Methods of Ground Engineering

Slurry walls, or cut-off slurry walls, are constructed by excavating a trench under a cement/bentonite slurry. Typically, trenches are excavated from 12 to 15 metres deep using hydraulic back-actors (backhoes). For greater depths up to 45 metres, diaphragm wall grabs or cutters are used for difficult ground conditions.

A hydrated cement-bentonite mix is pumped into the trench. Once hardened, it provides a low-permeability barrier to groundwater and polluted or contaminated materials. Self-hardening slurry walls are commonly used in dam core cut-offs, flood water control and landfills to prevent gas lateral migration. Slurry walls are quick to install, cost-effective and resistant to various heavy metals and organic contaminants.

Soil nailing is a ground engineering technique used for reinforcing in situ soils and creating a gravity retaining wall. Soil nail walls are commonly used for stabilising slopes or landslides, supporting permanent or temporary excavations and repairing existing retaining walls.

The slope or wall is predrilled from top to bottom at a 70° to 90° angle to create slightly downward holes. Long tension-resisting or steel nails are inserted into the holes and grouted firmly to hold the soil in place. If the soil contains corrosive elements, the steel nails must be coated with an anti-corrosive coating beforehand. After installation, shotcrete is used to fix the nails in place and a permanent wall facing is applied in layers. Soil nailing is quick, economical, has little impact on the environment compared to other methods and is suitable for restricted sites.

Ground anchors are permanent or temporary components used to hold, restrain and support buildings and other structures. They work by transferring tensile loads into the ground. To install, a borehole is drilled at the desired diameter and depth. Next, the hole is grouted and the anchor is installed in place.

Once the grout has hardened, a specified load is applied using hydraulic pumps and jacks. The anchor can be tested for correct installation by applying the load to the end of the anchor and attempting to pull it out. Finally, the anchor cap, drainage and final facing are installed.

Ground anchors are primarily used to prevent horizontal movement while stabilising retaining structures such as bored concrete piles, sheet piles and steel tubular walls. Additionally, this ground improvement method is used for stabilising slopes and embankments, retaining bridge adjustments and erosion control. Ground anchors are versatile, durable, quick to install and suitable for a variety of ground conditions.

Compaction grouting is a ground improvement method for increasing soil density and stabilising sinkholes or subsurface voids. It is carried out by injecting stiff mortar grout under pressure through a cased borehole to form a column of grout bulbs. Typically, grouting is performed using grid patterns of 4 to 9 m2. As the grout expands, it pushes the surrounding soil aside and increases its density and bearing capacity.

Grouting is cost-effective and ideal for limited spaces. It is also suitable for rubble or poorly placed fills, soluble rock and a wide variety of loose granular soils with good drainage and a depth of 2 metres or more. When completed properly, compaction grouting can prevent the collapse of granular soils, groundwater movement and settlement under adjacent foundations.

In conjunction with grouting, bulk infill drilling and grouting is a ground stabilisation method for preventing the migration of abandoned mines and other voids that pose a risk to buildings and sub-surface infrastructure. This method can also be used for soil mixing, soil stabilisation and compaction grouting.

The drilling process involves rotary percussive rigs, installing casing seated into the rockhead, and drilling to 1m below the base of the seam. Commencing at the lowest dip side, grout is injected through a flexible tube called a tremie from the base of the drilling hole to the surface. Bulk infill drilling and grouting can be carried out in limited spaces.