Description
This technique was conceived and developed in France. It consists of isolated drainage wells of diameter 100 to 300 mm, equipped with a slotted PVC or a perforated or micro perforated steel pipe, with the annulus between pipe and soil filled with draining material, as explained in fact sheet 4.5.. The wells are pumped using a siphon driven by the fall in elevation of the slope (Fig. 2), overcoming the inconvenience of installing and operating a pump in each well.
Its use in slope stability is restricted for two reasons (Forrester, 2000):
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A siphon has a maximum theoretical lift of 10.2 m (equivalent to atmospheric pressure); however, it has a maximum practical lift of 8.3 m due to the vapor pressure of water and friction head loss.
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If a sufficient air enters the siphon at any time, the pumping is broken. Flow can only resume if priming is restored.
A siphon is a familiar device for moving water from one level to a lower one; in its simplest form this consists of an inverted U-tube, both legs being full of water, and the flow is generally calculated by equating the total head producing flow, i.e. the difference of heads in the two reservoirs, h, to the sum of the frictional and other losses in the pipe and of the velocity head produced (for details see Citrini, Noseda 1986). System flow would decrease as h decreases due to drawdown in the well. Equilibrium would occur at the drawdown, yielding the system flow capacity.
Siphons require priming (initial filling of line) to initiate flow. After priming, the siphon will passively convey liquid from the point of higher hydraulic head to the one of lower head indefinitely so long as the head differential is maintained and the prime is not lost. For flow tio be maintained, it is necessary that at all times:
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inlets and outlets are submerged, to prevent air from being drawn into the siphon line,
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gases which tend to accumulate in the siphon line as they come out of soultion due to the sub-atmospheric pressures. are removed
In fact, as the summit (minimum) pressure decreases, dissolved gases in the groundwater come out of solution and help form intermittent discontinuities as the pressure approaches a true vacuum. A break in the siphoning action occurs at a point less than the theoretical limit as the summit pressure continues to decrease.
One or both of the following methods may be used to remove the gases which have degassed from the liquid, thus maintaining full siphon flow:
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Maintenance of the minimum flushing velocity required to transport gases out to the end of the siphon.
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Use of air chambers at the siphon crest This makes the system less than entirely passive, since the chambers require periodic recharging.
Management of gas within the siphon line is considered to be of the greatest importance in the maintenance of siphon flow. Gas bubble transport, accumulation, agglomeration, and entrapment are controlled by fluid flow velocity, gas buoyancy, and siphon line grades and inside diameter discontinuities (i.e. fittings). Gas bubble transport in the upward leg of the siphon line is facilitated by higher fluid flow velocities, by a continuous upward siphon line grade (no localized high points), and the minimization or elimination of fittings which produce discontinuities in the internal diameter of the siphon line. The direction of gas bubble transport, if any, in the siphon line downward leg is determined by whether transport due to fluid flow velocity or gas buoyancy is dominant. In order to utilize the minimum flushing velocity to maintain full flow in the siphon line downward leg, the fluid flow velocity must be dominant in the downward leg. Additionally a continuous, downward, siphon line, grade (i.e. no localized high points) and the minimization or elimination of fittings which produce discontinuities in the internal diameter of the siphon line, is necessary.
Design methods
The siphon system is a very effective solution to slope stability problems in terms of adaptability and durability. The water table can be lowered to 8.5 m vertically below the surface when the suction inlet is placed at 10 m below the crown of the siphon. Depending on the gradient of the slope, it is possible to achieve greater effective lowering of the water table if the length and the slope of the wells are modified. Both the diameter and the number of siphon pipes depend on the drainage flow. Diameters range from 10 mm for 150 litres/hour per well, to 25 mm for 1 m3/hour per well. This system proves to be economically advantageous and relatively simple to set up even if it necessitates a programme of controls and maintenance.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | This system usually is adopted to stabilize landslides characterized by a circular surface. Depending on the gradient of the slope, it is possible to achieve greater effective as regards the lowering of the water table if the length and the slope of the wells are adjusted, taking into account each situation. |
| Topple | 0 | |
| Slide | 6 | |
| Spread | 2 | |
| Flow | 0 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 6 | . |
| Debris | 6 | |
| Rock | 4 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 0 | The siphon can lower the water table to 8.5 - 9 m below ground level, thus it can be most effective for slip surfaces up to 10-11 m.. deep |
| Shallow (0.5 to 3 m) | 4 | |
| Medium (3 to 8 m) | 6 | |
| Deep (8 to 15 m) | 8 | |
| Very deep (> 15 m) | 4 | |
Rate of movement |
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| 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 acquifer properties |
| Slow | 2 | |
| Very slow | 8 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 6 | This system is suitable for shallow freatic water-table |
| High | 8 | |
| Low | 6 | |
| Absent | 0 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 2 | Not suitable to drain shallow water. |
| Snowmelt | 2 | |
| Localized | 0 | |
| Stream | 0 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 6 | The siphon system is a very effective solution to slope stability problems in terms of adaptability and durability. The good working depends strongly on the maintenance in particular by the management of gas inside pipes.. |
| Feasibility and Manageability | 5 | These technique and design processes are used especially in France, at least in Italy and in U.K |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 7 | System implementation is easy, especially in absence of pumps. |
| Environmental suitability | 4 | will be updated |
| Economic suitability (cost) | 5 | Costs depend on the maintenance |
References
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Citrini D., Noseda G.(1986). Idraulica. Casa Editrice Ambrosiana Milano.
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Forrester K. (2001). Subsurface drainage for slope stabilization. ASCE Press.
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GE.A.S.Servizi ed Applicazioni geotecniche. Il sistema di drenaggio mediante “dreno sifone”. Esempi di realizzazione: Casale Monferrato (Alessandria) 1999,Villacollemandina (Lucca) 2002.Conv.I sistemi drenanti nei dissesti del territorio, Torino7aprile 2006.
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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.
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WJ Groundwater Ltd. Slope stability - Siphon drain system: Permanent gravity-fed siphon drains®.