How To Set A Diving Rock Bolt

Auger

Strap Connection

Strap

Cable

Strap Connection

Strap

Cable

Auger Anchor Bolts

Auger

Saddle Connection

Figure 6-12. Pin anchor fasteners.

Auger

Saddle Connection

Figure 6-13. U-rod configuration.

Figure 6-12. Pin anchor fasteners.

• Grouted Fasteners - In areas of significant wave and current action, grouted-in copper-nickel or stainless steel U-rods, as illustrated in Figure 6-13, can be used to immobilize split-pipe covered cable. Properly installed U-rods provide an excellent immobilization technique, but require good diving conditions and considerable time. U-rods must be installed tight against the top of the split-pipe or the resulting split-pipe movement and abrasion may lead to failure. Grouted fasteners can develop considerable holding strength in some weak seafloor rock, making them a better alternative than expansion anchors, suchasrockbolts. The clearance inside the U is set to just clear the projections on the type of split-pipe used, and the legs should allow a minimum of 10 inches insertion into the grout. The last 3 inches of each leg should be threaded and a nut installed before grouting to provide pullout resistance.

A drill jig, illustrated in Figure 6-14, is normally used to achieve proper spacing and alignment of the U-rod holes. Drilling of the holes, discussed in Section2.2.3, should allow at least 3/8 inch around the grouted fastener to permit distribution of the grout. The drill rod should be clearly marked to indicate when the hole is deep enough. A hole of insufficient depth will not allow the U-rod to fit snugly over the split-pipe, rendering it useless. The U-rods should then be grouted in place immediately using the techniques discussed in Section 2.11. Once the grout is dispensed in the hole, the U-rod must be pressed firmly against the top of the split-pipe. The grouted hole should be protected against grout washout.

• Rockbolts - Nongrouted rockbolts develop their anchoring strength by me-

Adjustable leg

Bridging channel

Locking c^-j pin

Pivot bolt

Pivoted clamping plate

Welded & ^ clamping plate

Bridging channel

Locking c^-j pin

Pivot bolt

Pivoted clamping plate

Figure 6-14. U-bolt drill jig.

chanically expanding the down hole end of the bolt, which develops an anchoring force through a combination of friction, adhesion between the anchor and rock, and physical penetration of the anchor into the rock. Rockbolts are classified into two types: drive-set and torque-set.

The drive-set fastener is assembled by placing a wedge into a slotted rod, which has a threaded upper end, and by positioning the rod into a predrilled hole. The anchor is then secured by driving the slotted rod over the wedge that rests on the bottom of the hole, causing the rod to expand into the rock.

A torque-set fastener has a wedge or cone threaded to the bottom of the bolt, and is surrounded by a sleeve. After inserting the bolt into the predrilled hole, torque is applied to the nut, pulling the bolt and cone up through the sleeve, thus securing the anchorage. The torque-set rockbolt is used extensively because of the availability of underwater tools that can supply the required torque.

When rockbolts are used in conjunction with split-pipe, the pipe itself can be used as the clamp. The holes in the flange are used as the drill jig and the rockbolts are installed through the flange into the predrilled hole. This limits the rockbolt size to 5/8-inch diameter, making it suitable only where hard rock seabeds exist. A minimum length of 12 inches is required to ensure that the rockbolt is adequately secured.

If rockbolts are to be used with split-pipe on weak or porous seafloor materials, then a larger bolt is required along with a saddle that spans the split-pipe. Figure 6-15 illustrates such an installation. The same type of rockbolts should be used on both sides of the saddle. If steel rockbolts are used, as shown on the left, sacrificial anodes should be attached to the rockbolt as shown. If titanium rockbolts are used, sacrificial anodes are not required. Titanium rockbolts are not commercially available and must be specially fabricated for the application.

6.2.2.3 Burial. Burial provides protection by allowing the cable to be placed below the surface of the seafloor. There are many techniques for cable burial: self-burial, jetting, controlled blasting, dredging and mechanical trenching, and drilled hole.

