Fascines with Subsurface Drain

FASCINES WITH SUBSURFACE DRAIN

1. CATEGORY

4.0 – Slope Stabilization

2. DESIGN STATUS

Level II

3. ALSO KNOWN AS

Fascines with Axial Subdrains.

4. DESCRIPTION

Rows of fascines are installed on contour on a slope in the conventional manner (see Technique: Live Fascines). In addition, a subsurface drain, oriented downslope and perpendicular to the fascines, is placed in a trench beneath the rows of fascines to intercept and collect seepage (Gray and Sotir, 1996; Kropp, 1989). The subsurface drain consists of a perforated pipe wrapped in a geo-composite drainage medium placed at the bottom of a trench. The trench is backfilled with clean, coarse aggregate or gravel.

5. PURPOSE

This system is used on very wet sites where there is evidence of substantial subsurface seepage that is causing piping and slope instability. The subsurface drain improves the effectiveness of the fascines, which might otherwise fail because of extremely wet conditions. The system can be considered as an alternative to fascines used as live pole drains (Gray and Sotir, 1986).

6. PLANNING

Useful for Erosion Processes:

	Toe erosion with upper bank failure
	Scour of middle and upper banks by currents
	Local scour
	Erosion of local lenses or layers of noncohesive sediment
	Erosion by overbank runoff
	General bed degradation
	Headcutting
	Piping
	Erosion by navigation waves
	Erosion by wind waves
	Erosion by ice and debris gouging
	General bank instability or susceptibility to mass slope failure

Hydrologic / Geomorphic Setting

	Resistive
	Redirective
	Continuous
	Discontinuous
	Outer Bend
	Inner Bend
	Incision
	Lateral Migration
	Aggradation

Conditions Where Practice Applies:

The system can be considered for use in filled gully areas or tributary swales where groundwater is likely to collect and concentrate.

Complexity:

Moderate to High. Live Fascines are relatively simple and straightforward to fabricate and install. The subsurface drain adds another level of complexity because an axial trench must first be excavated, drain placed in the bottom, and then backfilled before the fascines can be installed across and over the trench.

Design Guidelines / Typical Drawings:

The seepage collection trench is excavated first, and a perforated pipe wrapped in a composite geodrain is placed in the bottom of the trench. The trench is then backfilled with gravel or coarse aggregate. The fascines are installed over and across the trench and subsurface drain as shown schematically in Figure 1. Fascines are prepared and installed in the conventional manner as described elsewhere (see Technique: Live Fascines). A cross section view of the subsurface drain or "geodrain" is shown in Figure 2. The geodrain is formed by first wrapping the perforated pipe in a three-dimensional open mat or matrix comprised of semi rigid polymeric fibers, e.g., Enkamat ™, followed by another wrap of filter fabric or filter cloth. The porous core of the geocomposite should face in towards the pipe, with the filter cloth backing facing outward. All drains should be constructed and installed with clean-out access tubes, as shown schematically in Figure 1. The perforated pipe in the subdrain should have sufficient capacity to handle and transmit intercepted groundwater flow or seepage. Guidelines for computing the appropriate size can be determined from published nomographs (see Technique: Trench Drain). Normally, a 10 cm (4 in) diameter polymeric pipe should suffice.

Figure 1. Profile view of a fascine system constructed over a subsurface interceptor drain

Figure 2. Cross-section view of a composite geodrain/ perforated pipe interceptor drain installed under a fascine system

After construction of the drain trench, the live fascines are installed over the trench in the normal manner (see Technique: Live Fascines). A schematic drawing of a typical live fascine installation is shown in Figure 3. Other than the presence of a clean out tube in the treatment including subsurface drainage, both techniques have the same external appearance.

Figure 3. Schematic drawing (profile view) of live fascine installation

Fascines with Subsurface Interceptor Drain Typical Drawing
.dwg	.dgn

7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

Wet conditions or excessive emergent seepage in a streambank can undo the effectiveness of protective surface cover. The conjunctive use of fascines and a subsurface interceptor drain helps to prevent this problem. Fascines are made of live cuttings that root and grow, ensuring rapid establishment of riparian vegetation with associated environmental benefits

8. HYDRAULIC LOADING

The considerations that apply to hydraulic loading of live fascines installed on a streambank apply here as well (see Technique: Live Fascines). The subdrain system is buried and not subjected to hydraulic shear or tractive stresses during flood events.

9. COMBINATION OPPORTUNITIES

The conjunctive use of live fascines with an interceptor drain already constitutes a combined practice of sorts. The system can also be used in combination with Live Gully Fill Repair (see Technique: Live Gully Repair) and with various types of river training techniques, particularly protective toe structures (see Techniques: Longitudinal Stone Toe, Longitudinal Dikes with Toe Spurs).

10. ADVANTAGES

The same advantages that apply to the use of Live Fascines apply here as well. In addition, the conjunctive use of a subsurface drain extends the use of fascines in very wet areas, e.g., tributary swales, filled gullies, and water bearing strata or lenses.

