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Earth Retention Utilizing Shotcrete

Shotcrete is mortar or concrete that is applied as a spray, through a high-pressure hose and application nozzle. In the hands of a skilled operator, shotcrete offers the versatility to construct, repair and reinforce walls, bridges, tunnels, channels and other structural elements.

The history of concrete is punctuated with innovations that have expanded the usefulness of this basic material. Post-tensioning technology creates concrete planks with incredible strength and spanning capabilities. Admixtures allow builders to pour concrete in freezing weather, reduce water content for greater strength, and produce lightweight insulated concrete full of tiny bubbles. Advanced pumping equipment makes it possible to move and place just-mixed “mud” into places where most folks wouldn’t believe it could go.

Shotcrete is an especially noteworthy innovation in concrete technology. Sometimes referred to as “wet gunning,” shotcreting involves spraying concrete or mortar through a high-pressure hose to fill a form, repair a damaged surface or cover an armature or other substrate.

 

Soil Nails

Soil nails can provide a cost-effective solution for soil stabilization

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How is Shotcrete Applied?

A shotcrete application can be done using concrete (sand, aggregate, Portland cement and water) or mortar (sand, Portland cement and water). In either case, the ingredients are thoroughly mixed before being fed into a concrete pump that forces the liquid through a high-pressure hose. Air from a powerful air compressor is added to liquid at the application nozzle, enabling the operator to apply thin layers of concrete (or mortar) to a target surface. Because the air propels and lightens the mixture, shotcrete can be applied against vertical and sloping surfaces.

Dry-Gunning vs. Wet-Gunning

Before delving into the different uses of shotcrete, it’s important to clear up some terminology. Shotcrete is often confused with “Gunite,” which is a trade name for another type of spray application.

A Gunite application involves spraying a dry mortar mix through a hose, with water rather than air being added at the application nozzle. To avoid confusion, some people refer to a Gunite application as “dry-gunning.” Choosing between dry-gunning and wet-gunning is best left to the contractor, since there are advantages and limitations with both application techniques.

Applications for Shotcrete

Shotcrete’s spray-on application makes this technique popular for building swimming pools, as shown in the photo. However, foundation repair contractors exploit the versatility of shotcrete by using this technique to construct and repair retaining walls, foundation walls, tunnels, dams, bridges and shear walls. Shotcrete is also useful for lining drainage channels and for reinforcing walls as part of a seismic or shear wall retrofit.

The success of a shotcrete or Gunite application depends largely on the skill and experience of the application crew. For any high-pressure spray-on concrete project, it’s important to have a contractor who is familiar with the tools and techniques.

Soil Nail and Shotcrete Installation for Commercial Projects

Experienced foundation repair specialists and geotechnical engineers appreciate soil nails for their ability to solve soil stabilization problems in cost-effective ways.

For example, when a basement foundation must be excavated right next to an existing building with a slab foundation, it’s critical to prevent the soil next to the slab from caving into the excavation.

Soil nails, used in combination with shotcrete, enable the contractor to quickly and inexpensively construct a retaining wall that stabilizes the excavation while the new basement foundation is under construction.

It’s also possible to construct permanent retaining walls with soil nails and shotcrete. This technique is faster, easier, less expensive and less disruptive than building the same wall with poured concrete or concrete masonry units (CMUs).

A soil nail retaining wall can be built from the top down, in increments of 6ft. or so (See “Building a soil nail and shotcrete retaining wall” below). But a poured concrete or CMU retaining wall must be built from the bottom up. This necessitates a complete, full-depth excavation at the start of the project, a scope of work that may be too major and too disruptive in some locations. Soil nailing has become popular because it eliminates or minimizes these impacts.

Building a Soil Nail and Shotcrete Retaining Wall

Like a conventional concrete retaining wall, a retaining wall that utilizes soil nails is an engineering exercise involving soil analysis, loading calculations and other important design factors. Once the work begins, there are three basic steps:

  • Making a shallow excavation. The first cut (typically 3ft.-6ft. deep) defines the top of the retaining wall. Curved walls are nearly as easy to construct as straight walls —another advantage with this technique.
  • Installing soil nails. Soil nails come in different sizes, lengths and designs. Helical soil nails are designed to be self-drilling, while others are designed to be jacketed in grout that is pumped into a pre-drilled hole. During the wall design process, engineers will determine the type, size and spacing of soil nails to be used based on wall height and soil characteristics. Workers use hydraulic equipment to drive each soil nail into the excavated wall, aiming the nail slightly downwards.
  • Adding a shotcrete facing. When all nails have been driven, workers apply a shotcrete facing that’s usually between 3 in. and 6 in. thick. Drainage matting material and steel mesh are sometimes incorporated into this spay-applied wall. Promoting good drainage helps to prevent hydrostatic pressure from building up behind the wall. The steel mesh is included to increase the strength of the concrete.

When these three steps are completed, a second excavation is made and the same three steps are repeated. This 3-step construction cycle continues until the wall reaches its final design height.

Used in combination, soil nails and shotcrete give foundation repair contractors a number of problem-solving options that are useful to residential and commercial customers alike. Being able to build retaining walls quickly and inexpensively makes it possible to create workable level areas on sloping sites that would otherwise be unusable. Soil nail and shotcrete techniques can also stabilize slopes and create temporary walls so that more permanent structural work can take place safely.

To find out more, contact Thrasher Commercial at 1 (800) 827-0702.