Drainage tunnels, Adits, Galleries with secondary drains or as outlet for wells

Category: MODIFYING THE GROUNDWATER REGIME – Deep drainage

Description

Drainage galleries constitute a rather expensive stabilization measure for large, deep landslide movements (30-35 m), to be used where the subsoil is unsuitable for trenches or drainage wells and when it is impossible to work on the surface owing to lack of space for the work machinery. In fact galleries are expensive, if compared with inclined bored drains but they may be advantageous where seepage takes place from closely-spaced fissures or laminations in a rock formation. Moreover, these drainage systems have to be built on the stable part of the slope (Figs. 1-2).

Figure 1: Drainage galleries with drain pipes
Figure 1: Drainage galleries with drain pipes
Figure 2: a) Typical shape of freatic level before and after the construction of galleries; b) typical transversal section of galleries
Figure 2: a) Typical shape of freatic level before and after the construction of galleries; b) typical transversal section of galleries

The gallery can be tunnelled to intercept the source of seepage and then continued along the water-bearing horizon to the extent necessary to achieve the lowering of piezometric pressures behind the slope. Drainage galleries provide a means of access for supplementary stabilization measures such as transverse adits, inclined bored drains, or grouting. The drainage systems are placed inside the galleries and are made up of micro drains, with lengths that can reach 50–60 m and are spatially oriented in suitable directions. The sizes of galleries are conditioned by the need to insert the drain drilling equipment. For this reason the minimum transversal internal size of galleries vary from a minimum of 2 m, when using special reduced size equipment, to at least 3.5 m, when using traditional equipment (Fig. 3).

Figure 3: Microtunneling system to insert microdrains.
Figure 3:  Microtunneling system to insert microdrains.

Galleries are constructed on an upward gradient to permit drainage by gravity towards the portal through a piped drain constructed beneath the floor of the gallery. The drain should have a removable cover for easy inspection and maintenance.

Where a gallery is constructed in highly-weathered rocks, permanent support is required in the form of reinforced concrete liling. In this case, the permanent lining should be surrounded with a properly designed drainage filter so that there is a good hydraulic connection with the material being drained. Weepholes then have to be provided through the lining in order to drain the filter.


Design methods

Drainage galleries can be very effective in dewatering the slope, because of high surface area exposed for drainage. They are however expensive; therefore careful consideration about costs and benefits is required. The position and size of the gallery is important as shown by Sharp (1970).  In fact the knowledge of the ground-water flow is necessary to design the position, the path, and the size of the galleries and the length of any micro drains installed inside galleries.

The flow to drain apart from the permeability of surrounding soils depends on the size and the inner surface of galleries.

The flow per unit length of the gallery, with only one side at the contact with freatic aquifer can be calculated as (with
f: = hydraulic permeability, Milano, 2005):

Figure 3: Microtunneling system to insert microdrains.

 


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 2 Drainage galleries constitute an effective but expensive mitigation measure for landslide movements.
Topple 4
Slide 6
Spread 6
Flow 6

Material type

Descriptor Rating Notes
Earth 6 They may be advantageous where seepage takes place from closely-spaced fissures or laminations in a rock formation.
Debris 6
Rock 6

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 0 rainage galleries constitute a mitigation measure for large, deep landslide movements (30-35 m).
Shallow (0.5 to 3 m) 0
Medium (3 to 8 m) 2
Deep (8 to 15 m) 6
Very deep (> 15 m) 8

Rate of movement

Descriptor Rating Notes
Moderate to fast 4 The steady-state condition is attained when the cone of depression reaches the equilibrium; time is a function of the acquifer properties.
Slow 8
Very slow 8
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 6 This system is very suitable for freatic acquifer.
High 8
Low 8
Absent 0

Surface water

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

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 7 They may be advantageous where seepage takes place from closely-spaced fissures or laminations in a rock formation.
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 6 The same technologies used for tunnels are suitable. Recent applications are carried out by microtunneling system (Angeli & Pontoni, 2002).
Environmental suitability 2 will be updated
Economic suitability (cost) 1 They are the most expensive mitigation system for slope stability and are built where the slope is unsuitable for trenches or drainage wells and when it is impossible to work on the surface owing to a lack of space for the work machinery.

References

  • Angeli M.G., Pontoni F., (2002). Stabilizzazione di grandi frane in aree urbane mediante i microtunnels drenanti: due casi di intervento in Italia Centrale. XXI Convegno Nazionale di Geotecnica, L’Aquila. Associazione Geotecnica Italiana.

  • APAT,Agenzia per la Protezione dell’Ambiente e per i servizi Tecnici (2002). Atlante delle opere di sistemazione dei versanti:Manuale e linee Guida.

  • 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

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

  • Siddle H.J.(1986). Groundwater control by drainage gallery at Aberfan, South Wales. Geological Society, London, Engineering Geology Special Publications 1986; v. 3; p. 533-540

  • Sharp J.C. (1970).’’ Drainage Characteristics of Subsurface Galleries’’Proceeding, 2nd Congresso f the Intrnational Society of Rock Mechanics, Belgrade, Thema 6,  No 10, 8pp

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