Introduction

Like many other geotechnical projects, surface water and groundwater flow can cause problems during construction, increase cost and duration, compromise the long-term integrity of the structure, and weaken the performance of soil-nailed retaining walls. To minimize these issues, both surface and groundwater flow must be controlled during and after the construction of soil-nailed retaining walls. Moreover, when an effective and efficient drainage system is implemented to control the groundwater level behind a soil-nailed retaining wall, the wall’s performance significantly improves. Below is a brief description of the drainage control systems used in soil-nailed retaining walls.

Control of Groundwater and Surface Water in Soil Nailing Excavation Support

Water discharge measures during construction

These include controlling surface runoff and upwelling groundwater or flow associated with localized leakage zones.
Excavating a surface water separation ditch along the excavation crest, lined with concrete during the first-stage shotcrete application, is a recommended solution for controlling surface water flow in soil-nailed retaining wall construction.
Additionally, if the design engineers believe that groundwater impacts are local or short-term, temporary well points or wells can be installed behind the length of nails (or soil nail reinforcements) to lower the groundwater level.
However, this method may increase construction costs and cause project delays.

Long-Term Control of Groundwater and Surface Water in Soil Nailing Excavation Support

Geocomposite Strip Drains

These elements are strips of geosynthetic material approximately 200 to 400 mm wide.
They are installed vertically along the entire depth of the soil-nailed retaining wall in front of the excavation face (see Figure 1).
The horizontal spacing of the drains usually matches the horizontal spacing of the nails (or soil nail reinforcements).
The lower ends of the strips drain into a collector pipe along the base of the retaining wall or discharge into weep holes in the wall base.
For very irregular excavation faces, placing prefabricated strip drains is difficult and often impractical.
In some cases, strip drains may be sandwiched between the shotcrete facing and permanent cast-in-place concrete facing, and combined with drainage through weep holes 50 to 75 mm in diameter passing through the facing.
To ensure that the drainage system is not damaged during shotcrete application, the design engineer must provide clear construction and monitoring guidelines.
If proper function is not guaranteed, the influence of groundwater level must be considered in the analyses.

Surface Drainage (Weep Holes)

Typically, PVC pipes 300 to 400 mm long with diameters of 50 to 100 mm extend through the facing and are placed at localized leakage points.
Weep holes are also used at the base of vertical strip drains to drain collected water from the soil-nailed retaining wall.

Drainage Pipes

Horizontal or gently sloping drainage pipes may be installed where controlling groundwater pressure on the retained soil mass is necessary.
Drainage pipes typically consist of prefabricated or perforated PVC tubes, 50 mm in diameter, installed at a 5 to 10-degree slope.
They are usually longer than the nails (or soil nail reinforcements) and prevent groundwater from contacting the nails or the nailed soil block, as shown in Figure 1.
The length of the drains depends on their purpose.
For surface drainage or perched water near the facing, pipes 0.3 to 0.5 m long (sometimes 1 m) may be used.
Typical drain spacing is approximately one drain per 10 m² of facing.
To prevent grout intrusion into perforated pipes, drainage pipes are usually installed after nail installation.
They are generally located between the facing and the soil-nailed retaining wall.

PVC Pipe Details

As shown in Figure 2, the PVC pipes should be perforated.
Although drainage pipes are usually installed after nails are placed and shotcrete applied to avoid grout or shotcrete intrusion, they can also be installed before shotcrete.
In this case, cement plugs (dry-pack) and temporary PVC caps must be used to prevent shotcrete from entering and blocking the pipe holes.

Permanent Surface Water Control

Permanent surface water control measures include installing a separation and diversion ditch behind the soil-nailed retaining wall to prevent surface runoff from penetrating behind or flowing over the top edge of the soil-nailed wall.
Vegetative cover may also be used to reduce, delay, or repel water infiltration into the soil.

Figures:

• Figure 1: Drainage system for soil-nailed retaining walls

• Figure 2: Detail of perforated drainage pipes for groundwater control in soil-nailed walls