8.5 Drainage installation

8.5.1 General

8.5.1.1 The effective control of surface and groundwater is essential during construction and service life of a soil nail construction. Drainage measures shall therefore be constructed to a high standard to protect the soil nail works from the detrimental effects of surface water and groundwater, both during construction and the design life.

8.5.1.2 If groundwater or surface water flows are greater than anticipated, then the design shall be reviewed.

8.5.1.3 The effects of surface water can be controlled by a variety of measures, such as cut-off trenches, channels, bunds, sumps and sheeting. Measures to control surface water should generally be installed prior to commencing excavation or the stripping of vegetation.

8.5.1.4 The effects of groundwater can be controlled by internal drainage measures, such as upwardly inclined well screen drains and trenched drains, or by the construction of a drainage blanket immediately behind the facing, often in combination with weep holes.

NOTE Pockets of water or perched water tables known to exist, or revealed during soil nail execution, are normally controlled through the deep drain system.

8.5.1.5 Where adverse hydraulic conditions are encountered, then measures required for controlling water during soil nail construction and preventing erosion of excavated faces shall be implemented and agreed before further excavation.

8.5.1.6 Surface water and groundwater from drains should be channelled to collection points, where it can be discharged safely and in accordance with environmental regulations.

8.5.2 Surface drainage

8.5.2.1 If surface drains are employed, they shall have sufficient capacity to collect and control rainwater flows arising from the storm with a return period equivalent to the design life of the works.

8.5.2.2 If sheeting is employed to collect surface water, attention should be paid to the jointing and overlapping of sheets to prevent water entering between the sheeting and ground. Where necessary, it should be pinned or weighted to keep it in intimate contact with the ground surface and prevent it from lifting under wind loading. Sheeting should be inspected regularly for damage and repaired as necessary to maintain serviceability.

8.5.2.3 If drainage channels are constructed, they should have a continuous fall to a collection point and shall prevent ponding of water. If collecting surface water from a slope, they should be detailed to ensure the water runs into the channel and does not pass into the ground below the channel. Where constructed directly on the ground surface, the ground should be well compacted. Construction joints should be watertight to prevent ingress of water and erosion of the soil below the channel. Details should be included to prevent damage to the channel due to differential or thermal movement.

8.5.2.4 Trenched drains should be excavated to ensure that the invert falls continuously to the collection point and ponding of water is prevented. Excavation should be carried out in controlled lengths to minimise the period the trench is open and where necessary trench support should be employed. If excavated in wet conditions, the excavation should commence from the lowest point working upwards.

8.5.2.5 Prior to back filling the trench, the sides and base of the trench can be lined with an appropriate geotextile to prevent fines from clogging the drain in the long-term.

8.5.2.6 If a perforated pipe or well screen is to be placed in the base of the trench, it should be inspected for damage and any joints checked to ensure the drain will act as a continuous duct.

8.5.2.7 On completion of the above, the trench should be back-filled, using a granular filter material complying with the design requirements with respect to grading and durability. Samples of backfill shall be tested for compliance as specified in the design.

8.5.3 Facing drainage

8.5.3.1 If used, a filter layer behind the face normally comprises strips of drainage filter running up and down the face or diagonally across it.

8.5.3.2 If required by the design, the drainage filter should not adversely affect the quality of the facing concrete or the friction between ground and facing.

8.5.3.3 Where there is a risk of water build-up behind the face, weep holes shall be constructed to allow free flow of water through them. They shall have an internal diameter that allows for cleaning and inspection.

NOTE Where placed directly in contact with the soil, they should be lined or wrapped with a geotextile filter fabric or similar. If placed before construction of a sprayed concrete facing, they should be fixed securely in position and protected to prevent blockage or damage during spraying. When fixed into predrilled holes through a facing, then the annulus between the weep hole and facing should be sealed to ensure that seepage water passes through the weep hole.

8.5.3.4 The connection of the facing drainage system and the weep holes shall be inspected and tested prior to the application of the facing.

8.5.4 Subsurface drainage

8.5.4.1 If subsurface drainage is used, it should be installed with a minimal fall of 5 % towards the facing of the construction.

8.5.4.2 The location, diameter, length and inclination shall be specified in the design.

8.5.4.3 The filter characteristic shall be specified in the design in order to ensure the compatibility between the soil grading and the filter characteristics.

8.5.4.4 The method of installing deep drains shall ensure that the pipe is not damaged and that soil is not smeared over the filter surface impairing its efficiency. Joints between sections of pipe shall be securely fastened.

8.5.4.5 Where the subsurface drain meets the facing or surface of the works, a seal shall be provided between the filter and surrounding ground to ensure that water exits through the pipe and does not erode the soil in this region. A mortar, concrete or other suitable compound may provide this seal.

8.5.4.6 Observation of water flows, fine particles, or otherwise, from deep drains shall be recorded and investigated if necessary.

8.5.5 De-watering systems

If soil nailing is carried out below the ground water level, or if unexpected flows of water are causing construction difficulties, a de-watering system shall be employed.