1. CATEGORY
4.0 – Slope Stabilization
2. DESIGN STATUS
Level II
3. ALSO KNOWN AS
Interceptor Drain, French Drain, Longitudinal Drain.
4. DESCRIPTION
A drainage trench is excavated parallel to and just behind the crest of a streambank. Ideally, the bottom of the trench should be keyed into an impermeable layer in the slope. The trench should be backfilled with a coarse graded aggregate that meets filtration criteria; i.e., it should allow unimpeded flow of groundwater while excluding fines from the seepage water. Alternatively, the trench can first be lined with a filter fabric (geotextile) that meets the filtration requirements and then be backfilled with a coarse aggregate.
5. PURPOSE
The purpose of the trench is to intercept and divert shallow seepage away from the face of the streambank (Gray and Sotir, 1996).
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
Spatial Application:
Instream Toe Midbank Top of Bank
Hydrologic / Geomorphic Setting
Resistive Redirective Continuous Discontinuous Outer Bend Inner Bend Incision Lateral Migration Aggradation Conditions Where Practice Applies:
Should be considered when shallow, water bearing strata conduct groundwater that emerges (daylights) at the streambank. A good example would be relatively permeable surface strata or water bearing sands up to 3 m (10 ft) thick, e.g., outwash sand or coarse alluvium, overlying relatively impermeable silty clay deposits, e.g., clay till or fine alluvium. This is a fairly common stratigraphic sequence in glaciated terrain and alluvial valleys.Complexity:
Low. Technique is relatively simple and straight forward to implement. Basically requires the use of a backhoe to excavate and backfill a narrow trench parallel to the crest of a streambank.
Design Guidelines / Typical Drawings:
Trench Drains constructed without a pipe at the bottom are commonly known as French Drains (see Figure 1a). An efficient, well-constructed Trench Drain requires the use of perforated, jointed, slotted or porous pipe placed near the bottom of a trench (see Figure 1b) that is surrounded with pea gravel or selected pervious filter aggregate. When a drain is excavated in erodible materials, synthetic filter fabrics (geotextiles) should be used (see Figure 1c) to line the sides and bottom of the trench to prevent soil fines from entering the coarse backfill in the drain (see Special Topic: Role of Geotextiles). The main backfill should be specially selected pervious filter aggregate designed to allow unrestricted flow of water to the pipes. The minimum required permeability, k, of the trench fill material can be determined by Darcy's law, using a downward gradient of 1.0 and a factor of 10 to 20 to allow for seepage flow reduction caused by contamination. Assume, for example, a flow of 5.7 m3/day (200 ft3/day) will enter a longitudinal drain with a width of 0.5 m (1.5 ft). The minimum required k = 20 (5.7)/0.5 = 228 m/day (k = 20 (200)/1.5 = 2670 ft/day). Pea gravel, stone chips, or comparable materials would be needed to ensure this level of permeability and flow throughout.
Most drains should be equipped with pipes because gravel or rock-filled trenches have limited discharge capabilities even when clean aggregates are used. The discharge capabilities of drainage trenches backfilled with clean stone or coarse gravel, as estimated by Darcy's law, are given in Table 1. The required diameters of both corrugated metal, concrete, and polymeric (smooth) drain pipes for a wide range of discharge quantities can be determined from the nomograph in Figure 2 (USACE, 1974).
The location of perforations and open joints in pipes should always be placed to allow unobstructed flow to pipes. If a drainage pipe is completely surrounded with specially selected coarse filter aggregate (refer to Figure 1b), perforations can completely surround the pipe. Unjointed sections of pipe should be used to convey water across areas where the discharge of water into the soil from drains must be prevented. The same injunction holds for the final discharge of collected drain water, viz., it must not be allowed to discharge on to a slope and instead must be conducted safely down a slope using a chute or slope drain (see Technique: Slope Drains).
Trench Drain Typical Drawing
7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS
Groundwater that emerges or daylights at the face of streambanks causes piping
and seepage erosion. This piping greatly exacerbates slope instability and
degradation. Interception and collection of this seepage behind the crest
of the slope not only corrects this problem, it does so in a non-intrusive
and visually transparent manner. No grading and disturbance of the streambank
itself are required. In addition, this technique lends itself readily for
use in combination with virtually any other type of streambank protection
system and to the establishment of natural (woody) riparian species on the
stabilized bank slope.