11. LIMITATIONS

Because of requirements for a trench in which to place a subsurface drain, the maximum bank slope should be limited to no more than 1V:2H. The main limitation is the feasibility and extra effort of excavating a trench down the face of a streambank. The drain also has to be checked occasionally to insure that it has not become clogged.

12. MATERIALS AND EQUIPMENT

In addition to the live cuttings required for the fascines, additional materials are also required for manufacturing the drains, viz., perforated polymeric pipe, filter cloth, and a turf reinforcement mat, e.g., Enkamat™, for wrapping the pipe.

13. CONSTRUCTION / INSTALLATION

Fascines are prepared and installed in the conventional manner as described elsewhere (see Technique: Live Fascines). The seepage collection trench is excavated first, and a perforated pipe wrapped in a composite geodrain is placed in the bottom of the trench. The trench is then backfilled with gravel or coarse aggregate. The fascines are then installed on contour over and across the trench and subsurface drain. Row spacing guidelines for fascine installations are presented in Table 1.

TABLE 1: Recommended Spacings For Live Fascines On Slopes

Slope Steepness (V:H)	Slope Distance Between Fascine Rows m(ft)
Slope Steepness (V:H)	On Contour	On Angle
1:1 to 1:1.5	0.9 – 1.2 (3 - 4)	0.6 – 0.9 (2 – 3)
1:1.5 to 1:2	1.2 – 1.5 (4 – 5)	0.9 – 1.5 (3 – 5)
1:2 to 1:2.5	1.5 – 1.8 (5 – 6)	0.9 – 1.5 (3 – 5)
1:2.5 to 1:3	1.8 – 2.4 (6 – 8)	1.2 – 1.5 (4 – 5)
1:3.5 to 1:4	2.4 – 2.7 (8 – 9)	1.5 – 2.1 (5 – 7)
1:4.5 to 1:5	2.7 – 3.0 (9 – 10)	1.8 – 2.4 (6 – 8)

14. COST

The installed cost of live fascines has been estimated at $26.24 - $328/m ($8 - 100/ft) (Washington, 2002). This wide discrepancy probably reflects the effect of site specific considerations (proximity and availability of live cuttings, ease of access, steepness and height of slope, etc). The requirement for installation of a subsurface interceptor drain will drive the cost towards the higher estimation. Given a bank 5 m (16.5 ft) high, with a 1V:2H slope and a swale or gully 20 m (65 ft) wide, installation of fascines with axial subdrain would be between $1,560 and $19,500 (including labor and materials, but not planning, design, administration or monitoring).

15. MAINTENANCE / MONITORING

The exit end of the subsurface drain should be checked periodically to insure that water is flowing out of the drain. The subsurface drain pipe should be flushed on a regular basis via the clean-out access tube to insure that it flows freely and does not become clogged.

16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE

The most common reasons for failure are improper design of the interceptor drain placed at the bottom of the axial trench. The perforated pipe should be correctly sized and wrapped with a suitable geodrain composite that excludes fines but that lets water through. Failure to inspect and periodically flush the drain via the clean-out access tube can also lead to clogging and poor performance.

17. CASE STUDIES AND EXAMPLES

A fascine and subsurface interceptor drain system was used to stabilize the track of a debris flow scar on a hillside in Pacifica, California (Kropp, 1989). The debris flow left a shallow swale that contained sufficient residual soil in a vulnerable condition that was susceptible to both erosion and future sliding. A fascine system was selected to stabilize the slope, but significant amounts of both subsurface seepage and concentrated surface runoff made additional drainage imperative. The tendency of both subsurface seepage and runoff to concentrate naturally in the middle of the swale suggested the use of a single, axial subsurface drain running up and down the slope beneath the fascines. A photo of the combined system is shown in Figure 4. The outline of the central (axial) drainage trench is visible in the photo. Grass and other native forbs were planted between fascine rows to improve protection against surficial erosion. A view of the clean-out access tube is shown in Figure 5. The site has remained stable with little or no evidence of surface runoff or erosion.

Figure 4. Fascines placed in a wet swale. A drain was installed beneath the fascines to intercept and collect sub-surface water.

Figure 5. Fascine and subsurface interceptor drain system. A clean-out access tube is visible in the photo.

18. RESEARCH OPPORTUNITIES

The combined use of Live Gully Repair and Live Fascines with Subsurface Interceptor Drain and possible modifications deserve additional investigation.

19. REFERENCES

Gray, D. H. & Sotir, R. (1996). Biotechnical and Soil Bioengineering Slope Stabilization. John Wiley and Sons, New York, N. Y.

Kropp, A., Thomas, M., & Lucas, A. (1989). Biotechnical stabilization of a debris flow scar. Proceedings, XX International Erosion Control Association Conference, Vancouver, pp. 413-429.

Washington Dept of Fish & Wildlife (2003). Integrated Streambank Protection Guidelines, published in co-operation with Washington Dept. of Transportation and Washington Dept. of Ecology, June 2003. (Chapter 6 pdf) (Appendix L pdf) (Appendix H pdf) http://www.wa.gov/wdfw/hab/ahg/ispgdoc.htm (April 2003)

	Instream
	Toe
	Midbank
	Top of Bank