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Soil Nailing

Soil nailing technique

NSS Engineers & Geotechnical Services is please to be the consultant for execution and to design the soil nailing technique for stabilization of the natural or manmade soil slope. We have been regularly using this technique for the stabilization of the soil slope in different challenging environments with satisfactory approach. The theory of the soil nailing and its application is discussed below for observation and consideration of the facts of this technique. It is completely our pleasure to take an opportunity for implementing the applications of this technique.

The soil nailing is the technique which is use to reinforce the existing ground by increasing the normal loads (and hence the shear strength) on potential sliding surfaces through the nails, which is simply consist of steel tendon, but most commonly the tendon is encapsulated in a cement grouted body to provide corrosion and improved load transfer to the ground.

It is an effective an economical method to stabilize the excavated soil face or landslide zone. Soil nail technology was first used in France to build a permanent retaining wall cut in soft rock, whereas the first soil nail wall to use modern soil nailing techniques was built near Versailles in 1972. After that it has been common methodology for the stabilization of the soil mass in different applications. Destabilization of the soil mass caused by the increase in tensile force compare to the compressive force or normal force (resisting force). In this technique nails work predominantly in tension but are considered by some work also in bending/shear in certain circumstances. The effect of the nail reinforcement is to improve stability by a) increasing the normal force and hence the soil shear resistance along potential slip surfaces in frictional soils and b) reducing the driving force along potential slip surfaces in both frictional and cohesive soils. A construction facing is also usually required and is typically shotcrete reinforced by welded wire mesh. In soil nailing, the reinforcement is installed horizontally or sub – horizontally, which would be approximately parallel to the direction of major tensile straining in the soil to minimize the tensile force and to maximize the resisting potential of the soil mass through nail-ground interactions.

Best suited conditions for soil nailing

  • In general, the economical use of soil nailing requires, the ground has to be stand unsupported in a vertical or steeply – sloped cut of 1 to 2 m in height for one to two days.
  • In addition, it is highly desirable that an open drill hole can maintain its stability for at least several hours neither the execution of the work would be more challenging.
  • Geologically the best suited conditions for soil nailing would be characterized as the stiff to hard fine-grained soils which include stiff to hard clays, clayey silts, silty clays, sandy clays, and sandy silts are preferred soils.
  • Sand, gravels, cobbles, pebbles and boulder, which are dense to very dense soils with some apparent cohesion also work well for soil nailing.
  • Weathered rocks is also favorable for soil nailing while it would be weathered throughout and there should not be the weak planes.
  • The soil which has been deposited by glacial event is also favorable for soil nailing.

Design of soil nailing

Design of soil nailing for stabilization of respective soil mass could be executed with two approaches i.e. the selection of limit states and design approaches. Again there are two common limits has to be considered i.e. strength limit and service limit states. The strength limit state is the limit state that addresses potential failure mechanisms or collapse states of the soil nail wall system. The service limit state is the limit state that addresses loss of service function resulting from excessive wall deformation and is defined by restriction in stress, deformation and facing crack width under regular service conditions.
In design approaches the initial design considerations include wall layout (wall height and length), soil nail vertical and horizontal spacing, inclination of soil nail, length of soil nail and distribution, soil nail material and relevant ground properties. After considering these all parameters the design engineer has to be evaluating the nail length and maximum nail force. Nail length, diameter and spacing typically control external and internal stability of the wall. These parameters could be adjusted during design until all external and internal stability requirements are met. After the initial design is completed, final design progresses where the soil nail wall has to be tested for external and internal failure modes, seismic considerations and aesthetic qualities. Drainage, frost penetration and external loads such as wind and hydrostatic forces also have to be determined and included in the final examination of the design.

Construction of soil nail

With the design complete, the execution of the soil nailing and the construction of soil nail wall is the next step. Most soil nail wall construction follows a specific procedure. Its construction process is faster than other similar methods. The construction process start by cutting the soil face, which would be approximately vertical and after that put the temporary bracing at that excavated soil face, if necessary. This can be manually performed or can be used earth moving equipment. After that start the drilling into the soil, where the nail, steel bar, is going to be placed. The holes for the soil nails are drilled at predetermined locations as specified by the design engineer. The equipment used for this step is dependent on the stability of the material in which the soil nail is supporting. After inserting the nails into drilled holes, it must be grouted into the soil to create structure similar to a gravity wall.
With the holes drilled, the next step is to install and grout the nails into place. After all nails are inserted, a drainage system is put into place. Synthetic drainage mat is placed vertically between the nails heads, which are extended down to the base of the wall where they are most commonly connected to a footing drain. A layer of shotcrete is applied and bearing plates are installed before a final facing is put in place to complete the soil nail wall. Variations of the steps described above may be necessary to accommodate additional preparation tasks or supplementary activities for specific project conditions.

Advantage of soil nailing

  •  It is cost effective over other alternatives for stabilization of the natural soil slope or any man made excavated soil face.
  • It has a less impact on environment and the other natural phenomena because of the minimum excavation and backfilling of the respective region.
  • The method is well-suited to sites with difficult or remote access because of the relatively small size and the mobility of the required construction equipment.
  • The method is also well suited to urban construction where noise, vibration and access can pose problems.
  • Minimize right-of-way requirement.
  • Can be used as both temporary and permanent support

Application of soil nailing

  • Providing an earth retention system for deep excavations
  • It can be useful to create permanent retaining walls.
  • This has been regularly used for stabilizing slopes and landslide.
  • It can also be used for natural hillsides stabilization and disturbed terrain.
  • It can be used to stabilize the man made soil cut slopes.
  • It would be helpful to preserve the natural areas.
  • Supporting and strengthening ground around tunnel excavations.

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