m.

Submersible pump (Fig. 2c, 3c, 4c): This type also required a cased boreholes with a well screen as part of the casing. The lowest component of the pump is an electric motor connected by a cable to the power source at ground level. Above the motor are the water inlet and pump screen, and the several pump rotors, one above the other. To prevent the motor from overheating, water must be kept moving past it while it is operating. Therefore, a switch that turns off the power at a lower water level is required. Submersible pumps are available in a wide variety of sizes and capacities. Switching is controlled by electrical contacts, similar in principle to those used to monitor waste levels in standpipe piezometers. A remote alarm system, also required for each pump, warns of pump malfunction or failure. The submersible pump is suitable for deep wells. Bore diameter: 150 mm to 500 mm - Well depth: up to 150 m.

However in Figure 5 the application field of this pumping system is shown as a function of soil permeability and design drawdown.

 

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Design methods

Each pump system can work up to a maximum depth. Wells are uneconomic compared to other drainage systems, especially when shallow depth must be reached; therefore well points are usually not adopted for slope stabilization. Deep wells (around 30 m) with submersible pumps are the most common system.

The type of pump to be adopted is a function of the pumping flow. Having determined how much the piezometric level must be lowered to obtain the required condition of slope stability, the flow is determined by means of the characteristic curve of the well (see fact sheet 4.5) or the Thiem equation. All suppliers provide the characteristic curve of their pumps to choose the most appropriate model and the optimum working point according to the design parameters. Typical submersible pump-characteristic curves are shown in the figure 6, for each pump type (motor type) at assigned diameter.

 

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Functional suitability criteria

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Reliability and feasibility criteria

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Urgency and consequence suitability

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.

• BS: 6031 1981. Code of Practice for Earthworks. British Standard Institution.

• Cedergren H.R.(1989). Seepage, drainage and Flow Nets. Third edition. Wiley Professional Paperback series. Jhon Wiley and Sons, 1989

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

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

• 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).

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

• Quinion D.W.,  Quinion  G.R. (1987). Control of Groundwater. ICE WORKS COSTRUCTION GUIDES, Thomas Telford, London.

• 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.

• WJ Groundwater Ltd. Slope stability – Well points system.

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