U

Drive rod

Manta Ray Duckbill

Sheet metal

.1-inch diam jet nozzle

Jetted Cone

Single or multi-helix

Screw

Figure 2-44. Direct embedment anchors.

Plate

Figure 2-45 Simple fabricated plate anchor. (Note: The length should be 1 to 1-1/2 times the width.)

Plate

Figure 2-45 Simple fabricated plate anchor. (Note: The length should be 1 to 1-1/2 times the width.)

Water is pumped into the upper end of the pipe and discharged at the bottom, thereby dislodging soil and permitting the cone and pipe to settle into the liquefied soil. The soil should be tamped thoroughly after jetting to redensify the soil and increase anchor capacity. Another method of increasing capacity is to pump grout through the pipe to fill any voids, enlarge the effective area of the cone shape, and to further compact the soil. If there are gravel or clay layers in the seabed, the cone should be eliminated from the assembly to simplify installation. Then, grout can be pumped to create a small bulb of grout at the base of the pipe to increase capacity. Studs can be located on the pipe to enhance grout-to-pipe bonding.

2.8.33 Screw Anchors. This anchor comes in single and multi-helix configurations. High capacity screw anchors are installed on land easily with truck- or jeep-mounted equipment, while lower capacity anchors are installed by hand or with a power digger. Small 4-, 6-, and 8-inch anchors are usually installed by hand by placing a bar through the shaft eye. Anchoring capacity depends on soil type, number and spacing of helixes, torsional strength of the shaft, and depth of anchor embedment. For safety, it is recommended that only the lower helix of a multi-helix screw anchor be considered when calculating anchor capacity.

Due to difficulties associated with torque resistance during installation, screw anchors are better suited to onshore anchoring applications. Use of screw anchors underwater is more difficult because of the need to resist the applied torque. To eliminate this problem for pipeline anchoring, a counter-rotating twin-screw anchor system that is surface controlled was developed by Anchoring Incorporated of Houston, Texas, to clamp the pipeline to the seafloor.

2.8.3.4 Plate Anchor Holding Capacity. There is a significant amount of data on the capacity of the various anchor types that is often conflicting and difficult to interpret. The following methods for estimating plate anchor capacity, Fa, in sands and clays are based on NFESC testing and are suitable for routine anchoring applications. These equations are valid for anchors embedded to at least five times the minimum anchor dimension after setting:

Sand

Fa = 15 * D * A — Offshore anchors and beach anchors below water table.

Fa = 25 * D * A — Beach anchors above water table.

Note: For jetted anchors in sand, reduce Fa by 50 percent to account for soil disturbance during jetting. For horizontal loading in sand, increase Fa by 25 percent.

Mud and Clay

Fa = anchor holding capacity (pounds)

D = anchor depth of embedment after setting (feet); for driven anchors, use driven depth minus one anchor length.

A = anchor area (square inches)

C = soil undrained shear strength (psi); this can be determined with the vane shear device provided in the geotechnical tool kit.

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