Applications of Soil Nail Walls

Applications of Soil Nail Walls

Soil nail walls can be used in the following roadway applications:
  • Roadway cuts 
  • Road widening under existing bridge abutments 
  • Tunnel portals 
  • Repair and reconstruction of existing retaining structures 
  • Hybrid soil nail systems 
  • Shored Mechanically Stabilized Earth (SMSE) walls
Roadway Cuts: 
Soil nailing is attractive in roadway cuts because a limited excavation and reasonable right-of-way (ROW) and clearing limits are required. These factors help to reduce the environmental impacts along the transportation corridor. 

The impact to traffic may also be reduced because the equipment for installing soil nails is relatively small. Variations in the details may exist, particularly to those details related to runoff control. 
Road Widening Under Existing Bridge Abutments:
Soil nail walls can be advantageous for underpass widening when the removal of an existing bridge abutment slope is necessary. While the cost of installing a soil nail wall under a bridge abutment may be comparable to that of other applicable systems, the advantage of soil nailing is that the size of the soil nail drill rig is relatively small. Soil nailing equipment can operate within limited overhead, and traffic flow along the underpass road may not need to be totally interrupted during the widening. 
The location, length, and inclination of soil nails need to be carefully planned so that the nails do not interfere with the existing bridge girders and do not intersect the existing abutment foundation. 

The upper soil nails must be positioned within the clear space between bridge girders and must be parallel to them. The remaining, lower rows of soil nails must be positioned and oriented to avoid hitting the foundation elements below. 
Vertical micropiles and soil nails have also been used in modified abutments for road widening projects where an existing bridge was originally supported on shallow foundations constructed on top of an existing embankment. The micropiles support the modified abutment and prevent settlements, as they transfer bridge loads below the new road grades. 

At the same time, the added soil nail wall retains the excavated soil. Because overhead clearances are small in most roadway widening projects, the combined use of soil nails and micropiles is attractive in these applications as they can help expedite construction by allowing the bridge to remain operational during widening of the underpass lanes. 
Tunnel Portals:
Tunnel portals can also be stabilized using soil nails. Although the principle behind the use of soil nails in tunnel portals is similar to that for road cuts, other aspects must be considered in the design and construction of this application.
 First, the vertical stability of the shotcrete facing above the tunnel must be considered. The potential transfer of soil nail loads to the tunnel structure at the portal must also be taken into account. The interaction between soil nails and the initial shotcrete support and lining of the tunnel near the portal need to be fully evaluated. 

In addition, the layout of soil nails may be different than that in the conventional use in roadway applications. Soil nails must be installed with an appropriate horizontal splay and a suitable vertical orientation to avoid interfering with the tunnel support components.

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