The un bonded length is that portion of the pre stressing steel that is free to elongate elastically and transfer the resisting force from the bond length to the structure. A bond breaker is a smooth plastic sleeve that is placed over the tendon in the un bonded length to prevent the pre stressing steel from bonding to the surrounding grout. It enables the pre stressing steel in the un bonded length to elongate without obstruction during testing and stressing and leaves the pre stressing steel un bonded after lock-off. The tendon bond length is that length of the pre stressing 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.
These and other components of a ground anchor are shown. 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
Types of Ground Anchor
There are three main ground anchor types that are currently used in U.S. practice: straight shaft gravity-grouted ground anchors; straight shaft pressure-grouted ground anchors; and post-grouted ground anchors. Although not commonly used today in U.S. practice, another type of anchor is the under reamed anchor.
Straight Shaft Gravity-Grouted Ground Anchors
Straight shaft gravity-grouted ground anchors are typically installed in rock and very stiff to hard
cohesive soil deposits using either rotary drilling or hollow-stem auger methods. Tremie (gravity
displacement) methods are used to grout the anchor in a straight shaft borehole. The borehole may
be cased or uncased depending on the stability of the borehole. Anchor resistance to pullout of the
grouted anchor depends on the shear resistance that is mobilized at the grout/ground interface.
Straight Shaft Pressure-Grouted Ground Anchors
Post-grouted ground anchors use delayed multiple grout injections to enlarge the grout body of
straight shafted gravity grouted ground anchors. Each injection is separated by one or two days.
Post grouting is accomplished through a sealed grout tube installed with the tendon. The tube is
equipped with check valves in the bond zone. The check valves allow additional grout to be injected under high pressure into the initial grout which has set. The high pressure grout fractures the initial grout and wedges it outward into the soil enlarging the grout body. Two fundamental types of post grouted anchors are used. One system uses a packer to isolate each valve. The other system pumps the grout down the post-grout tube without controlling which valves are opened.
Underreamed anchors consist of tremie grouted boreholes that include a series of enlargement bells
or underreams. This type of anchor may be used in firm to hard cohesive deposits. In addition to
resistance through side shear, as is the principal load transfer mechanism for other anchors, resistance may also be mobilized through end bearing. Care must be taken to form and clean the under reams.
Application of Soil Nails
Soil Nails are commonly used on retaining walls, for slope stabilization, to uplift slabs and even for concrete dam stabilization. They can also applied Ground Anchor techniques on the following:
Execute excavations neatly to create large construction plan without using props in order to make mechanized excavation.
Keep excavation walls sustainable, make very deep excavations without depending on the basement structure.
Anchors combine with soft retaining walls to redistribute the intemal forces of wall structure, so this can reduce the size, depth of steel bars in retaining walls.
It is necessary to use specifed equipment, exprienced professional engineers.
It is difficult to apply anchors in weak soil and to implement anchors with great depth.
Anchor execution would affect the land of surrounding construction works, which must be accepted by their owners.
Disadvantages of Soil Nails
Some of the disadvantages of soil nails are:
Soil nails are not applicable for large amount of groundwater seeps on excavation.
Permanent long term applications are not recommended.
Sand and gravels are not compatible on this process.
Not practical to use on areas with a high water table.