Live pole drains (NBS)

Category: MODIFYING THE SURFACE WATER REGIME – Surface drainage

Description

This measure consists in cylindrical bundles made of live cuttings with roots used as drain collector against the underground seepage rising. Traditional engineering solutions for addressing this problems are often the so-called "French drains" or, in other cases the loading of the face of the slope with rock. Live pole drains can be designed as alternative and sustainable solution to these techniques that provides double effect on both the drain of excess soil moisture and the reinforcement of soil with roots (Pacas, 1999). Usually these drains create a pattern with a "Y" shape along the slope in order to collect excess moisture from a diffuse seepage zone. These drains are used when excess of water in loose materials or debris scar areas results in soil instabilities (Polster, 2003). Soil slumps such as those which occur the first spring after road resloping operations can be stabilized using live pole drains (Pacas, 1999). For a better effectiveness in collection of the diffuse seepage, these drains can be combined with lateral drain fascines (Figure 1, C).  

Advantages:

  • Protection from additional slumping of soil;

  • Helpful in the revegetation of already landslide affected areas;

  • They can be installed without requirement of machine access thus reducing the operational costs.

Disadvantages:

  • If not well designed the water can flow out from the lateral parts (the height of the center should be lower than the wings);

 

Figure 1. Figure A) Live pole drain system installed along the scar of a landslide occurred in 2003 in British Columbia, Canada, Figure B) design of the installed system in combination with deep rooting techniques (brush layering) and Figure C) View of live pole drains and lateral drain installed (Stokes et al., 2014).  

 

Figure 2. Live pole drains were installed in the scarp to control movement of the large block of soil on left). The soil was so wet rocks were needed to hold down the drain (Polster, 2003).

 

Figure 3. Live pole drains provide immediate drainage of “soupy” soils. Bundles of willow are inserted and act as a preferred flow path for subsurface flows, moving the water through sensitive soils (Polster, 2003).

 


Design methods

The shape of the live pole drains depends mostly on the site conditions where they should be installed. Usually a "Y" shape pattern can be used to collect a diffuse seepage problem (lateral and linear), while a linear shape pattern is preferred where a discrete seepage site exists (Polster, 2003). Once the shape is designed, shallow trenches are excavated from the site of seepage down the slope and away from the problem area. The bundles consisting of cuttings with tips and butts alternating are tightly tied with bailing twine or mechanics wire and placed into the trenches, thus lightly buried with local materials, by leaving some twigs from the bundle aboveground (Figure 4). The cuttings used for bundles sprout and grow thanks to the moisture continuing to drain, allowing other species to invade.

Period of installation: preferably during dry season

Materials: long live fascines, baling twine.

Figure 4. The view shows the layout of live pole drains in a slump with the covering soils removed for clarity. The section shows a typical covering. (Polster, 2003).

 


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 0 Most suited to all types of slides and, subject to circumstances in flows. In spreads, only useful as remediation, not as a preventive measure.
Topple 0
Slide 9
Spread 6
Flow 7

Material type

Descriptor Rating Notes
Earth 9 Mainly applicable to land sliding involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode. Potential difficulties in coarse debris.

Filling in the fascine with soil is very difficult and made not very reliable this technique .
Debris 9
Rock 0

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 10 Mainly applicable to landslides of between surficial and shallow depth of movement, with its relative effectiveness decreasing as the depth of movement increases.
Shallow (0.5 to 3 m) 7
Medium (3 to 8 m) 2
Deep (8 to 15 m) 0
Very deep (> 15 m) 0

Rate of movement

Descriptor Rating Notes
Moderate to fast 0 Can be carried out without difficulty when the rate of movement is slow (5 cm/day) or less. It provides protection from additional slumping of soil along an already landslide affected areas.
Slow 3
Very slow 7
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 4 It is aimed at draining water from the area, it is more effective along slopes with high ground water conditions.
High 10
Low 5
Absent 5

Surface water

Descriptor Rating Notes
Rain 9 Not applicable along water courses.

Submerged fascine filled with water are very reliable
Snowmelt 6
Localized 7
Stream 4
Torrent 4
River 2

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 8 Remedial measure to reduce the occurrence of additional slumping along a landslide area, it is reliable only if well established and well implemented along the slope.
Feasibility and Manageability 10 Simple technique, it can be installed without requirement of machine access thus reducing the operational costs.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 10 Easily implemented in a short time without extra equipment
Environmental suitability 10 Highly environmental suitable, it can be used also as restoration measure in the revegetation of a landslide area (derbis deposits at the toe of the slope)
Economic suitability (cost) 10 Low, where applicable.

References

  • Polster, D. F. (2003). Soil bioengineering for slope stabilization and site restoration. Mining and the Environment III, 25-28.

  • Pacas, C. (1999). Restoration of the Vermilion Wetlands. In Fourth Annual Roads, Rails and Environment Workshop:“Impacts and Solutions for Aquatic Ecosystems” (p. 28).

  • Polster Environmental Service Ltd. (2003). Alternatives for bank stabilization – literature review. http://www.dfo-mpo.gc.ca/Library/277708.pdf

  • Stokes, A., Douglas, G. B., Fourcaud, T., Giadrossich, F., Gillies, C., Hubble, T., ... & Mickovski, S. B. (2014). Ecological mitigation of hillslope instability: ten key issues facing researchers and practitioners. Plant and Soil, 377(1-2), 1-23.

 

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