Soil nail and root technology - SNART (Hybrid)

Category: TRANSFER OF LOADS TO MORE COMPETENT STRATA

Description

SNART (soil nail and root technology) is a combined technology which involves deep mechanically anchored soil nails combined with shallow vegetation (Figure 1). This technology was developed by DST Consulting Engineers, Inc., in Canada in the 1990s (Tozer and Fabius, 2000) with the same function of the traditional Soil nailing (see 6.5). In addition, instead of the typical surface covering, such as shotcrete and welded wire mesh, geogrid/geotextiles sheets and cast-in-place concrete or prefabricated panels, the facing is designed with roots from 15 to 30 cm deep depending on the slope angle (Geoengineering.org).  The length and the spacing of nails are engineered to address all potential modes of failures: from deeper failures to surficial slipping failures within the nails depth zone (Bo et al., 2015). The performance of these environmental friendly slope stabilization methods is demonstrated by several case studies successfully applying SNART (Fabius et al., 2008; Bo et al., 2015).

 

Advantages

  • Only the vegetation is visible once the system is installed;

  • Technically, economically effective and environmental friendly stabilization method.

Disadvantages

  • The disadvantages of the technique are mainly linked to its constructability, in relation to nature of ground to be reinforced and/or presence of groaundwater percolating through the face (see Soil nailing – 6.5).

 

Figure 1.(Bo et al., 2015)

 


Design methods

As for soil nailing (see 6.5) some primarily considerations for designing the measures need to be addressed. The installation then means of pushing using static or dynamic driving forces depending on the type of soil and resistance, using lightweight equipment operating on or adjacent to the slope (see the schematic construction sequence in 6.5).

The shallow soil between nails is reinforced with long roots vegetation system which both provides reinforcing and soil suction at the shallow depth. In order to initially enhance vegetation establishment, biodegradable coconut fibers are placed along the slope surface as protection of the top-soil from possible surface erosion (Bo et al., 2015).


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 7 Applicable to slides and in special circumstances to falls and topples in cemented or stiff/hard cohesive soils.

Use of adequate vegetal species is recommended.
Topple 4
Slide 7
Spread 4
Flow 4

Material type

Descriptor Rating Notes
Earth 10 Applicable to earth and debris. In very coarse debris drilling can be problematical and launching is precluded.
Debris 10
Rock 2

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 4 Practical soil nail lengths and the need to achieve sufficient anchorage in the underlying stable soil limit the application of this technique to situations where the residual thickness of the actual or potential landslide to be stabilized is significant. In the meantime, the vegetation roots can improve the soil stability at shallowest layers.
Shallow (0.5 to 3 m) 6
Medium (3 to 8 m) 5
Deep (8 to 15 m) 3
Very deep (> 15 m) 3

Rate of movement

Descriptor Rating Notes
Moderate to fast 2 Workers’ safety and end result require construction to take place when movement is extremely slow or very slow (maximum 1.5 m/year, corresponding to approximately 5 mm/day).
Under special conditions and taking due precautions it may be carried out when movement is ”slow” (up to 1.5 m/month, corresponding to 5 cm/day) .
Slow 4
Very slow 9
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 5 Drillhole stability where groundwater may be encountered should be reviewed carefully, since the use of temporary casing, if required, would normally make this technique excessively expensive. Groundwater seepage at the surface must be avoided, incorporating suitable drainage works, with the risk of local slumping before the draingae works are effective.
High 7
Low 9
Absent 9

Surface water

Descriptor Rating Notes
Rain 5 Where sliding is due to channelized water, construction difficulties may be expected and there may be special requirements for the facing.
The presence of vegetation can increase the soil suction at shallow layer where localized soil moisture is available.
Snowmelt 5
Localized 3
Stream 3
Torrent 3
River 3

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 6 Successful application depends on correct schematization and characterization of the landslide, design and construction detail, correct application.
Feasibility and Manageability 6 There is over 25 years of experience with the technique, but it is still susceptible to technological and design improvements.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 6 Requires specialist equipment; special arrangements may be required for access on existing slopes; simplified by launching but durability is questionable.
Environmental suitability 8 The use of vegetation for slope covering improves the environmental suitability of the measure.
Economic suitability (cost) 6 Moderate. Can become quite high if drillholes require temporary casing and/or special access arrangements.

References

  • Bo, M. W., Fabius, M., Arulrajah, A., & Horpibulsuk, S. (2015). Environmentally friendly slope stabilization using a soil nail and root system in Canada. In Ground Improvement Case Histories(pp. 629-654). Butterworth-Heinemann.

  • Fabius, M., Bo, M. W., & Villegas, B. (2008). Stabilization of a 30 m High Riverbank in Canada with Nails, Plates and Roots.

  • Fabius, M., Eng, P., & Principal, D. S. T. STEEP NAILED EMBANKMENT TECHNOLOGY: 2 SNART CASE STUDIES.

  • Geoengineering.org. An unusual application of Soil Nails - https://www.geoengineer.org/virtual/an-unusual-application-of-soil-nails

  • Tozer, S. E., & Fabius, M. (2000). Steep nailed embankment technology: 2 case studies. In 51st Annual Highway Geology Symposium, Seattle, USA.

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