Ground Anchors & Nailing

Nailing
Soil nailing consists of the passive reinforcement (i.e., no post-tensioning) of existing ground by installing closely spaced steel bars (i.e., nails), which are subsequently encased in grout. As construction proceeds from the top to bottom, shotcrete or concrete is also applied on the excavation face to provide continuity. Soil nailing is typically used to stabilize existing slopes or excavations where top-to-bottom construction is advantageous compared to other retaining wall systems. For certain conditions, soil nailing offers a viable alternative from the viewpoint of technical feasibility, construction costs, and construction duration when compared to ground anchor walls, which is another popular top-to bottom retaining system.


 Wall stabilization with application of nailing and ground anchor system, Imam Khomeini hospital, Terhan, Iran (Omran Ista)


Wall stabilization with application of nailing system, Aghdasieh, Terhan, Iran (Omran Ista)

Application of soil nail walls
Soil nail walls are particularly well suited to excavation applications for ground conditions that require vertical or near-vertical cuts. They have been used successfully in highway cuts; end slope removal under existing bridge abutments during underpass widening; for the repair, stabilization, and reconstruction of existing retaining structures; and tunnel portals. Soil nail walls have been shown to be particularly well suited in the following temporary or permanent applications:
• roadway cut excavations;
• road widening under an existing bridge end;
• repair and reconstruction of existing retaining structures; and
• temporary or permanent excavations in an urban environment.

Basic elements of a soil nail wall
The most common practice for soil nailing consists of drilled soil nails, in which a steel bar is placed in a pre-drilled hole and then grouted. The following Figure shows a cross-section of a typical soil nail wall where the following components are illustrated.


Typical cross-section of a soil nail wall

1. Steel reinforcing bars – The solid steel reinforcing bars are the main component of the soil nail wall system. These elements are placed in pre-drilled drill holes and grouted in place. Tensile stress is applied passively to the nails in response to the deformation of the retained materials during subsequent excavation activities.
2. Grout – Grout is placed in the pre-drilled borehole after the nail is placed. The grout serves the primary function of transferring stress from the ground to the nail. The grout also provides a level of corrosion protection to the soil nail.
3. Nail head – The nail head is the threaded end of the soil nail that protrudes from the wall facing.
4. Hex nut, washer, and bearing plate – These components attach to the nail head and are used to connect the soil nail to the facing.
5. Temporary and permanent facing – The facing provides structural connectivity. The temporary facing serves as the bearing surface for the bearing plate and support the exposed soil.
6. Geocomposite strip drainage – The geocomposite strip drainage system media is placed prior to application of the temporary facing to allow collection and transmission of seepage water that may migrate to the temporary facing.

1-3- Construction sequence

The typical sequence of construction for a soil nail wall using solid steel nail bars is described below and shown schematically in following figure.
 


Typical soil nail wall construction sequence

Step 1. Excavation. Initial excavation is carried out to a depth for which the face of the excavation has the ability to remain unsupported for a short period of time, typically on the order of 24 to 48 hours. The depth of the excavation lift is usually between 1 and 2 m (3 and 6 ft) and reaches slightly below the elevation where nails will be installed.
Step 2. Drilling Nail Holes. Drillholes are drilled to a specified length, diameter, inclination, and horizontal spacing from this excavated platform.
Step 3. Nail Installation and Grouting. Nail bars are placed in the pre-drilled hole. The bars are most commonly solid, although hollow steel nails can be also be used. Centralizers are placed around the nails prior to insertion to help maintain alignment within the hole and allow sufficient protective grout coverage over the nail bar. A grout pipe (tremie) is also inserted in the drillhole at this time. The drillhole is then filled with cement grout through the tremie pipe. Prior to Step 4 (facing placement), geocomposite drainage strips are installed on the excavation face approximately midway between each set of adjacent nails. The drainage strips are then unrolled to the next wall lift.
Step 4. Construction of Temporary Shotcrete Facing. A temporary facing system is then
constructed to support the open-cut soil section before the next lift of soil is excavated. The reinforcement typically consists of welded wire mesh (WWM), which is placed at approximately the middle of the facing thickness. Following appropriate curing time for the temporary facing, a steel bearing plate is placed over the nail head protruding from the drillhole. The bar is then lightly pressed into the first layer of fresh shotcrete. A hex nut and washers are subsequently installed to secure the nail head against the bearing plate.
Step 5. Construction of Subsequent Levels. Steps 1 through 4 are repeated for the remaining excavation lifts. At each excavation lift, the vertical drainage strip is unrolled downward to the subsequent lift. A new panel of WWM is then placed overlapping at least one full mesh cell. The temporary shotcrete is continued with a cold joint with the previous shotcrete lift. At the bottom of the excavation, the drainage strip is tied to a collecting toe drain.
Step 6. Construction of a Final, Permanent Facing. After the bottom of the excavation is reached and nails are installed and load tested, a final facing may be constructed. Final facing may consist of cast-in-place (CIP) reinforced concrete, reinforced shotcrete, or prefabricated panels.
Variations of the steps described above may be necessary to accommodate additional preparation tasks or supplementary activities for specific project conditions.


