Earth Anchor Installation

Earth anchor installation:

The installation procedures differ with the type of soil and anchor.

For small anchors, such as a 50.8 mm (2 in) Arrowhead ground anchor, in soft soils, the earth anchor is driven into the ground with a drive rod and a sledge hammer or with a driving device similar to that used for steel fenceposts. The drive rod is inserted into the hole at the top of the anchor and is driven at the desired angle until it cannot be driven further or the desired depth is reached. The rod is removed, and then the anchor must be "set" or "keyed" by pulling on the anchor strap. You should be able to detect an increase in the resistance to the pull on the strap when the anchor begins to set. Experience will assist in developing a feel for the set point for a particular soil/anchor combination.

For the larger Arrowheads (152.4-mm [6-in] size or larger) and Manta Ray MR-1's, a hydraulic, pneumatic, or gasoline-driven impact hammer is needed to install the anchor. The installation procedure is the same as for the drive rod and sledge hammer except that the hammer is placed on the drive rod before the rod is inserted into the anchor.

In stiff soils, such as dense clays, a pilot hole can be augered before the anchor is driven. With a Manta Ray MR-1 anchor, for example, a 101.6-mm (4-in) pilot hole is augered at least 152.4 mm (6 in) deeper than the design depth. This leaves an area at the bottom of the hole for loose soil to accumulate. After the hole is augered, the anchor is driven by using the pilot hole as a guide. Because the Manta Ray MR-1 anchor is 177.8 mm (7 in) wide, 76.2 mm (3 in) of its width is driven through the undisturbed soil. Once the anchor is at the desired depth, the rod is pulled out, the hole is filled with soil and tamped, the anchor must be set.

The additional time and equipment required to auger a pilot hole before the anchor is driven is offset by several advantages. With fixed length drive rods, retrieving the rod after the anchor is driven becomes difficult if a pilot hole is not used. The friction of the soil on the rod is enough to require a mechanical pulling device for retrieval. It takes less time to drive an anchor by using a pilot hole than to proceed without a pilot hole. If an obstacle is encountered during the augering process, the auger can be pulled up and moved to a new hole site. However, if an obstacle is first encountered when the anchor is driven, it would be very difficult to retrieve the anchor to resite it, and the anchor could become damaged.

In rocky soils, it may be easier to make a pilot hole using a rock bit and impact hammer. In soft soils and at shallow depths, driving a pointed rod into the soil may be adequate for a pilot hole.

Both the Arrowhead anchor and the Manta Ray MR-1 anchor are installed by being driven into the ground. The soil toggle anchor is installed by dropping it down an augered hole. The smaller soil toggle requires a hole 152.4 mm (6 in) in diameter, and the larger model requires a hole 203.2 mm (8 in) in diameter. The soil toggle has a wing on each end, one blunt and the other sharp (figure 4). The anchor is placed blunt end down in the hole. This allows the anchor to slide down the hole without catching on a root or other obstacle. The hole is then filled and tamped. As the anchor is set, the pointed wing at the top will dig into the side of the hle and cause the anchor to rotate to its load-holding position.

The minimum depth of installation for the anchor types should be 0.9 m (3 ft) for the 50.8- or 101.6-mm (2- or 4- in) arrowhead anchors, 1.5 m (5 ft) for the Manta Ray MR-1 and small soil toggle anchor, or 2.4 (8 ft) for the large soil toggle anchor. If these depths cannot be reached, the installer should move a few feet and try to attain the proper depth. The production anchors should not be installed at a depth less than that at which feasibility tests are conducted. In stiff soils all anchors should be installed with the strap facing away from the direction of pull (figure 5).

The angle of installation should be decided after the direction of pull relative to the slope of the ground is determined. In general, for upslope and downslope pulls, the anchor should be installed perpendicular to the ground surface. As the angle of pulls nears perpendicular to the ground, the anchor should be installed vertically. The objective is to avoid having the direction of pull in line with the direction of installation and to maximize the distance of undisturbed soil between the installed anchor and the ground surface in the direction of pull (refer to figure 6). It is recommended that a trench be dug along the direction of pull so that the attachment cable tends to dig into the side of the installation hole and pulls the anchor toward undisturbed soil.