Counterfort drains (trench drains intersecting basal shear plane)

Category: TRANSFER OF LOADS TO MORE COMPETENT STRATA

Description

Deep trench drains that intercept the slip plane and provide additional frictional resistance are generally called counterfort drains. Trench drains are commonly used to stabilize landslides of small to moderate depth in clay slopes. They contribute to slope stability only through their drainage action, as discussed in detail in the relevant fact sheets. If trench drains are deep enough to intersect the basal failure plane, they provide additional mechanical stabilization, by the replacement of the weak slipped material by the stronger material in the drain, thus improving the average shear resistance that can be mobilized on the failure plane for any given pore pressure regime (Lee and Clark, 2002). While deep trench drains intersecting the failure plane are generally referred to as “counterfort drains”, the term is often used loosely to indicate trench drains aligned along or close to the direction of  maximum inclination of the slope, irrespective of whether they do or do not intersect the slip plane.

Figure 1: Schematic section. Counterfort drains provide additional stabilization by intercepting the slip plane (source: Carter, 1992)

One of the earliest formally reported applications of counterfort drains to stabilize landslide is the construction of deep gravel filled counterfort drains through the shear surface to the undisturbed clay below to remediate rotational movements observed in London Clay in railway cuttings at New Cross (Gregory, 1844).  

Deep counterfort drains are reported by Tianchi (1996) to be the main measure used to treat small and medium scale landslides because of the combined benefits of the drainage and mechanical effects.

Many slip planes are less than 5 m deep and counterfort drains can be excavated to 6 m deep using hydraulic backactor excavators; greater depths up to 7 or 8 m deep can be reached using machines equipped with long reach booms. They are typically 0.5 to 1.0 m wide and they are back-filled with suitable free-draining material. They are design as invertes filters, with a gravel core surrounded by sand, to prevent them becoming chocked with fines, which renders them ineffective. Geotextile filters are widely used for this purpose to simplify construction. A porous pipe may be placed at the base to collect and remove the water. Provision to prevent clogging must be incorporated in the design. The mechanical benefits are increased if free draining concrete is used in lieu of the gravel fill.

Picture 1: Deep trench drains (counterfort drains) under construction (source: http://www.svr-vlo.org.uk/floodline_no50-59.htm)

 


Design methods

For the hydraulic aspect of the design, reference shall be made to the relevant fact-sheets.

Provided the length, thickness and spacing of the counterfort drains are such that load transfer from the sliding mass to the counterforts and from the these to the underlying stable soil is guaranteed, the mechanical benefit of partially replacing the shear surface with more competent material may be taken into account simply by calculating the post construction average strength as the weighted average strength of the original soil and the drain material.

Clearly, this is most effective when remediating pre-existing planar slides in clay, which often exist close to limit equilibrium and are cyclically reactivated. Assuming a residual angle of friction on the failure plane equal to 14° and an angle of friction of the drain material equal to 32°, a replacement ratio of 20% would result in a 30% improvement in the factor of safety of the slope. Clearly, lower replacement ratios are sufficient to provide a similar result if the drainage effect is also taken into account.

For the full mechanical effect to be mobilized, the proportions between the length and the spacing of the drains must be such that arching takes place between adjacent drains.  

 


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 0 Applicable to planar slides and, to a lesser extent, to rotational slides.
Topple 0
Slide 8
Spread 0
Flow 0

Material type

Descriptor Rating Notes
Earth 8 Most suitable in clays, both in terms of ease and local stability of excavations and in terms of relative effectivness. In debris it may be useful if carried out with free draining concrete.
Debris 4
Rock 0

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 8 Depths up to 4 – 5 m can be reached without special difficulty; higher depths up to 7 to 8 m, suitable for slides up to 6 m deep, may be achieved using special equipment (long reach booms).
Shallow (0.5 to 3 m) 8
Medium (3 to 8 m) 4
Deep (8 to 15 m) 0
Very deep (> 15 m) 0

Rate of movement

Descriptor Rating Notes
Moderate to fast 0 Should be carried out preferably on very or extremely slow landslides; with due care it can be carried out in slow landslide.
Slow 4
Very slow 8
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 4 High groundwater levels imply the maximum effectiveness in terms of drainage, but may pose problems during construction; applicability to situations with arrtesian conditions to be reviewed carefully.
High 8
Low 6
Absent 2

Surface water

Descriptor Rating Notes
Rain 6 Suitable to deal with diffused surface water. Concentrated flows should be prevented or diverted from the slope.
Snowmelt 6
Localized 4
Stream 2
Torrent 0
River 0

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 8 Generally reliable. Exact location of slip surface can be confirmed by inspection during installation. Effective almost immediately.
Feasibility and Manageability 8 Traditional technique, widely applied, mainly on an empirical basis without formal design.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 6 Deep excavation in potentially unstable soil causes significant safety hazard. must be well planned. Arrangements must be made to avoid man entry.
Environmental suitability 4 will be updated
Economic suitability (cost) 8 Relatively low cost, unless free draining concrete is used and provided suitable material is readily available.

References

  • Carter P. (1992) “Dewatering”. In: Drainage Design, P. Smart and J.G. Herbertson editors,  blackie & Son Ltd, Glasgow

  • Gregory C. H. (1844) “On railway cuttings and embankments with an account of some slips in London Clay”. Minutes of Proceedings of the Institution of Civil Engineers, 3, 135-173

  • Lee E.M., Clark A.R. (2002) “Investigation and management of soft rock cliffs”. Thomas Telford, London, UK

  • Tianchi L. (1996). “Landslide Hazard mapping and management in China”. International Centre for Integrated Mountain Development, Kathmandu, Nepal.

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