Drilling nail holes with casings


Drilling nail holes

Ground Anchors
A prestressed grouted ground anchor is a structural element installed in soil or rock that is used to transmit an applied tensile load into the ground. Grouted ground anchors, referenced simply as ground anchors, are installed in grout filled drill holes. Grouted ground anchors are also referred to as “tiebacks”. The basic components of a grouted ground anchor include the: (1) anchorage; (2) free stressing (unbonded) length; and (3) bond length.

 


Components of a ground anchor


Anchorage components for a bar tendon

 


Anchorage components for a strand tendon

The anchorage is the combined system of anchor head, bearing plate, and trumpet that is capable of transmitting the prestressing force from the prestressing steel (bar or strand) to the ground surface or the supported structure. Anchorage components for a bar tendon and a strand tendon are shown in below figures.
 


Construction of a ground anchor wall
 


A completed project with ground anchor system
 


 

The unbounded length is that portion of the prestressing steel that is free to elongate elastically and transfer the resisting force from the bond length to the structure. A bondbreaker is a smooth plastic sleeve that is placed over the tendon in the unbonded length to prevent the prestressing steel from bonding to the surrounding grout. It enables the prestressing steel in the unbounded length to elongate without obstruction during testing and stressing and leaves the prestressing steel unbonded after lock-off. The tendon bond length is that length of the prestressing steel that is bonded to the grout and is capable of transmitting the applied tensile load into the ground. The anchor bond length should be located behind the critical failure surface.
A portion of the complete ground anchor assembly is referred to as the tendon. The tendon includes the prestressing steel element (strands or bars), corrosion protection, sheaths (also referred to as sheathings), centralizers, and spacers, but specifically excludes the grout. The sheath is a smooth or corrugated pipe or tube that protects the prestressing steel in the unbounded length from corrosion. Centralizers position the tendon in the drill hole such that the specified minimum grout cover is achieved around the tendon. For multiple element tendons, spacers are used to separate the strands or bars of the tendons so that each element is adequately bonded to the anchor grout. The grout is a Portland cement based mixture that provides load transfer from the tendon to the ground and provides corrosion protection for the tendon.
 


Main types of grouted ground anchors

 


Components of a hollow bar anchor system

2-1- Types of Ground Anchors

There are three main ground anchor types that are currently used in practice: (1) straight shaft gravity-grouted ground anchors (Type A); (2) straight shaft pressure-grouted ground anchors (Type B); and (3) post-grouted ground anchors (Type C). Although not commonly used today in  practice, another type of anchor is the underreamed anchor (Type D).


Construction sequence of a hollow bar anchor system

Hollow bar anchor system
Another system which its application has extended over the past years is “hollow bar anchor system”. The self-drilling hollow bar anchor system is comprised of a hollow threaded bar with an attached drill bit that performs drilling, anchoring and grouting in a single operation. The hollow bar allows air and water to freely pass through the bar during drilling to remove debris and then allow grout to be injected immediately after drilling is completed. Grout fills the hollow bar and completely covers the entire bolt. Couplers can be used to join hollow bars and extend the bolt length while nuts and plates are used to provide the required tension. The self-drilling hollow bars are known for their excellent load capacity and are available in various diameters from 1 to 4 in. (25 to 103 mm).


Hollow bar anchor system

Features
• Drilling, anchor installation and grouting in single operation
• Separate anchor installation or removal of casing and drill rods is not required
• Similar installation methods for all ground conditions
• Choice of drill bits for different ground conditions
• Hollow core is used for flushing with air or water during drilling along with grouting the anchor tendon
• Flexibility in length by using couplers
• Ability to work with small drill rigs without casing in restricted headroom conditions
• Enhanced corrosion protection available upon request.