A.4 Examples of drainage systems used in a soil nail structure

A.4.1 General remarks

Water is detrimental to slope stability and has to be drained away from the surface as much as possible. In this way, general or local erosion, etc. and critical water pressures behind facings may be minimised (specially important in case of a full cover or with a vegetation layer).

Three essential measures have to be distinguished:

  • a) prevention of surface runoff water;
  • b) surface drainage;
  • c) subsurface drainage.

A.4.2 Interception of surface water run off

Figures A.12 and A.13 show examples of drainage above the soil nailing structure.

Trenched drains above the soil nail structure guided to the sides of the slope
Figure A.12 — Trenched drains above the soil nail structure guided to the sides of the slope
Surface drainage above the soil nail structure
Figure A.13 — Surface drainage above the soil nail structure (e.g. in case of stratum water) (e.g. Y- drains)

A.4.3 Surface drainage

Systems for flexible and soft facings with vegetation layers but also possible behind hard facings (sprayed concrete).

Seepage

Key

  • 1 foot drainage
Figure A.14 — Seepage

A.4.4 Drainage systems for hard and impermeable facings

In case of concrete walls, prefabricated or cast in place, spread filters made of drainage material and collector drains can be applied.

In any case, with impermeable facings, sufficient leakage holes have to be placed, as shown in Figure A.15.

Hard and impermeable facings

Key

  • 1 drainage material
  • 2 collector drain
  • 3 "weep-hole" drain
Figure A.15 — Hard and impermeable facings

A.4.5 Subsurface drainage

Subsurface drainage will be required if water-bearing strata are predicted or encountered. Subsurface drainage may be required if the groundwater table has to be lowered.

Drainage boreholes normally contain slotted or perforated pipes. They are normally wrapped with a geotextile filter to prevent the ingress of fines. The characteristic opening size of the geotextile should be chosen to minimise clogging while permitting water into the pipe.

The number, length and pattern of the drainage pipes depend on the expected amount and regime of water. The inclination of the boreholes is typically ≥ 5 %.

Subsurface drainage
Figure A.16 — Subsurface drainage

A.5 Tolerances

A.5.1 The completed soil nail location at the ground surface should be within ± 100 mm of that intended within the final structural face. Where pre-cast facing is used, a greater accuracy may be required.

A.5.2 The orientation measured at the head of the completed soil nail should be within ± 5° of the design alignment.

A.5.3 Although borehole deviation over the length of the bore may not be critical, it should rarely exceed 1/30 of the bore length.

A.5.4 Where soil nails are installed closely, or are located in close proximity to existing services, drains or structures, the designer may specify tighter tolerances.

A.6 Long-term monitoring

The following points are intended as guide of long-term monitoring:

  • a) crest levels (minimum intervals 20 m);
  • b) toe levels (minimum intervals 20 m);
  • c) crest line (minimum intervals 20 m);
  • d) partial height line and level (recommended for retained heights over 5 m);
  • e) inclinometers;
  • f) ground water level;
  • g) water flow through weep holes;
  • h) corrosion test samples (wired for recovery).

EN 14490:2010 Execution of special geotechnical works – Soil nailing