GROUND ANCHORS

Ground anchors consisting of cables or rods connected to a bearing plate are often used for the stabilization of steep slopes or slopes consisting of softer soils, as well as the enhancement of embankment or foundation soil capacity, or to prevent excessive erosion and landslides. The use of steel ground anchors is often constrained by overall durability in placement (due to weight), and the difficulty in maintaining tension levels in the anchor. Anchor systems fabricated from fiber reinforced composite materials show a number of benefits compared to conventional systems for the following reasons:

• Enhanced durability including resistance to corrosion and resistance to alkalis and solutions in soils increase their life and greatly reduce the need for maintenance, thereby decreasing life-cycle costs.
• Lighter weight results in easier transportation of cables to site, and increases the efficiency of handling and placement.
• Enhanced tensile strength coupled with lighter weight and enhanced mechanical properties results in greater safety during installation in areas with limited clearance.

In most cases, it is possible to use conventional jacking systems and still realize greater flexibility in placement and tensioning in difficult ground formations.

Composite ground anchors generically consist of three parts:

1. The anchorage is generally a stainless steel sheath with an anchor nut/plate through which the composite cable is run. The anchorage is usually filled with a non-shrink expansive cement mortar that ensures fixity and no slippage. The anchorage also is used to fasten the system to the outside structure.
2. The cable can consist of multiple rods that are separate or braided together, or a single rod.
3. A sheath or sleeve made from polyethylene or PVC that is fitted around the free anchor length of the cables.

System Details

Four different composite ground anchor systems are available.

1. Leadline Type System: Marketed by Mitsubishi Chemical Corporation and Chemical Grouting Company, Ltd., this type uses carbon fiber reinforced epoxy cables that usually have nine 8 mm diameter rods arranged in a circle with a single rod in the center. Each rod has cross-type indentations or spirals cut into it to provide interlock and stress transfer. A set of these in the anchorage are shown in Fig. 3.101.
2. CFCC Type System: Marketed by Tokyo Rope Manufacturing Co., Ltd., this type uses carbon fiber reinforced epoxy cables that are formed from 7 12.5 mm diameter rods twisted together and covered with epoxy. Figure 3.102 shows an example of this in its anchorage.


Fig. 3.101. Leadline type anchor system.



Fig. 3.102. CFCC type anchor system.

3. FiBRA Type System: Marketed by Shinko Wire Co., Ltd., this type uses aramid fiber reinforced epoxy cables that are formed through the braiding of individual strands into a tight bundle with 10.4 mm nominal diameter.
4. Technora Rod Type System: Marketed by Sumitomo Construction Co., Ltd., and Teijin, Ltd., this type uses aramid fiber reinforced vinylester cables that are formed through the use of nine individual 7.4 mm diameter rods that, like the Leadline system, are isolated but brought together in the anchorage. Unlike the Leadline system, wherein the rods are arranged in a circle with a single rod at the center, these rods are brought into close contact.

Table 3.25 provides a comparison of these different systems.

The components of a Technora aramid ground anchor are shown in Fig. 3.103. The front/top end consists of a stainless steel anchorage fixed to a bearing plate using nuts (which make initial tensioning and subsequent retensioning simple). The anchorage is filled with a non-shrink mortar. The free anchor length is enclosed in a polyethylene sheath joined to the anchorage protection pipe that extends beyond the filled anchorage section. Individual rods along the fixed anchor length are held together by plastic grips. The ends are enclosed in an end anchorage similar to the front anchorage. The entire system is fitted into a predrilled hole in the soil, which is then filled with grout.

Fig. 3.103. Details of an aramid ground anchor system.The jacking/tensioning of individual anchors is shown in Fig. 3.104. The components of a "Leadline" ground anchor system are shown in Fig. 3.105. The anchors at the front and at the end are both made of stainless steel and use an expansive non-shrink mortar. Set loss in the system is accounted for by using the tensioning nuts that facilitate adjustment through the service life of the system. The anchor system is inserted into a bore hole, whose diameter varies between 115 mm and 170 mm, depending on the designed anchor force and the prevalent ground conditions. A flowchart showing details of the installation procedure is given in Fig. 3.106 along with diagrams showing details of some of the steps.

