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Soil anchors

A soil anchor is a safety addition to buildings and construction processes, hence the importance of the anchor component as complex and unusual construction elements and their geotechnical and constructive aspects.

Soil anchors are used to stabilize buildings and to transfer loads from the front and surface to the depth of the ground. The soil anchor was developed as a modern solution for temporary strengthening of the soil and when it is necessary to carry out a deep excavation near existing buildings and/or to great depths that require the use of retaining walls. The anchor makes it possible to strengthen the lining walls and prevent collapse and in a way of transferring and distributing the loads in depth and width.

From a review of the Israeli standards and general and special specifications such as TI 940 part 4.2, it appears that these elements constitute a complex category, which includes a constructive aspect on the one hand, and a geotechnical aspect on the other hand. Engineer Moti Yuger, owner and CEO of Yuger Soil Consultants, explains: "Every year in Israel about -20,000 anchors are used, in many and varied projects. Most of the anchors (about 95%) are temporary anchors, replaced during construction by permanent structural elements, such as ceilings. The rest (about 5%) are permanent anchors that are supposed to be used for the entire length of the structure's existence, which reaches 120 years in public projects" (Yuger consultants, soil anchors in Israel - vision and reality).

Hence its great centrality in complex construction processes and their geotechnical and constructive consequences.

The soil anchor

Soil anchors are used in a variety of applications including: walled excavations for basements, in places where open excavations are not possible, due to considerations of space and risk to buildings and infrastructure, a means of protection against sliding of slopes, to receive the troubles expected from the planned construction such as dynamic forces caused by wind and earthquake disturbances, as well as lifting forces caused by Underground construction, below the groundwater level.

Sometimes the excavation is open and when it comes to excavation near an existing building and sometimes closed excavation under an existing building (for example digging a parking lot or basement) and for the benefit of strengthening the building against collapse or in a wide variety of needs such as, "walled excavations for basements, in places where open excavations are not possible, due to space considerations and risk to buildings and infrastructures, a means of protection against landslides, to receive the expected inconveniences from the planned construction such as dynamic forces caused by wind and earthquake disturbances, as well as uplift forces caused by underground construction, below the groundwater level".

In essence, soil anchors are used as a safety supplement in the construction process and therefore in 2011, a detailed Israeli standard was issued for the first time, dealing with soil anchors, TI 940 part 4.2, called "Geotechnical design: strengthening and stabilization of buildings for engineering purposes - soil anchors made of piles". This standard is defined as a recommended standard, which is not legally binding (unless it is determined as such in another legal framework, which adopts it as binding)" .

"Soil anchors have been used for years in the construction industry to stabilize buildings, slopes, retaining walls, dam piers, and more. The primary and most important purpose of a soil anchor is to transfer forces from the front of the building to a stable area in the subsoil. This stability is achieved through a significant increase in the normal forces acting on the planes potential destruction. Treading the soil anchors for labor required in advance, may reduce future displacements of the anchored structure. Soil anchor alternatives should take into account the advanced technology used to recruit relatively high labor forces, while immediately checking the short-term and long-term endurance" (Wikipedia).

In planning and characterizing the soil anchors, a number of technical and environmental indicators are taken into account, such as the type of soil, groundwater levels in the immediate area, nearby buildings, the type of soil and its coefficient of density and weight, and more.

Soil anchors can be a horizontal ground anchor or a vertical ground anchor and according to the strengthening requirements of the structure in question and have been found to be effective in a wide variety of soil types and uses. The depth of the anchor should be at least 15 meters and the average anchor can support up to 120 tons of weight and its load distribution is about 12 square meters from the retaining wall.


Soil anchor types

Soil chemical anchor - an anchor that differs in the type of material from which it is produced and the production method in the field based on injection, mixing or capsule (mixing materials from containers).

Mechanical anchor - an anchor whose locking is done by screwing or closing a nut head.

Another division is the way the anchor is used:

Temporary soil anchors - used to support decking walls for an acceptable limited period of up to 24 months (subject to the 940.4.2 standard). At the end of the castings, the load passes from the anchors to the ceiling and the walls and anchors themselves are practically disconnected.

Retractable soil anchors - function similarly to temporary anchors, but at the end of the work they are pulled out of the ground and in accordance with various space and environmental limitations.

Fixed soil anchors - long-term permanent support of the structure for a period of at least 120 years (subject to TI standard 940.4.2). Relying on fixed anchors is required in the absence of support alternatives such as ceiling and walls and they are made of reinforced materials and protection against corrosion and more.

Polymer soil anchors - the use of polymer is intended to enable future excavation and soil drilling in the construction area and for example in the future planning of transportation tunnels for trains or vehicles. The anchor itself is a temporary ground anchoring solution and is not used for long-term support of the upper structure.

Other anchors - soil anchors such as rock screws or earth nails and more.

It is important to note that choosing between a mechanical or chemical soil anchor varies and depends on many different variables and as a result of the intended use as permanent or temporary.

Possible failures in soil anchors
Anchors are a complex mechanical element in its structure, in the way it is installed and in the way it functions and in the existing length of the structure. Failures in anchors can be due to the following reasons:

Inadequate design of the support system, which includes the anchor + the constructive element it supports (conventional wall, reinforced concrete façade, bridge, etc.). An area under the responsibility of the planning team, mainly.

Failure in the structure of the anchor itself, on its various components, which includes mechanical components, protections against corrosion, drilling and installation of the anchor. This area, for the most part, is the responsibility of the anchor contractor.

Failure to test the anchor and guide it, including full monitoring of the anchor.

Anchors develop relatively high service tolerances for each anchor. This makes it possible to reduce the number of anchors per square meter of wall façade and makes them more economical than other anchoring methods. This advantage is accompanied by a disadvantage, since the system has lower redundancy, due to the fact that failure of a single anchor may create progressive failures in the anchors.

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