• Self-Burial - Cables lying on sands, silts, and soft clays will be self-burying because of the high unit weight of the cable and the strength reduction and scour of the underlying seabed caused by wave and current action. Sufficient slack must be provided to permit the cable system to follow the seabed surface, and the cable route should not pass over rock that will be exposed at some time during the design life of the system. Self-burial should not be used where damage from dragging anchors may occur. This technique involves no physical work out utvutzn mm / en hvo /mllm /»UN rtl(JCtUUHt6

Steel roclcbolt-

Anodes (used with steel rockbolts) PVC clamp

Locknut Washer

Steel roclcbolt-

Anodes (used with steel rockbolts) PVC clamp

Locknut Washer

Split-pipe

Titanium rockbolt " (No need for cathodic protection on titianium)

Split-pipe

Titanium rockbolt " (No need for cathodic protection on titianium)

Figure 6-15. Split-pipe installation using rockbolts, using either titanium bolts (shown on the right) or steel bolts with an anode.

side of the cable-laying operation and thus is the first choice where conditions permit. Split-pipe may be applied to provide additional weight to assist in promoting self-burial.

• Jetting - Cables can be buried in most noncohesive materials and in many cohesive soils by jetting techniques. These techniques are discussed in Section 2.6.

• Controlled Blasting - Successful cable stabilization requires either burying the cable in the seabed or securing the cable to the seabed. Where hard rock or coral seabeds exist, a relatively smooth route must be provided that minimizes bending in the cable and provides adequate support along its length. If not naturally occurring, the route can be prepared by either smoothing a path or trenching and providing a relatively smooth trench bottom. In very hard materials or in extremely rugged terrain, controlled blasting is a viable technique for preparing the route and excavating a trench. Controlled blastim? is discussed in <spotion 2.7.

• Dredging and Mechanical Trenching - The burial of cable by a dredge-type trencher generally requires laying the cable first along the cable route followed by trench excavation and cable insertion by the towed or self-propelled trencher. Trenching in the near shore zone is usually not performed because wave and current action tends to backfill the trench as soon as it is excavated. Dredging and mechanical trenching are not typically UCT operations.

• Drilled Hole - Cables can be installed in drilled holes starting just above the high water mark, running through the nearshore zone at some distance below the seafloor, and emerging from the seabed in deep water beyond the more aggressive environment. This technique requires sophisticated drilling machines and highly experienced personnel and is generally not in the scope of UCT operations. Commercial slant drilling has been successfully used to install pipeline through the surf zone at a distance to 6,000 feet from the shore entry point.

6.2.2.4 Tensioning. Tensioning of a cable provides a degree of stabilization by limiting the movement of a cable. The nearshore cable is anchored at either the shore or sea end while it is still supported by float balloons. The cable is then tensioned at the other end and that tension is maintained by a mushroom or clump anchor while the float balloons are removed and die cable is placed on the seabed.

Tensioning is best used as a supplement to other permanent-type cable stabilization techniques to reduce cable displacement produced by wave and current forces while the permanent stabilization is installed.

6.2.3 Planning and Estimating Data

The time required to carry out a nearshore cable installation project is highly dependent on the site conditions and varies widely. Bearing this in mind, Table 6-1 has been developed as an example to provide planning and estimating guidance for a nearshore cable installation project. The table is based on a project consisting of the installation of about 18,000 feet of SD List 3 cable. The inshore end is placed in an excavated trench extending from mean low water to a position about 500 feet inland. The nearshore soil is sand. Split-pipe stabilization is provided 300 feet seaward and 100 feet shoreward of the MLW mark. (Installation times for grouted anchors and rockbolts have been included in the table for planning projects with rock or coral sea-beds.) The cables are anchored to a precast deadman anchor installed shoreward of the mean low water mark. A sand dune located in the vicinity of the beach will be excavated to provide a 50-foot-wide level access for the cable landing and trenching operation. All trenches, excavations, and sand dunes created or disturbed during project activities will be restored to their original configuration. The cable will be laid to a point where the water depth reaches about 120 feet, assumed to be 3 miles offshore for this example.

Column 1 of Table 6-1 describes the general tasks of the cable installation operation. Column 2 lists the variable factors that affect the time required to carry out each task. Columns 3 and 4 list the low and high estimates of the number of days required to carry out the tasks described in Column 1. The difference in the low and high estimates is caused by variations in site conditions, weather conditions, remoteness of the site, and the skill of the UCT performing the work. These time estimates should be adiusted if the listed variafciec expected on a particular project are different than those indicated in Column 1 of the table.

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    How to set a diving rock bolt?
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