Fig. 3.104. Jacking of Technora ground anchors.

Fig. 3.105. Components of a "Leadline" ground anchor system (all dimensions in mm).

Fig. 3.106. Flowchart of the installation process.

Examples of Applications

A list of some application sites showing anchor types, locations and dimensions is given in Table 3.26 (p. 83). Some examples are described in more depth below.

Slope Stabilization Along the Meishin Expressway

During the widening of the Kajiwara section of the Meishin Expressway near Osaka, a test section was used to validate the efficacy of using Technora ground anchors for slope stabilization. The site had electric pylons located at the crown of the slope, necessitating the stabilization of the slope to prevent future deformation. Thirty six anchors were installed at the site using bundles of 9 spiral wound rods of 7.4 mm diameter each. This configuration achieved a design stabilization force of 400 kN in a strata consisting of a top layer of weathered soil 3 to 5 m deep, covering a fractured zone of slate. Anchors 19 m to 30 m long were placed as shown in a representative cross-section in Fig. 3.107. The anchors (6.5 m long) were fixed in the slate layer in holes with a bore diameter of 115 mm. After completion of construction in September 1994, lightweight composite pressure plates were used at the surface as shown in Fig. 3.108. A variety of other, non-composite ground anchoring systems were also tested at the same site as part of an overall test program conducted by the Japan Highways Corp. and the Osaka Construction Bureau.

Fig. 3.107. Representative cross-section of slope.

Fig. 3.108. Meishin Expressway site showing composite pressure plates used with Technora ground anchors.

Slope Protection Along a National Highway in Kumamato Prefecture

In 1995, the Ministry of Construction commissioned the use of Technora aramid ground anchors in a pilot project. The aim was the technical certification of the system along the Kosedo section of a national road in the Kumamoto Prefecture. Road construction and related slope stabilization was necessary because of the rerouting of a section of the national highway due to construction of the Kawabe Dam. Sixty-five aramid ground anchors were used with lengths ranging from 7.8 m to 11 m along a section 30 m long and 8.5 m high. The top strata was a gravely soil to a depth of 4 m, below which was soft rock. Each ground anchor consisted of 9 deformed aramid rods of 7.4 mm diameter, each with a designed tensile load capacity of 392 kN. The anchor plates were set into a grid of reinforced concrete as shown in Fig. 3.109.

Fig. 3.109. Slope along the Kosedo section of the National Highway stabilized through the use of Technora aramid ground anchors.

Slope Construction for the Hokkaido Highway

CFCC ground anchors consisting of 7 12.5 mm rods were used for stabilization of a newly constructed slope along the Abuta section of the Jukan Highway in Hokkaido.

Fig. 3.110. Site with CFCC ground anchors.

The site used 28 ground anchors affixed to specially designed concrete pressure caps (Fig. 3.110) using a 6-piece stainless steel anchor clamped together as shown in the cross-section in Fig. 3.111.

Fig. 3.111. Representative section showing details of placement of the CFCC ground anchor.

Ground Anchors for a Stress Ribbon Bridge

Thirty two Leadline ground anchors (9 rods of 8 mm diameter each) were used to provide the anchorage for the stress ribbon bridge built for the Southern Yard Country Club. The anchors were post-tensioned through the concrete abutments as shown in the schematic in Fig. 3.112. Anchors were initially stressed to 80% of ultimate strength, which reduced to 70% at transfer for a final level of 60% under design load. Average anchor length was 26.5 m.

Fig. 3.112. Schematic showing placement of Leadline ground anchors in the Birdie stress ribbon bridge.

Table 3.26 lists examples of ground anchor applications.