8. HYDRAULIC LOADING
Hydraulic loading is not a factor in the design/installation of trench drains
because they are buried in the ground behind the crest and are not exposed
to shear or tractive stresses of streamflow. The only hydraulic consideration
is that the drain has sufficient drainage capacity to handle the seepage flows
that are intercepted by the drain (see Design Guidelines).
9. COMBINATION OPPORTUNITIES
Use with slope drains to convey the collected discharge safely down the face
of a slope to the stream below. Can also be used in combination with virtually
all other bank and channel protection measures.
10. ADVANTAGES
Visually unobtrusive, relatively easy to install, can be used in conjunction
with other bank protection measures without interference problems, mitigates
problems associated with piping and spring sapping caused by emergent seepage
at the bank face.
11. LIMITATIONS
Can only be used to intercept shallow seepage in near surface water bearing
strata. Interception depth is limited by practical limits dictated by maximum
depth of trench. Drainage trench can become clogged with fines over time and
lose its hydraulic transmission capacity.
12. MATERIALS AND EQUIPMENT
Suitable drainage rock or gravel (see Design Guidelines)
in addition to a perforated polymeric pipe. A small backhoe is required for
excavating and backfilling the trench. A geotextile filter fabric will be
required if the trench is to be lined.
13. CONSTRUCTION / INSTALLATION
Guidelines for trench drain construction have been published by Cedergren (1989).
Maximum trench depths are restricted to the reach of a backhoe/excavator or approximately 2 to 2.7 m (6 to 8 ft). Trench widths are also determined by the width of the excavator bucket, which can range from 0.3 to 0.6 m (12 to 24 in). The water transmission characteristics of the drainage trench can be improved by placing a perforated or slotted drainage pipe on a slight grade at the bottom. The discharge from a trench drain should be conveyed in a safe, non-eroding manner down the slope directly to the stream.
14. COST
Costs for trench drains vary with the type of drain and dimensions of the trench. Each cubic m of stone or gravel is equivalent to 2 metric tons (each cubic yd of stone or gravel used is equivalent to 1.7 tons), which translates into 5 metric tons of rock per linear m for a 1 m wide by 2.5 m deep trench (half a ton of rock per linear ft for a 1 ft wide by 8 ft deep trench). The material cost of filter gravel (placed) runs from $52-78/m3 ($40-60/yd3), river gravel runs $52-104/m3 ($40-80/ yd3) and pit run rock ranges from $39-52/m3 ($30-40/yd3). The delivered cost of gravel or coarse aggregate will depend on local availability and proximity of the project site to a rock or gravel quarry. Use of a pipe in the bottom or filter cloth liner will add to the material costs. The services of a backhoe operator will also be required for excavating the trench.
15. MAINTENANCE / MONITORING
Subsurface drains, including trench drains, are difficult to access and
inspect once installed. A possible way to monitor the performance of a trench
drain is to check the outflow from the pipe at the bottom of the interceptor
trench. If there is steady shallow seepage towards a streambank, this exit
pipe should flow continuously. The effectiveness of a trench drain for intercepting
shallow seepage can be monitored indirectly by examining for signs of seepage
and/or slumping/sliding at the bank face.
16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE
The limitations of trench drains cited previously are the most common reasons
for failure. Failure to excavate the trench deep enough to reach the impermeable
base of a perched groundwater system may let groundwater pass under the
trench. Loss of drainage capacity from clogging of a drain can lead to
the saturation and buildup of pore pressure in the streambank itself.
Either of these conditions can lead to mass stability failure of a streambank
or seepage induced erosion of the bank face.
17. CASE STUDIES AND EXAMPLES
Information Unavailable
18. RESEARCH OPPORTUNITIES
Information Unavailable
19. REFERENCES
Cedergren, H. R. (1989). Seepage, Drainage, and Flow Nets. John
Wiley and Sons, 3rd edition, New York, N. Y.
Gray, D. H. & Sotir, R. (1996). Biotechnical and Soil
Bioengineering Slope Stabilization. John Wiley and Sons, New York, N.
Y.
US Army Corps of Engineers (USACE) (1974). Methodology and effectiveness
of drainage systems for airfield pavements. Technical Report C-13,
Construction Engr. Research Laboratory (CERL), Urbana, IL